# Showing papers in "Physical Review B in 2015"

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TL;DR: In this article, a comprehensive study of phonon lifetimes and thermal conductivity for 33 zincblende- and wurtzite compounds using linearized phonon Boltzmann equation and first-principles anharmonic phonon calculations is presented.

Abstract: A collaboration of researchers from Japan and France present a comprehensive study of phonon lifetimes and thermal conductivity for 33 zincblende- and wurtzite compounds using linearized phonon Boltzmann equation and first-principles anharmonic phonon calculations. The software that the authors created for this study will be released as an open source package and should be of help in the search of new materials for thermoelectric applications.

921 citations

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TL;DR: In this article, the authors used a shift-inverse exact diagonalization approach to identify the edge of the many-body localization edge in a random field Heisenberg chain.

Abstract: The authors study the phenomena of many-body localization in a random field Heisenberg chain. In this paper the authors use a shift-inverse exact diagonalization approach that allows them to study the mid-spectrum spectral properties of the model for system sizes of up to N=22. This has allow the authors to identify the many-body localization edge.

791 citations

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TL;DR: In this article, the authors investigate the stability and electronic properties of the honeycomb structure of the arsenene system based on first-principles calculations and find that both buckled and puckered arsenenes possess indirect gaps.

Abstract: Recently, phosphorene, a monolayer honeycomb structure of black phosphorus, was experimentally manufactured and has attracted rapidly growing interest. Motivated by phosphorene, here we investigate the stability and electronic properties of the honeycomb structure of the arsenic system based on first-principles calculations. Two types of honeycomb structures, buckled and puckered, are found to be stable. We call them arsenenes, as in the case of phosphorene. We find that both buckled and puckered arsenenes possess indirect gaps. We show that the band gap of puckered and buckled arsenenes can be tuned by applying strain. The gap closing occurs at 6% strain for puckered arsenene, where the bond angles between the nearest neighbors become nearly equal. An indirect-to-direct gap transition occurs by applying strain. Specifically, 1% strain is enough to transform puckered arsenene into a direct-gap semiconductor. We note that a bulk form of arsenic called gray arsenic exists which can be used as a precursor for buckled arsenene. Our results will pave the way for applications to light-emitting diodes and solar cells.

708 citations

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TL;DR: In this paper, two different classes of symmetry protected nodal lines in the absence and in the presence of spin-orbital coupling (SOC), respectively, are studied. But unlike nodal line in the same symmetry class, each nodal can only be created (annihilated) in pairs.

Abstract: We theoretically study three-dimensional topological semimetals (TSMs) with nodal lines protected by crystalline symmetries. Compared to TSMs with point nodes, e.g., Weyl semimetals and Dirac semimetals, where the conduction and the valence bands touch at discrete points, in these TSMs the two bands cross at closed lines in the Brillouin zone. We propose two different classes of symmetry protected nodal lines in the absence and in the presence of spin-orbital coupling (SOC), respectively. In the former, we discuss nodal lines that are protected by a combination of inversion symmetry and time-reversal symmetry, yet, unlike previously studied nodal lines in the same symmetry class, each nodal line has a ${Z}_{2}$ monopole charge and can only be created (annihilated) in pairs. In the second class, with SOC, we show that a nonsymmorphic symmetry (screw axis) protects a four-band crossing nodal line in systems having both inversion and time-reversal symmetries.

700 citations

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TL;DR: In this paper, the authors reported an all-carbon MTC with topologically nontrivial electronic states by exhibiting node lines in bulk, which can evolve into a 3D Dirac point in the absence of inversion symmetry.

Abstract: Graphene, a two-dimensional (2D) carbon sheet, acquires many of its amazing properties from the Dirac point nature of its electronic structures with negligible spin-orbit coupling. Extending to 3D space, graphene networks with negative curvature, called Mackay-Terrones crystals (MTCs), have been proposed and experimentally explored, yet their topological properties have yet to be discovered. Based on the first-principle calculations, we report an all-carbon MTC with topologically nontrivial electronic states by exhibiting node lines in bulk. When the node lines are projected onto surfaces to form circles, ``drumhead''-like flat surface bands nestled inside of the circles are formed. The bulk node line can evolve into a 3D Dirac point in the absence of inversion symmetry, the existence of which has been shown to be plausible in recent experiments.

