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  • 1. 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.

  • 2.
    B. Arnardottir, Kristin
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Iceland, Iceland.
    Kyriienko, O.
    Portnoi, M. E.
    Shelykh, I. A.
    One-dimensional Van Hove polaritons2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 12, article id 125408Article in journal (Refereed)
    Abstract [en]

    We study the light-matter coupling of microcavity photons and an interband transition in a one-dimensional (1D) nanowire. Due to the Van Hove singularity in the density of states, resulting in a resonant character of the absorption line, the achievement of strong coupling becomes possible even without the formation of a bound state of an electron and a hole. The calculated absorption in the system and corresponding energy spectrum reveal anticrossing behavior characteristic of the formation of polariton modes. In contrast to the case of conventional exciton polaritons, the formation of 1D Van Hove polaritons will not be restricted to low temperatures and can be realized in any system with a singularity in the density of states.

  • 3.
    B. Arnardottir, Kristin
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Iceland, Iceland.
    Kyriienko, O.
    Shelykh, I. A.
    Hall effect for indirect excitons in an inhomogeneous magnetic field2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 24, article id 245311Article in journal (Refereed)
    Abstract [en]

    We study the effect of an inhomogeneous out-of-plane magnetic field on the behavior of two-dimensional (2D) spatially indirect excitons. Due to the difference of the magnetic field acting on electrons and holes, the total Lorentz force affecting the center of mass motion of an indirect exciton appears. Consequently, an indirect exciton acquires an effective charge proportional to the gradient of the magnetic field. The appearance of the Lorentz force causes the Hall effect for neutral bosons, which can be detected by measurement of the spatially inhomogeneous blueshift of the photoluminescence using a counterflow experiment.

  • 4. Balakrishna, S. G.
    et al.
    de Wijn, Astrid S.
    Stockholm University, Faculty of Science, Department of Physics.
    Bennewitz, Roland
    Preferential sliding directions on graphite2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 24, p. 245440-Article in journal (Refereed)
    Abstract [en]

    The anisotropy of friction on graphitic surfaces is investigated by a combined friction force microscopy and modeling study. Friction vectors deviate up to 15 degrees from pulling directions. The strongest deviations are found for pulling directions which lie almost along one zigzag direction of the honeycomb structure, the preferred sliding direction on graphite surfaces and epitaxial graphene grown on SiC(0001). Atomic stick-slip events along and across molecular rows determine direction and magnitude of friction. Simulation and modeling reveal the role of temperature and of the two-dimensional character of the surface potential for the friction anisotropy.

  • 5.
    Baym, Gordon
    et al.
    Univ Illinois, Dept Phys, Urbana, IL, USA.
    Beck, D. H.
    Niels Bohr Inst, Niels Bohr Int Acad, Copenhagen O, Denmark.
    Pethick, Christopher
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Niels Bohr Inst, Niels Bohr Int Acad, Copenhagen , Denmark.
    Transport in very dilute solutions of He-3 in superfluid He-42013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 1, p. 014512-Article in journal (Refereed)
    Abstract [en]

    Motivated by a proposed experimental search for the electric dipole moment of the neutron (nEDM) utilizing neutron-He-3 capture in a dilute solution of He-3 in superfluid He-4, we derive the transport properties of dilute solutions in the regime where the He-3 are classically distributed and rapid He-3-He-3 scatterings keep the He-3 in equilibrium. Our microscopic framework takes into account phonon-phonon, phonon-He-3, and He-3-He-3 scatterings. We then apply these calculations to measurements by Rosenbaum et al. [J. Low Temp. Phys. 16, 131 (1974)] and by Lamoreaux et al. [Europhys. Lett. 58, 718 (2002)] of dilute solutions in the presence of a heat flow. We find satisfactory agreement of theory with the data, serving to confirm our understanding of the microscopics of the helium in the future nEDM experiment.

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  • 6. Baym, Gordon
    et al.
    Beck, D. H.
    Pethick, Christopher J.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Illinois, USA; University of Copenhagen, Denmark.
    Transport in ultradilute solutions of He-3 in superfluid He-42015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 2, article id 024504Article in journal (Refereed)
    Abstract [en]

    We calculate the effect of a heat current on transporting He-3 dissolved in superfluid He-4 at ultralow concentration, as will be utilized in a proposed experimental search for the electric dipole moment of the neutron (nEDM). In this experiment, a phonon wind will be generated to drive (partly depolarized) 3He down a long pipe. In the regime of 3He concentrations less than or similar to 10(-9) and temperatures similar to 0.5 K, the phonons comprising the heat current are kept in a flowing local equilibrium by small angle phonon-phonon scattering, while they transfer momentum to the walls via the He-4 first viscosity. On the other hand, the phonon wind drives the 3He out of local equilibrium via phonon-He-3 scattering. For temperatures below 0.5 K, both the phonon and He-3 mean free paths can reach the centimeter scale, and we calculate the effects on the transport coefficients. We derive the relevant transport coefficients, the phonon thermal conductivity, and the He-3 diffusion constants from the Boltzmann equation. We calculate the effect of scattering from the walls of the pipe and show that it may be characterized by the average distance from points inside the pipe to the walls. The temporal evolution of the spatial distribution of the He-3 atoms is determined by the time dependent He-3 diffusion equation, which describes the competition between advection by the phonon wind and He-3 diffusion. As a consequence of the thermal diffusivity being small compared with the He-3 diffusivity, the scale height of the final He-3 distribution is much smaller than that of the temperature gradient. We present exact solutions of the time dependent temperature and He-3 distributions in terms of a complete set of normal modes.

