Atomically thin $\mathrm{Mo}{\mathrm{S}}_2$ grown by chemical vapor deposition (CVD) is a promising candidate for next-generation electronics due to inherent CVD scalability and controllability. However, it is well known that the stacking sequence in few-layer $\mathrm{Mo}{\mathrm{S}}_2$ can significantly impact electrical and optical properties. Herein we report different intrinsic stacking sequences in CVD-grown few-layer $\mathrm{Mo}{\mathrm{S}}_2$ obtained by atomic-resolution annular-dark-field imaging in an aberration-corrected scanning transmission electron microscope operated at 50 keV. Trilayer $\mathrm{Mo}{\mathrm{S}}_2$ displays a new stacking sequence distinct from the commonly observed $2H$ and $3R$ phases of $\mathrm{Mo}{\mathrm{S}}_2$. Density functional theory is used to examine the stability of different stacking sequences, and the findings are consistent with our experimental observations.

The entanglement entropy in many gapless quantum systems receives a contribution from the corners in the entangling surface in 2+1d, which is characterized by a universal function $a\left(\theta \right)$ depending on the opening angle $\theta$, and contains pertinent low energy information. For conformal field theories (CFTs), the leading expansion coefficient in the smooth limit $\theta \to \pi$ yields the stress tensor two-point function coefficient $C_T$. Little is known about $a\left(\theta \right)$ beyond that limit. Here, we show that the next term in the smooth limit expansion contains information beyond the two- and three-point correlators of the stress tensor. We conjecture that it encodes four-point data, making it much richer. Further, we establish strong constraints on this and higher-order smooth-limit coefficients. We also show that $a\left(\theta \right)$ is lower-bounded by a nontrivial function multiplied by the central charge $C_T$, e.g., $a(\pi /2)\ge ({\pi }^{2}ln2)C_T/6$. This bound for 90-degree corners is nearly saturated by all known results, including recent numerics for the interacting Wilson-Fisher quantum critical points (QCPs). A bound is also given for the Rényi entropies. We illustrate our findings using $\text{O}\left(N\right)$ QCPs, free boson and Dirac fermion CFTs, strongly coupled holographic ones, and other models. Exact results are also given for Lifshitz quantum critical points, and for conical singularities in 3+1d.

We investigate the optical control possibilities of spin-valley qubit carried by single electrons localized in nanostructures of monolayer TMDs, including small quantum dots formed by lateral heterojunction and charged impurities. The quantum controls are discussed when the confinement induces valley hybridization and when the valley hybridization is absent. We show that the bulk valley and spin optical selection rules can be inherited in different forms in the two scenarios, both of which allow the definition of spin-valley qubit with desired optical controllability. We also investigate nuclear spin-induced decoherence and quantum control of electron-nuclear spin entanglement via intervalley terms of the hyperfine interaction. Optically controlled two-qubit operations in a single quantum dot are discussed.

We study a system of two quantum dots, each with several discrete levels, which are coherently coupled to a microwave oscillator. They are attached to electronic leads and coupled to a phonon bath, both leading to inelastic processes. For a simpler system with a single level in each dot it has been shown that a population inversion can be created by electron tunneling, which in a resonance situation leads to lasing-type properties of the oscillator. In the multilevel system several resonance situations may arise, some of them relying on a sequence of tunneling processes which also involve nonresonant, inelastic transitions. The resulting photon number in the oscillator and the current-voltage characteristic are highly sensitive to these properties and accordingly can serve as a probe for microscopic details.

We report a new, cleavable, strong topological metal, ${\mathrm{Zr}}_2{\mathrm{Te}}_2\mathrm{P}$, which has the same tetradymite-type crystal structure as the topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_2\mathrm{Se}$. Instead of being a semiconductor, however, ${\mathrm{Zr}}_2{\mathrm{Te}}_2\mathrm{P}$ is metallic with a pseudogap between 0.2 and 0.7 eV above the Fermi energy $\left(E_F\right)$. Inside this pseudogap, two Dirac dispersions are predicted: one is a surface-originated Dirac cone protected by time-reversal symmetry (TRS), while the other is a bulk-originated and slightly gapped Dirac cone with a largely linear dispersion over a 2 eV energy range. A third surface TRS-protected Dirac cone is predicted, and observed using angle-resolved photoemission spectroscopy, making ${\mathrm{Zr}}_2{\mathrm{Te}}_2\mathrm{P}$ the first system, to our knowledge, to realize TRS-protected Dirac cones at $\overline{M}$ points. The high anisotropy of this Dirac cone is similar to the one in the hypothetical Dirac semimetal $\mathrm{Bi}{\mathrm{O}}_2$. We propose that if $E_F$ can be tuned into the pseudogap where the Dirac dispersions exist, it may be possible to observe ultrahigh carrier mobility and large magnetoresistance in this material.

