Showing papers in "Physical Review B in 2012"
TL;DR: In this article, it was shown that crystalline A(3)Bi (A = Na, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry, and they possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries.
Abstract: Three-dimensional (3D) Dirac point, where two Weyl points overlap in momentum space, is usually unstable and hard to realize. Here we show, based on the first-principles calculations and effective model analysis, that crystalline A(3)Bi (A = Na, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry. They possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries. Giant diamagnetism, linear quantum magnetoresistance, and quantum spin Hall effect will be expected for such compounds.
1,599 citations
TL;DR: In this paper, the Bethe-Salpeter equation (BSE) was used to detect two strongly bound excitons below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the $K$ point of the Brillouin zone.
Abstract: Quasiparticle band structures and optical properties of MoS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, WS${}_{2}$, and WSe${}_{2}$ monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons ($A$ and $B$) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the $K$ point of the Brillouin zone. The transition energies for monolayer MoS${}_{2}$, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers are predicted to lie in the range of 1--2 eV. Systematic trends are identified for quasiparticle band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of layered transition-metal dichalcogenides at the nanoscale.
1,282 citations
TL;DR: In this article, self-consistent calculations of the band structures and related effective mass parameters are carried out for bulk, monolayer, and bilayer MoS, including excitonic effects within the Mott-Wannier theory.
Abstract: Quasiparticle self-consistent $GW$ calculations of the band structures and related effective-mass parameters are carried out for bulk, monolayer, and bilayer MoS${}_{2}$. Including excitonic effects within the Mott-Wannier theory, quantitative agreement is obtained between the $A$, $B$ excitons, measured by absorption [Phys. Rev. Lett. 105, 136805 (2010)], and the calculated exciton gap energies at $K$. The $A$-$B$ splitting arises from the valence-band splitting which in the monolayer is entirely due to spin-orbit coupling and leads to spin-split states, while in the bilayer it is a combined effect of interlayer and spin-orbit coupling.
1,158 citations
TL;DR: In this paper, the authors explore the electronic structures of 2H-$M{X}_{2}$ and find that when the number of layers reduces to a single layer, the indirect gap becomes a direct gap with larger gap and the band curvatures are found to lead to drastic changes of effective masses.
Abstract: Using the first-principles calculations, we explore the electronic structures of 2H-$M{X}_{2}$ ($M$ $=$ Mo, W; $X$ $=$ S, Se, Te). When the number of layers reduces to a single layer, the indirect gap of bulk becomes a direct gap with larger gap and the band curvatures are found to lead to the drastic changes of effective masses. On the other hand, when the strain is applied on the single layer, the direct gap becomes an indirect gap and the effective masses vary. Especially, the tensile strain reduces the gap energy and effective masses while the compressive strain enhances them. Furthermore, the much larger tensile stress leads to become metallic.
1,107 citations
TL;DR: In this paper, a semi-empirical density functional optimization method using regularization and cross-validation methods from machine learning is developed to avoid the overfitting found when standard least-squares methods are applied to high-order polynomial expansions.
Abstract: A methodology for semiempirical density functional optimization, using regularization and cross-validation methods from machine learning, is developed. We demonstrate that such methods enable well-behaved exchange-correlation approximations in very flexible model spaces, thus avoiding the overfitting found when standard least-squares methods are applied to high-order polynomial expansions. A general-purpose density functional for surface science and catalysis studies should accurately describe bond breaking and formation in chemistry, solid state physics, and surface chemistry, and should preferably also include van der Waals dispersion interactions. Such a functional necessarily compromises between describing fundamentally different types of interactions, making transferability of the density functional approximation a key issue. We investigate this trade-off between describing the energetics of intramolecular and intermolecular, bulk solid, and surface chemical bonding, and the developed optimization method explicitly handles making the compromise based on the directions in model space favored by different materials properties. The approach is applied to designing the Bayesian error estimation functional with van der Waals correlation (BEEF--vdW), a semilocal approximation with an additional nonlocal correlation term. Furthermore, an ensemble of functionals around BEEF--vdW comes out naturally, offering an estimate of the computational error. An extensive assessment on a range of data sets validates the applicability of BEEF--vdW to studies in chemistry and condensed matter physics. Applications of the approximation and its Bayesian ensemble error estimate to two intricate surface science problems support this.
