arXiv subjects

Zhiren He

Publications and source records attributed to Zhiren He.

6 recordsLinked to original sources

arXiv2026

Universal unconventional responses controlled by ferroaxial order

Ferroaxial materials are an emerging class of ferroic materials that, in contrast to ferromagnets and ferroelectrics, are fully robust against stray fields, making them ideally suited for data storage and other nonvolatile applications. However, detection of the ferroaxial state remains challenging, as ferroaxiality does not manifest directly through a measurable macroscopic electric polarization or magnetization, and probes are mostly limited to optical methods. Here, we theoretically establish that ferroaxial order universally generates unconventional components of responses to external stimuli, such as the spin Hall effect, magneto-Seebeck effect, or Faraday effect, across a broad range of linear and nonlinear transport, optical and equilibrium phenomena. These unconventional responses are always directly coupled to ferroaxial order and can distinguish ferroaxial domains, providing probes of ferroaxial order. This general connection also reveals a largely unexplored class of ferroaxial metals, in which unconventional transport provides a natural probe of ferroaxial order, as demonstrated by our first-principles calculations for representative materials. Our results reveal a fundamentally different way for ferroic order to manifest -- ferroaxiality primarily shows unconventional material responses rather than the directly measurable order parameters, broadening the possibilities of nonvolatile ferroic control and flexible device design.

cond-mat.mtrl-sci↗

arXiv2025

Geometric control of the moire twist angle in heterobilayer flakes

We demonstrate a finite twist-angle stabilization mechanism in lattice-mismatched 2D heterobilayers, which results from the geometric alignment between the flake edges and its moire pattern. Using atomistic simulations of graphene on hexagonal boron nitride flakes with diameters of up to $\sim 2500$Å, we identify robust metastable angles at $\sim 0.61^\circ$ for armchair and $\sim1.89^\circ$ for zigzag-edged flakes, tunable via in-plane heterostrain. This locking mechanism, which relies on energy barriers that are an order of magnitude larger than those of nearby metastable twist angles, provides a geometric route to precision twist-angle control of two-dimensional heterostructures and to understand the self-orientation of macroscopic flakes.

cond-mat.mes-hall↗

arXiv2025

Lattice-induced spin dynamics in Dirac magnet CoTiO3

Spin-lattice coupling is crucial for understanding the spin transport and dynamics for spintronics and magnonics applications. Recently, cobalt titanate (CoTiO3), an easy-plane antiferromagnet, has been found to host axial phonons with a large magnetic moment, which may originate from spin-lattice coupling. Here, we investigate the effect of light-driven lattice dynamics on the magnetic properties of CoTiO3 using time-resolved spectroscopy with a THz pump and a magneto-optic probe. We found resonantly driven Raman active phonons, phonon-polariton-induced excitation of the antiferromagnetic magnons, and a slow increase in the polarization rotation of the probe, all indicating symmetry breaking that is not intrinsic to the magnetic space group. The temperature dependence confirmed that the observed spin dynamics is related to the magnetic order, and we suggest surface effects as a possible mechanism. Our results of THz-induced spin-lattice dynamics signify that extrinsic symmetry breaking may contribute strongly and unexpectedly to light-driven phenomena in bulk complex oxides.

cond-mat.mtrl-sci↗

arXiv2025

Persistent spin textures, altermagnetism and charge-to-spin conversion in metallic chiral crystals TM$_{3}$X$_{6}$

Chiral crystals, due to the lack of inversion and mirror symmetries, exhibit unique spin responses to external fields, enabling physical effects rarely observed in high-symmetry systems. Here, we show that materials from the chiral dichalcogenide family TM$_3$X$_6$ (T = 3d, M = 4d/5d, X = S) exhibit persistent spin texture (PST) - unidirectional spin polarization of states across large regions of the reciprocal space - in their nonmagnetic metallic phase. Using the example of NiTa$_{3}$S$_{6}$ and NiNb$_{3}$S$_{6}$, we show that PSTs cover the full Fermi surface, a rare and desirable feature that enables efficient charge-to-spin conversion and suggests long spin lifetimes and coherent spin transport above magnetic ordering temperatures. At low temperatures, the materials that order antiferromagnetically become chiral altermagnets, where spin textures originating from spin-orbit coupling and altermagnetism combine in a way that sensitively depends on the orientation of the Neel vector. Using symmetry analysis and first-principles calculations, we classify magnetic ground states across the family, identify cases with weak ferromagnetism, and track the evolution of spin textures and charge-to-spin conversion across magnetic phases and different Neel vector orientations, revealing spin transport signatures that allow one to distinguish Neel vector directions. These findings establish TM$_3$X$_6$ as a tunable platform for efficient charge-to-spin conversion and spin transport, combining structural chirality, persistent spin textures, and altermagnetism.

cond-mat.mtrl-sci↗

arXiv2025

Optical control of the moiré twist angle

In this theoretical work, we propose an all-optical method for fast, precise manipulation of two-dimensional multilayers by transferring orbital angular momentum from phase-structured light (e.g. vortex beams) to a 2D material flake. We model the light-matter interaction, analyze the twist dynamics, and develop a phase diagram for optical twists by mapping the system onto an impulsively forced nonlinear pendulum. Our findings reveal rich dynamical responses spanning single- and multi-pulse twist angle control to (quasi)stable dynamical trajectories, and suggest a pathway for all-optical measurement of the twist potential energy. Aided by classical potential estimates for the interlayer energy and numerical simulation, we demonstrate the feasibility of this approach with hexagonal boron nitride bilayers and extend the results to dichalcogenides with first-principles calculations. These results can be generalized to other 2D multilayers, paving the way for scalable and customizable moiré electronics and photonics.

cond-mat.mtrl-sci↗

arXiv2024

Optical Control of Ferroaxial Order

Materials that exhibit ferroaxial order hold potential for novel multiferroic applications. However, in pure ferroaxials, domains are not directly coupled to stress or static electric field due to their symmetry, limiting the ability to pole and switch between domains -- features required for real-world applications. Here we propose a general approach to selectively condense and switch between ferroaxial domains with light. We show that circularly polarized light pulses on resonance with infrared-active phonons manifest helicity-dependent control over ferroaxial domains. Nonlinear contributions to the lattice polarizability play an essential role in this phenomenon. We illustrate the feasibility of our approach using first-principle calculations and dynamical simulations for the archetypal ferroaxial material RbFe(MoO$_4$)$_2$. Our results are discussed in the context of future pump-probe optical experiments, where polarization, carrier frequency, and fluence threshold are explored.

cond-mat.mtrl-sci↗