arXiv subjects

Yun-Peng Wang

Publications and source records attributed to Yun-Peng Wang.

18 recordsLinked to original sources

arXiv2026

Polymorphic crystallites model for monolayer amorphous materials

Establishing atomistic models of amorphous materials remains a central challenge in materials science. Here, we examine three representative multicomponent monolayer systems using first-principles-trained machine-learning potentials (MLPs) and an energy-guided Monte Carlo structural-search workflow. For monolayer amorphous boron nitride (maBN), independent density-functional-theory (DFT) single-point calculations confirm that the ensemble generated by the MLP-driven search is lower in energy on average than an extended-Tersoff-generated ensemble. Across the sampled maBN structures, local-ring classification combined with strict connected-domain analysis identifies h-BN-like, o-B$_2$N$_2$-like, and o-B$_4$N$_4$-like crystallite domains together with mixed medium-range order. Only individual samples containing at least two distinct phase-pure crystallite-domain types are classified as polymorphic crystallites. The same analysis finds coexisting hexagonal and tetragonal domains in monolayer amorphous LiCl and graphene-like, h-BN-like, borophene-like, and mixed-order regions in monolayer amorphous BCN. Five-nanosecond MLP molecular-dynamics trajectories at 300 K show that the candidate structures are dynamically persistent on the simulated timescale. These results support a polymorphic-crystallite description for the selected model systems, while experimental discrimination from alternative continuous-random-network descriptions will require medium-range-order-sensitive measurements.

cond-mat.mtrl-sci↗

arXiv2023

Structure and mechanical properties of monolayer amorphous carbon and boron nitride

Amorphous materials exhibit various characteristics that are not featured by crystals and can sometimes be tuned by their degree of disorder (DOD). Here, we report results on the mechanical properties of monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (maBN) with different DOD. The pertinent structures are obtained by kinetic-Monte-Carlo (kMC) simulations using machine-learning potentials (MLP) with density-functional-theory (DFT)-level accuracy. An intuitive order parameter, namely the areal fraction Fx occupied by crystallites within the continuous random network, is proposed to describe the DOD. We find that Fx captures the essence of the DOD: Samples with the same Fx but different sizes and distributions of crystallites have virtually identical radial distributions functions as well as bond-length and bond-angle distributions. Furthermore, by simulating the fracture process with molecular dynamics, we found that the mechanical responses of MAC and maBN before fracture are solely determined by Fx and are insensitive to the sizes and specific arrangements of the crystallites. The behavior of cracks in the two materials is analyzed and found to mainly propagate in meandering paths in the CRN region and to be influenced by crystallites in distinct ways that toughen the material. The present results reveal the relation between structure and mechanical properties in amorphous monolayers and may provide a universal toughening strategy for 2D materials.

cond-mat.mtrl-sci↗

arXiv2022

The structure of amorphous two-dimensional materials: Elemental monolayer amorphous carbon versus binary monolayer amorphous boron nitride

The structure of amorphous materials has been debated since the 1930's as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely honeycomb regions made of noncanonical hexagons. Implications for other non-elemental 2D and bulk amorphous materials are discussed.

cond-mat.mtrl-sci↗

arXiv2022

First-Principles investigation of the First-order Phase Transition in Kagome Na$_2$Ti$_3$Cl$_8$

The antiferromagnetic kagome material Na2Ti3Cl8 exhibits a first-order phase transition; the low-temperature phase is characterized by the trimerizations of Ti ions. In this work, we carry out first-principles calculations on the crystal and electronic structure of Na$_2$Ti$_3$Cl$_8$ at the high- and low-temperature phases. The crystal structures, including the lattice constant and the trimerization of Ti ions, are well reproduced by taking account a small Hubbard correction of 1 eV. The calculated total energy landscape reveals a first-order phase transition with the total energy barrier of 80 meV per formula unit. Analysis of the electronic structure indicates a direct metal bonding among Ti in the trimierized low-temperature phase.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast Optical Spectroscopy Evidence of Pseudogap and Electron-Phonon Coupling in an Iron-Based Superconductor KCa$_2$Fe$_4$As$_4$F$_2$

