Profiles

Vladimir Krasnov

Vladimir Krasnov

Professor i kondenserade materiens fysik

Visa sidan på svenska
Works at Department of Physics
Telephone 08-553 786 06
Email krasnov@fysik.su.se
Visiting address Roslagstullsbacken 21
Room A2:1054
Postal address Fysikum 106 91 Stockholm

About me

Professor in Experimental Condensed Matter Physics with focus on mesoscopic phenomena

Teaching

FK7053 Superconductivity

http://ekmf.fysik.su.se/Supraledning/WWW/Supraledning.htm

 

FK7054 Nanotechnology

http://ekmf.fysik.su.se/Nanoteknologi/WWW/Nanotechnology.htm

 

Evaluation of Master and Bachelor projects

Research

Prof. Vladimir Krasnov is leading the Experimental Condensed Matter Physics Group at Stockholm University from February 2005. The research in the group is focused at the study of mesoscopic phenomena in condensed matter physics with emphasis on superconductivity, magnetism and nanotechnology.

Current research projects:
* THz applications of superconducting tunnel (Josephson) junctions;
* Study of hybrid superconductor/ferromagnet devices for spintronic applications (specifically for cryogenic memory);
* Vortices in Josephson junctions;
* Intrinsic tunneling spectroscopy of high temperature superconductors;
* Analysis of non-equilibrium phenomena in Josephson junctions.
(For more details see "Research" and "Publications")

Methodology and experimental techniques

Nano-Fab clean room: We make our samples using advanced sample engineering and micro/nano-fabrication in the clean-room environment. Nano-fabrication is usually performed in the Nano-Fab Lab, a joint SU-KTH facility, located here at AlbaNova University Center.

Low-temperature lab: Measurements are performed in the Low-T lab, containing various cryogen-free cryostats for measurements at temperatures 0.25-300 K, high magnetic fields (up to 17T) and high (THz) frequencies.

Publications

A selection from Stockholm University publication database
  • 2020. Vladimir M. Krasnov. Physical Review B 101 (14)

    A Josephson junction can be subjected to a local, strongly inhomogeneous magnetic field in various experimental situations. Here this problem is analyzed analytically and numerically. A modified sine-Gordon type equation in the presence of time-dependent local field is derived and solved numerically in static and dynamic cases. Two specific examples of local fields are considered: induced either by an Abrikosov vortex, or by a tip of a magnetic force microscope (MFM). It is demonstrated that a time-dependent local field can induce a dynamic flux-flow state in the junction with shuttling or unidirectional ratchetlike Josephson vortex motion. This provides a mechanism for detection and manipulation of Josephson vortices by an oscillating MFM tip. In a static case, the local field leads to a distortion of the critical current versus magnetic field, I-c(H), pattern. The distortion is sensitive to both the shape and the amplitude of the local field. Therefore, the I-c(H) pattern carries information about the local field distribution within the junction. This opens the possibility for employing a single Josephson junction as a scanning probe sensor with spatial resolution not limited by its geometrical size, thus obviating a known problem of a trade-off between the field sensitivity and the spatial resolution of a sensor.

  • 2019. Roberto de Andrés Prada (et al.). Journal of Superconductivity and Novel Magnetism 32 (9), 2721-2726

    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" role="presentation" style="box-sizing: border-box; display: inline-table; line-height: normal; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">TCurie∼160TCurie∼160 K, typical for colossal magnetoresistive manganites, as well as a kink in the current-voltage characteristics at large bias (V∼1" role="presentation" style="box-sizing: border-box; display: inline-table; line-height: normal; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">V∼1V∼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 ∼" role="presentation" style="box-sizing: border-box; display: inline-table; line-height: normal; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">∼∼ 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.

  • 2019. Viacheslav V. Dremov (et al.). Nature Communications 10

    Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2 pi-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.

  • 2019. Roberto de Andrés Prada (et al.). Physical Review B 99 (21)

    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-T-c cuprate superconductors YBa2Cu3O7-x and the colossal magnetoresistive (CMR) manganite ferromagnets La2/3X1/3MnO3+delta (X=Ca or Sr). 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.

  • 2019. Taras Golod, Olena M. Kapran, Vladimir M. Krasnov. Physical Review Applied 11 (1)

    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.

