arXiv subjects

Woochang Kim

Publications and source records attributed to Woochang Kim.

8 recordsLinked to original sources

arXiv2026

Moir\'e Phonons and Emergent Exciton-Phonon Coupling in a Moir\'e Heterobilayer

Moir\'e superlattices have emerged as a new platform for engineering electronic and optical properties in van der Waals heterostructures, enabling control over correlated and excitonic phenomena. Yet the impact of moir\'e superlattices on exciton-phonon coupling remains largely unexplored. Here we demonstrate emergent, layer-selective coupling between moir\'e phonons and moir\'e excitons in angle-aligned WS2/WSe2 heterobilayers. Using a broadband terahertz phonon transducer, we coherently launch moir\'e phonons that resonantly perturb the excitonic states. We show that the exciton-phonon coupling is intrinsically modified by the moir\'e superlattice in a layer-selective manner. A driven oscillator model captures the dynamics, revealing three moir\'e phonon resonances with distinct coupling to the moir\'e excitons. First principles calculations show that many moir\'e phonon modes can arise with distinct strongly hybridized in-plane and out-of-plane vibrations in the moir\'e unit cells. The calculations further identify the three experimentally observed moir\'e phonons and their emergent characteristic coupling to the moir\'e excitons.

cond-mat.mtrl-sci↗

arXiv2026

Imaging Interacting Two-Dimensional Anisotropic Electrons

We directly visualize a two-dimensional anisotropic Wigner crystal and its quantum melting in monolayer 1T-ReSe2 using non-invasive scanning tunnelling microscopy. In crystals with anisotropic effective mass, an electron's quantum wavefunction becomes elongated along the light-mass direction to reduce kinetic energy. At low electron density, such anisotropic electrons are predicted to form an oblique Wigner crystal rather than the familiar triangular lattice of isotropic systems. Despite longstanding theoretical interest, this physics has been little explored experimentally. Here we first image the anisotropic shape of individual electrons in gated monolayer ReSe2, whose wavefunctions are strongly elongated along the light-mass direction. At low density, these electrons crystallize into an oblique Wigner lattice. As the density increases, quantum fluctuations grow more rapidly along the light-mass direction than along the heavy-mass direction, driving a one-dimensional melting of the crystal. The resulting state retains order along one direction but melts along the other, consistent with a smectic electron liquid crystal between the electron solid and Fermi liquid phases. Our work establishes monolayer ReSe2 as a platform for studying anisotropic correlated electrons, quantum melting, and coupled one-dimensional electron chains.

cond-mat.str-el↗

arXiv2026

Ab initio quantum embedding description of magic angle twisted bilayer graphene at even-integer fillings

Magic angle twisted bilayer graphene (MATBG) hosts narrow moir\'e bands with meV-scale energy splittings, making its correlated phases sensitive to both material parameters and modeling choices in low-energy downfolding. We develop an ab initio quantum-embedding workflow that derives interacting flat-band Hamiltonians from Kohn-Sham density functional theory (KS-DFT) of a relaxed, unstrained structure. Our model combines constrained random phase approximation (cRPA) screening, controlled double-counting subtraction, and an automated gauge-fixing procedure based on the selected columns of the density matrix (SCDM) that is compatible with symmetry-resolved many-body calculations. Solving the resulting models using Hartree-Fock (HF) and coupled cluster singles and doubles (CCSD), we recover robust insulating Kramers intervalley coherent (KIVC) states at charge neutrality ($\nu=0$) and at electron doping ($\nu=+2$). The main new physical effect appears on the hole-doped side: at $\nu=-2$ we observe a fragile semimetal with a weak $\sqrt{3}\times\sqrt{3}$ Kekul\'e modulation and enhanced intervalley-scattering peaks in the Fourier-transformed local density of states. Although the underlying KS-DFT band structure is nearly particle-hole symmetric, the effective interacting Hamiltonian exhibits a pronounced particle-hole asymmetry at $\nu=\pm 2$ that we trace to momentum-dependent single-particle renormalizations generated by subtraction terms constructed from reference densities consistent with the KS-DFT filling. Our work provides a first-principles route for connecting microscopic electronic structure, screened interactions, subtraction choices, and scanning tunneling microscopy signatures in MATBG.

cond-mat.str-el↗

arXiv2024

Learning to Scale Logits for Temperature-Conditional GFlowNets

GFlowNets are probabilistic models that sequentially generate compositional structures through a stochastic policy. Among GFlowNets, temperature-conditional GFlowNets can introduce temperature-based controllability for exploration and exploitation. We propose \textit{Logit-scaling GFlowNets} (Logit-GFN), a novel architectural design that greatly accelerates the training of temperature-conditional GFlowNets. It is based on the idea that previously proposed approaches introduced numerical challenges in the deep network training, since different temperatures may give rise to very different gradient profiles as well as magnitudes of the policy's logits. We find that the challenge is greatly reduced if a learned function of the temperature is used to scale the policy's logits directly. Also, using Logit-GFN, GFlowNets can be improved by having better generalization capabilities in offline learning and mode discovery capabilities in online learning, which is empirically verified in various biological and chemical tasks. Our code is available at \url{https://github.com/dbsxodud-11/logit-gfn}

