We find anomalously large diamagnetic responses in the cage compounds AV2Al20 where A = Y and La, not A = Al0.3, Sc0.4, and Lu, despite the apparent similarities in crystal and electronic structures among these compounds. The magnetic susceptibilitie
s of the Y and La compounds become -1.94 and -7.44 x 10-4 cm3 mol-1 at 10 K, respectively, the latter of which corresponds to approximately one-quarter of that of bismuth, a well-known diamagnetic material, in terms of unit volume. The origin is not clear but may be related to a specific evolution in the band structure, as the diamagnetic response increases with increasing lattice constant.
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.
A cage compound AxV2Al20 (Al10V), that 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 influence 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 recent study, and conduction electrons manifests itself as anomalous enhancement in resistivity at around low temperatures corresponding to the Einstein temperatures.
