Quantum matter in high magnetic fields

Abstract:

We investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1 xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. The unusually small and strongly elongated Fermi surface of FeSe increases monotonically with chemical pressure, x, due to the suppression of the in-plane anisotropy except for the smallest orbit which suffers a Lifshitz-like transition once nematicity disappears. Even outside the nematic phase the Fermi surface continues to increase, in stark contrast to the reconstructed Fermi surface detected in FeSe under applied external pressure. We detect signatures of orbital-dependent quasiparticle mass renomalization suppressed for those orbits with dominant dxz=yz character, but unusually enhanced for those orbits with dominant dxy character. The lack of enhanced superconductivity outside the nematic phase in FeSe1 xSx suggest that nematicity may not play the essential role in enhancing Tc in these systems

Abstract:

The presence of dxz−dyz orbital ordering is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but the details of the order parameter remain controversial. Here, we report a high-resolution angle-resolved photoemission spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90 K, but, in contrast, the changes to the electron pockets do not require fourfold symmetry breaking. Instead, there is an additional separation of the existing dxy and dxz/yz bands, which themselves are not split within resolution. These observations lead us to propose a scenario of “unidirectional nematic bond ordering” to describe the low-temperature electronic structure of FeSe, supported by good agreement with ten-orbital tight-binding model calculations.

Abstract:

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.

Abstract:

By solving the Boltzmann transport equation we investigate theoretically the general form of oscillations in the resistivity caused by varying the direction of an applied magnetic field for the case of quasi-two dimensional systems on hexagonal lattices. The presence of the angular magnetoresistance oscillations can be used to map out the topology of the Fermi surface and we study how this effect varies as a function of the degree of interplane warping as well as a function of the degree of isotropic scattering. We find that the angular dependent effect due to in-plane rotation follows the symmetry imposed by the lattice whereas for inter-plane rotation the degree of warping dictates the dominant features observed in simulations. Our calculations make predictions for specific angle-dependent magnetotransport signatures in magnetic fields expected for quasi-two dimensional hexagonal compounds similar to PdCoO2 and PtCoO2.