Abstract
Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of an Fe-based superconductor FeSe with applied magnetic field up to 16 T. We found that the temperature dependence of the upper critical field Hc2(T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of Hc2,∥c(0) and Hc2,∥ab(0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-Tc superconductors, lower values of Hc2(0) and higher superconducting anisotropy Γ(0) were observed in FeSe.
Highlights
Superconductivity in Fe-based superconductors has been widely studied since their first discovery in 2008 due to its potential application and the needs of its understanding in physics.1 The family of Fe-based superconductors can be mainly categorized into “1111-type” RFeAs(O,F) (R = rare earth),1–4 “122-type” material (e.g., BaFe2As2),5,6 “111-type” LiFeAs,7,8 and “11-type” iron chalcogenides9,10 according to their crystal structure
We found that the temperature dependence of the upper critical field Hc2(T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity
We reported the measurements of the superconducting anisotropy involving temperature-dependent upper critical field Hc2(T) of FeSe single crystals by a resonance frequency technique, which uses a capacitance tuning LC circuit newly designed by ourselves
Summary
One of the good candidates is the recently developed resonance frequency technique, which utilizes the direct inductance change in a resonant LC circuit induced by the bulk magnetic susceptibility and/or electrical resistivity change of the sample during the appearance of a superconducting phase transition.23–25 This novel method can sensitively track the effect of the superconducting vortex phases on the shielding of the radio frequency (RF) magnetic field in a sample coil and was used to determine Hc2 and Tc.. We showed that the experimental data for both field directions at H0 ∥ c and H0 ∥ a&b coincide with the Werthamer-HelfandHohenberg (WHH) model when we consider the Maki parameter α, suggesting an important role of spin-paramagnetic effect in the pair-breaking mechanism in the superconductivity of FeSe. The zero-temperature upper critical fields Hc2,∥c(0) and Hc2,∥ab(0) were derived to be 12 T and 29 T from the WHH fitting, for H0 ∥ c and H0 ∥ a&b, respectively, which give an anisotropy parameter Γ(0) ∼ 2.4. Lower upper critical field and somewhat higher superconducting anisotropy at zero temperature were observed in FeSe in comparison with those of other typical ironbased superconductors
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