290S/290K Quantum Materials Seminar speakers Yi Lin and Luke Pritchard Cairns (both UCB) on Wednesday, November 9 at 2:00 pm in Physics South 402

Time/Venue Wednesday, November 9 at 2:00 pm PST in Physics South 402
Host Bob Birgeneau
Speaker Yi Lin (Lanzara Group)
Mapping Ultrafast Excitonic Phenomena in 2D Materials by Using Time-Resolved ARPES
Excitonic correlations between electrons and holes play the key role for understanding the optical phenomena, many-body interactions and exotic phase transitions in semiconducting materials and excitonic insulators, which have been conventionally studied by optical spectroscopies. In this talk, we present our experimental and theoretical work toward probing ultrafast excitonic phenomena in 2D materials by using extreme-UV time- and angle-resolved photoemission spectroscopy (XUV-trARPES). By photoemitting the electrons from different types of correlated electron-hole gases in the material, we made ultrafast movies of electronic band structure impacted by the excitonic correlations. Our results visualized a non-resonant exciton formation process and revealed exciton-driven band renormalization effects, featuring surprising bandgap opening and band flattening. Our work demonstrates a new pathway for studying excitonic many-body phenomena by using photoemission techniques under a single-particle narrative fully in energy, momentum and time.
Luke Pritchard Cairns (Analytis Group)
Tracking the evolution from isolated dimers to many-body entanglement in NaLuxYb1−xSe2
Abstract NaYbSe2 belongs to a recently discovered class of Yb delafossites, the majority of which have been shown to exhibit the hallmarks of a quantum spin liquid (QSL) – for example an absence of long-range magnetic order and a large anomalous heat capacity at the lowest measurable temperatures. However, given the small regions of parameter space QSL states can occupy for the triangular lattice, it seems unlikely that such a varied collection of compounds should all satisfy the constraints. In this study we look to diverge from the typical investigation of a QSL, and have instead doped NaYbSe2 with non-magnetic Lu. In this way, we are able to investigate the evolution of magnetic correlations, from isolated dimers, through the percolation transition and building towards the highly-correlated many-body entangled state in NaYbSe2. We hope that this might shed light on the proposed QSL state in this compound, and also the broader class of materials.

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