[Paper Review] Pressure Study of BiS2-Based Superconductors Bi4O4S3 and La(O,F)BiS2

Hisashi Kotegawa, Yusuke Tomita+7 more|arXiv (Cornell University)|Jul 30, 2012

Metallurgical Processes and Thermodynamics4 citations

TL;DR

This study investigates pressure-induced superconductivity in BiS2-based materials Bi4O4S3 and La(O,F)BiS2 via electrical resistivity measurements under hydrostatic pressure. It reveals that superconducting transition temperature (Tc) decreases monotonically in metallic Bi4O4S3 but initially increases then decreases in semiconducting La(O,F)BiS2, indicating that high Tc emerges near the metal-insulator boundary, with Fermi surface instability playing a key role in enhancing superconductivity despite low carrier density and density of states.

ABSTRACT

We report the electrical resistivity measurements under pressure for the recently discovered BiS2-based layered superconductors Bi4O4S3 and La(O,F)BiS2. In Bi4O4S3, the transition temperature Tc decreases monotonically without a distinct change in the metallic behavior in the normal state. In La(O,F)BiS2, on the other hand, Tc initially increases with increasing pressure and then decreases above ? 1 GPa. The semiconducting behavior in the normal state is suppressed markedly and monotonically, whereas the evolution of Tc is nonlinear. The strong suppression of the semiconducting behavior without doping in La(O,F)BiS2 suggests that the Fermi surface is located in the vicinity of some instability. In the present study, we elucidate that the superconductivity in the BiS2 layer favors the Fermi surface at the boundary between the semiconducting and metallic behaviors.

Motivation & Objective

• To investigate the pressure dependence of superconducting transition temperature (Tc) in BiS2-based superconductors Bi4O4S3 and La(O,F)BiS2.
• To understand the role of electronic structure and Fermi surface topology in determining Tc, particularly near the metal-insulator transition.
• To clarify whether superconductivity is enhanced when the Fermi level lies near an instability, such as the edge of the conduction band or a pseudogap regime.
• To examine how pressure suppresses semiconducting behavior in La(O,F)BiS2 and its correlation with Tc evolution.

Proposed method

• Electrical resistivity measurements were performed under hydrostatic pressure up to ~4 GPa using a Daphne 7474-based indenter cell.
• Four-probe method with silver paint contacts was used to measure resistivity, ensuring accurate determination of Tc onset and zero-resistance transition.
• Pressure was calibrated using the superconducting transition temperature of a lead manometer.
• Polycrystalline samples of Bi4O4S3 and LaO0.5F0.5BiS2 were synthesized via solid-state reaction and high-pressure annealing, respectively.
• The pressure dependence of Tc was analyzed by tracking the onset (Tc,onset) and zero-resistance (Tc,zero) transitions in resistivity curves.
• Theoretical interpretation focused on Fermi surface instability, band structure proximity to a topological change, and the role of impurity bands in semiconducting behavior.

Experimental results

Research questions

• RQ1 How does pressure affect the superconducting transition temperature Tc in Bi4O4S3 and La(O,F)BiS2?
• RQ2 Why is Tc higher in La(O,F)BiS2 than in Bi4O4S3 despite lower carrier density?
• RQ3 What is the origin of the semiconducting behavior in LaO0.5F0.5BiS2, and how does pressure suppress it?
• RQ4 Is there a correlation between the proximity of the Fermi level to a band edge or instability and the enhancement of Tc?
• RQ5 Can the nonlinear Tc evolution in La(O,F)BiS2 be explained by nesting or electron correlation effects near a quantum critical point?

Key findings

• In Bi4O4S3, Tc decreases monotonically with pressure, with an initial slope of -1.1 K/GPa, and shows no significant change in metallic normal-state resistivity.
• In La(O,F)BiS2, Tc initially increases with pressure up to ~1 GPa, reaching a maximum before decreasing, indicating a non-monotonic superconducting response.
• The semiconducting behavior in La(O,F)BiS2 is strongly suppressed with increasing pressure, suggesting a shift toward metallic character and Fermi surface reconstruction.
• The suppression of semiconducting behavior occurs without doping, implying the Fermi level is near a critical instability, possibly at the edge of the conduction band or near an impurity band.
• The highest Tc (~10 K) in La(O,F)BiS2 occurs near the metal-insulator boundary, indicating that superconductivity is favored when the Fermi surface is near an instability rather than in a fully metallic state.
• The results suggest that high Tc in BiS2-based superconductors is not primarily driven by high density of states, but rather by proximity to a quantum critical point or Fermi surface instability.