Superconductivity in Cage Compounds La$Tr_{2}$Al$_{20}$ with $Tr$ = Ti, V, Nb, and Ta

TL;DR

This study investigates superconductivity in La$Tr_{2}$Al$_{20}$ compounds with $Tr$ = Ti, V, Nb, Ta, revealing weak-coupling superconductivity with varying transition temperatures and suggesting a correlation between cage guest free space and $T_c$.

Contribution

It provides the first detailed analysis of superconductivity and cage dynamics in La$Tr_{2}$Al$_{20}$ compounds, introducing the $d_{ m GFS}$ parameter to classify their superconducting behavior.

Findings

LaTi$_{2}$Al$_{20}$ is likely a rare type-I superconductor.

Superconducting transition temperatures vary from 0.15 to 1.05 K among the compounds.

Abstract

Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$Tr_{2}$Al$_{20}$ ($Tr$ = Ti, V, Nb, and Ta) revealed that these four compounds exhibit weak-coupling superconductivity with transition temperatures $T_{\rm c}$ = 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 $d_{\rm GFS}$ quantifying the "guest free space" of the cage-center ion, we demonstrate that nonmagnetic $RTr_{2}$Al$_{20}$ superconductors are classified into two groups, i.e., (A) $d_{\rm GFS} \ne 0$ and $T_{\rm c}$ correlates with $d_{\rm GFS}$, and (B) $d_{\rm GFS} \simeq 0$ and $T_{\rm c}$ seems to be governed by other factors.