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R. G. Mints

Publications and source records attributed to R. G. Mints.

At least 19 recordsLinked to original sources
arXiv2015

Effective model for a short Josephson junction with a phase discontinuity

We consider a short Josephson junction with a phase discontinuity $κ$ created, e.g., by a pair of tiny current injectors, at some point $x_0$ along the length of the junction. We derive the effective current-phase relation (CPR) for the system as a whole, i.e., reduce it to an effective point-like junction. From the effective CPR we obtain the ground state of the system and predict the dependence of its critical current on $κ$. We show that in a large range of $κ$ values the effective junction behaves as a $φ_0$ Josephson junction, i.e., has a unique ground state phase $φ_0$ within each $2π$ interval. For $κ\approxπ$ and $x_0$ near the middle of the junction one obtains a $φ_0\pmφ$ junction, i.e., the Josephson junction with degenerate ground state phase $φ_0\pmφ$ within each $2π$ interval. Further, in view of possible escape experiments especially in the quantum domain, we investigate the scaling of the energy barrier and eigenfrequency close to the critical currents and predict the behavior of the escape histogram width $σ(κ)$ in the regime of the macroscopic quantum tunneling.

cond-mat.supr-con↗
arXiv2015

Thermal and electromagnetic properties of Bi$_2$Sr$_2$CaCu$_2$O$_8$ intrinsic Josephson junction stacks studied via one-dimensional coupled sine-Gordon equations

We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a $300\,μ\mathrm{m}$ long intrinsic Josephson junction stack consisting of $N = 700$ junctions. The junctions in the stack are combined to $M$ segments where we assume that inside a segment all junctions behave identically. Most simulations are for $M = 20$. For not too high bath temperatures there is the appearence of a hot spot at high bias currents. In terms of electromagnetic properties, robust standing wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to $π$ kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with $M$, on the basis of simulations with $M$ between 10 and 350.

cond-mat.supr-con↗
arXiv2014

Effect of current injection into thin-film Josephson junctions

New thin-film Josephson junctions have recently been tested in which the current injected into one of the junction banks governs Josephson phenomena. One thus can continuously manage the phase distribution at the junction by changing the injected current. A method of calculating the distribution of injected currents is proposed for a half-infinite thin-film strip with source-sink points at arbitrary positions at the film edges. The strip width $W$ is assumed small relative to $Λ=2λ^2/d$, $λ$ is the bulk London penetration depth of the film material, $d$ is the film thickness.

cond-mat.supr-con↗
arXiv2014

Interaction of Josephson junction and distant vortex in narrow thin-film superconducting strips

The phase difference between the banks of an edge-type planar Josephson junction crossing the narrow thin-film strip depends on wether or not vortices are present in the junction banks. For a vortex close to the junction this effect has been seen by Golod, Rydh, and Krasnov, \prl {\bf 104}, 227003 (2010), who showed that the vortex may turn the junction into $π$-type. It is shown here that even if the vortex is far away from the junction, it still changes the 0-junction to $π$-junction when situated close to the strip edges. Within the approximation used, the latter effect is independent of the vortex-junction separation, a manifestation of topology of the vortex phase which extends to macroscopic distances of superconducting coherence.

cond-mat.supr-con↗
arXiv2013

Phase retrapping in a pointlike $φ$ Josephson junction: the Butterfly effect

We consider a $φ$ Josephson junction, which has a bistable zero-voltage state with the stationary phases $ψ=\pmφ$. In the non-zero voltage state the phase "moves" viscously along a tilted periodic double-well potential. When the tilting is reduced quasistatically, the phase is retrapped in one of the potential wells. We study the viscous phase dynamics to determine in which well ($-φ$ or $+φ$) the phase is retrapped for a given damping, when the junction returns from the finite-voltage state back to zero-voltage state. In the limit of low damping the $φ$ Josephson junction exhibits a butterfly effect --- extreme sensitivity of the destination well on damping. This leads to an impossibility to predict the destination well.

cond-mat.supr-con↗
arXiv2013

Modeling the linewidth dependence of coherent terahertz emission from intrinsic Josephson junction stacks in the hot-spot regime

Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions which are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation.

cond-mat.supr-con↗
arXiv2013

Hot-spot formation in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8

We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8 (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. The formation of hot spots in this system is shown to arise mainly from the strongly negative temperature coefficient of the c-axis resistivity of BSCCO at low temperatures. This leads to situations where the maximum temperature in the hot spot can be below or above the superconducting transition temperature Tc. The numerical simulations are in good agreement with the experimental observations.

cond-mat.supr-con↗
arXiv2012

Magnetic field dependence of the critical current in YBa_2Cu_3O_{7-δ}/Au/Nb ramp-zigzag Josephson junctions

We study the critical current I_c dependence on applied magnetic field H for multifacet YBa_2Cu_3O_{7-δ}-Au-Nb ramp-type zigzag Josephson junctions. For many experiments one would like to apply a homogeneous field in the junction plane. However, even tiny misalignments can cause drastic deviations from homogeneity. We show this explicitly by measuring and analyzing I_c vs. H for an 8 facet junction, forming an array of 4\times(0-π)-segments. The ramp angle is θ_r=8^\circ. The facet width is 10\,\mum. H is applied under different angles θrelative to the substrate plane and different angles ϕrelative to the in-plane orientation of the zigzags. We find that a homogeneous flux distribution is only achieved for an angle θ_h\approx 1^\circ - 2^\circ and that even a small misalignment \sim 0.1^\circ relative to θ_h can cause a substantial inhomogeneity of the flux density inside the junction, drastically altering its I_c vs. H interference pattern. We also show, that there is a dead angle θ^*_d relative to θ_h of similar magnitude, where the average flux density completely vanishes.

cond-mat.supr-con↗
arXiv2012

Josephson junction with magnetic-field tunable current-phase relation

We consider a 0-$π$ Josephson junction consisting of asymmetric 0 and $π$ regions of different lengths $L_0$ and $L_π$ having different critical current densities $j_{c,0}$ and $j_{c,π}$. If both segments are rather short, the whole junction can be described by an \emph{effective} current-phase relation for the spatially averaged phase $ψ$, which includes the usual term $\propto\sin(ψ)$, a \emph{negative} second harmonic term $\propto\sin(2ψ)$ as well as the unusual term $\propto H \cosψ$ tunable by magnetic field $H$. Thus one obtains an electronically tunable current-phase relation. At H=0 this corresponds to the $φ$ Josephson junction.

cond-mat.supr-con↗
arXiv2012

Experimental evidence of a ϕ Josephson junction

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase ψ=\pmϕ. The value of ϕ can be chosen by design in the interval 0<ϕ<π. The junctions used in our experiments are fabricated as 0-π Josephson junctions of moderate normalized length with asymmetric 0 and π regions. We show that (a) these ϕ Josephson junctions have two critical currents, corresponding to the escape of the phase ψ from -ϕ and +ϕ states; (b) the phase ψ can be set to a particular state by tuning an external magnetic field or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions.

cond-mat.supr-con↗
arXiv2011

Josephson junction with magnetic-field tunable ground state

We consider an asymmetric 0-pi Josephson junction consisting of 0 and pi regions of different lengths L_0 and L_pi. As predicted earlier this system can be described by an effective sine-Gordon equation for the spatially averaged phase psi so that the effective current-phase relation of this system includes a \emph{negative} second harmonic ~sin(2 psi). If its amplitude is large enough, the ground state of the junction is doubly degenerate psi=\pmvarphi, where varphi depends on the amplitudes of the first and second harmonics. We study the behavior of such a junction in an applied magnetic field H and demonstrate that H induces an additional term ~H cos(psi) in the effective current-phase relation. This results in a non-trivial ground state \emph{tunable} by magnetic field. The dependence of the critical current on H allows for revealing the ground state experimentally.

cond-mat.supr-con↗
arXiv2010

Interference patterns of multifacet 20x(0-pi-) Josephson junctions with ferromagnetic barrier