  • 7.
    Bergholtz, E. J.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kailasvuori, J.
    Stockholm University, Faculty of Science, Department of Physics.
    Wikberg, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Thors Hans
    Stockholm University, Faculty of Science, Department of Physics.
    Karlhede, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pfaffian quantum Hall state made simple: Multiple vacua and domain walls on a thin torus2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 8, p. 081308-Article in journal (Refereed)
    Abstract [en]

    We analyze the Moore-Read Pfaffian state on a thin torus. The known sixfold degeneracy is realized by two inequivalent crystalline states with a four- and twofold degeneracy, respectively. The fundamental quasihole and quasiparticle excitations are domain walls between these vacua, and simple counting arguments give a Hilbert space of dimension 2n−1 for 2n−k holes and k particles at fixed positions and assign each a charge ±e∕4. This generalizes the known properties of the hole excitations in the Pfaffian state as deduced using conformal field theory techniques. Numerical calculations using a model Hamiltonian and a small number of particles support the presence of a stable phase with degenerate vacua and quarter-charged domain walls also away from the thin-torus limit. A spin-chain Hamiltonian encodes the degenerate vacua and the various domain walls.

  • 8. 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.

  • 9. Black-Schaffer, Annica M.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Odd frequency superconducting pairing in topological insulators2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 14, p. 144506-Article in journal (Refereed)
    Abstract [en]

    We discuss the appearance of odd-frequency spin-triplet s-wave superconductivity, first proposed by Berezinskii [J. Exp. Theor. Phys. 20, 287 (1974)], on the surface of a topological insulator proximity coupled to a conventional spin- singlet s-wave superconductor. Using both analytical and numerical methods, we show that this disorder robust odd-frequency state is present whenever there is an in-surface gradient in the proximity induced gap, including superconductor-normal state junctions. The time-independent order parameter for the odd-frequency superconductor is proportional to the in-surface gap gradient. The induced odd-frequency component does not produce any low-energy states.

  • 10. 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.

  • 11. 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.

  • 12.
    Boris, Andrey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Quantization of the superconducting energy gap in an intense microwave field2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 17, article id 174506Article in journal (Refereed)
    Abstract [en]

    We study experimentally photon-assisted tunneling in Nb/AlOx/Nb Josephson junctions. We perform a quantitative calibration of the microwave field inside the junction. This allows direct verification of the quantum efficiency of microwave photon detection, which corresponds to tunneling of one electron per one absorbed microwave photon. We observe that voltages of photon-assisted tunneling steps vary both with the microwave power and the tunneling current. However, this variation is not monotonous but staircaselike. The phenomenon is caused by mutual locking of positive and negative step series. A similar locking is observed with Shapiro steps. As a result, the superconducting gap assumes quantized values equal to multiples of the quarter of the photon energy. The quantization is a manifestation of nonequilibrium tuning (suppression or enhancement) of superconductivity by the microwave field.

  • 13.
    Borysov, Stanislav S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Platz, Daniel
    de Wijn, Astrid S.
    Stockholm University, Faculty of Science, Department of Physics.
    Forchheimer, Daniel
    Tolen, Eric A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos Natl Lab, Div Theoret, Los Alamos, NM 87545 USA.
    Haviland, David B.
    Reconstruction of tip-surface interactions with multimodal intermodulation atomic force microscopy2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 11, p. 115405-Article in journal (Refereed)
    Abstract [en]

    We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.

  • 14.
    Budich, Jan Carl
    Stockholm University, Faculty of Science, Department of Physics.
    Charge conservation protected topological phases2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 16, p. 161103-Article in journal (Refereed)
    Abstract [en]

    We discuss the relation between particle number conservation and topological phases. In four spatial dimensions, we find that systems belonging to different topological phases in the presence of a U(1) charge conservation can be connected adiabatically, i.e., without closing the gap, upon intermediately breaking this local symmetry by a superconducting term. The time reversal preserving topological insulator states in two and three dimensions which can be obtained from the four-dimensional parent state by dimensional reduction inherit this protection by charge conservation. Hence, all topological insulators can be adiabatically connected to a trivial insulating state without breaking time reversal symmetry, provided an intermediate superconducting term is allowed during the adiabatic deformation. Conversely, in one spatial dimension, non-symmetry-protected topological phases occur only in systems that break U(1) charge conservation. These results can intuitively be understood by considering a natural embedding of the classifying spaces of charge conserving Hamiltonians into the corresponding Bogoliubov-de Gennes classes.

