Showing papers in "Physical Review B in 2014"
TL;DR: In this article, the authors present necessary and sufficient conditions for elastic stability in all crystal classes, as a concise and pedagogical reference to stability criteria in noncubic materials.
Abstract: While the Born elastic stability criteria are well known for cubic crystals, there is some confusion in the literature about the form they should take for lower-symmetry crystal classes Here we present closed form necessary and sufficient conditions for elastic stability in all crystal classes, as a concise and pedagogical reference to stability criteria in noncubic materials
2,199 citations
TL;DR: In this paper, the authors reported that the band structure of few-layer black phosphorous is anisotropic and the band gap increases with the decrease in number of staking layers.
Abstract: The authors report that the band structure of few-layer black phosphorous is anisotropic and the band gap increases with the decrease in number of staking layers. The optical absorption and excitonic effects are also anisotropic.
2,021 citations
TL;DR: In this paper, the complex in-plane dielectric function from 1.5 to 3 eV for monolayers of four transition metal dichalcogenides (MoSe 2, WSe2, MoS2, and WS2) was presented.
Abstract: This chapter presents the complex in-plane dielectric function from 1.5 to 3 eV for monolayers of four transition metal dichalcogenides: MoSe2, WSe2, MoS2, and WS2. The results were obtained from optical reflection spectra using a Kramers–Kronig constrained variational analysis. From the inferred dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the respective bulk materials [1].
1,080 citations
TL;DR: In this paper, the effect of axial strain on the electronic band structure of phosphorene was studied using first-principles methods including density functional theory (DFT) and hybrid functionals.
Abstract: Recently fabricated two-dimensional phosphorene crystal structures have demonstrated great potential in applications of electronics. In this paper, strain effect on the electronic band structure of phosphorene was studied using first-principles methods including density functional theory (DFT) and hybrid functionals. It was found that phosphorene can withstand a tensile stress and 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. Although the DFT method is known to underestimate band gap of semiconductors, it was proven to correctly predict the strain effect on the electronic properties with validation from a hybrid functional method in this work. 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 that 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.
822 citations
TL;DR: In this article, the authors consider fully many-body-localized systems, i.e., isolated quantum systems where all the manybody eigenstates of the Hamiltonian are localized, and define a sense in which such systems are integrable with localized conserved operators.
Abstract: We consider fully many-body-localized systems, i.e., isolated quantum systems where all the many-body eigenstates of the Hamiltonian are localized. We define a sense in which such systems are integrable, with localized conserved operators. These localized operators are interacting pseudospins, and the Hamiltonian is such that unitary time evolution produces dephasing but not ``flips'' of these pseudospins. As a result, an initial quantum state of a pseudospin can in principle be recovered via (pseudospin) echo procedures. We discuss how the exponentially decaying interactions between pseudospins lead to logarithmic-in-time spreading of entanglement starting from nonentangled initial states. These systems exhibit multiple different length scales that can be defined from exponential functions of distance; we suggest that some of these decay lengths diverge at the phase transition out of the fully many-body-localized phase while others remain finite.
810 citations
TL;DR: In this article, the role of spin-orbit coupling in the electronic structure of Ru ions in a honeycomb lattice has been examined, and it has been shown that Ru ions are spin-assisted Mott insulators.
Abstract: We examine the role of spin-orbit coupling in the electronic structure of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$, in which Ru ions in $4{d}^{5}$ configuration form a honeycomb lattice. Our x-ray absorption spectroscopy measurements at the Ru $L$ edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be important for understanding magnetism of this compound.
653 citations
TL;DR: In this article, the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian was calculated, and the role of interband coupling and disorder on observed anisotropic absorption spectra was discussed.
Abstract: Black phosphorus thin films might offer attractive alternatives to narrow-gap compound semiconductors for optoelectronics across mid- to near-infrared frequencies. In this work, we calculate the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian. The optical absorption spectra of multilayer black phosphorus are shown to vary sensitively with thickness, doping, and light polarization. In conjunction with experimental spectra obtained from infrared absorption spectroscopy, we also discuss the role of interband coupling and disorder on the observed anisotropic absorption spectra.