558 citations

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TL;DR: In this article, the orthorhombic phase of the layered transition-metal dichalcogenide (MoTe) was investigated as a Weyl semimetal candidate and the spacing between each pair of Weyl points was found to be as large as 4% of the reciprocal lattice.

Abstract: We investigate the orthorhombic phase $({T}_{d})$ of the layered transition-metal dichalcogenide ${\mathrm{MoTe}}_{2}$ as a Weyl semimetal candidate. ${\mathrm{MoTe}}_{2}$ exhibits four pairs of Weyl points lying slightly above $(\ensuremath{\sim}6\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ the Fermi energy in the bulk band structure. Different from its cousin ${\mathrm{WTe}}_{2}$, which was recently predicted to be a type-II Weyl semimetal, the spacing between each pair of Weyl points is found to be as large as 4% of the reciprocal lattice in ${\mathrm{MoTe}}_{2}$ (six times larger than that of ${\mathrm{WTe}}_{2}$). When projected onto the surface, the Weyl points are connected by Fermi arcs, which can be easily accessed by angle-resolved photoemission spectroscopy due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive ${\mathrm{MoTe}}_{2}$ from a type-II to a type-I Weyl semimetal.

540 citations

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TL;DR: In this paper, the effect of defects induced by ion bombardment on the Raman spectrum of single-layer molybdenum disulfide was determined by using density functional theory to calculate the phonon dispersion curves.

Abstract: We determine the effect of defects induced by ion bombardment on the Raman spectrum of single-layer molybdenum disulfide. The evolution of both the linewidths and frequency shifts of the first-order Raman bands with the density of defects is explained with a phonon confinement model, using density functional theory to calculate the phonon dispersion curves. We identify several defect-induced Raman scattering peaks arising from zone-edge phonon modes. Among these, the most prominent is the $\mathrm{LA}(M)$ peak at $\ensuremath{\sim}227\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and its intensity, relative to the one of first-order Raman bands, is found to be proportional to the density of defects. These results provide a practical route to quantify defects in single-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ using Raman spectroscopy and highlight an analogy between the $\mathrm{LA}(M)$ peak in $\mathrm{Mo}{\mathrm{S}}_{2}$ and the $D$ peak in graphene.

537 citations

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TL;DR: In this paper, first-principles lattice dynamics (phonon spectrum) for each phase of the hybrid halide perovskite were reported, and the equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction.

Abstract: The hybrid halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes, we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low-frequency modes associated with the inorganic (${\mathrm{PbI}}_{3}{}^{\ensuremath{-}}{)}_{n}$ network and high-frequency modes of the organic ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{}^{+}$ cation, significant coupling between them is found, which emphasizes the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the ${\mathrm{PbI}}_{6}$ octahedra around room temperature.

463 citations

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TL;DR: In this article, a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations was performed, and the authors found evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, in contrast with the currently assumed trigonal three-layer stacking periodicity.

Abstract: The layered honeycomb magnet $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled ${j}_{\mathrm{eff}}=\frac{1}{2}\phantom{\rule{4.pt}{0ex}}{\mathrm{Ru}}^{3+}$ magnetic moments. Here, we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the ${j}_{\mathrm{eff}}=\frac{1}{2}$ picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the $b$ axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below ${T}_{\mathrm{N}}\ensuremath{\approx}13$ K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

432 citations

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Aalto University

^{1}TL;DR: In this paper, the formation energies of neutral and charged defects, determine the charge transition levels, and from these self-consistently assess the concentration of defects at thermal equilibrium as well as the resulting positions of the Fermi level.

