arXiv subjects

Yen-Chen Tsui

Publications and source records attributed to Yen-Chen Tsui.

5 recordsLinked to original sources

arXiv2026

Local spectroscopy of anyons bound to charge traps

Fractional quantum Hall states host anyons, emergent quasiparticles with fractional charge and nontrivial exchange statistics. Controlling, trapping, and braiding anyons are central goals for both fundamental physics and topological quantum computation. A key step toward such control is understanding how anyons behave when confined in local potentials, where their internal structure can become relevant. Here, we use the scanning tunneling microscopy/spectroscopy (STM/STS) to study the excitation spectrum in integer and fractional quantum Hall states of monolayer graphene near individual charged impurities. In the integer quantum Hall states, the STS spectra show lifting of orbital degeneracy near defects, appearing as a band of discrete energy levels. In fractional states, (v=1/3 and 2/5), however, we observe an additional energy splitting of the lowest-energy spectral feature that occurs only when the chemical potential lies within a fractional gap and is absent in compressible or integer regimes. We attribute this to many-body configurations of anyons trapped by an impurity potential. Strikingly, numerical calculations show that the splitting requires an anisotropic confining potential, vanishing for a rotationally symmetric trap. The competing multi-anyon states carry nearly identical charge within the core of the potential but differ in how that charge is redistributed at larger radius. Our results establish local tunneling spectroscopy as a direct probe of anyon bound states, providing a key step toward understanding and controlling their behavior in confined geometries relevant for braiding and fusion.

cond-mat.mes-hall↗

arXiv2026

Visualizing Symmetry Broken Chern Insulators and their Quantum Melting

In the presence of a magnetic field, electronic states of moiré quantum materials develop a Hofstadter spectrum that provides a unique setting for studying the interplay between band topology and strong electron-electron interaction. Using scanning tunneling microscopy, we study Hofstadter's states in bilayer graphene aligned with hexagonal BN and directly visualize the formation of interaction-driven symmetry breaking Chern insulators. Our measurements reveal the formation of phases that double, triple or quadruple the moiré unit cell at fractional filling of the Hofstadter bands, as well as states with complex intra-unit-cell wave functions. We visualize two distinct quantum phenomena in different Chern states, including quantum melting driven by the appearance and proliferation of topological defects, and a quantum transition co-occurring with phase competition and separation.

cond-mat.mes-hall↗

arXiv2025

High-Resolution Tunneling Spectroscopy of Fractional Quantum Hall States

Strong interaction between electrons in two-dimensional systems in the presence of a high magnetic field gives rise to fractional quantum Hall states that host quasiparticles with fractional charge and fractional exchange statistics. Here, we demonstrate high-resolution scanning tunneling microscopy and spectroscopy of fractional quantum Hall states in ultra clean Bernal-stacked bilayer graphene devices. Spectroscopy measurements show sharp excitations that have been predicted to emerge when electrons fractionalize into bound states of quasiparticles. We find energy gaps for candidate non-abelian fractional states that are larger by a factor of five than other related systems - for example semiconductor heterostructures - and this suggests bilayer graphene is an ideal platform for the manipulation of these quasiparticles and for the creation of a topological quantum bit. We also find previously unobserved fractional states in our very clean graphene samples.

cond-mat.mes-hall↗

arXiv2024

Fingerprints of Composite Fermion Lambda Levels in Scanning Tunneling Microscopy

Composite fermion (CF) is a topological quasiparticle that emerges from a non-perturbative attachment of vortices to electrons in strongly correlated two-dimensional materials. Similar to non-interacting fermions that form Landau levels in a magnetic field, CFs can fill analogous ``Lambda'' levels, giving rise to the fractional quantum Hall (FQH) effect of electrons. Here, we show that Lambda levels can be directly visualized through the characteristic peak structure in the signal obtained via spectroscopy with the scanning tunneling microscopy (STM) on a FQH state. Complementary to transport, which probes low-energy properties of CFs, we show that \emph{high-energy} features in STM spectra can be interpreted in terms of Lambda levels. We numerically demonstrate that STM spectra can be accurately modeled using Jain's CF theory. Our results show that STM provides a powerful tool for revealing the anatomy of FQH states and identifying physics beyond the non-interacting CF paradigm.

cond-mat.mes-hall↗

arXiv2023

Direct observation of a magnetic field-induced Wigner crystal

Eugene Wigner predicted long ago that when the Coulomb interactions between electrons become much stronger than their kinetic energy, electrons crystallize into a closely packed lattice. A variety of two-dimensional systems have shown evidence for Wigner crystals; however, a spontaneously formed classical or quantum Wigner crystal (WC) has never been directly visualized. Neither the identification of the WC symmetry nor direct investigation of its melting has been accomplished. Here we use high-resolution scanning tunneling microscopy (STM) measurements to directly image a magnetic field-induced electron WC in Bernal-stacked bilayer graphene (BLG), and examine its structural properties as a function of electron density, magnetic field, and temperature. At high fields and the lowest temperature, we observe a triangular lattice electron WC in the lowest Landau Level (LLL) of BLG. The WC possesses the expected lattice constant and is robust in a range of filling factors between $ν\sim$ 0.13 and $ν\sim$ 0.38 except near fillings where it competes with fractional quantum Hall (FQH) states. Increasing the density or temperature results in the melting of the WC into a liquid phase that is isotropic but has a modulated structure characterized by the WC's Bragg wavevector. At low magnetic fields, the WC unexpectedly transitions into an anisotropic stripe phase, which has been commonly anticipated to form in higher LLs. Analysis of individual lattice sites reveals signatures that may be related to the quantum zero-point motion of electrons in the WC lattice.

cond-mat.mes-hall↗