Specific heat of single-crystal HfV2: Strong-coupling conventional superconductivity and the effect of the martensitic transition

Abstract

Specific-heat (C) measurements on single crystals of HfV2 were made from 1to150K in magnetic fields (B) to 14T applied along the [110] axis. The type-II superconductor HfV2 has a martensitic transition at TM∼118K and becomes superconducting at Tc∼8to9K. Specific heats are thermal-history dependent and Tc increases following repeated cooling cycles from ambient temperature through TM. This progression is probably related to an incomplete structural transition from cubic-to-orthorhombic symmetry at TM and the related strains that are produced. Differential scanning calorimetry through TM had a hysteresis of ∼1K related to cooling and warming cycles between 90 and 140K with no other effect on the martensitic transition. An x-ray determination of phase contents for one sample was used to establish a ratio (∼1:10) of cubic-to-orthorhombic phases below TM. Both phases are superconducting, but only a sharp, anomaly is observed at Tc, which can be rationalized if both phases have nearly identical Tc’s. From fits to the specific heat above Tc, a representative derived Debye temperature (ΘD), characterizing the low-temperature lattice specific heat, is 177K. At Tc the ratio ΔC(Tc)∕γTc=2.07, with Tc=8.00K and γ=42.1mJK−2mol−1, is representative of the specific-heat measurements and indicates strong coupling. This ratio is nearly independent of variations in Tc and C associated with repeated cooling from ambient temperature through TM. The conventional superconducting state specific heat can be fitted with the alpha model for strong coupling using an energy gap Δ(0)∕kBTc=2.1. It has an associated electron-phonon coupling constant λ=1.45. Both parameters are similar to those for the type-I superconductor Pb. In the normal state the Sommerfeld constant (γ) depends on the thermal history, with a general increase as Tc increases. For B>0 the superconducting anomaly shifts to lower temperatures and the Sommerfeld constant in the vortex state (γv) is linear in B with values and slopes that depend on the thermal history. Extrapolating γv(B) vs B to γ yields upper critical fields (Bc2) ranging from 29to35T.

Source-reported materials — not catalogue approval

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