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with $T_c=33.5$ K. Our results reveal a possible pseudogap ($Δ_{PG}$ = 2.4 $\pm$ 0.1 meV) below $T^*\approx 50$ K but prior to the opening of a superconducting gap ($Δ_{SC}$(0) $\approx$ 4.3 $\pm$ 0.1 meV). Measurements under high pump fluence real two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency $A_{1g}$(2) mode corresponds to the $c-$axis polarized vibrations of FeAs planes with a nominal electron-phonon coupling constant $λ_{A_{1g}(2)}$ = 0.194 $\pm$ 0.02. Our findings suggest that the pseudogap is likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa$_2$Fe$_4$As$_4$F$_2$.

cond-mat.supr-con↗

arXiv2021

Thermal transport of amorphous carbon and boron-nitride monolayers

Two-dimensional (2D) materials like graphene and h-BN usually show high thermal conductivity, which enables rich applications in thermal dissipation and nanodevices. Disorder, on the other hand, is often present in 2D materials. Structural disorder induces localization of electrons and phonons and alters the electronic, mechanical, thermal, and magnetic properties. Here we calculate the in-plane thermal conductivity of both monolayer carbon and monolayer boron nitride in the amorphous form, by reverse nonequilibrium molecular dynamics simulations. We find that the thermal conductivity of both monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (ma-BN) are about two orders of magnitude smaller than their crystalline counterparts. Moreover, the ultralow thermal conductivity is independent of the temperature due to the extremely short phonon mean free path in these amorphous materials. The relation between the structure disorder and the reduction of the thermal conductivity is analyzed in terms of the vibrational density of states and the participation ratio. ma-BN shows strong vibrational localization across the frequency range, while MAC exhibits a unique extended G' mode at high frequency due to its sp2 hybridization and the broken E2g symmetry. The present results pave the way for potential applications of MAC and ma-BN in thermal management.

cond-mat.mtrl-sci↗

arXiv2021

Synthesis and properties of free-standing monolayer amorphous carbon

Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks, recent experimental evidence favours the competing crystallite model in the case of amorphous silicon. In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition, of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model. We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics.

cond-mat.mtrl-sci↗

arXiv2020

Gate field effects on the topological insulator BiSbTeSe2 interface

Interfaces between two topological insulators are of fundamental interest in condensed matter physics. Inspired by experimental efforts, we study interfacial processes between two slabs of BiSbTeSe2 (BSTS) via first principles calculations. Topological surface states are absent for the BSTS interface at its equilibrium separation, but our calculations show that they appear if the inter-slab distance is greater than 6 Ang. More importantly, we find that topological interface states can be preserved by inserting two or more layers of hexagonal boron nitride between the two BSTS slabs. In experiments, the electric current tunneling through the interface is insensitive to back gate voltage when the bias voltage is small. Using a first-principles based method that allows us to simulate gate field, we show that at low bias the extra charge induced by a gate voltage resides on the surface that is closest to the gate electrode, leaving the interface almost undoped. This provides clues to understand the origin of the observed insensitivity of transport properties to back voltage at low bias. Our study resolves a few questions raised in experiment, which does not yet offer a clear correlation between microscopic physics and transport data. We provide a road map for the design of vertical tunneling junctions involving the interface between two topological insulators.

cond-mat.mtrl-sci↗

arXiv2019

DFT+DMFT calculations of the complex band and tunneling behavior for the transition metal monoxides MnO, FeO, CoO and NiO

We report complex band structure (CBS) calculations for the four late transition metal monoxides, MnO, FeO, CoO and NiO, in their paramagnetic phase. The CBS is obtained from density functional theory plus dynamical mean field theory (DMFT) calculations to take into account correlation effects. The so-called $β$ parameters, governing the exponential decay of the transmission probability in the non-resonant tunneling regime of these oxides, are extracted from the CBS. Different model constructions are examined in the DMFT part of the calculation. The calculated $β$ parameters provide theoretical estimation for the decay length in the evanescent channel, which would be useful for tunnel junction applications of these materials.

cond-mat.str-el↗

arXiv2017

Multi-Control Over Graphene-Molecule Hetereo-Junctions

The vertical configuration is a powerful tool recently developed experimentally to investigate field effects in quasi 2D systems. Prototype graphene-based vertical tunneling transistors can achieve an extraordinary control over current density utilizing gate voltages. In this work we study theoretically vertical tunneling junctions that consist of a monolayer of photo-switchable aryl-azobenzene molecules of sandwiched between two sheets of graphene. Azobenzene molecules transform between {\it trans} and {\it cis} conformations upon photoexcitation, thus adding a second knob that enhances control over physical properties of the junction. Using first-principles methods within the density functional framework, we perform simulations with the inclusion of field effects for both {\it trans} and {\it cis} configurations. We find that the interference of interface states resulting from molecule-graphene interactions at the Fermi energy introduces a dual-peak pattern in the transmission functions and dominates the transport properties of gate junctions, shedding new light on interfacial processes.

