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  • 1.
    Abergel, David S. L.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Edge, Jonathan M.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    The role of spin-orbit coupling in topologically protected interface states in Dirac materials2014In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, p. 065012-Article in journal (Refereed)
    Abstract [en]

    We highlight the fact that two-dimensional (2D) materials with Dirac-like low energy band structures and spin-orbit coupling (SOC) will produce linearly dispersing topologically protected Jackiw-Rebbi modes at interfaces where the Dirac mass changes sign. These modes may support persistent spin or valley currents parallel to the interface, and the exact arrangement of such topologically protected currents depends crucially on the details of the SOC in the material. As examples, we discuss buckled 2D hexagonal lattices such as silicene or germanene, and transition metal dichalcogenides such as MoS2.

  • 2. Ahmed, Towfiq
    et al.
    Albers, R. C.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Friedrich, C.
    Zhu, Jian-Xin
    GW quasiparticle calculations with spin-orbit coupling for the light actinides2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 3, p. 035104-Article in journal (Refereed)
    Abstract [en]

    We report on the importance of GW self-energy corrections for the electronic structure of light actinides in the weak-to-intermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu using a one-shot GW approximation that includes spin-orbit coupling within a full potential LAPW framework. We also present RPA screened effective Coulomb interactions for the f-electron orbitals for different lattice constants, and show that there is an increased contribution from electron-electron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems.

  • 3. Ahmed, Towfiq
    et al.
    Haraldsen, Jason T.
    Rehr, John J.
    Di Ventra, Massimiliano
    Schuller, Ivan
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Correlation dynamics and enhanced signals for the identification of serial biomolecules and DNA bases2014In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 25, no 12, p. 125705-Article in journal (Refereed)
    Abstract [en]

    Nanopore-based sequencing has demonstrated a significant potential for the development of fast, accurate, and cost-efficient fingerprinting techniques for next generation molecular detection and sequencing. We propose a specific multilayered graphene-based nanopore device architecture for the recognition of single biomolecules. Molecular detection and analysis can be accomplished through the detection of transverse currents as the molecule or DNA base translocates through the nanopore. To increase the overall signal-to-noise ratio and the accuracy, we implement a new 'multi-point cross-correlation' technique for identification of DNA bases or other molecules on the single molecular level. We demonstrate that the cross-correlations between each nanopore will greatly enhance the transverse current signal for each molecule. We implement first-principles transport calculations for DNA bases surveyed across a multilayered graphene nanopore system to illustrate the advantages of the proposed geometry. A time-series analysis of the cross-correlation functions illustrates the potential of this method for enhancing the signal-to-noise ratio. This work constitutes a significant step forward in facilitating fingerprinting of single biomolecules using solid state technology.

  • 4. Ahmed, Towfiq
    et al.
    Haraldsen, Jason T.
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Royal Institute of Technology, Sweden; Los Alamos National Laboratory, USA.
    Next-Generation Epigenetic Detection Technique: Identifying Methylated Cytosine Using Graphene Nanopore2014In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 5, no 15, p. 2601-2607Article in journal (Refereed)
    Abstract [en]

    DNA methylation plays a pivotal role in the genetic evolution of both embryonic and adult cells. For adult somatic cells, the location and dynamics of methylation have been very precisely pinned down with the 5-cytosine markers on cytosine-phosphate-guanine (CpG) units. Unusual methylation on CpG islands is identified as one of the prime causes for silencing the tumor suppressant genes. Early detection of methylation changes can diagnose the potentially harmful oncogenic evolution of cells and provide promising guideline for cancer prevention. With this motivation, we propose a cytosine methylation detection technique. Our hypothesis is that electronic signatures of DNA acquired as a molecule translocates through a nanopore would be significantly different for methylated and nonmethylated bases. This difference in electronic fingerprints would allow for reliable real-time differentiation of methylated DNA. We calculate transport currents through a punctured graphene membrane while the cytosine and methylated cytosine translocate through the nanopore. We also calculate the transport properties for uracil and cyanocytosine for comparison. Our calculations of transmission, current, and tunneling conductance show distinct signatures in their spectrum for each molecular type. Thus, in this work, we provide a theoretical analysis that points to a viability of our hypothesis.

  • 5. Altfeder, Igor
    et al.
    Voevodin, Andrey A.
    Check, Michael H.
    Eichfeld, Sarah M.
    Robinson, Joshua A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Scanning Tunneling Microscopy Observation of Phonon Condensate2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 43214Article in journal (Refereed)
    Abstract [en]

    Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase-and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

  • 6.
    Balatsky, Alexander
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Gudnason, Sven Bjarke
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Krikun, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Alikhanov Institute for Theoretical & Experimental Physics, Russia.
    Thorlacius, Lárus
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Iceland, Iceland.
    Zarembo, Konstantin
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden; Alikhanov Institute for Theoretical & Experimental Physics, Russia.
    Classical and quantum temperature fluctuations via holography2015In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 1, article id 011Article in journal (Refereed)
    Abstract [en]

    We study local temperature fluctuations in a 2+1 dimensional CFT on the sphere, dual to a black hole in asymptotically AdS spacetime. The fluctuation spectrum is governed by the lowest-lying hydrodynamic modes of the system whose frequency and damping rate determine whether temperature fluctuations are thermal or quantum. We calculate numerically the corresponding quasinormal frequencies and match the result with the hydrodynamics of the dual CFT at high temperature. As a by-product of our analysis we determine the appropriate boundary conditions for calculating low-lying quasinormal modes for a four-dimensional Reissner-Nordstrom black hole in global AdS.

