Superconductivity in Metal-Rich Chalcogenide Ta2Se

Abstract

The metal-metal bond in metal-rich chalcogenide is known to exhibit various structures and interesting physical properties. Ta₂Se can be obtained by both arc-melting and solid-state pellet methods. Ta₂Se crystallizes a layered tetragonal structure with space group P4/nmm (No. 129; Pearson symbol tP6). Each unit cell consists of four layers of body-centered close-packing Ta atoms sandwiched between two square nets of Se atoms, forming the Se-Ta-Ta-Ta-Ta-Se networks. Herein, we present magnetic susceptibility, resistivity, and heat capacity measurements on Ta₂Se, which together indicate bulk superconductivity with T_c = 3.8(1) K. According to first-principles calculations, the d orbitals in Ta atoms dominate the Fermi level in Ta₂Se. The flat bands at the Γ point in the Brillouin zone yield the van Hove singularities in the density of states around the Fermi level, which is intensified by introducing a spin-orbit coupling effect, and thus could be critical for the superconductivity in Ta₂Se. The physical properties, especially superconductivity, are completely different from those of Ta-rich alloys or transition-metal dichalcogenide TaSe₂.

Figure 1

(a) Crystal structure of Ta₂Se, where red and cyan balls represent Ta and Se atoms, respectively. (b) Refined powder XRD pattern for Ta₂Se. The black line with balls, red line, blue line, green vertical ticks, and orange vertical ticks stand for observed and calculated patterns, difference between the observed and calculated patterns, and Ta₂Se and 1T-TaSe₂ Bragg peaks. Insets show the magnified versions of powder XRD pattern fitting.

Figure 2

(a) Temperature dependence of the ZFC and FC volume magnetic susceptibilities for Ta₂Se. The data were collected between 1.8 and 5 K in applied magnetic field μ₀H = 5 mT. (b) Electrical resistivity ρ(T) of Ta₂Se measured in zero magnetic field. (c) Expanded plot of the low-temperature ρ(T) showing the superconducting transition for different magnetic fields from 0 to 0.5 T. Horizontal lines represent a residual resistivity and half of the transition, respectively. (d) Upper critical field μ₀H_c2 versus temperature of Ta₂Se determined from the electrical resistivity ρ(T,H) data in panel c. The red curve is a fit obtained using the G–L equation.

Figure 3

(a) Specific heat anomaly in a zero magnetic field at low temperatures with T_c = 3.77 K. (b) C_p/T versus T² plot under a μ₀H = 1 T magnetic field.

Figure 4

Band structure of Ta₂Se projected from the d orbitals of Ta atoms and the p orbitals of Se atoms, where the thicker the band, the higher the contribution of the corresponding orbital to the band (left three) and DOS of Ta₂Se (right): (a) without consideration of the SOC effect; (b) with consideration of the SOC effect.