Recently emerging large-area single-layer ${\mathrm{MoS}}_2$ grown by chemical vapor deposition has triggered great interest due to its exciting potential for applications in advanced electronic and optoelectronic devices. Unlike gapless graphene, ${\mathrm{MoS}}_2$ has an intrinsic band gap in the visible which crosses over from an indirect to a direct gap when reduced to a single atomic layer. In this paper, we report a comprehensive study of fundamental optical properties of ${\mathrm{MoS}}_2$ revealed by optical spectroscopy of Raman, photoluminescence, and vacuum ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by the absorption threshold of bulk ${\mathrm{MoS}}_2$ that shifts to 1.83 eV in monolayer ${\mathrm{MoS}}_2$. We extracted the high precision dielectric function up to 9.0 eV, which leads to the identification of many unique interband transitions at high symmetry points in the ${\mathrm{MoS}}_2$ momentum space. The positions of the so-called A and B excitons in single layers are found to shift upwards in energy compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. A very strong optical critical point predicted to correspond to a quasiparticle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the $M$ and $\Gamma$ points in the reduced Brillouin zone. The absence of the bulk ${\mathrm{MoS}}_2$ spin-orbit interaction peak at ∼3.0 eV in monolayer ${\mathrm{MoS}}_2$ is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurring in bulk semiconductors, is associated with a combination of several critical points. Additionally, extending into the vacuum ultraviolet energy spectrum are a series of newly observed oscillations representing optical transitions from valence bands to higher conduction bands of the monolayer ${\mathrm{MoS}}_2$ complex band structure. These optical transitions herein reported enhance our understanding of monolayer ${\mathrm{MoS}}_2$ as well as of two-dimensional systems in general and thus provide informative guidelines for ${\mathrm{MoS}}_2$ optical device designs and theoretical considerations.

An exact mapping of the tight-binding Hamiltonian for a graphene nanoribbon under any armchair uniaxial strain into an effective one-dimensional system is presented. As an application, for a periodic modulation we have found a gap opening at the Fermi level and a complex fractal spectrum, akin to the Hofstadter butterfly resulting from the Harper model. The latter can be explained by the commensurability or incommensurability nature of the resulting effective potential. When compared with the zig-zag uniaxial periodic strain, the spectrum shows much bigger gaps, although in general the states have a more extended nature. For a special critical value of the strain amplitude and wavelength, a gap is open. At this critical point, the electrons behave as relativistic Dirac fermions in one direction, while, in the other direction, a nonrelativistic Schrödinger behavior is observed. Also, some topological states were observed which have the particularity of not being completely edge states since they present some amplitude in the bulk. However, these are edge states of the effective system due to a reduced dimensionality through decoupling. These states also present the fractal Chern beating observed recently in quasiperiodic systems.

We consider adsorbates on metallic armchair-graphene nanoribbons (AGNRs). An adsorbed atom on a nodal site of the gapless subband in the AGNR is found to induce a bound state. The nodal-adsorbate bound state alone does not cause reflection in the AGNR quantum transport in the one propagating-channel regime. Yet its manifestation, as a Fano resonance in $G$, is brought forth by the presence of a non-nodal adsorbate (or a scatterer) located on a longitudinally separated site. Both the adsorbate-dependent nature and the significantly enhanced energy resolution of the Fano structure in $G$ invite adsorbate recognition. The Fano peak is shown to be robust against weak disorder such as from edge vacancies, due to the localized nature of the Fano resonant state. Adsorbates F and OH, described within an extended Hückel model, are considered as examples.

