Showing papers in "Physical Review B in 2013"
TL;DR: In this paper, a reformulation of the optimization is developed, including the ability to apply it to positive-energy atomic scattering states and to enforce greater continuity in the pseudopotential.
Abstract: Fully nonlocal two-projector norm-conserving pseudopotentials are shown to be compatible with a systematic approach to the optimization of convergence with the size of the plane-wave basis. A reformulation of the optimization is developed, including the ability to apply it to positive-energy atomic scattering states and to enforce greater continuity in the pseudopotential. The generalization of norm conservation to multiple projectors is reviewed and recast for the present purposes. Comparisons among the results of all-electron and one- and two-projector norm-conserving pseudopotential calculations of lattice constants and bulk moduli are made for a group of solids chosen to represent a variety of types of bonding and a sampling of the periodic table.
1,727 citations
TL;DR: It is demonstrated that certain widely used descriptors that initially look quite different are specific cases of a general approach, in which a finite set of basis functions with increasing angular wave numbers are used to expand the atomic neighborhood density function.
Abstract: We review some recently published methods to represent atomic neighborhood environments, and analyze their relative merits in terms of their faithfulness and suitability for fitting potential energy surfaces. The crucial properties that such representations (sometimes called descriptors) must have are differentiability with respect to moving the atoms and invariance to the basic symmetries of physics: rotation, reflection, translation, and permutation of atoms of the same species. We demonstrate that certain widely used descriptors that initially look quite different are specific cases of a general approach, in which a finite set of basis functions with increasing angular wave numbers are used to expand the atomic neighborhood density function. Using the example system of small clusters, we quantitatively show that this expansion needs to be carried to higher and higher wave numbers as the number of neighbors increases in order to obtain a faithful representation, and that variants of the descriptors converge at very different rates. We also propose an altogether different approach, called Smooth Overlap of Atomic Positions, that sidesteps these difficulties by directly defining the similarity between any two neighborhood environments, and show that it is still closely connected to the invariant descriptors. We test the performance of the various representations by fitting models to the potential energy surface of small silicon clusters and the bulk crystal.
1,621 citations
TL;DR: In this paper, the authors recover the silent topological nature of Cd3As2, a well known semiconductor with high carrier mobility, and find that it is a symmetry-protected topological semimetal with a single pair of three-dimensional (3D) Dirac points in the bulk and nontrivial Fermi arcs on the surfaces.
Abstract: Based on the first-principles calculations, we recover the silent topological nature of Cd3As2, a well known semiconductor with high carrier mobility. We find that it is a symmetry-protected topological semimetal with a single pair of three-dimensional (3D) Dirac points in the bulk and nontrivial Fermi arcs on the surfaces. It can be driven into a topological insulator and a Weyl semimetal state by symmetry breaking, or into a quantum spin Hall insulator with a gap more than 100 meV by reducing dimensionality. We propose that the 3D Dirac cones in the bulk of Cd3As2 can support sizable linear quantum magnetoresistance even up to room temperature.
1,386 citations
TL;DR: In this article, the classical magnetoresistance of a Weyl metal in which the electron Fermi surface possesses nonzero fluxes of the Berry curvature was considered, and it was shown that such a system may exhibit large negative magnetoreduction with unusual anisotropy as a function of the angle between the electric and magnetic fields.
Abstract: We consider the classical magnetoresistance of a Weyl metal in which the electron Fermi surface possesses nonzero fluxes of the Berry curvature. Such a system may exhibit large negative magnetoresistance with unusual anisotropy as a function of the angle between the electric and magnetic fields. In this case the system can support an additional type of plasma wave. These phenomena are consequences of the chiral anomaly in electron transport theory.
1,148 citations
TL;DR: In this paper, it was shown that the boundary excitations of SPT phases can be described by a nonlocal Lagrangian term that generalizes the Wess-Zumino-Witten term for continuous nonlinear σ models.