  • 15.
    Budich, Jan Carl
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Equivalent topological invariants for one-dimensional Majorana wires in symmetry class D2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 7, p. 075419-Article in journal (Refereed)
    Abstract [en]

    Topological superconductors in one spatial dimension exhibiting a single Majorana bound state at each end are distinguished from trivial gapped systems by aZ(2) topological invariant. Originally, this invariant was calculated by Kitaev in terms of the Pfaffian of the Majorana representation of the Hamiltonian: The sign of this Pfaffian divides the set of all gapped quadratic forms of Majorana fermions into two inequivalent classes. In the more familiar Bogoliubov de Gennes mean-field description of superconductivity, an emergent particle-hole symmetry gives rise to a quantized Zak-Berry phase, the value of which is also a topological invariant. In this work, we explicitly show the equivalence of these two formulations by relating both of them to the phase winding of the transformation matrix that brings the Majorana representation matrix of the Hamiltonian into its Jordan normal form.

  • 16.
    Budich, Jan Carl
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Fractional topological phase in one-dimensional flat bands with nontrivial topology2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 3, p. 035139-Article in journal (Refereed)
    Abstract [en]

    We consider a topologically nontrivial flat-band structure in one spatial dimension in the presence of nearest-and next-nearest-neighbor Hubbard interaction. The noninteracting band structure is characterized by a symmetry-protected topologically quantized Berry phase. At certain fractional fillings, a gapped phase with a filling-dependent ground-state degeneracy and fractionally charged quasiparticles emerges. At filling 1/3, the ground states carry a fractional Berry phase in the momentum basis. These features at first glance suggest a certain analogy to the fractional quantum Hall scenario in two dimensions. We solve the interacting model analytically in the physically relevant limit of a large band gap in the underlying band structure, the analog of a lowest Landau level projection. Our solution affords a simple physical understanding of the properties of the gapped interacting phase. We pinpoint crucial differences to the fractional quantum Hall case by studying the Berry phase and the entanglement entropy associated with the degenerate ground states. In particular, we conclude that the fractional topological phase in one-dimensional flat bands is not a one-dimensional analog of the two-dimensional fractional quantum Hall states, but rather a charge density wave with a nontrivial Berry phase. Finally, the symmetry-protected nature of the Berry phase of the interacting phase is demonstrated by explicitly constructing a gapped interpolation to a state with a trivial Berry phase.

  • 17.
    Budich, Jan Carl
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Topological invariant for generic one-dimensional time-reversal-symmetric superconductors in class DIII2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 13, p. 134523-Article in journal (Refereed)
    Abstract [en]

    A one-dimensional time-reversal-symmetric topological superconductor (symmetry class DIII) features a single Kramers pair of Majorana bound states at each of its ends. These holographic quasiparticles are non-Abelian anyons that obey Ising-type braiding statistics. In the special case where an additional U (1) spin rotation symmetry is present, this state can be understood as two copies of a Majorana wire in symmetry class D, one copy for each spin block. We present a manifestly gauge invariant construction of the topological invariant for the generic case, i.e., in the absence of any additional symmetries like spin rotation symmetry. Furthermore, we show how the presence of inversion symmetry simplifies the calculation of the topological invariant. The proposed scheme is suitable for the classification of both interacting and disordered systems and allows for a straightforward numerical evaluation of the invariant since it does not rely on fixing a continuous phase relation between Bloch functions. Finally, we apply our method to compute the topological phase diagram of a Rashba wire with competing s-wave and p-wave superconducting pairing terms.

  • 18.
    Budich, Jan Carl
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Innsbruck, Austria.
    Eisert, J.
    Bergholtz, E. J.
    Topological insulators with arbitrarily tunable entanglement2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 19, p. 195120-Article in journal (Refereed)
    Abstract [en]

    We elucidate how Chern and topological insulators fulfill an area law for the entanglement entropy. By explicit construction of a family of lattice Hamiltonians, we are able to demonstrate that the area law contribution can be tuned to an arbitrarily small value but is topologically protected from vanishing exactly. We prove this by introducing novel methods to bound entanglement entropies from correlations using perturbation bounds, drawing intuition from ideas of quantum information theory. This rigorous approach is complemented by an intuitive understanding in terms of entanglement edge states. These insights have a number of important consequences: The area law has no universal component, no matter how small, and the entanglement scaling cannot be used as a faithful diagnostic of topological insulators. This holds for all Renyi entropies which uniquely determine the entanglement spectrum, which is hence also nonuniversal. The existence of arbitrarily weakly entangled topological insulators furthermore opens up possibilities of devising correlated topological phases in which the entanglement entropy is small and which are thereby numerically tractable, specifically in tensor network approaches.

