arXiv subjects

Wen-Xuan Qiu

Publications and source records attributed to Wen-Xuan Qiu.

15 recordsLinked to original sources

arXiv2025

Quantum Phases in Twisted Homobilayer Transition Metal Dichalcogenides

Twisted homobilayer transition metal dichalcogenides - specifically twisted bilayer MoTe$_2$ and twisted bilayer WSe$_2$ - have recently emerged as a versatile platform for strongly correlated and topological phases of matter. These two-dimensional systems host tunable flat Chern bands in which Coulomb interactions can dominate over kinetic energy, giving rise to a variety of interaction-driven phenomena. A series of groundbreaking experiments have revealed a rich landscape of quantum phases, including integer and fractional quantum anomalous Hall states, quantum spin Hall states, anomalous Hall metals, zero-field composite Fermi liquids, and unconventional superconductors, along with more conventional topologically trivial correlated states including antiferromagnets. This review surveys recent experimental discoveries and theoretical progress in understanding these phases, with a focus on the key underlying mechanisms - band topology, electron interactions, symmetry breaking, and charge fractionalization. We emphasize the unique physics of twisted TMD homobilayers in comparison to other related systems, discuss open questions, and outline promising directions for future research.

cond-mat.str-el↗

arXiv2025

Topological Chiral Superconductivity Mediated by Intervalley Antiferromagnetic Fluctuations in Twisted Bilayer WSe$_2$

Motivated by the recent observations of superconductivity in twisted bilayer WSe$_2$ (tWSe$_2$), we theoretically investigate the superconductivity driven by electronic mechanism. We first demonstrate that the multi-band screened Coulomb interaction within the random phase approximation is insufficient to induce observable pairing instability. Nevertheless, by further including the intervalley antiferromagnetic fluctuations, the pairing interaction is substantially enhanced, yielding superconductivity with critical temperature $T_c$ of hundreds of millikelvin at van Hove singularities. The predicted $T_c$ increases with increasing the displacement field and corresponds to a doubly-degenerate $d$-wave-like pairing, which evolves into topological chiral $d \pm id$ superconductor below $T_c$. The interplay between superconductivity and intervalley antiferromagnetism results in a phase diagram consistent with experimental observations.These findings establish intervalley fluctuations as the primary pairing glue in tWSe$_2$.

cond-mat.supr-con↗

arXiv2025

Topological magnons and domain walls in twisted bilayer MoTe$_2$

We theoretically investigate the magnetic excitations in the quantum anomalous Hall insulator phase of twisted bilayer MoTe$_2$ at a hole filling factor of $ν=1$, focusing on magnon and domain wall excitations. Using a generalized interacting Kane-Mele model, we obtain the quantum anomalous Hall insualtor ground state with spin polarization. The magnon spectrum is then computed via the Bethe-Salpeter equation, revealing two low-energy topological magnon bands with opposite Chern numbers. To further explore the magnon topology, we construct a tight-binding model for the magnon bands, which is analogous to the Haldane model. We also calculate the energy cost of domain walls that separate regions with opposite Chern numbers and bind chiral edge states. Finally, we propose an effective spin model that describes both magnon and domain wall excitations, incorporating Heisenberg spin interactions and Dzyaloshinskii-Moriya interactions. The coupling constants in this model are determined from the numerical results for magnons and domain walls. This model accounts for the Ising anisotropy of the system, captures the magnon topology, and allows for the estimation of the magnetic ordering temperature. Our findings provide a comprehensive analysis of magnetic excitations in twisted MoTe$_2$ and offer new insights into collective excitations in moiré systems.

cond-mat.mes-hall↗

arXiv2025

Quantum Geometry Probed by Chiral Excitonic Optical Response of Chern Insulators

We theoretically derive the sum rule for the negative first moment of the absorptive optical conductivity with excitonic effects and establish its connection to the quantum weight $K$ and Chern number $C$ of the ground state. Applying this framework, we investigate the excitonic optical response of the Chern insulator at hole filling factor $ν=1$ in twisted bilayer MoTe$_2$. A single chiral exciton state, which selectively absorbs circularly polarized light of a specific handedness, dominates the optical sum rule. The chiral exciton state comprises two types of interlayer electron-hole transitions, which cancel out the total out-of-plane dipole moment. The absorption spectrum shows nearly perfect magnetic circular dichroism, which can be attributed to the nearly saturated bound $K \ge |C|$ of the Chern insulator under study. Our work illustrates the potential of using excitonic optical responses to probe quantum geometry encoded by $K$ and $C$ of Chern insulators in moiré superlattices.

