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

Abstract

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(εF), 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.

Pressure Dependence of the Structural and Superconducting Properties of the Bi-Based Superconductor Bi2Pd3Se2

“…The value of K 0 ( = − V d p d V = 1 κ ) was larger than those for β-FeSe ( K 0 = 3.0(1) × 10 1 GPa) and (NH 3 ) y Cs x FeSe ( K 0 = 3.96(1) × 10 1 GPa), , showing the smaller compressibility κ in Bi 2 Pd 3 Se 2 than in FeSe compounds. However, K 0 was consistent with that of Bi 2 Rh 3 Se 2 , 1.2(1) × 10 2 GPa, which was evaluated in this study from the V – p plot reported previously . The values of K 0 ’ and V 0 of Bi 2 Rh 3 Se 2 were evaluated to be 4(1) and 1.147(3) × 10 3 Å 3 , respectively.…”

“…The value of approximately 3.0 K was constant when increasing the pressure (Figure (d)), which differed from the T c – p plot in Bi 2 Rh 3 Se 2 , where the T c increased with an increase in pressure. Thus, the suppression of the possible CDW transition by the application of pressure (Figure (a,b)) did not directly enhance T c , which is different from the case of Bi 2 Rh 3 Se 2 …”

“…12) for both compounds. The unit cell volume V of Bi 2 Rh 3 Se 2 was 1144–1152 Å 3 for the case of β ∼ 90°; a = 11.414(10) Å, b = 8.3709(9) Å, c = 11.989(1) Å, and β = 89.153(3)° in ref , a = 11.416(1) Å, b = 8.365(1) Å, c = 11.982(2) Å, and β = 89.10(3)° in ref , and a = 11.4316(7) Å, b = 8.3872(4) Å, c = 12.0157(3) Å, and β = 89.225(4)° in ref , while the value of V for Bi 2 Pd 3 Se 2 determined in this study was 612.30(2) Å 3 for the case of β ∼ 133° ( a = 11.8156(5) Å, b = 8.4889(3) Å, c = 8.4686(3) Å, and β = 133.88(1)°). In addition, a study indicated that β ∼ 134° was also an optimal lattice choice even for the crystal lattice of Bi 2 Rh 3 Se 2 where a = 11.413(1) Å, b = 8.368(2) Å, c = 8.336(1) Å, and β = 134.05(4)° .…”

“…This suggests that to detect CDW transitions from the M / H plot is little difficult. By applying pressure for Bi 2 Pd 3 Se 2 , the CDW transition disappeared, but the value of T c did not clearly increase, in contrast to Bi 2 Rh 3 Se 2 . The CDW for Bi 2 Rh 3 Se 2 was recently investigated using optical spectroscopy and ultrafast pump–probe study, and the CDW gap energy of 120 mV at 10 K was obtained .…”

“…As described above, Bi 2 Rh 3 Se 2 is an exotic compound in which the superconducting and CDW ordered states coexist at ambient pressure. − The group 9 element Rh in Bi 2 Rh 3 Se 2 is commonly replaced by the group 10 element Pd to produce Bi 2 Pd 3 Se 2 . − The first study on the physical properties of Bi 2 Pd 3 Se 2 was reported in 2008, and the superconducting properties were also studied. No CDW transition was observed in Bi 2 Pd 3 Se 2 , while superconductivity was observed with a T c value of approximately 1.0 K.…”