arXiv subjects

W. T. Geng

Publications and source records attributed to W. T. Geng.

At least 19 recordsLinked to original sources

arXiv2020

Friedel oscillations and helium bubble ordering in molybdenum

Helium ions implanted into metals can evolve into ordered bubbles isomorphic to the host lattice. Long-range elastic interaction is generally believed to drive the formation of bubble superlattice, but little is known about the thermodynamics at the very initial stage. Our first-principles calculations demonstrate that in molybdenum, Friedel oscillations induced by individual helium generate both potential barriers and wells for helium clustering at short He-He distances. Such repulsion and attraction at high concentration provide a thermodynamic diving force to assist lining up randomly distributed He atoms into ordered bubbles. Friedel oscillations might have general impact on solute-solute interactions in alloys.

cond-mat.mtrl-sci↗

arXiv2020

Angle-dependence of interlayer coupling in twisted transition metal dichalcogenide heterobilayers

We reveal by first-principles calculations that the interlayer binding in a twisted MoS2/MoTe2 heterobilayer decreases with increasing twist angle, due to the increase of the interlayer overlapping degree, a geometric quantity describing well the interlayer steric effect. The binding energy is found to be a Gaussian-like function of twist angle. The resistance to rotation, an analogue to the interlayer sliding barrier, can also be defined accordingly. In sharp contrast to the case of MoS2 homobilayer, here the energy band gap reduces with increasing twist angle. We find a remarkable interlayer charge transfer from MoTe2 to MoS2 which enlarges the band gap, but this charge transfer weakens with greater twisting and interlayer overlapping degree. Our discovery provides a solid basis in twistronics and practical instruction in band structure engineering of van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2020

Moiré potential, lattice corrugation, and band gap spatial variation in a twist-free MoS2/MoTe2 heterobilayer

To have a fully ab initio description of the Moiré pattern in a transition metal dichalcogenide heterobilayer, we have carried out density functional theory calculations, taking accounts of both atomic registry in and the lattice corrugation out of the monolayers, on a MoTe2(9*9)/MoS2(10*10) system which has a moderate size of superlattice larger than an exciton yet not large enough to justify a continuum model treatment. We find that the local potential in the midplane of the bilayer displays a conspicuous Moiré pattern. It further leads us to reveal that the variation of the average local potential near Mo atoms in both MoTe2 and MoS2 layers make intralayer Moiré potentials. They are the result of mutual modulation and correlate directly with the spatial variation of the valence band maximum and conduction band minimum. The interlayer Moiré potential, defined as the difference between the two intralayer Moiré potentials, has a depth of 0.11 eV and changes roughly in proportion to the band gap variation in the Moiré cell, which has an amplitude of 0.04 eV. We find the lattice corrugation is significant in both MoTe2 (0.30Å) and MoS2 (0.77Å) layers, yet its effect on the electronic properties is marginal. The wrinkling of the MoTe2/MoS2 bilayer enhances the spatial variation of the local band gap by 5 meV, while its influence on the global band gap is within 1 meV. A simple intralayer band-coupling model is proposed to understand the correlation of Moiré potential and spatial variation of the band gap.

cond-mat.mtrl-sci↗

arXiv2018

One-for-multiple substitution in solid solutions

It is generally assumed that one solute atom will occupy only one lattice site in a substitutional solid solution. We here report an interesting discovery by first-principles calculations that a large solute atom can replace multiple matrix atoms in the elemental crystal of beryllium. Examination on Groups IIIB, IVB, VB, VIB, and VA elements shows that Cr will substitute for one, V and Mo for three, Sc, Y, Ti, Zr, Hf, W, Nb, Ta, As, Sb, and Bi for four, and La for five Be atoms. Dissolution of Zr, Hf, Sc, and Y is exothermic, suggesting a good solubility. At low concentration, the configurational entropy resulted from one-for-multiple substitution is larger than in the one-for-one substitution case. We find that Sc, Y, Zr, and Hf all have tendency to aggregate in Be, but Sc is the weakest among them and thus can be expected to improve the superplasticity of Be.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Band Gaps of In$_x$Ga$_{1-x}$N Alloys: From Bulk to Two-Dimensional Limit