  • 7.
    Balatsky, Alexander V.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Balatsky, Galina I.
    Borysov, Stanislav S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Resource Demand Growth and Sustainability Due to Increased World Consumption2015In: Sustainability, ISSN 2071-1050, E-ISSN 2071-1050, Vol. 7, no 3, p. 3430-3440Article in journal (Refereed)
    Abstract [en]

    The paper aims at continuing the discussion on sustainability and attempts to forecast the impossibility of the expanding consumption worldwide due to the planet's limited resources. As the population of China, India and other developing countries continue to increase, they would also require more natural and financial resources to sustain their growth. We coarsely estimate the volumes of these resources (energy, food, freshwater) and the gross domestic product (GDP) that would need to be achieved to bring the population of India and China to the current levels of consumption in the United States. We also provide estimations for potentially needed immediate growth of the world resource consumption to meet this equality requirement. Given the tight historical correlation between GDP and energy consumption, the needed increase of GDP per capita in the developing world to the levels of the U.S. would deplete explored fossil fuel reserves in less than two decades. These estimates predict that the world economy would need to find a development model where growth would be achieved without heavy dependence on fossil fuels.

  • 8.
    Banerjee, Saikat
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Royal Institute of Technology, Sweden; Los Alamos National Laboratory, USA.
    Fransson, J.
    Black-Schaffer, A. M.
    Ågren, H.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Granular superconductor in a honeycomb lattice as a realization of bosonic Dirac material2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 13, article id 134502Article in journal (Refereed)
    Abstract [en]

    We examine the low-energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in a honeycomb lattice structure. Using the example of graphene, we present evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the K and K' points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an intergrain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with fermionic Dirac materials. The Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of bosonic Dirac materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.

  • 9.
    Banerjee, Saikat
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Royal Institute of Technology, Sweden; Los Alamos National Laboratory, USA.
    Ågren, Hans
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Landau-like states in neutral particles2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 23, article id 235134Article in journal (Refereed)
    Abstract [en]

    We show the emergence of a new type of dispersion relation for neutral atoms with an interesting similarity to the spectrum of two-dimensional electrons in an applied perpendicular constant magnetic field. These neutral atoms can be confined in toroidal optical traps and give quasi-Landau spectra. In strong contrast to the equidistant infinitely degenerate Landau levels for charged particles, the spectral gap for such two-dimensional neutral particles increases in particular electric-field configurations. The idea in the paper is motivated by the development in cold atom experiments and builds on the seminal paper of Aharonov and Casher.

  • 10. Björnson, Kristofer
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; ETH Institute for Theoretical Studies, Switzerland.
    Black-Schaffer, Annica M.
    Superconducting order parameter pi-phase shift in magnetic impurity wires2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 10, article id 104521Article in journal (Refereed)
    Abstract [en]

    It has previously been found that amagnetic impurity in a conventional s-wave superconductor can give rise to a local pi-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the pi-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When nonresonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a pi-phase shift. We also show that for a finite spin-orbit coupling, the pi-phase shift is preserved and occurs in a large portion of the topologically nontrivial phase.

  • 11. Björnson, Kristofer
    et al.
    Pershoguba, Sergey S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Black-Schaffer, Annica M.
    Spin-polarized edge currents and Majorana fermions in one- and two-dimensional topological superconductors2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 21, article id 214501Article in journal (Refereed)
    Abstract [en]

    We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an s-wave superconductor with Rashba spin-orbit interaction. We solve self-consistently for the superconducting order parameter and investigate both two-dimensional blocks and one-dimensional wires of ferromagnetic impurities, with the magnetic moments pointing both perpendicular and parallel to the surface. We find that the topologically protected edge states of ferromagnetic blocks give rise to spin-polarized edge currents, but that the total persistent current flows in opposite direction to what is expected from the dispersion relation of the edge states. We also show that the Majorana fermions at the end points of one-dimensional wires are spin polarized, which can be directly related to the spin polarization of the edge currents of two-dimensional blocks. Connections are also made to the physics of the Yu-Shiba-Rusinov states for zero-dimensional impurities.

  • 12. Black-Schaffer, Annica M.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, United States Department of Energy (DOE) .
    Odd-frequency superconducting pairing in multiband superconductors2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 10, p. 104514-Article in journal (Refereed)
    Abstract [en]

    We point out that essentially all multiband superconductors have an odd-frequency pairing component, as follows from a general symmetry analysis of even- and odd-frequency pairing states. We show that odd-frequency superconducting pairing requires only a finite band hybridization, or scattering, and nonidentical intraband order parameters, of which only one band needs to be superconducting. Under these conditions odd-frequency odd-interband pairing is always present. From a symmetry analysis we establish a complete reciprocity between parity in band index and frequency.

  • 13. Black-Schaffer, Annica M.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory .
    Proximity-induced unconventional superconductivity in topological insulators2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 22, p. 220506-Article in journal (Refereed)
    Abstract [en]

    We study and classify the proximity-induced superconducting pairing in a topological insulator (TI)-superconductor (SC) hybrid structure for SCs with different symmetries. The Dirac surface state gives a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for p-wave SCs. We also find that all SCs induce pairing that is odd in both frequency and orbital (band) index, with oddness in frequency and orbital index being completely interchangeable. The different induced pairing amplitudes significantly modify the density of states in the TI surface layer.

