arXiv subjects

D. Y. Vodolazov

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

At least 19 recordsLinked to original sources

arXiv2013

Vortex-induced negative magnetoresistance and peak effect in narrow superconducting films

In framework of Ginzburg-Landau model it is shown that narrow superconducting film with width w ~ 3-8 ξ(T) (ξ(T) is a temperature dependent coherence length) exhibits unusual transport properties. In the absence of bulk pinning its critical current Ic nonmonotonically depends on perpendicular magnetic field H and has one minima (dip) and one maxima (peak) at some magnetic fields. At currents I << Ic(H) the finite magnetoresistance R(H) of such a samples due to thermo-activated vortex hopping via edge barriers also shows both local maxima(peak) and minima(dip) nearly at the same magnetic fields. In narrower films such an effect is absent due to absence of the vortices and in wider films the effect is weaker due to increased vortex-vortex interaction. Finite length of the film produces additional periodic variation in both Ic(H) and R(H) because of discrete change in the number of the vortices, which is superimposed on the above mentioned nonmonotonic dependence. The obtained results are directly related to many experiments on narrow superconducting films/bridges where such a nonmonotonic dependencies Ic(H) and R(H) were observed.

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arXiv2013

Differences in the effects of turns and constrictions on the resistive response in current-biased superconducting wire after single photon absorption

We study how turns and constrictions affect the resistive response of the superconducting wire after instant in time and local in space heating, which models the absorption of the single photon by the wire. We find that the presence of constriction favors detection of photons of various energies but the presence of turn increases only ability to detect relatively "low" energy photons. The main reason is that in case of constriction the current density is increased over whole length and width of the constriction while in case of the turn the current density is enhanced only near the inner corner of the turn. It results in inhomogeneous Joule heating near the turn and worsens the conditions for appearance of the normal domain at relatively small currents when the "high" energy photons already could create normal domain in straight part of the wire. We also find that the amplitude of the voltage pulse depends on the place where the photon is absorbed. It is the smallest one when photon is absorbed near the turn and it is the largest one when photon is absorbed near the constriction. This effect comes from the difference in resistance of constriction and the turn in the normal state from the resistance of the rest of the wire.

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arXiv2012

Saddle point states in 2D superconducting film biased near the depairing current

The structure and energy of saddle point (SP) states in 2D superconducting film of finite width $w$ with transport current $I$ are found in the framework of Ginzburg-Landau model. We show that very near depairing current $I_{dep}$ the SP state with a vortex does not exist and it transforms to 2D nucleus state, which is a finite region with partially suppressed order parameter. It is also shown that for slightly lower currents the contribution of the vortex core energy is important for SP state with a vortex and it cannot be neglected for $I\gtrsim 0.6 I_{dep}$. It is demonstrated that in the film with local current concentration near the bend the energy of SP state may be much smaller than one in the straight film and it favors the effect of fluctuations in such a samples.

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arXiv2011

Photon detection by current-carrying superconducting film: A time-dependent Ginzburg-Landau approach

We study dynamics of the order parameter in a superconducting film with transport current after absorption of a single photon. The system from the time-dependent Ginszburg-Landau equation, Poisson's equation for an electrical potential and the heat diffusion equation was solved numerically. For each photon energy in the absence of fluctuations, there exists a corresponding threshold current below which the superconducting state is stable and no voltage appears between the ends of the film. At larger currents the superconducting state collapses starting from the appearance of a vortex-antivortex pair in the center of the region with suppressed superconducting order parameter which has been created by the absorbed photon. Lorentz force causes motion of these vortices that heats the film locally and gives rise to a normal domain. When biased with the fixed current, the film latches in the normal state. In the regime when current via superconductor may change, which is more relevant for experiments, the normal domain exists only for a short time resulting in the voltage pulse with the duration controlled by the kinetic inductance of the superconducting film.

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arXiv2011

Enhancement of the retrapping current of superconducting microbridges of finite length

We theoretically find that the resistance of a superconducting microbridge/nanowire {\it decreases} while the retrapping current $I_r$ for the transition to the superconducting state {\it increases} when one suppresses the magnitude of the order parameter $|Δ|$ in the attached superconducting leads. This effect is a consequence of the increased energy interval for diffusion of the 'hot' nonequilibrium quasiparticles (induced by the oscillations of $|Δ|$ in the center of the microbridge) to the leads. The effect is absent in short microbridges (with length less than the coherence length) and it is relatively weak in long microbridges (with length larger than the inelastic relaxation length of the nonequilibrium distribution function). A nonmonotonous dependence of $I_r$ on the length of the microbridge is predicted. Our results are important for the explanation of the enhancement of the critical current and the appearance of negative magnetoresistance observed in many recent experiments on superconducting microbridges/nanowires.

