arXiv subjects

R. V. Vovk

Publications and source records attributed to R. V. Vovk.

At least 19 recordsLinked to original sources

arXiv2026

Hydrostatic pressure effect on the pseudogap in Y0.77Pr0.23Ba2Cu3O7-δsingle crystals

We studied the changes of resistivity $ρ(T)$, superconducting transition temperature $T_c$, fluctuation conductivity $σ'(T)$, and pseudogap $Δ^*(T)$ of $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ (YPrBCO) ($x = 0.23$) single crystal under hydrostatic pressure (HP) up to $\sim 1.1$ GPa. Defects created by magnetic inclusions of $PrBa_2Cu_3O_{7-δ}$ (PrBCO) were found to play a significant role in the behavior of the sample. The largest decrease in $ρ(T)$ was found depending on HP at a rate of $d\lnρ(100~K)/dP = -(29 \pm 0.2) \% \cdot GPa^{-1}$. This indicates that the mechanisms of the influence of HP on the $ρ(T)$ and $T_c$ of YPrBCO and $YBa_2Cu_3O_{7-δ}$ single crystals are different. From the analysis of the pseudogap, it was revealed that the mechanism of the interaction of charge carriers with magnetic PrBCO impurities changes three times with increasing HP. Relatively low HP, up to $\sim 0.5$ GPa, promotes the formation of defects caused by PrBCO magnetic inclusions. Starting from 0.6 GPa, the HP neutralizes the influence of magnetic impurities, and the magnetic maximum on $Δ^*(T)$ disappears. Above $\sim 1.0$ GPa, HP aligns the magnetic moments of PrBCO, and the magnetic maximum reappears on $Δ^*(T)$, more pronounced than at $P = 0$.

cond-mat.supr-con↗

arXiv2025

Evolution of the pseudogap and excess conductivity of YBa$_2$Cu$_3$O$_{7-δ}$ single crystals in the course of long-term aging

The temperature dependences of both fluctuation conductivity (FLC) $σ^\prime(T)$ and pseudogap (PG) $Δ^*(T)$ derived from measurements of resistivity $ρ(T)$ of an optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystal subjected to long-term storage have been studied. The as-grown sample S1 exhibits characteristics typical of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystals containing twins and twin boundaries. Analysis of both FLC and PG showed an unexpected improvement in all characteristics of the sample after 6 years of storage (sample S2), indicating that the effect of twin boundaries is somehow limited. After 17 years of storage, all characteristics of the sample changed dramatically, which indicates a strong influence of internal defects formed during the aging process. For the first time, the temperature dependences of both FLC and PG were obtained after 17 years of storage.

cond-mat.supr-con↗

arXiv2025

Influence of strong electron irradiation on fluctuation conductivity and pseudogap in YBa$_2$Cu$_3$O$_7$--$δ$ single crystals

The effect of high-energy electron irradiation on the temperature dependences of the resistivity $ρ(T)$, fluctuation conductivity (FLC), and pseudogap (PG) $Δ^{*}(T)$ of YBa$_2$Cu$_3$O$_7$--$δ$ (YBCO) single crystals without twins was studied. Irradiation causes a linear increase in $ρ(T)$ and a decrease in the superconducting transition temperature $T_c$ with dose $ϕ$. For small $ϕ$, the reduction of $T_c$ follows the Abrikosov--Gorkov (AG) pair-breaking theory, while for large $ϕ$ it is described by the Emery--Kivelson (EK) model, where quantum phase fluctuations dominate. At $ϕ_3 = 2.5 \times 10^{19}$ e/cm$^2$, which corresponds to the AG--EK crossover, the spacing between CuO$_2$ planes $d_{01}$, the coherence length $ξ_c(0)$, and the fluctuation region $T_{\mathrm{fl}}$ increase sharply, and the two-dimensional Maki--Thompson (2D--MT) contribution is replaced by the Aslamazov--Larkin (2D--AL) term. Surprisingly, no signatures of the crossover appear in $ρ(ϕ)$ or $T_c(ϕ)$. At the same $ϕ_3$, a sharp rise in the pseudogap opening temperature $T^{*}$ and in $Δ^{*}$ indicates a possible reduction of the density of states. With further increase in $ϕ$, both PG parameters and their energy scale decrease markedly, and $Δ^{*}(T)$ acquires an unusual form. However, at $ϕ_5 = 5.6 \times 10^{19}$ e/cm$^2$, the temperature dependences of FLC and PG again show behavior typical of well-structured YBCO, regardless of defect density.

