Pressure Dependence of Superconductivity in a Charge-Density-Wave Superconductor Bi₂Rh₃Se₂

The structural and superconducting properties of a Bi-based compound, Bi₂Rh₃Se₂, are investigated over a wide pressure range. Bi₂Rh₃Se₂ is a superconductor with a superconducting transition temperature, T_c, of 0.7 K. This compound is in a charge-density-wave (CDW) state below 240 K, which implies the coexistence of superconducting and CDW states at low temperatures. Here, the superconducting properties of Bi₂Rh₃Se₂ are studied from the perspective of the temperature dependence of electrical resistance (R) at high pressures (p's). The pressure dependence of T_c of Bi₂Rh₃Se₂ shows a slow increase in T_c at 0-15.5 GPa, and the T_c slowly decreases with pressure above 15.5 GPa, which is markedly different from that of normal superconductors because the value of T_c should simply decrease owing to the decrease in density of states (DOS) on the Fermi level, N(ε_F), driven by a simple shrinkage of the lattice under pressure. To ascertain the origin of such a dome-like T_c-p behavior, the crystal structure of Bi₂Rh₃Se₂ was explored over a wide pressure range of 0-20 GPa on the basis of powder X-ray diffraction; no structural phase transitions or simple shrinkage of the lattice was observed. This result implies that the increase in T_c against pressure cannot simply be explained from a structural point of view. In other words, a direct relation between superconductivity and crystal structure was not found. On the other hand, the CDW transition became ambiguous at pressures higher than 3.8 GPa, suggesting that the T_c had been suppressed by the CDW transition in a low pressure range. Thus, the findings suggest that for Bi₂Rh₃Se₂, T_c is enhanced through the suppression of CDW transition, which may be reasonable because the CDW-ordered state restrains the charge fluctuation to weaken the electron-phonon coupling and opens the gap to decrease the density of states on the Fermi level. The obtained dome-like T_c-p behavior indicates the possibility of Bi₂Rh₃Se₂ being an exotic superconductor.