Abstract: Symmetry protected topological (SPT) phases are gapped short-range-entangled quantum phases with a symmetry G. They can all be smoothly connected to the same trivial product state if we break the symmetry. The Haldane phase of spin-1 chain is the first example of SPT phases which is protected by SO(3) spin rotation symmetry. The topological insulator is another example of SPT phases which are protected by U(1) and time-reversal symmetries. In this paper, we show that interacting bosonic SPT phases can be systematically described by group cohomology theory: Distinct d-dimensional bosonic SPT phases with on-site symmetry G (which may contain antiunitary time-reversal symmetry) can be labeled by the elements in H^(1+d)[G,UT(1)], the Borel (1+d)-group-cohomology classes of G over the G module UT(1). Our theory, which leads to explicit ground-state wave functions and commuting projector Hamiltonians, is based on a new type of topological term that generalizes the topological θ term in continuous nonlinear σ models to lattice nonlinear σ models. The boundary excitations of the nontrivial SPT phases are described by lattice nonlinear σ models with a nonlocal Lagrangian term that generalizes the Wess-Zumino-Witten term for continuous nonlinear σ models. As a result, the symmetry G must be realized as a non-on-site symmetry for the low-energy boundary excitations, and those boundary states must be gapless or degenerate. As an application of our result, we can use H^(1+d)[U(1)⋊ Z^(T)_(2),U_T(1)] to obtain interacting bosonic topological insulators (protected by time reversal Z2T and boson number conservation), which contain one nontrivial phase in one-dimensional (1D) or 2D and three in 3D. We also obtain interacting bosonic topological superconductors (protected by time-reversal symmetry only), in term of H^(1+d)[Z^(T)_(2),U_T(1)], which contain one nontrivial phase in odd spatial dimensions and none for even dimensions. Our result is much more general than the above two examples, since it is for any symmetry group. For example, we can use H1+d[U(1)×Z2T,UT(1)] to construct the SPT phases of integer spin systems with time-reversal and U(1) spin rotation symmetry, which contain three nontrivial SPT phases in 1D, none in 2D, and seven in 3D. Even more generally, we find that the different bosonic symmetry breaking short-range-entangled phases are labeled by the following three mathematical objects: (G_H,G_Ψ,H^(1+d)[G_Ψ,U_T(1)]), where G_H is the symmetry group of the Hamiltonian and G_Ψ the symmetry group of the ground states.
1,001 citations
TL;DR: In this paper, the Dzyaloshinskii-Moriya interaction (DMI) was used to bend the magnetization at the edges of ultrathin film nanostructures.
Abstract: We study the modification of micromagnetic configurations in nanostructures, due to the Dzyaloshinskii-Moriya interaction (DMI), that appear at the interface of an ultrathin film. We show that this interaction leads to micromagnetic boundary conditions that bend the magnetization at the edges. We explore several cases of ultrathin film nanostructures that allow analytical calculations (one-dimensional systems, domain walls, cycloids, and skyrmions), compare with fully numerical calculations, and show that a good physical understanding of this type of micromagnetics can be reached. We particularly focus on skyrmions confined in circular nanodots and show that edges allow for the isolation of single skyrmions for a large range of the DMI parameter.
854 citations
TL;DR: In this article, an effective mass model of neutral excitons and charged trions in monolayers of transition metal dichalcogenides, including molybdenum disulfide, is presented.
Abstract: We present a microscopic theory of neutral excitons and charged excitons (trions) in monolayers of transition metal dichalcogenides, including molybdenum disulfide. Our theory is based on an effective mass model of excitons and trions, parameterized by ab initio calculations and incorporating a proper treatment of screening in two dimensions. The calculated exciton binding energies are in good agreement with high-level many-body computations based on the Bethe-Salpeter equation. Furthermore, our calculations for the more complex trion species compare very favorably with recent experimental measurements and provide atomistic insight into the microscopic features which determine the trion binding energy.
798 citations
TL;DR: In this article, a three-band tight-binding model for describing low-energy physics in monolayers of group-VIB transition metal dichalcogenides is presented.
Abstract: We present a three-band tight-binding (TB) model for describing the low-energy physics in monolayers of group-VIB transition metal dichalcogenides $M{X}_{2}$ ($M=\text{Mo}$, W; $X=\text{S}$, Se, Te). As the conduction- and valence-band edges are predominantly contributed by the ${d}_{{z}^{2}}$, ${d}_{xy}$, and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals of $M$ atoms, the TB model is constructed using these three orbitals based on the symmetries of the monolayers. Parameters of the TB model are fitted from the first-principles energy bands for all $M{X}_{2}$ monolayers. The TB model involving only the nearest-neighbor $M$-$M$ hoppings is sufficient to capture the band-edge properties in the $\ifmmode\pm\else\textpm\fi{}K$ valleys, including the energy dispersions as well as the Berry curvatures. The TB model involving up to the third-nearest-neighbor $M$-$M$ hoppings can well reproduce the energy bands in the entire Brillouin zone. Spin-orbit coupling in valence bands is well accounted for by including the on-site spin-orbit interactions of $M$ atoms. The conduction band also exhibits a small valley-dependent spin splitting which has an overall sign difference between Mo${X}_{2}$ and W${X}_{2}$. We discuss the origins of these corrections to the three-band model. The three-band TB model developed here is efficient to account for low-energy physics in $M{X}_{2}$ monolayers, and its simplicity can be particularly useful in the study of many-body physics and physics of edge states.