The canonical one-band Hubbard model is studied using a computational method that mixes the Monte Carlo procedure with the mean field approximation. This technique allows us to incorporate thermal fluctuations and the development of short-range magnetic order above ordering temperatures, contrary to the crude finite-temperature Hartree-Fock approximation, which incorrectly predicts a Néel temperature $T_N$ that grows linearly with the Hubbard $U/t$. The effective model studied here contains quantum and classical degrees of freedom. It thus belongs to the “spin fermion” model family widely employed in other contexts. Using exact diagonalization, supplemented by the traveling cluster approximation, for the fermionic sector, and classical Monte Carlo for the classical fields, the Hubbard $U/t$ vs temperature $T/t$ phase diagram is studied employing large three- and two-dimensional clusters. We demonstrate that the method is capable of capturing the formation of local moments in the normal state without long-range order, the nonmonotonicity of $T_N$ with increasing $U/t$, the development of gaps and pseudogaps in the density of states, and the two-peak structure in the specific heat. Extensive comparisons with determinant quantum Monte Carlo results suggest that the present approach is qualitatively, and often quantitatively, accurate, particularly at intermediate and high temperatures. Finally, we study the Hubbard model including plaquette diagonal hopping (i.e., the $t\text{−}{t}^{\prime }$ Hubbard model) in two dimensions and show that our approach allows us to study low-temperature properties where determinant quantum Monte Carlo fails due to the fermion sign problem. Future applications of this method include multiorbital Hubbard models such as those needed for iron-based superconductors.

Thermal properties are of great interest in modern electronic devices and nanostructures. Calculating these properties is straightforward when the device is made from a pure material, but problems arise when alloys are used. Specifically, only approximate band structures can be computed for random alloys and most often the virtual crystal approximation (VCA) is used. Unfolding methods [Boykin, Kharche, Klimeck, and Korkusinski, J. Phys.: Condens. Matter 19, 036203 (2007)] have proven very useful for tight-binding calculations of alloy electronic structure without the problems in the VCA, and the mathematical analogy between tight-binding and valence-force-field approaches to the phonon problem suggests they be employed here as well. However, there are some differences in the physics of the two problems requiring modifications to the electronic-structure approach. We therefore derive a phonon alloy band-structure (vibrational-mode) approach based on our tight-binding electronic-structure method, modifying the band-determination method to accommodate the different physical situation. Using the method, we study $\mathrm{In}\left(x\right)\mathrm{Ga}\left(1-x\right)\mathrm{As}$ alloys and find very good agreement with available experiments.

The appearance of nonhomogeneous structures of the indirect exciton density distribution in the region of the quantum well (in the region of the inner ring) is explained. The structure (the fragmentation) occurs due to the exciton condensed phase formation because of the interaction between excitons. The formation of the struc-ture is related with the nonequalibrity of the system, which is caused by the exciton finite lifetime and the presence of the pumping. The structure emerges in the shape of a set of islands or circles of the condensed phase. The structure type depends on the pumping intensity, the size of the laser spot, and disappears with increasing the temperature. The merging of two structures, created by different laser spots, is investigated at decreasing the distance between the centers of the spots.

We report on the spin-lattice interaction and coherent manipulation of electron spins in Mn-doped colloidal PbS quantum dots (QDs) by electron spin resonance. We show that the phase memory time,$T_M$, is limited by Mn–Mn dipolar interactions, hyperfine interactions of the protons (${}^1\mathrm{H}$) on the QD capping ligands with Mn ions in their proximity (<1 nm), and surface phonons originating from thermal fluctuations of the capping ligands. In the low Mn concentration limit and at low temperature, we achieve a long phase memory time constant $T_M\sim 0.9\mu \mathrm{s}$, thus enabling the observation of Rabi oscillations. Our findings suggest routes to the rational design of magnetic colloidal QDs with phase memory times exceeding the current limits of relevance for the implementation of QDs as qubits in quantum information processing.

The distribution of electron waiting times is useful to characterize quantum transport in mesoscopic structures. Here we consider a generic quantum-coherent conductor consisting of a mesoscopic scatterer in a two-terminal setup. We extend earlier results for single-channel conductors to setups with several (possibly spin degenerate) conduction channels and we discuss the effect of a finite electronic temperature. We present detailed investigations of the electron waiting times in a quantum point contact as well as in two mesoscopic interferometers with energy-dependent transmissions: a Fabry–Pérot interferometer and a Mach–Zehnder interferometer. We show that the waiting time distributions allow us to determine characteristic features of the scatterers, for instance, the number of resonant levels in the Fabry–Pérot interferometer that contribute to the electronic transport.

