Evolution of transport properties in FeSe thin flakes with thickness approaching the two-dimensional limit

TL;DR

This study investigates how the superconducting and nematic properties of FeSe thin flakes evolve as their thickness approaches the two-dimensional limit, revealing suppressed transition temperatures, 2D superconductivity signatures, and a linear relation between $T_c$ and inverse thickness.

Contribution

It provides a systematic analysis of the evolution of superconductivity and nematicity in FeSe flakes down to bilayer, highlighting 2D superconductivity and related phenomena.

Findings

Superconducting transition temperature $T_c$ decreases with thickness.

Signatures of Berezinskii-Kosterlitz-Thouless transition observed in ultrathin flakes.

$T_c$ shows linear dependence on inverse thickness for $d \,\le\, 13$ nm.

Abstract

Electronic properties of FeSe can be tuned by various routes. Here, we present a comprehensive study on the evolution of the superconductivity and nematicity in FeSe with thickness from bulk single crystal down to bilayer ($\sim$ 1.1 nm) through exfoliation. With decreasing flake thickness, both the structural transition temperature $T_{\rm s}$ and the superconducting transition temperature $T_{\rm c}^{\rm zero}$ are greatly suppressed. The magnetic field ($B$) dependence of Hall resistance $R_{xy}$ at 15 K changes from $B$-nonlinear to $B$-linear behavior up to 9 T, as the thickness ($d$) is reduced to 13 nm. $T_{\rm c}$ is linearly dependent on the inverse of flake thickness (1/$d$) when $d\le$ 13 nm, and a clear drop of $T_{\rm c}$ appears with thickness smaller than 27 nm. The $I$-$V$ characteristic curves in ultrathin flakes reveal the signature of Berezinskii-Kosterlitz-Thouless (BKT) transition, indicating the presence of two-dimensional superconductivity. Anisotropic magnetoresistance measurements further support 2D superconductivity in few-layer FeSe. Increase of disorder scattering, anisotropic strains and dimensionality effect with reducing the thickness of FeSe flakes, might be taken into account for understanding these behaviors. Our study provides systematic insights into the evolution of the superconducting properties, structural transition and Hall resistance of a superconductor FeSe with flakes thickness and provides an effective way to find two-dimensional superconductivity as well as other 2D novel phenomena.