1,071 citations
TL;DR: In this article, the authors studied the phonon-limited mobility in intrinsic $n$-type single-layer MoS for temperatures $Tg100$ K. The mobility is weakly dependent on the carrier density.
Abstract: We study the phonon-limited mobility in intrinsic $n$-type single-layer MoS${}_{2}$ for temperatures $Tg100$ K. The materials properties including the electron-phonon interaction are calculated from first principles and the deformation potentials and Fr\"ohlich interaction in single-layer MoS${}_{2}$ are established. The calculated room-temperature mobility of $\ensuremath{\sim}$410 cm${}^{2}$V${}^{\ensuremath{-}1}$s${}^{\ensuremath{-}1}$ is found to be dominated by optical phonon scattering via intra and intervalley deformation potential couplings and the Fr\"ohlich interaction. The mobility is weakly dependent on the carrier density and follows a $\ensuremath{\mu}\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\gamma}}$ temperature dependence with $\ensuremath{\gamma}=1.69$ at room temperature. It is shown that a quenching of the characteristic homopolar mode, which is likely to occur in top-gated samples, increases the mobility with $\ensuremath{\sim}$70 cm${}^{2}$V${}^{\ensuremath{-}1}$s${}^{\ensuremath{-}1}$ and can be observed as a decrease in the exponent to $\ensuremath{\gamma}=1.52$. In comparison to recent experimental findings for the mobility in single-layer MoS${}_{2}$ ($\ensuremath{\sim}$200 cm${}^{2}$V${}^{\ensuremath{-}1}$s${}^{\ensuremath{-}1}$), our results indicate that mobilities close to the intrinsic phonon-limited mobility can be achieved in two-dimensional materials via dielectric engineering that effectively screens static Coulomb scattering on, e.g., charged impurities.
1,036 citations
TL;DR: In this paper, the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms were analyzed.
Abstract: We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms. The electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal.
969 citations
TL;DR: Using in situ Raman scattering from a single-layer MoS2 electrochemically top-gated field effect transistor (FET), this paper showed softening and broadening of the A(1g) phonon with electron doping, whereas the other Raman-active E2g(1) mode remains essentially inert.
Abstract: A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS2 electrochemically top-gated field-effect transistor (FET), we show softening and broadening of the A(1g) phonon with electron doping, whereas the other Raman-active E-2g(1) mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why the A(1g) mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS2-based FETs, which have a high on-off ratio and are of technological significance.
916 citations
TL;DR: In this article, the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity $n$-type and $p$)-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride was studied.
Abstract: An accurate quantitative description of the Auger recombination rate in silicon as a function of the dopant density and the carrier injection level is important to understand the physics of this fundamental mechanism and to predict the physical limits to the performance of silicon based devices. Technological progress has permitted a near suppression of competing recombination mechanisms, both in the bulk of the silicon crystal and at the surfaces. This, coupled with advanced characterization techniques, has led to an improved determination of the Auger recombination rate, which is lower than previously thought. In this contribution we present a systematic study of the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity $n$-type and $p$-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride. Based on these measurements, we develop a general parametrization for intrinsic recombination in crystalline silicon at 300 K consistent with the theory of Coulomb-enhanced Auger and radiative recombination. Based on this improved description we are able to analyze physical aspects of the Auger recombination mechanism such as the Coulomb enhancement.
775 citations
TL;DR: In this article, the authors performed precise spectroscopic ellipsometry measurements on evaporated gold, template-stripped gold, and single-crystal gold to determine the optical dielectric function across a broad spectral range from 300 nm to 25 εm (0.05-4.14 eV) with high spectral resolution.
Abstract: In metal optics gold assumes a special status because of its practical importance in optoelectronic and nano-optical devices, and its role as a model system for the study of the elementary electronic excitations that underlie the interaction of electromagnetic fields with metals. However, largely inconsistent values for the frequency dependence of the dielectric function describing the optical response of gold are found in the literature. We performed precise spectroscopic ellipsometry measurements on evaporated gold, template-stripped gold, and single-crystal gold to determine the optical dielectric function across a broad spectral range from 300 nm to 25 $\ensuremath{\mu}$m (0.05--4.14 eV) with high spectral resolution. We fit the data to the Drude free-electron model, with an electron relaxation time ${\ensuremath{\tau}}_{D}=14\ifmmode\pm\else\textpm\fi{}3$ fs and plasma energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{p}=8.45$ eV. We find that the variation in dielectric functions for the different types of samples is small compared to the range of values reported in the literature. Our values, however, are comparable to the aggregate mean of the collection of previous measurements from over the past six decades. This suggests that although some variation can be attributed to surface morphology, the past measurements using different approaches seem to have been plagued more by systematic errors than previously assumed.
