arXiv subjects

Vladimir M. Krasnov

Publications and source records attributed to Vladimir M. Krasnov.

At least 19 recordsLinked to original sources

arXiv2026

In-situ tunable superconducting diode: towards field-free operation with infinite nonreciprocity

Efficient, scalable, and magnetic-field-free superconducting diodes are essential for future superconducting electronics; yet, despite significant efforts, such practical devices remain unrealized. The main challenge lies in achieving broad-range in-situ tunability, both for optimization and for achieving transistor-like operation. Here, we study diodes based on four-terminal niobium planar Josephson junctions. We show that the multiterminal structure eliminates the need for an external magnetic field and enables essentially unrestricted in-situ tunability, along with reconfigurability of the diode polarity, leading to new functionality. For example, we demonstrate that such diodes can operate as Gauss neurons via reentrant superconductivity. By deliberately tuning the junction parameters, we obtain effectively infinite nonreciprocity (within experimental resolution) leading to threshold-free ac-current rectification. Such technologically simple, reconfigurable, and broadly tunable diodes could be instrumental for future digital and neuromorphic computing.

physics.app-ph↗

arXiv2025

Planar Josephson junctions for sensors and electronics:Different geometry, new functionality

Josephson junctions are key elements in superconducting electronics. The most common type is the overlap (sandwich-type) junction, formed by vertically stacking two superconducting layers. In contrast, planar junctions are fabricated without overlap, at the edge of two superconducting films within a single plane. This geometric distinction has a significant impact on their physical properties. The planar geometry greatly enhances sensitivity to magnetic fields and improves impedance matching for terahertz (THz) devices. Its two-dimensional structure allows for simple and flexible electronic component design, enabling drastic miniaturization. Here I highlight recent advances in the application of planar junctions for novel technologies, including junction-on-cantilever sensors for super-resolution magnetic imaging, vortex-based memory cells, and programmable superconducting diodes. I will also discuss the general requirements, future perspectives, and key challenges in the evolving field of superconducting electronics.

cond-mat.supr-con↗

arXiv2025

Demonstration of an optical microwave rectification by a superconducting diode with near 100% efficiency

Superconducting electronics offer significant advantages in speed and power efficiency for next-generation computing and communication systems. However, their practical deployment is limited by the absence of simple, efficient, and scalable superconducting counterparts to key semiconductor components. In this work, we investigate diodes based on planar Josephson junctions fabricated from a conventional niobium superconductor. The nonreciprocity in these diodes arises from the self-field effect induced by the geometrical asymmetry of the junction. By deliberate tuning of the junction parameters, we achieved effectively infinite nonreciprocity (within experimental resolution), characterized by a complete suppression of the superconducting critical current in one direction while maintaining a significant current in the opposite direction. The key novelty of this work lies in the demonstration of the optical diode effect. We observed threshold-free rectification of 75 GHz microwave radiation, indicating that these diodes exhibit near-ideal optical nonreciprocity. Our results open new avenues for ultrafast superconducting electronics and lay the groundwork for wireless sub-THz signal processing.

cond-mat.supr-con↗

arXiv2025

Resonant switching current detector based on underdamped Josephson junctions

Current-biased Josephson junctions can act as detectors of electromagnetic radiation. At optimal conditions, their sensitivity is limited by fluctuations causing stochastic switching from the superconducting to the resistive state. This work provides a quantitative description of a stochastic switching current detector, based on an underdamped Josephson junction. It is shown that activation of a Josephson plasma resonance can greatly enhance the detector responsivity in proportion to the quality factor of the junction. The ways of tuning the detector for achieving optimal operation are discussed. For realistic parameters of Nb/AlOx/Nb tunnel junctions, the sensitivity can reach $S\simeq 5\times10^{12}$ (V/W).

cond-mat.supr-con↗

arXiv2025

Cascade switching current detectors based on arrays of Josephson junctions

Cascade multiplication is a common technique to enhance the sensitivity of photon detectors. In this study, we demonstrate novel cascade-amplified superconducting detectors utilizing arrays of Josephson junctions. The mutual coupling between junctions induces avalanche-like switching of multiple junctions upon photon absorption, leading to cascade amplification of the readout voltage. We present two prototypes featuring either low-Tc linear Nb/NbxSi1-x/Nb arrays, or high-Tc stacked intrinsic Josephson junctions. Both devices exhibit clear antenna effects in microwave directivities, indicating good impedance matching and absorption efficiency. The combination of high absorption efficiency and large cascade amplification has the potential to produce broadband THz sensors with sensitivity exceeding 10^13 V/W.

cond-mat.supr-con↗

arXiv2024

Numerical modelling of a vortex-based superconducting memory cell: dynamics and geometrical optimization of a fluxonic quantum dot

The lack of dense random-access memory is one of the main obstacles to the development of digital superconducting computers. It has been suggested that AVRAM cells, based on the storage of a single Abrikosov vortex, the smallest quantized object in superconductors, can enable drastic miniaturization to the nanometer scale. In this work, we present numerical modeling of such cells using time-dependent Ginzburg-Landau equations. The cell represents a fluxonic quantum dot containing a small superconducting island, an asymmetric notch for vortex entrance, a guiding track, and a vortex trap. We determine the optimal geometrical parameters for operation at zero magnetic field and the conditions for controllable vortex manipulation by short current pulses. We report ultra-fast vortex motion with velocities more than an order of magnitude faster than those expected for macroscopic superconductors. This phenomenon is attributed to strong interactions with the edges of a mesoscopic island, combined with the nonlinear reduction of flux-flow viscosity due to nonequilibrium effects in the track. Our results show that such cells can be scaled down to sizes comparable to the London penetration depth, 100 nm, and can enable ultrafast switching on the picosecond scale with ultra-low energy per operation, 0.1 aJ.

