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Superconductivity in the Einstein Solid AxV2Al20 (A= Al and Ga)
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
(Received November 6, 2011; Accepted November 28, 2011; Published January 5, 2012)
A cage compound of the form AxV2Al20 (Al10V), which was called an Einstein solid by Caplin and coworkers 40 years ago, is revisited to investigate the low-energy, local vibrations of the A atoms and their effects on the electronic and superconducting properties of the compound. Polycrystalline samples with A= Al, Ga, Y, and La are studied through resistivity and heat capacity measurements. Weak-coupling BCS superconductivity is observed below Tc = 1.49, 1.66, and 0.69 K for Ax= Al0.3, Ga0.2, and Y, respectively, but not above 0.4 K for Ax= La. Low-energy modes are detected only for A= Al and Ga, which are approximately described by the Einstein model with Einstein temperatures of 24 and 8 K, respectively. A weak but significant coupling between the low-energy modes, which are almost identical to those called rattling in a recent study, and conduction electrons manifests itself as anomalous enhancement in resistivity at low temperatures approximately corresponding to the Einstein temperatures.
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