arXiv subjects

Xiangzhou Zhu

Publications and source records attributed to Xiangzhou Zhu.

10 recordsLinked to original sources

arXiv2026

Light-Induced Transient Polarization Reversal in Rhombohedrally Stacked Bilayer Transition Metal Dichalcogenides via an Electronic Mechanism

Light-induced sliding ferroelectricity in two-dimensional van der Waals materials enables polarization control via relative layer motion. However, polarization switching occurs on the time scale of shear modes (tens of ps) and requires very large fluences, potentially damaging the samples. Here, using constrained density functional theory and many-body real-time simulations, we demonstrate an ultrafast electronic reversal of the total out-of-plane polarization sign in the photoexcited state, without requiring interlayer sliding, in rhombohedrally stacked transition-metal dichalcogenide bilayers. The polarization changes sign relative to its initial ground-state value at moderate fluences and within $\sim$ 200 fs, about 50 times faster than the typical shear-mode period. The ultrafast switching is driven by a rearrangement of localized dipoles around the tungsten sites. We establish a novel general mechanism for electronic control of low-dimensional ferroelectrics common to all polar multilayers having type II band alignment. Our work has direct implications for ultrahigh-speed volatile optical memory operating on sub-ps time scales.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast magnetization induced by linearly polarized pulses is widespread in nonmagnetic semiconductors

Ultrafast optical on-off switching of magnetic order promises near-petahertz information processing. Recently, it has been proposed that non-magnetic semiconductors with narrow band edges or strong exchange interactions could display ultrafast magnetization when photoexcited with linearly polarized femtoseconds pulses, but the experimental detection of this effect remains a challenge, mostly for the lack of suitable candidate compounds. Here, we present a high-throughput first-principles screening of the MC3D database of experimentally known inorganic crystals, identifying nearly 440 non-magnetic semiconductors that develop spin polarization under photoexcitation with linearly polarized pulses via a light-induced exchange-driven instability. We determine how the crystal field environment and band-edge orbital character govern the magnitude and the type of magnetic order of the photoinduced state and we unveil systematic chemical and periodic trends that provide intuitive guidance for materials selection. Our results argue that on-off switching of magnetization with linearly polarized femtosecond pulses is a widespread occurrence in non-magnetic semiconductors, opening novel avenues for experimental verification and application.

cond-mat.mtrl-sci↗

arXiv2025

Predicting the Thermal Behavior of Semiconductor Defects with Equivariant Neural Networks

The presence of defects strongly influences semiconductor behavior. However, predicting the electronic properties of defective materials at finite temperatures remains computationally expensive even with density functional theory due to the large number of atoms in the simulation cell and the multitude of thermally accessible configurations. Here, we present a neural network-based framework to investigate the electronic properties of defective semiconductors at finite temperatures efficiently. We develop an active learning approach that integrates two advanced equivariant graph neural networks: MACE for atomic energies and forces and DeepH-E3 for the electronic Hamiltonian. Focusing on representative point defects in GaAs, we demonstrate computational accuracy comparable to density functional theory at a fraction of the computational cost, predicting the temperature-dependent band gap of defective GaAs directly from larger scale molecular dynamics trajectories with an accuracy of few tens of meV. Our results highlight the potential of equivariant neural networks for accurate atomic-scale predictions in complex, dynamically evolving materials.

cond-mat.mtrl-sci↗

arXiv2024

The Effect of Overdamped Phonons on the Fundamental Band Gap of Perovskites

Anharmonic atomic motions can strongly influence the optoelectronic properties of materials but how these effects are connected to the underlying phonon band structure is not understood well. We investigate how the electronic band gap is influenced by overdamped phonons, which occur in an intriguing regime of phonon-phonon interactions where vibrational lifetimes fall below one oscillation period. We contrast the anharmonic halide perovskite CsPbBr$_3$, known to exhibit overdamped phonons in its cubic phase, with the anharmonic oxide perovskite SrTiO$_3$ where the phonons are underdamped at sufficiently high temperatures. Our results show that overdamped phonons strongly impact the band gap and cause slow dynamic fluctuations of electronic levels that have been implicated in the unique optoelectronic properties of halide perovskites. This finding is enabled by developing augmented stochastic Monte Carlo methods accounting for phonon renormalization and imaginary modes that are typically neglected. Our work provides guidelines for capturing anharmonic effects in theoretical calculations of materials.

cond-mat.mtrl-sci↗

arXiv2023

Anharmonic Fluctuations Govern the Band Gap of Halide Perovskites

We determine the impact of anharmonic thermal vibrations on the fundamental band gap of CsPbBr$_3$, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase-transitions, cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.

cond-mat.mtrl-sci↗

arXiv2022

Probing the Disorder inside the Cubic Unit Cell of Halide Perovskites from First-Principles

Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr$_3$. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr$_3$ and quantify the important local structural fluctuations on an atomic scale.

cond-mat.mtrl-sci↗

arXiv2022

Transversal Halide Motion Intensifies Band-To-Band Transitions in Halide Perovskites

Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the {finite-temperature optoelectronic properties} in the prototypical cubic CsPbBr$_3$, using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with halide perovskites enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. We find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr$_3$ but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. We conclude that the dynamic structural flexibility in halide perovskites and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.

cond-mat.mtrl-sci↗

arXiv2022

Robust excitons across the phase transition of two-dimensional hybrid perovskites

Two-dimensional halide perovskites are among intensely studied materials platforms profiting from solution based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase-transitions in two-dimensional perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in two-dimensional perovskites.

cond-mat.mes-hall↗

arXiv2020

Fast and anomalous exciton diffusion in two-dimensional hybrid perovskites

Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and -emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are still largely unexplored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton-phonon coupling, highlighting two-dimensional hybrids as promising platforms for many-body physics research and optoelectronic applications on the nanoscale.

cond-mat.mes-hall↗

arXiv2019

Magnetic exchange interactions in SrMnO$_3$

We calculate Heisenberg-type magnetic exchange interactions for SrMnO$_3$ under isotropic volume expansion using an approach that is based on total energy variations due to infinitesimal spin rotations around a given reference state. Our total energy calculations using density functional theory (DFT) indicate a transition from antiferromagnetic to ferromagnetic coupling for increasing interatomic distances, corresponding to a sign change of the nearest neighbor exchange interaction. This sign change cannot easily be understood from a standard superexchange mechanism. Furthermore, the exchange interaction strongly depends on the corresponding reference state. This "non-Heisenberg" behavior increases with increasing volume and is also confirmed through non-collinear DFT calculations. An orbital- and energy-resolved decomposition of the exchange coupling suggests that an increased partial occupancy of $e_g$ orbitals near the Fermi level is crucial both for the sign change and the non-Heisenberg behavior of the nearest neighbor interaction. Furthermore, even though both $e_g$ and $t_{2g}$ contributions to the exchange interactions decay exponentially for large inter-atomic distances, the $e_g$ contribution remains surprisingly strong over relatively large distances along the crystal axes.

cond-mat.mtrl-sci↗