Density functional theory (DFT) calculations reliably aid in understanding the relative stability of different crystal phases as functions of pressure and temperature. Our purpose here is to employ DFT to analyze the character of the melting process, with an emphasis on comparing normal and anomalous melting. The normal-anomalous distinction is the absence or presence, respectively, of a significant electronic structure change between crystal and liquid. We study the normal melters Na and Cu, which are metallic in both phases, and the anomalous melter Ga, which has a partially covalent crystal and a nearly free-electron liquid. We calculate free energies from lattice dynamics for the crystal and from vibration-transit (V-T) theory for the liquid, where the liquid formulation is similar to that of the crystal but has an additional term representing the diffusive transits. Internal energies $U$ and entropies $S$ calculated for both phases of Na and Cu were previously shown to be in good agreement with experiment; here we find the same agreement for Ga. The dominant theoretical terms in the melting $\Delta U$ and $\Delta S$ are the structural potential energy, the vibrational entropy, and the purely liquid transit terms in both $U$ and $S$. The melting changes in structural energy and vibrational entropy are much larger in Ga than in Na and Cu. This behavior arises from the change in electronic structure in Ga, and is the identifying characteristic of anomalous melting. We interpret our DFT results in terms of the physical effects of the relatively few covalent bonds in the otherwise metallic Ga crystal.

The anomalous compressibility of vitreous silica has been known for nearly a century, but the mechanisms responsible for it remain poorly understood. Using GHz-ultrasonic interferometry, we measured longitudinal and transverse acoustic wave travel times at pressures up to 5 GPa in vitreous silica with fictive temperatures $\left(T_f\right)$ ranging between 985 °C and 1500 °C. The maximum in ultrasonic wave travel times–corresponding to a minimum in acoustic velocities–shifts to higher pressure with increasing $T_f$ for both acoustic waves, with complete reversibility below 5 GPa. These relationships reflect polyamorphism in the supercooled liquid, which results in a glassy state possessing different proportions of domains of high- and low-density amorphous phases (HDA and LDA, respectively). The relative proportion of HDA and LDA is set at $T_f$ and remains fixed on compression below the permanent densification pressure. The bulk material exhibits compression behavior systematically dependent on synthesis conditions that arise from the presence of floppy modes in a mixture of HDA and LDA domains.

The phase diagram of ${(\mathrm{PbTiO}}_3{)}_m/{(\mathrm{BaTiO}}_3{)}_n$ ferroelectric superlattices was computed using the phase-field approach as a function of layer volume fraction and biaxial strain to tune ferroelectric properties through domain engineering. Two interesting domain structures are found: one with mixed Bloch-Néel-Ising domain wall structures and the other with stabilized monoclinic ${\mathrm{M}}_{\mathrm{c}}$ phases. The polarization of the monoclinic ${\mathrm{M}}_{\mathrm{c}}$ phase is able to rotate from out-of-plane to in-plane or vice versa under an electric field, and thus facilitates the domain reversal of rhombohedral domains. This contributes significantly to both reduced coercive fields and enhanced piezoelectric responses.

We show that von Neumann entropy grows algebraically in time for many-body localized systems with long-range interactions. The dephasing time of far-apart localized degrees of freedom is studied first. For a disentangled high-energy initial state, our analysis indicates that the entropy of one half of the system evolves as $t^{1/(p+1)}$, where $p$ controls the spatial decay of interactions. Numerical simulations of the entropy evolution in the Coulomb and dipolar cases also exhibit an algebraic growth. We find that the saturation value of von Neumann entropy is extensive but small for an ergodic state, and it does not depend on the shape of the interactions. Due to the algebraic entanglement spread, the signature of an extensive entropy should be much easier to observe in experiments with nonlocal interactions than in those with local ones.

We theoretically investigate the thermoelectric properties of a spin-polarized two-dimensional electron gas hosting a Kondo adatom hybridized with a STM tip. Such a setup is treated within the single-impurity Anderson model in combination with the atomic approach for the Green's functions. Due to the spin dependence of the Fermi wave numbers, the electrical and thermal conductances together with thermopower and Lorenz number reveal beating patterns as a function of the STM tip position in the Kondo regime. In particular, by tuning the lateral displacement of the tip with respect to the adatom vicinity, the temperature, and the position of the adatom level, one can change the sign of the Seebeck coefficient through charge and spin. This opens a possibility of the microscopic control of the heat flux analogously to that established for the electrical current.

