High-$T_c$ Superconductivity in FeSe at High Pressure: Dominant Hole Carriers and Enhanced Spin Fluctuations

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures ($T_c$). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-$T_c$ superconductors raises a fundamental question of whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-$T_c$ phase induced by pressure in bulk FeSe from magnetotransport measurements and first-principles calculations. With increasing pressure, the low-$T_c$ superconducting phase transforms into the high-$T_c$ phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-$T_c$ systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-$T_c$ phase. These results in FeSe highlight similarities with high-$T_c$ phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity.

Figure 1

Phase diagram and Hall coefficient of FeSe. Temperature-pressure phase diagram of FeSe is superimposed by a contour plot of Hall coefficient $R_H$, which is defined as the field derivative of ${\rho }_{xy}$, $R_H\equiv d{\rho }_{xy}/dH$, at the zero-field limit at each temperature and pressure. The structural (nematic) transition ($T_s$), pressure-induced magnetic transition ($T_m$), and superconducting (SC) transition ($T_c$) have been determined by the resistivity measurements under pressure [18]. The dashed curve is a guide for the eyes.

Figure 2

Field dependence of Hall resistivity ${\rho }_{xy}(H)$ of a FeSe single crystal measured under various temperatures at different pressures.

Figure 3

Zero-field resistivity curve $\rho (T)$ (red, left axis) and the temperature dependence of the Hall coefficient $R_H$ (blue, right axis) defined as the field derivative of ${\rho }_{xy}$, $R_H\equiv d{\rho }_{xy}/dH$, at the zero-field limit at each pressure. The vertical dotted lines in (a)–(e) mark the nematic order transition at $T_s$ and the magnetic order at $T_m$; the horizontal dotted lines in all figures indicated the zero $R_H$. The resistivity difference $\mathrm{\Delta }\rho$ (scaled by a factor of 2) was obtained by subtracting from the measured resistivity $\rho (T)$ the linear-fitting curve at high temperatures as indicated by the broken line (green).

Figure 4

(a) Results of first-principles calculations for the Fermi surface of tetragonal FeSe at 6 GPa, viewed along the $c$ axis (left) and at an angle (right), with hole bands shown in blue and electron bands shown in red. (b) Calculated 200 K Hall coefficient as a function of pressure for field along the $c$ axis ($z$, $R_{xy}$) corresponding to the experimental geometry, and for field in plane ($x/y$, $R_{xz}$).