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

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 Added by Yuji Aoki
 Publication date 2018
  fields Physics
and research's language is English




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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) $d_{rm GFS} e 0$ 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.



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Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$Tr_2$Al$_{20}$ with $Tr$ = Mo and W revealed that these compounds exhibit superconductivity with transition temperatures $T_c$ = 3.22 and 1.81 K, respectively, achieving the highest values in the reported La$Tr_2$Al$_{20}$ compounds. There appears a positive correlation between $T_c$ and the electronic specific heat coefficient, which increases with increasing the number of $4d$- and $5d$-electrons. This finding indicates that filling of the upper $e_g$ orbitals in the $4d$ and $5d$ bands plays an essential role for the significant enhancement of the superconducting condensation energy. Possible roles played by the $d$ electrons in the strongly correlated electron phenomena appearing in $RTr_{2}$Al$_{20}$ are discussed.
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