cond-mat.mes-hall↗

arXiv2025

Visualization of intervalley coherent phase in PtSe2/HOPG heterojunction

Intervalley coherent (IVC) phase in graphene systems arises from the coherent superposition of wave functions of opposite valleys, whose direct microscopic visualization provides pivotal insight into the emergent physics but remains elusive. Here, we successfully visualize the IVC phase in a heterostructure of monolayer PtSe2 on highly oriented pyrolytic graphite. Using spectroscopic imaging scanning tunneling microscopy, we observe a Root3 by Root3 modulation pattern superimposed on the higher-order moire superlattice of the heterostructure, which correlates with a small gap opening around the Fermi level and displays an anti-phase real-space conductance distribution of the two gap edges. Such modulation pattern and small-gap vanish on the heterostructure of monolayer PtSe2 on bilayer-graphene-covered SiC substrate, due to the increased carrier density in the bilayer graphene. We provide a theoretical mechanism that the Root3 by Root3 modulation pattern originates from the IVC phase of few-layer graphene, which is magnified by the higher-order moire superlattice. Our work achieves visualization of the IVC phase, and develops an avenue for its generation and amplification via a moiré interface.

cond-mat.mes-hall↗

arXiv2024

Observation of Ferromagnetic Phase in the Second Moiré Band of Twisted MoTe2

Flat bands and electron correlation in moiré lattices give rise to many exotic phases, including Mott insulators, superconductivity, and topological states. Within the first moiré band, integer and fractional quantum anomalous Hall effects have been observed in twisted bilayer MoTe2 (tMoTe2) at one hole doping and fractional doping per moiré unit cell, respectively. When the second moiré band is fully hole doped, quantum spin Hall insulator has also been reported in tMoTe2 at a certain twist angle. Exotic topological states together with ferromagnetic (FM) states in the high moiré band can potentially exist as well. In this study, we report the observation of a FM phase in the second moiré band in tMoTe2. The FM phase can be tuned by both the doping level and displacement field. At filling around 2.58 holes per moiré unit cell, the FM phase reaches a Curie temperature of 3.5 K. A large displacement field can suppress the FM phase, like the FM phase at the filling of -1. Our results demonstrate the realization of time-reversal symmetry-breaking states in the higher moiré bands in tMoTe2.

cond-mat.mes-hall↗

arXiv2024

Quantum simulation of honeycomb lattice model by high-order moiré pattern

Moiré superlattices have become an emergent solid-state platform for simulating quantum lattice models. However, in single moiré device, Hamiltonians parameters like lattice constant, hopping and interaction terms can hardly be manipulated, limiting the controllability and accessibility of moire quantum simulator. Here, by combining angle-resolved photoemission spectroscopy and theoretical analysis, we demonstrate that high-order moiré patterns in graphene-monolayered xenon/krypton heterostructures can simulate honeycomb model in mesoscale, with in-situ tunable Hamiltonians parameters. The length scale of simulated lattice constant can be tuned by annealing processes, which in-situ adjusts intervalley interaction and hopping parameters in the simulated honeycomb lattice. The sign of the lattice constant can be switched by choosing xenon or krypton monolayer deposited on graphene, which controls sublattice degree of freedom and valley arrangment of Dirac fermions. Our work establishes a novel path for experimentally simulating the honeycomb model with tunable parameters by high-order moiré patterns.

cond-mat.mtrl-sci↗

arXiv2024

Electrically tuned topology and magnetism in twisted bilayer MoTe$_2$ at $ν_h=1$

We present a theoretical study of an interaction-driven quantum phase diagram of twisted bilayer MoTe$_2$ at hole filling factor $ν_h=1$ as a function of twist angle $θ$ and layer potential difference $V_z$, where $V_z$ is generated by an applied out-of-plane electric field. At $V_z=0$, the phase diagram includes quantum anomalous Hall insulators in the intermediate $θ$ regime and topologically trivial multiferroic states with coexisting ferroelectricity and magnetism in both small and large $θ$ regimes. There can be two transitions from the quantum anomalous Hall insulator phase to topologically trivial out-of-plane ferromagnetic phase, and finally to in-plane 120$^\circ$ antiferromagnetic phase as $|V_z|$ increases, or a single transition without the intervening ferromagnetic phase. We show explicitly that the spin vector chirality of various 120$^\circ$ antiferromagnetic states can be electrically switched. We discuss the connection between the experimentally measured Curie-Weiss temperature and the low-temperature magnetic order based on an effective Heisenberg model with magnetic anisotropy.