cond-mat.supr-con↗

arXiv2023

Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped $HoBa_2Cu_3O_{7-δ}$ single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) $σ^\prime(T)$ and pseudogap (PG) $Δ^*(T)$ in the basal ab plane of $ReBa_2Cu_3O_{7-δ}$ (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near $T_c$ can be described by the Aslamazov$-$Larkin and Maki$-$Thompson fluctuation theories, demonstrating a 3D$-$2D crossover with increasing temperature. The crossover temperature $T_0$ was used to determine the coherence length along the $c$-axis, $ξ_c(0)$=2.82 A. At the inter-mediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in $HoBa_2Cu_3O_{7-δ}$ (HoBCO): $μ_{eff, Ho}$=9.7$μ_{B}$. For the quenched sample S1, the temperature dependence of the PG has a shape typical of single crystals with a large number of defects. However, $Δ^*(T)$ has two small additional maxima at high temperature, which is a feature of HoBCO single crystals with pronounced twins and indicates the two-phase nature of the sample. Upon annealing, the shape of $Δ^*(T)$ noticeably changes, very likely due to an increase in the magnetic interaction (sample S2). More important is the change in the slope of the data at high temperatures, which has become about 3.5 times steeper. The ordering of the oxygen distribution due to the diffusion process during annealing somewhat compensates for the influence of magnetic interaction. Interestingly, the slope turns out to be the same as for FeAs-based superconductors, suggesting the possibility of the existence of spin density waves in HoBCO in the PG state.

cond-mat.supr-con↗

arXiv2022

Influence of electron irradiation on fluctuation conductivity and pseudogap in $YBa_2Cu_3O_{7-δ}$ single crystals

The effect of electron irradiation with the energy of 2.5 MeV on the temperature dependences of the resistivity of an optimally doped $YBa_2Cu_3O_{7-δ}$ single crystal has been studied. The temperature dependences of both fluctuation conductivity $σ^\prime(T)$ and the pseudogap $Δ^*(T)$ on irradiation dose $ϕ$ have been calculated within the local pair model. Here we show that with an increase in $ϕ$, the value of $ρ(300 K)$ increases linearly, while $T_c$ decreases linearly. Concurrently, the value of $ρ{100 K}$ increases nonlinearly, demonstrating a feature for $ϕ_3$ = 4.3 10$^{18}$ e/$cm^2$, which is also observed in the number of other dose-dependent parameters.Regardless of the irradiation dose, in the temperature range from $T_c$ up to $T_{01}$, $σ^\prime(T)$ obeys the classical fluctuation theories of Aslamazov$-$Larkin and Maki$-$Thompson, demonstrating 3D$-$2D crossover with increasing temperature. The crossover temperature $T_0$ makes it possible to determine the coherence length along the $c$-axis, $ξ_c(T)$, which increases by $\approx 3$ times under irradiation. Furthermore, the range of superconducting fluctuations above $T_c$ also noticeably increases.At $ϕ$ = 0, the dependence $Δ^*(T)$ typical for single crystals containing pronounced twin boundaries is observed with a maximum at $T_{pair}$ ~ 120 K and a distinct minimum at $T = T_{01}$. It was determined for the first time that at $ϕ_3$ = 4.3 10$^{18}$ the shape of $Δ^*(T)$ changes strongly and becomes the same as in optimally doped $YBa_2Cu_3O_{7-δ}$ single crystals with a very low pseudogap opening temperature $T^*$ and noticeably reduced $T_{pair}$, while at $T_c(ϕ)$ there are no singularities.