  • 19.
    Campanini, Donato
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Diao, Zhu
    Stockholm University, Faculty of Science, Department of Physics.
    Fang, L.
    Kwok, W. -K.
    Welp, U.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Superconducting gap evolution in overdoped BaFe2(As1-xPx)(2) single crystals through nanocalorimetry2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 24, article id 245142Article in journal (Refereed)
    Abstract [en]

    We report on specific heat measurements on clean overdoped BaFe2(As1-xPx)(2) single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature gamma(r) = C/T backslash(T -> 0) is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Delta(0) similar to 5.3 meV, corresponding to Delta(0)/k(B)T(c) similar to 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Delta(0) similar to 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on gamma(r), all samples however show similar behavior [gamma(r)(H) -gamma(r)(H = 0) proportional to H-n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

  • 20. Chaparro, C.
    et al.
    Fang, L.
    Claus, H.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Crabtree, G. W.
    Stanev, V.
    Kwok, W. K.
    Welp, U.
    Doping dependence of the specific heat of single crystal bafe2(as1 xpx)(2)2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 18, p. 184525-Article in journal (Refereed)
    Abstract [en]

    We present specific heat measurements on a series of BaFe2(As1-xPx)(2) single crystals with phosphorous doping ranging from x = 0.3 to x = 0.55. Our results reveal that BaFe2(As1-xPx)(2) follows the scaling Delta C/T-c approximate to T-c(2) remarkably well. The clean-limit nature of this material imposes additional restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is Gamma approximate to 2.6, independent of doping.

  • 21. Chung, Sangwoo S.
    et al.
    Kakashvili, Paata
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Bolech, C. J.
    Numerical simulation of the Nernst effect in extreme type-II superconductors: A negative Nernst signal and its noise power spectra2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 13, p. 134525-Article in journal (Refereed)
    Abstract [en]

    Recently, different transport coefficients have been measured in high-T-c superconductors to pinpoint the nature of the pseudogap phase. In particular, the thermoelectric coefficients received considerable attention both theoretically and experimentally. We numerically simulate the Nernst effect in extreme type-II superconductors using the time-dependent Ginzburg-Landau equations. We report the sign reversal of the thermoelectric coefficient alpha(xy) at temperatures close to the mean-field transition temperature T-c(MF) (H), which qualitatively agrees with recent experiments on high-T-c materials. We also discuss the noise power spectrum of alpha(xy), which shows 1/f(beta) behavior. Based on this observation, we propose an experiment to distinguish among different regimes of vortex dynamics by measuring the noise correlations of the Nernst signal.

  • 22. Davenport, Simon C.
    et al.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Regnault, Nicolas
    Simon, Steven H.
    Spin-singlet Gaffnian wave function for fractional quantum Hall systems2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 4, p. 045310-Article in journal (Refereed)
    Abstract [en]

    We characterize in detail a wave function conceivable in fractional quantum Hall systems where a spin or equivalent degree of freedom is present. This wave function combines the properties of two previously proposed quantum Hall wave functions, namely the non-Abelian spin-singlet state and the nonunitary Gaffnian wave function. This is a spin-singlet generalization of the spin-polarized Gaffnian, which we call the spin-singlet Gaffnian (SSG). In this paper we present evidence demonstrating that the SSG corresponds to the ground state of a certain local Hamiltonian, which we explicitly construct, and, further, we provide a relatively simple analytic expression for the unique ground-state wave functions, which we define as the zero energy eigenstates of that local Hamiltonian. In addition, we have determined a certain nonunitary, rational conformal field theory which provides an underlying description of the SSG and we thus conclude that the SSG is ungapped in the thermodynamic limit. In order to verify our construction, we implement two recently proposed techniques for the analysis of fractional quantum Hall trial states: The spin dressed squeezing algorithm, and the generalized Pauli principle.

  • 23.
    de Wijn, Astrid S.
    Stockholm University, Faculty of Science, Department of Physics.
    (In)commensurability, scaling, and multiplicity of friction in nanocrystals and application to gold nanocrystals on graphite2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 8, p. 085429-Article in journal (Refereed)
    Abstract [en]

    The scaling of friction with the contact size A and (in)commensurabilty of nanoscopic and mesoscopic crystals on a regular substrate are investigated analytically for triangular nanocrystals on hexagonal substrates. The crystals are assumed to be stiff, but not completely rigid. Commensurate and incommensurate configurations are identified systematically. It is shown that three distinct friction branches coexist, an incommensurate one that does not scale with the contact size (A(0)) and two commensurate ones which scale differently (with A(1/2) and A) and are associated with various combinations of commensurate and incommensurate lattice parameters and orientations. This coexistence is a direct consequence of the two-dimensional nature of the contact layer, and such multiplicity exists in all geometries consisting of regular lattices. To demonstrate this, the procedure is repeated for rectangular geometry. The scaling of irregularly shaped crystals is also considered, and again three branches are found (A(1/4), A(3/4), A). Based on the scaling properties, a quantity is defined which can be used to classify commensurability in infinite as well as finite contacts. Finally, the consequences for friction experiments on gold nanocrystals on graphite are discussed.

  • 24.
    Diao, Zhu
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Campanini, Donato
    Stockholm University, Faculty of Science, Department of Physics.
    Fang, L.
    Kwok, W. -K.
    Welp, U.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe2(As1-xPx)(2) single crystals2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 1, article id 014509Article in journal (Refereed)
    Abstract [en]

    We investigate the electronic specific heat of superoptimally substituted BaFe2(As1-x P-x(x))(2) single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Delta, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump Delta C proportional to T-c(3) , found in many iron-based superconductors, in this system originates from a T-c-dependent ratio Delta/k(B)T(c) in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n(s) yields a penetration depth lambda that decreases with increasing T-c without sharp divergence at the quantum critical point. Uemura scaling indicates that T-c is governed by the Fermi temperature T-F for this multiband system.