633 citations
TL;DR: A machine learning model is constructed from a database of thousands of density functional theory calculations that can predict the thermodynamic stability of arbitrary compositions without any other input and with six orders of magnitude less computer time than DFT.
Abstract: Typically, computational screens for new materials sharply constrain the compositional search space, structural search space, or both, for the sake of tractability. To lift these constraints, we construct a machine learning model from a database of thousands of density functional theory (DFT) calculations. The resulting model can predict the thermodynamic stability of arbitrary compositions without any other input and with six orders of magnitude less computer time than DFT. We use this model to scan roughly 1.6 million candidate compositions for novel ternary compounds (${A}_{x}{B}_{y}{C}_{z}$), and predict 4500 new stable materials. Our method can be readily applied to other descriptors of interest to accelerate domain-specific materials discovery.
569 citations
TL;DR: In this article, the major effects of spin-orbit coupling on the optical spectra of these promising solar-cell materials are investigated, and it is shown that nonparabolic dispersion of valence bands is important to the device behavior of such absorbers.
Abstract: Challenging quasiparticle self-consistent $G\phantom{\rule{0}{0ex}}W$ calculations expose the major effects of spin-orbit coupling on the optical spectra of these promising solar-cell materials. Nonparabolic dispersion of valence bands is important to the device behavior of such absorbers.
556 citations
TL;DR: In this paper, the authors performed ab initio calculations for one-to four-layer black phosphorus within the $GW$ approximation, and obtained a significant difference in the band gap, which is in line with recent experimental data.
Abstract: By performing ab initio calculations for one- to four-layer black phosphorus within the $GW$ approximation, we obtain a significant difference in the band gap ($\ensuremath{\sim}$1.5 eV), which is in line with recent experimental data. The results are analyzed in terms of the constructed four-band tight-binding model, which gives accurate descriptions of the mono- and bilayer band structure near the band gap, and reveal an important role of the interlayer hoppings, which are largely responsible for the obtained gap difference.
535 citations
TL;DR: Lee et al. as mentioned in this paper proposed a van der Waals density functional (vdW-DF) that improves upon the description of energy and geometries of molecules, solids, and adsorption systems over the original vdWDF.
Abstract: I propose a van der Waals density functional (vdW-DF) that improves upon the description of energetics and geometries of molecules, solids, and adsorption systems over the original vdW-DF. The functional is based on the nonlocal correlation for the second version of the vdW-DF [Lee et al., Phys. Rev. B 82, 081101(R) (2010)] and an exchange functional that recovers the second-order gradient expansion approximation in the slowly varying limit, while reproducing the large density gradient behavior proposed by Becke [J. Chem. Phys. 85, 7184 (1986)]. A systematic assessment of the proposed functional is presented, which demonstrates the applicability of the proposed vdW-DF to a wide range of systems.
TL;DR: In this paper, the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) is used to describe all exchange-correlation components.
Abstract: Is the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) robust enough to describe all exchange-correlation components? To address this question, we design an exchange functional based on this plasmon description as well as recent analysis on exchange in the large-s regime. In the regime with reduced gradients s = |del n|/2nk(F)(n) smaller than approximate to 2.5, dominating the nonlocal correlation part of the binding energy, the enhancement factor F-x(s) closely resembles the Langreth-Vosko screened exchange. In the s regime beyond, dominated by exchange, F-x(s) passes smoothly over to the revised Perdew-Wang-86 form. We term the specific exchange functional LV-PW86r, wheras the full van der Waals functional version emphasizing consistent handling of exchange is termed vdW-DF-cx. Our tests indicate that vdW-DF-cx produces accurate separations and binding energies of the S22 data set of molecular dimers as well as accurate lattice constants and bulk moduli of layered materials and tightly bound solids. These results suggest that the plasmon description within vdW-DF gives a good description of both exchange and correlation effects in the low-to-moderate s regime.
TL;DR: In this paper, the binding energy and Van der Waals (vdW) interlayer spacing between the two layers of bilayer transition metal dichalcogenides were investigated for five different stacking patterns, and examined the stacking-induced modifications on the electronic and optical/excitonic properties within the GW approximation with a priori inclusion of spin-orbit coupling and by solving the twoparticle Bethe-Salpeter equation.