  • 14.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; KTH Royal Institute of Technology, Sweden.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Cross-Correlation Asymmetries and Causal Relationships between Stock and Market Risk2014In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 9, no 8, p. e105874-Article in journal (Refereed)
    Abstract [en]

    We study historical correlations and lead-lag relationships between individual stock risk (volatility of daily stock returns) and market risk (volatility of daily returns of a market-representative portfolio) in the US stock market. We consider the cross-correlation functions averaged over all stocks, using 71 stock prices from the Standard & Poor's 500 index for 1994-2013. We focus on the behavior of the cross-correlations at the times of financial crises with significant jumps of market volatility. The observed historical dynamics showed that the dependence between the risks was almost linear during the US stock market downturn of 2002 and after the US housing bubble in 2007, remaining at that level until 2013. Moreover, the averaged cross-correlation function often had an asymmetric shape with respect to zero lag in the periods of high correlation. We develop the analysis by the application of the linear response formalism to study underlying causal relations. The calculated response functions suggest the presence of characteristic regimes near financial crashes, when the volatility of an individual stock follows the market volatility and vice versa.

  • 15.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Geilhufe, R. Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). ETH Zürich, Germany.
    Organic materials database: An open-access online database for data mining2017In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 12, no 2, article id e0171501Article in journal (Refereed)
    Abstract [en]

    We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band structures, which is freely accessible online at http://omdb.diracmaterials.org. The OMDB focus lies on electronic structure, density of states and other properties for purely organic and organometallic compounds that are known to date. The electronic band structures are calculated using density functional theory for the crystal structures contained in the Crystallography Open Database. The OMDB web interface allows users to retrieve materials with specified target properties using non-trivial queries about their electronic structure. We illustrate the use of the OMDB and how it can become an organic part of search and prediction of novel functional materials via data mining techniques. As a specific example, we provide data mining results for metals and semiconductors, which are known to be rare in the class of organic materials.

  • 16.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Olsthoorn, Bart
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics.
    Gedik, M. Berk
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Geilhufe, R. Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Online search tool for graphical patterns in electronic band structures2018In: Npj Computational Materials, ISSN 2057-3960, Vol. 4, article id 46Article in journal (Refereed)
    Abstract [en]

    Many functional materials can be characterized by a specific pattern in their electronic band structure, for example, Dirac materials, characterized by a linear crossing of bands; topological insulators, characterized by a Mexican hat pattern or an effectively free electron gas, characterized by a parabolic dispersion. To find material realizations of these features, manual inspection of electronic band structures represents a relatively easy task for a small number of materials. However, the growing amount of data contained within modern electronic band structure databases makes this approach impracticable. To address this problem, we present an automatic graphical pattern search tool implemented for the electronic band structures contained within the Organic Materials Database. The tool is capable of finding user-specified graphical patterns in the collection of thousands of band structures from high-throughput calculations in the online regime. Using this tool, it only takes a few seconds to find an arbitrary graphical pattern within the ten electronic bands near the Fermi level for 26,739 organic crystals. The source code of the developed tool is freely available and can be adapted to any other electronic band structure database.

  • 17.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Roudi, Yasser
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). The Kavli Institute for Systems Neuroscience, NTNU, Norway.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, USA .
    US stock market interaction network as learned by the Boltzmann machine2015In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 88, no 12Article in journal (Refereed)
    Abstract [en]

    We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented results show that binarization preserves the correlation structure of the market. Properties of distributions of external fields and couplings as well as the market interaction network and industry sector clustering structure are studied for different historical dates and moving window sizes. We demonstrate that the observed positive heavy tail in distribution of couplings is related to the sparse clustering structure of the market. We also show that discrepancies between the model's parameters might be used as a precursor of financial instabilities.

  • 18. Boyko, D.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Haraldsen, J. T.
    Evolution of magnetic Dirac bosons in a honeycomb lattice2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014433Article in journal (Refereed)
    Abstract [en]

    We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

  • 19. Commeau, Benjamin
    et al.
    Geilhufe, R. Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Fernando, Gayanath W.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Structural and electronic properties of alpha-(BEDT-TTF)(2)I-3, ss-(BEDT-TTF)(2)I-3, and kappa-(BEDT-TTF)(2)X-3 (X = I, F, Br, Cl) organic charge transfer salts2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 12, article id 125135Article in journal (Refereed)
    Abstract [en]

    (BEDT-TFF)(2)I-3 charge transfer salts are reported to show superconductivity and pressure-induced quasi-twodimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases alpha, beta, and kappa(.) We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in kappa-(BEDT-TTF)(2)I-3 achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of kappa-(BEDT-TTF)(2)F-3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

  • 20. Creange, Nicole
    et al.
    Constantin, Costel
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Haraldsen, Jason T.
    Computational Investigation of the Electronic and Optical Properties of Planar Ga-Doped Graphene2015In: Advances in Condensed Matter Physics, ISSN 1687-8108, E-ISSN 1687-8124, article id 635019Article in journal (Refereed)
    Abstract [en]

    We simulate the optical and electrical responses in gallium-doped graphene. Using density functional theory with a local density approximation, we simulate the electronic band structure and show the effects of impurity doping (0-3.91%) in graphene on the electron density, refractive index, optical conductivity, and extinction coefficient for each doping percentage. Here, gallium atoms are placed randomly (using a 5-point average) throughout a 128-atom sheet of graphene. These calculations demonstrate the effects of hole doping due to direct atomic substitution, where it is found that a disruption in the electronic structure and electron density for small doping levels is due to impurity scattering of the electrons. However, the system continues to produce metallic or semimetallic behavior with increasing doping levels. These calculations are compared to a purely theoretical 100% Ga sheet for comparison of conductivity. Furthermore, we examine the change in the electronic band structure, where the introduction of gallium electronic bands produces a shift in the electron bands and dissolves the characteristic Dirac cone within graphene, which leads to better electron mobility.

