Visualization of quasiparticles in quantum materials at high magnetic fields

Laburpena

The electronic band structure of solids contains relevant information about their behavior, particularly in metals and superconductors. There is a recent thrust that allows achieving detailed understanding of the band structure in an energy range just a few meV close to the Fermi level. This is supported by experiments that provide detailed measurements of the electronic band structure and in some cases by calculations. Scanning Tunneling Microscopy (STM) stands out because of the superior resolution in energy, the spatial imaging capabilities and because it provides empty as well as filled electronic states. Here, I will address the problem of understanding better superconductivity, for which I have chosen a few model systems. Superconductivity often emerges close to vanishing magnetism. However, the emergence of superconductivity often obscures the processes that destroy magnetism. To understand such processes, I have first studied the bandstructure of Ce(Ru0.92Rh0.08)2Si2, with a magnetic ground state that can be easily modified by an applied magnetic field. My results show that the Zeeman splitting by the magnetic field eliminates magnetism, without radically modifying the low energy band structure, which is dominated by Kondo hybridization between Ce 4f-electrons and conduction electrons. I have then addressed the band structure of WTe2, finding a band structure close to the Fermi level that compares very well to density functional calculations. I have shown that in this case the band structure does not change when applying a magnetic field. I have furthermore addressed the surface band structure, finding relevant features that point to the presence of surface bands close to the topologically non-trivial type II Weyl points of the bulk band structure. I have then analyzed a superconducting system, Au2Pb, where superconductivity arises in a phase where a structural distortion has opened a gap in a Dirac cone of the band structure. I will show that the superconducting density of states is finite at the Fermi level and explore the possibility that this feature is connected to the closing of the Dirac cone at the surface. I will discuss possible superconducting states that might arise in this situation. Finally, I have studied the superconductor FeSe. I have measured the temperature and magnetic field dependence of the superconducting gap and the vortex lattice in a hitherto unexplored field range (up to 15 T). My results show that the bottom of an electron band lies completely within the superconducting gap. This peculiar situation produces a hitherto unobserved electron-hole asymmetry in the superconducting density of states. Furthermore, I find a new charge density wave at high magnetic fields, whose wavevector varies with field together with the wavevector of the vortex lattice. This unique effect might be related to the peculiar low energy band structure of this system.