arXiv subjects

N. V. Klenov

Publications and source records attributed to N. V. Klenov.

At least 19 recordsLinked to original sources

arXiv2026

Magnetically memorable inductance in superconducting multilayer resonators

Superconductor-ferromagnet hybrid structures with tunable kinetic inductance are promising elements for neuromorphic and quantum computing circuits. We report the fabrication and microwave characterization of split-ring resonators based on Nb/Co/Nb/Co/Nb/Al spin-trigger multilayers and demonstrate a non-volatile spin-valve effect on their resonant properties. Reversal of the relative magnetization orientation of the cobalt layers produces a reproducible shift of the resonant frequency up to 4 MHz at zero applied magnetic field, corresponding to a change in the kinetic inductance of the structure. The incorporation of a proximitized aluminum overlayer is shown to enhance the inductance contrast between the parallel and antiparallel magnetic states by a factor of approximately three relative to structures without this layer. The experimental results are in quantitative agreement with a microscopic model based on the Usadel equations. The demonstrated magnetic memory of the resonant frequency at zero field establishes spin-trigger multilayers as viable field-programmable inductive elements for superconducting digital and neuromorphic circuits.

cond-mat.supr-con↗

arXiv2024

Thouless energy in Josephson SN-N-NS bridges

We have studied the Thouless energy in Josephson superconductor-normal metal-superconductor (SN-N-NS) bridges analytically and numerically, taking into account the influence of the sub-electrode regions. We have found a significant suppression of the Thouless energy with increasing interfacial resistance, in agreement with experimental results. The analysis of the temperature dependence of the critical current in Josephson junctions in comparison with the expressions for the Thouless energy may allow the determination of the interface parameters of S and N-layers.

cond-mat.supr-con↗

arXiv2024

Optimisation challenge for superconducting adiabatic neural network implementing XOR and OR boolean functions

In this article, we consider designs of simple analog artificial neural networks based on adiabatic Josephson cells with a sigmoid activation function. A new approach based on the gradient descent method is developed to adjust the circuit parameters, allowing efficient signal transmission between the network layers. The proposed solution is demonstrated on the example of the system implementing XOR and OR logical operations.

cond-mat.supr-con↗

arXiv2021

Miniaturization of Josephson junction for digital superconducting circuits

In this work, we briefly overview various options for Josephson junctions which should be scalable down to nanometer range for utilization in nanoscale digital superconducting technology. Such junctions should possess high values of critical current, $I_c$, and normal state resistance, $R_n$. Another requirement is the high reproducibility of the junction parameters across a wafer in a fabrication process. We argue that Superconductor - Normal metal - Superconductor (SN-N-NS) Josephson junction of "variable thickness bridge" geometry is a promising choice to meet these requirements. Theoretical analysis of SN-N-NS junction is performed in the case where the distance between the S-electrodes is comparable to the coherence length of the N-material. The restriction on the junction geometrical parameters providing the existence of superconductivity in the S-electrodes is derived for the current flowing through the junction of an order of $I_c$. The junction heating, as well as available mechanisms for the heat removal, is analyzed. The obtained results show that an SN-N-NS junction with a high (sub-millivolt) value of $I_cR_n$ product can be fabricated from a broadly utilized combination of materials like Nb/Cu using well-established technological processes. The junction area can be scaled down to that of semiconductor transistors fabricated in the frame of a 40-nm process.

cond-mat.supr-con↗

arXiv2021

Superconducting circuits without inductors based on bistable Josephson junctions

Magnetic flux quantization in superconductors allows the implementation of fast and energy-efficient digital superconducting circuits. However, the information representation in magnetic flux severely limits their functional density presenting a long-standing problem. Here we introduce a concept of superconducting digital circuits that do not utilize magnetic flux and have no inductors. We argue that neither the use of geometrical nor kinetic inductance is promising for the deep scaling of superconducting circuits. The key idea of our approach is the utilization of bistable Josephson junctions allowing the representation of information in their Josephson energy. Since the proposed circuits are composed of Josephson junctions only, they can be called all-Josephson junction (all-JJ) circuits. We present a methodology for the design of the circuits consisting of conventional and bistable junctions. We analyze the principles of the circuit functioning, ranging from simple logic cells and ending with an 8-bit parallel adder. The utilization of bistable junctions in the all-JJ circuits is promising in the aspects of simplification of schematics and the decrease of the JJ count leading to space-efficiency.

cond-mat.supr-con↗

arXiv2020

Picosecond operations on superconducting quantum register based on Ramsey patterns

An ultrafast qubit control concept is proposed to reduce the duration of operations with a single and multiple superconducting qubits. It is based on the generation of Ramsey fringes due to unipolar picosecond control pulses. The key role in the concept is played by the interference of waves of qubit states population propagating forward and backward in time. The influence of the shape and duration of control pulses on the contrast of the interference pattern is revealed in the frame of Ramsey's paradigm. Protocols for observation of Ramsey oscillations and implementation of various gate operations are developed. We also suggest a notional engineering solution for creating the required picosecond control pulses with desired shape and amplitude. It is demonstrated that this makes it possible to control the quantum states of the system with the fidelity of more than 99%.

