Rattling and Superconducting Properties of the Cage Compound GaxV2Al20

TL;DR

This study investigates how low-energy rattling modes of Ga and Al atoms in GaxV2Al20 influence its superconducting properties, revealing that Ga rattling significantly enhances Tc and softens the cage structure.

Contribution

It provides new insights into the impact of low-energy rattling modes on superconductivity and cage dynamics in GaxV2Al20, highlighting the role of Ga atoms.

Findings

Superconductivity observed below 1.4-1.7 K across all samples.

Introduction of Ga rattling modes increases Tc by 8%.

Low-energy Ga rattling modes soften the cage structure.

Abstract

Low-energy rattling modes and their effects on superconductivity are studied in the cage compound GaxV2Al20. A series of polycrystalline samples of 0 < x =< 0.6 are examined through resistivity, magnetic susceptibility, and heat capacity measurements. A weak-coupling BCS superconductivity is observed below Tc = 1.4-1.7 K for all the samples. For small Ga contents below 0.20, approximately 30% of the cages are occupied by rattling Al atoms having an Einstein temperature TE of 23 K, probably with most Ga atoms substituting for the cage-forming Al atoms. For higher Ga contents, approximately 0.05 Ga and 0.25-0.35 Al atoms coexist statistically inside the cages and behave as rattlers with TE ~ 8 and 23 K, respectively. A significant effect of Ga rattling on the superconductivity is clearly evidenced by the observation of a sharp rise in Tc by 8% at x = 0.20 when 0.05 Ga atoms are introduced into the case. Probably, the electron-phonon interaction is significantly enhanced by an additional contribution to the phonon density of states from the extremely low energy rattling modes of Ga atoms. In addition, a large softening of the acoustic modes is observed for x => 0.20, suggesting that the cage itself becomes anomalously soft in the presence of low-energy Ga rattling modes.