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  • 1.
    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.

  • 2.
    Boris, Andrey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Klushin, A. M.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Evidence for Nonlocal Electrodynamics in Planar Josephson Junctions2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 11, article id 117002Article in journal (Refereed)
    Abstract [en]

    We study the temperature dependence of the critical current modulation I-c(H) for two types of planar Josephson junctions: a low-T-c Nb/CuNi/Nb and a high-T-c YBa2Cu3O7-delta bicrystal grain-boundary junction. At low T both junctions exhibit a conventional behavior, described by the local sine-Gordon equation. However, at elevated T the behavior becomes qualitatively different: the I-c(H) modulation field Delta H becomes almost T independent and neither Delta H nor the critical field for the penetration of Josephson vortices vanish at T-c. Such an unusual behavior is in good agreement with theoretical predictions for junctions with nonlocal electrodynamics. We extract absolute values of the London penetration depth lambda from our data and show that a crossover from local to nonlocal electrodynamics occurs with increasing T when lambda(T) becomes larger than the electrode thickness.

  • 3.
    Borodianskyi, Evgenii A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures2017In: Nature Communications, E-ISSN 2041-1723, Vol. 8, article id 1742Article in journal (Refereed)
    Abstract [en]

    Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity. 

  • 4.
    Cattaneo, Roger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kalenyuk, Aleksey A.
    Stockholm University, Faculty of Science, Department of Physics. National Academy of Sciences of Ukraine, Ukraine.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Superconducting Terahertz Sources with 12% Power Efficiency2021In: Physical Review Applied, E-ISSN 2331-7019, Vol. 16, no 6, article id L061001Article in journal (Refereed)
    Abstract [en]

    Low power efficiency is one of the main problems of terahertz (THz) sources, colloquially known as “the THz gap.” In this work we present prototypes of THz devices based on whisker crystals of a high-temperature superconductor Bi2Sr2CaCu2O8+δ with a record high-radiation power efficiency of 12% at a frequency of approximately 4 THz. We employ various on- and off-chip detection techniques and, in particular, use the radiative cooling phenomenon for accurate evaluation of the emission power. We conclude that such devices can be used for creation of tunable, monochromatic, cw, compact, and power-efficient THz sources.

  • 5.
    Cattaneo, Roger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Galin, Mikhail A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Observation of collective excitation of surface plasmon resonances in large Josephson junction arrays2022In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 13, p. 1578-1588Article in journal (Refereed)
    Abstract [en]

    Josephson junctions can be used as sources of microwave radiation. However, synchronization of many junctions is required for achieving a coherent amplification of the emitted power. In this work we present an experimental study of large arrays containing up to one thousand Nb/NbxSi1−x/Nb junctions. The arrays exhibit profound cavity mode resonances, corresponding to the formation of standing waves at the electrode/substrate interface. We observe that resonant steps in the current–voltage characteristics appear above some threshold number of junctions, Nth ≈ 100, and then progressively enhance in amplitude with further increment of the number of junctions in the resistive oscillating state. We use an external detector to measure the emission of electromagnetic waves. The emission power correlates with the step amplitude. Our results indicate that the emission is facilitated by the cavity modes in the electrodes. The modes are collectively excited by active junctions. In turn, the standing wave imprints its order on the array, facilitating mutual phase-locking of junctions. This provides an indirect coupling mechanism, allowing for the synchronization of junctions, which do not directly interact with each other. Our results demonstrate that electrodes can effectively work as a common external resonator, facilitating long-range phase-locking of large junction arrays with sizes larger than the emitted wavelength.

  • 6.
    de Andrés Prada, Roberto
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Fribourg, Switzerland.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Bernhard, C.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Growth and Nanofabrication of All-Perovskite Superconducting/Ferromagnetic/Superconducting Junctions2019In: Journal of Superconductivity and Novel Magnetism, ISSN 1557-1939, E-ISSN 1557-1947, Vol. 32, no 9, p. 2721-2726Article in journal (Refereed)
    Abstract [en]

    We fabricate and study experimentally all-perovskite-oxide superconductor/ferromagnetic insulator/superconductor (S/FI/S) tunnel junctions made out of the high-temperature cuprate superconductor YBa2Cu3O7−y (YBCO) and the colossal magnetoresistive manganite LaMnO3 (LMO) in the ferromagnetic insulator state. YBCO/LMO/YBCO heterostructures with different LMO thicknesses (5, 10, and 20 nm) are grown epitaxially via pulsed laser deposition. Nanoscale S/FI/S junctions with sizes down to 300 nm are made by three-dimensional nano-sculpturing with focused ion beam. Junctions with a thick (20 nm) LMO barrier exhibit a large negative magnetoresistance below TCurie∼160 K, typical for colossal magnetoresistive manganites, as well as a kink in the current-voltage characteristics at large bias (V∼1–2 Volts), attributed to Zener-type tunneling. However, they do not show a measurable Josephson current. On the contrary, junctions with the thinnest 5-nm LMO barrier exhibit a large supercurrent and no signs of magnetism. The latter may indicate the presence of pinholes due to thickness inhomogeneity and/or a ∼ 2 nm dead magnetic layer at the YBCO / LMO interface caused, e.g., by interdiffusion or strain. The junction with an intermediate 10-nm LMO barrier exhibited a desired S/FI/S junction behavior with significant negative magnetoresistance and signatures of a small Josephson current.