cond-mat.supr-con↗

arXiv2020

Moving flux quanta cool superconductors by a microwave breath

Almost any use of a superconductor implies a nonequilibrium state. Remarkably, the non-equilibrium states induced by a microwave stimulus and the dynamics of magnetic flux quanta (Abrikosov vortices) can give rise to strikingly contrary effects: A sufficiently high-power electromagnetic field of GHz frequency can stimulate superconductivity, whereas fast vortex motion can trigger an instability abruptly quenching the superconducting state. Here, we advance or delay such dynamical quenching of the vortex state in Nb thin films by tuning the power and frequency of the microwave ac stimulus added to a dc bias current. The experimental findings are supported by time-dependent Ginzburg-Landau simulations and they can be explained qualitatively based on a model of "breathing mobile hot spots", implying a competition of heating and cooling of quasiparticles along the trajectories of moving fluxons whose core sizes vary in time. In addition, we demonstrate universality of the stimulation effect on the thermodynamic and transport properties of type II superconductors.

cond-mat.supr-con↗

arXiv2019

Spin Seebeck effect and phonon energy transfer in heterostructures containing layers of a normal metal and a ferromagnetic insulator

In the framework of the kinetic approach based on the Boltzmann equation for the phonon distribution function, we analyze phonon heat transfer in a heterostructure containing a layer of a normal metal ($ N $) and a layer of a ferromagnetic insulator ($ F $). Two realistic methods for creating a temperature gradient in such a heterostructure are considered: by heating of the $N$-layer by an electric current and by placing the $N/F$-bilayer between massive dielectrics with different temperatures. The electron temperature $ T_e $ in the $ N $-layer and the magnon temperature $ T_m $ in the $ F $-layer are calculated. The difference in these temperatures determines the voltage $ V_{ISHE} $ on the $ N $-layer in the Seebeck spin effect regime. The dependence of $ V_{ISHE} $ on the bath temperature and on the thickness of the $ N $ and $ F $ layers is compared with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2019

Local flux-flow instability in superconducting films near Tc

Larkin and Ovchinnikov established that the viscous flow of magnetic flux quanta in current-biased superconductor films placed in a perpendicular magnetic field can lose stability due to a decrease in the vortex viscosity coefficient $η$ with increasing velocity of the vortices $v$. The dependence of $η$ on $v$ leads to a $nonlinear$ section in the current-voltage ($I$-$V$) curve which ends at the flux-flow instability point with a voltage jump to a highly resistive state. At the same time, in contradistinction with the nonlinear conductivity regime, instability jumps often occur in $linear$ $I$-$V$ sections. Here, for the elucidation of such jumps we develop a theory of local instability of the magnetic flux flow occurring not in the entire film but in a narrow strip across the film width in which vortices move much faster than outside it. The predictions of the developed theory are in agreement with experiments on Nb films for which the heat removal coefficients and the inelastic scattering times of quasiparticles are deduced. The presented model of local instability is also relevant for the characterization of superconducting thin films whose performance is examined for fast single-photon detection.

cond-mat.supr-con↗

arXiv2019

Spin-wave phase inverter upon a single nanodefect

Local modification of magnetic properties of nanoelements is a key to design future-generation magnonic devices, in which information is carried and processed via spin waves. One of the biggest challenges here is to fabricate simple and miniature phase-controlling elements with broad tunability. Here, we successfully realize such spin-wave phase shifter upon a single nanogroove milled by focused ion beam in a Co-Fe microsized magnonic waveguide. By varying the groove depth and the in-plane bias magnetic field we continuously tune the spin-wave phase and experimentally evidence a complete phase inversion. The microscopic mechanism of the phase shift is based on the combined action of the nanogroove as a geometrical defect and the lower spin-wave group velocity in the waveguide under the groove where the magnetization is reduced due to the incorporation of Ga ions during the ion-beam milling. The proposed phase shifter can easily be on-chip integrated with spin-wave logic gates and other magnonic devices. Our findings are crucial for designing nano-magnonic circuits and for the development of spin-wave nano-optics.

