arXiv subjects

D. Yu. Vodolazov

Publications and source records attributed to D. Yu. Vodolazov.

At least 37 records · Page 2Linked to original sources

arXiv2018

Peculiarities of superconducting properties of thin superconductor-normal metal bilayer with large ratio of resistivities

We demonstrate, both theoretically and experimentally, that thin dirty superconductor-normal metal bilayer with resistivity of normal metal $ρ_N$ much smaller than normal-state resistivity of superconductor $ρ_S$ has unique superconducting properties. First of all the normal layer provides the dominant contribution to the diamagnetic response of whole bilayer structure in wide temperature interval below the critical temperature due to proximity induced superconductivity. Secondly, the presence of the normal layer may increase the critical current $I_c$ in several times (the effect is not connected with enhanced vortex pinning), provides strong temperature dependence of both $I_c$ and effective magnetic field penetration depth even at temperatures much below the critical one and leads to the diode effect in parallel magnetic field. Besides of general interest we believe that the found results may be useful in construction of different kinds of superconducting detectors.

cond-mat.supr-con↗

arXiv2018

Optical single photon detection in micron-scaled NbN bridges

We demonstrate experimentally that single photon detection can be achieved in micron-wide NbN bridges, with widths ranging from 0.53 $μ$m to 5.15 $μ$m and for photon-wavelengths from 408 nm to 1550 nm. The microbridges are biased with a dc current close to the experimental critical current, which is estimated to be about 50 % of the theoretically expected depairing current. These results offer an alternative to the standard superconducting single-photon detectors (SSPDs), based on nanometer scale nanowires implemented in a long meandering structure. The results are consistent with improved theoretical modelling based on the theory of non-equilibrium superconductivity including the vortex-assisted mechanism of initial dissipation.

cond-mat.supr-con↗

arXiv2017

Soliton induced critical current oscillations in two-band superconducting bridges

Using time-dependent Ginzburg-Landau theory we find oscillations of critical current density $j_c$ as a function of the length $L$ of the bridge formed from two-band superconductor. We explain this effect by appearance of the phase solitons in the bridge at $j<j_c$, those number changes with change of $L$. In case of sufficiently strong interband coupling oscillations of $j_c$ disappear.

cond-mat.supr-con↗

arXiv2017

Proximity effect model of ultra-narrow NbN strips

We show that narrow superconducting strips in superconducting (S) and normal (N) states are universally described by the model presenting them as lateral NSN proximity systems in which the superconducting central band is sandwiched between damaged edge-bands with suppressed superconductivity.The width of the superconducting band was experimentally determined from the value of magnetic field at which the band transits from the Meissner state to the static vortex state. Systematic experimental study of 4.9 nm thick NbN strips with widths in the interval from 50 nm to 20 $μ$m, which are all smaller than the Pearl's length, demonstrates gradual evolution of the temperature dependence of the critical current with the change of the strip width.

physics.app-ph↗

arXiv2017

Threshold fluctuations in a superconducting current-carrying bridge

We calculate the energy of threshold fluctuation $δF_{thr}$ which triggers the transition of superconducting current-carrying bridge to resistive state. We show that the dependence $δF_{thr}(I)\propto I_{dep}\hbar(1-I/I_{dep})^{5/4}/e$, found by Langer and Ambegaokar for a long bridge with length $L \gg ξ$, holds far below the critical temperature both in dirty and clean limits (here $I_{dep}$ is the depairing current of the bridge and $ξ$ is a coherence length). We also find that even 'weak' local defect (leading to the small suppression of the critical current of the bridge $I_c \lesssim I_{dep}$) provides $δF_{thr}\propto I_c\hbar(1-I/I_c)^{3/2}/e$, typical for a short bridge with $L \ll ξ$ or a Josephson junction.

cond-mat.supr-con↗

arXiv2017

Enhancement of superconductivity in NbN nanowires by negative electron-beam lithography with positive resist