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arXiv2011

Heating of quasiparticles driven by oscillations of the order parameter in short superconducting microbridges

We predict 'heating' of quasiparticles driven by order parameter oscillations in the resistive state of short superconducting microbridges. The finite relaxation time of the magnitude of the order parameter $|Δ|$ and the dependence of the spectral functions both on $|Δ|$ and the supervelocity $Q$ are the origin of this effect. Our result is opposite to those of Aslamazov and Larkin (Zh. Eks. Teor. Fiz. {\bf 70}, 1340 (1976)) and Schmid, Schon and Tinkham (Phys. Rev. B {\bf 21} 5076 (1980)) where 'cooling' of quasiparticles was found.

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arXiv2008

Strong influence of a magnetic layer on the critical current of Nb bridge in finite magnetic fields due to surface barrier effect

We measured the critical current of the bilayer Nb/Co in the applied magnetic field. When the magnetic field was tilted to the axis which was perpendicular to the plane of the bilayer we observed a large difference in critical currents flowing in opposite directions. We found that the largest critical current of the bilayer exceeded the critical current of the superconductor without Co layer in a wide range of the tilted magnetic fields. The theory which takes into account the surface barrier effect for vortex entry and magnetic field of the magnetic layer gave a quantitative explanation of our experimental results.

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arXiv2007

Tunneling spectroscopy of persistent currents in superconducting microrings

It is shown that in a structure consisting of a superconducting ring-shaped electrode overlapped by a normal metal contact through a thin oxide barrier, measurements of the tunnel current in magnetic field can probe persistent currents in the ring. The effect manifests itself as periodic oscillations of the tunnel current through the junction at a fixed bias voltage as function of perpendicular magnetic field. The magnitude of oscillations depends on bias point. It reaches maximum at energy eV which is close to the superconducting gap and decreases with increase of temperature. The period of oscillations dF in units of magnetic flux is equal neither to h/e nor to h/2e, but significantly exceeds these values for larger loop circumferences. The phenomenon is explained by formation of metastable states with large vorticity. The pairing potential and the superconducting density of states are periodically modulated by the persistent currents at sub-critical values resulting in corresponding variations of the measured tunnel current.

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arXiv2007

Rearrangement of the vortex lattice due to instabilities of vortex flow

With increasing applied current we show that the moving vortex lattice changes its structure from a triangular one to a set of parallel vortex rows in a pinning free superconductor. This effect originates from the change of the shape of the vortex core due to non-equilibrium effects (similar to the mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The moving vortex creates a deficit of quasiparticles in front of its motion and an excess of quasiparticles behind the core of the moving vortex. This results in the appearance of a wake (region with suppressed order parameter) behind the vortex which attracts other vortices resulting in an effective direction-dependent interaction between vortices. When the vortex velocity $v$ reaches the critical value $v_c$ quasi-phase slip lines (lines with fast vortex motion) appear which may coexist with slowly moving vortices between such lines. Our results are found within the framework of the time-dependent Ginzburg-Landau equations and are strictly valid when the coherence length $ξ(T)$ is larger or comparable with the decay length $L_{in}$ of the non-equilibrium quasiparticle distribution function. We qualitatively explain experiments on the instability of vortex flow at low magnetic fields when the distance between vortices $a \gg L_{in} \gg ξ(T)$. We speculate that a similar instability of the vortex lattice should exist for $v>v_c$ even when $a<L_{in}$.

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arXiv2007

Symmetric and asymmetric states in mesoscopic superconducting wire in voltage driven regime

The response of a mesoscopic homogeneous superconducting wire, connected with bulk normal metal reservoirs, is theoretically investigated as function of the applied voltage. The finite relaxation length of the nonequilibrium quasiparticle distribution function $\bar{L_E}$ is included where we assumed that our wire is in the dirty limit. We found that {\it both} symmetric and asymmetric states can exist which are characterized by a stationary symmetric and asymmetric distribution of the order parameter with respect to the center of the wire. Current voltage characteristics of the wire with length $L>\bar{L_E}$ being in the symmetric state show a pronounced S-behavior. The asymmetric state may exist only for voltages larger than some critical value and coexist with the symmetric state in a finite voltage interval. For wires with $L \sim \bar{L_E}$ the asymmetric state survives up to higher values of the voltage than the symmetric one and may exist both in the voltage and the current driven regimes. We propose an experiment to observe reversible switching between those stationary symmetric and asymmetric states.

