arXiv subjects

Chengdong Wang

Publications and source records attributed to Chengdong Wang.

3 recordsLinked to original sources

arXiv2026

Suppression of Metallic Transport in Nitrogen-rich Two-Dimensional Transition Metal Nitrides

The recent experimental realization of two-dimensional (2D) transition metal nitrides (TMNs, e.g., Mo5N6, {\delta}-MoN, and W5N6) opens new opportunities for exploring their fundamental physical properties at the two-dimensional limit. In this work, we propose a unified picture of transport phenomena in the nitrogen-rich 2D W5N6 and Mo5N6, and the stoichiometric 2D {\delta}-MoN based on several observations and first-principles calculations. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) from Hall measurements consistently suggest disorder-induced transport mechanism at low temperatures (10-30 K). Notably, we observe a transition from metal to semimetal driven by the variation of nitrogen content in TMNs, supported by the suppressed density of states at the Fermi energy in nitrogen-rich TMNs (e.g. Mo5N6) from first-principle calculations. Carrier density calculations of bulk TMNs and 2D TMNs with -NH termination groups further reveal the switching of majority carrier type of Mo5N6 at reduced thickness, which is in great agreement with Hall measurement results. Our findings demonstrate that high nitrogen content in metallic molybdenum nitrides can induce the transition to a semimetallic phase at the 2D limit, shedding light on both the fundamental aspects of these materials and directions in future material design.

cond-mat.mtrl-sci↗

arXiv2026

Orbital Transformers for Predicting Wavefunctions in Time-Dependent Density Functional Theory

We aim to learn wavefunctions simulated by time-dependent density functional theory (TDDFT), which can be efficiently represented as linear combination coefficients of atomic orbitals. In real-time TDDFT, the electronic wavefunctions of a molecule evolve over time in response to an external excitation, enabling first-principles predictions of physical properties such as optical absorption, electron dynamics, and high-order response. However, conventional real-time TDDFT relies on time-consuming propagation of all occupied states with fine time steps. In this work, we propose OrbEvo, which is based on an equivariant graph transformer architecture and learns to evolve the full electronic wavefunction coefficients across time steps. First, to account for external field, we design an equivariant conditioning to encode both strength and direction of external electric field and break the symmetry from SO(3) to SO(2). Furthermore, we design two OrbEvo models, OrbEvo-WF and OrbEvo-DM, using wavefunction pooling and density matrix as interaction method, respectively. Motivated by the central role of the density functional in TDDFT, OrbEvo-DM encodes the density matrix aggregated from all occupied electronic states into feature vectors via tensor contraction, providing a more intuitive approach to learn the time evolution operator. We adopt a training strategy specifically tailored to limit the error accumulation of time-dependent wavefunctions over autoregressive rollout. To evaluate our approach, we generate TDDFT datasets consisting of 5,000 different molecules in the QM9 dataset and 1,500 molecular configurations of the malonaldehyde molecule in the MD17 dataset. Results show that our OrbEvo model accurately captures quantum dynamics of excited states under external field, including time-dependent wavefunctions, time-dependent dipole moment, and optical absorption spectra.

cs.LG↗

arXiv2025

Quantum spin Hall effects in van der Waals materials

The quantum spin Hall (QSH) effect, first predicted in graphene by Kane and Mele in 2004, has emerged as a prototypical platform for exploring spin-orbit coupling, topology, and electronic interactions. Initially realized experimentally in quantum wells exhibiting characteristic QSH signatures, the field has since expanded with the discovery of van der Waals (vdW) materials. This review focuses on vdW systems, which offer unique advantages: their exposed surfaces enable a combination of surface-sensitive spectroscopic and microscopic tools for comprehensive detection of the QSH state; mechanical stacking with other vdW layers facilitates symmetry engineering and proximity effects; and moir\'e engineering introduces layer skyrmion topological phases and strong correlation effects. We highlight two monolayer families, 1T$^\prime$-MX$_2$ and MM$^\prime$X$_4$, represented by WTe$_2$ and TaIrTe$_4$, respectively. These materials exhibit QSH phases intertwined with or in close proximity to other quantum phases, such as excitonic insulators, charge density waves, and superconductivity. Their low crystal symmetry and topology enable rich quantum geometrical responses, ranging from nonlinear Hall effects to circular photogalvanic effects. We also discuss moir\'e systems, which combine topology with flatband physics and enhanced correlations, driving spontaneous symmetry breaking and transitions from QSH to quantum anomalous Hall (QAH) states. Remarkably, fractionalized QAH and QSH states have recently been observed in moir\'e systems, significantly advancing the field of condensed matter physics. Finally, we explore emerging applications of QSH and derived materials, such as using nonlinear Hall effects for quantum rectification in microwave energy harvesting and harnessing fractional anomalous states for topological quantum computing.

cond-mat.mes-hall↗