749 citations
TL;DR: In this paper, it was shown that the topological response in Weyl semimetals is fully described by a $\ensuremath{\theta}$ term in the action for the electromagnetic field, where the field is not a constant parameter, like, for example, in topological insulators, but is a field which has a linear dependence on the space-time coordinates.
Abstract: We demonstrate that topological transport phenomena, characteristic of Weyl semimetals, namely the semiquantized anomalous Hall effect and the chiral magnetic effect (equilibrium magnetic-field-driven current), may be thought of as two distinct manifestations of the same underlying phenomenon, the chiral anomaly. We show that the topological response in Weyl semimetals is fully described by a $\ensuremath{\theta}$ term in the action for the electromagnetic field, where $\ensuremath{\theta}$ is not a constant parameter, like, for example, in topological insulators, but is a field, which has a linear dependence on the space-time coordinates. We also show that the $\ensuremath{\theta}$ term and the corresponding topological response survive for sufficiently weak translational symmetry breaking perturbations, which open a gap in the spectrum of the Weyl semimetal, eliminating the Weyl nodes.
TL;DR: In this paper, the authors present an equation of state (EOS) that describes how the hydrostatic change in surface area is related to two-dimensional in-plane pressure and yields the measure of a material's resilience to isotropic stretching.
Abstract: We present an equation of state (EOS) that describes how the hydrostatic change in surface area is related to two-dimensional in-plane pressure ($\mathcal{F}$) and yields the measure of a material's resilience to isotropic stretching (the layer modulus $\ensuremath{\gamma}$) as one of its fit parameters. We give results for the monolayer systems of graphene and boronitrene, and we also include results for Si, Ge, GeC, and SiC in the isostructural honeycomb structure for comparison. Our results show that, of the honeycomb structures, graphene is the most resilient to stretching with a value of ${\ensuremath{\gamma}}_{\text{C}}$ = $206.6$ N m${}^{\ensuremath{-}1}$, second is boronitrene with ${\ensuremath{\gamma}}_{\text{BN}}$ = $177.0$ N m${}^{\ensuremath{-}1}$, followed by ${\ensuremath{\gamma}}_{\text{SiC}}$ = 116.5 N m${}^{\ensuremath{-}1}$, ${\ensuremath{\gamma}}_{\text{GeC}}$ = 101.0 N m${}^{\ensuremath{-}1}$, ${\ensuremath{\gamma}}_{\text{Si}}$ = 44.5 N m${}^{\ensuremath{-}1}$, and ${\ensuremath{\gamma}}_{\text{Ge}}$ = 29.6 N m${}^{\ensuremath{-}1}$. We calculate the Young's and shear moduli from the elastic constants and find that, in general, they rank according to the layer modulus. We also find that the calculated layer modulus matches the one obtained from the EOS. We use the EOS to predict the isotropic intrinsic strength of the various systems and find that, in general, the intrinsic stresses also rank according to the layer modulus. Graphene and boronitrene have comparable strengths with intrinsic stresses of 29.4 and 26.0 N m${}^{\ensuremath{-}1}$, respectively. We considered four graphene allotropes including pentaheptite and graphdiyne and find that pentaheptite has a value for $\ensuremath{\gamma}$ comparable to graphene. We find a phase transition from graphene to graphdiyne at $\mathcal{F}$ = $\ensuremath{-}7.0$ N m${}^{\ensuremath{-}1}$. We also consider bilayer, trilayer, and four-layered graphene and find that the addition of extra layers results in a linear dependence of $\ensuremath{\gamma}$ with $\mathcal{F}$.
TL;DR: In this paper, the authors reported a superconducting artificial atom with a coherence time of ${T}_{2}^{*}=92$ $\ensuremath{\mu}$s and energy relaxation time of{T}{1}=70$
Abstract: We report a superconducting artificial atom with a coherence time of ${T}_{2}^{*}=92$ $\ensuremath{\mu}$s and energy relaxation time ${T}_{1}=70$ $\ensuremath{\mu}$s. The system consists of a single Josephson junction transmon qubit on a sapphire substrate embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.
