Interplay of superconductivity and structural phase transition in the clathrate Ba6Ge25

Abstract

The cubic compound Ba6Ge25 undergoes a structural phase transition around 200 K, below which the electrical resistivity increases abruptly. However, out of this “bad” metallic state (ρ0≃1.5mΩcm), BCS-like superconductivity occurs at very low temperature (Tc≈0.24K). By applying hydrostatic pressure, the structural phase transition is suppressed at pc≃2.8GPa, but the superconducting transition temperature Tc increases remarkably, reaching a maximum of Tc≃3.8K at p∼pc. To understand these unusual properties, we have measured the upper critical magnetic field Hc2 for Ba6Ge25 and its isostructural compound Na2Ba4Ge25 under pressure, from which the density of states at the Fermi energy N(EF) is determined. It demonstrates that, in Ba6Ge25, the structural phase transition causes a significant reduction of N(EF) (by a factor of about 4) and therefore largely weakens superconductivity. Furthermore, the drastic enhancement of Tc under pressure is found to be solely governed by N(EF).

Source-reported materials — not catalogue approval

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