Unconventional Quantum States in a Kagome Superconductor

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Abstract
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The kagome lattice offers a unique electronic landscape, characterized by flat bands and van Hove singularities. These features enhance electronic correlations and drive instabilities that lead to phenomena such as superconductivity and charge density waves. Here, I will discuss our investigation into the kagome-lattice superconductors AV3Sb5 (where A = K, Cs). First, I will describe our approach to probing broken symmetries in the normal state through the material's nonlinear electromagnetic response to circularly polarized light [1], along with ongoing research in my group related to this effort. Subsequently, I will share our discovery of two distinct superconducting regimes in CsV3Sb5, intriguingly observed without any clear phase transition between them [2]. These regimes exhibit markedly different characteristics: the low-temperature phase reveals a second superconducting gap and supports low-energy quasiparticles, which may arise from a nodal gap structure. Time permitting, I will conclude with recent experimental work in my group aimed at understanding the superconducting order parameter.

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About the Speaker
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Md Shafayat Hossain is an assistant professor in Materials Science and Engineering at the UCLA Samueli School of Engineering, effective in July 2025. Before UCLA, he was a lecturer and postdoctoral associate in Physics at Princeton University. He earned his Ph.D. in Electrical Engineering and Materials Science from Princeton. His group at UCLA seeks to (1) develop topological materials and devices that can function at room temperature and (2) build topological quantum computation platforms where anyons can be controlled and braided. His work has led to discoveries such as the first room-temperature, ambient-pressure quantum state and several elusive quantum phases, including the topological excitonic insulator, Bloch ferromagnet, band-selective superconductivity, and hybrid topological phases, with publications in Nature, Nature Physics, Nature Materials, and PRL.