arXiv subjects

Qi Ren

Publications and source records attributed to Qi Ren.

5 recordsLinked to original sources

arXiv2025

Low and Anisotropic Thermal Conductivity in Mixed-Valent Sn$_2$S$_3$

Compounds of Sn, such as SnSe and SnS, exhibit novel phonon characteristics and low thermal conductivity, making them emerging star materials in the thermoelectric family. In this work, through the Boltzmann transport equation scheme and the Wigner thermal transport model, quasi-1D mixed-valent Sn$_2$S$_3$ were found to exhibit a low thermal conductivity along c-axis with a weak temperature dependence. The low thermal conductivity is attributed to the anharmonic rattling vibrations of weakly bonded Sn(II) atoms, which are influenced by the coulomb interaction of lone pairs at adjacent Sn(II) atoms. The rattling of Sn(II) induces low-frequency flat optical phonons and avoids crossing behavior. The atomic displacements and mean square displacement (MSD) analysis reveal that Sn(II) atoms exhibit significantly greater and anisotropic displacements compared to Sn(IV) and S, confirming that Sn(II) behaves as a rattler. The results obtained from this work suggest an opportunity to discover low thermal conductivity in mixed-valent compounds.

cond-mat.mtrl-sci↗

arXiv2025

Polarization Vortices in a Ferromagnetic Metal via Twistronics

Recent advances in moire engineering provide new pathways for manipulating lattice distortions and electronic properties in low-dimensional materials. Here, we demonstrate that twisted stacking can induce dipolar vortices in metallic SrRuO3 membranes, despite the presence of free charges that would normally screen depolarizing fields and dipole-dipole interactions. These polarization vortices are correlated with moire-periodic flexoelectricity induced by shear strain gradients, and exhibit a pronounced dependence on the twist angle. In addition, multiferroic behavior emerges below the ferromagnetic Curie temperature of the films, whereby polarization and ferromagnetism coexist and compete, showing opposite twist-angle dependencies of their respective magnitudes. Density functional theory calculations provide insights into the microscopic origin of these observations. Our findings extend the scope of polarization topology design beyond dielectric materials and into metals.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear Breakdown of Antisymmetric Flexoelectric Coupling

Flexoelectric coupling is conventionally regarded as antisymmetric, such that reversing the strain gradient reverses the polarization direction while preserving its magnitude. Here we report that this antisymmetric coupling breaks down in noncentrosymmetric single crystals driven by large strain gradients, where nonlinearity becomes operative. Harmonic-resolved measurements reveal robust even-order flexoelectric harmonics that are forbidden in antisymmetric systems, as well as the three-wave mixing indicative of symmetry breaking under dual-frequency excitation. These frequency-relevant generations provide evidence for asymmetric flexoelectricity in the nonlinear regime. A reconstructed nonlinear constitutive framework consistently accounts for all observations by incorporating even-order strain-gradient terms. Our findings establish asymmetric flexoelectricity as an intrinsic electromechanical response and uncover a symmetry-breaking mechanism in flexoelectricity.

cond-mat.mtrl-sci↗

arXiv2024

Identifying Highly Deformable van der Waals Layered Chalcogenides with Superior Thermoelectric Performance Using Deformability Factors and Interpretable Machine Learning

Van der Waals layered chalcogenide-based flexible thermoelectric devices show great potential for applications in wearable electronics. However, materials that are both highly deformable and exhibit superior thermoelectric performance are extremely limited. There is an urgent need for methods that can efficiently predict both deformability and thermoelectric performance to enable high-throughput screening of these materials. In this study, over 1000 van der Waals layered chalcogenides were high-throughput screened from material databases, the deformability of which were predicted with our previously developed deformability factor. An accurate and efficient model based on machine learning methods were developed to predict the thermoelectric properties. Several candidate materials with both deformability and thermoelectric potential were successfully discovered. Among them, NbSe2Br2 was verified by first principles calculations, achieving ZTmax value of 1.35 at 1000K, which is currently the highest value among flexible inorganic thermoelectric materials. And the power factor value of 8.1 {\mu}Wcm-1K-2 at 300K also surpassed most organic and inorganic flexible thermoelectric materials. Its high deformability mainly attributed to the small slipping energy that allows interlayer slip and the small in-plane modulus that allows deformation before failure. The high ZTmax is mainly contributed by the extremely low thermal conductivity and the high Seebeck coefficient along the out-of-plane direction at high temperature. The high power factor at room temperature is mainly comes from the high conductivity in the in-plane direction. This study is expected to accelerate the development and application of flexible thermoelectric devices based on inorganic semiconductor materials.

cond-mat.mtrl-sci↗

arXiv2021

Zero Poisson' s Ratio and Suppressed Mechanical Anisotropy in BP/SnSe Van der Waals Heterostructure: A First-principles Study

Black phosphorene and its analogs have attracted intensive attention due to their unique puckered structures, anisotropic characteristics, and negative Poisson's ratio. The van der Waals heterostructures assembly by stacking different materials may show novel physical properties which the parent materials don't possess. In this work, the first-principles calculations were performed to study the mechanical properties of the BP/SnSe van der Waals heterostructure. Interestingly, a near-zero Poisson's ratio vzx was found in BP/SnSe heterostructure. In addition, compared to the parent materials BP and SnSe with strong in-plane anisotropic mechanical properties, the BP/SnSe heterostructure shows strongly suppressed anisotropy. Our findings suggest that the vdW heterostructure could show quite different mechanical properties from the parent materials and provide new opportunities for the mechanical applications of the heterostructures.

cond-mat.mtrl-sci↗