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ORAL CONTRIBUTIONS
The evolution of superconductivity in the ternary AnM3n-1B2n
(n=1:A= Ca, Sr; M=Rh, Ir and n=3:A= Ca, Sr; M=Rh) phases
M. ElMassalami
Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, 21941-972
Rio de Janeiro, Brazil
We present our work on the search for superconductivity in the titled homologous
series. We followed the evolution of the metallic properties of this series when n as well
as the atomic type of A and M are varied, covering the range: for n=1: A= Ca, Sr and
M=Rh, Ir while for n=3: A= Ca, Sr and M=Rh. The crystal structure of n =1 members is
known from previous studies to be the CaRh 2 B 2 -type (Fddd) while that of n =3 is the
Ca 3 Rh 8 B 6 -type (Fmmm): the later can be visualized as a stacking of structural fragments
from AM 3 B 2 and AM 2 B 2 . It is observed that the metallic properties of the n=1 and 3
members are distinctly di¤erent: on the one hand, the n=1 family exhibits linear coe¢
cient of the electronic specific heat γ≈3 mJ/molK 2 , Debye temperature Θ D ≈300 K, a
normal conductivity down to 2 K, and a relatively strong linear magnetoresistivity for
.elds up to 150 kOe. The n=3 family, on the other hand, exhibits γ≈18 mJ/molK 2 , ΘD ≈
330 K, a weak linear magnetoresistivity, and an onset of superconductivity (for
Ca 3 Rh 8 B 6 , Tc=4.0 K and H c2 = 14.5 kOe while for Sr 3 Rh 8 B 6 , Tc=3.4 K and H c2 ≈ 4.0
kOe). To understand these remarkable differences, we carried out electronic band
structure calculation. Indeed, the difference among the electronic properties of these two
families are found to be related to the distinctly different electronic band structures and
densities of states, DOS, N(E). In particular, the Fermi level, E F of Ca 3 Rh 8 B 6 lies at
almost the top of a DOS peak while that of CaRh2B2 lies at a bottom of a local valley.
Based on these results, we attributed the superconductivity of A 3 Rh 8 B 6 to the presence
of a pronounced local peak in the DOS curve. Furthermore, we observed that though all
atoms contribute to this peak, however the contribution of the Rh atoms (in particular at
the 16j sites) is the strongest. We found a satisfactorily agreement between the
measured and calculated γ.
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