  • 2019. Taras Golod, Alessandro Pagliero, Vladimir M. Krasnov. Physical Review B 100 (17)

    Abrikosov vortices contain magnetic fields and circulating currents that decay at a short range lambda similar to 100 nm. However, vortices can induce Josephson phase shifts at a long range r similar to mu m >> lambda. 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 less than or similar to lambda mechanism is due to circulating vortex currents inside a superconducting electrode without involvement of magnetic fields. The long range r >> lambda 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.

  • 2018. Mikhail A. Galin (et al.). EPJ Web of Conferences 195
  • 2018. Aleksey A. Kalenyuk (et al.). Physical Review Letters 120 (6)

    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. similar or equal to 3 mu V, 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.

  • 2018. Mikhail A. Galin (et al.). Physical Review Applied 9 (5)

    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.

  • 2017. Evgenii A. Borodianskyi, Vladimir M. Krasnov. Nature Communications 8

    Mesa structures made of Bi2Sr2CaCu2O8+delta 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 similar to 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.

  • 2017. Adrian Iovan, Vladimir M. Krasnov. Physical Review B 96 (1)

    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.

  • 2017. Aleksey A. Kalenyuk (et al.). Physical Review B 96 (13)

    We study angular-dependent magnetoresistance in iron-based superconductors Ba1-xNaxFe2As2 and FeTe1-xSex. Both superconductors have relatively small anisotropies gamma similar to 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 H-c2(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.

  • 2016. Vladimir M. Krasnov. Physical Review B 93 (6)

    A subharmonic structure in Josephson junctions appears due to Andreev reflections within the junction. Here we report on experimental observation of a subharmonic half-gap singularity in interlayer tunneling characteristics of a layered high temperature superconductor Bi2Sr2CaCu2O8+delta. The singularity is most pronounced in optimally doped crystals and vanishes with decreasing doping. It indicates the existence of nonvanishing electronic density of states and certain metallic properties in the intermediate BiO layers, which grows stronger with increasing doping. This provides an additional coherent interlayer transport channel and can explain a gradual transition from an incoherent quasi-two-dimensional c-axis transport in underdoped to a coherent metallic transport in overdoped cuprates. Furthermore, due to a very small subgap current, the singularity allows unambiguous extraction of the superconducting gap, without distortion by self-heating.

  • 2016. Thorsten Jacobs (et al.). Physical Review Letters 116 (6)

    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.

  • 2016. Thorsten Jacobs, Sven-Olof Katterwe, Vladimir M. Krasnov. Physical Review B 94 (22)

    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.

  • 2016. Arash Zeinali, Taras Golod, Vladimir M. Krasnov. Physical Review B 94 (21)

    We study the origin of broadening of superconducting transition in sputtered Nb films at high magnetic fields. From simultaneous tunneling and transport measurements we conclude that the upper critical field H-c2 always corresponds to the bottom of transition R similar to 0, while the top R similar to R-n occurs close to the critical field for destruction of surface superconductivity H-c3 similar or equal to 1.7H(c2). The two-dimensional nature of superconductivity at H > H-c2 is confirmed by cusplike angular dependence of magnetoresistance. Our data indicates that surface superconductivity is remarkably robust even in disordered polycrystalline films and, surprisingly, even in perpendicular magnetic fields. We conclude that surface superconductivity, rather than flux-flow phenomenon, inhomogeneity, or superconducting fluctuations, is the primary cause of broadening of superconducting transition in magnetic field.

  • 2015. Andrey A. Boris, Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 92 (17)

    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.

  • 2015. Taras Golod, Adrian Iovan, Vladimir M. Krasnov. Nature Communications 6

    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.

  • 2015. Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 91 (22)

    How does Fermi surface develop upon doping of cuprates, does it consist of large barrels or small pockets, which of them is responsible for superconductivity, and what is a role of the pseudogap? Those are actively debated questions, important for understanding of high temperature superconductivity. We study doping dependence of intrinsic tunneling in Bi2Sr2Ca1-x YxCu2O8+delta cuprates, which allows independent analysis of Cooper pair and quasiparticle transport. We observe that with decreasing doping the supercurrent is rapidly decreasing at a rate much faster than the doping level, but the quasiparticle resistance at a sufficiently high bias remains almost doping independent. This remarkable discrepancy indicates that Cooper pairs and quasiparticles are originating from different parts of the Brillouin zone: Cooper pairs are residing on small pockets, which are progressively shrinking with decreasing doping, but the majority of the quasiparticle current is integrated over large barrels, which are only weakly doping dependent. The expanding pseudogap areas along the barrels do not contribute to pair current. This provides direct evidence for nonsuperconducting origin of the pseudogap. We present numerical calculations, taking into account Fermi surface topology, that support our conclusions.

