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Register a new userEvolution of transport properties in FeSe thin flakes with thickness approaching the two-dimensional limit
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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 $dle$ 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.
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In contrast to bulk FeSe superconductor, heavily electron-doped FeSe-derived superconductors show relatively high Tc without hole Fermi surfaces and nodal superconducting gap structure, which pose great challenges on pairing theories in the iron-based superconductors. In the heavily electron-doped FeSe-based superconductors, the dominant factors and the exact working mechanism that is responsible for the high Tc need to be clarified. In particular, a clean control of carrier concentration remains to be a challenge for revealing how superconductivity and Fermi surface topology evolves with carrier concentration in bulk FeSe. Here, we report the evolution of superconductivity in the FeSe thin flake with systematically regulated carrier concentrations by liquid-gating technique. High-temperature superconductivity at 48 K can be achieved only with electron doping tuned by gate voltage in FeSe thin flake with Tc less than 10 K. This is the first time to achieve such a high temperature superconductivity in FeSe without either epitaxial interface or external pressure. It definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with Tc as high as 48 K in bulk FeSe. Intriguingly, our data also indicates that the superconductivity is suddenly changed from low-Tc phase to high-Tc phase with a Lifshitz transition at certain carrier concentration. These results help us to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on further pursuit of higher Tc in these materials.
Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate dielectric, we have successfully tuned carrier density in FeSe$_{0.5}$Te$_{0.5}$ thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe$_{0.5}$Te$_{0.5}$. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li$^+$ ions intercalation into the FeSe$_{0.5}$Te$_{0.5}$. It indicates that a systematic change of electronic properties in FeSe$_{0.5}$Te$_{0.5}$ arises from the electrostatic doping induced by the accumulation of Li$^+$ ions at the interface between FeSe$_{0.5}$Te$_{0.5}$ and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which $T_c$ is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions.
We performed systematic studies on the transport properties of FeSe thin films with controlled degrees of in-plane lattice strain, including both tensile and compressive strains. The superconducting transition temperature, $T_{mathrm c}$, increases up to 12 K for films with compressive strain while the superconductivity disappears for films with large tensile strains. On the other hand, the structural (nematic) transition temperature, $T_{mathrm s}$, slightly decreases as the in-plane strain is more compressive. This suggests that the structural transition can be extinguished by a smaller amount of Te substitution for films with more compressive strain, which may lead to higher $T_{mathrm c}$ in FeSe$_{1-x}$Te$_x$. It was also found that the carrier densities evaluated via transport properties increase as the in-plane strain becomes more compressive. A clear correlation between $T_{mathrm c}$ and the carrier densities suggests that it is essential to increase carrier densities for the $T_{mathrm c}$ enhancement of iron chalcogenides.
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 16T and temperatures down to 2K 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. In the thin limit we detect signatures of a crossover towards two-dimensional behaviour from the observation of the vortex-antivortex unbinding transition and strongly enhanced anisotropy. Our study provides detailed insights into the evolution of the superconducting properties from three-dimensional bulk behaviour towards the two-dimensional limit of FeSe in the absence of a dopant substrate.
We develop a novel field effect transistor (FET) device using solid ion conductor (SIC) as a gate dielectric, and we can tune the carrier density of FeSe by driving lithium ions in and out of the FeSe thin flakes, and consequently control the material properties and its phase transitions. A dome-shaped superconducting phase diagram was mapped out with increasing Li content, with $T_c$ $sim$ 46.6 K for the optimal doping, and an insulating phase was reached at the extremely overdoped regime. Our study suggests that, using solid ion conductor as a gate dielectric, the SIC-FET device can achieve much higher carrier doping in the bulk, and suit many surface sensitive experimental probes, and can stabilize novel structural phases that are inaccessible in ordinary conditions.
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