arXiv subjects

Qun Wei

Publications and source records attributed to Qun Wei.

At least 19 recordsLinked to original sources

arXiv2026

Design of a 60.8 K superconducting hydride LiMgZr2H12 at ambient pressure via Lithium doping

High-pressure hydrogen-rich compounds have long been regarded as promising room-temperature superconductor candidates; however, their practical applications are limited by their reliance on extreme compression. This study explores hydrogen-rich superconductors that may be stable at ambient pressures. Inspired by recent investigations of the MgZrH2n family, the LiMgZr2H12 structure with a Pmmm symmetry was constructed, and its thermodynamic, mechanical, and dynamical stability were evaluated using first-principles calculations. Electron-phonon coupling (EPC) analysis suggests that LiMgZr2H12 reaches a superconducting critical temperature (Tc) of 60.8 K at ambient pressure. Compared with MgZrH6, Li doping significantly increases the contribution of hydrogen atoms to the electron density of states near the Fermi level (EF) and enhances the EPC constant of the LiMgZr2H12 structure. LiMgZr2H12 exhibits a superconducting figure of merit of 1.56, which is significantly greater than that of MgZrH6, demonstrating its outstanding potential for practical applications. This work guides ambient-pressure design of high-Tc hydrides.

cond-mat.supr-con↗

arXiv2025

Localization mechanism of the Kalb-Ramond field on brane with codimension-two

The $2$-form Kalb-Ramond (KR) field, together with the metric tensor and dilaton, arises as one of the massless excitation mode of a closed string. Subsequently, this field plays an important role in both string theory and field theory. In this paper, we investigate the localization of the KR field on the brane with codimension-2. A general Kaluza-Klein (KK) decomposition is adopted, wherein the six-dimensional KR field is expanded into one four-dimensional (4D) KR field, two 4D vector fields, and one 4D scalar field. Then, for the case of the extra dimensions $\mathcal{R}_1\times\mathcal{R}_1$, only the 4D scalar field can be localized on the brane. In contrast, for the case of extra dimensions $\mathcal{R}_1\times\mathcal{S}_1$, one 4D vector field and the 4D scalar field can be localized on the brane at the same time. In both cases, the mass of the 4D scalar field remains zero. Next, we examine the localization of the KR field within a specific six-dimensional brane model with extra dimensions $\mathcal{R}_1\times\mathcal{S}_1$. By introducing the background scalar coupling, we show that the 4D KR field, along with the other three 4D fields, can be localized on the brane under the condition of the coupling parameter $t>v^2/12$. Additionally in this case, for both the 4D KR field and the one 4D vector field which acquires its mass from the non-compact extra dimension, the resonant KK modes could exist near the origin of this extra dimension.

hep-th↗

arXiv2024

First-Principles Study of High-Temperature Superconductivity in X2MH6 Compounds under 20 GPa

Research on high-temperature superconductors has primarily focused on hydrogen-rich compounds, however, the need for extreme pressures limits their practical applications. The X2MH6-type structure Mg2IrH6 stands out because it exhibits superconductivity at 160 K under ambient pressure. This study investigates methods to increase the superconducting transition temperature of this structure via atomic substitution and low-pressure treatment and assess the mechanical, thermodynamic, and dynamic stability of structures obtained by substituting Mg and Ir atoms in Mg2IrH6 with elements from the same groups using first-principles calculations. The findings identify 11 stable ternary compounds, 10 of which exhibit superconducting transition temperatures, with three compounds, Mg2CoH6, Mg2RhH6, and Mg2IrH6, exceeding 100 K, classifying them as high-temperature superconductors. Their superconducting figure of merit S values are 2.71, 3.35, and 3.83, respectively, suggesting strong practical application potential. The analysis results indicate that mid-frequency hydrogen phonons significantly enhance superconducting properties via electron-phonon coupling. The band structure study highlights the importance of van Hove singularities near the Fermi level. In addition, electron localization function and Fermi surface topology analyses reveal that the Fermi surface shape and density of states are crucial for increasing superconducting transition temperatures.