We derive the continuum equations and boundary conditions governing phonon-mediated heat transfer in the limit of a small but finite mean-free path from the asymptotic solution of the linearized Boltzmann equation in the relaxation time approximation. Our approach uses the ratio of the mean-free path to the characteristic system length scale, also known as the Knudsen number, as the expansion parameter to study the effects of boundaries on the breakdown of the Fourier description. We show that, in the bulk, the traditional heat conduction equation using Fourier's law as a constitutive relation is valid at least up to second order in the Knudsen number for steady problems and first order for time-dependent problems. However, this description does not hold within distances on the order of a few mean-free paths from the boundary; this breakdown is a result of kinetic effects that are always present in the boundary vicinity and require solution of a Boltzmann boundary layer problem to be determined. Matching the inner, boundary layer solution to the outer, bulk solution yields boundary conditions for the Fourier description as well as additive corrections in the form of universal kinetic boundary layers; both are found to be proportional to the bulk-solution gradients at the boundary and parametrized by the material model and the phonon-boundary interaction model (Boltzmann boundary condition). Our derivation shows that the traditional no-jump boundary condition for prescribed temperature boundaries and the no-flux boundary condition for diffusely reflecting boundaries are appropriate only to zeroth order in the Knudsen number; at higher order, boundary conditions are of the jump type. We illustrate the utility of the asymptotic solution procedure by demonstrating that it can be used to predict the Kapitza resistance (and temperature jump) associated with an interface between two materials. All results are validated via comparisons with low-variance deviational Monte Carlo simulations.

We demonstrate a robust experimental method for determining the depth of individual shallow nitrogen-vacancy (NV) centers in diamond with $\sim 1$ nm uncertainty. We use a confocal microscope to observe single NV centers and detect the proton nuclear magnetic resonance (NMR) signal produced by objective immersion oil, which has well understood nuclear spin properties, on the diamond surface. We determine the NV center depth by analyzing the NV NMR data using a model that describes the interaction of a single NV center with the statistically polarized proton spin bath. We repeat this procedure for a large number of individual, shallow NV centers and compare the resulting NV depths to the mean value expected from simulations of the ion implantation process used to create the NV centers, with reasonable agreement.

A single crystal of the ${\mathrm{Co}}^{2+}\text{−}\mathrm{based}$ pyrochlore ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$ was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to 1/60th of the Curie-Weiss temperature. Below $T_f=2.4$ K, diffuse elastic scattering develops and comprises $30\pm 10\%$ of the total magnetic scattering, as expected for $J_{\text{eff}}=1/2$ moments frozen on a time scale that exceeds $\hslash /\delta E=3.8$ ps. The diffuse scattering is consistent with short range $XY$ antiferromagnetism with a correlation length of 16 Å. The momentum $\left(\mathit{Q}\right)$ dependence of the inelastic intensity indicates relaxing $XY$-like antiferromagnetic clusters at energies below $\sim 5.5$ meV, and collinear antiferromagnetic fluctuations above this energy. The relevant $XY$ configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$ despite evidence for weak $\left(\sim 12\%\right)$ exchange disorder. Instead, ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$ freezes into short range ordered clusters that span this manifold.

Materials with a $5d^4$ electronic configuration are generally considered to have a nonmagnetic ground state ($J=0$). Interestingly, ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ (${\mathrm{Ir}}^{5+}$ having $5d^4$ electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted ${\mathrm{IrO}}_6$ octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ with cubic ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ may shed light on the source of magnetism in such ${\mathrm{Ir}}^{5+}$ materials with $5d^4$ configuration. Besides, ${\mathrm{Ir}}^{5+}$ materials having $5d^4$ are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$. We observe that in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear.

Surface waves are often excited by interdigitated transducers consisting of many nanostrips attached on a substrate, and it has been recognized that the mass and stiffness of the attached nanostrips affect surface-wave resonances to some extent. Here, we reveal the more noticeable influence of the interfacial stiffness between strips and substrate at high frequencies. This influence is confirmed by exciting and detecting surface-wave resonances up to $\sim 6$ GHz by picosecond ultrasound spectroscopy. The resonance frequency significantly decreases and attenuation increases as the interfacial stiffness decreases for silicon substrate. However, low-attenuation branches appear along the Rayleigh-wave-resonance dispersion curve for silica substrate, and the resonance frequencies remain nearly identical to those of the Rayleigh waves. Previous models fail to reproduce these surface-wave resonance behaviors. The proposed theoretical model, involving the interfacial stiffness, consistently explained them, indicating the importance of the interface bond strength in designing surface-wave resonators.

