Unconventional isotope effect on transition temperature in BiS₂-based superconductor Bi₄O₄S₃

Abstract

We report the sulfur isotope effect on transition temperature in a BiS2-based superconductor Bi4O4S3. Polycrystalline samples of Bi4O4S3 were prepared using 32 S and 34 S isotope chemicals.From magnetization analyses, the isotope exponent (S) was estimated as -0.1 < S < 0.1.Although the Tc estimated from electrical resistivity was scattered as compared to those estimated from the magnetization, we observed no clear correlation between Tc and the isotope mass. The present results suggest that unconventional paring states are essential in Bi4O4S3.

Possible pairing mechanism switching driven by structural symmetry breaking in BiS2-based layered superconductors

“…As summarised in Fig. 1, isotope effect suggesting the importance of phonon was observed for the high-P phase of (Sr,La)FBiS 2 , whereas unconventional isotope effects were observed in La(O,F)BiSSe and Bi 4 O 4 S 3 26,27 . Although there are some possible factors, which could affect isotope effect, other than pairing states 32 , we consider that the observed difference in isotope effect is essentially caused by the different pairing states between those systems.…”

“…Conventional shifts in T c between samples synthesised with 32 S and 34 S were observed, which suggests the importance of phonons for the pairing mechanisms in the compound. The conventional isotope effects in (Sr,La)FBiS, which has a monoclinic structure, are in contrast to the unconventional isotope effects observed in La(O,F)BiSSe and Bi 4 O 4 S 3 , which have a tetragonal structure 26,27 . Based on a combination of the discussion of previous and present isotope studies, we suggest that the structural difference between the tetragonal and monoclinic structures could be a switch of the pairing mechanisms in BiCh 2 -based superconductors.…”

“…In addition, we have recently reported on an unconventional isotope effect with 32 S and 34 S in Bi 4 O 4 S 3 (− 0.1 < α S < 0.1) (Fig. 1g) 27 . These two superconductors have a tetragonal crystal structure and show a relatively low T c of 3.8 K for LaO 0.6 F 0.4 BiSSe and 4.7 K for Bi 4 O 4 S 3 .…”

Structural Phase Diagram of LaO1−xFxBiSSe: Suppression of the Structural Phase Transition by Partial F Substitutions

“…However, an angle-resolved photoemission spectroscopy (ARPES) study reported the observation of the anisotropic superconducting gap, indicating that unconventional superconductivity is emerging in NdO 0.71 F 0.29 BiS 2 [10]. Furthermore, the absence of isotope effects was observed in tetragonal LaO 0.6 F 0.4 Bi(S,Se) 2 , which was examined using 76 Se and 80 Se isotopes [11], and Bi 4 O 4 S 3 , which was examined using 32 S and 34 S isotopes [12]. These results also imply that unconventional pairing is essential for BiCh 2 -based superconductors with a tetragonal structure.…”

High-pressure effects on La(O,F)BiS2 single crystal using diamond anvil cell with dual-probe diamond electrodes

“…Similar high-pressure effects with the structure change have been observed in various BiS2-based superconductors, such as EuFBiS2 [14] and Ce(O,F)BiS2 [15], suggesting a common mechanism for Tc enhancement. According to an investigation of isotope effects, the high-pressure phase of (Sr,La)FBiS2 with a monoclinic crystal structure exhibits conventional-type Tc shifts by substituting 32 S and 34 S [16], unlike the unconventional isotope effects observed in La(O,F)Bi(S,Se)2 [7] and Bi4O4S3 [17] with tetragonal structures. The high-pressure approach is also beneficial for inducing superconductivity in BiS2-based compounds, such as NdO0.8F0.2Sb1-xBixSe2 (x = 0 to 0.8) [18] and EuFBiS2 [19].…”