784 citations
TL;DR: In this article, the quasiparticle (QP) band structures of both strainless and strained monolayer MoS${}_{2}$ were investigated using more accurate many-body perturbation theory and maximally localized Wannier functions (MLWFs) approach.
Abstract: The quasiparticle (QP) band structures of both strainless and strained monolayer MoS${}_{2}$ are investigated using more accurate many-body perturbation $\mathit{GW}$ theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent $\mathit{GW}$${}_{0}$ (sc$\mathit{GW}$${}_{0}$) calculation, the predicted optical gap magnitude is in good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc$\mathit{GW}$${}_{0}$ and BSE calculations are also performed on monolayer WS${}_{2}$, similar characteristics are predicted and WS${}_{2}$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.
766 citations
TL;DR: In this paper, the spin Hall magnetoresistance in multilayers made from an insulating ferromagnet F, such as yttrium iron garnet (YIG), and a normal metal N with spin-orbit interactions was investigated.
Abstract: We present a theory of the spin Hall magnetoresistance (SMR) in multilayers made from an insulating ferromagnet F, such as yttrium iron garnet (YIG), and a normal metal N with spin-orbit interactions, such as platinum (Pt). The SMR is induced by the simultaneous action of spin Hall and inverse spin Hall effects and therefore a nonequilibrium proximity phenomenon. We compute the SMR in F|N and F|N|F layered systems, treating N by spin-diffusion theory with quantum mechanical boundary conditions at the interfaces in terms of the spin-mixing conductance. Our results explain the experimentally observed spin Hall magnetoresistance in N|F bilayers. For F|N|F spin valves we predict an enhanced SMR amplitude when magnetizations are collinear. The SMR and the spin-transfer torques in these trilayers can be controlled by the magnetic configuration.
678 citations
TL;DR: In this article, the nonlinear optical properties of few-layer MoS${}_{2}$ two-dimensional crystals were studied using femtosecond laser pulses, which showed a polarization intensity dependence that directly revealed the underlying symmetry and orientation of the crystal.
Abstract: The nonlinear optical properties of few-layer MoS${}_{2}$ two-dimensional crystals are studied using femtosecond laser pulses. We observed highly efficient second-harmonic generation from the odd-layer crystals, which shows a polarization intensity dependence that directly reveals the underlying symmetry and orientation of the crystal. Additionally, the measured second-order susceptibility spectra provide information about the electronic structure of the material. Our results open up opportunities for studying the nonlinear optical properties in these two-dimensional crystals.
TL;DR: In this paper, the second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10{}^{\ensuremath{-}7}$ m/V.
Abstract: We show that the lack of inversion symmetry in monolayer MoS${}_{2}$ allows strong optical second harmonic generation. The second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10${}^{\ensuremath{-}7}$ m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proof-of-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in the fast and noninvasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS${}_{2}$ and similar materials.
TL;DR: In this article, the authors propose an easy-to-build easyto-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor.
Abstract: We propose an easy-to-build easy-to-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor. Model calculations show that such chains can be easily tuned between trivial and topological ground states. In the latter, spatially resolved spectroscopy can be used to probe the Majorana fermion end states. Decoupled Majorana bound states can form even in short magnetic chains consisting of only tens of atoms. We propose scanning tunneling microscopy as the ideal technique to fabricate such systems and to probe their topological properties.
TL;DR: In this article, the effect of strain on the phonon modes of monolayer and few-layer MoS was investigated by observing the strain-induced shifts of the Raman-active modes.
