The upper critical field and its anisotropy in (Li_1−xFe_x)OHFe_1−ySe

Abstract

The temperature dependence of the upper critical field (H ) in a (Li Fe )OHFe Se single crystal ([Formula: see text] K) has been determined by means of magnetotransport measurements down to 1.4 K both for inter-plane ([Formula: see text], [Formula: see text]) and in-plane ([Formula: see text], [Formula: see text]) field directions in static magnetic fields up to 14 T and pulsed magnetic fields up to 70 T. [Formula: see text] exhibits a quasilinear increase with decreasing temperature below the superconducting transition and can be described well by an effective two-band model with unbalanced diffusivity, while [Formula: see text] shows a flattening below 35 K and follows the Werthamer-Helfand-Hohenberg (WHH) model incorporating orbital pair-breaking and spin-paramagnetic effects, yielding zero-temperature critical fields of [Formula: see text] T and [Formula: see text] T. The anisotropy of the upper critical fields, [Formula: see text] monotonically decreases with decreasing temperature from about 7 near T to 1.5 at 0 K. This reduced anisotropy, observed in most Fe-based superconductors, is caused by the Pauli limitation of [Formula: see text].

Strong Pauli paramagnetic effect in the upper critical field of KCa$_2$Fe$_4$As$_4$F$_2$

“…As the temperature decreases further, this steep tendency is interrupted severely, and the H ab c2 − T curve reveals strongly convex curvature in the temperature region 25-30 K. On the contrary, the out-of-plane H c c2 (T ) shows a linear temperature-dependent behavior in fields exceeding 8 T. Below 8 T, the H c c2 − T curve displays a tail-like feature with a low slope. Similar features are also observed in other systems of FeSCs 27,28,36 .…”

“…3. Note that the value of α obtained herein is very large, and the previous maximum value of α = 6.5 is found in the 11111 system 28 . This indicates a very strong spin-paramagnetic effect in the present system.…”

“…This difference in behavior is a consequence of the different types of vortices in the different field orientations in a quasi-two-dimensional layered material 35 . It is believed that the presence of SC fluctuation near the onset of the SC transition and vortex-liquid state near zero-resistivity will affect the determination of upper critical field 27,28 . Hence, the criteria 50%ρ n was adopted herein.…”

“…Generally, for type-II superconductors, the upper critical field can be affected by the interactions between the external magnetic field and orbital motion of the SC electrons, by the effect of the field on the electron spin magnetic moments (Pauli paramagnetic effect), and by the spin-orbit scattering [19][20][21][22] . In the case of FeSCs, the Pauli paramagnetic effect is significant, severely limiting the in-plane H ab c2 when approaching zero temperature [23][24][25][26][27][28] . Therefore, almost all the FeSCs exhibit isotropic characteristics in low temperature.…”

Coexistence of multiphase superconductivity and ferromagnetism in lithiated iron selenide hydroxide [(Li1−xFexUrban1, Valmianski2, Pachmayr3 et al. 2018Phys. Rev. B4020View full textAdd to dashboardCite

“…An experimentally observable magnetic crossover can be seen in figure 3 b five K lower, at T M = 12 K at which the H c2 (T) curvature changes similar to low temperature magnetic superconductors and to several related theoretical publications, which implies a change in magnetic ordering at this observed crossover temperature, T M = 12 K. 10,46,47 H c2 I (0K) = 9 T, derived from the high temperature region, is comparable to the upper critical field at zero temperature obtained from the experimental data in this regime with the Werthamer-Helfand Hohenberg (WHH) aproximation. 48 Earlier published values for LISH are in the same order of magnitude as H c2 II (0K) = 120 T. 49,50 The superconducting coherence lengths ξ I, II (T=0) were extracted using the Ginzburg Landau equations 51,52 with ! = 2•10 !!…”