Using first-principles calculations combined with a semi-empirical van der Waals dispersion correction, we have investigated structural parameters, mixing enthalpies, and band gaps of buckled and planar few-layer In$_x$Ga$_{1-x}$N alloys. We predict that the free-standing buckled phases are less stable than the planar ones. However, with hydrogen passivation, the buckled In$_x$Ga$_{1-x}$N alloys become more favorable. Their band gaps can be tuned from 6 eV to 1 eV with preservation of direct band gap and well-defined Bloch character, making them promising candidate materials for future light-emitting applications. Unlike their bulk counterparts, the phase separation could be suppressed in these two-dimensional systems due to reduced geometrical constraints. In contrast, the disordered planar thin films undergo severe lattice distortion, nearly losing the Bloch character for valence bands; whereas the ordered planar ones maintain the Bloch character yet with the highest mixing enthalpies.

cond-mat.mtrl-sci↗

arXiv2017

Lattice Defects and the Mechanical Anisotropy of Borophene

Using density functional theory combined with a semi-empirical van der Waals dispersion correction, we have investigated the stability of lattice defects including boron vacancy, substitutional and interstitial X (X=H, C, B, N, O) and $Σ$5 tilt grain boundaries in borophene and their influence on the anisotropic mechanical properties of this two-dimensional system. The pristine borophene has significant in-plane Young's moduli and Poisson's ratio anisotropy due to its strong and highly coordinated B-B bonds. The concentration of B vacancy and $Σ$5 grain boundary could be rather high given that their formation energies are as low as 0.10 eV and 0.06 eV/$Å$ respectively. In addition, our results also suggest that borophene can react easily with H$_2$, O$_2$ and N$_2$ when exposed to these molecules. We find that the mechanical properties of borophene are remarkably degraded by these defects. The anisotropy in Poisson's ratio, however, can be tuned by some of them. Furthermore, the adsorbed H or substitutional C may induce remarkably negative Poisson's ratio in borophene, and the substitutional C or N can significantly increase the Poisson's ratio by contrast.

cond-mat.mtrl-sci↗

arXiv2017

Volume Contraction at a Grain Boundary in Vanadium

It is a conventional wisdom that symmetry breakdown at grain boundaries in crystals introduces volume expansion and there has been no confirmed evidence of volume contraction at a grain boundary in any kind of crystals. We report surprising volume contraction at the Sigma3(111) grain boundary in vanadium. The lattice distortion near this grain boundary is found to drive the local structure toward a hypothetical ω phase, which is only slightly less stable than the bcc phase. Compressing lattice constant a of the ω-phase down to the value of bcc, as is the case at the Sigma3(111) grain boundary, results in smaller c than in bcc structure, and hence the volume contraction. We also find such a volume contraction impedes the segregation of Ti and H, while enhances that of Cr, and hence a significant influence on material properties. The discovery adds fundamental new knowledge of condensed matter, and may also point to new techniques in grain boundary engineering of novel materials through volume control.

cond-mat.mtrl-sci↗

arXiv2016

Excess Volume at Grain Boundaries in hcp Metals

The excess volume associated with grain boundaries represents a key structural parameter for the characterization of grain boundaries. It plays a critical role in segregation of impurity and alloy elements to grain boundaries, and influences significantly the mechanical and functional properties of materials. We have carried out first-principles density functional calculations on the atomic structure of the (10-12) coherent twin boundary in hexagonal close packed (hcp) Ti and Zr and the basal-prismatic boundary in Zn, Cd, and Zr. We find the calculated excess volume has a vanishing magnitude at the (10-12) coherent twin boundary in Ti and Zr; whereas it is remarkable at the basal-prismatic boundary in Zn, Cd, and Zr.