Abstract: Employing the random phase approximation we investigate the binding energy and Van der Waals (vdW) interlayer spacing between the two layers of bilayer transition metal dichalcogenides ${\mathrm{MoS}}_{2}$, ${\mathrm{MoSe}}_{2}$, ${\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ for five different stacking patterns, and examine the stacking-induced modifications on the electronic and optical/excitonic properties within the GW approximation with a priori inclusion of spin-orbit coupling and by solving the two-particle Bethe-Salpeter equation. Our results show that for all cases, the most stable stacking order is the high symmetry $A{A}^{\ensuremath{'}}$ type, distinctive of the bulklike $2H$ symmetry, followed by the $AB$ stacking fault, typical of the $3R$ polytypism, which is by only 5 meV/formula unit less stable. The conduction band minimum is always located in the midpoint between K and $\ensuremath{\Gamma}$, regardless of the stacking and chemical composition. All $M{X}_{2}$ undergo an direct-to-indirect optical gap transition going from the monolayer to the bilayer regime. The stacking and the characteristic vdW interlayer distance mainly influence the valence band splitting at K and its relative energy with respect to $\ensuremath{\Gamma}$, as well as, the electron-hole binding energy and the values of the optical excitations.
TL;DR: In this paper, a band unfolding technique was used to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations, which involves intrinsic p...
Abstract: We use a band unfolding technique to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations. This involves intrinsic p ...
TL;DR: In this article, a classification of topological phases and their topological defects in crystalline insulators and superconductors is presented, and the topological periodic table shows a periodicity in the number of flipped coordinates under the order-two spatial symmetry.
Abstract: We complete a classification of topological phases and their topological defects in crystalline insulators and superconductors. We consider topological phases and defects described by noninteracting Bloch and Bogoliubov--de Gennes Hamiltonians that support additional order-two spatial symmetry, besides any of 10 classes of symmetries defined by time-reversal symmetry and particle-hole symmetry. The additional order-two spatial symmetry we consider is general and it includes ${\mathbit{Z}}_{2}$ global symmetry, mirror reflection, twofold rotation, inversion, and their magnetic point group symmetries. We find that the topological periodic table shows a periodicity in the number of flipped coordinates under the order-two spatial symmetry, in addition to the Bott periodicity in the space dimensions. Various symmetry-protected topological phases and gapless modes will be identified and discussed in a unified framework. We also present topological classification of symmetry-protected Fermi points. The bulk classification and the surface Fermi point classification provide a realization of the bulk-boundary correspondence in terms of the $K$ theory.
TL;DR: In this article, the authors derived an analytical formula for the harmonic Hall voltages to evaluate the effective field for both out-of-plane and in-plane magnetized systems, and the results illustrate the versatility of harmonic Hall voltage measurement for studying current induced torques in magnetic heterostructures.
Abstract: Solid understanding of current induced torques is a key to the development of current and voltage controlled magnetization dynamics in ultrathin magnetic heterostructures. To evaluate the size and direction of such torques, or effective fields, a number of methods have been employed. Here, we examine the adiabatic (low-frequency) harmonic Hall voltage measurement that has been used to study the effective field. We derive an analytical formula for the harmonic Hall voltages to evaluate the effective field for both out of plane and in-plane magnetized systems. The formula agrees with numerical calculations based on a macrospin model. Two different in-plane magnetized films, Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The effective field obtained for the latter system shows relatively good agreement with that estimated using spin torque switching phase diagram measurements reported previously. Our results illustrate the versatile applicability of harmonic Hall voltage measurement for studying current induced torques in magnetic heterostructures.
TL;DR: It is found that conventional representations of the input data, such as the Coulomb matrix, are not suitable for the training of learning machines in the case of periodic solids and proposes a novel crystal structure representation for which learning and competitive prediction accuracies become possible within an unrestricted class of spd systems of arbitrary unit-cell size.
