Research Seminar - Yanqi Wang

One-dimensional electron systems (1DESs) host unique quantum coherent states that are fundamentally different from those in higher-dimensions. For example, electron-electron interactions in 1DESs have been predicted to induce Tomonaga-Luttinger liquid (TLL) behavior. Naturally-occurring grain boundaries in single-layer semiconducting transition metal dichalcogenides provide 1D conducting channels that have been proposed to host TLL, but charge density wave physics has also been suggested to explain their behavior. Clear identification of the electronic ground state of this system has been hampered by an inability to electrostatically gate such boundaries and thereby tune their charge carrier concentration.  Here we present a scanning tunneling microscopy/spectroscopy (STM/STS) study of gate-tunable mirror twin boundaries (MTBs) in single-layer 1H-MoSe2 devices. Gating here enables STM spectroscopy to be performed for different MTB electron densities, thus allowing precise characterization of electron-electron interaction effects. Visualization of MTB electronic structure under these conditions allows unambiguous identification of collective density wave excitations having two distinct velocities. Theoretical analysis based on finite-length TLL model quantitatively explained the experimental observations originated from spin-charge separation. Our collaborative experimental and theoretical efforts demonstrated a novel pathway to measure, manipulate, and understand correlated quantum coherence at engineered interfaces of 2D materials.

Yanqi is a theoretical physicist studying condensed matter in the Moore group. He’s interested in topological materials and correlated systems, in particular the quantum coherence in engineered interfaces. Many interacting 1D electronic systems can be described by Luttinger liquid. Unlike the Fermi liquid which normally breaks down in 1D metals, there are no Landau quasiparticle-like excitations in Luttinger liquid whose dynamics are collective. This leads to numerous interesting phenomena such as spin-charge separation and Friedel oscillations. The interfaces or boundaries of 2D materials naturally serve as interacting 1D systems. For example, the mirror twin boundaries in single-layer 1H-MoSe2 could be captured by the finite-length Luttinger liquid model, and the boundary of 2D quantum spin Hall insulators such as WTe2 is predicted to hold topologically protected Helical Luttinger liquid. Yanqi is actively interacting with the experimental efforts in aforementioned materials from the theory side, and trying to extend the boundary physics into other areas like higher order topological insulators and Moiré systems. Additionally, he’s working on transport properties and optical response of quantum materials.