arXiv subjects

M. Nashaat

Publications and source records attributed to M. Nashaat.

16 recordsLinked to original sources

arXiv2025

Originality of resonance and locking phenomena in SFS $\varphi_0$ Josephson junction

We demonstrate the realization and interplay of two ferromagnetic resonances in one SFS $\varphi_0$ Josephson junction. First resonance that is realized under microwave radiation is the famous Kittel resonance. The other is Buzdin one appearing as a result of interaction of superconducting current and ferromagnetic interlayer magnetization. Transformations of one type of resonance to another under variation of external electromagnetic radiation and the $\varphi_0$ junction parameters open an interesting way to manipulation both of them. The combined ferromagnetic resonance that exhibits the features of both resonances is demonstrated too. The coupling of the Josephson phase with the magnetization of the ferromagnetic layer, caused by the spin-orbit interaction, leads to double synchronization, namely, synchronization of both magnetic precession and Josephson oscillations by external radiation. The obtained results demonstrate reach physics and unique opportunities for various applications.

cond-mat.supr-con↗

arXiv2025

The uniqueness of the driven $φ_0$ Josephson junction: when steps are not Shapiro

The $φ_0$ superconductor-ferromagnet-superconductor Josephson junction exhibits unique locking phenomena under the external periodic signal when the magnetic component is taken into account. Contrary to the well-known Shapiro steps that come from the locking with the electric component, locking of the Josephson oscillations with the magnetic one results in the appearance of Buzdin steps in the current-voltage characteristic and a much more complex response of the system. These steps possess distinctive properties that are indications of their unique origins and locking mechanisms. The width of the Buzdin step oscillates with the amplitude of the magnetic component, nevertheless, it exhibits anomalies in the Bessel-like behavior. In addition, we perform an analytical analysis that supports the numerical results and shows that the width of the Buzdin step represents a product of two Bessel functions. Investigation of the effects that simultaneously appear in the magnetic subsystem reveals the presence of destructive interference and magnetization reorientation that accompany the appearance of Buzdin steps.

cond-mat.supr-con↗

arXiv2025

Intrinsic space-time crystalline order in a hybrid Josephson junction

We demonstrate the emergence of an intrinsic space-time crystalline order in a long ferromagnetic $\varphi_{0}$ Josephson junction on a topological insulator without any external periodic modulation. The presence of the exchange and Dzyaloshinskii-Moriya interactions in a ferromagnetic layer with broken inversion symmetry internally modulates the critical current due to the coupling between the magnetic moment and the Josephson phase. This breaks the time translation symmetry, leading to the appearance of the space-time crystalline pattern in the spatiotemporal dependence of the in-plane current, which oscillates with almost twice the ferromagnetic resonance frequency. In the limit where the critical current is not modulated internally, the space-time crystalline order does not occur. In this case, only when an external parametric modulation is applied, the system exhibits a typical classical discrete space-time crystalline order that oscillates at half of the modulation frequency. Considering the still-pending problem of experimental detection, we demonstrate that a recently developed magnetometry device, which visualizes the supercurrent flow at the nanoscale, can be used to detect space-time crystalline patterns in hybrid Josephson junctions experimentally.

cond-mat.supr-con↗

arXiv2024

Buzdin, Shapiro and Chimera Steps in $φ_0$ Josephson Junctions. II. Bifurcation, Switching, and Hysteresis

The dynamics of magnetization and current-voltage characteristics of the superconductor-ferromagnet-superconductor $φ_0$ Josephson junction in the presence of external electromagnetic radiation have been studied. Effects of radiation magnetic component are investigated in the frameworks of one- and two-signal models. The implementation of two types of dynamical states of magnetization is demonstrated. These states have a phase shift of $π$ in the synchronization region of magnetic precession and Josephson oscillations and differ in the nature of their time dependence. Transitions between these states with increasing and decreasing bias current show hysteresis, which is reflected in the bifurcation diagram and the current-voltage characteristics. We also provide an experimental way to test the obtained results by measuring the phase shift in voltage temporal dependence at fixed current value for both sweeping directions. The results obtained can find application in various fields of superconducting spintronics and quantum computing.

cond-mat.supr-con↗

arXiv2023

Thermal local quantum uncertainty in a two-qubit-superconducting system under decoherence

By considering the local quantum uncertainty (LQU) as a measure of quantum correlations, the thermal evolution of a two-qubit-superconducting system is investigated. We show that the thermal LQU can be increased by manipulating the Hamiltonian parameters such as the mutual coupling and Josephson energies, however, it undergoes sudden transitions at specific temperatures. Furthermore, a detailed analysis is presented regarding the impact of decohering channels on thermal LQU. This controllable LQU in engineering applications can disclose the advantage enabled in the superconducting charge qubits for designing quantum computers and quantum batteries.