  • 7.
    de Andrés Prada, Roberto
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Fribourg, Switzerland.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Kapran, Olena M.
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Bernhard, Ch.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Memory-functionality superconductor/ferromagnet/superconductor junctions based on the high-Tc cuprate superconductors YBa2Cu3O7−x and the colossal magnetoresistive manganite ferromagnets La2/3X1/3MnO3+δ(X=Ca,Sr)2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 21, article id 214510Article in journal (Refereed)
    Abstract [en]

    Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nanoscale superconductor/ferromagnet/ superconductor junctions with the high-Tc cuprate superconductors YBa2Cu3O7−x and the colossal magnetoresistive (CMR) manganite ferromagnets La2/3X1/3MnO3+δ(X=CaorSr). We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a monodomain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in a finite magnetic field. The CMR facilitates a large readout signal in a small applied field. We argue that such a simple single-layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.

  • 8. Dremov, Viacheslav V.
    et al.
    Grebenchuk, Sergey Yu.
    Shishkin, Andrey G.
    Baranov, Denis S.
    Hovhannisyan, Razmik A.
    Skryabina, Olga V.
    Golovchanskiy, Igor A.
    Chichkov, Vladimir I.
    Brun, Chritophe
    Cren, Tristan
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Golubov, Alexander A.
    Roditchev, Dimitri
    Stolyarov, Vasily S.
    Local Josephson vortex generation and manipulation with a Magnetic Force Microscope2019In: Nature Communications, E-ISSN 2041-1723, Vol. 10, article id 4009Article in journal (Refereed)
    Abstract [en]

    Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2π-phase singularities, their observation and manipulation are challenging. Here we show that in Superconductor—Normal metal—Superconductor lateral junctions Josephson vortices have a peculiar magnetic fingerprint that we reveal in Magnetic Force Microscopy (MFM) experiments. Based on this discovery, we demonstrate the possibility of the Josephson vortex generation and manipulation by the magnetic tip of a MFM, thus paving a way for the remote inspection and control of individual nano-components of superconducting quantum circuits. 

  • 9. Galin, M. A.
    et al.
    Rudau, F.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kurin, V. V.
    Koelle, D.
    Kleiner, R.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Klushin, A. M.
    Direct Visualization of Phase-Locking of Large Josephson Junction Arrays by Surface Electromagnetic Waves2020In: Physical Review Applied, E-ISSN 2331-7019, Vol. 14, no 2, article id 024051Article in journal (Refereed)
    Abstract [en]

    Phase-locking of oscillators leads to super-radiant amplification of the emission power. This is particularly important for development of terahertz sources, which suffer from low emission efficiency. In this work we study large Josephson junction arrays containing several thousand Nb-based junctions. Using low-temperature scanning laser microscopy, we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting themselves as global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon-type electromagnetic waves propagating at the electrode-substrate interface. Thus, we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.

  • 10.
    Galin, Mikhail A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute for Physics of Microstructures RAS, Russia.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kurin, V. V.
    Shereshevskiy, I. A.
    Vdovicheva, N. K.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Klushin, A. M.
    Synchronization of Large Josephson-Junction Arrays by Traveling Electromagnetic Waves2018In: Physical Review Applied, E-ISSN 2331-7019, Vol. 9, no 5, article id 054032Article in journal (Refereed)
    Abstract [en]

    Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

  • 11.
    Galin, Mikhail A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute for Physics of Microstructures RAS, Russia.
    Borodianskyi, Ievgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kurin, V. V.
    Shereshevskiy, I. A.
    Vdovicheva, N. K.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Klushin, A. M.
    Evidence of synchronization of large Josephson-junction arrays by traveling electromagnetic waves2018In: EPJ Web of Conferences, E-ISSN 2100-014X, Vol. 195, article id 02004Article in journal (Refereed)
  • 12. Galin, Mikhail A.
    et al.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Shereshevsky, Ilya A.
    Vdovicheva, Nadezhda K.
    Kurin, Vladislav V.
    Coherent amplification of radiation from two phase-locked Josephson junction arrays2022In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 13, p. 1445-1457Article in journal (Refereed)
    Abstract [en]

