arXiv subjects

Jia-An Yan

Publications and source records attributed to Jia-An Yan.

At least 19 recordsLinked to original sources

arXiv2026

Electronic Structure of Multilayer Graphene with Arbitrary Stackings

Stacking geometry in multilayer graphene (MLG) provides an interesting degree of freedom to engineer its electronic structure near the Fermi level, wherein the linear bands in single layer graphene could retain or evolve into parabolic or flat bands. Using a tight-binding model, we carried out a detailed analytical analysis of the electronic band structures for arbitrarily stacked MLGs. We show that their low energy band dispersions near the Fermi level may be deduced from its substacks in isolation. The analytical solutions of the momenta with zero eigenvalue for an AA stacking allows us to generalize the results of the zero energy momenta for arbitrarily stacked MLGs. Moreover, we find that an interplay of parallel and rhombohedral stackings allows for flat band engineering and enhancement in arbitrarily stacked MLGs. The existence of flat bands in MLGs might offer another interesting platform for exploring the superconductivity in graphene systems beyond the twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2024

Accelerated Data-Driven Discovery and Screening of Two-Dimensional Magnets Using Graph Neural Networks

In this study, we employ Graph Neural Networks (GNNs) to accelerate the discovery of novel 2D magnetic materials which have transformative potential in spintronics applications. Using data from the Materials Project database and the Computational 2D materials database (C2DB), we train three GNN architectures on a dataset of 1190 magnetic monolayers with energy above the convex hull ($E_{\text{hull}}$) less than 0.3 eV/atom. Our Crystal Diffusion Variational Auto Encoder (CDVAE) generates 11,100 candidate crystals. Subsequent training on two Atomistic Line Graph Neural Networks (ALIGNN) achieves a 93$\%$ accuracy in predicting magnetic monolayers and a mean average error of 0.039 eV/atom for $E_{\text{hull}}$ predictions. After narrowing down candidates based on magnetic likelihood and predicted energy, constraining the atom count in the monolayers to five or fewer, and performing dimensionality checks, we identify 190 candidates. These are validated using Density-Functional Theory (DFT) to confirm their magnetic and energetic favorability resulting in 167 magnetic monolayers with $E_{\text{hull}} < 0.3$ eV/atom and a total magnetization of $\geq$ $0.5 μ_{B}$. Our methodology offers a way to accelerate exploring and predicting potential 2D magnetic materials, contributing to the ongoing computational and experimental efforts aimed at the discovery of new 2D magnets.

cond-mat.dis-nn↗

arXiv2023

Magnetic Stability, Fermi Surface Topology, and Spin-Correlated Dielectric Response in Monolayer 1T-CrTe2

We have carried out density-functional theory (DFT) calculations to study the magnetic stability of both ferromagnetic (FM) and anti-ferromagnetic (AFM) states in monolayer 1T-CrTe2. Our results show that the AFM order is lower in energy and thus is the ground state. By tuning the lattice parameters, the AFM order can transition to the FM order, in good agreement with experimental observation. We observe a commensurate SDW alongside the previously predicted CDW, and attribute the AFM order to the SDW. This results in distinct hole and electron Fermi pockets and a pronounced optical anisotropy, suggesting quasi-one-dimensional behavior in this material.

cond-mat.mtrl-sci↗

arXiv2021

Structural Monoclinicity and Its Coupling to Layered Magnetism in Few-Layer $\mathrm{CrI_{3}}$

Using polarization-resolved Raman spectroscopy, we investigate layer number, temperature, and magnetic field dependence of Raman spectra in one- to four-layer $\mathrm{CrI_{3}}$. Layer-number-dependent Raman spectra show that in the paramagnetic phase a doubly degenerated $E_{g}$ mode of monolayer $\mathrm{CrI_{3}}$ splits into one $A_{g}$ and one $B_{g}$ mode in N-layer (N > 1) $\mathrm{CrI_{3}}$ due to the monoclinic stacking. Their energy separation increases in thicker samples until an eventual saturation. Temperature-dependent measurements further show that the split modes tend to merge upon cooling but remain separated until 10 K, indicating a failed attempt of the monoclinic-to-rhombohedral structural phase transition that is present in the bulk crystal. Magnetic-field-dependent measurements reveal an additional monoclinic distortion across the magnetic-field-induced layered antiferromagnetism-to-ferromagnetism phase transition. We propose a structural change that consists of both a lateral sliding toward the rhombohedral stacking and a decrease in the interlayer distance to explain our experimental observations.