TL;DR: In this paper, the Haldane phase in integer spin chains is shown to be a topologically nontrivial phase which is protected by any one of the following three global symmetries: (i) the dihedral group of rotation about the $x, $y, and $z$ axes, (ii) time-reversal symmetry, and (iii) link inversion symmetry.
Abstract: We discuss the characterization and stability of the Haldane phase in integer spin chains on the basis of simple, physical arguments. We find that an odd-$S$ Haldane phase is a topologically nontrivial phase which is protected by any one of the following three global symmetries: (i) the dihedral group of $\ensuremath{\pi}$ rotations about the $x$, $y$, and $z$ axes, (ii) time-reversal symmetry ${S}^{x,y,z}\ensuremath{\rightarrow}\ensuremath{-}{S}^{x,y,z}$, and (iii) link inversion symmetry (reflection about a bond center), consistent with previous results [Phys. Rev. B 81, 064439 (2010)]. On the other hand, an even-$S$ Haldane phase is not topologically protected (i.e., it is indistinct from a trivial, site-factorizable phase). We show some numerical evidence that supports these claims, using concrete examples.
TL;DR: In this article, the Fourier components of the spatially modulated hopping amplitudes of the twisted graphene bilayer were calculated analytically for any type of commensurate structures in the low-twist-angle limit.
Abstract: The continuum model of the twisted graphene bilayer [Lopes dos Santos, Peres, and Castro Neto, Phys. Rev. Lett. 99, 256802 (2007)] is extended to include all types of commensurate structures. The essential ingredient of the model, the Fourier components of the spatially modulated hopping amplitudes, can be calculated analytically for any type of commensurate structures in the low-twist-angle limit. We show that the Fourier components that could give rise to a gap in the sublattice exchange symmetric (SE-even) structures discussed by Mele [Phys. Rev. B 81, 161405 (2010)] vanish linearly with angle, whereas the amplitudes that saturate to finite values, as $\ensuremath{\theta}\ensuremath{\rightarrow}0$, ensure that all low-angle structures share essentially the same physics. We extend our previous calculations beyond the validity of perturbation theory to discuss the disappearance of Dirac cone structure at angles below $\ensuremath{\theta}\ensuremath{\lesssim}1$${}^{\ensuremath{\circ}}$.
TL;DR: In this paper, the authors studied the dependence of the entanglement entropy on the shape of entangling region(s), on the total charge density, on temperature, and on the presence of additional visible Fermi surfaces of gauge-neutral fermions.
Abstract: General scaling arguments, and the behavior of the thermal entropy density, are shown to lead to an infrared metric holographically representing a compressible state with hidden Fermi surfaces. This metric is characterized by a general dynamic critical exponent, z, and a specic hyperscaling violation exponent, . The same metric exhibits a logarithmic violation of the area law of entanglement entropy, as shown recently by Ogawa et al. (arXiv:1111.1023). We study the dependence of the entanglement entropy on the shape of the entangling region(s), on the total charge density, on temperature, and on the presence of additional visible Fermi surfaces of gauge-neutral fermions; for the latter computations, we realize the needed metric in an Einstein-Maxwell-dilaton theory. All our results support the proposal that the holographic theory describes a metallic state with hidden Fermi surfaces of fermions carrying gauge charges of deconned gauge elds.
TL;DR: Tran et al. as mentioned in this paper examined the possibility to further improve over the original TB-mBJ potential by either reparametrizing its coefficients using a larger test set of solids or defining a parametrization for small/medium-size band-gap semiconductors only.
Abstract: The modified Becke-Johnson exchange potential [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)] (TB-mBJ) yields very accurate electronic band structures and gaps for various types of semiconductors and insulators (e.g., $sp$ semiconductors, noble-gas solids, and transition-metal oxides). However, the TB-mBJ potential has, for a few groups of solids, the tendency to underestimate the band gap. This has led us to examine the possibility to further improve over the original TB-mBJ potential by either reparametrizing its coefficients using a larger test set of solids or defining a parametrization for small-/medium-size band-gap semiconductors only. We also checked alternatives to the average of $|\ensuremath{
abla}\ensuremath{\rho}|/\ensuremath{\rho}$ in the unit cell for the determination of parameter $c$, which determines the amount of the screening contribution. Among these different possibilities, the best one seems to be a reparametrization of the coefficients, which leads to a much more balanced description of the band gaps.