    We analyze experimentally and theoretically mutual phase locking and electromagnetic interaction between two linear arrays with a large number of Josephson junctions. Arrays with different separation, either on the same chip or on two separate substrates are studied. We observe a large coherent gain, up to a factor of three, of emitted power from two simultaneously biased arrays, compared to the sum of powers from two individually biased arrays. The phenomenon is attributed to the phase locking of junctions in different arrays via a common electromagnetic field. Remarkably, the gain can exceed the factor of two expected for a simple constructive interference of two oscillators. The larger gain is explained by an additional consequence of mutual interaction between two large arrays. Mutual phase locking of large arrays does not only result in constructive interference outside the arrays, but also improved synchronization of junctions inside each array. Our conclusion is supported by numerical modelling.

  • 13.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Frederiksen, Henrik
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nb-PtNi-Nb Josephson junctions made by 3D FIB nano-sculpturing2009In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 150, no 5, p. 052062-Article in journal (Refereed)
    Abstract [en]

    We use Focused Ion Beam (FIB) for fabrication of nano-scale Superconductor-Ferromagnet-Superconductor (SFS) Josephson junctions, aiming to achieve a uniform, mono-domain state in the F-layer within the junction. We employ a Pt1-xNix alloy, characterized by the perfect solubility of the two components, for obtaining a homogeneous diluted ferromagnet. We perform a systematic analysis of both chemical composition, and ferromagnetic properties of Pt1—xNix thin films for different Ni—concentrations. The nano-scale homogeneity of the Pt1—xNix films is confirmed by energy dispersive X-ray spectroscopy. The Curie temperature of Pt1—xNix films decreases in a non-linear manner with Ni concentration. We observe that the critical current density of NbPt1—xNixNb junctions decreases non-monotonously with increasing Ni-concentration: at x 30% it exhibits a minimum, which we attribute to switching into the π state as a function of Ni-concentration.

  • 14.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hovhannisyan, Razmik A.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Kapran, Olena M.
    Stockholm University, Faculty of Science, Department of Physics.
    Dremov, Vyacheslav V.
    Stolyarov, Vasily S.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Reconfigurable Josephson Phase Shifter2021In: Nano Letters, ISSN 1530-6984, E-ISSN 1530-6992, Vol. 21, no 12, p. 5240-5246Article in journal (Refereed)
    Abstract [en]

    Phase shifter is one of the key elements of quantum electronics. In order to facilitate operation and avoid decoherence, it has to be reconfigurable, persistent, and nondissipative. In this work, we demonstrate prototypes of such devices in which a Josephson phase shift is generated by coreless superconducting vortices. The smallness of the vortex allows a broad-range tunability by nanoscale manipulation of vortices in a micron-size array of vortex traps. We show that a phase shift in a device containing just a few vortex traps can be reconfigured between a large number of quantized states in a broad [−3π, +3π] range.

  • 15.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Iovan, Adrian
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Single Abrikosov vortices as quantized information bits2015In: Nature Communications, E-ISSN 2041-1723, Vol. 6, article id 8628Article in journal (Refereed)
    Abstract [en]

    Superconducting digital devices can be advantageously used in future supercomputers because they can greatly reduce the dissipation power and increase the speed of operation. Non-volatile quantized states are ideal for the realization of classical Boolean logics. A quantized Abrikosov vortex represents the most compact magnetic object in superconductors, which can be utilized for creation of high-density digital cryoelectronics. In this work we provide a proof of concept for Abrikosov-vortex-based random access memory cell, in which a single vortex is used as an information bit. We demonstrate high-endurance write operation and two different ways of read-out using a spin valve or a Josephson junction. These memory cells are characterized by an infinite magnetoresistance between 0 and 1 states, a short access time, a scalability to nm sizes and an extremely low write energy. Non-volatility and perfect reproducibility are inherent for such a device due to the quantized nature of the vortex.

  • 16.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kapran, Olena M.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, State University, Russia.
    Planar Superconductor-Ferromagnet-Superconductor Josephson Junctions as Scanning-Probe Sensors2019In: Physical Review Applied, E-ISSN 2331-7019, Vol. 11, no 1, article id 014062Article in journal (Refereed)
    Abstract [en]

    We propose a magnetic scanning-probe sensor based on a single-planar Josephson junction with a magnetic barrier. The planar geometry together with the high magnetic permeability of the barrier facilitates a double flux-focusing effect, which helps to guide magnetic flux into the junction and thus enhances field sensitivity of the sensor. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic Cu−Ni and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nanometer scale and facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feedback operation over a broad dynamic range. We argue that the proposed sensor can outperform equally sized superconducting quantum-interference devices (SQUIDs) both in terms of magnetic-field sensitivity and spatial resolution, which makes it advantageous for scanning-probe microscopy.