  • 25. Dion, C. M.
    et al.
    Jukimenko, O.
    Modestov, Mikhai
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Marklund, M.
    Bychkov, V.
    Anisotropic properties of spin avalanches in crystals of nanomagnets2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 1, p. 014409-Article in journal (Refereed)
    Abstract [en]

    Anisotropy effects for spin avalanches in crystals of nanomagnets are studied theoretically with the external magnetic field applied at an arbitrary angle to the easy axis. Starting with the Hamiltonian for a single nanomagnet in the crystal, two essential quantities characterizing spin avalanches are calculated: the activation and Zeeman energies. The calculation is performed numerically for a wide range of angles and analytical formulas are derived within the limit of small angles. The anisotropic properties of a single nanomagnet lead to anisotropic behavior of the magnetic deflagration speed. Modifications of the magnetic deflagration speed are investigated for different angles between the external magnetic field and the easy axis of the crystals. Anisotropic properties of magnetic detonation are also studied, which concern, first of all, the temperature behind the leading shock and the characteristic time of spin switching in the detonation. DOI: 10.1103/PhysRevB.87.014409

  • 26. Disch, S.
    et al.
    Hermann, R. P.
    Wetterskog, Erik
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Podlesnyak, A. A.
    An, K.
    Hyeon, T.
    Salazar-Alvarez, German
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bergström, Lennart
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Brueckel, Th.
    Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 6, p. 064402-Article in journal (Refereed)
    Abstract [en]

    We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Lande factor g = 2.05(2) as expected for maghemite (gamma-Fe2O3) nanoparticles. A large effective anisotropy field of B-A,B-eff = 0.45(16) T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles.

  • 27.
    Edge, Jonathan M.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Asboth, Janos K.
    Localization, delocalization, and topological transitions in disordered two-dimensional quantum walks2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 10, article id 104202Article in journal (Refereed)
    Abstract [en]

    We investigate time-independent disorder on several two-dimensional discrete-time quantum walks. We find numerically that, contrary to claims in the literature, random onsite phase disorder, spin-dependent or otherwise, cannot localize the Hadamard quantum walk; rather, it induces diffusive spreading of the walker. In contrast, split-step quantum walks are generically localized by phase disorder. We explain this difference by showing that the Hadamard walk is a special case of the split-step quantum walk, with parameters tuned to a critical point at a topological phase transition. We show that the topological phase transition can also be reached by introducing strong disorder in the rotation angles. We determine the critical exponent for the divergence of the localization length at the topological phase transition, and find nu = 2.6, in both cases. This places the two-dimensional split-step quantum walk in the universality class of the quantum Hall effect.

  • 28. Efthimiopoulos, Ilias
    et al.
    Benson, Daryn E.
    Konar, Sumit
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nylén, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liebig, Stefan
    Ruschewitz, Uwe
    Vazhenin, Grigory V.
    Loa, Ingo
    Hanfland, Michael
    Syassen, Karl
    Structural transformations of Li2C2 at high pressures2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 6, article id 064111Article in journal (Refereed)
    Abstract [en]

    Structural changes of Li2C2 under pressure were studied by synchrotron x-ray diffraction in a diamond anvilcell under hydrostatic conditions and by using evolutionary search methodology for crystal structure prediction.We show that the high-pressure polymorph of Li2C2, which forms from the Immm ground-state structure (Z = 2)at around 15 GPa, adopts an orthorhombic Pnma structure with Z = 4. Acetylide C2 dumbbells characteristic ofImmm Li2C2 are retained in Pnma Li2C2. The structure of Pnma Li2C2 relates closely to the anticotunnite-typestructure. C2 dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structureof Immm Li2C2. First-principles calculations predict a transition of Pnma Li2C2 at 32 GPa to a topologicallyidentical phase with a higher Cmcm symmetry. The coordination of C2 dumbbell units by Li atoms is increasedto 11. The structure of Cmcm Li2C2 relates closely to the Ni2 In-type structure. It is calculated that Cmcm Li2C2becomes metallic at pressures above 40 GPa. In experiments, however, Pnma Li2C2 is susceptible to irreversibleamorphization.

  • 29. Euchner, H.
    et al.
    Mihalkovic, M.
    Gaehler, F.
    Johnson, M. R.
    Schober, H.
    Rols, S.
    Suard, E.
    Bosak, A.
    Ohhashi, S.
    Tsai, A. -P
    Lidin, Sven
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gomez, C. Pay
    Custers, J.
    Paschen, S.
    de Boissieu, M.
    Anomalous vibrational dynamics in the Mg(2)Zn(11) phase2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 14, p. 144202-Article in journal (Refereed)
    Abstract [en]

