arXiv subjects

Lingzhi Cao

Publications and source records attributed to Lingzhi Cao.

4 recordsLinked to original sources

arXiv2026

Optical-Phonon-Enabled Large Lattice Thermal Conductivity Anisotropy in Hexagonal Perovskites Cs$BX_3$ ($B$ = Mg, Cd; $X$ = Cl, Br, I)

Materials exhibiting strongly anisotropic lattice thermal conductivity are desirable for thermal-management applications, yet such behavior is commonly associated with layered or quasi-one-dimensional van der Waals crystals and highly anisotropic elastic properties. Here, we investigate lattice thermal transport in the hexagonal perovskites Cs$BX_3$ ($B=$ Mg, Cd; $X=$ Cl, Br, I) using first-principles calculations. At 300~K, the calculated in-plane and out-of-plane lattice thermal conductivities range from 0.13--0.83 and 0.34--6.26~Wm$^{-1}$K$^{-1}$, respectively, corresponding to anisotropy ratios of 2.6--7.5. This pronounced anisotropy is remarkable given the relatively modest elastic anisotropy, characterized by $C_{33}/C_{11}$ = 0.994--1.842. Our analysis reveals that medium-frequency optical phonons provide an efficient out-of-plane heat-transport channel, contrary to the conventional picture in which heat transport is dominated by acoustic phonons. These findings identify face-sharing octahedral frameworks as a promising platform for engineering strong thermal-conductivity anisotropy in mechanically near-isotropic, non--van der Waals crystals.

cond-mat.mtrl-sci↗

arXiv2026

Polarization Rotation Drives a Spin-Topological Transition in Ferroelectric Bismuth Monolayer

Bismuth monolayer is the first two-dimensional elemental ferroelectric and an appealing platform for coupling polar order to spin-orbit-driven topology. However, its microscopic switching mechanism remains elusive. Here, using first-principles lattice dynamics and symmetry-adapted mode analysis, we identify a previously overlooked rotational pathway for in-plane polarization switching. Its energy barrier is more than four times lower than that of direct reversal, naturally explaining the vortexlike domain textures observed in molecular dynamics simulations. Remarkably, this polarization rotation also drives a spin-topological transition, changing the spin Chern number from $C_s=-2$ to $0$. Directional uniaxial strain further steers the polarization orientation and tunes the associated topological transition. These results establish polarization rotation as the switching mechanism of ferroelectric Bi monolayer and as an efficient route to electrically and mechanically programmable topology in two-dimensional ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

AgNbO$_3$ is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group $R3$, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses--including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics--can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO$_3$ but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic Origin of the Ultralow Lattice Thermal Conductivity in Vacancy-Ordered Halide Double Perovskites Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$= Cl, Br, and I)

Vacancy-ordered halide double perovskites Cs$_2BX_6$ have recently attracted significant attention due to their intrinsically ultralow lattice thermal conductivity ($κ_{\mathrm{L}}$), which is highly desirable for thermal insulation and thermoelectric applications. In this work, we systematically investigate the anharmonic lattice dynamics and thermal transport properties of Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$ = Cl, Br, and I) using state-of-the-art first-principles calculations, based on a unified theory of thermal transport for crystals and glasses. All studied compounds are found to exhibit ultralow $κ_{\mathrm{L}}$ below 1.0~W\,m$^{-1}$\,K$^{-1}$ at room temperature and large derivation from the conventional $T^{-1}$ temperature dependence. Our analysis combining with machine-learning approach show that low sound velocities (1100 -- 1600~m\,s$^{-1}$), which originates from the intrinsically weak chemical bonding, play a crucial role in suppressing heat transport of the most compounds, instead of the strong scattering of rattling phonon modes expected from the large void in the structure. Furthermore, the influence of $B$ and $X$-site elements on phonon dispersion, anharmonicity, and scattering phase space is clarified. Our results provide microscopic insights into the origin of ultralow $κ_{\mathrm{L}}$ in Cs$_2BX_6$ and offer guiding principles for the rational design of halide-based materials with tailored thermal transport properties.

cond-mat.mtrl-sci↗