cond-mat.mes-hall↗

arXiv2024

Majorana zero modes in twisted transition metal dichalcogenides homobilayers

Semiconductor moiré superlattices provide a highly tunable platform to study the interplay between electron correlation and band topology. For example, the generalized Kane-Mele-Hubbard model can be simulated by the topological moiré flat bands in twisted transition metal dichalcogenides homobilayers. For this system, we obtain the filling factor, twist angle, and electric field-dependent quantum phase diagrams with a plethora of phases, including the quantum spin Hall insulator, the in-plane antiferromagnetic state, the out-of-plane antiferromagnetic Chern insulator, the spin-polarized Chern insulator, the in-plane ferromagnetic state, and the 120$^\circ$ antiferromagnetic state. We predict that a gate-defined junction formed between the quantum spin Hall insulator phase with proximitized superconductivity and magnetic phases with in-plane magnetization (either ferromagnetic or antiferromagnetic) can realize one-dimensional topological superconductor with Majorana zero modes. Our proposal introduces semiconductor moiré homobilayers as an electrically tunable Majorana platform with no need of an external magnetic field.

cond-mat.supr-con↗

arXiv2023

Interaction-driven topological phase diagram of twisted bilayer MoTe$_2$

Twisted bilayer MoTe$_2$ is a promising platform to investigate the interplay between band topology and many-body interaction. We present a theoretical study of its interaction-driven quantum phase diagrams based on a three-orbital model, which can be viewed as a generalization of the Kane-Mele-Hubbard model with one additional orbital and long-range Coulomb repulsion. We predict a cascade of phase transitions tuned by the twist angle $θ$. At the hole filling factor $ν=1$ (one hole per moiré unit cell), the ground state can be in the multiferroic phase with coexisting spontaneous layer polarization and magnetism, the quantum anomalous Hall phase, and finally the topologically trivial magnetic phases, as $θ$ increases from $1.5^{\circ}$ to $5^{\circ}$. At $ν=2$, the ground state can have a second-order phase transition between an antiferromagnetic phase and the quantum spin Hall phase as $θ$ passes through a critical value. The dependence of the phase boundaries on model parameters such as the gate-to-sample distance, the dielectric constant, and the moiré potential amplitude is examined. The predicted phase diagrams can guide the search for topological phases in twisted transition metal dichalcogenide homobilayers.

cond-mat.mes-hall↗

arXiv2021

Efficient Method for Prediction of Meta-stable/Ground Multipolar Ordered States and its Application in Monolayer $α$-\ce{RuX3} (X=Cl,I)

Exotic high-rank multipolar order parameters have been found to be unexpectedly active in more and more correlated materials in recent years. Such multipoles are usually dubbed as "Hidden Orders" since they are insensitive to common experimental probes. Theoretically, it is also difficult to predict multipolar orders via \textit{ab initio} calculations in real materials. Here, we present an efficient method to predict possible multipoles in materials based on linear response theory under random phase approximation. Using this method, we successfully predict two pure meta-stable magnetic octupolar states in monolayer $α$-\ce{RuCl3}, which is confirmed by self-consistent unrestricted Hartree-Fock calculations. We then demonstrate that these octupolar states can be stabilized in monolayer $α$-\ce{RuI3}, one of which becomes the octupolar ground state. Furthermore, we also predict a fingerprint of orthogonal magnetization pattern produced by the octupole moment, which can be easily detected by experiment. The method and the example presented in this work serve as a guidance for searching multipolar order parameters in other correlated materials.

cond-mat.str-el↗

arXiv2021

Making artificial $\textit{p}_{x,y}$-orbital honeycomb electron lattice on metal surface