cond-mat.mtrl-sci↗

arXiv2016

Role of Interlayer Coupling on the Evolution of Band Edges in Few-Layer Phosphorene

Using first-principles calculations, we have investigated the evolution of band-edges in few-layer phosphorene as a function of the number of P layers. Our results predict that monolayer phosphorene is an indirect band gap semiconductor and its valence band edge is extremely sensitive to strain. Its band gap could undergo an indirect-to-direct transition under a lattice expansion as small as 1% along zigzag direction. A semi-empirical interlayer coupling model is proposed, which can well reproduce the evolution of valence band-edges obtained by first-principles calculations. We conclude that the interlayer coupling plays a dominated role in the evolution of the band-edges via decreasing both band gap and carrier effective masses with the increase of phosphorene thickness. A scrutiny of the orbital-decomposed band structure provides a better understanding of the upward shift of valence band maximum surpassing that of conduction band minimum.

cond-mat.mtrl-sci↗

arXiv2015

Migration of helium-pair in metals

Understanding helium accumulation in plasma-facing or structural materials in a fusion reactor starts from uncovering the details of the migration of single and paired He interstitials. We have carried out a first-principles density functional theory investigation into the migration of both a single interstitial He atom and an interstitial He-pair in bcc (Fe, Mo and W) and fcc (Cu, Pd and Pt) metals. By identifying the most stable configurations of an interstitial He-pair in each metal and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our first-principles calculations reveal that the migration trajectories and barriers are determined predominantly by the relatively stable He-pair configurations which depend mainly on the stability of a single He in different interstices. Contrary to atomistic studies reported in literature, the migration barrier in bcc Fe, Mo, and W is 0.07, 0.07, and 0.08 eV respectively, always slightly higher than for a single interstitial He (0.06 eV for all three). Configurations of a He-pair in fcc metals are much more complicated, due to the stability closeness of different interstitial sites for a single He atom. In both Cu and Pd, the migration of a He-pair proceeds by moving one He at a time from one tetrahedral site to neighboring octahedral site; whereas in Pt the two He move simultaneously because the bridge interstitial site presents an extremely low barrier. The migration barrier for a He-pair is 0.05, 0.15, and 0.04 eV for Cu, Pd, and Pt, slightly lower than (in Cu), or similar to (in Pd and Pt) a single He, which is 0.08, 0.15, and 0.03 eV, respectively. The associative motions of a He-pair are ensured by the strong He-He interactions in metals which are chemical bonding-like and can be described very well with Morse potentials.

cond-mat.mtrl-sci↗

arXiv2015

Native point defects in few-layer phosphorene

Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene.

cond-mat.mtrl-sci↗

arXiv2014

Waltzing of a Helium Pair in Tungsten: Migration Barrier and Trajectory Revealed from First-Principles

Despite well documented first-principles theoretical determination of the low migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical calculations on the migration of a He pair still present a challenge due to the complexity of its trajectory. By identifying the six most stable configurations of the He pair in W and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our density functional theory calculations demonstrate a He pair has three modes of motion: a close or open circular two-dimensional motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along [001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along [010] direction with the two He swinging in the plane (100) and a barrier of 0.31 eV. The graceful associative movements of a He pair are related to the chemical-bonding-like He-He interaction being much stronger than its migration barrier in W. The excellent agreement with available experimental measurements (0.24-0.32 eV) on He migration makes our first-principles result a solid input to obtain accurate He-W interatomic potentials in molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2012

Helium under high pressure: A comparative study of all-electron and pseudopotential methods within density functional theory