We show that artificial magnetism of periodic dielectric or metal/dielectric structures has limitations and is subject to at least two “uncertainty principles.” First, the stronger the magnetic response (the deviation of the effective permeability tensor from identity), the less accurate (“certain”) the predictions of any homogeneous model. Second, if the magnetic response is strong, then homogenization cannot accurately reproduce the transmission and reflection parameters and, simultaneously, power dissipation in the material. These principles are general and not confined to any particular method of homogenization. Our theoretical analysis is supplemented with a numerical example: a hexahedral lattice of cylindrical air holes in a dielectric host. Even though this case is highly isotropic, which might be thought of as conducive to homogenization, the uncertainty principles remain valid.

We present exact diagonalization and momentum average approximation (MA) results for the single-polaron properties of a one-dimensional two-band model with phonon-modulated hopping. At strong electron-phonon coupling, we find a sharp transition in which the polaron ground state momentum jumps discontinuously from $k=\pi$ to $k=0$. The nature and origin of this transition is investigated and compared to that of the Su-Schrieffer-Heeger model, where a sharp but smooth transition was previously reported. We argue that such discontinuous transitions are a consequence of the multiband nature of the model, and are unlikely to be observed in one-band models. We also show that MA describes qualitatively and even quantitatively accurately this polaron and its transition. Given its computationally efficient generalization to higher dimensions, MA thus promises to allow for accurate studies of electron-phonon coupling in multiband models in higher dimensions.

We investigate the vertical ionization potential (IP) and electron affinity (EA) of organic semiconductors in the solid state that govern the optoelectrical property of organic devices using a fully ab initio way. The present method combines the density functional theory and many-body perturbation theory based on $GW$ approximations. To demonstrate the accuracy of this approach, we carry out calculations on several prototypical organic molecules. Since IP and EA depend on the molecular orientation at the surface, the molecular geometry of the surface is explicitly considered through the slab model. The computed IP and EA are in reasonable and consistent agreements with spectroscopic data on organic surfaces with various molecular arrangements. However, the transport gaps are slightly underestimated in calculations, which can be explained by different screening effects between surface and bulk regions.

A pressure-induced phase transition, associated with an increase of the coordination number of In and Ta, is detected beyond 13 GPa in $\mathrm{InTa}{\mathrm{O}}_4$ by combining synchrotron x-ray diffraction and Raman measurements in a diamond-anvil cell with ab initio calculations. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure that shares the same space group with the low-pressure phase ($P2/c$). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell volume collapse and an electronic band-gap collapse observed by experiments and calculations. Additionally, a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calculations also provide information on infrared-active phonons and bond distances. These findings provide insights into the effects of pressure on the physical properties of $\mathrm{InTa}{\mathrm{O}}_4$.

It is well known that a current driven through a two-dimensional electron gas with Rashba spin-orbit coupling induces a spin polarization in the perpendicular direction (Edelstein effect). This phenomenon has been extensively studied in the linear response regime, i.e., when the average drift velocity of the electrons is a small fraction of the Fermi velocity. Here we investigate the phenomenon in the nonlinear regime, meaning that the average drift velocity is comparable to or exceeds the Fermi velocity. This regime is realized when the electric field is very large or when electron-impurity scattering is very weak. We consider the limiting case of a two-dimensional noninteracting electron gas with no impurities. In this case, the quantum kinetic equation for the density matrix is exactly and analytically solvable, reducing to a problem of spin dynamics for “unpaired” electrons near the Fermi surface. The crucial parameter is $\gamma =eE{L}_s/E_F$, where $E$ is the electric field, $e$ is the absolute value of the electron charge, $E_F$ is the Fermi energy, and $L_s=\hslash /\left(2m\alpha \right)$ is the spin-precession length in the Rashba spin-orbit field with coupling strength $\alpha$. If $\gamma \ll 1$, the evolution of the spin is adiabatic, resulting in a spin polarization that grows monotonically in time and eventually saturates at the maximum value $n(\alpha /v_F)$, where $n$ is the electron density and $v_F$ is the Fermi velocity. If $\gamma \gg 1$ the evolution of the spin becomes strongly nonadiabatic and the spin polarization is progressively reduced and eventually suppressed for $\gamma \to \infty$. We also predict an inverse nonlinear Edelstein effect, in which an electric current is driven by a magnetic field that grows linearly in time. The “conductivities” for the direct and the inverse effects satisfy generalized Onsager reciprocity relations, which reduce to the standard ones in the linear response regime.