  • 21. Crook, Charles B.
    et al.
    Houchins, Gregory
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Constantin, Costel
    Haraldsen, Jason T.
    Spatial dependence of the super-exchange interactions for transition-metal trimers in graphene2018In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 123, no 1, article id 013903Article in journal (Refereed)
    Abstract [en]

    This study examines the magnetic interactions between spatially variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce a Ruderman-Kittel-Kasuya-Yosidalike super-exchange coupling. It is hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.

  • 22. Das, Tanmoy
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory .
    Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures2013In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, p. 1972-Article in journal (Refereed)
    Abstract [en]

    Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the G-point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.

  • 23. Das, Tanmoy
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory United States Department of Energy (DOE) .
    Origin of pressure induced second superconducting dome in AyFe(2-x)Se(2) [A = K, (TI, Rb)]2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. 093045-Article in journal (Refereed)
    Abstract [en]

    Recent observation of a pressure induced second superconducting phase in A(y)Fe(2-x)Se(2) [A = K, (TI, Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose the model where pressure induces renormalization of band parameters in such a way that it leads to changes in Fermi surface topology even for a fixed electron number. We develop a low-energy effective model, derived from first-principles band-structure calculation at finite pressure, to suggest the phase assignment where a low pressure superconducting state with no hole pocket at the 0 point is a nodeless d-wave state. It evolves into a s(+/-) state at higher pressure when the Fermi surface topology changes and the hole pocket appears. We analyze the pairing interactions using a five band tight binding fitted band structure and find that a strong pairing strength is dependent on pressure. We also evaluate the energy and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.

  • 24.
    Dunnett, Kirsty
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Narayan, Awadhesh
    Spaldin, N. A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; University of Connecticut, USA.
    Strain and ferroelectric soft-mode induced superconductivity in strontium titanate2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 14, article id 144506Article in journal (Refereed)
    Abstract [en]

    We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviors under uniaxial and biaxial strain can be understood.

  • 25.
    Dunnett, Kirsty
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Zhu, J. -X.
    Spaldin, N. A.
    Juricic, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Dynamic Multiferroicity of a Ferroelectric Quantum Critical Point2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 5, article id 057208Article in journal (Refereed)
    Abstract [en]

    Quantum matter hosts a large variety of phases, some coexisting, some competing; when two or more orders occur together, they are often entangled and cannot be separated. Dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetization, is an example where the two orders are impossible to disentangle. Here we demonstrate an elevated magnetic response of a ferroelectric near the ferroelectric quantum critical point (FE QCP), since magnetic fluctuations are entangled with ferroelectric fluctuations. We thus suggest that any ferroelectric quantum critical point is an inherent multiferroic quantum critical point. We calculate the magnetic susceptibility near the FE QCP and find a region with enhanced magnetic signatures near the FE QCP and controlled by the tuning parameter of the ferroelectric phase. The effect is small but observable-we propose quantum paraelectric strontium titanate as a candidate material where the magnitude of the induced magnetic moments can be similar to 5 x 10(-7) mu(B) per unit cell near the FE QCP.

  • 26.
    Edge, J. M.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Upper Critical Field as a Probe for Multiband Superconductivity in Bulk and Interfacial STO2015In: Journal of Superconductivity and Novel Magnetism, ISSN 1557-1939, E-ISSN 1557-1947, Vol. 28, no 8, p. 2373-2384Article in journal (Refereed)
    Abstract [en]

    We investigate the temperature dependence of the upper critical field H-c2 as a tool to probe the possible presence of multiband superconductivity at the interface between LaAlO3 and SrTiO3 (LAO/STO). The behaviour of H-c2 can clearly indicate two-band superconductivity through its nontrivial temperature dependence. For the disorder scattering dominated two-dimensional LAO/STO interface, we find a characteristic non-monotonic curvature of the H-c2(T). We also analyse the H-c2 for multiband bulk STO and find similar behaviour.

  • 27.
    Edge, Jonathan M.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Aschauer, Ulrich
    Spaldin, Nicola A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Quantum Critical Origin of the Superconducting Dome in SrTiO32015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 24, article id 247002Article in journal (Refereed)
    Abstract [en]

    We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped SrTiO3 using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing O-18 isotope substitution, a scenario that is experimentally verifiable. Our model is applicable to other quantum paraelectrics, such as KTaO3.

  • 28. Fang, Kun
    et al.
    Fernando, G. W.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Institute for Materials Science, Los Alamos, USA.
    Kocharian, A. N.
    Possible phase separation in square and honeycomb Hubbard model: A variational cluster study2015In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 379, no 37, p. 2230-2238Article in journal (Refereed)
    Abstract [en]

    The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole doping manifested by spatial inhomogeneities of coexisting different electron densities at equilibrium. Phase separation is accompanied by spectral weight loss and first Brillouin zone boundary deformation. Such an instability is observed in square structures and it is absent in honeycomb lattices. To our knowledge, no previous publications have revealed relationship between a Fermi surface instability and phase separation. Our VCA calculations provide strong support for this spontaneous instability, driven by electron correlations in specific lattice geometries, proposed in our earlier publications using exact quantum cluster calculations.