cond-mat.mes-hall↗

arXiv2017

Tunneling Field-Effect Junctions with WS$_2$ barrier

Transition metal dichalcogenides (TMDCs), with their two-dimensional structures and sizable bandgaps, are good candidates for barrier materials in tunneling field-effect transistor (TFET) formed from atomic precision vertical stacks of graphene and insulating crystals of a few atomic layers in thickness. We report first-principles study of the electronic properties of the Graphene/WS$_2$/Graphene sandwich structure revealing strong interface effects on dielectric properties and predicting a high ON/OFF ratio with an appropriate WS$_2$ thickness and a suitable range of the gate voltage. Both the band spin-orbit coupling splitting and the dielectric constant of the WS$_2$ layer depend on its thickness when in contact with the graphene electrodes, indicating strong influence from graphene across the interfaces. The dielectric constant is significantly reduced from the bulk WS$_2$ value. The effective barrier height varies with WS$_2$ thickness and can be tuned by a gate voltage. These results are critical for future nanoelectronic device designs.

cond-mat.mtrl-sci↗

arXiv2017

Ferroelectricity in $\mathbf{BiMnO_3}$ Thin Films

The existence of ferroelectricity in {$\mathrm{BiMnO}_3$} has been a long-standing question for both experimentalists and theorists. In addition to a highly distorted bulk structure, the ionic crystal planes cause a large roughness in thin films that makes it extremely difficult to nail down the physical mechanisms underlying a possible ferroelectric-ferromagnetic phase. We approach the problem by including the substrate explicitly to study the polarization. With this model, we investigate mono-, di-, and trilayer {$\mathrm{BiMnO}_3$} thin films on $\mathrm{SrTiO}_3$ substrates. We find that thin film systems have both strong ferromagnetism and strong ferroelectricity. Substrate constraints weaken the competition between displacements induced by stereochemically active Bi-$6s^2$ lone pairs and by Jahn-Teller distortions around Mn ions found in the bulk, such that the sum of off-center displacements of Bi ions in bulk {$\mathrm{BiMnO}_3$} nearly cancel. In {$\mathrm{BiMnO}_3$} thin films, in contrast, all Bi ions displace roughly in parallel, resulting in a strongly polar structure. We also find spontaneous charge disproportionation of Mn ion pairs in {$\mathrm{BiMnO}_3$} thin films.

cond-mat.mtrl-sci↗

arXiv2016

All-electron self-consistent GW in the Matsubara-time domain: implementation and benchmarks of semiconductors and insulators

The GW approximation is a well-known method to improve electronic structure predictions calculated within density functional theory. In this work, we have implemented a computationally efficient GW approach that calculates central properties within the Matsubara-time domain using the modified version of Elk, the full-potential linearized augmented plane wave (FP-LAPW) package. Continuous-pole expansion (CPE), a recently proposed analytic continuation method, has been incorporated and compared to the widely used Pade approximation. Full crystal symmetry has been employed for computational speedup. We have applied our approach to 18 well-studied semiconductors/insulators that cover a wide range of band gaps computed at the levels of single-shot G0W0, partially self-consistent GW0, and fully self-consistent GW (scGW). Our calculations show that G0W0 leads to band gaps that agree well with experiment for the case of simple s-p electron systems, whereas scGW is required for improving the band gaps in 3-d electron systems. In addition, GW0 almost always predicts larger band gap values compared to scGW, likely due to the substantial underestimation of screening effects. Both the CPE method and Pade approximation lead to similar band gaps for most systems except strontium titantate, suggesting further investigation into the latter approximation is necessary for strongly correlated systems. Our computed band gaps serve as important benchmarks for the accuracy of the Matsubara-time GW approach.