We present a study of the model spin-glass ${\mathrm{LiHo}}_{0.5}{\mathrm{Er}}_{0.5}{\mathrm{F}}_4$ using simultaneous ac susceptibility, magnetization, and magnetocaloric effect measurements along with small angle neutron scattering (SANS) at sub-Kelvin temperatures. All measured bulk quantities reveal hysteretic behavior when the field is applied along the crystallographic $c$ axis. Furthermore, avalanchelike relaxation is observed in a static field after ramping from the zero-field-cooled state up to 200–300 Oe. SANS measurements are employed to track the microscopic spin reconfiguration throughout both the hysteresis loop and the related relaxation. Comparing the SANS data to inhomogeneous mean-field calculations performed on a box of one million unit cells provides a real-space picture of the spin configuration. We discover that the avalanche is being driven by released Zeeman energy, which heats the sample and creates positive feedback, continuing the avalanche. The combination of SANS and mean-field simulations reveal that the conventional distribution of cluster sizes is replaced by one with a depletion of intermediate cluster sizes for much of the hysteresis loop.

We investigate the topological change in a Belavin-Polyakov skyrmion under the action of a spin-polarized current. The dynamics is described by the Schrödinger equation for the electrons carrying the current coupled to the Landau-Lifshitz equation for the evolution of the magnetic texture in a square lattice. We show that the addition of an exchange dissipation term tends to smooth the transition from the skyrmion state to the ferromagnetic state. We demonstrate that this topological change in the continuum dissipationless limit can be described as a self-similar finite-time singularity by which the skyrmion core collapses.

We have studied the local structure of a ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ superconductor across the phase separation and iron-vacancy order-disorder temperatures (respectively at $\sim 520$ K and $\sim 580$ K). The combination of Fe K edge and Se K edge extended x-ray absorption fine-structure (EXAFS) measurements reveal the glassy local structure of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$, with anomalous behavior of local atomic displacements. We find that the Fe-Se distance remains temperature independent while the Fe-Fe distance suffers a substantial effect of the phase separation. Furthermore, the Fe-Se network shows a reduced disorder in the phase separation regime at lower temperature. The x-ray absorption near-edge structure features follow the local structure anomaly observed by EXAFS and reveal substantially reduced Fe $3d$–Se $4p$ hybridization in the iron-vacancy disordered phase at higher temperature. The results provide further information on the role of nanoscale atomic displacements in the peculiar coexistence of different electronic phases in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ with a filamentary superconducting state embedded in the iron-vacancy ordered magnetic texture.

We predict spin Hall angles up to 80% for ultrathin noble-metal films with substitutional Bi impurities. The colossal spin Hall effect is caused by enhancement of the spin Hall conductivity in reduced sample dimension and a strong reduction of the charge conductivity by resonant impurity scattering. These findings can be exploited to create materials with high efficiency of charge to spin current conversion by strain engineering.

The intrinsic critical ferroelectric thickness of epitaxial ultrathin capacitors of incipient ferroelectric $\mathrm{BaZr}{\mathrm{O}}_3$ (BZO) films with realistic $\mathrm{SrRu}{\mathrm{O}}_3$ (SRO) electrodes is investigated by first-principles calculations based on density functional theory. We reveal that polarization can stably exist even in one-unit-cell thick BZO films, i.e., absence of critical thickness, whereas the widely investigated proper ferroelectrics like $\mathrm{BaTi}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ films have no polarization. The influences of realistic ferroelectric-electrode interface and misfit strain on the ionic and electronic structures of the BZO-SRO thin film system have been examined under the short-circuited boundary condition. It is found that the ionic polarization of conductive SRO electrodes can effectively strengthen the screening of bound charges at the interface, which greatly reduces the depolarization field in the BZO films. Furthermore, the epitaxial misfit strain remarkably enhances the polarization through the enhancement of hybridization of $\mathrm{Zr}$ and $\mathrm{O}$ electron orbitals, resulting in the disappearance of ferroelectric critical thickness. Our findings are beyond the critical thickness of proper ferroelectrics and are thus promising for future nanometer-scale ferroelectric device such as high-density ferroelectric memory.

The validity of Haldane's conjecture entails that the mass gap of the two-dimensional O(3) nonlinear sigma model with a $\theta$ term must tend to zero as $\theta$ approaches the value $\pi$ by following a precise law. In the present paper we extract the related critical exponents by simulating the model at imaginary $\theta$.