Abstract: The effect of strain on the phonon modes of monolayer and few-layer MoS${}_{2}$ has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS${}_{2}$ sandwiched between two layers of optically transparent polymer. The resulting band shifts of the ${E}_{2g}^{1}$ ($\ensuremath{\sim}$$385.3\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) and ${A}_{1g}$ ($\ensuremath{\sim}$$402.4\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) Raman modes were found to be small but observable. First-principles plane-wave calculations based on density functional perturbation theory were used to determine the Gr\"uneisen parameters for the ${E}_{1g}$, ${E}_{2g}^{1}$, ${A}_{1g}$, and ${A}_{2u}$ modes and predict the experimentally observed band shifts for the monolayer material. The polymer--MoS${}_{2}$ interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS${}_{2}$ into devices.
TL;DR: In this paper, a simple modification of the VV10 nonlocal density functional for dispersion interactions is presented. But unlike the original functional, this modification allows nonlocal correlation energy and its derivatives to be efficiently evaluated in a plane wave framework along the lines pioneered by Rom\'an-P\'erez and Soler.
Abstract: We present a simple revision of the VV10 nonlocal density functional by Vydrov and Van Voorhis [J. Chem. Phys. 133, 244103 (2010)] for dispersion interactions. Unlike the original functional our modification allows nonlocal correlation energy and its derivatives to be efficiently evaluated in a plane wave framework along the lines pioneered by Rom\'an-P\'erez and Soler [Phys. Rev. Lett. 103, 096102 (2009)]. Our revised functional maintains the outstanding precision of the original VV10 in noncovalently bound complexes and performs well in representative covalent, ionic, and metallic solids.
TL;DR: In this article, a two-dimensional diluted magnetic semiconductor (MoS2) is proposed, which is doped by transition metal atoms from the IIIB to VIB groups.
Abstract: Using first-principles calculations, we propose a two-dimensional diluted magnetic semiconductor: monolayer MoS2 doped by transition metals. Doping of transition metal atoms from the IIIB to VIB groups results in nonmagnetic states, since the number of valence electrons is smaller or equal to that of Mo. Doping of atoms from the VIIB to IIB groups becomes energetically less and less favorable. Magnetism is observed for Mn, Fe, Co, Zn, Cd, and Hg doping, while for the other dopants from these groups it is suppressed by Jahn-Teller distortions. Analysis of the binding energies and magnetic properties indicates that (Mo,X)S2 (X = Mn, Fe, Co, and Zn) are promising systems to explore two-dimensional diluted magnetic semiconductors.
TL;DR: In this article, density functional theory is used to systematically study the electronic properties of doped MoS2 monolayers, wherethedopantsare incorporated both via S/Mosubstitution and ad-sorbates.
Abstract: Density functional theory is used to systematically study the electronic properties of doped MoS2 monolayers, wherethedopantsareincorporatedbothviaS/Mosubstitutionorasadsorbates.Amongthepossiblesubstitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS2 conduction band, making the system n type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both n -a nd p-type conductivity, depending on the charge polarity of the adsorbed species.
TL;DR: In this paper, Raman scattering the shear and layer breathing modes in multilayer MoS was used to identify polarization measurements and symmetry analysis, with different scaling for odd and even layers.
Abstract: We study by Raman scattering the shear and layer breathing modes in multilayer MoS${}_{2}$. These are identified by polarization measurements and symmetry analysis. Their positions change significantly with the number of layers, with different scaling for odd and even layers. A chain model can explain the results, with general applicability to any layered material, allowing a reliable diagnostic of their thickness.
TL;DR: In this paper, transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact are presented, and a crossover in magnetoconductance is observed: magnetic fields above similar to 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance (multichannel) regime.
Abstract: We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above similar to 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.
TL;DR: In this article, the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and non-ferromagnet hybrid structures was investigated and quantitatively analyzed.
Abstract: We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator, we use either yttrium iron garnet, nickel ferrite, or magnetite and for the nonferromagnet, copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin-mixing conductance of 4×1014 ??1m?2, we obtain a spin Hall angle of 0.11±0.08 and a spin diffusion length of (1.5±0.5) nm for Pt in our thin-film samples
TL;DR: In this paper, a comparative study of spin Hall related effects and magnetoresistance in YIG$|$Pt and yttrium iron garnet bilayers is presented.
Abstract: We report on a comparative study of spin Hall related effects and magnetoresistance in YIG$|$Pt and YIG$|$Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to yttrium iron garnet (YIG): the spin mixing conductance ${G}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ at the YIG$|$normal metal interface, the spin Hall angle ${\ensuremath{\Theta}}_{\mathrm{SH}}$, and the spin diffusion length ${\ensuremath{\lambda}}_{\mathrm{sd}}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be ${\ensuremath{\lambda}}_{\mathrm{sd}}^{\mathrm{Ta}}=1.8\ifmmode\pm\else\textpm\fi{}0.7$ nm. Both the YIG$|$Pt and YIG$|$Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.