cond-mat.supr-con↗

arXiv2024

Determination of crystal structure and physical properties of Ru2Al5 intermetallic from first-principles calculations

Novel ordered intermetallic compounds have stimulated much interest. Ru-Al alloys are a prominent class of high-temperature structural materials, but the experimentally reported crystal structure of the intermetallic Ru2Al5 phase remains elusive and debatable. To resolve this controversy, we extensively explored the crystal structures of Ru2Al5 using first-principles calculations combined with crystal structure prediction technique. Among the calculated X-ray diffraction patterns and lattice parameters of five candidate Ru2Al5 structures, those of the orthorhombic Pmmn structure best aligned with recent experimental results. The structural stabilities of the five Ru2Al5 structures were confirmed through formation energy, elastic constants, and phonon spectrum calculations. We also comprehensively analyzed the mechanical and electronic properties of the five candidates. This work can guide the exploration of novel ordered intermetallic compounds in Ru-Al alloys.

cond-mat.mtrl-sci↗

arXiv2024

Localization mechanism of $q-$form field on the brane-world by coupling with gravity

It is known that the scalar fields can be trapped on branes of different types, and the $U(1)$ gauge vector fields can be localized on the thick de Sitter brane, or the thick Minkowski brane via coupling with gravity. The Kalb-Ramond fields can be localized on the Minkowski brane and the thick de-Sitter brane, with certain couplings. In this paper, with considering a coupling mechanism between the kinetic term of the $q-$form fields and the background spacetime, we investigate the localization of Kaluza-Klein modes for the $q-$form fields in $D-$dimensional spacetime. Concrete $q-$form fields are discussed within five-dimensional brane models with typical spacetime geometries: Minkowski, de Sitter, and Anti-de Sitter. In the Minkowski brane case, the zero modes of various $q-$form fields can be localized on the brane. In the de Sitter brane case, the zero mode of the $U(1)$ gauge vector fields can be localized on the brane. Lastly in the Anti-de Sitter brane case, the zero mode of the Kalb-Ramond fields can be localized on the brane. For the massive Kaluza-Klein modes of these $q-$form fields, they could be localized or quasi-localized on the brane of different types. Besides, subtle and detailed behaviors of the Kaluza-Klein modes for $q-$form fields are observed: the zero modes could be localized on both sides of the brane, and the massive modes could be localized or quasi-localized at the brane position.

hep-th↗

arXiv2023

Degenerate perturbation theory to quantum search

We utilize degenerate perturbation theory to investigate continuous-time quantum search on second-order truncated simplex lattices. In this work, we show that the construction of the Hamiltonian must consider the structure of the lattice. This idea enables effective application of degenerate perturbation theory to third- and higher-order lattices. We identify two constraints on the reduction of the dimension of the Hamiltonian. In addition, we elucidate the influence of the distinct configurations of marked vertices on the quantum search.

quant-ph↗

arXiv2021

A new 2D auxetic CN2 nanostructure with high energy density and mechanical strength

The existence of a new two dimensional CN2 structure was predicted using ab-initio molecular dynamics (AIMD) and density-functional theory calculations. It consists tetragonal and hexagonal rings with C-N and N-N bonds arranged in a buckling plane, isostructural to tetrahex-carbon allotrope. It is thermodynamically and kinetically stable suggested by its phonon spectrum and AIMD. This nanosheet has high concentration of N and contains N-N single bonds with an energy density of 6.3 kJ/g, indicating potential applications as high energy density materials. It possesses exotic mechanical properties with negative Poisson's ratio and an anisotropic Young's modulus. The modulus in the zigzag direction is predicted to be 340 N/m, stiffer than h-BN and penta-CN2 sheets and comparable to graphene. Its ideal strength of 28.8 N/m outperforms that of penta-graphene. The material maintains phonon stability upon the application of uniaxial strain up to 10% (13%) in the zigzag (armchair) direction or biaxial strain up to 5%. It possesses a wide indirect HSE band gap of 4.57 eV which is tunable between 3.37 - 4.57 eV through strain. Double-layer structures are also explored. Such unique properties may have potential applications in high energy density materials, nanomechanics and electronics.