  • 2014. Adrian Iovan, Taras Golod, Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 90 (13), 134514

    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.

  • 2014. Sven Olof Katterwe (et al.). Physical Review B 89 (21)

    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.

  • 2013. Taras Golod (et al.). Physical Review B. Condensed Matter and Materials Physics 87 (10), 104407

    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. DOI: 10.1103/PhysRevB.87.104407

  • 2013. Andrey A. Boris (et al.). Physical Review Letters 111 (11)

    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.

  • 2013. Taras Golod (et al.). Physical Review B. Condensed Matter and Materials Physics 87 (13), 134520

    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.

  • 2013. Vladimir M. Krasnov, Sven-Olof Katterwe, Andreas Rydh. Nature Communications 4

    Understanding the pairing mechanism that gives rise to high-temperature superconductivity is one of the longest-standing problems of condensed-matter physics. Almost three decades after its discovery, even the question of whether or not phonons are involved remains a point of contention to some. Here we describe a technique for determining the spectra of bosons generated during the formation of Cooper pairs on recombination of hot electrons as they tunnel between the layers of a cuprate superconductor. The results obtained indicate that the bosons that mediate pairing decay over micrometre-scale distances and picosecond timescales, implying that they propagate at a speed of around 10(6) m s(-1). This value is more than two orders of magnitude greater than the phonon propagation speed but close to Fermi velocity for electrons, suggesting that the pairing mechanism is mediated by unconventional repulsive electron-electron, rather than attractive electron-phonon, interactions.

  • 2013. Holger Motzkau (et al.). Physica. C, Superconductivity 491, 51-55

    Bi2Sr2CaCu2O8+x single crystals represent natural stacks of atomic scale intrinsic Josephson junctions, formed between metallic CuO2–Ca–CuO2 and ionic insulating SrO–2BiO–SrO layers. Electrostriction effect in the insulating layers leads to excitation of c-axis phonons by the ac-Josephson effect. Here we study experimentally the interplay between and velocity matching (Eck) electromagnetic resonances in the flux-flow state of small mesa structures with c-axis optical phonons. A very strong interaction is reported, which leads to formation of phonon-polaritons with infrared and Raman-active transverse optical phonons. A special focus in this work is made on analysis of the angular dependence of the resonances. We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices in fields up to 17 T, which is essential for achieving high-quality resonances at record high frequencies up to 13 THz.

  • 2012. Stella Tagliati, Vladimir M. Krasnov, Andreas Rydh. Review of Scientific Instruments 83 (5), 055107

    A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-mu g in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1 K. The device has several distinctive features: (i) The resistive thermometer, made of a Ge1-xAux alloy, displays a high dimensionless sensitivity |dlnR/dlnT| greater than or similar to 1 over the entire temperature range. (ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. (iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. (iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 mu g piece of superconducting Pb in various magnetic fields.

  • 2012. Thorsten Jacobs (et al.). Physical Review B 86 (21)

    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.

  • Sven-Olof Katterwe, Vladimir M. Krasnov.

    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.

  • 2012. Holger Motzkau (et al.). Physical Review B 85 (14)

    Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here, we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping, the critical temperature and the superconducting gap decrease; with underdoping, the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even-in-voltage contribution, attributed to a charge transfer effect, and (ii) odd-in-voltage contribution, attributed to reordering of oxygen vacancies.

  • 2012. Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 85 (13), 134525

    In the sine-Gordon equation solitons and antisolitons in the absence of perturbations do not annihilate. Here, I present numerical analysis of soliton-antisoliton collisions in the coupled sine-Gordon equation. It is shown that in such a system, soliton-antisoliton pairs (breathers) do annihilate even in the absence of perturbations. The annihilation occurs via a logarithmic-in-time decay of a breather caused by emission of plasma waves in every period of breather oscillations. This also leads to a significant coupling between breathers and propagating waves, which may lead to self-oscillations at the geometrical resonance conditions in a dc-driven system. The phenomenon may be useful for achieving superradiant emission from coupled oscillators.

