Study on Susceptibilities of Superconductors BaPtSb and BaPtAs with Honeycomb Structure

N. Furutani, Y. Imai, T. Imazu, and J. Goryo

The low-energy electronic properties of the new superconductors BaPtSb and BaPtAs with an ordered honeycomb
network are investigated in the normal phase, where the former compound is a candidate for time-reversal symmetry-
breaking superconductors. By means of the first-principles calculation, we show that there exist two-dimensional
cylinder-like Fermi surfaces around the kz axis and one outer spherelike Fermi surface around the K and KA points in both
compounds, which are mainly composed of Pt 5d and Sb 5p=As 4p electrons. We construct low-energy effective
models, which are well described by using three bands consisting of two Pt 5d orbitals and one Sb 5p=As 4p orbital. By
evaluating susceptibilities using effective models, we find that dominant contributions to those susceptibilities result
from the outer spherelike Fermi surface. Whereas out-of-plane fluctuations are enhanced in both compounds in the
higher-temperature region, in-plane fluctuations become dominant in the very low-temperature region in BaPtSb owing
to a better nesting condition in the kz = 0 plane from the outer spherelike Fermi surface. These fluctuations yield
instabilities to ordered states, such as superconductivity, and might be associated with the occurrence of the
superconducting state with time-reversal symmetry breaking in BaPtSb.

J. Phys. Soc. Jpn.

92

094708