arXiv subjects

Lotan Attias

Publications and source records attributed to Lotan Attias.

4 recordsLinked to original sources

arXiv2025

Superconducting diode effect in Ising superconductors

We study the superconducting diode effect (SDE) in an Ising superconductor with broken basal mirror symmetry in a parallel magnetic field. We show that in the presence of a small Rashba spin splitting, $Δ_R$, the dominant Ising spin-orbit coupling ($Δ_I >> Δ_R$) dramatically enhances the SDE efficiency compared to a Rashba superconductor with $Δ_I = 0$ and the same $Δ_R$. The suppression of the SDE for $Δ_I = 0$ at $Δ_R$ much larger than the critical temperature ($T_c$) is accidental. At $Δ_R << T_c$, the SDE efficiency is small because, to linear order, $Δ_R$ can be removed by a gauge transformation. These two factors -- the accidental suppression of the SDE at large $Δ_R$ and the systematic suppression at small $Δ_R$ -- are eliminated by Ising spin-orbit coupling. As a result, SDE efficiency is substantially enhanced for $Δ_I >> Δ_R \neq 0$.

cond-mat.supr-con↗

arXiv2024

Intrinsic anomalous Hall effect in altermagnets

We study the anomalous Hall effect arising from the altermagnetic order and spin-orbit interaction in doped FeSb$_2$. To investigate the anomalous transport, we have constructed a tight-binding model of FeSb$_2$. We separately considered the constraints imposed on the model parameters by the spin symmetry group and magnetic symmetry group at zero and finite spin-orbit interaction, respectively. The resulting model includes the effect of exchange splitting and is applicable at both zero and finite spin-orbit interaction. In the case of spin symmetry, the analysis covers the spin-only subgroup arising from collinear magnetism, as well as non-trivial symmetry elements. This allows us to explore changes in the hopping amplitudes as symmetry is reduced by spin-orbit interaction from the spin group to the magnetic group. While the anomalous Hall effect is forbidden by spin symmetry, it is allowed by the symmetries of the magnetic group. The intrinsic Hall conductivity is shown to vanish linearly with spin-orbit interaction. This non-analytic behavior is universal to altermagnets. It originates from the singularity of the Berry curvature localized along lines on a Fermi surface confined to symmetry planes. These planes host spin degeneracy protected by spin symmetry, which is lifted by spin-orbit interaction.

cond-mat.str-el↗

arXiv2017

Spectroscopy of bulk and few-layer superconducting NbSe$_2$ with van der Waals tunnel junctions

Tunnel junctions, a well-established platform for high-resolution spectroscopy of superconductors, require defect-free insulating barriers with clean engagement to metals on both sides. Extending the range of materials accessible to tunnel junction fabrication, beyond the limited selection which allows high-quality oxide formation, requires the development of alternative fabrication techniques. Here we show that van-der-Waals (vdW) tunnel barriers, fabricated by stacking layered semiconductors on top of the transition metal dichalcogenide (TMD) superconductor NbSe$_2$, sustain a stable, low noise tunneling current, and exhibit strong suppression of sub-gap tunneling. We utilize the technique to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) devices at 70 mK. The spectra exhibit two distinct energy gaps, the larger of which decreases monotonously with thickness and $T_C$, in agreement with BCS theory. The spectra are analyzed using a two-band model modified to account for depairing. We show that in the bulk, the smaller gap exhibits strong depairing in an in-plane magnetic field, consistent with a high Fermi velocity. In the few-layer devices, depairing of the large gap is negligible, consistent with out-of-plane spin-locking due to Ising spin-orbit coupling. Our results demonstrate the utility of vdW tunnel junctions in mapping the intricate spectral evolution of TMD superconductors over a range of magnetic fields.

cond-mat.supr-con↗

arXiv2017

Hard superconducting gap and vortex-state spectroscopy in NbSe$_2$ van der Waals tunnel junctions

Device-based tunnel spectroscopy of superconductors was first performed by Giaever, whose seminal work provided clear evidence for the spectral gap in the density of states (DOS) predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. Since then, tunnel-barrier-based heterostructure devices have revealed myriad physical phenomena and found a range of applications. Most of these devices rely on a limited number of oxides, which form high-quality insulating, non-magnetic barriers. These barriers, however, do not grow well on all surfaces. Promising alternatives are van der Waals (vdW) materials, ultrathin layers of which can be precisely positioned on many surfaces; they have been shown to form tunnel barriers when engaged with graphene. Here we demonstrate that vdW semiconductors MoS$_2$ and WSe$_2$ deposited on the superconductor NbSe$_2$ form high quality tunnel barriers, with transparencies in the $10^{-8}$ range. Our measurements of the NbSe$_2$ DOS at 70mK show a hard superconducting gap, and a quasiparticle peak structure with clear evidence of contributions from two bands, with intrinsic superconductivity in both bands. In both perpendicular and parallel magnetic fields, we observe a sub-gap DOS associated with vortex bound states. The linear dependence of the zero-bias signal on perpendicular field allows us to confirm the s-wave nature of superconductivity in NbSe$_2$. As vdW tunnel barriers can be deployed on many solid surfaces, they extend the range of superconducting and other materials addressable not only by high resolution tunneling spectroscopy but also non-equilibrium and/or non-local transport.

cond-mat.supr-con↗