TL;DR: In this paper, the effects of quantum confinement on the electronic structure of monolayer transition metal dichalcogenides have been investigated using the Bethe-Salpeter equation.
Abstract: Using $GW$ first-principles calculations for few-layer and bulk MoS${}_{2}$, we study the effects of quantum confinement on the electronic structure of this layered material. By solving the Bethe-Salpeter equation, we also evaluate the exciton energy in these systems. Our results are in excellent agreement with the available experimental data. Exciton binding energy is found to dramatically increase from 0.1 eV in the bulk to 1.1 eV in the monolayer. The fundamental band gap increases as well, so that the optical transition energies remain nearly constant. We also demonstrate that environments with different dielectric constants have a profound effect on the electronic structure of the monolayer. Our results can be used for engineering the electronic properties of MoS${}_{2}$ and other transition-metal dichalcogenides and may explain the experimentally observed variations in the mobility of monolayer MoS${}_{2}$.
TL;DR: In this article, the lattice thermal conductivity of thermoelectric materials, such as Mg${}_{2}$Si, Mg{}_{1\ensuremath{-kappa}$], Mg {1}$Ge{1}{2}{3}{4}, and Mg ${}_{ 2}µ$Siµ, and their alloys, were calculated for bulk and nanowires, without adjustable parameters.
Abstract: The lattice thermal conductivity ($\ensuremath{\kappa}$) of the thermoelectric materials, Mg${}_{2}$Si, Mg${}_{2}$Sn, and their alloys, are calculated for bulk and nanowires, without adjustable parameters. We find good agreement with bulk experimental results. For large nanowire diameters, size effects are stronger for the alloy than for the pure compounds. For example, in 200 nm diameter nanowires $\ensuremath{\kappa}$ is lower than its bulk value by 30$%$, 20$%$, and 20$%$ for Mg${}_{2}$Si${}_{0.6}$Sn${}_{0.4}$, Mg${}_{2}$Si, and Mg${}_{2}$Sn, respectively. For nanowires less than 20 nm thick, the relative decrease surpasses 50$%$, and it becomes larger in the pure compounds than in the alloy. At room temperature, $\ensuremath{\kappa}$ of Mg${}_{2}$Si${}_{x}$Sn${}_{1\ensuremath{-}x}$ is less sensitive to nanostructuring size effects than Si${}_{x}$Ge${}_{1\ensuremath{-}x}$, but more sensitive than PbTe${}_{x}$Se${}_{1\ensuremath{-}x}$. This suggests that further improvement of Mg${}_{2}$Si${}_{x}$Sn${}_{1\ensuremath{-}x}$ as a nontoxic thermoelectric may be possible.
TL;DR: In this article, an approach based on GGA $+$ $U$ calculations, including the spin-orbit coupling, which involves fitted elemental-phase reference energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compound enthalpies of formation was presented.
Abstract: Despite the great success that theoretical approaches based on density functional theory have in describing properties of solid compounds, accurate predictions of the enthalpies of formation ($\ensuremath{\Delta}{H}_{f}$) of insulating and semiconducting solids still remain a challenge. This is mainly due to incomplete error cancellation when computing the total energy differences between the compound total energy and the total energies of its elemental constituents. In this paper we present an approach based on GGA $+$ $U$ calculations, including the spin-orbit coupling, which involves fitted elemental-phase reference energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compound enthalpies of formation. We use an extensive set of 252 binary compounds with measured $\ensuremath{\Delta}{H}_{f}$ values (pnictides, chalcogenides, and halides) to obtain FERE energies and show that after the fitting, the 252 enthalpies of formation are reproduced with the mean absolute error $\text{MAE}=0.054$ eV/atom instead of $\text{MAE}\ensuremath{\approx}0.250$ eV/atom resulting from pure GGA calculations. When applied to a set of 55 ternary compounds that were not part of the fitting set the FERE method reproduces their enthalpies of formation with $\text{MAE}=0.048$ eV/atom. Furthermore, we find that contributions to the total energy differences coming from the spin-orbit coupling can be, to a good approximation, separated into purely atomic contributions which do not affect $\ensuremath{\Delta}{H}_{f}$. The FERE method, hence, represents a simple and general approach, as it is computationally equivalent to the cost of pure GGA calculations and applies to virtually all insulating and semiconducting compounds, for predicting compound $\ensuremath{\Delta}{H}_{f}$ values with chemical accuracy. We also show that by providing accurate $\ensuremath{\Delta}{H}_{f}$ the FERE approach can be applied for accurate predictions of the compound thermodynamic stability or for predictions of Li-ion battery voltages.
