Pressure-driven superconductivity in the transition-metal pentatelluride $\mathrm{HfT}{\mathrm{e}}_5$

The discovery of superconductivity in hafnium pentatelluride $\mathrm{HfT}{\mathrm{e}}_5$ under high pressure is reported. Two structural phase transitions and metallization with superconductivity developing at around 5 GPa are observed. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure of the study (42 GPa). The combination of electrical transport and crystal structure measurements as well as theoretical electronic structure calculations enables the construction of a phase diagram of $\mathrm{HfT}{\mathrm{e}}_5$ under high pressure.

Figure 1

Crystal structure and electrical and thermal properties of $\mathrm{HfT}{\mathrm{e}}_5$ at ambient pressure. (a) Crystal structure of $\mathrm{HfT}{\mathrm{e}}_5$ with Cmcm space group. The red spheres represent Hf atoms, and both green and blue spheres represent Te atoms at different crystallographic positions. (b) Side view of the $\mathrm{HfT}{\mathrm{e}}_5$ crystal structure. The $\mathrm{HfT}{\mathrm{e}}_3$ chains that run along the $a$ axis are linked via zigzag chains of Te atoms. (c) Temperature-dependent resistivity of $\mathrm{HfT}{\mathrm{e}}_5$ along the $a$ axis. A large resistivity anomaly appears at around 40 K. Inset: Specific heat capacity of $\mathrm{HfT}{\mathrm{e}}_5$ crystals in the representation $C_P/T$ vs $T^2$. (d) Magnetoresistance (MR) of $\mathrm{HfT}{\mathrm{e}}_5$ at a temperature of 2 K and in a maximum field of 9 T with current and field along [100] and [010], respectively.

Figure 2

Evolution of superconductivity as a function of pressure. Plots of electrical resistivity as function of temperature for $P<5\mathrm{GPa}$ (a) and for $P>5\mathrm{GPa}$ (b). At $P=5.5\mathrm{GPa}$ and 6.2 GPa, superconductivity is observed although the normal state still exhibits the resistivity anomaly. (c) shows the electrical resistivity drop and zero-resistance behavior at low temperatures. The superconducting critical temperature $T_c$ increases with increasing pressure, the maximum $T_c=4.8\mathrm{K}$ is observed at 20 GPa. (d) Temperature dependence of resistivity under various magnetic fields up to ${\mu }_{0}H=4\mathrm{T}$ at 19.5 GPa.

Figure 3

Raman spectroscopy of $\mathrm{HfT}{\mathrm{e}}_5$ and possible crystal structure under high pressure. (a) Pressure-dependent Raman spectroscopic signals for $\mathrm{HfT}{\mathrm{e}}_5$ at room temperature. (b), (c) Crystal structures of the $C2/m$ and $P\overline{1}$ phases. The red spheres represent Hf atoms, and both green and blue spheres represent Te atoms with different positions. (d) and (e) show the Te layers of the $C2/m$ phase and the Te chains of the $P\overline{1}$ phase.

Figure 4

Enthalpy curves (relative to the Cmcm structure) of various structures of $\mathrm{HfT}{\mathrm{e}}_5$ as function of pressure. Enthalpies are given per unit cell ($\mathrm{H}{\mathrm{f}}_2\mathrm{T}{\mathrm{e}}_{10}$).

Figure 5

The calculated phonon spectra of the $C2/m$ and $P\overline{1}$ phases of $\mathrm{HfT}{\mathrm{e}}_5$. The phonon spectra of the high-pressure phases demonstrate that both the $C2/m$ (a) and $P\overline{1}$ (b) phases are stable.

Figure 6

Electronic phase diagram of $\mathrm{HfT}{\mathrm{e}}_5$. The black and magenta squares denote $T^{*}$, the peak temperature of the electrical resistivity anomaly defined as the temperature of the discontinuity in the resistivity derivative. The green and blue circles represent $T_c$ extracted from different runs of electrical resistivity measurements. Colored areas are a guide to the eye indicating the distinct phases.

Figure 7

Electronic band structure for $\mathrm{HfT}{\mathrm{e}}_5$ as function of pressure. The DFT calculated electronic band structures of $\mathrm{HfT}{\mathrm{e}}_5$ at 0 GPa (a) and 2.2 GPa (b) in Cmcm phase, 2.3 GPa (c) and 8.1 GPa (d) in $C2/m$ phase, 10.8 GPa (e) and 17.5 GPa (f) in $P\overline{1}$ phase. (a)–(f) The size of red filled circles (blue filled triangles) represents the fraction of Te $5p$ (Hf $5d$) states. [Γ: (0.0, 0.0, 0.0); Z: (0.0, 0.0, π/$a$); T: (π/$a$, π/$a$, π/$a$); Y: (π/$a$, π/$a$, 0.0); S: (0.0, π/$a$, 0.0); R: (0.0, π/$a$, π/$a$)].

Figure 8

Electronic band structure for $\mathrm{HfT}{\mathrm{e}}_5$ as function of pressure calculated by DFT at 8.1 GPa in the $C2/m$ phase (a) and at 17.5 GPa in the $P\overline{1}$ phase (b). The size of the red spheres represents the fraction of the in-layer (in-chain) Te $5p$ states in the $C2/m$ ($P\overline{1}$) phase, while the size of blue filled triangles represents the fraction of the out-of-layer (out-of-chain) Te $5p$ states in $C2/m$ ($P\overline{1}$) phase.

Figure 9

The evolution of the electronic density of states (DOS) at the Fermi level with the increase of pressure for $\mathrm{HfT}{\mathrm{e}}_5$.