Possible field-tuned superconductor-insulator transition in high-Tc superconductors: implications for pairing at high magnetic fields

The behavior of some high temperature superconductors (HTSC), such as La(2-x)Sr(x)CuO(4) and Bi(2)Sr(2-x)La(x)CuO(6 + delta), at very high magnetic fields, is similar to that of thin films of amorphous InOx near the magnetic-field-tuned superconductor-insulator transition. Analyzing the InOx data at high fields in terms of persisting local pairing amplitude, we argue by analogy that the local pairing amplitude also persists well into the dissipative state of the HTSCs, the regime commonly denoted as the "normal state" in very high magnetic field experiments.     

Optical nonreciprocity in magnetic structures related to high-Tc superconductors

Rotation of the plane of polarization of reflected light (Kerr effect) is a direct manifestation of broken time-reversal symmetry and is generally associated with the appearance of a ferromagnetic moment. Here I identify magnetic structures that may arise within the unit cell of cuprate superconductors that generate polarization rotation despite the absence of a net moment. For these magnetic symmetries the Kerr effect is mediated by magnetoelectric coupling, which can arise when antiferromagnetic order breaks inversion symmetry. The structures identified are candidates for a time-reversal breaking phase in the pseudogap regime of the cuprates.     

Layered superconductors in high magnetic fields

We investigate the possible occurrance of partially depaired states in superconducting intercalated layered systems. Those states are discussed as a possible explanation of the high critical fields found in some of these materials. It is shown that the Chandrasekhar-Clogston limit does not apply to those states mentioned above and that the maximum field compatible with superconductivity is a sensitive function of the shape of the Fermi surface. Mean free path and spin-orbit effects on the partially depaired state are investigated. An experiment is proposed to decide between the partially depaired state and a large spin-orbit scattering rate as possible explanations for the large critical fields.     

Iron-based superconductors in high magnetic fields

Here we review measurements of the normal and superconducting state properties of iron-based superconductors using high magnetic fields. We discuss the various physical mechanisms that limit superconductivity in high fields, and the information on the superconducting state that can be extracted from the upper critical field, but also how thermal fluctuations affect its determination by resistivity and specific heat measurements. We also discuss measurements of the normal state electronic structure focusing on measurement of quantum oscillations, particularly the de Haas–van Alphen effect. These results have determined very accurately, the topology of the Fermi surface and the quasi-particle masses in a number of different iron-based superconductors, from the 1111, 122 and 111 families.     

Fermi-liquid-based theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductors

Using a generalized RPA-type theory we calculate the in-plane anisotropy of the magnetic excitations in hole-doped high-Tc superconductors. Extending our earlier Fermi-liquid-based studies on the resonance peak by inclusion of orthorhombicity we still find two-dimensional spin excitations, however, being strongly anisotropic. This reflects the underlying anisotropy of the hopping matrix elements and of the resultant superconducting gap function. We compare our calculations with new experimental data on fully untwinned YBa2Cu3O6.85 and find good agreement. Our results are in contrast to earlier interpretations on the in-plane anisotropy in terms of stripes [H. Mook, Nature (London) 404, 729 (2000)], but reveal a conventional solution to this important problem.