TL;DR: In this paper, MEC-Spain (Grants FIS2010-21883-C02-01, and CONSOLIDER CSD2007-0010) and Generalitat Valenciana, Grant Prometeo 2012-11
Abstract: This work has been financially supported by MEC-Spain (Grants FIS2010-21883-C02-01, and CONSOLIDER CSD2007-0010) and Generalitat Valenciana, Grant Prometeo 2012-11
TL;DR: In this paper, a model based on the Boltzmann equation that unifies these approaches is presented. But the model fails to describe the experimentally observed thickness dependencies, which is a limitation of the drift-diffusion model.
Abstract: In bilayer nanowires consisting of a ferromagnetic layer and a nonmagnetic layer with strong spin-orbit coupling, currents create torques on the magnetization beyond those found in simple ferromagnetic nanowires. The resulting magnetic dynamics appear to require torques that can be separated into two terms, dampinglike and fieldlike. The dampinglike torque is typically derived from models describing the bulk spin Hall effect and the spin transfer torque, and the fieldlike torque is typically derived from a Rashba model describing interfacial spin-orbit coupling. We derive a model based on the Boltzmann equation that unifies these approaches. We also consider an approximation to the Boltzmann equation, the drift-diffusion model, that qualitatively reproduces the behavior, but quantitatively differs in some regimes. We show that the Boltzmann equation with physically reasonable parameters can match the torques for any particular sample, but in some cases, it fails to describe the experimentally observed thickness dependencies.
TL;DR: In this paper, the optical properties of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS${}_{2}$ layers sandwiched with WS${}{2}$, MoSe${}µ, MoTe${} µ, BN, or graphene sheets were investigated.
Abstract: We calculate from first principles the electronic structure and optical properties of a number of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS${}_{2}$ layers sandwiched with WS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, BN, or graphene sheets. Contrary to previous works, the systems are constructed in such a way that the unstrained lattice constants of the constituent incommensurate monolayers are retained. We find strong interaction between the $\ensuremath{\Gamma}$-point states in all TMD/TMD heterostructures, which can lead to an indirect gap. On the other hand, states near the $K$ point remain as in the monolayers. When TMDs are paired with BN or graphene layers, the interaction around the $\ensuremath{\Gamma}$-point is negligible, and the electronic structure resembles that of two independent monolayers. Calculations of optical properties of the MoS${}_{2}$/WS${}_{2}$ system show that, even when the valence- and conduction-band edges are located in different layers, the mixing of optical transitions is minimal, and the optical characteristics of the monolayers are largely retained in these heterostructures. The intensity of interlayer transitions is found to be negligibly small, a discouraging result for engineering the optical gap of TMDs by heterostructuring.
TL;DR: In this paper, a thermodynamically rigorous nonlinear elastic constitutive equation was derived for two-dimensional molybdenum disulfide, and the authors used first-principles density functional theory (DFT) calculations to predict the behavior of suspended monolayer MoS{}$ subjected to a spherical indenter load at finite strains in a multiple-length-scale finite element analysis model.
Abstract: This research explores the nonlinear elastic properties of two-dimensional molybdenum disulfide. We derive a thermodynamically rigorous nonlinear elastic constitutive equation and then calculate the nonlinear elastic response of two-dimensional MoS${}_{2}$ with first-principles density functional theory (DFT) calculations. The nonlinear elastic properties are used to predict the behavior of suspended monolayer MoS${}_{2}$ subjected to a spherical indenter load at finite strains in a multiple-length-scale finite element analysis model. The model is validated experimentally by indenting suspended circular MoS${}_{2}$ membranes with an atomic force microscope. We find that the two-dimensional Young's modulus and intrinsic strength of monolayer MoS${}_{2}$ are 130 and 16.5 N/m, respectively. The results approach Griffith's predicted intrinsic strength limit of ${\ensuremath{\sigma}}_{\mathrm{int}}\ensuremath{\sim}\frac{E}{9}$, where $E$ is the Young's modulus. This study reveals the predictive power of first-principles density functional theory in the derivation of nonlinear elastic properties of two-dimensional MoS${}_{2}$. Furthermore, the study bridges three main gaps that hinder understanding of material properties: DFT to finite element analysis, experimental results to DFT, and the nanoscale to the microscale. In bridging these three gaps, the experimental results validate the DFT calculations and the multiscale constitutive model.
