Vortex-lattice melting and paramagnetic depairing in the nematic superconductor FeSe

Abstract

The full H-T phase diagram in the nematic superconductor FeSe is mapped out using specific-heat and thermal-expansion measurements down to 0.7 K and up to 30 T for both field directions. A clear thermodynamic signal of an underlying vortex-melting transition is found in both datasets and could be followed down to low temperatures. The existence of significant Gaussian thermal superconducting fluctuations is demonstrated by a scaling analysis, which also yields the mean-field upper critical field Hc2(T). For both field orientations, Hc2(T) shows Pauli-limiting behavior. Whereas the temperature dependence of the vortex-melting line is well described by the model of Houghton et al., Phys. Rev. B 40, 6763 (1989) down to the lowest temperatures for H $\perp$ FeSe layers, the vortex-melting line exhibits an unusual behavior for fields parallel to the planes, where the Pauli limitation is much stronger. Here, the vortex-melting anomaly is only observed down to T*= 2-3 K, and then merges with the Hc2(T) line as predicted by Adachi and Ikeda, Phys. Rev. B 68 184510 (2003). Below T*, Hc2(T) also exhibits a slight upturn possibly related to the occurence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state.

7 figures, 11 pages

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• Multi-band description of the upper critical field of bulk FeSe
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• Vortex Lattice Melting Line in Superconductors with Paramagnetic Pair-Breaking
• Dimensionality of the superconductivity in the transition metal pnictide WP
• Chemical pressure tuning of competing orders in ${\text{Ba}}_{1-x}{\text{Ca}}_x{\text{Ni}}_2{\text{As}}_2$
• Interplay of stripe and double-Q magnetism with superconductivity in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ under the influence of magnetic fields
• On the existence of nematic-superconducting states in the Ginzburg-Landau regime
• Vanishing of Resistivity upon Freezing of Vortex Liquid in Clean Superconductors
• Designing high-performance superconductors with nanoparticle inclusions: comparisons to strong pinning theory