Abstract: High-throughput density functional calculations of solids are highly time-consuming. As an alternative, we propose a machine learning approach for the fast prediction of solid-state properties. To achieve this, local spin-density approximation calculations are used as a training set. We focus on predicting the value of the density of electronic states at the Fermi energy. We find that conventional representations of the input data, such as the Coulomb matrix, are not suitable for the training of learning machines in the case of periodic solids. We propose a novel crystal structure representation for which learning and competitive prediction accuracies become possible within an unrestricted class of $spd$ systems of arbitrary unit-cell size.
TL;DR: In this article, a numerical model for high-resolution atomic force microscopy and scanning tunneling microscopy (STM) imaging with functionalized tips is presented, which takes into account the relaxation of the probe due to the tip-sample interaction.
Abstract: High-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) imaging with functionalized tips is well established, but a detailed understanding of the imaging mechanism is still missing. We present a numerical STM/AFM model, which takes into account the relaxation of the probe due to the tip-sample interaction. We demonstrate that the model is able to reproduce very well not only the experimental intra- and intermolecular contrasts, but also their evolution upon tip approach. At close distances, the simulations unveil a significant probe particle relaxation towards local minima of the interaction potential. This effect is responsible for the sharp submolecular resolution observed in AFM/STM experiments. In addition, we demonstrate that sharp apparent intermolecular bonds should not be interpreted as true hydrogen bonds, in the sense of representing areas of increased electron density. Instead, they represent the ridge between two minima of the potential energy landscape due to neighboring atoms.
TL;DR: In this article, the authors investigate the Dzyaloshinskii-Moriya interactions in perpendicularly magnetized thin films of Pt/Co/Pt, and show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right-to the left-handed Neel wall.
Abstract: We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Neel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.
TL;DR: In this article, the anharmonic behavior of phonons and intrinsic thermal conductivity associated with the umklapp scattering in monolayer MoS${}_{2}$ sheet were investigated via first-principles calculations within the framework of density functional perturbation theory.
Abstract: The anharmonic behavior of phonons and intrinsic thermal conductivity associated with the umklapp scattering in monolayer MoS${}_{2}$ sheet are investigated via first-principles calculations within the framework of density functional perturbation theory. In contrast to the negative Gr\"uneissen parameter ($\ensuremath{\gamma}$) occurring in low-frequency modes in graphene, positive $\ensuremath{\gamma}$ in the whole Brillouin zone is demonstrated in monolayer MoS${}_{2}$ with much larger $\ensuremath{\gamma}$ for acoustic modes than that for the optical modes, suggesting that monolayer MoS${}_{2}$ sheet possesses a positive coefficient of thermal expansion. The calculated phonon lifetimes of the infrared active modes are 5.50 and 5.72 ps for ${E}^{\ensuremath{'}}$ and ${A}_{2}^{\ensuremath{'}\ensuremath{'}}$, respectively, in good agreement with experimental results obtained by fitting the dielectric oscillators with the infrared reflectivity spectrum. The lifetime of the Raman ${A}_{1}^{\ensuremath{'}}$ mode (38.36 ps) is about seven times longer than those of the infrared modes. The dominated phonon mean free path of monolayer MoS${}_{2}$ is less than 20 nm, about 30-fold smaller than that of graphene. Combined with the nonequilibrium Green's function calculations, the room temperature thermal conductivity of monolayer MoS${}_{2}$ is found to be around 23.2 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$, two orders of magnitude lower than that of graphene.
TL;DR: In this article, the intrinsic electron and hole-phonon interactions were investigated in monolayer transition-metal dichalcogenides based on a density functional theory formalism.
Abstract: Intrinsic electron- and hole-phonon interactions are investigated in monolayer transition-metal dichalcogenides $M{X}_{2}$ (M=Mo, W; X=S, Se) based on a density functional theory formalism. Due to their structural similarities, all four materials exhibit qualitatively comparable scattering characteristics with the acoustic phonons playing a dominant role near the conduction and valence band extrema at the K point. However, substantial differences are observed quantitatively leading to disparate results in the transport properties. Of those considered, ${\mathrm{WS}}_{2}$ provides the best performance for both electrons and holes with high mobilities and saturation velocities in the full-band Monte Carlo analysis of the Boltzmann transport equation. It is also found that monolayer $M{X}_{2}$ crystals with an exception of ${\mathrm{MoSe}}_{2}$ generally show hole mobilities comparable to or even larger than the value for bulk silicon at room temperature, suggesting a potential opportunity in p-type devices. The analysis is extended to estimate the effective deformation potential constants for a simplified treatment as well.