  • 17.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Demonstration of a superconducting diode-with-memory, operational at zero magnetic field with switchable nonreciprocity2022In: Nature Communications, E-ISSN 2041-1723, Vol. 13, no 1, article id 3658Article in journal (Refereed)
    Abstract [en]

    Superconducting diodes, operational at zero magnetic field, can be used in supercomputers. Here, the authors demonstrate prototypes of diodes-with-memory, based on Nb Josephson junctions, with a large and switchable nonreciprocity at zero field. Diode is one of the basic electronic components. It has a nonreciprocal current response, associated with a broken space/time reversal symmetry. Here we demonstrate prototypes of superconducting diodes operational at zero magnetic field. They are based on conventional niobium planar Josephson junctions, in which space/time symmetry is broken by a combination of self-field effect from nonuniform bias and stray fields from a trapped Abrikosov vortex. We demonstrate that nonreciprocity of critical current in such diodes can reach an order of magnitude and rectification efficiency can exceed 70%. Furthermore, we can easily change the diode polarity and switch nonreciprocity on/off by changing the bias configuration and by trapping/removing of a vortex. This facilitates a memory functionality. We argue that such a diode-with-memory can be used for a future generation of in-memory superconducting computers.

  • 18.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Morlet-Decarnin, Lise
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Word and bit line operation of a 1 x 1 μm2 superconducting vortex-based memory2023In: Nature Communications, E-ISSN 2041-1723, Vol. 14, no 1, article id 4926Article in journal (Refereed)
    Abstract [en]

    The lack of dense random access memory is one of the main bottlenecks for the creation of a digital superconducting computer. In this work we study experimentally vortex-based superconducting memory cells. Three main results are obtained. First, we test scalability and demonstrate that the cells can be straightforwardly miniaturized to submicron sizes. Second, we emphasize the importance of conscious geometrical engineering. In the studied devices we introduce an asymmetric easy track for vortex motion and show that it enables a controllable manipulation of vortex states. Finally, we perform a detailed analysis of word and bit line operation of a 1 x 1 mu m(2) cell. High-endurance, non-volatile operation at zero magnetic field is reported. Remarkably, we observe that the combined word and bit line threshold current is significantly reduced compared to the bare word-line operation. This could greatly improve the selectivity of individual cell addressing in a multi-cell RAM. The achieved one square micron area is an important milestone and a significant step forward towards creation of a dense cryogenic memory.

  • 19.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pagliero, Alessandro
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. State University, Russia.
    Two mechanisms of Josephson phase shift generation by an Abrikosov vortex2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 17, article id 174511Article in journal (Refereed)
    Abstract [en]

    Abrikosov vortices contain magnetic fields and circulating currents that decay at a short range λ∼100 nm. However, vortices can induce Josephson phase shifts at a long range r∼μm≫λ. Mechanisms of this puzzling phenomenon are not clearly understood. Here we present a systematic study of vortex-induced phase shifts in planar Josephson junctions. We make two key observations: (i) The cutoff effect: Although vortex-induced phase shift is a long-range phenomenon, it is terminated by the junction and does not persist beyond it. (ii) A linear to superlinear crossover with a rapid upturn of the phase shift occurs upon approaching a vortex to a junction. The crossover occurs at a vortex-junction distance comparable to the penetration depth. Together with theoretical and numerical analysis this allows unambiguous identification of two distinct and independent mechanisms. The short range r≲λ mechanism is due to circulating vortex currents inside a superconducting electrode without involvement of magnetic fields. The long range r≫λ mechanism is due to stray magnetic fields outside electrodes without circulating vortex currents. We argue that understanding of controlling parameters of vortex-induced Josephson phase shift can be used for development of novel compact cryoelectronic devices.

  • 20.
    Golod, Taras
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir
    Stockholm University, Faculty of Science, Department of Physics.
    Detection of the Phase Shift from a Single Abrikosov Vortex2010In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 104, p. 227003-Article in journal (Refereed)
    Abstract [en]

    We probe a quantum mechanical phase rotation induced by a single Abrikosov vortex in a superconducting lead, using a Josephson junction, made at the edge of the lead, as a phase-sensitive detector. We observe that the vortex induces a Josephson phase shift equal to the polar angle of the vortex within the junction length. When the vortex is close to the junction it induces a π step in the Josephson phase difference, leading to a controllable and reversible switching of the junction into the 0-π state. This in turn results in an unusual Φ0/2 quantization of the flux in the junction. The vortex may hence act as a tunable “phase battery” for quantum electronics.