TL;DR: In this article, the phonon dispersion for uniaxially strained transition metal dichalcogenides (TMDs) is calculated and compared to the measured Raman spectrum.
Abstract: Typical Raman spectra of transition metal dichalcogenides (TMDs) display two prominent peaks, E2g and A1g, that are well separated from each other. We find that these modes are degenerate in bulk WSe2 yielding one single Raman peak. As the dimensionality is lowered, the observed peak splits in two as a result of broken degeneracy. In contrast to our experimental findings, our phonon dispersion calculations reveal that these modes remain degenerate independent of the number of layers. Interestingly, for minuscule biaxial strain the degeneracy is preserved but once the crystal symmetry is broken by uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for uniaxially strained WSe2 shows a perfect match to the measured Raman spectrum which suggests that uniaxial strain exists in WSe2 flakes possibly induced during the sample preparation and/or as a result of interaction between WSe2 and the substrate. Furthermore, we find that WSe2 undergoes an indirect to direct bandgap transition from bulk to monolayers which is ubiquitous for semiconducting TMDs. These results not only allow us to understand the vibrational properties of WSe2 but also provides detailed insight to their physical properties.
TL;DR: In this paper, density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, with surfaces terminated by O, F, H, and OH groups.
Abstract: Density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, ${\mathrm{Ti}}_{n+1}{\mathrm{C}}_{n}$ and ${\mathrm{Ti}}_{n+1}{\mathrm{N}}_{n}$ ($n=1$--9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and HSE06 give similar results. Without functional groups, MXenes have magnetically ordered ground states. All the studied materials are metallic except for ${\mathrm{Ti}}_{2}{\mathrm{CO}}_{2}$, which we predict to be semiconducting. The calculated density of states at the Fermi level of the thicker MXenes ($n\ensuremath{\geqslant}5$) is much higher than for thin MXenes, indicating that properties such as electronic conductivity and surface chemistry will be different. In general, the carbides and nitrides behave differently with the same functional groups.
TL;DR: In this article, the intrinsic electrical transport properties of monolayer silicene and MoS were investigated from first principles, based on the first principles of the first principle of first principles.
Abstract: Article on intrinsic electrical transport properties of monolayer silicene and MoS₂ from first principles.
TL;DR: By combining high-resolution transmission electron microscopy experiments and first-principles calculations, this paper studied production, diffusion, and agglomeration of sulfur vacancies in monolayer MoS${}_{2}$ under electron irradiation.
Abstract: By combining high-resolution transmission electron microscopy experiments and first-principles calculations, we study production, diffusion, and agglomeration of sulfur vacancies in monolayer MoS${}_{2}$ under electron irradiation. Single vacancies are found to be mobile under the electron beam and tend to agglomerate into lines. Different kinds of such extended defects are identified in the experiments, and their atomic structures and electronic properties are determined with the help of calculations. The orientation of line defects is found to be sensitive to mechanical strain. Our calculations also indicate that the electronic properties of the extended defects can be tuned by filling vacancy lines with other atomic species, thereby suggesting a way for strain and electron-beam-assisted engineering of MoS${}_{2}$-based nanostructures.
TL;DR: In this article, the electronic and magnetic properties of Mn-doped monolayer MoS were investigated using a combination of first-principles density functional theory (DFT) calculations and Monte Carlo simulations.
Abstract: We investigate the electronic and magnetic properties of Mn-doped monolayer MoS${}_{2}$ using a combination of first-principles density functional theory (DFT) calculations and Monte Carlo simulations. Mn dopants that are substitutionally inserted at Mo sites are shown to couple ferromagnetically via a double-exchange mechanism. This interaction is relatively short ranged, making percolation a key factor in controlling long-range magnetic order. The DFT results are parameterized using an empirical model to facilitate Monte Carlo studies of concentration- and temperature-dependent ordering in these systems, through which we obtain Curie temperatures in excess of room temperature for Mn doping in the range of 10--15$%$. Our studies demonstrate the potential for engineering a new class of atomically thin dilute magnetic semiconductors based on Mn-doped MoS${}_{2}$ monolayers.