Events

An alternative to the “standard” quantum spin liquid model, which was constructed as a gauge theory, will be presented. This new framework is a Landau-type effective theory. We will explore both quasiparticle excitations and collective modes, along with their physical implications.

We present recent progress in exploring the detection of superconducting collective modes in NbN, MgB2, and YBa2Cu3O6+x using innovative terahertz spectroscopy techniques. Transient Higgs oscillations and a higher-order nonlinear light-Higgs mode coupling effect were detected in NbN superconductors. In MgB2, the Higgs mode contribution from the dirty 3D pi-band and the Leggett mode contribution were identified. In YBCO samples, a beat pattern in the terahertz signal revealed a coupling effect between the Higgs mode and the mode developed below T*, indicating the pseudogap phase may be a distinct order, not a precursor to superconductivity. This strong coupling effect provides fresh insights into superconductivity and the pseudogap.

During this colloquium, the unnoticed beauty of multiband of iron-based superconductors will be highlighted, including i) emerge of topological band which coexists with superconductivity, leading to the discovery of Majorana zero mode; ii) Interplay of multicomponent superconductivity, leading to the discovery of exotic paring with time-reversal symmetry breaking and fractional vortex; iii) Contribution of Hund coupling towards pairing in this unique class of superconductors.