  • 29. Fang, Kun
    et al.
    Fernando, G. W.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory United States Department of Energy (DOE) .
    Kocharian, A. N.
    Palandage, K.
    Pairing modulations and phase separation instabilities in Bi2Sr2CaCu2O8+delta2014In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 378, no 3, p. 243-248Article in journal (Refereed)
    Abstract [en]

    There is growing evidence that the unconventional spatial inhomogeneities in the doped high-T-c superconductors are accompanied by the pairing of electrons, subsequent phase transitions and condensation into coherent states. We show that such pairing states can be obtained from phase separation instabilities near level crossings. Conditions for coherent pairing instabilities are examined using exact diagonalization of Hubbard-like pyramid structures under variation of coupling and interaction strengths. We also evaluate the behavior of the energy charge gap in the vicinity of level crossings using a parametrization of coupling to the apical site to represent out-of-plane effects. These results provide a simple microscopic explanation of (correlation induced) supermodulation of the coherent pairing gap observed in scanning tunneling microscopy measurements at atomic scale in Bi2Sr2CaCu2O8+delta.

  • 30. Fernandes, R. M.
    et al.
    Haraldsen, J. T.
    Woelfle, P.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Two-band superconductivity in doped SrTiO3 films and interfaces2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 1, p. 014510-Article in journal (Refereed)
    Abstract [en]

    We investigate the possibility of multiband superconductivity in SrTiO3 films and interfaces using a two-dimensional two-band model. In the undoped compound, one of the bands is occupied whereas the other is empty. As the chemical potential shifts due to doping by negative charge carriers or application of an electric field, the second band becomes occupied and gives rise to a strong enhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO3, we find that intraband pairing dominates over interband pairing, unlike other known multiband superconductors. Given the similarities with the value of the transition temperature and with the band structure of LaAlO3/SrTiO3 heterostructures, we speculate that the superconductivity observed in SrTiO3 interfaces may be similar in nature to that of bulk SrTiO3, involving multiple bands with distinct electronic occupations.

  • 31. Fransson, J.
    et al.
    Black-Schaffer, A. M.
    Balatsky, A. V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Institute for Materials Science, USA.
    Engineered near-perfect backscattering on the surface of a topological insulator with nonmagnetic impurities2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 24, p. 241409-Article in journal (Refereed)
    Abstract [en]

    We show how to engineer enhanced skew scattering and gaplike openings in the surface state of three-dimensional topological insulators using only nonmagnetic impurities. Enhanced skewscattering off nonmagnetic impurities is obtained as a finite size effect of the scattering potential. Intimately related to the generated skew scattering is the emergence of a gaplike density of electron states locally around the impurities and surrounded by sharp resonances, with an extended energy gap appearing in engineered impurity structures.

  • 32. Fransson, J.
    et al.
    Black-Schaffer, A. M.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Institute for Materials Science, Los Alamos, USA.
    Magnon Dirac materials2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 7, article id 075401Article in journal (Refereed)
    Abstract [en]

    We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.

  • 33. Fransson, J.
    et al.
    She, J. -H
    Pietronero, L.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Inelastic electron tunneling spectroscopy at local defects in graphene2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 24, p. 245404-Article in journal (Refereed)
    Abstract [en]

    We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks or dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy d(2)I/dV(2) using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative U center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative U formation and its evolution from weak to strong coupling. The negative U site effectively acts as a local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac-cone-like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.

  • 34.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; University of Connecticut, USA.
    Symmetry analysis of odd- and even-frequency superconducting gap symmetries for time-reversal symmetric interactions2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 2, article id 024507Article in journal (Refereed)
    Abstract [en]

    Odd-frequency superconductivity describes a class of superconducting states where the superconducting gap is an odd function in relative time and Matsubara frequency. We present a group theoretical analysis based on the linearized gap equation in terms of Shubnikov groups of the second kind. By discussing systems with spin-orbit coupling and an interaction kernel which is symmetric under the reversal of relative time, we show that both even-and odd-frequency gaps are allowed to occur. Specific examples are discussed for the square lattice, the octahedral lattice, and the tetragonal lattice. For irreducible representations that are even under the reversal of relative time the common combinations of s- and d-wave spin singlet and p-wave spin triplet gaps are revealed, irreducible representations that are odd under reversal of relative time give rise to s- and d-wave spin triplet and p-wave spin singlet gaps. Furthermore, we discuss the construction of a generalized Ginzburg-Landau theory in terms of the associated irreducible representations. The result complements the established classification of superconducting states of matter.

  • 35.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Borysov, Stanislav S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Bouhon, Adrien
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; ETH Institute for Theoretical Studies, Switzerland.
    Data Mining for Three-Dimensional Organic Dirac Materials: Focus on Space Group 192017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 7298Article in journal (Refereed)
    Abstract [en]

    We combined the group theory and data mining approach within the Organic Materials Database that leads to the prediction of stable Dirac-point nodes within the electronic band structure of three-dimensional organic crystals. We find a particular space group P2(1)2(1)2(1) (#19) that is conducive to the Dirac nodes formation. We prove that nodes are a consequence of the orthorhombic crystal structure. Within the electronic band structure, two different kinds of nodes can be distinguished: 8-fold degenerate Dirac nodes protected by the crystalline symmetry and 4-fold degenerate Dirac nodes protected by band topology. Mining the Organic Materials Database, we present band structure calculations and symmetry analysis for 6 previously synthesized organic materials. In all these materials, the Dirac nodes are well separated within the energy and located near the Fermi surface, which opens up a possibility for their direct experimental observation.

