arXiv subjects

Chengxin Xiao

Publications and source records attributed to Chengxin Xiao.

14 recordsLinked to original sources

arXiv2026

Excitons probe intrinsic flat band Mottness in a van der Waals heterostructure

Excitons provide a sensitive optical probe of electronic correlations in nearby two-dimensional materials, yet their coupling to intrinsic flat-band Mott systems remains largely unexplored. Here we combine gate-tunable optical spectroscopy with first-principles calculations to study monolayer WSe$_2$ in direct contact with the van der Waals Mott insulator Nb$_3$Cl$_8$. The gate evolution of WSe$_2$ excitonic resonances reveals signatures of a correlation-reconstructed Mott gap in Nb$_3$Cl$_8$ that is absent from the single-particle band picture. In the electron-doped regime, the WSe$_2$ 2s Rydberg exciton undergoes a multistage evolution and develops into interlayer attractive and repulsive polaron branches, showing that a Rydberg exciton can be dressed by strongly correlated flat-band electrons in an adjacent Mott layer. Under an out-of-plane magnetic field, spin-polarized Nb$_3$Cl$_8$ states further induce valley-selective exciton coupling, producing a strongly enhanced circular polarization of the WSe$_2$ exciton emission. These results extend exciton-based sensing and exciton-polaron physics to intrinsic flat-band Mott materials, providing an optical route to probe and engineer correlation-driven interfacial quasiparticles.

cond-mat.mtrl-sci↗

arXiv2026

Bridging distributed quantum materials via multi-hotspot vacuum: remote Cooper pairing and Andreev teleportation

We introduce an architecture where mesoscopic quantum matter distributed over spatially separated nodes can be correlated in equilibrium, creating an unprecedented form of many-body quantum system. Central to the design is a multi-gap split-ring resonator (SRR) where the cavity photon has multiple hot spots -- each with deep-subwavelength volume at a split gap and separated by millimeter-scale distances. The cavity's vacuum fluctuations can then mediate a many-body interaction that bridges the distributed quantum materials embedded in the multiple gaps, coupling them into a single correlated mesoscopic system in equilibrium. As an example, we consider a THz SRR with two split gaps, each proximitized to a metallic moir\'e superlattice, where virtual exchange of a photon in the cavity vacuum mediates a current-current interaction across the gaps. The inherent attractive interaction channels lead to remote Cooper pairing reminiscent of mesoscopic superconductivity, demonstrated with density matrix renormalization group and exact diagonalization calculations. With the two constituents of a Cooper pair now paired across a millimeter-scale separation, a hole incident at one split gap can be converted into an outgoing electron at the remote gap, a process we term Andreev teleportation. The entanglement entropy between the two mesoscopic superlattices is shown to scale linearly with the area (total number of sites) of the mesoscopic lattice. Our results suggest an intriguing paradigm for equilibrium quantum networks of mesoscopic matter that enable emergent nonlocal functionalities and distributed quantum resources.

cond-mat.mes-hall↗

arXiv2026

Interlayer electric multipole Hall effect in twisted multilayers

Electrons in layered van der Waals materials possess a layer pseudospin characterizing their wave-function distribution among layers. In twisted structures, this pseudospin forms nontrivial textures, leading to intriguing phenomena such as the layer Hall effect (LHE), where distinct layer Hall currents flow despite the presence of time-reversal symmetry. In chiral bilayers, LHE manifests as an interlayer electric dipole Hall effect with Hall counterflows and a concomitant in-plane magnetic dipole. Multilayers host richer layer-dependent Hall currents, generating interlayer electric multipole Hall effects and in-plane magnetic multipoles. We start from exploring the interlayer electric quadrupole Hall effect in mirror-symmetric twisted trilayers. At small twist angles, interlayer translation efficiently tunes layer Hall current magnitudes. At large angles and low doping, the currents can be well accounted for by adding the contributions from the two individual twisted interfaces. This decomposition allows obtaining layer-resolved Hall currents in large-angle twisted multilayers even without well-defined periodicity.

cond-mat.mes-hall↗

arXiv2024

Anomalously Enhanced Diffusivity of Moir\'e Excitons via Manipulating the Interplay with Correlated Electrons