  • 21.
    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.
    Anomalous Hall effect in NiPt thin films2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 3, p. 033909-Article in journal (Refereed)
    Abstract [en]

    We study Hall effect in sputtered NixPt1-x thin films with different Ni concentrations. Temperature, magnetic field andangular dependencies are analyzed and the phase diagram of NiPt thin films is obtained. It is found that films with sub-critical Ni concentration exhibit cluster-glass behavior at low temperatures with a perpendicular magnetic anisotropy below the freezing temperature. Films with super-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the state of the film is strongly frustrated. Such films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather than a single second-order phase transition. But most remarkably, the extraordinary Hall effect changes sign at the critical concentration. We suggest that this is associated with a reconstruction of the electronic structure of the alloy at the normal metal - ferromagnet quantum phase transition.

  • 22.
    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.
    Application of nano-scale Josephson junction as phase sensitive detector for analysis of vortex states in mesoscopic superconductors2010In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 570, no 19, p. 890-892Article in journal (Refereed)
    Abstract [en]

    We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt1−xNix–Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt1−xNix–Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at 40 at.% Ni, which is an indication of switching into the π state.

  • 23.
    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.

  • 24.
    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.

  • 25.
    Grebenchuk, S. Yu
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute for Physics of Microstructures RAS, Russia.
    Cattaneo, Roger
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nonlocal Long-Range Synchronization of Planar Josephson-Junction Arrays2022In: Physical Review Applied, E-ISSN 2331-7019, Vol. 17, no 6, article id 064032Article in journal (Refereed)
    Abstract [en]

    We study arrays of planar Nb Josephson junctions with contacts to intermediate electrodes, which allow measurements of individual junctions and, thus, provide an insight into intricate array dynamics. We observe strong indications for array phase locking, despite a significant interjunction separation. Several unusual phenomena are reported, such as a bistable critical current with reentrant superconductivity upon switching of nearby junctions; and “incorrect” Shapiro steps, occurring at mixing frequencies between the external rf radiation and the internal Josephson frequency in nearby junctions. Our results reveal a surprisingly strong and long-range interjunction interaction, which is attributed to nonlocality of planar-junction electrodynamics, caused by the long-range spreading of stray electromagnetic fields. The nonlocality greatly enhances the high-frequency interjunction coupling and enables large-scale synchronization. Therefore, we conclude that planar geometry is advantageous for the realization of coherent Josephson electronics.

  • 26. Grebenchuk, Sergey Yu
    et al.
    Hovhannisyan, Razmik A.
    Dremov, Viacheslav V.
    Shishkin, Andrey G.
    Chichkov, Vladimir I.
    Golubov, Alexander A.
    Roditchev, Dimitri
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Stolyarov, Vasily S.
    Observation of interacting Josephson vortex chains by magnetic force microscopy2020In: Physical Review Research, E-ISSN 2643-1564, Vol. 2, no 2, article id 023105Article in journal (Refereed)
    Abstract [en]

    The ability to control Josephson vortices is instrumental for development of superconducting cryoelectronics. However, direct visualization of multivortex states in Josephson junctions is a challenging task. Here, we employ a magnetic force microscopy (MFM) for the analysis of planar Josephson junctions. We observe a specific MFM response, seen as a chain of small rings. By changing the applied field, we show that the number of rings is equal to the number of flux quanta in the junction. Therefore, each ring represents an individual vortex in a one-dimensional vortex chain within the junction. Our observation demonstrates that the MFM technique can be used for visualization of Josephson vortices and for probing their spatial configurations and mutual interaction.

  • 27.
    Hovhannisyan, Razmik A.
    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.
    Holographic reconstruction of magnetic field distribution in a Josephson junction from diffraction-like Ic(H) patterns2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 21, article id 214513Article in journal (Refereed)
    Abstract [en]

    A general problem of magnetic sensors is a trade-off between spatial resolution and magnetic-field sensitivity. With decreasing sensor size its resolution is improved but the sensitivity is deteriorated. Obviation of such a trade-off requires development of super-resolution imaging technique not limited by sensor size. Here we present a proof of concept for a super-resolution method of magnetic imaging by a Josephson junction (JJ). It is based on a solution of an inverse problem—reconstruction of a local magnetic-field distribution within a junction from the dependence of the critical current on an external magnetic field, Ic(H). The method resembles the Fourier-transform holography, with the diffractionlike Ic(H) pattern serving as a hologram. A simple inverse problem solution, valid for an arbitrary symmetric case, is derived. We verify the method numerically and show that the accuracy of reconstruction does not depend on the junction size and is only limited by the field range of the Ic(H) pattern. Finally, the method is tested experimentally using planar Nb JJs. Super-resolution reconstruction of stray magnetic fields from an Abrikosov vortex, trapped in the junction electrodes, is demonstrated. Thus our method facilitates both high field sensitivity and high spatial resolution, obviating the trade-off problem of magnetic sensors. We conclude that the holographic magnetic imaging by a planar JJ can be used in scanning probe microscopy