cond-mat.mtrl-sci↗

arXiv2020

New stable two dimensional silicon carbide nanosheets

We predict the existence of new two dimensional silicon carbide nanostructure employing ab initio density-functional theory calculations. These structures are composed of tetragonal and hexagonal rings with C-C and Si-C bonds arranged in a buckling plane. They are proven to be thermodynamically and mechanically stable with relatively low formation energy, implying potential fabrication in lab. They exhibit strong ductility and anisotropicity from their strain-stress relation and directional dependence of mechanical moduli. The materials maintain phonon stability upon the application of mechanical strain up to 27% with fantastic ductile property. The SiC2 structure possesses a tiny direct band gap of 0.02 eV predicted using HSE06 functional and the band gap can be opened up through multiple approaches such as hydrogenation and strain application. The gap values can be strategically tuned in the range of 0.02 ~ 1.72 eV and the direct/indirect gap nature can be further manipulated. In contrast, a closely related structure of SiC shows an indirect HSE band gap of 1.80 eV and strain engineering its value between 0.0 ~ 1.95 eV. The unique properties in these newly proposed structures might have potential applications in future nanomechanics and electronics.

cond-mat.mtrl-sci↗

arXiv2020

Accelerating inverse crystal structure prediction by machine learning: a case study of carbon allotropes

Based on structure prediction method, the machine learning method is used instead of the density function theory (DFT) method to predict the material properties, thereby accelerating the material search process. In this paper, we established a data set of carbon materials by high-throughput calculation with available carbon structures obtained from the Samara Carbon Allotrope Database. We then trained an ML model that specifically predicts the elastic modulus (bulk modulus, shear modulus, and the Young's modulus) and confirmed that the accuracy is better than that of AFLOW-ML in predicting the elastic modulus of a carbon allotrope. We further combined our ML model with the CALYPSO code to search for new carbon structures with a high Young's modulus. A new carbon allotrope not included in the Samara Carbon Allotrope Database, named Cmcm-C24, which exhibits a hardness greater than 80 GPa, was firstly revealed. The Cmcm-C24 phase was identified as a semiconductor with a direct bandgap. The structural stability, elastic modulus, and electronic properties of the new carbon allotrope were systematically studied, and the obtained results demonstrate the feasibility of ML methods accelerating the material search process.

cond-mat.mtrl-sci↗

arXiv2020

Enhanced carrier mobility in anisotropic 2D tetrahex-carbon through strain engineering

A recently predicted two dimensional (2D) carbon allotrope, tetrahex-carbon consisting of tetragonal and hexagonal rings, draws research interests due to its unique mechanical and electronic properties. Tetrahex-C shows ultrahigh strength, negative Poisson ratio, a direct band gap and high carrier mobility. In this work, we employ first-principles density-functional theory calculations to explore the directional dependence of electronic properties such as carrier effective mass and mobility in tetrahex-C. Tetrahex-C demonstrates strong anisotropicity in effective mass of charge carrier and therefore its mobility (electric conductance) exhibits a strong orientation preference. More interesting, we find that such unique anisotropic carrier effective mass and mobility can be controlled by simple uniaxial strain. The orientation dependence of effective mass can be dramatically rotated by 90 degrees through applying uniaxial tensile strain beyond ~ 7% (11%) in the armchair direction for the hole (electron). As a result, the intrinsic carrier mobility in tetrahex-C is significantly enhanced. The results are useful for potential electronic and mechanical applications in tetrahex-C.