cond-mat.mtrl-sci↗

arXiv2015

Electronic resistances of multilayered two-dimensional crystal junctions

We carry out a layer-by-layer investigation to understand electron transport across metal-insulator-metal junctions. Interfacial structures of junctions were studied and characterized using first-principles density functional theory within the generalized gradient approximation. We found that as a function of the number of crystal layers the calculated transmission coefficients of multilayer silicene junctions decay much slower than for BN-based junctions We revisited the semiclassical Boltzmann theory of electronic transport and applied to multilayer silicene and BN-based junctions. The calculated resistance in the high-transmission regime is smaller than that provided by the Landauer formula. As the thickness of the barrier increases, results from the Boltzmann and the Landauer formulae converge. We provide a upper limit in the transmission coefficient below which, the Landauer method becomes valid. Quantitatively, when the transmission coefficient is lower than $ \sim 0.05 $ per channel, the error introduced by the Landauer formula for calculating the resistance is negligible. In addition, we found that the resistance of a junction is not entirely determined by the averaged transmission, but also by the distribution of the transmission over the first Brillouin zone.

cond-mat.mes-hall↗

arXiv2015

First-principles Simulations of a Graphene Based Field-Effect Transistor

We improvise a novel approach to carry out first-principles simulations of graphene-based vertical field effect tunneling transistors that consist of a graphene$|${\it h}-BN$|$graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of {\it h}-BN thin films sandwiched between graphene layers are computed layer-by-layer following the theory of microscopic permittivity. We find that the permittivities of BN layers are very close to that of crystalline {\it h}-BN. The effect of interface with graphene on the dielectric properties of {\it h}-BN is weak, according to an analysis on the interface charge redistribution.

cond-mat.mtrl-sci↗

arXiv2013

Electron Transport Through Ag-Silicene-Ag Junctions

For several years the electronic structure properties of the novel two-dimensional system silicene have been studied extensively. Electron transport across metal-silicence junctions, however, remains relatively unexplored. To address this issue, we developed and implemented a theoretical framework that utilizes the tight-binding Fisher-Lee relation to span non-equilibrium Green's function (NEGF) techniques, the scattering method, and semiclassical Boltzmann transport theory. Within this hybrid quantum-classical, two-scale framework, we calculated transmission and reflection coefficients of monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in conjunction with density functional theory; derived and calculated the group velocities; and computed resistance using the semi-classical Boltzmann equation. We found that resistances of these junctions are $\sim${}$ 0.08 \fom$ for monolayer silicene junctions and $\sim${}$ 0.3 \fom$ for bilayer ones, factors of $\sim$8 and $\sim$2, respectively, smaller than Sharvin resistances estimated via the Landauer formalism.

cond-mat.mtrl-sci↗

arXiv2013

Does Silicene on Ag(111) Have a Dirac Cone?

We investigate the currently debated issue of the existence of the Dirac cone in silicene on an Ag(111) surface, using first-principles calculations based on density functional theory to obtain the band structure. By unfolding the band structure in the Brillouin zone of a supercell to that of a primitive cell, followed by projecting onto Ag and silicene subsystems, we demonstrate that the Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate that the linear dispersions observed in both angular-resolved photoemission spectroscopy (ARPES) [P. Vogt et al, Phys. Rev. Lett. 108, 155501 (2012)] and scanning tunneling spectroscopy (STS) [L. Chen et al, Phys. Rev. Lett. 109, 056804 (2012)] come from the Ag substrate and not from silicene.

cond-mat.mtrl-sci↗

arXiv2010

Accurate projected augmented wave datasets for BaFe$_2$As$_2$

By carefully choosing parameters and including more semi-core orbitals as valence electrons, we have constructed a high quality projected augmented wave (PAW) dataset that yields results comparable to existing full-potential linearized augmented plane-wave calculations. The dataset was then applied to BaFe$_2$As$_2$ to study the effects of different levels of structure optimization, as well as different choices of exchange-correlation functionals. It is found that the LDA exchange-correlation functional fails to find the correct SDW-AFM ground state under full optimization, while PBE exchange-correlation functional obtains the correct state but significantly overestimates the magnetism. The electronic structure of the SDW-AFM state is not very sensitive to structure optimizations with the PBE exchange-correlation functional because the position of the As atoms are preserved under optimizations. We further investigated the Ba atom diffusion process on the BaFe$_2$As$_2$ surface using the nudged elastic bands (NEB) method. The Ba atom was found to be stable above the center of the squares formed by the surface As atoms, and a diffusion barrier of 1.2 eV was found. Our simulated STM image suggests an ordered surface Ba atom structure, in agreement with Ref.

cond-mat.str-el↗