quant-ph↗

arXiv2023

Buzdin, Shapiro and Chimera Steps in $φ_0$ Josephson Junctions

The unique resonance and locking phenomena in the superconductor-ferromagnet-superconductor $φ_0$ Josephson junction under external electromagnetic radiation are demonstrated when not just the electric but also the magnetic component of external radiation is taken into account. Due to the coupling of superconductivity and magnetism in this system, the magnetic moment precession of the ferromagnetic layer caused by the magnetic component of external radiation can lock the Josephson oscillations, which results in the appearance of a particular type of steps in the current-voltage characteristics, completely different from the well-known Shapiro steps. We call these steps the Buzdin steps in the case when the system is driven only by the magnetic component and the Chimera steps in the case when both magnetic and electric components are present. Unlike the Shapiro steps where the magnetization remains constant along the step, here it changes though the system is locked. The spin-orbit coupling substantially contributes to the amplitude, i.e., the size of these steps. Dramatic changes in their amplitudes are also observed at frequencies near the ferromagnetic resonance. Combinations of the Josephson and Kittel ferromagnetic resonances together with different types of locking pronounced in dynamics and current-voltage characteristics make the physics of this system very interesting and open up a series of new applications.

cond-mat.supr-con↗

arXiv2023

Resonance phenomena in a nanomagnet coupled to a Josephson junction under external periodic drive

We investigate resonance phenomena in a system consisting of a nanomagnet coupled to a Josephson junction under external periodic drive. The coupling in the system leads to appearance of additional resonance peaks whose properties depend on the periodic signal and Josephson junction dynamics. In the linear regime, we derive an analytical description of the resonance phenomena which is then confirmed by numerical simulations. This analytical method is universal and can be also applied to Josephson junctions with anomalous phase shift in current phase relation. This work provides a new method of controlling the resonances of hybrid structures, which may be interesting for applications.

cond-mat.supr-con↗

arXiv2022

Kapitsa pendulum effects in Josephson junction + nanomagnet under external periodic drive

We investigate reorientation effects under external periodic drive in the nanomagnet dynamics coupled to a Josephson junction. The Kapitsa pendulum is introduced as a mechanical analog to this system and we demonstrate the reorientation of the easy axis of the nanomagnet. The magnetic field generated by the Josephson junction and external drive plays the role of the oscillating force of the suspension point in the Kapitsa pendulum. The high frequency oscillations change the orientation of the magnetic moment. The magnetic field of the quasiparticle current determines the frequency dependence of the magnetic moment's orientation. We obtain simple analytical formulas for the stable position of the magnetic moment, both under the external periodic drive and without it. The influence of external periodic drive on the voltage of complete reorientation have been demonstrated.

cond-mat.other↗

arXiv2022

Locking of magnetization and Josephson oscillations at ferromagnetic resonance in $φ_0$ junction under external radiation

We demonstrate the locking by external electromagnetic radiation of magnetic precession in the $ φ_0 $ Josephson junction through Josephson oscillations in the region of ferromagnetic resonance. This leads to a step in the dependence of the magnetization on the bias current, the position of which is determined by the radiation frequency and the shape of the resonance curve. In junctions with a strong spin-orbit coupling, states with negative differential resistance appear on the IV-characteristic, resulting in an additional locking step. A detailed study of the time dependence of voltage and magnetization and their Fourier transforms shows that the corresponding oscillations have the same frequency as the oscillations at the first step, but they have a different amplitude and different dependence on the radiation frequency. This makes it possible to control not only the frequency, but also the amplitude of the magnetic precession in the locking region. It opens up unique perspectives for the control and manipulation of magnetic moment in hybrid systems such as anomalous Josephson junctions.

cond-mat.supr-con↗

arXiv2021

Bifurcation structure and chaos in nanomagnet coupled to Josephson junction

Recently an irregular easy axis reorientation demonstrating the Kapitza pendulum features were observed in numerical simulations of nanomagnet coupled to the Josephson junction. To explain its origin we study the magnetization bifurcations and chaos which appear in this system due to interplay of superconductivity and magnetism. The bifurcation structure of the magnetization under the variation of Josephson to magnetic energy ratio as a control parameter demonstrates several precessional motions. They are related to chaotic behavior, bistability, and multiperiodic orbits in the ferromagnetic resonance region. Effect of external periodic drive on the bifurcation structure is investigated. The results demonstrate high-frequency modes of periodic motion and chaotic response near resonance. Far from the ferromagnetic resonance we observe a quasiperiodic behavior.