Abstract: We present an extensive first-principles study of a large set of native defects in ${\mathrm{MoS}}_{2}$ in order to find out the types and concentrations of the most important defects in this system. The calculations are carried out for both bulk and monolayer forms of ${\mathrm{MoS}}_{2}$, which allows us to study how defect properties change between these two limiting cases. We consider single- and few-atom vacancies, antisites, adatoms on monolayer, and interstitials between layers in the bulk material. We calculate the formation energies of neutral and charged defects, determine the charge transition levels, and from these self-consistently assess the concentration of defects at thermal equilibrium as well as the resulting positions of the Fermi level. The chemical potential values corresponding to different growth conditions are carefully accounted for, and for all values of chemical potentials relevant to the growth of ${\mathrm{MoS}}_{2}$, the S vacancies are found to be the most abundant defects. However, they are acceptors and cannot be the cause of the often observed $n$-type doping. At the same time, Re impurities, which are often present in natural ${\mathrm{MoS}}_{2}$ samples, naturally provide good $n$-type doping behavior. We also calculate migration barriers for adatoms and interstitials and discuss how they can affect the growth process.

425 citations

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TL;DR: In this paper, the spin Hall torque efficiency in a Pt-ferromagnetic (FM) structure is strongly dependent on the interface properties and the choice of the ferromagnetic materials.

Abstract: A systematic study of both the harmonic response and spin torque ferromagnetic resonance demonstrates that the spin Hall torque efficiency in a Pt-ferromagnetic (FM) structure is strongly dependent on the interface properties and the choice of the ferromagnetic materials, a key point that has until now not been made in spin Hall studies.

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TL;DR: In this article, the structural, electronic, and optical properties of the group-IV monochalcogenides SnS, SnSe, GeS, and GeSe were investigated.

Abstract: The group-IV monochalcogenides SnS, SnSe, GeS, and GeSe form a family within the wider group of semiconductor ``phosphorene analogues.'' Here, we used first-principles calculations to investigate systematically their structural, electronic, and optical properties, analyzing the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. We show that all those binary phosphorene analogues are semiconducting, with band-gap energies covering part of the infrared and visible range, and in most cases higher than phosphorene. Further, we found that they have multiple valleys in the valence and conduction band, the latter with spin-orbit splitting of the order of 19--86 meV.

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TL;DR: In this article, the Boltzmann transport theory was combined with first-principles calculations to predict the thermal and electrical transport properties of tin selenide and tin sulfide.

Abstract: Tin selenide (SnSe) and tin sulfide (SnS) have recently attracted particular interest due to their great potential for large-scale thermoelectric applications. A complete prediction of the thermoelectric performance and the understanding of underlying heat and charge transport details are the key to further improvement of their thermoelectric efficiency. We conduct comprehensive investigations of both thermal and electrical transport properties of SnSe and SnS using first-principles calculations combined with the Boltzmann transport theory. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical behaviors. The cross-plane lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ is $40--60%$ lower than the in-plane values. Extremely low ${\ensuremath{\kappa}}_{L}$ is found for both materials because of high anharmonicity, while the average ${\ensuremath{\kappa}}_{L}$ of SnS is $\ensuremath{\sim}8%$ higher than that of SnSe from 300 to 750 K. It is suggested that nanostructuring would be difficult to further decrease ${\ensuremath{\kappa}}_{L}$ because of the short mean free paths of dominant phonon modes (1--30 nm at 300 K), while alloying would be efficient in reducing ${\ensuremath{\kappa}}_{L}$ considering that the relative ${\ensuremath{\kappa}}_{L}$ contribution $(\ensuremath{\sim}65%)$ of optical phonons is remarkably large. On the electrical side, the anisotropic electrical conductivities are mainly due to the different effective masses of holes and electrons along the $a, b$, and $c$ axes. This leads to the highest optimal $\mathit{ZT}$ values along the $b$ axis and lowest ones along the $a$ axis in both $p$-type materials. However, the $n$-type ones exhibit the highest $\mathit{ZT}\mathrm{s}$ along the $a$ axis due to the enhancement of power factor when the chemical potential gradually approaches the secondary conduction band valley that causes significant increase in electron mobility and density of states. Owing to the larger mobility and smaller ${\ensuremath{\kappa}}_{L}$ along the given direction, SnSe exhibits larger optimal ZTs compared with SnS in both $p$- and $n$-type materials. For both materials, the peak $\mathit{ZT}\mathrm{s}$ of $n$-type materials are much higher than those of $p$-type ones along the same direction. The predicted highest $\mathit{ZT}$ values at 750 K are 1.0 in SnSe and 0.6 in SnS along the $b$ axis for the $p$-type doping, while those for the $n$-type doping reach 2.7 in SnSe and 1.5 in SnS along the $a$ axis, rendering them among the best bulk thermoelectric materials for large-scale applications. Our calculations show reasonable agreements with the experimental results and quantitatively predict the great potential in further enhancing the thermoelectric performance of SnSe and SnS, especially for the $n$-type materials.