  • 28.
    Hovhannisyan, Razmik A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Kapran, Olena M.
    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. Moscow Institute of Physics and Technology, Russia.
    Accurate Determination of the Josephson Critical Current by Lock-In Measurements2021In: Nanomaterials, E-ISSN 2079-4991, Vol. 11, no 8, article id 2058Article in journal (Refereed)
    Abstract [en]

    Operation of Josephson electronics usually requires determination of the Josephson critical current Ic, which is affected both by fluctuations and measurement noise. Lock-in measurements allow obviation of 1/f noise, and therefore, provide a major advantage in terms of noise and accuracy with respect to conventional dc measurements. In this work we show both theoretically and experimentally that the Ic can be accurately extracted using first and third harmonic lock-in measurements of junction resistance. We derived analytical expressions and verified them experimentally on nano-scale Nb-PtNi-Nb and Nb-CuNi-Nb Josephson junctions.

  • 29.
    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.

  • 30.
    Iovan, Adrian
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Signatures of the spin-triplet current in a Josephson spin valve: A micromagnetic analysis2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 1, article id 014511Article in journal (Refereed)
    Abstract [en]

    A Josephson spin valve is a ferromagnetic spin valve sandwiched between two superconducting electrodes. It has been predicted theoretically that such a device may exhibit a long-range proximity effect due to generation of unconventional odd-frequency spin-triplet and long-range spin-singlet components of the supercurrent. In this work we present a comprehensive numerical analysis of Josephson spin-valve characteristics. Our analysis is based on micromagnetic simulations for Ni-based spin valves. The supercurrent through the spin valve depends on shapes and sizes of components, the magnetic domain structure, and the flux quantization. For very small monodomain spin valves, the triplet current is manifested by a dissimilar double maximum in the magnetic field dependence of the critical current Ic (H). However, this feature is washed away in larger devices due to appearance of domains and flux quantization. The only remaining signature of the triplet current in this case are beatings in Ic (H) with a half-flux quantum periodicity. The complexity of the device can make it difficult to identify the spin-triplet supercurrent without a detailed knowledge of the spin-valve state. However, we argue that unambiguous conclusions can be made from a systematic analysis of size, thickness, and shape dependencies of the Josephson spin-valve characteristics.

  • 31.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Maljuk, A.
    Helm, T.
    Putzke, C.
    Kampert, E.
    Kartsovnik, M. V.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Electron-tunneling measurements of low-T-c single-layer Bi2+xSr2-yCuO6+delta: Evidence for a scaling disparity between superconducting and pseudogap states2012In: Physical Review B, ISSN 2469-9950, Vol. 86, no 21, article id 214506Article in journal (Refereed)
    Abstract [en]

    We experimentally study intrinsic tunneling and high magnetic field (up to 65 T) transport characteristics of the single-layer cuprate Bi2+xSr2-yCuO6+delta, with a very low superconducting critical temperature T-c less than or similar to 4 K. It is observed that the superconducting gap, the collective bosonic mode energy, the upper critical field, and the fluctuation temperature range are scaling down with T-c, while the corresponding pseudogap characteristics remain the same as in high-T-c cuprates with 20 to 30 times higher T-c. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.

  • 32.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Superconducting correlations above T-c in the pseudogap state of Bi2Sr2CaCu2O8+delta cuprates revealed by angular-dependent magnetotunneling2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 22, article id 220501Article in journal (Refereed)
    Abstract [en]

    We present an angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered Bi2Sr2CaCu2O8+delta cuprates. Our data indicate persistence of superconducting correlations at temperatures up to 1.5T(c) in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

  • 33.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Simsek, Yilmaz
    Stockholm University, Faculty of Science, Department of Physics. Universität Erlangen-Nürnberg, Germany.
    Koval, Y.
    Müller, P.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Sequence of Quantum Phase Transitions in Bi2Sr2CaCu2O8+δ Cuprates Revealed by In Situ Electrical Doping of One and the Same Sample2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 6, article id 067001Article in journal (Refereed)
    Abstract [en]

    Our recently discovered electrical doping technique allows a broad-range variation of carrier concentration without changing the chemical composition. We show that it is possible to induce superconductivity in a nondoped insulating sample and to tune it reversibly all the way to an overdoped metallic state. This way, we can investigate the whole doping diagram of one and the same sample. Our study reveals two distinct critical points. The one at the overdoped side is associated with the onset of the pseudogap and with the metal-to-insulator transition in the c-axis transport. The other at optimal doping is associated with the appearance of a dressed electron energy. Our study confirms the existence of multiple phase transitions under the superconducting dome in cuprates.

