Pressure-induced superconductivity and structural transition in ferromagnetic ${\mathrm{CrSiTe}}_3$

Layered structural materials have been a fertile playground to investigate mechanisms of fundamental physics and explore potential applications. Here, we report investigations on ferromagnetic van der Waals $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ via high-pressure synchrotron x-ray diffraction, electrical resistance, and magnetoresistance measurements. Under compression, $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ undergoes an insulator-metal transition and a structural transition at $\sim 7.5$ GPa. Concomitantly with the structural transition, the magnetoresistance changes sign, the negative Hall coefficient increases dramatically, and superconductivity emerges at 3 K. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum $T_c\approx$ 4.5 K at $\sim 30$ GPa. Our results suggest that $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ is paramagnetic in the pressure range of superconductivity. The discoveries of superconductivity and magnetic transition in ferromagnetic $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ under pressure provide new perspectives to explore the interplay between superconductivity and magnetism in Cr-based two-dimensional van der Waals materials.

Figure 1

Properties of $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ at ambient pressure. (a) Schematics of the crystal structure of $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ viewed from the $a$ axis (left) and $c$ axis (right) directions. The dashed line indicates a unit cell. The view in the right panel shows the honeycomb lattice. (b) Experimental and indexed XRD pattern measured on an exfoliated surface of a single crystal, as shown in the inset image. (c) Magnetizations of $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ against temperature measured on a single crystal under field cooling (FC) and zero-field cooling (ZFC) with a magnetic field of 2000 Oe applied along the $c$ axis and $ab$ plane, respectively.

Figure 2

(a) XRD diffraction patterns under pressures from 1.1 to 26.9 GPa. A clear structural transition takes place between 6.0 and 9.1 GPa. The background has been shifted for visualization. (b) Color plot of the intensities of the Bragg peaks transformed from (a) with intensity normalized in the pressure and $d$ spacing space. A first-order structural transition appears at $\sim 7.5$ GPa. The Bragg peaks are indexed in the $R$-3 space group.

Figure 3

Pressure dependence of the electrical resistance for $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$. (a) Temperature dependence of the resistance at pressures from 1.0 to 13.1 GPa shown on a logarithm scale. The arrows mark the temperatures of the humps in resistance. (b) Representative resistance measured at 1.0, 2.0, 4.0, and 8.0 GPa on a linear scale, showing the evolution of the IMT against temperature at varied pressures. The $T_{\mathrm{hump}}$ increases and finally vanishes as the pressure increases. A superconducting transition emerges at 8.0 GPa. (c) A zoom-in plot of the resistance against temperature between 1 and 7 K at pressures from 9.1 to 43.8 GPa, demonstrating SC occurs at $\sim 3.0$–4.5 K. (d) Magnetic field dependence of the superconducting transition at 19.5 GPa at 0, 0.25, 0.5, 1.0, 2.0, 3.0, and 4.0 T. The inset is a fitting of the upper critical field ${\mu }_0{H}_{c2}$ at 19.5 GPa. The solid line represents the Ginzburg-Landau (GL) fitting ${\mu }_0{H}_{c2}\left(T\right)={\mu }_0{H}_{c2}\left(0\right)[1-{(T/T_c)}^2]$, yielding ${\mu }_0{H}_{c2}\left(0\right)\approx 4.0$ T. The data in (a) and (b) and (c) and (d) are measured on two single-crystal samples.

Figure 4

(a) Magnetic field dependence of the resistance changes $(R-R_0)/R_0$ measured at 10 K and various applied pressures, where $R$ is resistance and $R_0$ is the resistance at zero field. The open points in (a) were measured at 9.1, 11.2, 15.4, 18.5, and 30.1 GPa. The solid points in (b) correspond to 1.0, 2.0, 3.1, 5.0, and 7.1 GPa. (c) Measurement of the resistance at 2 GPa and 300 K. The data have been symmetrized to exclude the influence of the Hall effect.

Figure 5

Representative Hall resistance measured at (a) 10 K, 4 GPa, (b) 40 K, 4 GPa, (c) 10 K, 5.9 GPa, (d) 40 K, 5.9 GPa, and (e) 10 K, 9.9 GPa. The black (red) points are collected for scanning the magnetic field from 1 to $-1$ T ($-1$ to 1 T). The difference in the resistance on the negative side is shaded green. (f) Differences in the integrated areas for two measured directions (the shaded areas) on the negative magnetic field side as functions of temperature and pressure. The solid lines are a guide to the eyes. The dashed line indicates a kink at the Curie temperature of $\sim 32$ K.

Figure 6

(a) Hall resistance $(R_{xy}^{+}-R_{xy}^{-})/2$ measured at 10 K with various pressures against positive ($R_{xy}^{+}$) and negative ($R_{xy}^{-}$) magnetic field. (b) Hall coefficient and carrier density as a function of pressure derived from the Hall resistance in (a), corresponding to the left and right axes, respectively. Due to the hysteresis of the Hall resistance, only the data in the range of $0.5\le \mid {\mu }_{0}H\mid \le 1.0$ T are considered. $R_H$ and $n_e$ have been averaged between two scanned directions.

Figure 7

DFT calculation of the total density of states (DOS) of $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$ at (a) ambient pressure, (b) 2 GPa, and (c) 4 GPa. (d) The partial density of states (PDOS) consists of the contributions from Te, Cr, and Si atoms at 6 GPa.

Figure 8

Temperature-pressure phase diagram. The IMT and superconducting transition temperatures are determined from the resistance measurements. The structural (str.) transition pressure is measured with the XRD at room temperature. The FM region is plotted based on the magnetic susceptibility at ambient pressure together with indications of the Hall resistance hysteresis measurements. $T_c$'s have been multiplied by a factor of 5. The pink and orange points in the SC area are $T_c$'s determined from the intersection points of the extended tangents of the resistance in the normal state just above and below the SC transition. The purple points are defined as the starting temperature of zero resistance.