Semiconducting transitional metal dichalcogenides (TMDCs) moir\'e superlattice provides an exciting platform for manipulating excitons. The in-situ control of moir\'e potential confined exciton would usher in unprecedented functions of excitonic devices but remains challenging. Meanwhile, as a dipolar composite boson, interlayer exciton in the type-II aligned TMDC moir\'e superlattice strongly interacts with fermionic charge carriers. Here, we demonstrate active manipulation of the exciton diffusivity by tuning their interplay with correlated carriers in moir\'e potentials. At fractional fillings where carriers are known to form generalized Wigner crystals, we observed suppressed diffusivity of exciton. In contrast, in Fermi liquid states where carriers dynamically populate all moir\'e traps, the repulsive carrier-exciton interaction can effectively reduce the moir\'e potential confinement seen by the exciton, leading to enhanced diffusivity with the increase of the carrier density. Notably, the exciton diffusivity is enhanced by orders of magnitude near the Mott insulator state, and the enhancement is much more pronounced for the 0-degree than the 60-degree aligned WS2/WSe2 heterobilayer due to the more localized nature of interlayer excitons. Our study inspires further engineering and controlling exotic excitonic states in TMDC moir\'e superlattices for fascinating quantum phenomena and novel excitonic devices.

cond-mat.mes-hall↗

arXiv2024

An active metasurface enhanced with moiré ferroelectricity

Semiconductor moiré systems, characterized by their periodic spatial light emission, unveil a new paradigm of active metasurfaces. Here, we show that ferroelectric moiré domains formed in a twisted hexagonal boron nitride (t-hBN) substrate can modulate light emission from an adjacent semiconductor MoSe$_2$ monolayer, enhancing its functionality as an active metasurface. The electrostatic potential at the surface of the t-hBN substrate provides a simple way to confine excitons in the MoSe$_2$ monolayer. The excitons confined within the domains and at the domain walls are spectrally separated due to a pronounced Stark shift. Moreover, the patterned light emission can be dynamically controlled by electrically gating the ferroelectric domains, introducing a novel functionality beyond conventional semiconductor moiré systems. Our findings chart an exciting pathway for integrating nanometer-scale moiré ferroelectric domains with various optically active functional layers, paving the way for advanced nanophotonic applications.

cond-mat.mes-hall↗

arXiv2024

Magnetic and nematic order of Bose-Fermi mixtures in moir\'e superlattices of 2D semiconductors

We investigate the magnetic orders in a mixture of Boson (exciton) and Fermion (electron or hole) trapped in transition-metal dichalcogenides moir\'e superlattices. A sizable antiferromagnetic exchange interaction is found between a carrier and an interlayer exciton trapped at different high symmetry points of the moir\'e supercell. This interaction at a distance much shorter than the carrier-carrier separation dominates the magnetic order in the Bose-Fermi mixture, where the carrier sublattice develops ferromagnetism opposite to that in the exciton sublattice. We demonstrate the possibility of increasing the Curie temperature of moir\'e carriers through electrical tuning of the exciton density in the ground state. In a trilayer moir\'e system with a p-n-p type band alignment, the exciton-carrier interplay can establish a layered antiferromagnetism for holes confined in the two outer layers. We further reveal a spontaneous nematic order in the Bose-Fermi mixture, arising from the interference between the Coulomb interaction and p-wave interlayer tunneling dictated by the stacking registry.

cond-mat.str-el↗

arXiv2024

Emergence of moir\'e superlattice potential in graphene by twisted-hBN layers

Moir\'e superlattices formed in stacks of two or more 2D crystals with similar lattice structures have recently become excellent platforms to reveal new physics in low-dimensional systems. They are, however, highly sensitive to the angle and lattice constant differences between the associated crystals, limiting the range of the material choice and the possible moir\'e patterns for a given 2D crystal. Here, we present a novel approach to realize an atomically flat substrate with a periodic moir\'e pattern that can induce the moir\'e potential on the material on top by van der Waals (vdW) interactions, without suffering from the lattice and angle mismatch. By constructing a twisted hBN (thBN) moir\'e substrate at an angle of about 1$^\circ$, we show that the graphene on top, aligned around 15$^\circ$ with the neighboring hBN layers, exhibits typical transport properties under a hexagonal moir\'e potential, including multiple satellite Dirac points (DPs), Hofstadter butterfly effect, and Brown-Zak oscillations. All features point to the existence of the moir\'e potential in graphene formed by thBN with $\sim$1$^\circ$ twist angle. Further statistical study shows that the twist from a parallel interface between the hBN layers is critical to induce the moir\'e potential. Our study demonstrates that the thBN moir\'e substrate can be used to investigate moir\'e physics in arbitrary materials without being constrained by their lattice constants.