quant-ph↗

arXiv2020

Density of states and current-voltage characteristics in SIsFS junctions

We study the density of states (DOS) inside superconducting Josephson SIsFS junctions with complex interlayer consisting of a thin superconducting spacer 's' between insulator I and a ferromagnetic metal F. The consideration is focused on the local density of states in the vicinity of a tunnel barrier, and it permits to estimate the current-voltage characteristics in the resistive state of such junctions. We study the influence of the proximity effect and Zeeman splitting on the properties of the system, and we find significant sub-gap regions with non-vanishing DOS. We also find manifestations of the 0-$π$ transition in the behavior of DOS in a thin s-layer. These properties lead to appearance of new characteristic features on I-V curves which provide additional information about electronic states inside the junction.

cond-mat.supr-con↗

arXiv2019

A linear magnetic flux-to-voltage transfer function of differential DC SQUID

A superconducting quantum interference device with differential output or "DSQUID" was proposed earlier for operation in the presence of large common-mode signals. The DSQUID is the differential connection of two identical SQUIDs. Here we show that besides suppression of electromagnetic interference this device provides effective linearization of DC SQUID voltage response. In the frame of the resistive shunted junction model with zero capacitance, we demonstrate that Spur-Free Dynamic Range (SFDR) of DSQUID magnetic flux-to-voltage transfer function is higher than SFDR > 100 dB while Total Harmonic Distortion (THD) of a signal is less than THD < $10^{-3}\%$ with a peak-to-peak amplitude of a signal being a quarter of half flux quantum, $2Φ_a = Φ_0/8$. Analysis of DSQUID voltage response stability to a variation of the circuit parameters shows that DSQUID implementation allows doing highly linear magnetic flux-to-voltage transformation at the cost of a high identity of Josephson junctions and high-precision current supply.

cond-mat.supr-con↗

arXiv2019

Dynamic properties of asymmetric double Josephson junction stack with quasiparticle imbalance

We study analytically and numerically the influence of the quasiparticle charge imbalance on the dynamics of the asymmetric Josephson stack formed by two inequivalent junctions: the fast capacitive junction $JJ_{1}$ and slow non-capacitive junction $JJ_{2}$. We find, that the switching of the fast junction into resistive state leads to significant increase of the effective critical current of the slow junction. At the same time, the initial switching of the slow junction may either increase or decrease the effective critical current of the fast junction, depending on ratio of their resistances and the value of the capacitance. Finally, we have found that the slow quasiparticle relaxation (in comparison with Josephson times) leads to appearance of the additional hysteresis on current-voltage characteristics.

cond-mat.supr-con↗

arXiv2018

Periodic Co/Nb pseudo spin-valve for cryogenic memory

We present a new study of magnetic structures with controllable effective exchange energy for Josephson switches and memory. As a basis for a weak link we propose to use a periodic structure comprised of ferromagnetic (F) layers spaced by thin superconductors (s). Our calculations based on Usadel equations show that switching from parallel (P) to antiparallel (AP) alignment of neighboring F layers can lead to a significant enhancement of the critical current through the junction. To control magnetic alignment we propose to use periodic system where unit cell is a pseudo spin-valve $F_1$/s/$F_2$/s with $F_1$ and $F_2$ two magnetic layers having different coercive fields. In order to check feasibility of controllable switching between AP and P states through the \emph{whole} periodic structure we prepared a superlattice [Co(1.5nm)/Nb(8nm)/Co(2.5nm)/Nb(8nm)]$_6$ between two superconducting layers of Nb(25nm). Neutron scattering showed that parallel and antiparallel alignment can be organized by using of magnetic fields of only several tens of Oersted.

cond-mat.supr-con↗

arXiv2018

Protected 0-pi states in SIsFS junctions for Josephson memory and logic

We study the peculiarities in current-phase relations (CPR) of the SIsFS junction in the region of $0$ to $π$ transition. These CPR consist of two independent branches corresponding to $0-$ and $π-$ states of the contact. We have found that depending on the transparency of the SIs tunnel barrier the decrease of the s-layer thickness leads to transformation of the CPR shape going in the two possible ways: either one of the branches exists only in discrete intervals of the phase difference $φ$ or both branches are sinusoidal but differ in the magnitude of their critical currents. We demonstrate that the difference can be as large as $10\%$ under maintaining superconductivity in the s layer. An applicability of these phenomena for memory and logic application is discussed.

cond-mat.supr-con↗

arXiv2018

Adiabatic Superconducting Artificial Neural Network: Basic Cells

We consider adiabatic superconducting cells operating as an artificial neuron and synapse of a multilayer perceptron (MLP). Their compact circuits contain just one and two Josephson junctions, respectively. While the signal is represented as magnetic flux, the proposed cells are inherently nonlinear and close-to-linear magnetic flux transformers. The neuron is capable of providing a one-shot calculation of sigmoid and hyperbolic tangent activation functions most commonly used in MLP. The synapse features by both positive and negative signal transfer coefficients in the range ~ (-0.5,0.5). We briefly discuss implementation issues and further steps toward multilayer adiabatic superconducting artificial neural network which promises to be a compact and the most energy-efficient implementation of MLP.

