Upper critical magnetic field in Ba_0.68K_0.32Fe_2As_2 and Ba(Fe_0.93Co_0.07)_2As_2

TL;DR

This study measures the upper critical magnetic field in two iron-based superconductors, revealing different behaviors and anisotropies, and models their temperature dependence considering Pauli spin paramagnetism and multi-gap effects.

Contribution

It provides detailed H-T phase diagrams and analyzes the anisotropic Hc2(T) behavior, highlighting differences between hole-doped and electron-doped compounds.

Findings

Concave curvature of Hc2(T) in Ba_0.68K_0.32Fe_2As_2 with decreasing anisotropy.

Pauli spin paramagnetism explains Hc2(T) in Ba_0.68K_0.32Fe_2As_2, but not in Ba(Fe_0.93Co_0.07)_2As_2.

Hc2(T) in Ba(Fe_0.93Co_0.07)_2As_2 fits Werthamer Helfand Hohenberg model for H || ab and shows two-gap behavior for H || c.

Abstract

We report measurements of the temperature dependence of the radio-frequency magnetic penetration depth in Ba_0.68K_0.32Fe_2As_2 and Ba(Fe_0.93Co_0.07)_2As_2 single crystals in pulsed magnetic fields up to 60 T. From our data, we construct an H-T phase diagram for the inter-plane (H || c) and in-plane (H || ab) directions for both compounds. For both field orientations in Ba_0.68K_0.32Fe_2As_2, we find a concave curvature of the Hc2(T) lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we can describe Hc2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair breaking is not essential for Ba(Fe_0.93Co_0.07)_2As_2. For this electron-doped compound, the data support a Hc2(T) dependence that can be described by the Werthamer Helfand Hohenberg model for H || ab and a two-gap behavior for H || c.