  • 2012. Holger Motzkau (et al.). 26th International Conference on Low Temperature Physics (LT26)

    We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+xsingle crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λJ(Jc) and occur at about 0.4 and 1.3 fluxons per λJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

  • 2011. Taras Golod, Andreas Rydh, Vladimir M. Krasnov. Journal of Applied Physics 110 (3), 033909

    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.

  • 2011. Sven-Olof Katterwe (et al.). Physical Review B Condensed Matter 83 (10)

    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.

  • 2011. Vladimir M. Krasnov (et al.). Physical Review B Condensed Matter 84 (5)

    We perform a detailed comparison of magnetotunneling in conventional low-Tc Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2−yPbySr2CaCu2O8+δ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field Hc2. For Nb an extended region of surface superconductivity at Hc2<H<Hc3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that Hc2(0) for Bi(Pb)-2212 is ≃T→Tc T and decreases significantly at T→Tc. The amplitude of subgap magnetoresistance is suppressed exponentially at T>Tc/2, but remains negative, although very small, above Tc. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at Tc.

  • 2011. Sven Olof Katterwe, Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 84 (21), 214519

    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.

  • 2011. Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 83 (17), 174517

    I propose a new mechanism of intense high-frequency electromagnetic wave generation by spatially uniform stacked Josephson junctions at zero magnetic field. The ac-Josephson effect converts the dc-bias voltage into ac supercurrent; however, in the absence of spatial variation of the Josephson phase difference it does not provide dc-to-ac power conversion, needed for emission of electromagnetic waves. Here I demonstrate that at geometrical resonance conditions, spatial homogeneity of the phase can be spontaneously broken by the appearance of breathers (bound fluxon-antifluxon pairs), facilitating effective dc-to-ac power conversion. This leads to self-oscillations at cavity-mode frequencies, accompanied by the emission of radiation. The proposed mechanism explains all major features of recently observed THz radiation from large-area Bi(2)Sr(2)CaCu(2)O(8+x) mesa structures.

  • 2010. Taras Golod, Andreas Rydh, Vladimir M. Krasnov. Physica. C, Superconductivity 570 (19), 890-892

    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.

  • 2010. Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 82 (13), 134524

    I derive simple nonlocal dynamic boundary conditions, suitable for modeling of radiation emission from stacked Josephson junctions in an arbitrary dynamic state. Coherent flux-flow emission from intrinsic Josephson junctions in high-Tc superconductors is analyzed. It is shown that due to the lack of Lorentz contraction of fluxons in stacked junctions, high-quality geometrical resonances are prerequisite for high power flux-flow emission from the stack. This leads to a dual role of the radiative impedance: on one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Optimal conditions are achieved when radiative losses are comparable to resistive losses inside the stack.

  • 2010. Taras Golod, Andreas Rydh, Vladimir Krasnov. Physical Review D. Particles and fields 104, 227003

    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.

  • 2010. Sven-Olof Katterwe, Andreas Rydh, Vladimir M. Krasnov. Journal of Physics, Conference Series 234 (4), 042016

    Resonant phenomena are important for the use of intrinsic Josephson junction as THz-oscillators, due to the decreased linewidth of emitted radiation when biasing the junctions near a resonance. We perform a detailed study of flux-flow characteristics and phonon resonances in small Bi(Pb)2Sr2CaCu2O8+x mesa structures. Magnetic field dependence of flux-flow characteristics up to 17 T and temperature and magnetic field dependence of phonon resonances at temperatures from 2 K to 80 K and in fields up to 15 T are analyzed. A shift of the phonon resonances in the presence of external magnetic fields and an interaction between flux-flow and phonon resonances are observed.

  • 2010. Sven-Olof Katterwe (et al.). Physical Review B. Condensed Matter and Materials Physics 82 (2)

    We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only a few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high-quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the terahertz frequency range.

  • 2009. Andreas Rydh, Taras Golod, Vladimir M. Krasnov. Journal of Physics, Conference Series 153 (1), 012027

    We study the controllable manipulation of vortices in a mesoscopic, superconducting "island" of Nb, using an integrated Josephson junction as a field-sensitive vortex detector. The island, divided by a single Josephson junction and suspended by Nb microbridges, was fabricated from a Nb/P11-xNix/Nb tri-layer using a focused ion beam. We find that the system at select magnetic fields behaves as a vortex memory cell, where current pulses can be used to switch the vortex configuration between metastable states of distinctly different junction critical currents. Non-destructive read-out of a state is then easily done with an intermediate current. Furthermore, we show that the Josephson junction displays a strong magnetoresistive effect at current bias well above the junction critical current but below the onset of flux flow. This enables the junction to be used as a quantitative probe of magnetic field with better than single flux quantum resolution.