cond-mat.mes-hall↗

arXiv2019

Stacking-Dependent Interlayer Phonons in 3R and 2H MoS$_{2}$

We have investigated the interlayer shear and breathing phonon modes in MoS$_{2}$ with pure 3R and 2H stacking order by using polarization-dependent ultralow-frequency Raman spectroscopy. We observe up to three shear branches and four breathing branches in MoS$_{2}$ with thickness from 2 to 13 layers. The breathing modes show the same Raman activity behavior for both polytypes, but the 2H breathing frequencies are consistently several wavenumbers higher than the 3R breathing frequencies, signifying that 2H MoS$_{2}$ has slightly stronger interlayer lattice coupling than 3R MoS$_{2}$. In contrast, the shear-mode Raman spectra are strikingly different for 2H and 3R MoS$_{2}$. While the strongest shear mode corresponds to the highest-frequency branch in the 2H structure, it corresponds to the lowest-frequency branch in the 3R structure. Such distinct and complementary Raman spectra of the 3R and 2H polytypes allow us to survey a broad range of shear modes in MoS$_{2}$, from the highest to lowest branch. By combining the linear chain model, group theory, effective bond polarizability model and first-principles calculations, we can account for all the major observations in our experiment.

cond-mat.mes-hall↗

arXiv2018

Distinct spin-lattice and spin-phonon interactions in monolayer magnetic CrI$_3$

We apply the density-functional theory to study various phases (including non-magnetic (NM), anti-ferromagnetic (AFM), and ferromagnetic (FM)) in monolayer magnetic chromium triiodide (CrI$_3$), a recently fabricated 2D magnetic material. It is found that: (1) the introduction of magnetism in monolayer CrI$_3$ gives rise to metal-to-semiconductor transition; (2) the electronic band topologies as well as the nature of direct and indirect band gaps in either AFM or FM phases exhibit delicate dependence on the magnetic ordering and spin-orbit coupling; and (3)the phonon modes involving Cr atoms are particularly sensitive to the magnetic ordering, highlighting distinct spin-lattice and spin-phonon coupling in this magnet. First-principles simulations of the Raman spectra demonstrate that both frequencies and intensities of the Raman peaks strongly depend on the magnetic ordering. The polarization dependent $A_{1g}$ modes at 77 cm$^{-1}$ and 130 cm$^{-1}$ along with the $E_g$ mode at about 50 cm$^{-1}$ in the FM phase may offer a useful fingerprint to characterize this material. Our results not only provide a detailed guiding map for experimental characterization of CrI$_3$, but also reveal how the evolution of magnetism can be tracked by its lattice dynamics and Raman response.

cond-mat.mes-hall↗

arXiv2018

Strain-tunable magnetic anisotropy in monolayer CrCl$_3$, CrBr$_3$, and CrI$_3$

Recent observation of intrinsic ferromagnetism in two-dimensional (2D) CrI$_3$ is associated with the large magnetic anisotropy due to strong spin-orbit coupling (SOC) of I. Magnetic anisotropy energy (MAE) defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work we apply the density functional theory to study the strain dependence of MAE in 2D monolayer chromium trihalides CrX$_3$ (with X = Cl, Br, and I). Detailed calculations of their energetics, atomic structures and electronic structures under the influence of a biaxial strain $\varepsilon$ have been carried out. It is found that all three compounds exhibit ferromagnetic ordering at the ground state (with $\varepsilon$=0) and upon applying a compressive strain, phase transition to antiferromagnetic state occurs. Unlike in CrCl$_3$ and CrBr$_3$, the electronic band gap in CrI$_3$ increases when a tensile strain is applied. The MAE also exhibits a strain dependence in the chromium trihalides: it increases when a compressive strain is applied in CrI$_3$, while an opposite trend is observed in the other two compounds. In particular, the MAE of CrI$_3$ can be increased by 47\% with a compressive strain of $\varepsilon$ = 5\%.

cond-mat.mes-hall↗

arXiv2017

Layered material GeSe and vertical GeSe/MoS2 p-n heterojunctions

Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass anisotropy explains the observed anisotropic conductance. The crystallographic orientation of GeSe is confirmed by angleresolved polarized Raman measurements, which are further supported by calculated Raman tensors for the orthorhombic structure. Novel GeSe/MoS2 p-n heterojunctions are fabricated, combining the natural p-type doping in GeSe and n-type doping in MoS2. The temperature dependence of the measured junction current reveals that GeSe and MoS2 have a type-II band alignment with a conduction band offset of ~0.234 eV. The anisotropic conductance of GeSe may enable the development of new electronic and optoelectronic devices, such as high-efficiency thermoelectric devices and plasmonic devices with resonance frequency continuously tunable through light polarization direction. The unique GeSe/MoS2 p-n junctions with type-II alignment may become essential building blocks of vertical tunneling field-effect transistors for low-power applications. The novel p-type layered material GeSe can also be combined with n-type TMDCs to form heterogeneous complementary metal oxide semiconductor (CMOS) circuits.

