Research Interests
Two-dimensional moiré materials and exotic electronic structure
When two monolayers of crystalline patterns are stacked with a small misalignment, their interference creates a moir´e superlattice with long wave- length. This allows the long-range Coulomb interaction to dominate when a dilute gas of carriers is doped into the bilayer. I use computational algorithms, such as Hartree-Fock (HF), density functional theory (DFT), quantum Monte Carlo (QMC) to explore and predict the properties of these exotic electronic phases.
Ferromagnetic semimetal and charge-density wave phases of interacting electrons in a honeycomb moiré potential
Yubo Yang, Miguel A. Morales, Shiwei Zhang
The exploration of quantum phases in moir´e systems has drawn intense experimental and theo- retical efforts. The realization of honeycomb symmetry has been a recent focus. The combination of strong interaction and honeycomb symmetry can lead to exotic electronic states such as fractional Chern insulator, unconventional superconductor, and quantum spin liquid. Accurate computations in such systems, with reliable treatment of strong long-ranged Coulomb interaction and approaching the large system sizes to extract thermodynamic phases, are mostly missing. We study the two- dimensional electron gas on a honeycomb moir´e lattice at quarter filling, using fixed-phase diffusion Monte Carlo. The ground state phases of this important model are determined in the parameter regime relevant to current experiments. With increasing moir´e potential, the systems transitions from a paramagnetic metal to an itinerant ferromagnetic semimetal and then a charge-density-wave insulator.
Observation of an electronic microemulsion phase emerging from a quantum crystal-to-liquid transition
Jiho Sung, Jue Wang, Ilya Esterlis, Pavel A. Volkov, Giovanni Scuri, You Zhou, Elise Brutschea, Takashi Taniguchi, Kenji Watanabe, Yubo Yang, Miguel A. Morales, Shiwei Zhang, Andrew J. Millis, Mikhail D. Lukin, Philip Kim, Eugene Demler, Hongkun Park
Strongly interacting electronic systems possess rich phase diagrams resulting from the competition between different quantum ground states. A general mechanism that relieves this frustration is the emergence of microemulsion phases, where regions of different phase self-organize across multiple length scales. The experimental characterization of these phases often poses significant challenges, as the long-range Coulomb interaction microscopically mingles the competing states. Here, we use cryogenic reflectance and magneto-optical spectroscopy to observe the signatures of the mixed state between an electronic Wigner crystal and an electron liquid in a MoSe2 monolayer. We find that the transit into this 'microemulsion' state is marked by anomalies in exciton reflectance, spin susceptibility, and Umklapp scattering, establishing it as a distinct phase of electronic matter. Our study of the two-dimensional electronic microemulsion phase elucidates the physics of novel correlated electron states with strong Coulomb interactions.
Metal-insulator transition in transition metal dichalcogenide heterobilayer: accurate treatment of interaction
Yubo Yang, Miguel A. Morales, Shiwei Zhang
Transition metal dichalcogenide superlattices provide an exciting new platform for exploring and understanding a variety of phases of matter. The moiré continuum Hamiltonian, of two-dimensional jellium in a modulating potential, provides a fundamental model for such systems. Accurate computations with this model are essential for interpreting experimental observations and making predictions for future explorations. In this work, we combine two complementary quantum Monte Carlo (QMC) methods, phaseless auxiliary field quantum Monte Carlo and fixed-phase diffusion Monte Carlo, to study the ground state of this Hamiltonian. We observe a metal-insulator transition between a paramagnetic and a 120∘ Néel ordered state as the moiré potential depth and the interaction strength are varied. We find significant differences from existing results by Hartree-Fock and exact diagonalization studies. In addition, we benchmark density-functional theory, and suggest an optimal hybrid functional which best approximates our QMC results.
Quantum Monte Carlo (QMC) and “Neural Quantum States” (NQS)
The wavefunction of a correlated many-body system is a map from the high-dimensional configurations space of all N quantum particles to a complex-valued “wavefunction amplitute”. The complexity and high-dimensional nature of this wavefunction precludes simple analytical treatment, typically built on a non-interacting approximation. Quantum Monte Carlo (QMC) is a computational method that efficiently sample the high-dimensional space to directly work with accurate wavefunctions to compute properties.
Ground state phases of the two-dimension electron gas with a unified variational approach
Conor Smith, Yixiao Chen, Ryan Levy, Yubo Yang, Miguel A. Morales, Shiwei Zhang
The two-dimensional electron gas (2DEG) is a fundamental model, which is drawing increasing interest because of recent advances in experimental and theoretical studies of 2D materials. Current understanding of the ground state of the 2DEG relies on quantum Monte Carlo calculations, based on variational comparisons of different ansatze for different phases. We use a single variational ansatz, a general backflow-type wave function using a message-passing neural quantum state architecture, for a unified description across the entire density range. The variational optimization consistently leads to lower ground-state energies than previous best results. Transition into a Wigner crystal (WC) phase occurs automatically at rs = 37 +/- 1, a density lower than currently believed. Between the liquid and WC phases, the same ansatz and variational search strongly suggest the existence of intermediate states in a broad range of densities, with enhanced short-range nematic spin correlations.