    We present a combined experimental and theoretical study of the structure and the lattice dynamics in the complex metallic alloy Mg(2)Zn(11), by means of neutron and x-ray scattering, as well as ab initio and empirical potential calculations. Mg(2)Zn(11) can be seen as an intermediate step in structural complexity between the simple Laves-phase MgZn(2) on one side, and the complex 1/1 approximants and quasicrystals ZnMgAl and Zn(Mg)Sc on the other. The structure can be described as a cubic packing of a triacontahedron whose center is partially occupied by a Zn atom. This partially occupied site turned out to play a major role in understanding the lattice dynamics. Data from inelastic neutron scattering evidence a Van Hove singularity in the vibrational spectrum of Mg(2)Zn(11) for an energy as low as 4.5 meV, which is a unique feature for a nearly-close-packed metallic alloy. This corresponds to a gap opening at the Brillouin zone boundary and an interaction between a low-lying optical branch and an acoustic one, as could be deduced from the dispersion relation measured by inelastic x-ray scattering. Second, the measured phonon density of states exhibits many maxima, indicating strong mode interactions across the whole energy range. The origin of the low-energy modes in Mg(2)Zn(11) and other features of the vibrational spectra are studied, using both ab initio and empirical potential calculations. A detailed analysis of vibrational eigenmodes is presented, linking features in the vibrational spectrum to atomic motions within structural building blocks.

  • 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. Fischer, A.
    et al.
    Scheidt, E. -W.
    Scherer, W.
    Benson, D. E.
    Wu, Y.
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Haussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 22, article id 224309Article in journal (Refereed)
    Abstract [en]

    The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.

  • 32. 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.

  • 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.
    Fremling, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Thors Hans
    Stockholm University, Faculty of Science, Department of Physics.
    Suorsa, Juha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hall viscosity of hierarchical quantum Hall states2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 12, article id 125303Article in journal (Refereed)
    Abstract [en]

    Using methods based on conformal field theory, we construct model wave functions on a torus with arbitrary flat metric for all chiral states in the abelian quantum Hall hierarchy. These functions have no variational parameters, and they transform under the modular group in the same way as the multicomponent generalizations of the Laughlin wave functions. Assuming the absence of Berry phases upon adiabatic variations of the modular parameter tau, we calculate the quantum Hall viscosity and find it to be in agreement with the formula, given by Read, which relates the viscosity to the average orbital spin of the electrons. For the filling factor nu = 2/5 Jain state, which is at the second level in the hierarchy, we compare our model wave function with the numerically obtained ground state of the Coulomb interaction Hamiltonian in the lowest Landau level, and find very good agreement in a large region of the complex t plane. For the same example, we also numerically compute the Hall viscosity and find good agreement with the analytical result for both the model wave function and the numerically obtained Coulomb wave function. We argue that this supports the notion of a generalized plasma analogy that would ensure that wave functions obtained using the conformal field theory methods do not acquire Berry phases upon adiabatic evolution.

  • 35. Garaud, Julien
    et al.
    Sellin, Karl A. H.
    Jäykkä, Juha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Babaev, Egor
    Skyrmions induced by dissipationless drag in U(1)xU(1) superconductors2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 10, p. 104508-Article in journal (Refereed)
    Abstract [en]

    Rather generically, multicomponent superconductors and superfluids have intercomponent current-current interaction. We show that in superconductors with substantially strong intercomponent drag interaction, the topological defects which form in an external field are characterized by a skyrmionic topological charge. We then demonstrate that they can be distinguished from ordinary vortex matter by a very characteristic magnetization process due to the dipolar nature of inter-skyrmion forces. The results provide an experimental signature to confirm or rule out the formation p-wave state with reduced spin stiffness in p-wave superconductors.

  • 36.
    Genkin, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Effects of screened Coulomb impurities on autoionizing two-electron resonances in spherical quantum dots2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 12, p. 125315-Article in journal (Refereed)
    Abstract [en]

    In a recent paper [Phys. Rev. B 78, 075316 (2008)], Sajeev and Moiseyev demonstrated that the bound-to-resonant transitions and lifetimes of autoionizing states in spherical quantum dots can be controlled by varying the confinement strength. In the present paper, we report that such control can in some cases be compromised by the presence of Coulomb impurities. It is demonstrated that a screened Coulomb impurity placed in the vicinity of the dot center can lead to bound-to-resonant transitions and to avoided-crossinglike-behavior when the screening of the impurity charge is varied. It is argued that these properties also can have impact on electron transport through quantum dot arrays.

  • 37.
    Genkin, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Waltersson, Erik
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Estimation of the spatial decoherence time in circular quantum dots2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. B79, no 24, p. 245310-Article in journal (Refereed)
    Abstract [en]

    We propose a simple phenomenological model to estimate the spatial decoherence time in quantum dots. The dissipative phase space dynamicsis described in terms of the density matrix and the corresponding Wigner function, which are derived from a master equation with Lindblad operatorslinear in the canonical variables. The formalism was initially developed to describe diffusion and dissipation in deep inelastic heavy ioncollisions, but also an application to quantum dots is possible.It allows us to study the dependence of the decoherence rate on the dissipation strength, the temperature and an external magnetic field, which isdemonstrated in illustrative calculations on a circular GaAs one-electron quantum dot.

