Valerii Vinokour

An underlying mechanism of the high temperature superconductivity (HTS) and the nature of the adjacent phases remains the biggest mystery of condensed matter physics. It is believed that the key to understanding of HTS lies in revealing the origin of the enigmatic properties of the pseudogap state that settles in the underdoped region between the superconducting transition temperature Tc and the pseudogap temperature T* > Tc. The experiments indicate that the pseudogap state is a distinct thermodynamic phase that exhibits metallic transport, magnetoelectric effect and the nematicity. We develop a unified theory that offers a quantitative description of the pseudogap phase properties and describes the observed phase diagram. The proposed mechanism of the superconductivity is the emergence of the condensate of dyons, the composite particles carrying both electric and magnetic charge, in our case the Cooper pair bound with the magnetic monopole. In the HTS phase, the dyon condensate coexists with the fundamental Cooper pair condensate and the elevated Tc results from the stabilizing effect of the monopole condensate. We show that in the pseudogap phase charged magnetic monopole condensate realizes the oblique confinement of Cooper pairs. The universality of the HTS phase diagram for different materials reflects the unique topological mechanism responsible for formation of the emerging phases. Our findings provide a topological reason for the high critical temperature in HTS.

Edwin Herrera

URu2Si2 is a heavy fermion system which crystallizes in a tetragonal structure and where superconductivity emerges inside the misterious hidden order phase. The latter consists of a still unknown type of order that appears together with a strong entropy reduction below 17.5 K. URu2Si2 becomes superconducting below 1.5 K. The hidden order phase is characterized by dynamical spin modes at q0=(0 0 1) and q1=(0.6 0 0). These quench into an antiferromagnetic order under pressure (q0=(0 0 1)) and at high magnetic fields (q1=(0.6 0 0)). Here I will show recent Scanning Tunneling Microscopy experiments (STM) at very low temperatures (0.1 K). I will report on the discovery of a charge modulation with a wavevector that is a moiré combination of the atomic lattice periodicity and q1, produced by fracturing the crystal in presence of the dynamical spin mode at q1. Our results suggest that charge interactions are a fundamental ingredient that competes with hidden order in URu2Si2 and advance controlled fracture as powerful means to obtain ground states derived from strong electronic correlations [1]. Furthermore, I will show results at surfaces with large amounts of atomically flat steps showing the U fourfold lattice. There we find a new heavy fermion 2D-electron gas type of surface state with an effective mass 17 times the free electron mass. We discuss lateral quantization of such 2D heavy electrons and the interaction of the surface band with bulk superconductivity.