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TL;DR: Using broadband spectroscopic ellipsometry, the complex valued dielectric function of silver films from 0.05 eV to 4.14 eV with a statistical uncertainty of less than 1% was determined in this article.

Abstract: Using broadband spectroscopic ellipsometry, the authors determine the complex valued dielectric function of silver films from 0.05 eV (\ensuremath{\lambda}=25 \ensuremath{\mu}) to 4.14 eV (\ensuremath{\lambda} = 300 nm) with a statistical uncertainty of less than 1%. While several previous similar measurements exist, they span considerably shorter energy ranges and report partially inconsistent results. In view of the wide-ranging applications of silver in nanophotonics, plasmonics and optical metamaterials, we anticipate this paper to become a standard reference for many scientists and engineers.

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TL;DR: In this article, the authors report magnetic and thermodynamic properties of single crystal ion and show that the low temperature peak in specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, consistent with zigzag order predicted within the framework of the Kitaev-Heisenberg model.

Abstract: We report magnetic and thermodynamic properties of single crystal $\ensuremath{\alpha}\ensuremath{-}{\text{RuCl}}_{3}$, in which the ${\text{Ru}}^{3+}\phantom{\rule{0.28em}{0ex}}(4{d}^{5})$ ion is in its low spin state and forms a honeycomb lattice. Two features are observed in both magnetic susceptibility and specific heat data; a sharp peak at 7 K and a broad hump near 10--15 K. In addition, we observe a metamagnetic transition between 5 and 10 T. Our neutron diffraction study of single crystal samples confirms that the low temperature peak in the specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, which is consistent with zigzag order, one of the types of magnetic order predicted within the framework of the Kitaev-Heisenberg model.

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TL;DR: Using first-principles calculations within density functional theory, the magnetic ground states of monolayers of Mn-and Cr-based semiconducting trichalcogenides were investigated in this article.

Abstract: Layered transition-metal trichalcogenides with the chemical formula $AB{X}_{3}$ have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions (${J}_{2}$ and ${J}_{3}$) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer ${\mathrm{CrSiTe}}_{3}$ is an antiferromagnet with a zigzag spin texture due to significant contribution from ${J}_{3}$, whereas ${\mathrm{CrGeTe}}_{3}$ is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (${\mathrm{MnPS}}_{3}$ and ${\mathrm{MnPSe}}_{3}$) always show antiferromagnetic N\'eel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that $AB{X}_{3}$ can be a promising platform to explore two-dimensional magnetic phenomena.

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TL;DR: In this paper, the authors show that spatiotemporal variations over a surface may greatly extend the degree of wave manipulation in metasurfaces, and break several of their constraints associated with symmetries.

Abstract: Metasurfaces characterized by a transverse gradient of local impedance have recently opened exciting directions for light manipulation at the subwavelength scale. Here we add a temporal gradient to the picture, showing that spatiotemporal variations over a surface may greatly extend the degree of wave manipulation in metasurfaces, and break several of their constraints associated with symmetries. As an example, we synthesize a nonreciprocal classical analog to electromagnetically induced transparency, opening a narrow window of one-way efficient transmission in an otherwise opaque surface. These properties pave the way to magnetic-free, planarized, nonreciprocal ultrathin surfaces for free-space isolation.

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TL;DR: In this paper, Brillouin light spectroscopy in the Damon-Eshbach geometry was used to study spin wave propagation in perpendicularly magnetized ultrathin films with varying Co thickness.