physics.app-ph↗

arXiv2023

Word and bit line operation of a 1x1 μm2 superconducting vortex-based memory

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 1x1 μm2 cell. High-endurance, nonvolatile 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 multibit RAM. The achieved one square micron area is an important milestone and a significant step forward towards creation of a dense cryogenic memory.

cond-mat.supr-con↗

arXiv2022

Holographic reconstruction of magnetic field distribution in a Josephson junction from diffraction-like Ic(H) patterns

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 the trade-off requires development of super-resolution imaging technique, not limited by the sensor size. Here we present a proof of concept for a super-resolution method of magnetic imaging by a Josephson junction. 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 diffraction-like 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 Josephson junctions. Super-resolution reconstruction of stray magnetic fields from an Abrikosov vortex, trapped in the junction electrodes, is demonstrated. Thus, our method facilitate both the 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 Josephson junction can be used in scanning probe microscopy.

cond-mat.supr-con↗

arXiv2022

How to write a contemporary scientific article

Today scientists are drowned in information and have no time for reading all publications even in a specific area. The information is sifted and only a small fraction of articles is being read. Under circumstances, scientific articles have to be properly adjusted to pass through the superficial sifting. Here I present written down instructions that I used to give to my students with almost serious advises on how to write (and how not to write) a contemporary scientific article. I argue that it should tell a story and should answer on the three main questions: Why, What and So what?

physics.ed-ph↗

arXiv2021

Design aspects of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ THz sources: optimization of thermal and radiative properties

Impedance matching and heat management are important factors influencing performance of THz sources. In this work we analyze thermal and radiative properties of such devices based on mesa structures of a layered high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. Two types of devices are considered, containing either a conventional large single crystal, or a whisker. We perform numerical simulations for various geometrical configurations and parameters and make a comparison with experimental data for the two types of devices. It is demonstrated that the structure and the geometry of both the superconductor and the electrodes are playing important roles. In crystal-based devices an overlap between the crystal and the electrode leads to appearance of a large parasitic capacitance, which shunts THz emission and prevents impedance matching with open space. The overlap is avoided in whisker-based devices. Furthermore, the whisker and the electrodes form a turnstile (crossed-dipole) antenna facilitating good impedance matching. This leads to more than an order of magnitude enhancement of the radiation power efficiency in whisker-based, compared to crystal-based devices. These results are in good agreement with presented experimental data.

cond-mat.supr-con↗

arXiv2021

Accurate determination of the Josephson critical current by lock-in measurements

Operation of Josephson electronics usually requires determination of the Josephson critical current $I_c$, 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 $I_c$ can be accurately extracted using first and third harmonic lock-in measurements of junction resistance. We derive analytic expressions and verify them experimentally on nano-scale Nb-PtNi-Nb and Nb-CuNi-Nb Josephson junctions.

cond-mat.supr-con↗

arXiv2021

Transport characterization of magnetic states in Superconductor/Ferromagnet Nb/Co multilayers

Employment of the non-trivial proximity effect in Superconductor/Ferromagnet (S/F) heterostructures for creation of novel superconducting devices requires an accurate control of magnetic states in complex thin-film multilayers composing such devices. In this work we study experimentally in-plane transport properties of micro-structured Nb/Co multilayers. We apply various experimental techniques for characterization of multilayers, including the anisotropic magnetoresistance, the Hall effect and the first-order-reversal-curves analysis. We demonstrate that a combination of those techniques can provide a detailed knowledge of the magnetic state of the multilayer. In particular, we identify the range of existence of the coherently rotating, monodomain scissor-like state. It is anticipated, that in this noncollinear magnetic state the unconventional odd-frequency spin-triplet order parameter should appear. The non-hystertic nature of this state allows reversible tuning of the magnetic orientation. Thus, we identify the range of parameters and the procedure for controllable operation of devices based on such S/F heterostructures.

cond-mat.supr-con↗

arXiv2014

Controllable generation of a spin-triplet supercurrent in a Josephson spin-valve

It has been predicted theoretically that an unconventional odd-frequency spin-triplet component of superconducting order parameter can be induced in multilayered ferromagnetic structures with non-collinear magnetization. In this work we study experimentally nano-scale 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 magnetization of the two ferromagnetic layers and is enhanced in the non-collinear state of the spin valve. This provides a direct prove of controllable generation of the spin-triplet superconducting component in a ferromagnet.

cond-mat.supr-con↗

arXiv2012

Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure

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 Tesla, which is essential for achieving high-quality resonances at record high frequencies up to 13 THz.

cond-mat.supr-con↗

arXiv2012

The anti-ordinary Hall effect in NiPt thin films

We study the anomalous Hall effect in binary alloys between the group-10 elements Ni and Pt. It is observed that the ordinary Hall effect is negative (electron-like) at any composition of the alloy. The extraordinary Hall effect is also negative except in the vicinity of the ferromagnetic quantum critical point. Close to the critical point the sign of the extraordinary Hall effect can be changed to positive (hole-like) by tuning either the temperature or the composition of the alloy. We attribute such an "anti-ordinary" Hall effect with opposite signs of the ordinary and the extraordinary contributions to a Berry phase singularity, moving away from the Fermi energy with increasing the ferromagnetic exchange energy.

cond-mat.mtrl-sci↗

arXiv2012

Radiative annihilation of a soliton and an antisoliton in the coupled sine-Gordon equation

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.

cond-mat.other↗

arXiv2012

Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures

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 impurities.

cond-mat.supr-con↗

arXiv2011

Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

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+x single 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 lambdaJ(Jc) and occur at about 0.4 and 1.3 fluxons per lambdaJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

cond-mat.supr-con↗