TL;DR: In this article, hybrid density functional theory is used to explore the band structure and effective masses of MoS and the effects of strain on the electronic properties, and the transition in band structure from bulk to monolayer is investigated.
Abstract: We use hybrid density functional theory to explore the band structure and effective masses of MoS${}_{2}$, and the effects of strain on the electronic properties. Strain allows engineering the magnitude as well as the nature (direct versus indirect) of the band gap. Deformation potentials that quantify these changes are reported. The calculations also allow us to investigate the transition in band structure from bulk to monolayer, and the nature and degeneracy of conduction-band valleys. Investigations of strain effects on effective masses reveal that small uniaxial stresses can lead to large changes in the hole effective mass.
TL;DR: In this paper, the authors investigated the behavior of holes in the valence band of a range of wide-band-gap oxides including ZnO, MgO, In, Naor, Ga, Al, SnO, SiO, and TiO.
Abstract: We investigate the behavior of holes in the valence band of a range of wide-band-gap oxides including ZnO, MgO, In${}_{2}$O${}_{3}$, Ga${}_{2}$O${}_{3}$, Al${}_{2}$O${}_{3}$, SnO${}_{2}$, SiO${}_{2}$, and TiO${}_{2}$. Based on hybrid functional calculations, we find that, due to the orbital composition of the valence band, holes tend to form localized small polarons with characteristic lattice distortions, even in the absence of defects or impurities. These self-trapped holes (STHs) are energetically more favorable than delocalized, free holes in the valence band in all materials but ZnO and SiO${}_{2}$. Based on calculated optical absorption and emission energies we show that STHs provide an explanation for the luminescence peaks that have been observed in many of these oxides. We demonstrate that polaron formation prohibits $p$-type conductivity in this class of materials.
TL;DR: In this paper, polarization resolved photoluminescence from monolayer polysilicon (MoS) was reported, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space.
Abstract: We report polarization resolved photoluminescence from monolayer ${\mathrm{MoS}}_{2}$, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90$%$ polarized emission at 4 K is observed with 40$%$ polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T indicating robust, selective valley excitation.
TL;DR: In this article, the authors investigate the variation of thermal conductivity of hybrid graphene nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) using equilibrium molecular dynamics.
Abstract: Chemical and structural diversity present in hexagonal boron nitride ($h$-BN) and graphene hybrid nanostructures provide avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity ($\ensuremath{\kappa}$) of hybrid graphene/$h$-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parametrized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material, exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher $\ensuremath{\kappa}$ in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the $\ensuremath{\kappa}$ of $h$-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affect the thermal transport properties more strongly than superlattices. The largest reduction in thermal conductivity is observed at 50$%$ dot concentration, but the dot radius appears to have little effect on the magnitude of reduction around this concentration.
TL;DR: Ju et al. as discussed by the authors showed that a much richer resonant picture would be observable for higher relaxation times of charge carriers: more resonances appear and transmission can be totally suppressed, and the resonant features of the periodic array are associated with leaky plasmonic modes.
Abstract: Resonance diffraction in the periodic array of graphene microribbons is theoretically studied following a recent experiment [L. Ju et al., Nature Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters are presented. It is shown that a much richer resonant picture would be observable for higher relaxation times of charge carriers: More resonances appear and transmission can be totally suppressed. The comparison with the absorption cross-section of a single ribbon shows that the resonant features of the periodic array are associated with leaky plasmonic modes. The longest-wavelength resonance provides the highest visibility of the transmission dip and has the strongest spectral shift and broadening with respect to the single-ribbon resonance, due to collective effects.
TL;DR: In this paper, superconductivity in a new bismuth-oxysulfide compound BiS${}{4}$O${}_{ 4}$S${]_{ 3}$ was shown.