Abstract: Spin waves in perpendicularly magnetized ${\text{Pt/Co/AlO}}_{x}/\text{Pt}$ ultrathin films with varying Co thicknesses (06--12 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thickness This nonreciprocity, attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), is significantly stronger than asymmetries resulting from surface anisotropies for such modes Results are consistent with an interfacial DMI constant ${D}_{\mathrm{s}}=\ensuremath{-}17\ifmmode\pm\else\textpm\fi{}011\phantom{\rule{028em}{0ex}}\text{pJ}$/m, which favors left-handed chiral spin structures This suggests that such films below 1 nm in thickness should support chiral states such as skyrmions at room temperature

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TL;DR: In this paper, the authors observe the changing Fermi surface, the position of the lowest energy electronic excitations, as well as the propensity towards nematic order and its manifestation under strain.

Abstract: Improvements in experimental resolution allow this group to elucidate how the electronic nematic transition evolves in FeSe. They observe the changing Fermi surface, the position of the lowest energy electronic excitations, as well as the propensity towards nematic order and its manifestation under strain.

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TL;DR: In this article, the authors introduce exactly solvable models of interacting Majorana fermions with extensive topological ground-state degeneracy and a hierarchy of pointlike, topological excitations that are only free to move within submanifolds of the full lattice.

Abstract: Exactly solvable models often provide valuable insights in theoretical studies of topological phases. Here the authors introduce exactly solvable models of interacting Majorana fermions each with extensive topological ground-state degeneracy and a hierarchy of pointlike, topological excitations that are only free to move within submanifolds of the full lattice. These very different models make up a new kind of topological quantum order.

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TL;DR: In this article, the authors investigated the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principles calculations and showed that the OH-terminated MXenes attain ultralow work functions between 1.6 and 2.8 eV.

Abstract: MXenes are a set of two-dimensional transition metal carbides and nitrides that offer many potential applications in energy storage and electronic devices. As an important parameter to design new electronic devices, we investigate the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principles calculations. From our calculations, it turns out that the OH-terminated MXenes attain ultralow work functions between 1.6 and 2.8 eV. Moreover, depending on the type of the transition metal, the F or O functionalization affects increasing or decreasing the work functions. We show that the changes in the work functions upon functionalizations are linearly correlated with the changes in the surface dipole moments. It is shown that the work functions of the F- or O-terminated MXenes are controlled by two factors: the induced dipole moments by the charge transfers between F/O and the substrate, and the changes in the total surface dipole moments caused by surface relaxation upon the functionalization. However, in the cases of the OH-terminated MXenes, in addition to these two factors, the intrinsic dipole moments of the OH groups play an important role in determining the total dipole moments and consequently justify their ultralow work functions.

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TL;DR: In this article, the authors present an ab initio framework to calculate anharmonic phonon frequency and phonon lifetime that is applicable to severely anharmonicity systems, including thermoelectric, ferroelectric, and superconducting materials.

Abstract: We present an ab initio framework to calculate anharmonic phonon frequency and phonon lifetime that is applicable to severely anharmonic systems. We employ self-consistent phonon (SCPH) theory with microscopic anharmonic force constants, which are extracted from density functional calculations using the least absolute shrinkage and selection operator technique. We apply the method to the high-temperature phase of ${\mathrm{SrTiO}}_{3}$ and obtain well-defined phonon quasiparticles that are free from imaginary frequencies. Here we show that the anharmonic phonon frequency of the antiferrodistortive mode depends significantly on the system size near the critical temperature of the cubic-to-tetragonal phase transition. By applying perturbation theory to the SCPH result, phonon lifetimes are calculated for cubic ${\mathrm{SrTiO}}_{3}$, which are then employed to predict lattice thermal conductivity using the Boltzmann transport equation within the relaxation-time approximation. The presented methodology is efficient and accurate, paving the way toward a reliable description of thermodynamic, dynamic, and transport properties of systems with severe anharmonicity, including thermoelectric, ferroelectric, and superconducting materials.

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TL;DR: In this article, the spectral weights used for the unfolding of two-component spinor eigenstates are decomposed as the sum of the spectral values of the corresponding spinor states.