cond-mat.mtrl-sci↗

arXiv2019

Auxetic tetrahex-carbon with ultrahigh strength and direct band gap

Tetrahex-carbon is a recently predicted two dimensional (2D) carbon allotrope which is composed of tetragonal and hexagonal rings. Unlike flat graphene, this new 2D carbon structure is buckled, possesses a direct band gap ~ 2.6 eV and high carrier mobility with anisotropic feature. In this work, we employ first-principles density-functional theory calculations to explore mechanical properties of tetrahex-C under uniaxial tensile strain. We find that tetrahex-C demonstrates ultrahigh ideal strength, outperforming both graphene and penta-graphene. It shows superior ductility and sustains uniaxial tensile strain up to 20% (16%) till phonon instability occurs, and the corresponding maximal strength is 38.3 N/m (37.8 N/m) in the zigzag (armchair) direction. It shows intrinsic negative Poisson's ratio. This exotic in-plane Poisson's ratio takes place when axial strain reaches a threshold value of 7% (5%) in the zigzag (armchair) direction. We also find that tetrahex-C holds a direct band gap of 2.64 eV at the center of Brillouin zone. This direct-band-gap feature maintains intact upon strain application with no direction-indirect gap transition. The ultrahigh ideal strength, negative Poisson's ratio and integrity of direct-gap under strain in tetrahex-C suggest it may have potential applications in nanomechanics and nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2019

Hybrid direct state tomography by weak value

Compared with the conventional quantum state tomography (QST), the direct state tomography (DST) using weak value is easily manipulated in experiments. However, the efficiency of the DST is lower than that of the conventional QST, especially for high-dimensional systems. For a pure state, the DST is revised to improve the efficiency. In the revised DST, the real or imaginary parts of the weak values can be obtained by measuring only one system observable. We constructed a hybrid DST by combining the original DST and the revised DST. By using the appropriate measurement strength, the efficiency of the hybrid DST is significantly larger than that of the conventional QST, especially for high-dimensional systems. The state reconstruction strategy investigated in this paper may be useful in actual experiments.

quant-ph↗

arXiv2017

Pressure and strain effects on the optical properties of K4 phosphorus

An investigation of the mechanical, electronic, and optical properties of the recently reported material K4 phosphorus was made in this work. The K4 phosphorus has been proved to be mechanically and dynamically stable up to 7 GPa under hydrostatic pressure. We compared the elastic anisotropy, average acoustic velocity and Debye temperature of K4 phosphorus at 0 and 7 GPa. The ideal tensile at large strains of K4 phosphorus was also examined, with the results showing that it would cleave under the tensile strength of 8.5 GPa with the strain of 0.3. In addition, the effect of tensile strain and pressure on optical properties and band gap were studied.

cond-mat.mtrl-sci↗

arXiv2014

Chemical scissors cut phosphorene nanostructures and their novel electronic properties

Phosphorene, a recently fabricated two dimensional puckered honeycomb structure of phosphorus, showed promising properties for applications of nano-electronics. In this work, we report our findings of chemical scissors effects on phosphorene, using first principles density functional theory methods. It was found that several chemical species, such as H, F, Cl and OH group, can act effectively as scissors to cut phosphorene. Phosphorus chains and nanoribbons can be obtained using different surface coverage of the chemical species. The scissor effects of these species are resulted from their strong chemical bonds with the P atoms. Species such as O, S and Se were not able to cut phosphorene nanostructures due to their lack of strong binding with P. The electronic structure calculations of the produced P-chains reveal that the saturated chain is an insulator while the pristine chain demonstrates a Dirac point at X with a Fermi velocity of 8*10E5 m/s. The obtained zigzag phosphorene nanoribbons show either metallic or semiconducting behaviors, depending on the treatment of the edge P atoms.

