arXiv subjects

Fangyuan Gu

Publications and source records attributed to Fangyuan Gu.

4 recordsLinked to original sources

arXiv2025

2D ferroelectricity accompanying antiferro-orbital order in semi-metallic WTe$_2$

The first switchable electric polarization in metals was recently discovered in bilayer and trilayer WTe2. Strangely, despite the tininess of the ordered polarization, the ferroelectricity survives up to 350 K, rendering the mechanism of such ferroelectricity challenging for standard understandings. Here, via a density-functional-based multi-energy-scale analysis of the system's broken symmetries, we identify a weak out-of-plane ferroelectricity accompanying a strong in-plane antiferro-orbital order. This unusual low-energy correlation, which emerges from an antiferroelectric structure formed at much higher energy, naturally explains the above puzzling observation. This result reveals an unprecedented paradigm of electronic ferroelectricity generally applicable to 2D polar metals with ultrafast-switchable polarization ideal for the next-generation non-volatile memory and other devices.

cond-mat.mtrl-sci↗

arXiv2024

`Interaction annealing' to determine effective quantized valence and orbital structure: an illustration with ferro-orbital order in WTe$_2$

Correlated materials are known to display qualitatively distinct emergent behaviors at low energy. Conveniently, upon absorbing rapid quantum fluctuations, these rich low-energy behaviors can always be effectively described by dressed particles with fully quantized charge, spin, and orbital structure. Such a powerful and simple description is, however, difficult to access through bare particles used in most many-body computations, especially when fluctuations are strong such as in $4d$ and $5d$ compounds. To decipher the dominant quantized structure, we propose an easy-to-implement `interaction annealing' approach that utilizes suppressed charge fluctuation through enhancing ionic charging energy. We establish its theoretical foundation using an exactly treated two-site Hubbard model as a generic example. We then demonstrate its applications with more affordable density functional calculations to a representative $3d$ Mott insulator La${_2}$CuO${_4}$ and a highly fluctuating $5d$ semi-metal WTe${_2}$. In the latter, it reveals an emergent local electronic structure that makes possible an unprecedented explanation of several experimental observations. Finally, we demonstrate the effectiveness of this approach in studying competing local electronic structures in functional materials.

cond-mat.str-el↗

arXiv2023

Ultrafast polarization switching in BaTiO$_3$ by photoactivation of its ferroelectric and central modes

We use molecular dynamics simulations with machine-learned atomistic force fields to simulate photoexcitation of BaTiO3 by a femtosecond laser pulse whose photon energy exceeds the optical gap. We demonstrate selective displacive excitation of coherent zone-center ferroelectric mode phonons and of the strongly anharmonic central mode. We show that the direction of P can either be reversed by a pulse in hundreds of femtoseconds or, on a longer time scale and when combined with a weak field, switched to any one of its symmetry-equivalent directions.

cond-mat.mtrl-sci↗

arXiv2023

Quantum fluctuation of ferroelectric order in polar metals

Since its discovery a decade ago, "polar metallic phase" has ignited significant research interest, as it further functionalizes the switchable electric polarization of materials with additional transport capability, granting them great potential in next-generation electronic devices. The polar metallic phase is an unusual metallic phase of matter containing long-range ferroelectric (FE) order in the electronic and atomic structure. Distinct from the typical FE insulating phase, this phase spontaneously breaks the inversion symmetry but without global polarization. Unexpectedly, the FE order is found to be dramatically suppressed by carriers and destroyed at moderate ~10% carrier density. Here, we propose a general mechanism based on carrier-induced quantum fluctuations to explain this puzzling phenomenon. Basically, the quantum kinetic effect would drive the formation of polaronic quasi-particles made of the carriers and their surrounding dipoles. The disruption in dipolar directions can therefore weaken or even destroy the FE order. We demonstrate such polaron formation and the associated FE suppression via a simple model using exact diagonalization, perturbation, and quantum Monte Carlo approaches. This quantum mechanism also provides an intuitive picture for many puzzling experimental findings, thereby facilitating new designs of multifunctional FE electronic devices augmented with quantum effects.

cond-mat.mtrl-sci↗