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arXiv2007

Negative magnetoresistance and phase slip process in superconducting nanowires

We argue that the negative magnetoresistance of superconducting nanowires, which was observed in recent experiments, can be explained by the influence of the external magnetic field on the critical current of the phase slip process. We show that the suppression of the order parameter in the bulk superconductors made by an external magnetic field can lead to an enhancement of both the first $I_{c1}$ and the second $I_{c2}$ critical currents of the phase slip process in nanowires. Another mechanism of an enhancement of $I_{c1}$ can come from decreasing the decay length of the charge imbalance $λ_Q$ at weak magnetic fields because $I_{c1}$ is inversely proportional to $λ_Q$. The enhancement of the first critical current leads to a larger intrinsic dissipation of the phase slip process. It suppresses the rate of both the thermo-activated and/or quantum fluctuated phase slips and results in decreasing the fluctuated resistance.

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arXiv2006

Microscopic model for multiple flux transitions in mesoscopic superconducting loops

A microscopic model is constructed which is able to describe multiple magnetic flux transitions as observed in recent ultra-low temperature tunnel experiments on an aluminum superconducting ring with normal metal - insulator - superconductor junctions [Phys. Rev. B \textbf{70}, 064514 (2004)]. The unusual multiple flux quantum transitions are explained by the formation of metastable states with large vorticity. Essential in our description is the modification of the pairing potential and the superconducting density of states by a sub-critical value of the persistent current which modulates the measured tunnel current. We also speculate on the importance of the injected non-equilibrium quasiparticles on the stability of these metastable states.

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arXiv2005

Considerable enhancement of the critical current in a superconducting film by magnetized magnetic strip

We show that a magnetic strip on top of a superconducting strip magnetized in a specified direction may considerably enhance the critical current in the sample. At fixed magnetization of the magnet we observed diode effect - the value of the critical current depends on the direction of the transport current. We explain these effects by a influence of the nonuniform magnetic field induced by the magnet on the current distribution in the superconducting strip. The experiment on a hybrid Nb/Co structure confirmed the predicted variation of the critical current with a changing value of magnetization and direction of the transport current.

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arXiv2005

Masking effect of heat dissipation on the current-voltage characteristics of a mesoscopic superconducting square with leads

A theoretical analysis based on a numerical solution of the coupled time-dependent Ginzburg-Landau and heat dissipation equations shows a strong dependence of the critical currents on the applied magnetic field in a mesoscopic square with attached contacts. In agreement with experiment we found hysteresis which are caused by a strong heat dissipation in the sample at currents close to the depairing Ginzburg-Landau current and/or the dynamics of the superconducting condensate. The theoretically obtained nonmonotonous dependence of the switching current (from superconducting to the resistive state) on the applied magnetic field, arising from the changes in the vorticity, agrees quantitatively with the experimental data. Our results show that heat dissipation leads to an increase of the hysteresis in the current-voltage characteristic and hence masks the actual dynamics of the superconducting condensate.

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arXiv2004

Long-range nonlocal flow of vortices in narrow superconducting channels

We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of inter-vortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice, even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.

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arXiv2003

Current-voltage characteristics of quasi-one-dimensional superconductors: An S-curve in the constant voltage regime

Applying a constant voltage to superconducting nanowires we find that its IV-characteristic exhibits an unusual S-behavior. This behavior is the direct consequence of the dynamics of the superconducting condensate and of the existence of two different critical currents: j_{c2} at which the pure superconducting state becomes unstable and j_{c1}<j_{c2} at which the phase slip state is realized in the system.

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arXiv2003

Condition of the occurrence of phase slip centers in superconducting nanowires under applied current or voltage

Experimental results on the phase slip process in superconducting lead nanowires are presented under two different experimental conditions: constant applied current or constant voltage. Based on these experiments we established a simple model which gives us the condition of the appearance of phase slip centers in a quasi-one-dimensional wire. It turns out that the competition between two relaxations times (relaxation time of the absolute value of the order parameter $τ_{|ψ|}$ and relaxation time of the phase of the order parameter in the phase slip center $τ_ϕ$) governs the phase slip process. Phase slip phenomena, as periodic oscillations in time of the order parameter, is possible only if the gradient of the phase grows faster than the value of the order parameter in the phase slip center, or equivalently if $τ_ϕ<τ_{|ψ|}$.

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arXiv2002

Multiple flux jumps and irreversible behavior of thin Al superconducting rings

An experimental and theoretical investigation was made of flux jumps and irreversible magnetization curves of mesoscopic Al superconducting rings. In the small magnetic field region the change of vorticity with magnetic field can be larger than unity. This behavior is connected with the existence of several metastable states of different vorticity. The intentional introduction of a defect in the ring has a large effect on the size of the flux jumps. Calculations based on the time-dependent Ginzburg-Landau model allows us to explain the experimental results semi-quantitatively.

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