Upper critical field ofKFe2As2under pressure: A test for the change in the superconducting gap structure

Abstract

We report measurements of electrical resistivity under pressure to 5.8 GPa, magnetization to 6.7 GPa, and ac susceptibility to 7.1 GPa in KFe2As2. The previously reported change of slope in the pressure dependence of the superconducting transition temperature Tc(p) at a pressure p * ∼ 1.8 GPa is confirmed, and Tc(p) is found to be nearly constant above p * up to 7.1 GPa. The T -p phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of Tc. Across p * , a change in the behavior of the upper critical field is revealed through a scaling analysis of the slope of Hc 2 with the effective mass as determined from the A coefficient of the T 2 term of the temperature-dependent resistivity. We show that this scaling provides a quantitative test for the changes of the superconducting gap structure and suggests the development of a kz modulation of the superconducting gap above p * as a most likely explanation.

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductorsKFe2As2,RbFe2As2, andTafti1, Ouellet2, Juneau-Fecteau3 et al. 2015Phys. Rev. B

“…We show that the slope of H c2 (T ) jumps by a factor 2 across P c , in a manner that also appears to be universal. This is compelling evidence for a sudden change in the gap structure, 8 which we attribute to a change of pairing symmetry. The Fermi surface of KFe 2 As 2 is known in detail from quantum oscillations studies.…”

“…5,6 Our results on KFe 2 As 2 have been reproduced by other groups. [7][8][9] We interpreted the transition at P c as a change of pairing state, possibly from d to s ± , where the decreasing T c of the former meets the growing T c of the latter at P c , resulting in a V-shaped phase diagram.…”

Superconducting properties ofK1−xNaxFe2As2under pressure

“…The value of the critical pressure is nearly independent of the amount of the disorder in K-122 but it is reduced in the unsubstituted clean Cs-122 21 . However, V. Taufour et al, showed that the pressure dependence of T c for K-122 is sensitive to the pressure medium 19 , and that for the best pressure (hydrostatic) conditions the reversal behavior of T c is replaced by a broad minimum followed by a nearly pressure independent T c value above 2.5 GPa. From the almost pressure independent behavior of the Hall coefficient extrapolated to T = 0 as well as from the residual resistivity, it was concluded that the topology of the Fermi surface does not change across the critical pressure.…”

“…21 Alternatively, V. Taufour et al proposed that a change of symmetry under pressure may not occur at all. 19 These authors empirically supposed that the critical pressure could be a consequence of anisotropic pressure derivatives, since the pressure derivatives along the a axis and the c axis are negative and positive, respectively. In this case, the critical pressure corresponds to the critical pressure in the ab plane above which the slope of T c (P ) depends on the value of ∂T c /∂P c > 0, only, whereas ∂T c /∂P ab ≡ 0 for P ≥ 2 GPa.…”

High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase ofKFe2As2under high pressure

“…The enhancement (rather than suppression) of T c in SC1 can be possibly explained by the reduction in hydrostatic pressure conditions produced by the steatite powder in our DAC experiment, as compared to the previous clamp-cell experiments. While the level of hydrostaticity at low pressures is generally con- sidered good in this configuration, the sensitivity of T c in KFe 2 As 2 to uniaxial components may be susceptible to such differences and may explain the variation of T c values reported at low pressures [12,13,[16][17][18]. We can estimate the slope of T c as a function of uniaxial pressure by using the thermodynamic relation for the second-order transition, or the Ehrenfest relation [25,26],…”