Abstract: Exotic superconductivity has often been discovered in materials with a layered (two-dimensional) crystal structure. The low dimensionality can affect the electronic structure and can realize high transition temperatures (${T}_{\mathrm{c}}$) and/or unconventional superconductivity mechanisms. We show superconductivity in a new bismuth-oxysulfide compound Bi${}_{4}$O${}_{4}$S${}_{3}$. Crystal structure analysis indicates that this superconductor has a layered structure composed of a stacking of spacer layers and BiS${}_{2}$ layers. Band calculation suggests that the Fermi level for Bi${}_{4}$O${}_{4}$S${}_{3}$ is just on the peak position of the partial density of states of the Bi 6$p$ orbital within the BiS${}_{2}$ layer. The BiS${}_{2}$ layer will be a basic structure which provides another universality class for a layered superconducting family, and this opens up a new field in the physics and chemistry of low-dimensional superconductors.
TL;DR: In this article, a survey of quantized bulk physical observables in two-and three-dimensional topological band insulators invariant under translational symmetry and crystallographic point group symmetries is presented.
Abstract: We survey various quantized bulk physical observables in two- and three-dimensional topological band insulators invariant under translational symmetry and crystallographic point group symmetries (PGS). In two-dimensional insulators, we show that (i) the Chern number of a ${C}_{n}$-invariant insulator can be determined, up to a multiple of $n$, by evaluating the eigenvalues of symmetry operators at high-symmetry points in the Brillouin zone; (ii) the Chern number of a ${C}_{n}$-invariant insulator is also determined, up to a multiple of $n$, by the ${C}_{n}$ eigenvalue of the Slater determinant of a noninteracting many-body system; and (iii) the Chern number vanishes in insulators with dihedral point groups ${D}_{n}$, and the quantized electric polarization is a topological invariant for these insulators. In three-dimensional insulators, we show that (i) only insulators with point groups ${C}_{n}$, ${C}_{nh}$, and ${S}_{n}$ PGS can have nonzero 3D quantum Hall coefficient and (ii) only insulators with improper rotation symmetries can have quantized magnetoelectric polarization ${P}_{3}$ in the term ${P}_{3}\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$, the axion term in the electrodynamics of the insulator (medium).
TL;DR: In this paper, the authors present a comparison between reaction energies provided by experiments and computed by DFT in the generalized gradient approximation (GGA), using a Hubbard U parameter for some transitionmetalelements.
Abstract: The evaluation of reaction energies between solids using density functional theory (DFT) is of practical importance in many technological fields and paramount in the study of the phase stability of known and predicted compounds In this work, we present a comparison between reaction energies provided by experiments and computed by DFT in the generalized gradient approximation (GGA), using a Hubbard U parameter for some transitionmetalelements(GGA +U)Weuseadatasetof135reactionsinvolvingtheformationofternaryoxides from binary oxides in a broad range of chemistries and crystal structures We find that the computational errors can be modeled by a normal distribution with a mean close to zero and a standard deviation of 24 meV/atom The significantly smaller error compared to the more commonly reported errors in the formation energies from the elements is related to the larger cancellation of errors in energies when reactions involve chemically similar compounds This result is of importance for phase diagram computations for which the relevant reaction energies are often not from the elements but from chemically close phases (eg, ternary oxides versus binary oxides) In addition, we discuss the distribution of computational errors among chemistries and show that the use of a Hubbard U parameter is critical to the accuracy of reaction energies involving transition metals even when no major change in formal oxidation state is occurring
TL;DR: In this paper, the ideal tensile strength and elastic moduli of single-layer MoS${}_{2}$ were investigated based on first-principle calculation, for biaxial tension and uniaaxial tension along zigzag and armchair directions.
Abstract: Ideal tensile stress strain relations for single-layer MoS${}_{2}$ are investigated based on first-principle calculation, for biaxial tension and uniaxial tension along zigzag and armchair directions. The predicted ideal tensile strengths and elastic moduli are in excellent agreement with the very recent experimental measurements of Bertolazzi et al. [ACS Nano 5, 9703 (2011)] and Castellanos-Gomez et al. [Adv. Mater. 24, 772 (2012)]. It is identified that the tensile strength of single-layer MoS${}_{2}$ are dictated by out-of-plane soft-mode phonon instability under biaxial tension and uniaxial tension along the armchair direction. This failure mechanism, different from that of the truly two-dimensional material graphene, is attributed to the out-of-plane atomic relaxation upon tensile strain. Investigation of the electronic structures of single-layer MoS${}_{2}$ under tensile strain shows the material becomes an indirect semiconductor at small tensile strain ($l2$$%$) and turns into metallic before reaching the ideal tensile strength.