arXiv subjects

Eyal Walach

Publications and source records attributed to Eyal Walach.

5 recordsLinked to original sources

arXiv2025

Quantum Phases of a Strongly Disordered Two-Legged Josephson Ladder

Disordered superconductors in low dimensions provide an exemplary manifestation for the role of quantum fluctuations in a many-body system. Specifically in Josephson arrays with comparable Josephson and charging energies ($E_J\sim E_C$), disorder tends to change the nature of the paradigmatic Superconductor-Insulator Transition (SIT) and potentially leads to formation of multiple distinct phases. We address this problem in a model of a two-legged Josephson ladder subjected to a wide spatial distribution of its parameters along the legs. In contrast, we assume the system to have a perfect $\mathbb{Z}_2$ symmetry to interchange between the legs, and investigate the effects of spatial randomness which preserves this symmetry in the strong-disorder limit. To this end, we apply a strong randomness real-space renormalization group technique and explore the resulting phase diagram. We identify three disorder-dominated phases, including an intermediate phase between a disordered superconductor and a disordered insulator. The latter insulating phase can be mapped to a XY spin-chain in a spin glass phase, while the intermediate phase turns out to be a Bose glass.

cond-mat.supr-con↗

arXiv2025

Energy Gap Modulation in Proximitized Superconducting Puddles of Graphene

We investigated proximity-induced superconductivity in a graphene-insulating InO bilayer system through gate-controlled transport measurements. Distinct oscillations in the differential conductance are observed across both the electron and hole doping regimes, with oscillation amplitudes increasing as the chemical potential moves away from the Dirac point. These findings are explained using a theoretical model of a normal-superconductor-normal (NSN) junction, which addresses reflection and transmission probabilities at normal incidence. From this model, we extract key parameters for the proximitized graphene, including the superconducting energy gap Delta and the effective length scale Ls of the superconducting regions. Near the Dirac point, we observe a minimal Ls and a maximal Delta, aligning with the theory that the gap in strongly disordered superconductors increases as the coherence length of localized pairs decreases. This suggests that spatial confinement in a low-density superconductor leads to an effective increase in the superconducting gap.

cond-mat.supr-con↗

arXiv2022

Enhancement of Superconductivity upon reduction of carrier density in proximitized graphene

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

cond-mat.supr-con↗

arXiv2021

Quantum Phases of a Weakly Disordered Josephson Ladder

The interplay of interactions and disorder in low-dimensional superconductors supports the formation of multiple quantum phases, as possible instabilities of the Superconductor-Insulator Transition (SIT) at a singular quantum critical point. We explore a one-dimensional model which exhibits such variety of phases in the strongly quantum fluctuations regime. Specifically, we study the effect of weak disorder on a two-leg Josephson ladder with comparable Josephson and charging energies ($E_J$~$E_C$). An additional key feature of our model is the requirement of perfect $\mathbb{Z}_2$-symmetry, respected by all parameters including the disorder. Using a perturbative renormalization-group (RG) analysis, we derive the phase diagram and identify at least one intermediate phase between a full-fledged superconductor and a disorder-dominated insulator. Most prominently, for repulsive interactions on the rungs we identify two distinct mixed phases: in both of them the longitudinal charge mode is a gapless superconductor, however one phase exhibits a dipolar charge density order on the rungs, while the other is disordered. This latter phase is characterized by coexisting superconducting (phase-locked) and charge-ordered rungs, and encompasses the potential of evolving into a Griffith's phase characteristic of the random-field Ising model in the strong disorder limit.

cond-mat.supr-con↗

arXiv2020

Superconducting Dirac point in proximetized graphene

Two-dimensional (2D) materials, composed of single atomic layers, have attracted vast research interest since the breakthrough discovery of graphene. One major benefit of such systems is the simple ability to tune the chemical potential by back-gating, in-principle enabling to vary the Fermi level through the charge neutrality point, thus tuning between electron and hole doping. For 2D Superconductors, this means that one may potentially achieve the strongly-coupled superconducting regime described by Bose Einstein Condensation physics of small bosonic tightly bound electron pairs. Furthermore, it should be possible to access both electron and hole based superconductivity in a single system. However, in most 2D materials, an insulating gap opens up around the charge neutrality point, thus preventing approach to this regime. Graphene is unique in this sense since it is a true semi-metal in which the un-gapped Dirac point is protected by the symmetries. In this work we show that single layer graphene, in which superconducting pairing is induced by proximity to regions of a low density superconductor, can be tuned from hole to electron superconductivity through the strong coupling regime. We study, both experimentally and theoretically, the vicinity of this "Superconducting Dirac point" and find an unusual situation where reflections at interfaces between normal and superconducting regions within the graphene, suppress the conductance and, at the same time, Andreev reflections maintain a large phase breaking length. In addition, the Fermi level can be adjusted so that the momentum in the normal and superconducting regimes perfectly match giving rise to ideal Andreev reflection processes.

cond-mat.supr-con↗