cond-mat.supr-con↗

arXiv2019

Microwave induced tunable subharmonic steps in superconductor-ferromagnet-superconductor Josephson junction

We investigate the coupling between ferromagnet and superconducting phase dynamics in superconductor-ferromagnet-superconductor Josephson junction. The current-voltage characteristics of the junction demonstrate a pattern of subharmonic current steps which forms a devil's staircase structure. We show that a width of the steps becomes maximal at ferromagnetic resonance. Moreover, we demonstrate that the structure of the steps and their widths can be tuned by changing the frequency of the external magnetic field, ratio of Josephson to magnetic energy, Gilbert damping and the junction size.

cond-mat.supr-con↗

arXiv2019

Electrical control of magnetization in S/F/S junctions on a 3D topological insulator

Strong dependence of the Josephson energy on the magnetization orientation in Josephson junctions with ferromagnetic interlayers and spin-orbit coupling opens a way to control magnetization by Josephson current or Josephson phase. Here we investigate the perspectives of magnetization control in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions on the surface of a 3D topological insulator hosting Dirac quasiparticles. Due to the spin-momentum locking of these Dirac quasiparticles a strong dependence of the Josephson current-phase relation on the magnetization orientation is realized. It is demonstrated that this can lead to splitting of the ferromagnet's easy-axis in the voltage driven regime. We show that such a splitting can lead to stabilization of an unconventional four-fold degenerate ferromagnetic state.

cond-mat.supr-con↗

arXiv2019

Devil's staircase structures in $φ_{0}$ junction

The superconductor-ferromagnet-superconductor $φ_{0}$ junction provides a direct coupling between Josephson phase and magnetic moment of ferromagnetic barrier. We demonstrate an appearance of additional fractional subharmonic steps in the IV-characteristics of $φ_{0}$ junction under external electromagnetic radiation due to spin-orbit coupling. An origin of subharmonic steps is related to the locking of magnetic moment precession to the Josephson oscillations. We prove that the positions of those steps follow a continued fraction algorithm.

cond-mat.supr-con↗

arXiv2019

Stationary Charge Imbalance Effect in System of Coupled Josephson Junction

We investigate stationary charge imbalance effect in the system of coupled overdamped Josephson junctions. We show that coupling between junction and nonzero stationary charge imbalance in the resistive state bring to a decrease of the Josephson frequency in the Josephson junctions of the stack. The formed difference in Josephson frequency leads to the nonuniform switch to the Shapiro step regime in the presence of external electromagnetic radiation and appearance of kinks of voltage on the IV-characteristics of the stack. We also show that stationary charge imbalance brings to the slope of the Shapiro step due to the difference of the charge imbalance potential on the edges of the step. The theoretical and experimental results for voltage bias coupled Josephson junctions have been compared with the current bias case.

cond-mat.supr-con↗

arXiv2018

Devil's Staircases in SFS Josephson Junctions

We study the effect of coupling between the superconducting current and magnetization in the superconductor/ferromagnet/superconductor Josephson junction under an applied circularly polarized magnetic field. Manifestation of ferromagnetic resonance in the frequency dependence of the amplitude of the magnetization and the average critical current density is demonstrated numerically. The IV-characteristics show subharmonic steps that form devil's staircases, following a continued fraction algorithm. The origin of the found steps is related to the effect of the magnetization dynamics on the phase difference in the Josephson junction. The dynamics of our system is described by a generalized RCSJ model coupled to the Landau-Lifshitz-Gilbert equation. In the suplement we justify analytically the appearance of the fractional steps in IV-characteristics of the superconductor/ferromagnet/superconductor Josephson junction.

cond-mat.supr-con↗

arXiv2016

Shapiro Step at Nonequilibrium Conditions

Detailed numerical simulations of intrinsic Josephson junctions of high temperature superconductors under external electromagnetic radiation are performed taking into account a charge imbalance effect. We demonstrate that the charge imbalance is responsible for a slope in the Shapiro step the value of which increases with a nonequilibrium parameter. Coupling between junctions leads to the distribution of the slope's values along the stack. The nonperiodic boundary conditions shift the Shapiro step in IV-characteristics from the canonical position determined by $V_{ss}=\hbar f /(2e)$, where $f$ is a frequency of external radiation. This fact makes ambiguous the interpretation of the experimentally found Shapiro step shift by the charge imbalance effect.

cond-mat.supr-con↗