Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2

TL;DR

This study explores the phase diagram and strong coupling effects in Ba1-xKxFe2As2 through thermodynamic measurements, revealing high critical field slopes, low anisotropy, and extreme type-II superconductivity.

Contribution

It provides detailed thermodynamic characterization of Ba1-xKxFe2As2, highlighting its strong coupling behavior and anisotropic properties near the superconducting transition.

Findings

High slopes of upper critical fields (dHc2/dT)

Low anisotropy gamma ~ 2.6 near Tc

Identification as an extreme type-II superconductor

Abstract

We present a thermodynamic study of the phase diagram of single-crystal Ba1-xKxFe2As2 using specific heat measurements. In zero-magnetic field a clear step in the heat capacity of deltaC/Tc = 0.1 J/f.u.K2 is observed at Tc = 34.6K for a sample with x = 0.4. This material is characterized by extraordinarily high slopes of the upper critical field of dHc2,c/dT= -6.5 T/K and dHc2,ab/dT= -17.4 T/K and a surprisingly low anisotropy of gamma ~ 2.6 near Tc. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of kappac ~ 100 and kappaab ~ 260 identifying Ba1-xKxFe2As2 as extreme type-II. The large value of the normalized discontinuity of the slopes of the specific heat at Tc, (Tc/deltaC)times delta(dC/dT)~ 6 indicates strong coupling effects in Ba1-xKxFe2As2.