TL;DR: In this article, the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged trions (trions) in photoluminescence were investigated.
Abstract: Optical interband transitions in monolayer transition metal dichalcogenides such as ${\mathrm{WSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K valley in momentum space. Here we probe the valley dynamics in monolayer ${\mathrm{WSe}}_{2}$ by monitoring the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4 ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of $\ensuremath{\approx}20%$, for which a typical valley polarization decay time of the order of 1 ns can be inferred.
TL;DR: In this paper, the impact of carbon impurities on the electrical and optical properties of GaN, AlN, and InN using density functional calculations based on a hybrid functional was investigated.
Abstract: Carbon is a common impurity in the group-III nitrides, often unintentionally incorporated during growth. Nevertheless, the properties of carbon impurities in the nitrides are still not fully understood. We investigate the impact of carbon impurities on the electrical and optical properties of GaN, AlN, and InN using density functional calculations based on a hybrid functional. We examine the stability of substitutional and interstitial configurations as a function of the Fermi-level position and chemical potentials. In all nitrides studied here, C${}_{\mathrm{N}}$ acts as a deep acceptor and gives rise to deep, broad photoluminescence bands. Carbon on the cation site acts as a shallow donor in InN and GaN, but behaves as a $DX$ center in AlN. A split interstitial is the most stable configuration for the C impurity in InN, where it acts as a double donor and likely contributes to $n$-type conductivity.
TL;DR: In this article, the effect of the $d$-band shape on the local electronic structure of adsorbates was investigated, e.g., energy and filling of adorbate-metal antibonding states.
Abstract: The $d$-band shape of a metal site, governed by the local geometry and composition of materials, plays an important role in determining trends of the surface reactivity of transition-metal alloys. We discuss this phenomenon using the chemisorption of various adsorbates such as C, N, O, and their hydrogenated species on Pd bimetallic alloys as an example. For many alloys, the $d$-band center, even with consideration of the $d$-band width and $\mathit{sp}$ electrons, can not describe variations in reactivity from one surface to another. We investigate the effect of the $d$-band shape, represented by higher moments of the $d$ band, on the local electronic structure of adsorbates, e.g., energy and filling of adsorbate-metal antibonding states. The upper $d$-band edge ${\ensuremath{\varepsilon}}_{u}$, defined as the highest peak position of the Hilbert transform of the density of states projected onto $d$ orbitals of an active metal site, is identified as an electronic descriptor for the surface reactivity of transition metals and their alloys, regardless of variations in the $d$-band shape. The utilization of the upper $d$-band edge with scaling relations enables a considerable reduction of the parameter space in search of improved alloy catalysts and further extends our understanding of the relationship between the electronic structure and chemical reactivity of metal surfaces.
TL;DR: In this paper, a first-principles based structure search algorithm was used to identify low free energy phases at various pressures and temperatures of hafnia, and these phases were then recognized by exploiting group theoretical principles for the symmetry-allowed displacive transitions between nonpolar and polar phases.
Abstract: The question of whether one can systematically identify (previously unknown) ferroelectric phases of a given material is addressed, taking hafnia $({\mathrm{HfO}}_{2})$ as an example. Low free energy phases at various pressures and temperatures are identified using a first-principles based structure search algorithm. Ferroelectric phases are then recognized by exploiting group theoretical principles for the symmetry-allowed displacive transitions between nonpolar and polar phases. Two orthorhombic polar phases occurring in space groups $Pca{2}_{1}$ and $Pmn{2}_{1}$ are singled out as the most viable ferroelectric phases of hafnia, as they display low free energies (relative to known nonpolar phases), and substantial switchable spontaneous electric polarization. These results provide an explanation for the recently observed surprising ferroelectric behavior of hafnia, and reveal pathways for stabilizing ferroelectric phases of hafnia as well as other compounds.