  • 34.
    Kalenyuk, Aleksey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute of Metal Physics of National Academy of Sciences of Ukraine, Ukraine; Kyiv Academic University, Ukraine.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kordyuk, A. A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Influence of the Fermi surface geometry on the Josephson effect between iron-pnictide and conventional superconductors2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 21, article id 214507Article in journal (Refereed)
    Abstract [en]

    We study hybrid Josephson junctions between a multiband Ba1-xNaxFe2As2 iron-pnictide and Nb. We observe that the insertion of a Cu interlayer in such junctions leads to a dramatic enhancement of the IcRn product, despite the weaker proximity-induced superconductivity of Cu. This counterintuitive phenomenon is attributed to the differences in Fermi surface geometries of Nb and Cu, which affect the selectivity of tunneling in sign-reversal s± bands of pnictide. Our results indicate that the sensitivity to Fermi surface geometries provides a new tool for phase-sensitive studies and paves the way to conscious Fermi surface engineering of pnictide junctions.

  • 35.
    Kalenyuk, Aleksey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute of Metal Physics of National Academy of Sciences of Ukraine, Ukraine.
    Pagliero, Alessandro
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Aswartham, S.
    Wurmehl, S.
    Büchner, B.
    Chareev, D. A.
    Kordyuk, A. A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Unusual two-dimensional behavior of iron-based superconductors with low anisotropy2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 13, article id 134512Article in journal (Refereed)
    Abstract [en]

    We study angular-dependent magnetoresistance in iron-based superconductors Ba1−xNaxFe2As2 and FeTe1−xSex. Both superconductors have relatively small anisotropies γ∼2 and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field Hc2(T) for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.

  • 36.
    Kalenyuk, Aleksey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute of Metal Physics of National Academy of Sciences of Ukraine, Ukraine.
    Pagliero, Alessandro
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Kordyuk, A. A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Phase-Sensitive Evidence for the Sign-Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1−xNaxFe2As2 Josephson Junctions2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 6, article id 067001Article in journal (Refereed)
    Abstract [en]

    Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional s-wave superconductor Nb and a multiband iron-pnictide Ba1−xNaxFe2As2. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the IcRn product is very small ≃3μV, which is not consistent with the sign-preserving s++ symmetry either. We argue that the small IcRn value, along with its unusual temperature dependence, provides evidence for the sign-reversal s± symmetry of the order parameter in Ba1−xNaxFe2As2. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.

  • 37.
    Kapran, Olena M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Iovan, Adrian
    Stockholm University, Faculty of Science, Department of Physics. Royal Institute of Technology, Sweden.
    Sidorenko, A. S.
    Golubov, A. A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, State University, Russia.
    Crossover between short- and long-range proximity effects in superconductor/ferromagnet/superconductor junctions with Ni-based ferromagnets2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 9, article id 094509Article in journal (Refereed)
    Abstract [en]

    We study superconductor/ferromagnet/superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in Pt1−xNix with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.

  • 38.
    Kapran, Olena M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Iovan, Adrian
    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.
    Observation of the dominant spin-triplet supercurrent in Josephson spin valves with strong Ni ferromagnets2020In: Physical Review Research, E-ISSN 2643-1564, Vol. 2, no 1, article id 013167Article in journal (Refereed)
    Abstract [en]

    We study experimentally nanoscale Josephson junctions and Josephson spin valves containing strongly ferromagnetic Ni interlayers. We observe that in contrast to junctions, spin valves with the same geometry exhibit anomalous Ic(H) patterns with two peaks separated by a dip. We develop several techniques for in situ characterization of micromagnetic states in our nanodevices, including magnetoresistance, absolute Josephson fluxometry, and first-order-reversal-curves analysis. They reveal a clear correlation of the dip in supercurrent with the antiparallel state of a spin valve and the peaks with two noncollinear magnetic states, thus providing evidence for generation of spin-triplet superconductivity. A quantitative analysis, based on micromagnetic simulations, brings us to the conclusion that the triplet current in our Ni-based spin valves is approximately three times larger than the conventional spin-singlet supercurrent.

  • 39.
    Kapran, Olena M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Morari, Roman
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Boian, Vladimir
    Prepelita, Andrei
    Klenov, Nikolay
    Sidorenko, Anatoli S.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. State University, Russia.
    In situ transport characterization of magnetic states in Nb/Co superconductor/ferromagnet heterostructures2021In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 12, p. 913-923Article in journal (Refereed)
    Abstract [en]

    Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state. The polydomain state is manifested by a profound enhancement of resistance caused by a flux-flow phenomenon, triggered by domain stray fields. The scissor state represents the noncollinear magnetic state in which the unconventional odd-frequency spin-triplet order parameter should appear. The non-hysteretic nature of this state allows for reversible tuning of the magnetic orientation. Thus, we identify the range of parameters and the procedure for in situ control of devices based on S/F heterostructures.

