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Abstract:

This data set was created as a part of the transport and torque measurements to understand the behaviour of the upper critical field of FeSe. These data correspond to the publication with the same name entitled: Multi-band description of the upper critical field of bulk FeSe, by M. Bristow et al. to appear in Physical Review B. The measurements were performed as a function of magnetic fields at constant temperatures using a low temperature cryostat in Oxford and at the High Field Magnet Laboratory (HFML) in Nijmegen up to 35T. This data set contains ASCII files and the different folders and subfolders contain the files corresponding to particular figure and subfigure, respectively. The data were analysed using the WHH model (Phys. Rev.147, 295 (1966)) and the two-band model, as described in Phys. Rev. B 82, 184504 (2010). The Fermi surface was calculated using the Wien2k and using the experimental parameters and the .struct file is provided.

Abstract:

Superconductivity in FeSe is strongly enhanced under applied pressure and it is proposed to emerge from anomalously coupled structural and magnetic phases. Small impurities inside the Fe plane can strongly disrupt the pair formation in FeSe at ambient pressure and can also reveal the interplay between normal and superconducting phases. Here, we investigate how an impurity inside the Fe plane induced by the Cu substitution can alter the balance between competing electronic phases of FeSe at high pressures. In the absence of an applied magnetic field, at low pressures the nematic and superconducting phases are suppressed by a similar factor. On the other hand, at high pressures, above 10 kbar, the superconductivity remains unaltered despite the lack of any signature in transport associated to a magnetic phase in zero-magnetic field. However, by applying a magnetic field, the resistivity displays an anomaly preceding the activated behavior in temperature, assigned to a magnetic anomaly. We find that the high-pressure superconducting phase of FeSe is robust and remains enhanced in the presence of Cu impurity, whereas the magnetic phase is not. This could suggest that high-Tc superconductivity has a sign-preserving order parameter in the presence of a rather glassy magnetic phase.