cond-mat.mes-hall↗

arXiv2019

Supersonic dynamics of guided magnetic flux quanta

The dynamics of Abrikosov vortices in superconductors is usually limited to vortex velocities $v\simeq1$ km/s above which samples abruptly transit into the normal state. In the Larkin-Ovchinnikov framework, near the critical temperature this is because of a flux-flow instability triggered by the reduction of the viscous drag coefficient due to the quasiparticles leaving the vortex cores. While the existing instability theories rely upon a uniform spatial distribution of vortex velocities, the measured (mean) value of $v$ is always smaller than the maximal possible one, since the distribution of $v$ never reaches the $δ$-functional shape. Here, by guiding magnetic flux quanta at a tilt angle of $15^\circ$ with respect to a Co nanostripe array, we speed up vortices to supersonic velocities. These exceed $v$ in the reference as-grown Nb films by almost an order of magnitude and are only a factor of two smaller than the maximal vortex velocities observed in superconductors so far. We argue that such high $v$ values appear in consequence of a collective dynamic ordering when all vortices move in the channels with the same pinning strength and exhibit a very narrow distribution of $v$. Our findings render the well-known vortex guiding effect to open prospects for investigations of ultrafast vortex dynamics.

cond-mat.supr-con↗

arXiv2019

Tuning electric charge scattering in YBCO single crystals via irradiation with MeV electrons

Irradiation with electrons is an efficient approach to inducing a large number of defects with a minimal impact on the material itself. Analysis of the energy transfer from an accelerated particle smashing into the crystal lattice shows that only electrons with MeV energies produce point defects in the form of interstitial ions and vacancies that form perfect scattering centers. Here, we investigate the changes in the resistive characteristics of YBCO single crystals from the 1-2-3 system after several steps of low-temperature irradiation with $0.5-2.5$\,MeV electrons and irradiation doses of up to $8.8\times10^{18}$\,cm$^{-2}$. The penetration depth of such electrons is much larger than the crystal thickness. We reveal that defects appearing in consequence of such electron irradiation not only increase the residual resistance, but they affect the phonon spectrum of the system and lower the superconducting transition temperature linearly with increase of the irradiation dose. Furthermore, the irradiation-induced defects are distributed non-uniformly, that manifests itself via a broadening of the superconducting transition. Interestingly, the excess conductivity remains almost unaffected after such electron irradiation.

cond-mat.supr-con↗

arXiv2019

Influence of annealing on the electrical resistance of YBCO single crystals

The effect of annealing on the basal-plane electrical resistivity of the YBa$_2$Cu$_3$O$_{7-δ}$ single crystals is studied in a broad range of oxygen contents. Within the framework of s-d scattering of electrons by phonons, an increase in the oxygen deficit index, $δ$, leads to a significant increase in the Debye temperature, $θ$, which is associated with the isotropization of the phonon spectrum as the concentration of oxygen vacancies increases. Near the optimal doping, the role of the paraconductivity becomes crucial, whereas its contribution decreases with increasing $δ$. At large values of $δ$ some deviations from the s-d model of electron scattering by phonons are observed at room temperature, while no paraproductivity is observed. In the superconducting transition region, a 2D-3D crossover is observed, which shifts in the direction of $T_c$ with increasing $δ$. The estimate for the transverse coherence length is about $1$\,Å.

cond-mat.supr-con↗

arXiv2019

Annealing effects on the normal-state resistive properties of underdoped cuprates

The influence of room-temperature annealing on the parameters of the basal-plane electrical resistance of underdoped YBa$_2$Cu$_3$O$_{7-δ}$ and HoBa$_2$Cu$_3$O$_{7-δ}$ single crystals in the normal and superconducting states is investigated. The form of the derivatives $dρ(T)/dT$ makes it possible to determine the onset temperature of the fluctuation conductivity and indicates a nonuniform distribution of the labile oxygen. Annealing has been revealed to lead to a monotonic decrease in the oxygen deficiency, that primarily manifests itself as a decrease of the residual resistance, an increase of $T_c$, and a decrease of the Debye temperature.