  • 40.
    Katterwe, Sven Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jacobs, Thorsten
    Stockholm University, Faculty of Science, Department of Physics.
    Maljuk, A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Low anisotropy of the upper critical field in a strongly anisotropic layered cuprate Bi2.15Sr1.9CuO6+δ: Evidence for a paramagnetically limited superconductivity2014In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 89, no 21, article id 214516Article in journal (Refereed)
    Abstract [en]

    We study angular-dependent magnetoresistance in a low-T-c layered cuprate Bi2.15Sr1.9CuO6+delta. The low Tc similar to 4 K allows complete suppression of superconductivity by modest magnetic fields and facilitates accurate analysis of the upper critical field H-c2. We observe a universal exponential decay of fluctuation conductivity in a broad range of temperatures above Tc and propose a method for extraction of H-c2(T) from the scaling analysis of the fluctuation conductivity at T > T c. Our main result is observation of a surprisingly low H-c2 anisotropy similar to 2, which is much smaller than the effective mass anisotropy of the material similar to 300. We show that the anisotropy is decreasing with increasing field and saturates at a small value when the field reaches the paramagnetic limit. We argue that the dramatic discrepancy of high-field and low-field anisotropies is clear evidence for paramagnetically limited superconductivity.

  • 41.
    Katterwe, Sven Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Temperature dependences of geometrical and velocity-matching resonances in Bi(2)Sr(2)CaCu(2)O(8+x) intrinsic Josephson junctions2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 21, p. 214519-Article in journal (Refereed)
    Abstract [en]

    We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux-flow state of small Bi(2)Sr(2)CaCu(2)O(8+x) mesa structures. It is shown that the quality factor of resonances is high at low T, but rapidly decreases with increasing temperature. We also study T dependencies of resonant voltages and the speed of electromagnetic waves (the Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits a flat T dependence at low T, following T dependence of the c-axis critical current, rather than the expected linear T dependence of the London penetration depth. Our data indicate that self-heating is detrimental for operation of mesas as coherent THz oscillators because it limits the emission power via suppression of the quality factor. On the other hand, significant temperature dependence of the Swihart velocity allows broad-range tunability of the output frequency.

  • 42.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir
    Stockholm University, Faculty of Science, Department of Physics.
    Stabilization of the in-phase fluxon state by geometrical confinement in small Bi2Sr2CaCu2O8+x mesa structures2009In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 80, no 2, p. 020502(R)-Article in journal (Refereed)
    Abstract [en]

    Thein-phase (rectangular) fluxon lattice is required for achieving coherent THzemission from stacked Josephson junctions. Unfortunately, it is usually unstabledue to mutual repulsion of fluxons in neighbor junctions, whichfavors the out-of-phase (triangular) lattice. Here we experimentally study magneticfield modulation of the critical current in small Bi-2212 mesastructures with different sizes. Clear Fraunhofer-like modulation is observed whenthe field is aligned parallel to CuO planes. For longmesas the periodicity of modulation is equal to half theflux quantum per intrinsic Josephson junction, corresponding to the triangularfluxon lattice. However, the periodicity is changed to one fluxquantum, characteristic for the rectangular fluxon lattice, both by decreasingthe length of the mesas and by increasing magnetic field.Thus, we demonstrate that the stationary in-phase fluxon state canbe effectively stabilized by geometrical confinement in small Bi-2212 mesastructures.

  • 43.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Temperature dependence of geometrical and velocity matching resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctionsManuscript (preprint) (Other academic)
    Abstract [en]

    We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux flow state of small Bi2Sr2CaCu2O8+x mesa structures. It is shown that the quality factor of resonances is high at low T, but rapidly decreases with increasing temperature already at T > 10 K. This indicates that self-heatingis strongly detrimental for operation of mesas as coherent THz oscillators and ultimately limits the emission power via suppression of the quality factor. We also study T-dependence of the resonant voltage and the speed of electromagnetic waves (Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits very weak T-dependence at low T, following T−dependence of the Josephson plasma frequency, rather than the expected linear T-dependence of the London penetration depth.

  • 44.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    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.
    Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions2011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 83, no 10, article id 100510Article in journal (Refereed)
    Abstract [en]

    The Bi2Sr2CaCu2O8+x high-temperature superconductor represents a natural layered metamaterial composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and self-detection of electromagnetic waves in Bi2Sr2CaCu2O8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to unusually strong electron-phonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm2 power densities.

  • 45.
    Katterwe, Sven-Olof