We derive the low-energy Hamiltonian for a honeycomb lattice with anisotropy in the hopping parameters. Taking the reported Dirac Hamiltonian for the anisotropic honeycomb lattice, we obtain its optical conductivity tensor and its transmittance for normal incidence of linearly polarized light. Also, we characterize its dichroic character due to the anisotropic optical absorption. As an application of our general findings, which reproduce the previous case of uniformly strained graphene, we study the optical properties of graphene under a nonmechanical distortion.

The plasmon structure of intrinsic and extrinsic bilayer graphene is investigated in the framework of ab initio time-dependent density-functional theory (TDDFT) at the level of the random-phase approximation (RPA). A two-step scheme is adopted, where the electronic ground state of a periodically repeated slab of bilayer graphene is first determined with full inclusion of the anisotropic band structure and the interlayer interaction; a Dyson-like equation is then solved self-consistently in order to calculate the so-called density-response function of the many-electron system. A two-dimensional correction is subsequently applied in order to eliminate the artificial interaction between the replicas. The energy range below $\sim 30$ eV is explored, focusing on the spectrum of single-particle excitations and plasmon resonances induced by external electrons or photons. The high-energy loss features of the $\pi$ and $\sigma +\pi$ plasmons, particularly their anisotropic dispersions, are predicted and discussed in relation with previous calculations and experiments performed on monolayer and bilayer graphene. At the low-energy end, the energy-loss function is found to be (i) very sensitive to the injected charge carrier density in doped bilayer graphene and (ii) highly anisotropic. Furthermore, various plasmon modes are predicted to exist and are analyzed with reference to the design of novel nanodevices.

In the present paper we introduce a way of identifying quantum phase transitions of many-body systems by means of local time correlations and Leggett-Garg inequalities. This procedure allows us to experimentally determine the quantum critical points not only of finite-order transitions but also those of infinite order, as the Kosterlitz-Thouless transition that is not always easy to detect with current methods. By means of simple analytical arguments for a general spin-$1/2$ Hamiltonian, and matrix product simulations of one-dimensional $XXZ$ and anisotropic $XY$ models, we argue that finite-order quantum phase transitions can be determined by singularities of the time correlations or their derivatives at criticality. The same features are exhibited by corresponding Leggett-Garg functions, which noticeably indicate violation of the Leggett-Garg inequalities for early times and all the Hamiltonian parameters considered. In addition, we find that the infinite-order transition of the $XXZ$ model at the isotropic point can be revealed by the maximal violation of the Leggett-Garg inequalities. We thus show that quantum phase transitions can be identified by purely local measurements and that many-body systems constitute important candidates to observe experimentally the violation of Leggett-Garg inequalities.

So far, several transition metal dichalcogenide (TMDC)–based two-dimensional (2D) topological insulators (TIs) have been discovered, all of them based on a tetragonal lattice. However, in 2D crystals, the hexagonal rather than the tetragonal symmetry is the most common motif. Here, based on first principles calculations, we propose a class of stable 2D TMDCs of composition ${\mathit{MX}}_2\left(M=\mathrm{Mo},\mathrm{W};X=\mathrm{S},\mathrm{Se},\mathrm{Te}\right)$ with a hexagonal lattice. They are all in the same stability range as other 2D TMDC allotropes that have been demonstrated experimentally, and they are identified to be practical 2D TIs with large band gaps ranging from 41 to 198 meV, making them suitable for applications at room temperature. Besides, in contrast to tetragonal 2D TMDCs, their hexagonal lattice will greatly facilitate the integration of theses novel TI state van der Waals crystals with other hexagonal or honeycomb materials and thus provide a route for 2D material–based devices for wider nanoelectronic and spintronic applications. The nontrivial band gaps of both $\mathrm{WS}{\mathrm{e}}_2$ and $\mathrm{WT}{\mathrm{e}}_2$ 2D crystals are 198 meV, which are larger than that in any previously reported TMDC-based TIs. These large band gaps entirely stem from the strong spin orbit coupling strength within the $d$ orbitals of Mo/W atoms near the Fermi level. Our findings broaden the scientific and technological impact of both 2D TIs and TMDCs.