cond-mat.mes-hall↗

arXiv2023

Exciton-exciton Interaction in Monolayer MoSe$_2$ from Mutual Screening of Coulomb Binding

The potential for low-threshold optical nonlinearity has received significant attention in the fields of photonics and conceptual optical neuron networks. Excitons in two-dimensional (2D) semiconductors are particularly promising in this regard as reduced screening and dimensional confinement foster their pronounced many-body interactions towards nonlinearity. However, experimental determination of the interactions remains ambiguous, as optical pumping in general creates a mixture of excitons and unbound carriers, where the impacts of band gap renormalization and carrier screening on exciton energy counteract each other. Here by comparing the influences on exciton ground and excited states energies in the photoluminescence spectroscopy of monolayer MoSe$_2$, we are able to identify separately the screening of Coulomb binding by the neutral excitons and by charge carriers. The energy difference between exciton ground state (A-1s) and excited state (A-2s) red-shifts by 5.5 meV when the neutral exciton density increases from 0 to $4\times 10^{11}$ cm$^{-2}$, in contrast to the blue shifts with the increase of either electron or hole density. This energy difference change is attributed to the mutual screening of Coulomb binding of neutral excitons, from which we extract an exciton polarizability of $\alpha_{2D}^{\rm exciton} = 2.55\times 10^{-17}$ eV(m/V)$^2$. Our finding uncovers a new mechanism that dominates the repulsive part of many-body interaction between neutral excitons.

cond-mat.mtrl-sci↗

arXiv2023

Remote gate control of topological transitions in moiré superlattices via cavity vacuum fields

Placed in cavity resonators with three-dimensionally confined electromagnetic wave, the interaction between quasiparticles in solids can be induced by exchanging virtual cavity photons, which can have a non-local characteristic. Here we investigate the possibility of utilizing this nonlocality to realize the remote control of the topological transition in mesoscopic moiré superlattices at full filling (one electron/hole per supercell) embedded in a split-ring terahertz electromagnetic resonator. We show that gate tuning one moiré superlattice can remotely drive a topological band inversion in another moiré superlattice not in contact but embedded in the same cavity. Our study of remote on/off switching of a topological transition provides a novel paradigm for the control of material properties via cavity vacuum fields.

cond-mat.mtrl-sci↗

arXiv2022

Every-other-layer Dipolar Excitons in a Spin-Valley locked Superlattice

Monolayer semiconducting transition metal dichalcogenides possess broken inversion symmetry and strong spin-orbit coupling, which leads to unique spin-valley locking effect. In 2H stacked pristine multilayers, the spin-valley locking yields an electronic superlattice structure, where alternating layers correspond to barrier and quantum well respectively, conditioned on the spin-valley indices. Here, we show that the spin-valley locked superlattice hosts a new kind of dipolar excitons with the electron and hole constituents separated in an every-other-layer configuration, i.e., either in two even or two odd layers. Such excitons become optically bright via hybridization with intralayer excitons, displaying multiple anti-crossing patterns in optical reflection spectrum as the dipolar exciton is tuned through the intralayer resonance by electric field. The reflectance spectra also reveal an excited state orbital of the every-other-layer exciton, pointing to a sizable binding energy in the same order of magnitude as the intralayer exciton. As layer thickness increases, the dipolar exciton can form one-dimensional Bose-Hubbard chain displaying a layer number dependent fine-structures in the reflectance spectra. Our work reveals a distinct valleytronic superlattice with highly tunable dipolar excitons for exploring light-matter interactions.

cond-mat.mes-hall↗

arXiv2022

Dynamic generation of spin spirals of moiré trapped carriers via exciton mediated spin interactions

