arXiv subjects

Ya. V. Fominov

Publications and source records attributed to Ya. V. Fominov.

At least 19 recordsLinked to original sources

arXiv2025

Superconducting spin valve effect in Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe heterostructures

The structures of the superconducting spin valve (SSV) Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe (where Si$_3$N$_4$ is a dielectric insulating layer of controlled thickness) were investigated. The dependence of the magnitude of the SSV effect on the thicknesses of the superconducting (S) and insulating (I) layers was studied. Optimization of the S and I layer thicknesses enabled a complete switching between the normal and superconducting states when the mutual orientation of the magnetizations of the ferromagnetic (F) layers changed from antiparallel to parallel. A maximal SSV effect value of 0.36\,K was achieved in an external magnetic field of 1\,kOe. These results demonstrate that SSV structures with tunable S/F interface transparency controlled by insulating interlayers are promising for achieving a significant magnitude of the effect. This opens new avenues for the development of such systems and their potential applications in spintronic devices.

cond-mat.supr-con↗

arXiv2024

Expanding the operational temperature window of a superconducting spin valve

To increase the efficiency of the superconducting spin valve (SSV), special attention should be paid to the choice of ferromagnetic materials for the F1/F2/S SSV multilayer. Here, we report the preparation and the superconducting properties of the SSV heterostructures where Pb is used as the superconducting S layer. In the magnetic part of the structure, we use the same starting material, the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$, for both F1 and F2 layers. We utilize the tunability of the magnetic properties of this alloy, which, depending on the deposition conditions, forms either an almost fully spin-polarized half-metallic F1 layer or a weakly ferromagnetic F2 layer. We demonstrate that the combination of the distinct properties of these two layers boosts the generation of the long-range triplet component of the superconducting condensate in the fabricated SSV structures and yields superior values of the triplet spin-valve effect of more than 1 K and of the operational temperature window of the SSV up to 0.6 K.

cond-mat.supr-con↗

arXiv2024

Influence of capacitance and thermal fluctuations on the Josephson diode effect in asymmetric higher-harmonic SQUIDs

Asymmetric two-junction SQUIDs with different current-phase relations in the two Josephson junctions, involving higher Josephson harmonics, demonstrate a flux-tunable Josephson diode effect (asymmetry between currents flowing in the opposite directions, which can be tuned by the magnetic flux through the interferometer loop). We theoretically investigate influence of junction capacitance and thermal fluctuations on performance of such Josephson diodes. Our main focus is on the "minimal model" with one junction in the SQUID loop possessing the sinusoidal current-phase relation and the other one featuring additional second harmonic. Capacitance generally weakens the diode effect in the resistive branch (R state) of the current-voltage characteristic (CVC) both in the absence and in the presence of external ac irradiation. At the same time, it leads to qualitatively new features of the Josephson diode effect such as asymmetry of the retrapping currents (which are a manifestation of hysteretic CVC). In particular, the limiting case of the single-sided hysteresis becomes accessible. In its turn, thermal fluctuations are known to lead to nonzero average voltages at any finite current, even below the critical value. We demonstrate that in the diode regime, the fluctuation-induced voltage can become strongly (exponentially) asymmetric. In addition, we find asymmetry of the switching currents arising both due to thermal activation and due to Josephson plasma resonances in the presence of ac irradiation.

cond-mat.supr-con↗

arXiv2024

Superconducting spin valve effect in the Co/Pb/Co heterostructure with insulating interlayers

We report the superconducting properties of the Co/Pb/Co heterostructures with thin insulating interlayers. The main specific feature of these structures is the intentional oxidation of both superconductor/ferromagnet (S/F) interfaces. We study variation of the critical temperature of our systems due to switching between parallel and antiparallel configurations of the magnetizations of the two magnetic layers. Common wisdom suggests that this spin valve effect, which is due to the S/F proximity effect, is most pronounced in the case of perfect metallic contact at the interfaces. Nevertheless, in our structures with intentionally deteriorated interfaces, we observed a significant full spin valve effect. A shift of the superconducting transition temperature $T_c$ by switching the mutual orientation of the magnetizations of the two ferromagnetic Co layers from antiparallel to parallel amounted to $ΔT_c =0.2$ K at the optimal thickness of the superconducting Pb layer. Our finding verifies the so far unconfirmed earlier results by Deutscher and Meunier on an F1/S/F2 heterostructure with oxidized interlayers [https://doi.org/10.1103/PhysRevLett.22.395, G. Deutscher and F. Meunier, Phys. Rev. Lett. 22, 395 (1969)] and suggests an alternative route to optimize the performance of the superconducting spin valves.