  • 36.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Borysov, Stanislav S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kalpakchi, Dmytro
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Towards novel organic high-T-c superconductors: Data mining using density of states similarity search2018In: Physical review materials, ISSN 2475-9953, Vol. 2, no 2, article id 024802Article in journal (Refereed)
    Abstract [en]

    Identifying novel functional materials with desired key properties is an important part of bridging the gap between fundamental research and technological advancement. In this context, high-throughput calculations combinedwith data-mining techniques highly accelerated this process in different areas of research during the past years. The strength of a data-driven approach for materials prediction lies in narrowing down the search space of thousands of materials to a subset of prospective candidates. Recently, the open-access organic materials database OMDBwas released providing electronic structure data for thousands of previously synthesized three-dimensional organic crystals. Based on the OMDB, we report about the implementation of a novel density of states similarity search tool which is capable of retrieving materials with similar density of states to a reference material. The tool is based on the approximate nearest neighbor algorithm as implemented in the ANNOY library and can be applied via the OMDB web interface. The approach presented here is wide ranging and can be applied to various problems where the density of states is responsible for certain key properties of a material. As the first application, we report about materials exhibiting electronic structure similarities to the aromatic hydrocarbon p-terphenyl which was recently discussed as a potential organic high-temperature superconductor exhibiting a transition temperature in the order of 120 K under strong potassium doping. Although the mechanism driving the remarkable transition temperature remains under debate, we argue that the density of states, reflecting the electronic structure of a material, might serve as a crucial ingredient for the observed high T-c. To provide candidates which might exhibit comparable properties, we present 15 purely organic materials with similar features to p-terphenyl within the electronic structure, which also tend to have structural similarities with p-terphenyl such as space group symmetries, chemical composition, and molecular structure. The experimental verification of these candidates might lead to a better understanding of the underlying mechanism in case similar superconducting properties are revealed.

  • 37.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Bouhon, Adrien
    Borysov, Stanislav S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Three-dimensional organic Dirac-line materials due to nonsymmorphic symmetry: A data mining approach2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 4, article id 041103Article in journal (Refereed)
    Abstract [en]

    A datamining study of electronic Kohn-Sham band structures was performed to identify Dirac materials within the Organic Materials Database. Out of that, the three-dimensional organic crystal 5,6-bis(trifluoromethyl)-2-methoxy-1H-1,3-diazepine was found to host different Dirac-line nodes within the band structure. From a group theoretical analysis, it is possible to distinguish between Dirac-line nodes occurring due to twofold degenerate energy levels protected by the monoclinic crystalline symmetry and twofold degenerate accidental crossings protected by the topology of the electronic band structure. The obtained results can be generalized to all materials having the space group P2(1)/c (No. 14, C-2h(5)) by introducing three distinct topological classes.

  • 38.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Olsthoorn, Bart
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ferella, Alfredo D.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Koski, Timo
    Kahlhoefer, Felix
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA; Los Alamos National Laboratory, USA.
    Materials Informatics for Dark Matter Detection2018In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 12, no 11, article id 1800293Article in journal (Refereed)
    Abstract [en]

    Dark Matter particles are commonly assumed to be weakly interacting massive particles (WIMPs) with a mass in the GeV to TeV range. However, recent interest has shifted toward lighter WIMPs, which are more difficult to probe experimentally. A detection of sub-GeV WIMPs will require the use of small gap materials in sensors. Using recent estimates of the WIMP mass, we identify the relevant target space toward small gap materials (100 to 10 meV). Dirac Materials, a class of small- or zero-gap materials, emerge as natural candidates for sensors for Dark Matter detection. We propose the use of informatics tools to rapidly assay materials band structures to search for small gap semiconductors and semimetals, rather than focusing on a few preselected compounds. As a specific example of the proposed strategy, we use the organic materials database () to identify organic candidates for sensors: the narrow band gap semiconductors BNQ-TTF and DEBTTT with gaps of 40 and 38 meV, and the Dirac-line semimetal (BEDT-TTF)center dot Br which exhibits a tiny gap of approximate to 50 meV when spin-orbit coupling is included. We outline a novel and powerful approach to search for dark matter detection sensor materials by means of a rapid assay of materials using informatics tools.

  • 39. Hafiz, Hasnain
    et al.
    Khair, Adnan Ibne
    Choi, Hongchul
    Mueen, Abdullah
    Bansil, Arun
    Eidenbenz, Stephan
    Wills, John
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Ahmed, Towfiq
    A high-throughput data analysis and materials discovery tool for strongly correlated materials2018In: Npj Computational Materials, ISSN 2057-3960, Vol. 4, article id 63Article in journal (Refereed)
    Abstract [en]

    Modeling of f-electron systems is challenging due to the complex interplay of the effects of spin-orbit coupling, electron-electron interactions, and the hybridization of the localized f-electrons with itinerant conduction electrons. This complexity drives not only the richness of electronic properties but also makes these materials suitable for diverse technological applications. In this context, we propose and implement a data-driven approach to aid the materials discovery process. By deploying state-of-the-art algorithms and query tools, we train our learning models using a large, simulated dataset based on existing actinide and lanthanide compounds. The machine-learned models so obtained can then be used to search for new classes of stable materials with desired electronic and physical properties. We discuss the basic structure of our f-electron database, and our approach towards cleaning and correcting the structure data files. Illustrative examples of the applications of our database include successful prediction of stable superstructures of double perovskites and identification of a number of physically-relevant trends in strongly correlated features of f-electron based materials.