  • 38. Gillis, San
    et al.
    Jäykkä, Juha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Milosevic, Milorad V.
    Vortex states in mesoscopic three-band superconductors2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 2, p. 024512-Article in journal (Refereed)
    Abstract [en]

    Using multicomponent Ginzburg-Landau simulations, we show a plethora of vortex states possible in mesoscopic three-band superconductors. We find that mesoscopic confinement stabilizes chiral states, with nontrivial phase differences between the band condensates, as the ground state of the system. As a consequence, we report the broken-symmetry vortex states, the chiral states where vortex cores in different band condensates do not coincide (split-core vortices), as well as fractional-flux vortex states with broken time-reversal symmetry.

  • 39. Gils, C.
    et al.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Trebst, S.
    Huse, D. A.
    Ludwig, A. W. W.
    Troyer, M.
    Wang, Z.
    Anyonic quantum spin chains: Spin-1 generalizations and topological stability2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 23, p. 235120-Article in journal (Refereed)
    Abstract [en]

    There are many interesting parallels between systems of interacting non-Abelian anyons and quantum magnetism occurring in ordinary SU(2) quantum magnets. Here we consider theories of so-called SU(2)(k) anyons, well-known deformations of SU(2), in which only the first k + 1 angular momenta of SU(2) occur. In this paper, we discuss in particular anyonic generalizations of ordinary SU(2) spin chains with an emphasis on anyonic spin S = 1 chains. We find that the overall phase diagrams for these anyonic spin-1 chains closely mirror the phase diagram of the ordinary bilinear-biquadratic spin-1 chain including anyonic generalizations of the Haldane phase, the AKLT construction, and supersymmetric quantum critical points. A novel feature of the anyonic spin-1 chains is an additional topological symmetry that protects the gapless phases. Distinctions further arise in the form of an even/odd effect in the deformation parameter k when considering su(2)(k) anyonic theories with k >= 5, as well as for the special case of the su(2)(4) theory for which the spin-1 representation plays a special role. We also address anyonic generalizations of spin-1/2 chains with a focus on the topological protection provided for their gapless ground states. Finally, we put our results into the context of earlier generalizations of SU(2) quantum spin chains, in particular so-called (fused) Temperley-Lieb chains.

  • 40.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Marozau, I.
    Uribe-Laverde, M. A.
    Satapathy, D. K.
    Wagner, Th.
    Bernhard, C.
    High bias anomaly in YBa2Cu3O7-x/LaMnO3+delta/YBa2Cu3O7-x superconductor/ferromagnetic insulator/superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 13, p. 134520-Article in journal (Refereed)
    Abstract [en]

    We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa2Cu3O7-x/LaMnO3+delta/YBa2Cu3O7-x) tunnel junctions. At a large bias voltage V similar to 1 V we observe a steplike onset of excess current that occurs below the superconducting transition temperature T < T-c and is easily suppressed by a magnetic field. The phenomenon is attributed to a different type of the superconducting proximity effect of nonequilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly nonequilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

  • 41.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Svedlindh, Peter
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Anti-ordinary Hall effect near the ferromagnetic quantum phase transition in NixPt1−x thin films2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 10, p. 104407-Article in journal (Refereed)
    Abstract [en]

    We study the Hall effect in NixPt1-x thin films. It is observed that the ordinary Hall coefficient is always negative (electron-like). The anomalous Hall coefficient is also negative, except in the vicinity of the ferromagnetic quantum phase transition, where it exhibits a sign reversal and turns positive (hole-like). This leads to an anti-ordinary Hall effect with opposite signs of ordinary and anomalous contributions. It clearly shows that the anomalous Hall effect does not reflect the overall topology of the Fermi surface (which remains unchanged), but originates from singular hot spots. We attribute the anti-ordinary contribution to the intrinsic (Berry-phase) origin and propose a spectroscopic explanation of its tunability as a function of temperature and composition.

  • 42. Grönbeck, Henrik
    et al.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Photoemission core-level shifts reveal the thiolate-Au(111) interface2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, p. 085416-Article in journal (Refereed)
    Abstract [en]

    The nature of the thiolate/Au(111) interface is a long-standing puzzle. It has been suggested that thiolates drive surface reconstruction, however, a consensus regarding the adsorption configuration is missing. Herein, the density-functional theory is used to evaluate surface core-level shifts (SCLSs) for methyl thiolates on Au(111) assuming a representative set of different surface reconstructions. The SCLSs are found to provide sensitive fingerprints of the anchoring configuration, and it is only thiolate adsorption in the form of MeS-Au-SMe complexes that can be reconciled with experimental data.

  • 43.
    Hansson, T. H.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Chang, C. -C
    Jain, J. K.
    Viefers, S.
    Composite-fermion wave functions as correlators in conformal field theory2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 7, p. 75347-Article in journal (Refereed)
    Abstract [en]