Abstract: We show that the spectral weights W mK (k ) used for the unfolding of two-component spinor eigenstates ∣ ∣ ψ SC mK ⟩=|α⟩|ψ SC mK ,α⟩+|β⟩|ψ SC mK ,β⟩ can be decomposed as the sum of the pa ...

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TL;DR: In this paper, the authors investigated high harmonic generation in solids and found that interband emission is dominant for the midinfrared laser driver frequencies, whereas intraband emission dominates the far-infrared range.

Abstract: High harmonic generation (HHG) in solids is investigated. We find that interband emission is dominant for the midinfrared laser driver frequencies, whereas intraband emission dominates the far-infrared range. Interband HHG is similar to atomic HHG and therewith opens the possibility to apply atomic attosecond technology to the condensed matter phase. Interband emission is investigated with a quasiclassical method, by which HHG can be modeled based on the classical trajectory analysis of electron-hole pairs. This analysis yields a simple approximate cutoff law for HHG in solids. Differences between HHG in atoms and solids are identified that are important for adapting atomic attosecond technology to make it applicable to condensed matter.

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TL;DR: In this paper, first-principles calculations of graphene on monolayer molecular lattice were performed for optical spin transfer between two-dimensional transition-metal dichalcogenides and graphene.

Abstract: Hybrids of graphene and two-dimensional transition-metal dichalcogenides (TMDCs) have the potential to bring graphene spintronics to the next level. As we show here by performing first-principles calculations of graphene on monolayer ${\mathrm{MoS}}_{2}$, there are several advantages of such hybrids over pristine graphene. First, Dirac electrons in graphene exhibit a giant global proximity spin-orbit coupling, without compromising the semimetallic character of the whole system at zero field. Remarkably, these spin-orbit effects can be very accurately described by a simple effective Hamiltonian. Second, the Fermi level can be tuned by a transverse electric field to cross the ${\mathrm{MoS}}_{2}$ conduction band, creating a system of coupled massive and massless electron gases. Both charge and spin transport in such systems should be unique. Finally, we propose to use graphene/TMDC structures as a platform for optospintronics, in particular, for optical spin injection into graphene and for studying spin transfer between TMDCs and graphene.

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TL;DR: In this paper, the proximity-induced Zeeman effect was used to create a giant valley splitting in monolayer transition-metal dichalcogenides, which is continuously tunable by rotating the substrate magnetization.

Abstract: Valleys in monolayer transition-metal dichalcogenides seamlessly connect two basic carriers of quantum information, namely, the electron spin and photon helicity. Lifting the valley degeneracy is an attractive route to achieve further optoelectronic manipulations. However, the magnetic field only creates a very small valley splitting. We propose a strategy to create giant valley splitting by the proximity-induced Zeeman effect. Our first principles calculations of monolayer ${\mathrm{MoTe}}_{2}$ on a EuO substrate show that valley splitting over 300 meV can be generated. Interband transition energies become valley dependent, leading to selective spin-photon coupling by optical frequency tuning. The valley splitting is also continuously tunable by rotating the substrate magnetization. The giant and tunable valley splitting adds a different dimension to the exploration of unique optoelectronic devices based on magneto-optical coupling and magnetoelectric coupling.

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TL;DR: In this article, a strong-coupling spin model for these correlation-assisted 1/2-bands is derived, in which large antiferromagnetic Kitaev interactions emerge along with ferromagnetic Heisenberg interactions.

Abstract: Intensive studies of the interplay between spin-orbit coupling (SOC) and electronic correlations in transition-metal compounds have recently been undertaken. In particular, ${j}_{\mathrm{eff}}=1/2$ bands on a honeycomb lattice provide a pathway to realize Kitaev's exactly solvable spin model. However, since current wisdom requires strong atomic SOC to make ${j}_{\mathrm{eff}}=1/2$ bands, studies have been limited to iridium oxides. Contrary to this expectation, we demonstrate how Kitaev interactions arise in $4d$-orbital honeycomb $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, despite having significantly weaker SOC than the iridium oxides, via assistance from electron correlations. A strong-coupling spin model for these correlation-assisted ${j}_{\mathrm{eff}}=1/2$ bands is derived, in which large antiferromagnetic Kitaev interactions emerge along with ferromagnetic Heisenberg interactions. Our analyses suggest that the ground state is a zigzag-ordered phase lying close to the antiferromagnetic Kitaev spin liquid. Experimental implications for angle-resolved photoemission spectroscopy, neutron scattering, and optical conductivities are discussed.

