arXiv subjects

Vadim Geshkenbein

Publications and source records attributed to Vadim Geshkenbein.

8 recordsLinked to original sources

arXiv2026

Vortex Dynamics in Magic-Angle Twisted Graphene

We use a gate-defined Josephson junction (JJ) device made from twisted-layer graphene for studying vortex dynamics in two dimensions. The JJ sensor signals the presence of individual vortices in the superconducting leads nearby the junction through shifts in the Fraunhofer interference pattern of the magnetic-field-dependent critical current $I_c(B)$ across the junction. Rapid vortex fluctuations manifest as telegraph-type noise in time traces of the junction voltage $V(t)$. Measurements of $I_c(B)$ and $V(t)$ are interpreted in terms of multi-vortex processes where fast vortex fluctuations in the leads are modulated by quasi-stationary vortices trapped in the leads. The different timescales associated with these processes allow for their disentangling and quantitative analysis. Tracking the temperature dependence of the vortex-dynamical rates between $T = 7$ mK and $T = 120$ mK, we find that the creep type vortex motion is thermally activated above $T \approx 100$ mK, while the saturation of rates below $T \approx 80$ mK is suggestive of a sharp transition to macroscopic quantum tunneling of vortices.

cond-mat.supr-con↗

arXiv2026

Vortex Tunneling and Critical State in an Oxide Heterostructure

Two-dimensional superconductors offer an excellent platform for the study of vortex matter due to their low superfluid stiffness and inability to effectively screen applied magnetic fields. Here we explore vortices in a two-dimensional superconductor formed at the surface of the complex oxide KTaO$_3$. Multiple regimes of vortex-mediated transport are identified and studied, revealing switching behaviour attributed to nucleation of individual vortices. Analysis of this regime allows us to identify the quantum tunneling of vortices, which transitions to thermally activated behaviour at elevated temperatures. Magnetic field dependence reveals rich histograms of the switching currents which we attribute to different configurations of pinned vortices.

cond-mat.supr-con↗

arXiv2026

Pearl-Vortex Tunneling in Magic-Angle Twisted Graphene

Twisted graphene provides a tunable platform for studying superconductivity in two dimensions. In the presence of electric currents and magnetic fields, vortices determine the phenomenological properties of the material. Related studies usually address bulk properties averaging over ensembles of vortices. Here, we employ a gate-defined Josephson junction as a single-vortex sensor, enabling direct access to individual vortex dynamical events. Our measurements reveal that, at elevated temperatures (T > 100 mK), vortices enter the superconducting leads via classical thermal activation over energy barriers. At lower temperatures (T < 90 mK), we observe macroscopic quantum tunneling through these barriers. The data are consistent with a sharp, first-order type quantum-to-classical transition. From our measurements, we extract vortex entry and exit energy barriers on the order of a few Kelvin and estimate the barrier thickness to be approximately 100 nm, corresponding to about one tenth of the device width.

cond-mat.supr-con↗

arXiv2024

Experimental detection of vortices in magic-angle graphene

The tunability of superconducting magic-angle twisted-layer graphene films elevates this material system to a promising candidate for superconducting electronics. We implement a gate-tuned Josephson junction in a magic-angle twisted four-layer graphene film. Field-dependent measurements of the critical current show a Fraunhofer-like pattern that differs from the standard pattern with characteristics typical for a weak transverse screener. We observe sudden shifts associated with vortices jumping into and out of the leads. By tuning the leads to the edge of the superconducting dome, we observe fast switching between superconducting and normal states, an effect associated with vortex dynamics. Time-dependent measurements provide us with the vortex energy scale and an estimate for the London penetration depth, in agreement with recent kinetic inductance measurements on twisted graphene films. Our results prove the utility of our junction as a sensor for vortex detection, allowing us to extract fundamental properties of the 2D superconductor.

cond-mat.mes-hall↗

arXiv1996

Electrostatics of Vortices in Type II Superconductors

In a type II superconductor the gap variation in the core of a vortex line induces a local charge modulation. Accounting for metallic screening, we determine the line charge of individual vortices and calculate the electric field distribution in the half space above a field penetrated superconductor. The resulting field is that of an atomic size dipole ${\bf d} \sim e a_{\rm B} {\bf {\hat z}}$, $a_{\rm B} = \hbar^2/m e^2$ is the Bohr radius, acting on a force microscope in the pico to femto Newton range.

cond-mat↗

arXiv1995

Low-Field Phase Diagram of Layered Superconductors: The Role of Electromagnetic Coupling

We determine the position and shape of the melting line in a layered superconductor taking the electromagnetic coupling between layers into account. In the limit of vanishing Josephson coupling we obtain a new generic reentrant low-field melting line. Finite Josephson coupling pushes the melting line to higher temperatures and fields and a new line shape $B_{\rm m} \propto (1-T/T_c)^{3/2}$ is found. We construct the low-field phase diagram including melting and decoupling lines and discuss various experiments in the light of our new results.

supr-con↗

arXiv1995

Vortex Dynamics and the Hall-Anomaly: a Microscopic Analysis

We present a microscopic derivation of the equation of motion for a vortex in a superconductor. A coherent view on vortex dynamics is obtained, in which {\it both} hydrodynamics {\it and} the vortex core contribute to the forces acting on a vortex. The competition between these two provides an interpretation of the observed sign change in the Hall angle in superconductors with mean free path $l$ of the order of the coherence length $ξ$ in terms of broken particle-hole symmetry, which is related to details of the microscopic mechanism of superconductivity.

supr-con↗

arXiv1995

Vortex Entanglement and Broken Symmetry

Based on the London approximation, we investigate numerically the stability of the elementary configurations of entanglement, the twisted-pair and the twisted-triplet, in the vortex-lattice and -liquid phases. We find that, except for the dilute limit, the twisted-pair is unstable and hence irrelevant in the discussion of entanglement. In the lattice phase the twisted-triplet constitutes a metastable, confined configuration of high energy. Loss of lattice symmetry upon melting leads to deconfinement and the twisted-triplet turns into a low-energy helical configuration.

cond-mat↗