We theoretically demonstrate that the desired $p_{x,y}$-orbital honeycomb electron lattice can be readily realized by arranging CO molecules into a hexagonal lattice on Cu(111) surface with scanning tunneling microscopy (STM). The electronic structure of the Cu surface states in the presence of CO molecules is calculated with various methods, \textit{i.e.}~DFT simulations, muffin-tin potential model and tight-binding model. Our calculations indicate that, by measuring the LDOS pattern using STM, the $p$-orbital surface bands can be immediately identified in experiment. We also give an analytic interpretation of the $p$-orbital LDOS pattern with $k \cdot p$ method. Meanwhile, different from the case of graphene, the $p$-orbital honeycomb lattice has two kinds of edge states, which can also be directly observed in STM experiment. Our work points out a feasible way to construct a $p_{x,y}$-orbital honeycomb electron lattice in a real system, which may have exotic properties, such as Wigner crystal, ferromagnetism, $f$-wave superconductivity, quantum anomalous Hall (QAH) effect. Furthermore, we also propose a simple way to calculate and identify the modified Cu surface bands in the Cu/CO systems with the DFT simulations. Considering the recent works about $p$-orbital square lattice in similar systems [M. R. Slot, \textit{et al.} Nat. Phys. \textbf{13}, 672 (2017); Liang Ma, \textit{et al.} Phys. Rev. B \textbf{99}, 205403 (2019)], our work once again illustrates that the artificial electron lattice on metal surface is an ideal platform to study the orbital physics in a controllable way.

cond-mat.mes-hall↗

arXiv2017

Orbital degree of freedom in artificial electron lattices on metal surface

Orbital degree of freedom plays a fundamental role in condensed matter physics. Recently, a new kind of artificial electron lattice has been realized in experiments by confining the metal surface electrons with adsorbed molecular lattice. A most recent example is the Lieb lattice realized by CO adsorption on Cu(111) surface [M. R. Slot, et al., Nat. Phys. 13, 672(2017)]. The Lieb lattice is of special interest due to its flat band physics. Here, by first-principles calculations, muffin-tin potential model and tight binding model, we demonstrate that, the high energy states observed in the experiment actually correspond to the artificial $p$-orbitals of the electron lattice. Our numerical results, together with the experimental observation, show that artificial $p$-orbital fermionic lattice has already been realized in solid state system. This opens a new avenue to investigate the orbital degree of freedom in a controllable way.

cond-mat.mes-hall↗

arXiv2016

Desiging Artificial Lieb Lattice on Metal Surface

Recently, several experiments have illustrated that metal surface electrons can be manipulated to form a two dimensional (2D) lattice by depositing a designer molecule lattice on metal surface. This offers a promising new technique to construct artificial 2D electron lattices. Here we theoretically propose a molecule lattice pattern to realize an artificial Lieb lattice on metal surface, which shows a flat electronic band due to the lattice geometry. We show that the localization of electrons in the flat band may be understood from the viewpoint of electron interference, which may be probed by measuring the local density of states with the scanning tunnelling microscopy. Our proposal may be readily implemented in experiment and may offer an ideal solid state platform to investigate the novel flat band physics of the Lieb lattice.

cond-mat.mes-hall↗

arXiv2016

The Effective Model of the Molecule Graphene System and One Application Beyond Graphene

Recently, a new kind of two dimensional (2D) artificial electron lattice, i.e. molecule graphene, has drawn a lots of interest, where the metal surface electrons are transformed into a honeycomb lattice via absorbing a molecule lattice on metal surface. In this work, we would like to point out that the technique used to build molecule graphene actually gives a promising way to explore the interesting physics of other novel 2D lattice beyond graphene. The key issue is that this system is an antidot system, where the absorbed molecule normally gives a repulsive potential. So, we need to establish a map between the molecule arrangement and the desired surface electron lattice. To give a concrete example, we first analyse the effective models of molecule graphene, and estimate the model parameters for the Cu/CO system through numerical fitting the exerimental data. Then, we design a molecule lattice, and show that, with this kind of lattice, the surface electrons can be transformed into a Kagome like lattice. Using the estimated parameters of Cu/CO system, we calculate the corresponding energy bands and LDOS, which can be readily tested in experiment. We hope that our work can stimulate further theoretical and experimental interest in this novel artificial 2D electron lattice.

cond-mat.mes-hall↗