Pressure Effects on Rattling and Superconductivity in the Einstein Solids

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Pressure effects on the rattling and superconductivity in the Einstein solids; A<inf>x</inf>V<inf>2</inf>Al<inf>20</inf>(A<inf>x</inf>= Ga<inf>0.2</inf>, Al<inf>0.3</inf>, and Y) are investigated by means of electrical resistivity measurements under high pressure. We observed bumps in resistivity at low temperatures only for Ga<inf>0.2</inf>and Al<inf>0.3</inf>, which can be explained by scattering of conduction electrons by local phonons with Einstein temperatures of 8.5 and 22 K, respectively. The bumps gradually fade out with increasing pressure, and the temperature variations of resistivity for Ga<inf>0.2</inf>and Al<inf>0.3</inf>approach those of Y. This indicates that the low-energy local phonons in the Ga and Al compounds disappear at high pressures. We also observed T^{2} behavior with large A coefficients in the low-temperature limit for Ga and Al compounds, but not for Y. With increasing pressure, the A coefficient is largely suppressed, while H_{\text{c2}}' \equiv |\mathrm{d}H_{\text{c2}}/\mathrm{d}T|_{T_{\text{c}}} that also reflects the density of states at Fermi energy decreases more slowly. The T^{2} behavior in Ga and Al compounds probably originates from el--ph scatterings enhanced by rattling of Ga and Al atoms rather than el--el scatterings. An enhancement in the superconducting transition temperatures by the rattling is also demonstrated by comparing the pressure responses of T_{\text{c}} between the three compounds. The enhancements of T_{\text{c}} and el--ph interaction \lambda is relatively small for these Einstein solids in contrast to those of the \beta-pyrochlore oxides.
2013-06-15

Pressure Effects on Rattling and Superconductivity in the Einstein Solids

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