We performed comparative experimental investigation of superconducting NbN nanowires which were prepared by means of positive-and negative electron-beam lithography with the same positive tone Poly-methyl-methacrylate (PMMA) resist. We show that nanowires with a thickness 4.9 nm and widths less than 100 nm demonstrate at 4.2 K higher critical temperature and higher density of critical and retrapping currents when they are prepared by negative lithography. Also the ratio of the experimental critical-current to the depairing critical current is larger for nanowires prepared by negative lithography. We associate the observed enhancement of superconducting properties with the difference in the degree of damage that nanowire edges sustain in the lithographic process. A whole range of advantages which is offered by the negative lithography with positive PMMA resist ensures high potential of this technology for improving performance metrics of superconducting nanowire singe-photon detectors.

physics.app-ph↗

arXiv2017

Theory of single photon detection by 'dirty' current-carrying superconducting strip based on the kinetic equation approach

Using kinetic equation approach we study dynamics of electrons and phonons in current-carrying superconducting nanostrips after absorption of single photon of near-infrared or optical range. We find that the larger the ratio $C_e/C_{ph}|_{T_c}$ ($T_c$ is a critical temperature of superconductor, $C_e$ and $C_{ph}$ are specific heat capacities of electrons and phonons, respectively) the larger part of photon's energy goes to electrons, they become stronger heated and, hence, could thermalize faster during initial stage of hot spot formation. Thermalization time $τ_{th}$ could be less than one picoseconds for superconductors with $C_e/C_{ph}|_{T_c}\gg 1$ and small diffusion coefficient $D\simeq 0.5 cm^2/s$ when thermalization occurs mainly due to electron-phonon and phonon-electron scattering in relatively small volume $\sim ξ^2d$ ($ξ$ is a superconducting coherence length, $d<ξ$ is a thickness of the strip). At larger times due to diffusion of hot electrons effective temperature inside the hot spot decreases, the size of hot spot increases, superconducting state becomes unstable and normal domain spreads in the strip at current larger than so-called detection current. We find dependence of detection current on the photon's energy, place of its absorption in the strip, width of the strip, magnetic field and compare it with existing experiments. Our results demonstrate that materials with $C_e/C_{ph}|_{T_c} \ll 1$ are bad candidates for single photon detectors due to small transfer of photon's energy to electronic system and large $τ_{th}$. We also predict that even several microns wide dirty superconducting bridge is able to detect single near-infrared or optical photon if its critical current exceeds 70 $\%$ of depairing current and $C_e/C_{ph}|_{T_c} \gtrsim 1$.

cond-mat.supr-con↗

arXiv2016

Rise times of voltage pulses in NbN superconducting single-photon detectors

We have found experimentally that the rise times of voltage pulses in NbN superconducting single photon detectors increase nonlinearly with increasing detector length. We fabricated superconducting single photon detectors based on NbN thin films with a meander-like sensitive region of area from 2x2um2 to 11x11um2. The effect is connected with the dependence of the detector resistance, which appears after photon absorption, on its kinetic inductance and hence on detector length. This conclusion is confirmed by our calculations in the framework of the two-temperature model.

cond-mat.supr-con↗

arXiv2016

Threshold Perturbations in Current-Carrying Superconducting Bridges with a Finite Length near the Critical Temperature

Near the critical temperature of a superconducting transition, the energy of the threshold perturbation $δF_{thr}$ that transfers a superconducting bridge to a resistive state at a current below the critical current $I_c$ has been determined. It has been shown that $δF_{thr}$ increases with a decrease in the length of a bridge for short bridges with lengths $L<ξ$ (where $ξ$ is the coherence length) and is saturated for long bridges with $L\gg ξ$. At certain geometrical parameters of banks and bridge, the function $δF_{thr}(L)$ at the current $I\to 0$ has a minimum at $L\sim 2-3 ξ$. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths $\sim 2-3 ξ$ is enhanced.