cond-mat.mes-hall↗

arXiv2016

Interlayer breathing and shear modes in NbSe2 atomic layers

Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with considerably different crystallographic structure and electronic properties from other TMDs, such as MoS2, MoSe2, WS2 and WSe2. Properties of TMD atomic layers are sensitive to interlayer coupling. Here we investigate the interlayer phonons of few-layer NbSe2 by ultralow-frequency Raman spectroscopy. We observe both the interlayer breathing modes and shear modes at frequencies below 40 cm-1 for samples of 2 to 15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. We account for these results utilizing a combination of the linear-chain model, group-theory analysis and first-principles calculations. Although NbSe2 possesses different stacking order from MoS2, MoSe2, WS2 and WSe2, it exhibits the same symmetry and Raman selection rules, as well as similar interlayer coupling strength and thickness dependence of interlayer phonon modes.

cond-mat.mes-hall↗

arXiv2015

Coupling and stacking order of ReS2 atomic layers revealed by ultralow-frequency Raman spectroscopy

We investigate the ultralow-frequency Raman response of atomically thin ReS2, a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS2 exhibit rich Raman spectra at frequencies below 50 cm-1, where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS2 layers are coupled and stacked orderly, in contrast to the general belief that the ReS2 layers are decoupled from one another. While the interlayer breathing modes can be described by a linear chain model as in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS2 are non-degenerate and well separated in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes, and determine the stacking order in bilayer ReS2.

cond-mat.mes-hall↗

arXiv2015

Probing the uniaxial strains in MoS$_2$ using polarized Raman spectroscopy: A first-principles study

Characterization of strain in two-dimensional (2D) crystals is important for understanding their properties and performance. Using first-principles calculations, we study the effects of uniaxial strain on the Raman-active modes in monolayer MoS$_2$. We show that the in-plane $E'$ mode at 384 cm$^{-1}$ and the out-of-plane $A_1'$ mode at 403 cm$^{-1}$ can serve as fingerprints for the uniaxial strain in this 2D material. Specifically, under a uniaxial strain, the doubly degenerate $E'$ mode splits into two non-degenerate modes: one is $E_{\parallel}'$ mode in which atoms vibrate in parallel to the strain direction, and the other is $E_\perp'$ mode in which atoms vibrate perpendicular to the strain direction. The frequency of the $E_{\parallel}'$ mode blue-shifts for a compressive strain, but red-shifts for a tensile strain. In addition, due to the strain-induced anisotropy in the MoS$_2$ lattice, the polarized Raman spectra of the $E_{\parallel}'$ and $E_{\perp}'$ modes exhibit distinct angular dependence for specific laser polarization setups, allowing for a precise determination of the direction of the uniaxial strain with respect to the crystallographic orientation. Furthermore, we find that the polarized Raman intensity of the $A_1'$ mode also shows evident dependence on the applied strain, providing additional effective clues for determining the direction of the strain even without knowledge of the crystallographic orientation. Thus, polarized Raman spectroscopy offers an efficient non-destructive way to characterize the uniaxial strains in monolayer MoS$_2$.

cond-mat.mtrl-sci↗

arXiv2015

Tuning the electronic structures of silicene and germanene by biaxial strain and electric field

We present a first-principles study of effects of small biaxial strain ($|\varepsilon|\le 5\%$) and perpendicular electric field (E-field) on the electronic and phonon properties of low-buckled silicene and germanene. With an increase of the biaxial strain, the conduction bands at the high symmetric $Γ$ and $M$ points of the first Brillouin zone shift significantly towards the Fermi level in both silicene and germanene. In contrast, the E-field changes the band dispersions near the $Γ$ and open a small band gap at the K point in silicene. We found that the field-induced gap opening in silicene could be enhanced by a compressive strain while mitigated by a tensile strain. This result highlights the tunability of the electronic structures of silicene by combining the mechanical strain and the electric field.

cond-mat.mtrl-sci↗

arXiv2015

Strain-tunable topological quantum phase transition in buckled honeycomb lattices

