Superconductivity in Cage Compounds La$T{r}_2$Al${}_{20}$ with $Tr$ = Ti, V, Nb, and Ta

Abstract

Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$T{r}_2$Al${}_{20}$ ($Tr$ = Ti, V, Nb, and Ta) revealed that these four compounds exhibit weak-coupling superconductivity with transition temperatures $T_{\mathrm{c}}$ = 0.46, 0.15, 1.05, and 1.03 K, respectively. LaTi${}_2$Al${}_{20}$ is most probably a type-I superconductor, which is quite rare among intermetallic compounds. Single-crystal X-ray diffraction suggests "rattling" anharmonic large-amplitude oscillations of Al ions (16$c$ site) on the Al${}_{16}$ cage, while no such feature is suggested for the cage-center La ion. Using a parameter $d_{\mathrm{GFS}}$ quantifying the "guest free space" of the cage-center ion, we demonstrate that nonmagnetic $RT{r}_2$Al${}_{20}$ superconductors are classified into two groups, i.e., (A) $d_{\mathrm{GFS}}\ne 0$ and $T_{\mathrm{c}}$ correlates with $d_{\mathrm{GFS}}$, and (B) $d_{\mathrm{GFS}}\simeq 0$ and $T_{\mathrm{c}}$ seems to be governed by other factors.

5 pages, 5 figures. To appear in J. Phys. Soc. Jpn