Quantum Matter Seminar

Kramers degeneracy (KD) is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry (TRS). For electrons in solids, KD can be extended to finite momentum by additional crystalline symmetries, while breaking TRS lifts KD and reshapes the topology and spin texture of Bloch electrons.

In the first part of the talk, using angle-resolved photoemission spectroscopy (ARPES) with spin resolution and ab initio calculations, we demonstrate the topological consequences of KD through Kramers nodal lines in intercalated transition metal dichalcogenide (TMD)superconductors. On the other hand, with TRS broken, we provide strong evidence that KDlifted by magnetism gives rise to unconventional momentum-space spin textures in ultra-thin epitaxially strained RuO2 films, a debated altermagnet candidate that remains poorly understood in the ultra-thin regime where substrate strain can be substantial.

In the second part of the talk, we go beyond the single-particle picture and explore how TMD platforms can be designed to become strongly correlated in bulk single-crystal form. We introduce a class of underexplored Fe-based TMDs where ARPES experiments unambiguously reveal the emergence of flat bands at the Fermi level, consistently captured by our density functional theory plus dynamical mean-field theory predictions. The associated coherence temperature scales are corroborated by transport measurements. Such observations establish a new material platform at the intersection of strong correlations, TMDs, and potentially two-dimensional magnetism.