cond-mat.supr-con↗

arXiv2015

Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips

Current crowding in the bends of superconducting nano-structures not only restricts measurable critical current in such structures but also redistributes local probabilities for dark and light counts to appear. Using structures from strips in the form of a square spiral which contain bends with the very same curvature with respect to the directions of bias current and external magnetic field, we have shown that dark counts as well as light counts at small photon energies originate from areas around the bends. The minimum in the rate of dark counts reproduces the asymmetry of the maximum critical current density as function of the magnetic field. Contrary, the minimum in the rate of light counts demonstrate opposite asymmetry. The rate of light counts become symmetric at large currents and fields. Comparing locally computed absorption probabilities for photons and the simulated threshold detection current we found the approximate locations of areas near bends which deliver asymmetric light counts. Any asymmetry is absent in Archimedean spiral structures without bends.

physics.optics↗

arXiv2015

Defect controlled vortex generation in current-carrying narrow superconducting strips

We experimentally study effect of single circular hole on the critical current $I_c$ of narrow superconducting strip with width $W$ much smaller than Pearl penetration depth $Λ$. We found nonmonotonous dependence of $I_c$ on the location of a hole across the strip and a weak dependence of $I_c$ on radius of hole has been found in case of hole with $ξ\ll R \ll W$ ($ξ$ is a superconducting coherence length) which is placed in the center of strip. The observed effects are caused by competition of two mechanisms of destruction of superconductivity - the entrance of vortex via edge of the strip and the nucleation of the vortex-antivortex pair near the hole. The mechanisms are clearly distinguishable by difference in dependence of $I_c$ on weak magnetic field.

cond-mat.supr-con↗

arXiv2015

Vortex assisted mechanism of photon counting in superconducting nanowire single photon detector revealed by external magnetic field

We use external magnetic field to probe the detection mechanism of superconducting nanowire single photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $Δ$ across the whole width of the superconducting nanowire after absorption of the single photon) does not explain observed weak field dependence of the photon count rate (PCR) for photons with $λ$=450 nm and noticeable {\it decrease} of PCR (with increasing the magnetic field) in some range of the currents for photons with wavelengths $λ$ =450-1200 nm. Found experimental results for all studied wavelengths $λ= 450-1550$ nm could be explained by the vortex hot spot model (which assumes partial suppression of $Δ$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $Δ$.

cond-mat.supr-con↗

arXiv2014

Current dependence of the 'red' boundary of superconducting single photon detectors in the modified hot spot model

We find the relation between the energy of the absorbed photon and the threshold current at which the resistive state appears in the current-carrying superconducting film with the probability about unity. In our calculations we use the modified hot spot model, which assumes different strength of suppression of the superconducting order parameter in the finite area of the film around the place where the photon is absorbed. To find the threshold current we solve the Ginzburg-Landau equation for superconducting order parameter, which automatically includes the current continuity equation and it allows us to consider the back effect of current redistribution near the hot spot on the stability of the superconducting state. We find quantitative agreement with the recent experiments, where we use the single fitting parameter which describes what part of the energy of the photon goes for the local destruction of the superconductivity in the film.

cond-mat.supr-con↗

arXiv2014

Temporary cooling of quasiparticles and delay in voltage response of superconducting bridges after abrupt switching on the supercritical current

We revisit the problem of the dynamic response of a superconducting bridge after abruptly switching on the supercritical current $I>I_c$. In contrast to previous theoretical works we take into account spatial gradients and use both the local temperature approach and the kinetic equation for the distribution function of quasiparticles. In both models the finite delay time $t_d$ in the voltage response is connected with temporary cooling of quasiparticles due to the suppression of the superconducitng order parameter by current. We find that $t_d$ has different values and different temperature dependencies in the considered models. In turns out that the presence of even small inhomogeneities in the bridge or of bulk leads/contacts at the ends of the {\it homogenous} bridge favors a local suppression of the superconducting order parameter $|Δ|$ during the dynamic response. It results in a decrease of the delay time, in comparison with the spatially uniform model, due to the diffusion of nonequilibrium quasiparticles from the region with locally suppressed $|Δ|$. In case the current distribution is spatially nonuniform across the bridge the delay time is mainly connected with the time needed for the nucleation of the first vortex at the position where the current density is maximal (at $I\sim I_c$ and for not very wide films). We also find that a short alternating current pulse (sinusoid like) with zero time-average may result in a nonzero time-averaged voltage response where its sign depends on the phase of the ac current.