cond-mat.supr-con↗

arXiv2023

Asymmetric higher-harmonic SQUID as a Josephson diode

We theoretically investigate asymmetric two-junction SQUIDs with different current-phase relations in the two Josephson junctions, involving higher Josephson harmonics. Our main focus is on the "minimal model" with one junction in the SQUID loop possessing the sinusoidal current-phase relation and the other one featuring additional second harmonic. The current-voltage characteristic (CVC) turns out to be asymmetric, $I(-V) \neq -I(V)$. The asymmetry is due to the presence of the second harmonic and depends on the magnetic flux through the interferometer loop, vanishing only at special values of the flux such as integer or half-integer in the units of the flux quantum. The system thus demonstrates the flux-tunable Josephson diode effect (JDE), the simplest manifestations of which is the direction dependence of the critical current. We analyze asymmetry of the overall $I(V)$ shape both in the absence and in the presence of external ac irradiation. In the voltage-source case of external signal, the CVC demonstrates the Shapiro spikes. The integer spikes are asymmetric (manifestation of the JDE) while the half-integer spikes remain symmetric. In the current-source case, the CVC demonstrates the Shapiro steps. The JDE manifests itself in asymmetry of the overall CVC shape, including integer and half-integer steps.

cond-mat.supr-con↗

arXiv2022

Peculiarities of the density of states in SN junctions

We study the density of states (DoS) $ν(E)$ in a normal-metallic (N) film contacted by a bulk superconductor (S). We assume that the system is diffusive and the SN interface is transparent. In the limit of thin N layer (compared to the coherence length), we analytically find three different types of the DoS peculiarity at energy equal to the bulk superconducting order parameter $Δ_0$. (i) In the absence of the inverse proximity effect, the peculiarity has the check-mark form with $ν(Δ_0)=0$ as long as the thickness of the N layer is smaller than a critical value. (ii) When the inverse proximity effect comes into play, the check-mark is immediately elevated so that $ν(Δ_0)>0$. (iii) Upon further increasing of the inverse proximity effect, $ν(E)$ gradually evolves to the vertical peculiarity (with an infinite-derivative inflection point at $E=Δ_0$). This crossover is controlled by a materials-matching parameter which depends on the relative degree of disorder in the S and N materials.

cond-mat.supr-con↗

arXiv2021

Superconducting phases and the second Josephson harmonic in tunnel junctions between diffusive superconductors

We consider a planar SIS-type Josephson junction between diffusive superconductors (S) through an insulating tunnel interface (I). We construct fully self-consistent perturbation theory with respect to the interface conductance. As a result, we find correction to the first Josephson harmonic and calculate the second Josephson harmonic. At arbitrary temperatures, we correct previous results for the nonsinusoidal current-phase relation in Josephson tunnel junctions, which were obtained with the help of conjectured form of solution. Our perturbation theory also describes the difference between the phases of the order parameter and of the anomalous Green functions.

cond-mat.supr-con↗

arXiv2019

Surface density of states in superconductors with inhomogeneous pairing constant: Analytical results

We consider a superconductor with surface suppression of the BCS pairing constant $λ(x)$. We analytically find the gap in the surface density of states (DOS), behavior of the DOS $ν(E)$ above the gap, a "vertical" peculiarity of the DOS around an energy equal to the bulk order parameter $Δ_0$, and a perturbative correction to the DOS at higher energies. The surface gap in the DOS is parametrically different from the surface value of the order parameter due to a difference between the spatial scale $r_c$, at which $λ(x)$ is suppressed, and the coherence length. The vertical peculiarity implies an infinite-derivative inflection point of the DOS curve at $E=Δ_0$ with square-root behavior as $E$ deviates from $Δ_0$. The coefficients of this dependence are different at $E<Δ_0$ and $E>Δ_0$, so the peculiarity is asymmetric.

