Superconducting properties and thermal expansion of YbGaxSi2−x

Abstract

YbGaxSi2−x forms the hexagonal AlB2-type crystal structure in the chemical composition range of 1.12 ≤ x ≤ 1.49 and shows superconductivity with critical temperatures TC ≤ 2.5 K. In this paper, we investigate the electronic and thermal properties of the YbGaxSi2−x compounds by means of electrical resistivity (ρ) and specific heat (C) measurements under magnetic fields, as well as by the temperature dependent x-ray diffraction study. The onset temperature of superconductivity, TonsetC, decreases with the increase of the Ga content x, from 2.5 K for x = 1.15 to 1.9 K for x = 1.31. Estimated upper critical field at zero temperature, Hc2(0), concomitantly decreases from 5.9 kOe for x = 1.15 to 3.1 kOe for x = 1.31. The normal-state electronic specific-heat coefficient is found to be almost independent to x; γ = 8mJ/molK2 for x = 1.18, 1.31, and 1.41. The small value of γ indicates electron-electron correlation is weak in this compound. An irreversible behavior is observed in ρ(T). ρ(T) of YbGa1.2Si0.8 shows a hump-like increase below about 200 K on cooling while it shows a rather monotonic increase on heating. Powder x-ray diffraction of this sample taken at 12 K shows no evidence of structural phase transition. Temperature dependence of the lattice parameters in this sample show normal behavior: both a and c decrease with decreasing temperature with thermal expansion coefficients similar to those in YbGaGe. Possible mechanism for the irreversible behavior in ρ(T) is discussed.