cs.LG↗

arXiv2024

Imaging Tunable Luttinger Liquid Systems in van der Waals Heterostructures

One-dimensional (1D) interacting electrons are often described as a Luttinger liquid1-4 having properties that are intrinsically different from Fermi liquids in higher dimensions5,6. 1D electrons in materials systems exhibit exotic quantum phenomena that can be tuned by both intra- and inter-1D-chain electronic interactions, but their experimental characterization can be challenging. Here we demonstrate that layer-stacking domain walls (DWs) in van der Waals heterostructures form a broadly tunable Luttinger liquid system including both isolated and coupled arrays. We have imaged the evolution of DW Luttinger liquids under different interaction regimes tuned by electron density using a novel scanning tunneling microscopy (STM) technique. Single DWs at low carrier density are highly susceptible to Wigner crystallization consistent with a spin-incoherent Luttinger liquid, while at intermediate densities dimerized Wigner crystals form due to an enhanced magneto-elastic coupling. Periodic arrays of DWs exhibit an interplay between intra- and inter-chain interactions that gives rise to new quantum phases. At low electron densities inter-chain interactions are dominant and induce a 2D electron crystal composed of phased-locked 1D Wigner crystal in a staggered configuration. Increased electron density causes intra-chain fluctuation potentials to dominate, leading to an electronic smectic liquid crystal phase where electrons are ordered with algebraical correlation decay along the chain direction but disordered between chains. Our work shows that layer-stacking DWs in 2D heterostructures offers new opportunities to explore Luttinger liquid physics.

cond-mat.str-el↗

arXiv2023

Topology, magnetism and charge order in twisted MoTe2 at higher integer hole fillings

Twisted homobilayer transition metal dichalcogenide (TMD) attracts an expanding experimental interest recently for exhibiting a variety of topological and magnetic states even at zero magnetic field. Most of the studies right now focus on hole filling nu_h < 1, while a rich phase diagram at higher hole filling calls for more investigation. We perform a thorough survey of possible interaction-driven phases at higher integer hole fillings. We first construct the continuum model from a first-principles calculation, and then perform a self-consistent Hartree-Fock study of the interacting ground states. We identify various valley polarized (VP) states at odd integer fillings and intervalley coherent (IVC) states at even integer fillings and discuss the energetics competition among them. We also discuss the origin and the experimental implications of the curious Chern insulator at nu_h = 2.

cond-mat.str-el↗

arXiv2023

Terahertz phonon engineering with van der Waals heterostructures

Phononic engineering at gigahertz (GHz) frequencies form the foundation of microwave acoustic filters, acousto-optic modulators, and quantum transducers. Terahertz (THz) phononic engineering could lead to acoustic filters and modulators at higher bandwidth and speed, as well as quantum circuits operating at higher temperatures. Despite its potential, methods for engineering THz phonons have been limited due to the challenges of achieving the required material control at sub-nanometer precision and efficient phonon coupling at THz frequencies. Here, we demonstrate efficient generation, detection, and manipulation of THz phonons through precise integration of atomically thin layers in van der Waals heterostructures. We employ few-layer graphene (FLG) as an ultrabroadband phonon transducer, converting femtosecond near-infrared pulses to acoustic phonon pulses with spectral content up to 3 THz. A monolayer WSe$_2$ is used as a sensor, where high-fidelity readout is enabled by the exciton-phonon coupling and strong light-matter interactions. Combining these capabilities in a single heterostructure and detecting responses to incident mechanical waves, we perform THz phononic spectroscopy. Using this platform, we demonstrate high-Q THz phononic cavities and show that a monolayer WSe$_2$ embedded in hexagonal boron nitride (hBN) can efficiently block the transmission of THz phonons. By comparing our measurements to a nanomechanical model, we obtain the force constants at the heterointerfaces. Our results could enable THz phononic metamaterials for ultrabroadband acoustic filters and modulators, and open novel routes for thermal engineering.

cond-mat.mes-hall↗

arXiv2023

Imaging Moiré Excited States with Photocurrent Tunneling Microscopy

Moiré superlattices provide a highly tunable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators1-17 to moiré excitons7-10,18. Scanning tunneling microscopy has played a key role in probing microscopic behaviors of the moiré correlated ground states at the atomic scale1,11-15,19. Atomic-resolution imaging of quantum excited state in moiré heterostructures, however, has been an outstanding experimental challenge. Here we develop a novel photocurrent tunneling microscopy by combining laser excitation and scanning tunneling spectroscopy (laser-STM) to directly visualize the electron and hole distribution within the photoexcited moiré exciton in a twisted bilayer WS2 (t-WS2). We observe that the tunneling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the exotic in-plane charge-transfer (ICT) moiré exciton in the t-WS2 that emerges from the competition between the electron-hole Coulomb interaction and the moiré potential landscape. Our photocurrent maps are in excellent agreement with our GW-BSE calculations for excitonic states in t-WS2. The photocurrent tunneling microscopy creates new opportunities for exploring photoexcited non-equilibrium moiré phenomena at the atomic scale.

cond-mat.mes-hall↗