  • 40. Houchins, Gregory
    et al.
    Crook, Charles B.
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Haraldsen, Jason T.
    Voltage-dependent spin flip in magnetically substituted graphene nanoribbons: Towards the realization of graphene-based spintronic devices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 15, article id 155450Article in journal (Refereed)
    Abstract [en]

    We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as a spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup where a magnetic dimer has been substituted into the lattice. Through this first-principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configurations. By calculating the total energy of each magnetic configuration, we determine that the initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the GNR, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct shift in the induced magnetization within graphene.

  • 41. Huang, Zhoushen
    et al.
    Arovas, Daniel P.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Impurity scattering in Weyl semimetals and their stability classification2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. art.nr.-123019Article in journal (Refereed)
    Abstract [en]

    Weyl semimetals (WS) are a new class of Dirac-type materials exhibiting a phase with bulk energy nodes and an associated vanishing density of states (DOS). We investigate the stability of this nodal DOS suppression in the presence of local impurities and consider whether or not such a suppression can be lifted by impurity-induced resonances. We find that while a scalar (chemical potential type) impurity can always induce a resonance at arbitrary energy and hence lift the DOS suppression at Dirac/Weyl nodes, for many other impurity types (e.g. magnetic or orbital mixing), resonances are forbidden in a wide range of energy. We investigate a four-band tight-binding model of WS adapted from a physical heterostructure construction due to Burkov et al (2011 Phys. Rev. B 84 235126), and represent a local impurity potential by a strength g as well as a matrix structure Lambda. A general framework is developed to analyze this resonance dichotomy and make connection with the phase shift picture in scattering theory, as well as to determine the relation between resonance energy and impurity strength g. A complete classification of impurities based on Lambda, based on their effect on nodal DOS suppression, is tabulated. We also discuss the differences between continuum and lattice approaches.

  • 42. Huang, Zhoushen
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Dynamical Quantum Phase Transitions: Role of Topological Nodes in Wave Function Overlaps2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 8, article id 086802Article in journal (Refereed)
    Abstract [en]

    A sudden quantum quench of a Bloch band from one topological phase toward another has been shown to exhibit an intimate connection with the notion of a dynamical quantum phase transition (DQPT), where the returning probability of the quenched state to the initial state-i.e., the Loschmidt echo-vanishes at critical times {t*}. Analytical results to date are limited to two-band models, leaving the exact relation between topology and DQPT unclear. In this Letter, we show that, for a general multiband system, a robust DQPT relies on the existence of nodes (i.e., zeros) in the wave function overlap between the initial band and the postquench energy eigenstates. These nodes are topologically protected if the two participating wave functions have distinctive topological indices. We demonstrate these ideas in detail for both one and two spatial dimensions using a three-band generalized Hofstadter model. We also discuss possible experimental observations.

  • 43. Huang, Zhoushen
    et al.
    Das, Tanmoy
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory.
    Arovas, Daniel P.
    Stability of Weyl metals under impurity scattering2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 15, p. 155123-Article in journal (Refereed)
    Abstract [en]

    We investigate the effects of bulk impurities on the electronic spectrum of Weyl semimetals, a recently identified class of Dirac-type materials. Using a T-matrix approach, we study resonant scattering due to a localized impurity in tight-binding versions of the continuum models recently discussed by [Burkov, Hook, and Balents, Phys. Rev. B 84, 235126 (2011)], describing perturbed four-component Dirac fermions in the vicinity of a critical point. The impurity potential is described by a strength g as well as a matrix structure Lambda. Unlike the case in d-wave superconductors, where a zero energy resonance can always be induced by varying the scalar and/or magnetic impurity strength, we find that for certain types of impurity (Lambda), the Weyl node is protected and that a scalar impurity will induce an intragap resonance over a wide range of scattering strength. A general framework is developed to address this question, as well as to determine the dependence of resonance energy on the impurity strength.

  • 44. Huang, Zhoushen
    et al.
    Woelfle, P.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Odd-frequency pairing of interacting Majorana fermions2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 12, article id 121404Article in journal (Refereed)
    Abstract [en]

    Majorana fermions are rising as a promising key component in quantum computation. Although the prevalent approach is to use a quadratic (i.e., noninteracting) Majorana Hamiltonian, when expressed in terms of Dirac fermions, generically the Hamiltonian involves interaction terms. Here we focus on the possible pair correlations in a simple model system. We study a model of Majorana fermions coupled to a boson mode and show that the anomalous correlator between different Majorana fermions, located at opposite ends of a topological wire, exhibits odd-frequency behavior. It is stabilized when the coupling strength g is above a critical value g(c). We use both, conventional diagrammatic theory and a functional integral approach, to derive the gap equation, the critical temperature, the gap function, the critical coupling, and a Ginzburg-Landau theory that allows discussing a possible subleading admixture of even-frequency pairing.

  • 45. Huang, Zhoushen
    et al.
    Zhu, W.
    Arovas, Daniel P.
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Invariance of Topological Indices Under Hilbert Space Truncation2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 1, article id 016403Article in journal (Refereed)
    Abstract [en]

    We show that the topological index of a wave function, computed in the space of twisted boundary phases, is preserved under Hilbert space truncation, provided the truncated state remains normalizable. If truncation affects the boundary condition of the resulting state, the invariant index may acquire a different physical interpretation. If the index is symmetry protected, the truncation should preserve the protecting symmetry. We discuss implications of this invariance using paradigmatic integer and fractional Chern insulators, Z(2) topological insulators, and spin-1 Affleck-Kennedy-Lieb-Tasaki and Heisenberg chains, as well as its relation with the notion of bulk entanglement. As a possible application, we propose a partial quantum tomography scheme trom which the topological index of a generic multicomponent wave function can be extracted by measuring only a small subset of wave function components, equivalent to the measurement of a bulk entanglement topological index.