TL;DR: In this article, the authors present a consistent method to separate thermoelectric and spin-orbit torque (SOT) measurements in ferromagnet/normal-metal bilayers, in which thermal gradient produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude.
Abstract: We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal-metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light-metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy-metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6 nm)/Co(2.5 nm) and negligible in Pt(6 nm)/Co(2.5 nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the fieldlike SOTs are about an order of magnitude smaller.
TL;DR: In this paper, a rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach was employed to examine the lattice thermal conductivity of strained and unstrained graphene.
Abstract: A rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach is employed to examine the lattice thermal conductivity, ${\ensuremath{\kappa}}_{L}$, of strained and unstrained graphene. First principles calculations show that the out-of-plane, flexural acoustic phonons provide the dominant contribution to ${\ensuremath{\kappa}}_{L}$ of graphene for all strains, temperatures, and system sizes considered, supporting a previous prediction that used an optimized Tersoff empirical interatomic potential. For the range of finite system sizes considered, we show that the ${\ensuremath{\kappa}}_{L}$ of graphene is relatively insensitive to strain. This provides validation for use of the BPE approach to calculate ${\ensuremath{\kappa}}_{L}$ for unstrained graphene, which has recently been called into question. The temperature and system size dependence of the calculated ${\ensuremath{\kappa}}_{L}$ of graphene is in good agreement with experimental data. The enhancement of ${\ensuremath{\kappa}}_{L}$ with isotopic purification is found to be relatively small due to strong anharmonic phonon-phonon scattering. This work provides insight into the nature of phonon thermal transport in graphene, and it demonstrates the power of first principles thermal transport techniques.
TL;DR: In this paper, the authors obtained defect energies with errors less than 0.2 eV for 17 defects in 10 compounds; the method is widely applicable to a range of defects and was improved on a previous method for correcting these errors.
Abstract: First-principles defect formation energy calculations can include errors up to several eV. Improving on a previous method for correcting these errors by Freysoldt et al (PRL 102, 016402), the authors obtain defect energies with errors less than 0.2 eV for 17 defects in 10 compounds; the method is widely applicable to a range of defects.
TL;DR: In this article, the excitonic dynamics in MoSe${}_{2}$ monolayer and bulk samples by femtosecond transient absorption were investigated by measuring a differential reflection of a probe pulse tuned in the range 790-820 nm.
Abstract: We investigate the excitonic dynamics in MoSe${}_{2}$ monolayer and bulk samples by femtosecond transient absorption. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by measuring a differential reflection of a probe pulse tuned in the range 790--820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in a bulk sample under comparable conditions. We also observe the saturated absorption induced by excitons in both monolayers and the bulk in the differential reflection measurements, which indicates their potential applications as saturable absorbers.
TL;DR: In this paper, the same authors showed that the electronic and optical properties of single-layer black phosphorus (BP) depend strongly on the applied strain and the orientation of applied strain.
Abstract: Using first principles calculations we showed that the electronic and optical properties of single-layer black phosphorus (BP) depend strongly on the applied strain. Due to the strong anisotropic atomic structure of BP, its electronic conductivity and optical response are sensitive to the magnitude and the orientation of the applied strain. We found that the inclusion of many body effects is essential for the correct description of the electronic properties of monolayer BP; for example, while the electronic gap of strainless BP is found to be 0.90 eV by using semilocal functionals, it becomes 2.31 eV when many-body effects are taken into account within the ${G}_{0}{W}_{0}$ scheme. Applied tensile strain was shown to significantly enhance electron transport along zigzag direction of BP. Furthermore, biaxial strain is able to tune the optical band gap of monolayer BP from 0.38 eV (at $\ensuremath{-}8%$ strain) to 2.07 eV (at 5.5%). The exciton binding energy is also sensitive to the magnitude of the applied strain. It is found to be 0.40 eV for compressive biaxial strain of $\ensuremath{-}8%$, and it becomes 0.83 eV for tensile strain of 4%. Our calculations demonstrate that the optical response of BP can be significantly tuned using strain engineering which appears as a promising way to design novel photovoltaic devices that capture a broad range of solar spectrum.