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TL;DR: In this article, a physically motivated construction of local integrals of motion (LIOMs) in the MBL phase is presented, and the resulting LIOMs are quasi-local, and use their decay to extract the localization length and establish the location of the transition between the many-body localized and ergodic phases.

Abstract: Many-body localization provides a generic mechanism of ergodicity breaking in quantum systems. In contrast to conventional ergodic systems, many-body localized (MBL) systems are characterized by extensively many local integrals of motion (LIOM), which underlie the absence of transport and thermalization in these systems. Here we report a physically motivated construction of local integrals of motion in the MBL phase. We show that any local operator (e.g., a local particle number or a spin flip operator), evolved with the system's Hamiltonian and averaged over time, becomes a LIOM in the MBL phase. Such operators have a clear physical meaning, describing the response of the MBL system to a local perturbation. In particular, when a local operator represents a density of some globally conserved quantity, the corresponding LIOM describes how this conserved quantity propagates through the MBL phase. Being uniquely defined and experimentally measurable, these LIOMs provide a natural tool for characterizing the properties of the MBL phase, both in experiments and numerical simulations. We demonstrate the latter by numerically constructing an extensive set of LIOMs in the MBL phase of a disordered spin chain model. We show that the resulting LIOMs are quasi-local, and use their decay to extract the localization length and establish the location of the transition between the MBL and ergodic phases.

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TL;DR: In this article, the buckled antimonene has been shown to exhibit strong directional mechanical properties, which may give rise to a strong influence of strain on the electronic properties of antimony.

Abstract: The pseudolayered character of 3D bulk crystals of antimony has led us to predict its 2D single-layer crystalline phase named antimonene in a buckled honeycomb structure like silicene. Sb atoms also form an asymmetric washboard structure like black phospherene. Based on an extensive analysis comprising ab initio phonon and finite-temperature molecular dynamics calculations, we show that these two single-layer phases are robust and can remain stable at high temperatures. They are nonmagnetic semiconductors with band gaps ranging from 0.3 eV to 1.5 eV, and are suitable for 2D electronic applications. The washboard antimonene displays strongly directional mechanical properties, which may give rise to a strong influence of strain on the electronic properties. Single-layer antimonene phases form bilayer and trilayer structures with wide interlayer spacings. In multilayers, this spacing is reduced and eventually the structure changes to 3D pseudolayered bulk crystals. The zigzag and armchair nanoribbons of the antimonene phases have fundamental band gaps derived from reconstructed edge states and display a diversity of magnetic and electronic properties depending on their width and edge geometry. Their band gaps are tunable with the widths of the nanoribbons. When grown on substrates, such as germanene or Ge(111), the buckled antimonene attains a significant influence of substrates.

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TL;DR: In this paper, the authors present their results on the synthesis and extensive characterization of 2D OIPC single-crystalline bilayers of (C${}_{4}$H$-BN layers, transition metal dichalcogenide layers, etc.

Abstract: The new family of layered organic-inorganic perovskite crystals (OIPCs) is a unique addition to the set of available 2D materials. It is a crystalline inorganic solid that is surrounded by well-ordered organic ligands, both of which can be modified separately. This hybrid character endows the material with an unusual degree of tunability and flexibility. Here, the authors present their results on the synthesis and extensive characterization of 2D OIPC single-crystalline bilayers of (C${}_{4}$H${}_{9}$NH${}_{3}$)${}_{2}$PbI${}_{4}$. These materials could significantly expand the range of van der Waals heterostructures that can be produced by their combination with, e.g., graphene, $h$-BN layers, transition metal dichalcogenide layers, etc.