cond-mat.supr-con↗

arXiv2019

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $ΔT_c$ assisted by the triplet superconducting spin-valve effect $ΔT_c^{trip}$. In the second case, we observed a giant magnitude of $ΔT_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in Appendix. We find a good agreement between our experimental data and theoretical results.

cond-mat.supr-con↗

arXiv2019

Superconducting switching due to triplet component in the Pb/Cu/Ni/Cu/Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ spin-valve structure

We report the superconducting properties of the Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$/Cu/Ni/Cu/Pb spin-valve structure which magnetic part comprises the Heusler alloy layer HA = Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ with a high degree of spin polarization (DSP) of the conduction band and the Ni layer of variable thickness. We obtained that the separation between the superconducting transition curves measured for the parallel ($α= 0^\circ$) and perpendicular ($α= 90^\circ$) orientation of the magnetizations of the HA and Ni layers reaches up to 0.5 K ($α$ is an angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature $T_c$ on $α$ demonstrates a deep minimum in the vicinity of the perpendicular configuration of magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of the superconducting spin-valve multilayer comprising a half-metallic ferromagnet recently proposed by A. Singh et al., Phys. Rev. X 5, 021019 (2015) in achieving large values of the switching effect.

cond-mat.supr-con↗

arXiv2018

Mesoscopic supercurrent fluctuations in diffusive magnetic Josephson junctions

We study the supercurrent in quasi-one-dimensional Josephson junctions with a weak link involving magnetism, either via magnetic impurities or via ferromagnetism. In the case of weak links longer than {\color{black}the magnetic pair-breaking} length, the Josephson effect is dominated by mesoscopic fluctuations. We establish the supercurrent-phase dependence $I(φ)$ along with statistics of its sample-dependent properties in junctions with transparent contacts between leads and link. High transparency gives rise to the inverse proximity effect, while the direct proximity effect is suppressed by magnetism in the link. We find that all harmonics are present in $I(φ)$. Each harmonic has its own sample-dependent amplitude and phase shift with no correlation between different harmonics. Depending on the type of magnetic weak link, the system can realize a $φ_0$ or $φ$ junction in the fluctuational regime. Full supercurrent statistics is obtained at arbitrary relation between temperature, superconducting gap, and the Thouless energy of the weak link.

cond-mat.mes-hall↗

arXiv2018

Local impedance on a rough surface of a chiral $p$-wave superconductor

We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral $p$-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for Sr$_2$RuO$_4$ and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material.

cond-mat.supr-con↗

arXiv2018

Cooper pair splitting in diffusive magnetic SQUIDs

We study Josephson junctions with weak links consisting of two parallel disordered arms with magnetic properties -- ferromagnetic, half-metallic or normal with magnetic impurities. In the case of long links, the Josephson effect is dominated by mesoscopic fluctuations. In this regime, the system realises a $φ_0$ junction with sample-dependent $φ_0$ and critical current. Cooper pair splitting between the two arms plays a major role and leads to $2Φ_0$ periodicity of the current as a function of flux between the arms. We calculate the current and its flux and polarization dependence for the three types of magnetic links.

cond-mat.mes-hall↗

arXiv2017

Cooper pair splitting in ballistic ferromagnetic SQUIDs

We consider ballistic SQUIDs with spin filtering inside half-metallic ferromagnetic arms. A singlet Cooper pair cannot pass through an arm in this case, so the Josephson current is entirely due to the Cooper pair splitting, with two electrons going to different interferometer arms. In order to elucidate the mechanisms of Josephson transport due to split Cooper pairs, we assume the arms to be single-channel wires in the short-junction limit. Different geometries of the system (determined by the length of the arms and the phases acquired by quasiparticles during splitting between the arms) lead to qualitatively different behavior of the SQUID characteristics (the Andreev levels, the current-phase relation, and the critical Josephson current) as a function of two control parameters, the external magnetic flux and misorientation of the two spin filters. The current-phase relation can change its amplitude and shape, in particular, turning to a pi-junction form or acquiring additional zero crossings. The critical current can become a nonmonotonic function of the misorientation of the spin filters and the magnetic flux (on half of period). Periodicity with respect to the magnetic flux is doubled, in comparison to conventional SQUIDs.