    It is known that a subset of fractional quantum Hall wave functions has been expressed as conformal field theory (CFT) correlators, notably the Laughlin wave function [Phys. Rev. Lett. 50, 1395 (1983)] at filling factor nu=1/m (m odd) and its quasiholes, and the Pfaffian wave function at nu=1/2 and its quasiholes. We develop a general scheme for constructing composite-fermion (CF) wave functions from conformal field theory. Quasiparticles at nu=1/m are created by inserting anyonic vertex operators, P-1/m(z), that replace a subset of the electron operators in the correlator. The one-quasiparticle wave function is identical to the corresponding CF wave function, and the two-quasiparticle wave function has correct fractional charge and statistics and is numerically almost identical to the corresponding CF wave function. We further show how to exactly represent the CF wave functions in the Jain series nu=s/(2sp+1) [Phys. Rev. Lett. 63, 199 (1989); Composite Fermions (Cambridge University Press, Cambridge, 2007)] as the CFT correlators of a new type of fermionic vertex operators, V-p,V-n(z), constructed from n free compactified bosons; these operators provide the CFT representation of composite fermions carrying 2p flux quanta in the nth CF Landau level. We also construct the corresponding quasiparticle and quasihole operators and argue that they have the expected fractional charge and statistics. For filling fractions 2/5 and 3/7, we show that the chiral CFTs that describe the bulk wave functions are identical to those given by Wen's general classification [Int. J. Mod. Phys. B 6, 1711 (1992); Adv. Phys. 44, 405 (1995)] of quantum Hall states in terms of K matrices and l and t vectors, and we propose that to be generally true. Our results suggest a general procedure for constructing quasiparticle wave functions for other fractional Hall states, as well as for constructing ground states at filling fractions not contained in the principal Jain series.

  • 44.
    Hansson, Thors Hans
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kvorning, Thomas
    Stockholm University, Faculty of Science, Department of Physics.
    Nair, V. P.
    Sreejith, Ganesh Jaya
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max Planck Society, Germany.
    Effective field theory for a p-wave superconductor in the subgap regime2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 7, article id 075116Article in journal (Refereed)
    Abstract [en]

    We construct an effective field theory for the 2d spinless p-wave paired superconductor that faithfully describes the topological properties of the bulk state, and also provides a model for the subgap states at vortex cores and edges. In particular, it captures the topologically protected zero modes and has the correct ground-state degeneracy on the torus. We also show that our effective field theory becomes a topological field theory in a well defined scaling limit and that the vortices have the expected non-Abelian braiding statistics.

  • 45. Horsdal, Mats
    et al.
    Rypestol, Marianne
    Hansson, Hans
    Stockholm University, Faculty of Science, Department of Physics.
    Leinaas, Jon Magne
    Charge fractionalization on quantum Hall edges2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 11, p. 115313-Article in journal (Refereed)
    Abstract [en]

    We discuss the propagation and fractionalization of localized charges on the edges of quantum Hall bars of variable widths, where interactions between the edges give rise to Luttinger liquid behavior with a nontrivial interaction parameter g. We focus in particular on the separation of an initial charge pulse into a sharply defined front charge and a broader tail. The front pulse describes an adiabatically dressed electron that carries a noninteger charge, which is root g times the electron charge. We discuss how the presence of this fractional charge can, in principle, be detected through measurements of the noise in the current created by tunneling of electrons into the system. The results are illustrated by numerical simulations of a simplified model of the Hall bar.

  • 46. 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.

  • 47. 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.

  • 48. Hudl, M.
    et al.
    Campanini, Donato
    Stockholm University, Faculty of Science, Department of Physics.
    Caron, L.
    Höglin, V.
    Sahlberg, M.
    Nordblad, P.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Thermodynamics around the first-order ferromagnetic phase transition of Fe2P single crystals2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 14, article id 144432Article in journal (Refereed)
    Abstract [en]

    The specific heat and thermodynamics of Fe2P single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at T-C similar or equal to 217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

  • 49.
    Hutter, Carsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Tholen, Erik A.
    Stannigel, Kai
    Stockholm University, Faculty of Science, Department of Physics.
    Lidmar, Jack
    Haviland, David B.
    Josephson junction transmission lines as tunable artificial crystals2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 1, p. 014511-Article in journal (Refereed)
    Abstract [en]

    We investigate one-dimensional Josephson junction arrays with generalized unit cells as a circuit approach to engineer microwave band gaps. An array described by a lattice with a basis can be designed to have a gap in the electromagnetic spectrum, in full analogy to electronic band gaps in diatomic or many-atomic crystals. We derive the dependence of this gap on the array parameters in the linear regime and suggest experimentally feasible designs to bring the gap below the single-junction plasma frequency. The gap can be tuned in a wide frequency range by applying external flux, and it persists in the presence of small imperfections.

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  • 50.
    Iovan, Adrian
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Controllable generation of a spin-triplet supercurrent in a Josephson spin valve2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 13, p. 134514-Article in journal (Refereed)
    Abstract [en]

    It has been predicted theoretically that an unconventional odd-frequency spin-triplet component of a superconducting order parameter can be induced in multilayered ferromagnetic structures with noncollinear magnetization. In this work, we study experimentally nanoscale devices, in which a ferromagnetic spin valve is embedded into a Josephson junction. We demonstrate two ways of in situ analysis of such Josephson spin valves: via magnetoresistance measurements and via in situ magnetometry based on flux quantization in the junction. We observe that supercurrent through the device depends on the relative orientation of magnetizations of the two ferromagnetic layers and is enhanced in the noncollinear state of the spin valve. We attribute this phenomenon to controllable generation of the spin-triplet superconducting component in a ferromagnet.

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