  • 46. Islam, M. F.
    et al.
    Canali, C. M.
    Pertsova, Anna
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mahatha, S. K.
    Carbone, C.
    Barla, A.
    Kokh, K. A.
    Tereshchenko, O. E.
    Jimenez, E.
    Brookes, N. B.
    Gargiani, P.
    Valvidares, M.
    Schatz, S.
    Peixoto, T. R. F.
    Bentmann, H.
    Reinert, F.
    Jung, J.
    Bathon, T.
    Fauth, K.
    Bode, M.
    Sessi, P.
    Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 15, article id 155429Article in journal (Refereed)
    Abstract [en]

    The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here, we report on a detailed and systematic investigation of transition metal (TM) doped Sb2Te3. By combining density functional theory calculations with complementary experimental techniques, i.e., scanning tunneling microscopy, resonant photoemission, and x-raymagnetic circular dichroism, we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM doped topological insulators. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V-and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity dependent and can vary from in plane to out of plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM doped Sb2Te3 in the ferromagnetic state.

  • 47.
    Juricic, Vladimir
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Abergel, David S. L.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; ETH Institute for Theoretical Studies, Switzerland.
    First-order quantum phase transition in three-dimensional topological band insulators2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 16, article id 161403Article in journal (Refereed)
    Abstract [en]

    Topological states of matter are characterized by global topological invariants which change their value across a topological quantum phase transition. It is commonly assumed that the transition between topologically distinct noninteracting gapped phases of fermions is necessarily accompanied by the closing of the band gap as long as the symmetries of the system are maintained. We show that such a quantum phase transition is possible without closing the gap in the case of a three-dimensional topological band insulator. We demonstrate this by calculating the free energy of the minimal model for a topological insulator, the Bernevig-Hughes-Zhang model, and show that as the band curvature continuously varies, a jump between the band-gap minima corresponding to the topologically trivial and nontrivial insulators occurs. Therefore, this first-order phase transition is a generic feature of three-dimensional topological band insulators. For a certain parameter range we predict a reentrant topological phase transition. We discuss our findings in connection with the recent experimental observation of a discontinuous topological phase transition in a family of topological crystalline insulators.

  • 48. Kaloni, T. P.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Schwingenschloegl, U.
    Substrate-enhanced superconductivity in Li-decorated graphene2013In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 104, no 4, p. 47013-Article in journal (Refereed)
    Abstract [en]

    We investigate the role of the substrate for the strength of the electron-phonon coupling in Li-decorated graphene. We find that the interaction with a h-BN substrate leads to a significant enhancement from lambda(0) = 0.62 to lambda(1) = 0.67, which corresponds to a 25% increase of the transition temperature from T-c0 = 10.33K to T-c1 = 12.98 K. The superconducting gaps amount to 1.56 meV (suspended) and 1.98 meV (supported). These findings open up a new route to enhanced superconducting transition temperatures in graphene-based materials by substrate engineering.

  • 49. Kanchanavatee, N.
    et al.
    Janoschek, M.
    Huang, K.
    White, B. D.
    Riseborough, P. S.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, Los Alamos, USA.
    Maple, M. B.
    Emergence of higher order rotational symmetry in the hidden order phase of URu2Si22017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 2, p. 144-154Article in journal (Refereed)
    Abstract [en]

    Electrical resistivity measurements were performed as functions of temperature, magnetic field, and angle theta between the magnetic field and the c-axis of a URu2Si2 single crystal. The resistivity exhibits a two-fold oscillation as a function of theta at high temperatures, which undergoes a 180 degrees-phase shift (sign change) with decreasing temperature at around 35 K. The hidden order transition is manifested as a minimum in the magnetoresistance and amplitude of the two-fold oscillation. Interestingly, the resistivity also showed four-fold, six-fold, and eight-fold symmetries at the hidden order transition. These higher order symmetries were also detected at low temperatures, which could be a sign of the formation of another pseudogap phase above the superconducting transition, consistent with recent evidence for a pseudogap from point-contact spectroscopy measurements and NMR. Measurements of the magnetisation of single crystalline URu2Si2 with the magnetic field applied parallel and perpendicular to the crystallographic c-axis revealed regions with linear temperature dependencies between the hidden order transition temperature and about 25 K. This T-linear behaviour of the magnetisation may be associated with the formation of a precursor phase or 'pseudogap' in the density of states in the vicinity of 30-35 K.

  • 50.
    Kedem, Yaron
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Unusual superconducting isotope effect in the presence of a quantum criticality2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 18, article id 184507Article in journal (Refereed)
    Abstract [en]

    The isotope effect in superconductivity (SC) is used to make a concrete connection to a quantum critical point (QCP) that is tunable by isotopic mass substitution. We find a distinct contribution to the isotope exponent in SC and derive an explicit relation to the critical exponent of a QCP. The relation between the two exponents is general and can be used as an experimental signature for the connection between SC and a QCP. We demonstrate it in a scenario where the SC pairing is due to modes related to a structural instability. Within this model the isotope exponent is derived in terms of microscopic parameters.

12 1 - 50 of 90
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