Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

Abstract

FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while its monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate.

Fragile pressure-induced magnetism in FeSe superconductors with a thickness reduction

“…In contrast to the thickness dependence of physical properties in many 2D materials [59][60][61], a reduction of thickness to hundreds of nanometers can already alter the physical properties of FeSe under pressure. Based on our T -p phase diagrams and previous results [28,30,36,44], we propose a schematic pressure-thickness phase diagram at T = 0 K showing the nematic and SDW phase boundaries, as shown in Fig. 4(g).…”

“…In addition to pressure and chemical substitution, the superconductivity in FeSe can also be tuned by sample thickness [26,[33][34][35][36][37][38][39][40]. Monolayer FeSe grown on SrTiO 3 substrate exhibits a remarkably high T c over 100 K [35], much higher than in the bulk sample under pressure.…”

“…Monolayer FeSe grown on SrTiO 3 substrate exhibits a remarkably high T c over 100 K [35], much higher than in the bulk sample under pressure. On the other hand, thin FeSe films cleaved from single crystals exhibit suppression of T c and T s as the thickness is reduced [26,[36][37][38]. Since the magnetically ordered phase in the bulk FeSe only appears with pressure applied, whether thickness reduction alters the magnetism in thin FeSe remains unknown yet an important topic.…”

Electronic Nematic States Tuned by Isoelectronic Substitution in Bulk FeSe1−xSx

“…ARPES is highly suited for the exploration of FeSe 1−x S x as these systems can be easily cleaved in-situ due to weak van der Waals bonds between the FeSe layers which also enable the development of devices of two-dimensional superconductors by mechanical exfoliation [22,26]. Furthermore, ARPES studies can evaluate the role of orbital character on the nematic electronic states, as the matrix element effects affect the intensity of different bands with different orbital character.…”

“…In a monolayer on FeSe, on a suitable substrate, the transition temperatures reach record values toward 65 K; a strong interfacial electron-phonon coupling and a charge transfer through the interface is proposed as a source for this twodimensional high-T c superconductivity [23,24]. This effect is surprisingly absent in a monolayer of FeS [25] and in the absence of substrate in thin flakes of FeSe [26].…”