Low-buckled silicene is a prototypical quantum spin Hall insulator with the topological quantum phase transition controlled by an out-of-plane electric field. We show that this field-induced electronic transition can be further tuned by an in-plane hydrostatic biaxial strain $\varepsilon$, owing to the curvature-dependent spin-orbit coupling (SOC): There is a $Z_2$ = 1 topological insulator phase for biaxial strain $|\varepsilon|$ smaller than 0.07, and the band gap can be tuned from 0.7 meV for $\varepsilon = +0.07$ up to a fourfold 3.0 meV for $\varepsilon = -0.07$. First-principles calculations also show that the critical field strength $E_c$ can be tuned by more than 113\%, with the absolute values nearly 10 times stronger than the theoretical predictions based on a tight-binding model. The buckling structure of the honeycomb lattice thus enhances the tunability of both the quantum phase transition and the SOC-induced band gap, which are crucial for the design of topological field-effect transistors based on two-dimensional materials.

cond-mat.mes-hall↗

arXiv2014

Stability and properties of high-buckled two-dimensional tin and lead

In realizing practical non-trivial topological electronic phases stable structures need to be determined first. Tin and lead do stabilize an optimal two-dimensional high-buckled phase --a hexagonal-close packed bilayer structure with nine-fold atomic coordination-- and they do not stabilize topological fullerenes, as demonstrated by energetics, phonon dispersion curves, and the structural optimization of finite-size samples. The high-buckled phases are metallic due to their high atomic coordination. The optimal structure of fluorinated tin lacks three-fold symmetry and it stabilizes small samples too. It develops two oblate conical valleys on the first Brillouin zone coupling valley, sublattice, and spin degrees of freedom with a novel $τ_zσ_xs_x$ term, thus making it a new 2D platform for valleytronics.

cond-mat.mes-hall↗

arXiv2014

A Note on the Monge-Kantorovich Problem in the Plane

The Monge-Kantorovich mass-transportation problem has been shown to be fundamental for various basic problems in analysis and geometry in recent years. Shen and Zheng (2010) proposed a probability method to transform the celebrated Monge-Kantorovich problem in a bounded region of the Euclidean plane into a Dirichlet boundary problem associated to a nonlinear elliptic equation. Their results are original and sound, however, their arguments leading to the main results are skipped and difficult to follow. In the present paper, we adopt a different approach and give a short and easy-followed detailed proof for their main results.

math.OC↗

arXiv2014

Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene

Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively-powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device's electrical conductance to its thermal conductance. High ZT (>2) has been achieved in materials via all-scale hierarchical architecturing. This efficiency holds at high temperatures (700K~900K) but quickly diminishes at lower temperatures. In this paper, a recently-fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal that phosphorene possesses spatially-anisotropic electrical and thermal conductances. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT can reach 2.5 (the criterion for commercial deployment) along the armchair direction of phosphorene at T=500K and is greater than 1 even at room temperature given moderate doping (~2 x 10^16 m-2). Ultimately, phosphorene stands out as an environmentally sound thermoelectric material with unprecedented qualities: intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (~ 300K) - one whose performance does not require any sophisticated engineering techniques.

cond-mat.mtrl-sci↗

arXiv2013

Electron-Phonon Coupling in Two-Dimensional Silicene and Germanene

Following the work in graphene, we report a first-principles study of electron-phonon coupling (EPC) in low-buckled (LB) monolayer silicene and germanene. Despite of the similar honeycomb atomic arrangement and linear band dispersion, the EPC matrix-element squares of the $Γ$-$E_g$ and K-$A_1$ modes in silicene are only about 50% of those in graphene. However, the smaller Fermi velocity in silicene compensates this reduction by providing a larger joint electronic density of states near the Dirac point. We predict that Kohn anomalies associated with these two optical modes are significant in silicene. In addition, the EPC-induced frequency shift and linewidth of the Raman-active $Γ$-$E_g$ mode in silicene are calculated as a function of doping. The results are comparable to those in graphene, indicating a similar non-adiabatic dynamical origin. In contrast, the EPC in germanene is found to be much reduced.

cond-mat.mes-hall↗

arXiv2013

Electric Field Effects on the Optical Vibrations in AB-Stacked Bilayer Graphene

Using first-principles methods, we show that an applied perpendicular electric field $E$ breaks the inversion symmetry of AB-stacked bilayer graphene (BLG), thereby slightly mixing the two in-plane high-energy optical vibrations ($E_g$ and $E_u$ modes). The mixed amplitudes increase parabolically with respect to the field strength when $E$$<$2.0 V/nm, and then exhibit linear dependence when $E$$>$2.0 V/nm. In contrast, the mixing effect on the out-of-plane vibrations ($A_{1g}$ and $A_{2u}$ modes) is found to be much stronger, with the mixed amplitudes nearly an order of magnitude larger than those for the in-plane modes. For the two in-plane modes, we then calculate their phonon linewidths and frequency shifts as a function of the electric field as well as the Fermi level. Our results reveal delicate interplay between electrons and phonons in BLG, tunable by the applied fields and charge carrier densities.

cond-mat.mes-hall↗