High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ under high pressure

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline ${\mathrm{KFe}}_2{\mathrm{As}}_2$ using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low-pressure regime, we show that the superconducting transition temperature $T_c$ finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum $T_c$ and reversal of pressure dependence at this pressure. Upon applying higher pressures, this $T_c$ is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band structure calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-$T_c$ superconductivity in iron pnictide superconductors.

Figure 1

Enhancement of $T_c$ under pressure. (a) Temperature dependence of resistance for ${\mathrm{KFe}}_2{\mathrm{As}}_2$ under pressure of sample no. 1. (b) Low-temperature zoom of resistivity measured at ambient pressure (without DAC) of sample no. 2 with $\text{RRR}\sim 1900$. (c) Suppression of superconducting transition at 14.4 GPa by applying magnetic field of 15 T.

Figure 2

$T-P$ phase diagram of ${\mathrm{KFe}}_2{\mathrm{As}}_2$, with two distinct superconducting phases occupying different pressure regimes. The superconducting transition temperature $T_c$ of SC1 phase (solid circles) is determined from the zero resistance state, and $T_c^{onset}$ (filled squares) in the high-pressure SC2 phase determined from the onset of resistive transition for samples no. 1 and no. 2 (open symbols) in Figs. 1 and 1, respectively. The critical pressure $P_c$ is defined as the pressure where the sign reversal of $T_c$ (triangles) in the low-pressure phase dependence has been observed previously [12]. The second superconducting phase SC2 appears above a structural collapse of the tetragonal unit cell at 13 GPa (see text). Blue dotted line is the pressure derivative of $T_c$ obtained from the Ehrenfest relation (see text).

Figure 3

(a) Pressure dependence of the residual resistance $R_0$ and the residual resistance ratio $R$(300 K)/$R_0$ for ${\mathrm{KFe}}_2{\mathrm{As}}_2$ (sample no. 1). Inset shows the schematic structural change from tetragonal to collapsed tetragonal phase. (b) Hall coefficient obtained from the low field Hall resistivity at 10 K (12 K for 14.4 GPa) as a function of pressure. Inset: typical Hall resistivity data at 10 K below and above the structural collapse. Dashed line indicates deviation from linear single-carrier expectation. (c) Room-temperature x-ray diffraction data, showing sudden decrease of the $c$-axis lattice parameter and increase of $a$ around 16 GPa, associated with a collapsed tetragonal transition. Open symbols at ambient pressure are obtained from Ref. [24]. Inset: pressure dependence of $c/a$.

Figure 4

Theoretical band calculations in ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Fermi surfaces in (a) tetragonal (T) phase (0 GPa), (b) collapsed tetragonal (cT) phase (20 GPa) obtained from the optimized arsenic height $z_{\mathrm{opt}}=0.366$, and (c) from preliminarily experimental arsenic height $z_{\exp }=0.386$. Density of states in (d) T phase and cT phase obtained from (e) $z_{\mathrm{opt}}$ and (f) from $z_{\exp }$.