We have calculated the ground state electronic structure of He under pressure from 0 to 1500 GPa using both all-electron full-potential and pseudopotential methods based on the density functional theory (DFT). We find that throughout this pressure range, pseudopotentials yield essentially the same energy-volume curve for all of bcc, fcc, and hcp configurations as does the full-potential method, a strong indication that pseudopotential approximation works well for He both as the common element in some giant planets and as detrimental impurities in fusion reactor materials. The hcp lattice is always the most stable structure and bcc the least stable one. Since the energy preference of hcp over fcc and bcc is within 0.01 eV below 100 GPa and about 0.1 eV at 1500 GPa, on the same order of the error bar in local or semi-local density approximations in DFT, phase transitions can only be discussed with more precise description of electron correlation in Quantum Monte Carlo or DFT-based GW methods.

cond-mat.mtrl-sci↗

arXiv2011

Impeding effect of cerium on the growth of helium bubble in iron

Our first-principles density functional theory calculations suggest that the rare earth element, Ce, has a stronger attraction (-1.31eV) to He than He-He (-1.18eV) in bcc Fe. Consequently, the mobile He atoms could be pinned to Ce, and hence a reduced merging of He clusters. Moreover, we find that the segregated Ce layer at the He bubble surface presents an energy barrier of 0.33 eV to the upcoming He atom and thus slows down the bubble growth.

cond-mat.mtrl-sci↗

arXiv2011

Gold slows down the growth of helium bubble in iron

We predict by first-principles calculations that Au have strong affinity to He in bcc Fe. The Au-Au bonding in the segregated Au layer at the He bubble surface is stronger than Fe-Fe and Au-Fe interactions; therefore this layer becomes an effective barrier to further He and slows down the bubble growth.

cond-mat.mtrl-sci↗

arXiv2011

Threshold concentration for H blistering in defect free W

Lattice distortion induced by high concentration of H is believed to be precursor of H blistering in single crystalline W (SCW) during H isotope irradiation. However, the critical H concentration needed to trigger bond-breaking of metal atoms presents a challenge to measure. Using density functional theory, we have calculated the formation energy of a vacancy and a self-interstitial atom (SIA) in supersaturated defect-free SCW with various H concentrations. When the ratio of H:W exceeds 1:2, the formation of both vacancies and self-interstitials becomes exothermic, meaning that spontaneous formation of micro-voids which can accommodate molecular H2 will occur. Molecular H2 is not allowed to form, and it is not needed either at the very initial stage of H blistering in SCW. With supersaturated H, the free volume at the vacancy or SIA is greatly smeared out with severe lattice distortion and more H can be trapped than in the dilute H case.

cond-mat.mtrl-sci↗

arXiv2011

Role of grain boundary and dislocation loop in H blistering in W: A Density functional theory assessment

We report a first-principles density functional theory study on the role of grain boundary and dislocation loop in H blistering in W. At low temperature, the {\Sugma}3(111) tilt grain boundary, when combined with a vacancy of vanishing formation energy, can trap up to nine H atoms per (1x1) unit in (111) plane. This amount of H weakens the cohesion across the boundary to an extent that a cleavage along the GB is already exothermic. At high temperature, this effect can be still significant. For an infinitely large dislocation loop in (100) plane, four H can be trapped per (1x1) unit even above room temperature, incurring a decohesion strong enough to break the crystal. Our numerical results demonstrate unambiguously the grain boundaries and dislocation loops can serve as precursors of H blistering. In addition, no H2 molecules can be formed in either environment before fracture of W bonds starts, well explaining the H blistering in the absence of voids during non-damaging irradiation.

cond-mat.mtrl-sci↗

arXiv2010

Nearsightedness of Electronic Matter and the Size of Viruses

I conjecture that the nearsightedness of component electronic matter largely determines the size of a virus. These two length scales, one from physics and one from biochemistry, are in fact the same dimension which connects our quantum and everyday worlds. Learning how viruses interact with microscopic molecules and macroscopic biological cells might help us understand the quantum-to-classical transition in general cases of multiscale phenomena.

physics.bio-ph↗