R. G. Herb Condensed Matter Seminars

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VdW heterostructures: a new route to designing quantum matters
Date: Thursday, February 23rd
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Xiameng Liu, Princeton
Abstract: From superconductivity to fractionalized particles, fascinating phenomena arise in quantum materials due to the collective behaviors of electrons. These quantum effects challenge our intuition about nature and provide new opportunities for future quantum information technologies. Recently, two-dimensional materials and their heterostructures have become a leading platform for realizing new quantum states of matter. Mechanically assembled layer-by-layer and held together by the van der Waals (vdW) force, vdW heterostructures break through the limitations of traditional material synthesis and offer entirely new ways to create quantum matters. My talk will feature two examples of designing quantum matters with vdW heterostructures. In the first example, I will illustrate how Coulomb interactions across separate atomic layers pair fermions (electrons and holes) into bosons to achieve a superfluid condensate state. Thanks to the tunability of the vdW platform, we can vary the pairing strength and change the nature of this fermion condensate from strong coupling to weak coupling, demonstrating a long-sought paradigm known as the BEC-BCS crossover. In the second example, I will introduce the idea of moiré band engineering, where the interference between two atomic lattices—named the moiré pattern—defines a new periodicity and reforms electronic band structures. In twisted double bilayer graphene, such moiré periodicity creates highly-degenerate bands tunable by a perpendicular electric field. We observed electron correlation effects, including interaction-driven insulation and spontaneous symmetry breaking of spins. Their evolution with the electric fields reveals their close connection with the moiré band features. Finally, I will briefly discuss applications of local probe techniques to uncover hidden quantum properties in vdW platforms and share visions of leveraging rich interplays across atomic interfaces to access major themes in condensed matter physics.
Host: Victor Brar
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