  • 2009. Taras Golod, Henrik Frederiksen, Vladimir M. Krasnov. Journal of Physics, Conference Series 150 (5), 052062

    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.

  • 2009. Vladimir Krasnov. Physical Review Letters 103 (22), 227002

    I solve numerically a full set of nonlinear kinetic balance equations for stacked Josephson junctions, which allows analysis of strongly nonequilibrium phenomena. It is shown that nonlinearity becomes significant already at very small disequilibrium. The following new, nonlinear effects are obtained: (i) At even-gap voltages V=2n/e (n=2,3,…) nonequilibrium bosonic bands overlap. This leads to enhanced emission of =2 bosons and to the appearance of dips in tunnel conductance. (ii) A new type of radiative solution is found at strong disequilibrium. It is characterized by the fast stimulated relaxation of quasiparticles. A stack in this state behaves as a light emitting diode and directly converts electric power to boson emission, without utilization of the ac-Josephson effect. The phenomenon can be used for realization of a new type of superconducting cascade laser in the THz frequency range.

  • 2009. Sven-Olof Katterwe, Vladimir Krasnov. Physical Review B Condensed Matter 80 (2), 020502(R)

    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.

  • 2009. Vladimir M. Krasnov. Physical Review B. Condensed Matter and Materials Physics 79, 214510

    Understanding of the puzzling phenomenon of high-temperature superconductivity requires reliable spectroscopic information about temperature dependence of the bulk electronic density of states. Here I present a detailed analysis of the T evolution of bulk electronic spectra in Bi2Sr2CaCu2O8+δ obtained by intrinsic tunneling spectroscopy on small mesa structures. Unambiguous spectroscopic information is obtained by obviation of self-heating problem and by improving the spectroscopic resolution. The obtained data allow accurate determination of the superconducting transition temperature and indicate that (i) the superconducting transition maintains the mean-field character down to moderate underdoping and is associated with a rapid opening of the superconducting gap, which is well described by the conventional BCS T dependence. (ii) The mean-field critical temperature reaches maximum at the optimal doping and decreases with underdoping. Such behavior is inconsistent with theories assuming intimate connection between superconducting and antiferromagnetic spin gaps and supports proposals associating high-temperature superconductivity with the presence of competing ground states and a quantum critical point near optimal doping.

  • 2008. Sven-Olof Katterwe, Andreas Rydh, Vladimir M. Krasnov. Physical Review Letters 101 (8), 087003

    We perform a detailed study of temperature, bias, and doping dependence of interlayer transport in the layered high temperature superconductor Bi2Sr2CaCu2O8+delta. We observe that the shape of interlayer characteristics in underdoped crystals exhibits a remarkable crossover at the superconducting transition temperature: from thermal activation-type above Tc to almost T-independent quantum tunneling-type below Tc. Our data provide insight into the nature of interlayer transport and indicate that its mechanism changes with doping: from the conventional single quasiparticle tunneling in overdoped to a progressively increasing Cooper pair contribution in underdoped crystals.

  • 2007. Vladimir M. Krasnov, Taras Golod, T Bauch and P. Delsing. Physical Review B. Condensed Matter and Materials Physics 76 (22), 224517

    Presented are the results of calculations suggesting that the quasi-one-dimensional organic superconductors (TMTSF)2X (where TMTSF represents tetramethyltetraselenafulvalene and X is PF6 AsF6, ClO4, etc.) may show a substantial increase in their superconducting and spin-density-wave ordering temperatures when the Fermi level is raised through application of an electrostatic gating voltage. A rich behavior is observed, strongly dependent on the form of the superconducting order parameter, as the Fermi level approaches the Van Hove singularity at ka=0. Included are predictions for the behavior of these materials under zero and moderate applied pressure. It is found that TSDW as high as 50 K and superconducting Tc as high as 20 K may be achieved at optimal gate voltages of approximately 100 mV.

Show all publications by Vladimir Krasnov at Stockholm University

Last updated: August 28, 2020

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