cond-mat.mes-hall↗

arXiv2016

Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve

We have studied the proximity-induced superconducting triplet pairing in CoO$_x$/Py1/Cu/Py2/Cu/Pb spin-valve structure (where Py = Ni$_{0.81}$Fe$_{0.19}$). By optimizing the parameters of this structure we found a triplet channel assisted full switching between the normal and superconducting states. To observe an "isolated" triplet spin-valve effect we exploited the oscillatory feature of the magnitude of the ordinary spin-valve effect $ΔT_c$ in the dependence of the Py2-layer thickness $d_{Py2}$. We determined the value of $d_{Py2}$ at which $ΔT_c$ caused by the ordinary spin-valve effect (the difference in the superconducting transition temperature $T_c$ between the antiparallel and parallel mutual orientation of magnetizations of the Py1 and Py2 layers) is suppressed. For such a sample a "pure" triplet spin-valve effect which causes the minimum in $T_c$ at the orthogonal configuration of magnetizations has been observed.

cond-mat.supr-con↗

arXiv2015

Superconducting spin valve effect and triplet superconductivity in CoO$_x$/Fe1/Cu/Fe2/Cu/Pb multilayer

We report magnetic and superconducting properties of the modified spin valve system CoOx/Fe1/Cu/Fe2/Cu/Pb. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin valve effect and of the manifestation of the long-range triplet component of the superconducting condensate.

cond-mat.supr-con↗

arXiv2015

Odd-frequency superconducting states with different types of Meissner response: Problem of coexistence

We consider physical properties of a superconductor with a recently proposed type of odd-frequency pairing that exhibits diamagnetic Meissner response ("odd-dia state"). Such a state was suggested in order to address stability issues arising in an odd-frequency superconducting state with paramagnetic Meissner response ("odd-para state"). Assuming the existence of an odd-dia state (due to a proper retarded interaction), we study its coexistence with an odd-para state. The latter is known to be generated as an induced superconducting component in, e.g., singlet superconductor/ferromagnet proximity structures or triplet superconductor/normal metal systems. Calculating the superfluid density of the mixed odd-para/odd-dia state and the Josephson current between the odd-para and odd-dia states, we find that the expressions for the currents in both cases have non-vanishing imaginary contributions and are therefore unphysical. We show that a realization of the odd-dia state implies the absence of a Hamiltonian description of the system, and suggest that there exists no physically realizable perturbation that could give rise to the spontaneous symmetry breaking necessary for an actual realization of the odd-dia superconducting state.

cond-mat.supr-con↗

arXiv2013

Superconducting proximity effect in quantum wires without time-reversal symmetry

We study the superconducting proximity effect in a quantum wire with broken time-reversal (TR) symmetry connected to a conventional superconductor. We consider the situation of a strong TR-symmetry breaking, so that Cooper pairs entering the wire from the superconductor are immediately destroyed. Nevertheless, some traces of the proximity effect survive: for example, the local electronic density of states (LDOS) is influenced by the proximity to the superconductor, provided that localization effects are taken into account. With the help of the supersymmetric sigma model, we calculate the average LDOS in such a system. The LDOS in the wire is strongly modified close to the interface with the superconductor at energies near the Fermi level. The relevant distances from the interface are of the order of the localization length, and the size of the energy window around the Fermi level is of the order of the mean level spacing at the localization length. Remarkably, the sign of the effect is sensitive to the way the TR symmetry is broken: In the spin-symmetric case (orbital magnetic field), the LDOS is depleted near the Fermi energy, whereas for the broken spin symmetry (magnetic impurities), the LDOS at the Fermi energy is enhanced.

cond-mat.mes-hall↗