cond-mat.supr-con↗

arXiv2014

Intrinsic detection efficiency of superconducting single photon detector in the modified hot spot model

We theoretically study the dependence of the intrinsic detection efficiency (IDE) of superconducting single photon detector on the applied current $I$ and magnetic field $H$. We find that the current, at which the resistive state appears in the superconducting film, depends on the position of the hot spot (region with suppressed superconductivity around the place where the photon has been absorbed) with respect to the edges of the film. It provides inevitable smooth dependence IDE(I) when IDE $\sim 0.05-1$ even for homogenous straight superconducting film and in the absence of fluctuations. When IDE $\lesssim 0.05$ much sharper current dependence comes from the fluctuation assisted vortex entry to the hot spot located near the edge of the film. We find that weak magnetic field strongly affects IDE when the photon detection is connected with fluctuation assisted vortex entry (IDE$\ll 1$) and it weakly affects IDE when the photon detection is connected with the current induced vortex entry to the hot spot or nucleation of the vortex-antivortex pair inside the hot spot (IDE$\sim 0.05-1$).

cond-mat.supr-con↗

arXiv2012

Geometry-induced reduction of the critical current in superconducting nanowires

Reduction of the critical current in narrow superconducting NbN lines with sharp and rounded bends with respect to the critical current in straight lines was studied at different temperatures. We compare our experimental results with the reduction expected in the framework of the London model and the Ginsburg-Landau model. We have experimentally found that the reduction is significantly less than either model predicts. We also show that in our NbN lines the bends mostly contribute to the reduction of the critical current at temperatures well below the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2010

Reverse-domain superconductivity in superconductor-ferromagnet hybrids: effect of a vortex-free channel on the symmetry of I-V characteristics

We demonstrate experimentally that the presence of a single domain wall in an underlying ferromagnetic BaFe_{12}O_{19} substrate can induce a considerable asymmetry in the current (I) - voltage (V) characteristics of a superconducting Al bridge. The observed diode-like effect, i.e. polarity-dependent critical current, is associated with the formation of a vortex-free channel inside the superconducting area which increases the total current flowing through the superconducting bridge without dissipation. The vortex-free region appears only for a certain sign of the injected current and for a limited range of the external magnetic field.

cond-mat.supr-con↗

arXiv2003

Surface impedance anisotropy of YBa$_2$Cu$_3$O$_{6.95}$ single crystals: electrodynamic basis of the measurements

An electrodynamic technique is developed for determining the components of surface impedance and complex conductivity tensors of HTSC single crystals on the basis of measured quantities of a quality factor and a resonator frequency shift. A simple formula is obtained for a geometrical factor of a crystal in the form of a plate with dimensions $b\gg a>c$ in a microwave magnetic field ${\bf H_ω}\perp ab$. To obtain the c-axis complex conductivity from measurements at ${\bf H_ω}\parallel ab$ we propose a procedure which takes account of sample size effects. With the aid of the technique involved temperature dependences of all impedance and conductivity tensors components of YBa$_2$Cu$_3$O$_{6.95}$ single crystal, grown in BaZrO$_3$ crucible, are determined at a frequency of $f=9.4$ GHz in its normal and superconducting states. All of them proved to be linear at $T<T_c/2$, and their extrapolation to zero temperature gives the values of residual surface resistance $R_{ab}(0)\approx 40$ $μΩ$ and $R_c(0)\approx 0.8$ m$Ω$ and magnetic field penetration depth $λ_{ab}(0)\approx 150$ nm and $λ_c(0)\approx 1.55$ $μ$m.

cond-mat.supr-con↗