cond-mat.mtrl-sci↗

arXiv2014

Edge effects on the electronic properties of phosphorene nanoribbons

Two dimensional few-layer black phosphorus crystal structures have recently fabricated and demonstrated great potential in applications of electronics. In this work, we employed first principles density functional theory calculations to study the edge effects and quantum confinement on the electronic properties of the phosphorene nanoribbons (PNR). Different edge functionalization groups, such as H, F, Cl, OH, O, S, and Se in addition to a pristine case, were studied for a series width of the ribbon up to 3.5 nm. It was found that the armchair-PNRs (APNRs) are semiconductors for all edge groups considered in this work. However, the zigzag-PNRs (ZPNRs) show either semiconductor or metallic behavior in dependence on their edge chemical groups. Family I edges (H, F, Cl, OH) form saturated bonds with P atoms and the edge states keep far away from the band gap. However, Family II edges (pristine, O, S, Se) form weak unsaturated bonds with the pz orbital of P atoms and bring edge states within the band gap. These edge states of Family II ribbons present around the Fermi level within the band gap, which close up the band gap of the ZPNRs. For the APNRs, these edge states are at the bottom of the conduction band and result in a reduced band gap.

cond-mat.mtrl-sci↗

arXiv2014

Superior mechanical flexibility of phosphorene and few-layer black phosphorus

Recently fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. Mechanical strain was demonstrated to be able to significantly modify the electronic properties of phosphorene and few-layer black phosphorus. In this work, we employed first principles density functional theory calculations to explore the mechanical properties of phosphorene, including ideal tensile strength and critical strain. It was found that a monolayer phosphorene can sustain tensile strain up to 27% and 30% in the zigzag and armchair directions, respectively. This enormous strain limit of phosphorene results from its unique puckered crystal structure. We found that the tensile strain applied in the armchair direction stretches the pucker of phosphorene, rather than significantly extending the P-P bond lengths. The compromised dihedral angles dramatically reduce the required strain energy. Compared to other 2D materials such as graphene, phosphorene demonstrates superior flexibility with an order of magnitude smaller Young modulus. This is especially useful in practical large-magnitude-strain engineering. Furthermore, the anisotropic nature of phosphorene was also explored. We derived a general model to calculate the Young modulus along different directions for a 2D system.

cond-mat.mtrl-sci↗

arXiv2014

Strain engineered direct-indirect band gap transition and its mechanism in 2D phosphorene

Recently fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. In this work, strain effect on the electronic band structure of phosphorene was studied using first principles methods. It was found that phosphorene can withstand a surface tension and tensile strain up to 10 N/m and 30%, respectively. The band gap of phosphorene experiences a direct-indirect-direct transition when axial strain is applied. A moderate -2% compression in the zigzag direction can trigger this gap transition. With sufficient expansion (+11.3%) or compression (-10.2% strains), the gap can be tuned from indirect to direct again. Five strain zones with distinct electronic band structure were identified and the critical strains for the zone boundaries were determined. The origin of the gap transition was revealed and a general mechanism was developed to explain energy shifts with strain according to the bond nature of near-band-edge electronic orbitals. Effective masses of carriers in the armchair direction are an order of magnitude smaller than that of the zigzag axis indicating the armchair direction is favored for carrier transport. In addition, the effective masses can be dramatically tuned by strain, in which its sharp jump/drop occurs at the zone boundaries of the direct-indirect gap transition.

cond-mat.mtrl-sci↗

arXiv2013

The negative probabilities and information gain in weak measurements

We study the outcomes in a general measurement with postselection, and derive upper bounds for the pointer readings in weak measurement. Using the idea of weak measurement, we study Hardy's gedanken experiment and show how the "negative probabilities" emerge in weak measurement. By calculating the information gain of the measuring device about which path the particles pass through, we show that the "negative probabilities" only emerge for cases when the information gain is little due to very weak coupling between the measuring device and the particles. When the coupling strength increases, we can unambiguously determine whether a particle passes through a given path every time, hence the average shifts always represent true probabilities, and the strange "negatives probabilities" disappear.

quant-ph↗