We have realized multifacet Josephson junctions with periodically alternating critical current density (MJJs) using superconductor-insulator-ferromagnet-superconductor heterostructures. We show that anomalous features of critical current vs. applied magnetic field, observed also for other types of MJJs, are caused by a non-uniform flux density (parallel to the barrier) resulting from screening currents in the electrodes in the presence of a (parasitic) off-plane field component.

cond-mat.supr-con↗
arXiv2009

Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and πsegments

Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness $d_F$ of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to $π$ ($π$ junction). When 0 and $π$ segments are joined to form a "0-$π$ junction", spontaneous supercurrents around the 0-$π$ boundary can appear. Here we report on the visualization of supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS) junctions by low-temperature scanning electron microscopy (LTSEM). We discuss data for rectangular 0, $π$, 0-$π$, 0-$π$-0 and 20 \times 0-$π$ junctions, disk-shaped junctions where the 0-$π$ boundary forms a ring, and an annular junction with two 0-$π$ boundaries. Within each 0 or $π$ segment the critical current density is fairly homogeneous, as indicated both by measurements of the magnetic field dependence of the critical current and by LTSEM. The $π$ parts have critical current densities $j_c^π$ up to $35\units{A/cm^2}$ at $T = 4.2\units{K}$, which is a record value for SIFS junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology is capable to produce Josephson devices with a unique topology of the 0-$π$ boundary.

cond-mat.supr-con↗
arXiv2008

Josephson junctions in narrow thin-film strips

We study the field dependence of the maximum supercurrent in narrow edge-type thin-film Josephson junctions. It is assumed that the junction extends across thin-film strip of width W that is much less than the Pearl length; the film thickness is much less than the London penetration depth. We calculate the maximum supercurrent within nonlocal Josephson electrodynamics, which takes into account the stray fields affecting tunneling currents. In the case when W is much less than the thin-film Josephson length, the phase difference along the junction depends only on the junction geometry and the applied field, but is independent of the Josephson critical current density, i.e., it is universal. Zeros of the maximum supercurrent are equidistant only in large fields (unlike the case of junctions with bulk banks); they are spaced by a field that is much smaller than the one of bulk junctions. Peaks of the maximum supercurrent decrease inversely proportional to the square root of the applied field, i.e., slower than 1/H for the bulk.

cond-mat.supr-con↗
arXiv2007

High-field vortices in Josephson junctions with alternating critical current density

We study long Josephson junctions with the critical current density alternating along the junction. New equilibrium states, which we call the field synchronized or FS states, are shown to exist if the applied field is from narrow intervals centered around equidistant series of resonant fields, $H_m$. The values of $H_m$ are much higher than the flux penetration field, $H_s$. The flux per period of the alternating critical current density, $ϕ_i$, is fixed for each of the FS states. In the $m$-th FS state the value of $ϕ_i$ is equal to an integer amount of flux quanta, $ϕ_i =mϕ_0$. Two types of single Josephson vortices carrying fluxes $ϕ_0$ or/and $ϕ_0/2$ can exist in the FS states. Specific stepwise resonances in the current-voltage characteristics are caused by periodic motion of these vortices between the edges of the junction.

cond-mat.supr-con↗
arXiv2007

Shapiro steps in Josephson junctions with alternating critical current density

We treat theoretically Shapiro steps in tunnel Josephson junctions with spatially alternating critical current density. Explicit analytical formulas for the width of the first integer (normal) and half-integer (anomalous) Shapiro steps are derived for short junctions. We develop coarse-graining approach, which describes Shapiro steps in the voltage-current curves of the asymmetric grain boundaries in YBCO thin films and different superconductor-ferromagnet-superconductor Josephson-type heterostructures.

cond-mat.supr-con↗
arXiv2005

Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T

We studied the tunneling density of states in YBCO films under strong currents flowing along node directions. The currents were induced by fields of up to 32.4T parallel to the film surface and perpendicular to the $CuO_{2}$ planes. We observed a remarkable change in the tunneling conductance at high fields where the gap-like feature shifts discontinuously from 15meV to a lower bias of 11meV, becoming more pronounced as the field increases. The effect takes place in increasing fields around 9T and the transition back to the initial state occurs around 5T in decreasing fields. We argue that this transition is driven by surface currents induced by the applied magnetic field.

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