Specific heat and magnetization of a ZrB12 single crystal: characterization of a type II/1 superconductor

TL;DR

This study characterizes a ZrB12 single crystal as a low-kappa, type II/1 superconductor, revealing its phase transition behavior, critical fields, and confirming its classification through specific heat, magnetization, and magnetoresistance measurements.

Contribution

It provides the first detailed thermodynamic and magnetic characterization of ZrB12 as a type II/1 superconductor, including phase transition behavior and critical field values.

Findings

Transition from first to second order with increasing magnetic field

Bulk upper critical field Hc2(0) = 550 G

Surface critical field Hc3(0) ≈ 1000 G

Abstract

We measured the specific heat, the magnetization, and the magnetoresistance of a single crystal of ZrB12, which is superconducting below Tc ~ 6 K. The specific heat in zero field shows a BCS-type superconducting transition. The normal- to superconducting-state transition changes from first order (with a latent heat) to second order (without latent heat) with increasing magnetic field, indicating that the pure compound is a low-kappa, type-II/1 superconductor in the classification of Auer and Ullmaier [J. Auer and H. Ullmaier, Phys. Rev.B 7, 136 (1973)]. This behavior is confirmed by magnetization measurements. The H-T phase diagram based on specific-heat and magnetization data yields Hc2(0) =550 G for the bulk upper critical field, whereas the critical field defined by vanishing resistance is a surface critical field Hc3(0) ~ 1000 G.