Uniaxial Compression-Induced Anisotropy and Electronic Dimensionality in the Iron-Based Superconductor FeSe

Abstract

The evolution of the superconducting transition temperature ($T_c$) in FeSe was investigated under in-plane, out-of-plane, and hydrostatic compression. For pressures up to 0.6 GPa, $T_c$ increases regardless of the compression mode, consistent with the suppression of nematic ordering. However, once nematicity is suppressed, $T_c$ exhibits a striking directional dependence: out-of-plane compression shows behavior similar to the hydrostatic case, with a sharp increase in $T_c$, whereas in-plane compression suppresses superconductivity. First-principles calculations suggest that in-plane compression shifts a hybridized band of Se $p_z$ and Fe $d_{x^2-y^2}$ character so that it crosses the Fermi level along the $Γ$-Z direction, leading to the emergence of an additional metallic band. This leads to an increased three-dimensionality of the electronic structure and may be interpreted as a possible Lifshitz-type change in the Fermi surface.

Figures (13)

• Figure 1: (a) Structure of tetragonal FeSe. Picture of the gasket hole showing the setting of the single crystals for the (b) in-plane and (c) out-of-plane compression measurements, and (d-e) the respective schematics of their impact on the unit cell.
• Figure 2: Temperature dependence of the in-phase magnetization $m'$ of FeSe for pressure up to 3 GPa using Pb as a manometer in the cases of (a) hydrostatic pressure conditions, (b) out-of-plane uniaxial compression, and (c) in-plane uniaxial compression. The small colored arrows indicate the temperature at which the magnetic shielding anomaly of FeSe appears. The magnetic signal from the cell without samples was subtracted, resulting in flat curves over $T_c$. The sharp signal at low temperature, indicated by a black dashed arrow, is the magnetic shielding signal of Pb. Pressure calibration details are shown in Fig. S2.
• Figure 3: (a) Experimental pressure dependence of the superconducting temperature $T_c$ obtained from Fig. 2 and (b) theoretical pressure dependence of the tetragonal distortion $a/c$ of FeSe for the different compression modes. Blue triangles correspond to the hydrostatic pressure case, black diamonds to the out-of-plane uniaxial compression, and red circles to the in-plane uniaxial compression.
• Figure 4: Fat-band representations based on maximally localized Wannier functions for FeSe under (a) ambient pressure, (b) hydrostatic pressure, (c) in-plane uniaxial compression, and (d) out-of-plane uniaxial compression with an applied pressure of 4 GPa. The Fermi energy is set to zero. (e) Enlarged view of the band structure near the Fermi level under in-plane uniaxial compression as a function of pressure. (f) Pressure dependence of the k points where the bands cross the Fermi level for the three compression modes along the M–$\Gamma$–Z–A path. Detailed calculations are shown in Figs. S3-S9.
• Figure S1: X-ray diffraction patterns of the FeSe single crystals used for the magnetization measurements as set in the gasket hole of the diamond anvil cell. A simulation pattern for the tetragonal $P4/nmm$ phase of FeSe is represented in red. Only the (00$l$) peaks are visible, confirming the orientation of the single crystals.