cond-mat.supr-con↗

arXiv2019

Zero-bias Shapiro steps in asymmetric pinning nanolandscapes

The coherent nonlinear dynamics of Abrikosov vortices in asymmetric pinning nanolandscapes is studied by theoretical modeling and combined microwave and dc electrical resistance measurements. The problem is considered on the basis of a single-vortex Langevin equation within the framework of a stochastic model of anisotropic pinning. When the distance over which Abrikosov vortices are driven during one half ac cycle coincides with one or a multiple of the nanostructure period, Shapiro steps appear in the current-voltage curves (CVCs) as a general feature of systems whose evolution in time can be described in terms of a particle moving in a periodic potential under combined dc and ac stimuli. While a dc voltage appears in response to the ac drive, the addition of a dc bias allows one to diminish the rectified voltage and eventually to change its sign when the extrinsic dc bias-induced asymmetry of the pinning potential starts to dominate the intrinsic one. This rectified negative voltage in the CVCs becomes apparent as \emph{zero-bias} Shapiro steps, which are theoretically predicted and experimentally observed for the first time.

cond-mat.supr-con↗

arXiv2019

Room-temperature annealing effects on the basal-plane resistivity of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystals

We reveal that the temperature dependence of the basal-plane normal-state electrical resistance of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystals can be with great accuracy approximated within the framework of the model of s-d electron-phonon scattering. This requires taking into account the fluctuation conductivity whose contribution exponentially increases with decreasing temperature and decreases with an increase of oxygen deficiency. Room-temperature annealing improves the sample and, thus, increases the superconducting transition temperature. The temperature of the 2D-3D crossover decreases during annealing.

cond-mat.supr-con↗

arXiv2019

Effect of electron irradiation and Pr doping on the charge transport in YBCO single crystals

The influence of irradiation by electrons with energies of $0.5-2.5$\,MeV at temperatures of about $10$\,K on the basal-plane resistivity of the YBa$_2$Cu$_3$O$_{7-δ}$ single crystals is investigated in the range from $T_c$ to $300$\,K. The resistivity temperature dependence is determined by defects arising due to the irradiation. These defects directly affect the superconducting transition, decreasing $T_c$ and increasing the transition width without significant distortions of its shape. The resulting defects also lead to an increase in the Debye temperature due to a reduction of the anisotropy, and a noticeable increase in the scattering by phonons in the sample. The excess conductivity does not change with the irradiation used.

cond-mat.supr-con↗

arXiv2019

Magnon-Fluxon interaction in a ferromagnet/superconductor heterostructure

Ferromagnetism and superconductivity are most fundamental phenomena in condensed matter physics. Entailing opposite spin orders, they share an important conceptual similarity: Disturbances in magnetic ordering in magnetic materials can propagate in the form of spin waves (magnons) while magnetic fields penetrate superconductors as a lattice of magnetic flux quanta (fluxons). Despite a rich choice of wave and quantum phenomena predicted, magnon-fluxon coupling has not been observed experimentally so far. Here, we clearly evidence the interaction of spin waves with a flux lattice in ferromagnet/superconductor Py/Nb bilayers. We demonstrate that, in this system, the magnon frequency spectrum exhibits a Bloch-like band structure which can be tuned by the biasing magnetic field. Furthermore, we observe Doppler shifts in the frequency spectra of spin waves scattered on a flux lattice moving under the action of a transport current in the superconductor.

cond-mat.supr-con↗

arXiv2018

Reduction of microwave loss by mobile fluxons in grooved Nb films

In the mixed state of type II superconductors penetrated by an external magnetic field in the form of a lattice of Abrikosov vortices the dc resistance is known to increase with increasing velocity of the vortex lattice. Accordingly, vortex pinning sites impeding the vortex motion are widely used to preserve the low-dissipative response of the system. Here, while subjecting superconducting Nb films with nanogrooves to a combination of dc and ac current stimuli and tuning the number of mobile and pinned vortices by varying the magnetic field around the so-called matching values, we observe a completely opposite effect. Namely, the vortex-related microwave excess loss for mobile vortices becomes smaller than for pinned vortices in a certain range of power levels at ac current frequencies above 100\,MHz. While a theoretical description of the observed effect is yet to be elaborated we interpret our findings in terms of a competition of the effective cooling of the system by the quasiparticles leaving the vortex cores with the conventional Joule heating caused by the current flow.

cond-mat.supr-con↗