Pressure-induced superconductivity in trigonal layered ${\mathrm{PtBi}}_2$ with triply degenerate point fermions

Materials with nonsaturating, extremely large magnetoresistance (XMR) attract continuous research interest due to their great potential in the fields of spintronics and magnetic sensors/switches. Here, we report the discovery of pressure-driven superconductivity (SC) in trigonal ${\mathrm{PtBi}}_2$, a semimetal material with nontrivial triply degenerate point fermions exhibiting a nonsaturating XMR effect, above a critical pressure of $\sim 5–6$ GPa. The superconducting transition onset temperature is $\sim 2$ K and displays a very weak pressure dependence across a broad pressure range of $\sim 5–48$ GPa. The lattice structure of trigonal ${\mathrm{PtBi}}_2$ is stable against pressure at least to 12.9 GPa. Around the same critical pressure where the SC emerges, the Hall coefficient varies violently and shows a sign crossover from low pressure positive (hole dominated) to high pressure negative (electron dominated), probably indicating a close relationship between them.

Figure 1

(a) Pressure dependence of dc electric resistance $R$ of trigonal ${\mathrm{PtBi}}_2$ in the temperature range of 1.8–300 K (run 1). Inset: Low-temperature $R$ at 5.0 GPa. $T_C^{\mathrm{Onset}}$ is defined as the onset temperature of the resistance drop in $R-T$. (b) and (c) Enlarged views of low-temperature resistance in run 1. (d) High-pressure electrical transport measurements in a second run (run 2). (e) and (f) Enlarged views of low-temperature resistance in run 2.

Figure 2

(a) The resistive superconducting phase transition of trigonal ${\mathrm{PtBi}}_2$ at 22.6 GPa in magnetic fields to 3.0 kOe. (b) Temperature dependence of the upper critical field ${\mu }_0{H}_{C2}$. The red dashed line is a fit to the data as described in the main text.

Figure 3

(a) The transverse MR of trigonal ${\mathrm{PtBi}}_2$ at 5 K under different pressures. The inset is a schematic of direction relationships of the applied current, the sample natural cleavage facet, and the applied magnetic field. At low fields MR $\propto H^2$, while beyond $H^{*}$ a linear field dependence is observed. (b) Plots of MR and $\text{RRR}=R_{300\mathrm{K}}/R_{5\mathrm{K}}$ versus pressure. (c) The crossover field $H^{★}$ and the slope of the high-field linear MR as a function of pressure.

Figure 4

Left column: representative experimental raw images at 2.1, 6.9, and 12.9 GPa. Middle column: XRD patterns at various pressures. The synchrotron x-ray wavelength is $\lambda =0.3100$ Å. Peaks marked with a star are from the cubic phase of ${\mathrm{PtBi}}_2$. Right column: lattice parameters and the volume as a function of pressure [equation of state (EOS)]. The EOS data are fitted by the third-order Birch-Murnaghan formula.

Figure 5

Hall resistivity as a function of field ${\rho }_{xy}\left(H\right)$ at different pressures and a fixed temperature of 5 K. The inset in (b) shows a schematic setup of the Hall effect measurement.

Figure 6

(a) Temperature-pressure phase diagram of trigonal ${\mathrm{PtBi}}_2$. $P_C$ indicates the critical pressure where superconductivity is initially observed. (b) Hall coefficient $R_H$ determined from the low-field or high-field limits of ${\rho }_{xy}\left(H\right)$.