cond-mat.supr-con↗

arXiv2017

Beyond Moore's technologies: operation principles of a superconductor alternative

The predictions of Moore's law are considered by experts to be valid until 2020 giving rise to "post-Moore's" technologies afterwards. Energy efficiency is one of the major challenges in high-performance computing that should be answered. Superconductor digital technology is a promising post-Moore's alternative for the development of supercomputers. In this paper, we consider operation principles of an energy-efficient superconductor logic and memory circuits with a short retrospective review of their evolution. We analyze their shortcomings in respect to computer circuits design. Possible ways of further research are outlined.

cond-mat.supr-con↗

arXiv2017

Current-phase relations in SIsFS junctions in the vicinity of 0-$π$ transition

We consider the current-phase relation (CPR) in the Josephson junctions with complex insulator-superconductor-ferromagnetic interlayers in the vicinity of 0-$π$ transition. We find a strong impact of the second harmonic on CPR of the junctions. It is shown that the critical current can be kept constant in the region of 0-pi transition, while the CPR transforms through multi-valued hysteretic states depending on the relative values of tunnel transparency and magnetic thickness. Moreover, CPR in the transition region has multiple branches with distinct ground states.

cond-mat.supr-con↗

arXiv2017

Observability of surface currents in p-wave superconductors

A general approach is formulated to describe spontaneous surface current distribution in a chiral p-wave superconductor. We use the quasiclassical Eilenberger formalism in the Ricatti parametrization to describe various types of the superconductor surface, including arbitrary roughness and metallic behaviour of the surface layer. We calculate angle resolved distributions of the spontaneous surface currents and formulate the conditions of their observability. We argue that local measurements of these currents by muSR technique may provide an information on the underlying pairing symmetry in the bulk superconductor.

cond-mat.supr-con↗

arXiv2016

Analytical derivation of DC SQUID response

We consider voltage and current responses formation in DC SQUID with overdamped Josephson junctions in resistive and superconducting state in the frame of resistively shunted junction (RSJ) model. For simplicity we neglect the junction capacitance and the noise effect. Explicit expressions for the responses in resistive state were obtained for a SQUID which is symmetrical with respect to bias current injection point. Normalized SQUID inductance $l = 2 e I_c L/\hbar$ (where $I_c$ is the critical current of Josephson junction, $L$ is the SQUID inductance, $e$ is the electron charge and $\hbar$ is the Planck constant) was assumed to be within the range $l \leq 1$, subsequently expanded up to $l \approx 7$ using two fitting parameters. SQUID current response in superconducting state was considered for arbitrary value of the inductance. Impact of small technological spread of parameters relevant for low-temperature superconductor (LTS) technology was studied with generalization of the developed analytical approach for a case of small difference of critical currents and shunt resistances of the Josephson junctions, and inequality of SQUID inductive shoulders for both resistive and superconducting states. Comparison with numerical calculation results shows that developed analytical expressions can be used in practical LTS SQUIDs and SQUID-based circuits design, e.g. large serial SQIF, drastically decreasing the time of simulation.

cond-mat.supr-con↗

arXiv2016

Superconducting Phase Domains for Memory Applications

In this work we study theoretically the properties of S-F/N-sIS type Josephson junctions in the frame of the quasiclassical Usadel formalism. The structure consists of two superconducting electrodes (S), a tunnel barrier (I), a combined normal metal/ferromagnet (N/F) interlayer and a thin superconducting film (s). We demonstrate the breakdown of a spatial uniformity of the superconducting order in the s-film and its decomposition into domains with a phase shift $π$ . The effect is sensitive to the thickness of the s layer and the widths of the F and N films in the direction along the sIS interface. We predict the existence of a regime where the structure has two energy minima and can be switched between them by an electric current injected laterally into the structure. The state of the system can be non-destructively read by an electric current flowing across the junction.

cond-mat.supr-con↗

arXiv2015

Josephson effect in SIFS-tunnel junctions with domain walls in weak link region

We study theoretically the properties of SIFS type Josephson junctions composed of two superconducting (S) electrodes separated by an insulating layer (I) and a ferromagnetic (F) film consisting of periodic magnetic domains structure with antiparallel magnetization directions in neighboring domains. The two-dimensional problem in the weak link area is solved analytically in the framework of the linearized quasiclassical Usadel equations. Based on this solution, the spatial distributions of the critical current density, $J_{C},$ in the domains and critical current, $I_{C},$ of SIFS structures are calculated as a function of domain wall parameters, as well as the thickness, $d_{F},$ and the width, $W,$ of the domains. We demonstrate that $I_{C}(d_{F},W)$ dependencies exhibit damped oscillations with the ratio of the decay length, $ξ_{1},$ and oscillation period, $ξ_{2},$ being a function of the parameters of the domains, and this ratio may take any value from zero to unity. Thus, we propose a new physical mechanism that may explain the essential difference between $ξ_{1}$ and $ξ_{2}$ observed experimentally in various types of SFS Josephson junctions.

cond-mat.supr-con↗