arXiv subjects

Zheyu Lu

Publications and source records attributed to Zheyu Lu.

6 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

Tunable Interlayer Charge-transfer States in MoSe$_2$/WS$_2$ Moir\'e Superlattices

Moir\'e superlattices formed by transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform for studying strongly correlated electronic, excitonic, and topological phenomena in solids. In particular, angle-aligned MoSe$_2$/WS$_2$ heterobilayers, which have a Type-I band alignment at zero vertical electric field, host rich correlated spin and charge physics. Here, combining large-scale first-principles calculations and optical reflection spectroscopy, we report a thorough study of the emergent moir\'e excitonic states and interlayer charge-transfer states in angle-aligned electron-doped MoSe$_2$/WS$_2$ moir\'e superlattices. The moir\'e excitonic states serve as sensitive optical probes to the localization profile of doped electrons. We observe a series of interlayer charge-transfer transitions from n/n$_0$ = 1 to 4 (where n$_0$ denotes the moir\'e density) when the vertical electric field switches the heterostructure band alignment from Type-I to Type-II. By tuning the vertical electric field, we can precisely control the interlayer electron localization, realizing a Fermi-Hubbard model with a tunable charge-transfer band on an effective honeycomb lattice. Furthermore, Monte Carlo simulation of the doping dependence of the electric-field susceptibility predicts that multiple correlated charge-ordered states appear at both integer and fractional fillings. Our results provide a holistic understanding of the emergent optical excitations and the correlated charge-transfer states in electron-doped MoSe$_2$/WS$_2$ moir\'e superlattices.

cond-mat.mes-hall↗

arXiv2024

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

Perfect Coulomb drag and exciton transport in an excitonic insulator

Strongly coupled two-dimensional electron-hole bilayers can give rise to novel quantum Bosonic states: electrons and holes in electrically isolated layers can pair into interlayer excitons, which can form a Bose-Einstein condensate below a critical temperature at zero magnetic field. This state is predicted to feature perfect Coulomb drag, where a current in one layer must be accompanied by an equal but opposite current in the other, and counterflow superconductivity, where the excitons form a superfluid with zero viscosity. Electron-hole bilayers in the strong coupling limit with an excitonic insulator ground state have been recently achieved in semiconducting transition metal dichalcogenide heterostructures, but direct electrical transport measurements remain challenging. Here we use a novel optical spectroscopy to probe the electrical transport of correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe perfect Coulomb drag in the excitonic insulator phase up to a temperature as high as ~15K. Strongly correlated electron and hole transport is also observed at unbalanced electron and hole densities, although the Coulomb drag is not perfect anymore. Meanwhile, the counterflow resistance of interlayer excitons remains finite. These results indicate the formation of an exciton gas in the excitonic insulator which does not condensate into a superfluid at low temperature. Our work also demonstrates that dynamic optical spectroscopy provides a powerful tool for probing novel exciton transport behavior and possible exciton superfluidity in correlated quantum electron-hole fluids.

cond-mat.mes-hall↗

arXiv2023

Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures

Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton superfluids. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~$0.8\times {10}^{12} \mathrm{cm}^{-2}$ and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

cond-mat.str-el↗

arXiv2020

Lithium intercalation in MoS$_2$ bilayers and implications for moiré flat bands

Intercalation of lithium atoms between layers of 2D materials can alter their atomic and electronic structure. We investigate effects of Li intercalation in twisted bilayers of the transition metal dichalcogenide MoS$_2$ through first-principles calculations, tight-binding parameterization based on the Wannier transformation, and analysis of moiré band structures through an effective continuum model. The energetic stability of different intercalation sites for Li between layers of MoS$_2$ are classified according to the local coordination type and the number of vertically aligned Mo atoms, suggesting that the Li atoms will cluster in certain regions of the moiré superlattice. The proximity of a Li atom has a dramatic influence on the interlayer interaction between sulfur atoms, deepening the moiré potential well and leading to better isolation of the flat bands in the energy spectrum. These results point to the usefulness for the use of chemical intercalation as a powerful means for controlling moiré flat-band physics in 2D semiconductors.

cond-mat.mes-hall↗