Stacking transition metal dichalcogenides (TMDs) to form moiré superlattices has provided exciting opportunities to explore many-body correlation phenomena of the moiré trapped carriers. TMDs bilayers, on the other hand, host long-lived interlayer exciton (IX), an elementary excitation of long spin-valley lifetime that can be optically or electrically injected. Here we find that, through the Coulomb exchange between mobile IXs and carriers, the IX bath can mediate both Heisenberg and Dzyaloshinskii-Moriya type spin interactions between moiré trapped carriers, controllable by exciton density and exciton spin current respectively. We show the strong Heisenberg interaction, and the extraordinarily long-ranged Dzyaloshinskii-Moriya interaction here can jointly establish robust spin spiral magnetic orders in Mott-Wigner crystal states at various filling factors, with spiral direction controlled by exciton current.

cond-mat.mes-hall↗

arXiv2022

Light-Induced Ferromagnetism in Moiré Superlattices

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would open the possibility to access and control complex electronic phase diagrams on demand. Recently, moiré superlattices formed by two-dimensional materials have emerged as a promising platform for quantum engineering such phenomena. The power of the moiré system lies in the high tunability of its physical parameters by tweaking layer twist angle, electrical field, moiré carrier filling, and interlayer coupling. Here, we report that optical excitation can drastically tune the spin-spin interactions between moiré trapped carriers, resulting in ferromagnetic order in WS2/WSe2 moiré superlattices over a small range of doping at elevated temperatures. Near the filling factor v = -1/3 (i.e., one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism (RMCD) signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photo-excited excitons mediate exchange coupling between moiré trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré trapped holes, and thus magnetic order can arise even in the dilute hole regime under optical excitation. This discovery adds a new and dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter.

cond-mat.mes-hall↗

arXiv2020

Universal superlattice potential for 2D materials from twisted interface inside h-BN substrate

Lateral superlattices in 2D materials are emerging as a powerful platform for exploring novel quantum phenomena, which can be realized through the proximity coupling in forming moiré pattern with another layer. This approach, however, is invasive, material-specific, and requires small lattice mismatch and suitable band alignment, largely limited to graphene and transition metal dichalcogenides (TMDs). Hexagonal boron nitride (h-BN) of anti-parallel (AA') stacking order has been an indispensable building block, as dielectric substrates and capping layers for realizing high quality van der Waals devices. There is also emerging interest on parallelly aligned h-BN of Bernal (AB) stacking, where the broken inversion and mirror symmetries lead to out-of-plane electrical polarization with sign controlled by interlayer translation. Here we show that the laterally patterned electrical polarization at a nearly parallel interface within the h-BN substrate or capping layer can be exploited to create non-invasively a universal superlattice potential in general 2D materials. The feasibility is demonstrated by first principle calculations for monolayer MoSe2, black phosphorus, and antiferromagnetic MnPSe3 on such h-BN substrate. The potential strength can reach 200 meV, customizable in this range through choice of vertical distance of target material from the interface in h-BN. We also find sizable out-of-plane electric field at the h-BN surface, which can realize superlattice potential for interlayer excitons in TMD bilayers as well as dipolar molecules. The idea is further generalized to AB stacked h-BN film subject to torsion with adjacent layers all twisted with an angle, which allows the potential and field strength to be scaled up with film thickness, saturating to a quasi-periodic one with chiral structure.

cond-mat.mes-hall↗

arXiv2020

Chiral channel network from magnetization textures in 2D MnBi2Te4

When atomically thin van der Waals (vdW) magnet forms long-period moiré pattern with a magnetic substrate, the sensitive dependence of interlayer magnetic coupling on the atomic registries can lead to moiré defined magnetization textures in the two-dimensional (2D) magnets. The recent discovery of 2D magnetic topological insulators such as MnBi2Te4 leads to the interesting possibility to explore the interplay of such magnetization textures with the topological surface states, which we explore here with a minimal model established for 2D MnBi2Te4. The sign flip of the exchange gap across a magnetization domain wall gives rise to a single in-gap chiral channel on each surface. In the periodic magnetization textures, such chiral spin channels at the domain walls couple to form a network and superlattice minibands emerge. We find that in magnetization textures with closed domain wall geometries, the formed superlattice miniband is a gapped Dirac cone featuring orbital magnetization from the current circulation in the close loops of chiral channels, while in magnetization textures with open domain wall geometries, gapless mini-Dirac cone is found instead. The miniband Bloch states feature a spatial texture of spin and local current density, which are clear manifestation of the spin-momentum locked chiral channels at the domain walls. The results suggest a new platform to engineer spin and current flows through the manipulation of magnetization domains for spintronic devices.

cond-mat.mes-hall↗