Photoabsorption spectra by circular polarized light in transition-metal dichalcogenides are calculated as a function of laser excitation energy. Although the 100% valley polarization occurs at the $K$ point of the Brillouin zone, the difference of the absorption intensity for left-handed and right-handed circular polarized light becomes maximum at about 1 eV higher energy than the direct energy band gap. The maximum intensity difference corresponds to the so-called $\mathrm{\Lambda }$ valley in the Brillouin zone. In order to understand valley polarization, analytic formula of optical absorption is given by tight-binding method.

Since the quantum anomalous Hall (QAH) effect was realized in magnetic topological insulators, research on the effect has become a hot topic. The very harsh realizing requirements of the effect in experiments, however, hinder its practical applications. Based on ab initio methods, we find that nonmagnetic $\mathrm{Pb}{\mathrm{I}}_2$ films are ideal substrates for the two-dimensional honeycomb stanene. The QAH effect with a pretty large band gap (up to 90 meV) can be achieved in the functionalized $\mathrm{stanene}/\mathrm{Pb}{\mathrm{I}}_2$ heterostructure. Despite van der Waals interactions in the heterostructure, band inversions are found to be happening between Sn $(s$ and $p_{x,y}$ ) and $\mathrm{Pb}\left(p_{x,y}\right)$ orbitals, playing a key role in determining the nontrivial topology and the large band gap of the system. Having no magnetic atoms is imperative to triggering the QAH effect. A very stable rudimentary device having QAH effects is proposed based on the $\mathrm{Sn}/\mathrm{Pb}{\mathrm{I}}_2$ heterostructure. Our results demonstrate that QAH effects can be easily realized in the $\mathrm{Sn}/\mathrm{Pb}{\mathrm{I}}_2$ heterostructures in experiments.

Advanced semiempirical calculations have been performed to compute simultaneously optical absorption and $K$ pre-edge x-ray absorption spectra of ${\mathrm{Fe}}^{2+}$ in four distinct site symmetries found in minerals. The four symmetries, i.e., a distorted octahedron, a distorted tetrahedron, a square planar site, and a trigonal bipyramidal site, are representative of the ${\mathrm{Fe}}^{2+}$ sites found in crystals and glasses. A particular attention has been paid to the definition of the $p\text{−}d$ hybridization Hamiltonian which occurs for noncentrosymmetric symmetries in order to account for electric dipole transitions. For the different sites under study, an excellent agreement between calculations and experiments was found for both optical and x-ray absorption spectra, in particular in terms of relative intensities and energy positions of electronic transitions. To our knowledge, these are the first calculations of optical absorption spectra on ${\mathrm{Fe}}^{2+}$ placed in such diverse site symmetries, including centrosymmetric sites. The proposed theoretical model should help to interpret the features of both the optical absorption and the $K$ pre-edge absorption spectra of $3d$ transition metal ions and to go beyond the usual fingerprint interpretation.

We present ab initio calculations of uniaxial absolute deformation potentials of the valence and the conduction bands in monolayer ${\mathrm{MoS}}_2,{\mathrm{MoSe}}_2,{\mathrm{WS}}_2,{\mathrm{WSe}}_2$, h-BN, and phosphorene. Calculations are performed using both semilocal and hybrid functionals. The absolute positions of the band edges in strained and unstrained materials are determined using the vacuum level as reference. For ${\mathrm{WSe}}_2$, we compare the obtained results with measured shifts of the valence band maximum (VBM) and the conduction band minimum (CBM) induced by uniaxial strain and find a very good agreement. The parameters describing the shifts in the VBM and CBM positions under strain can be used in the modeling of devices such as tunneling field-effect transistors.

We explore the physics of a Chern insulator subjected to a two-step Floquet drive. We analytically obtain the phase diagram and show that the system can exhibit different topological phases characterized by the presence and chirality of edge modes in the two bulk gaps of the Floquet quasienergy spectrum, around 0 and $\pi$. We find that the phase of the system depends on the mean but not on the amplitude of the drive. The bulk topological invariants characterizing the phases can be extracted by mapping the unitary evolution within a time period to an energetically trivial but topologically nontrivial time evolution. An extensive numerical study of the bulk topological invariants in the presence of quenched disorder reveals transitions induced by strong disorder (i) from the different topological to trivial insulator phases and (ii) from a trivial to a topological Anderson insulator phase at intermediate disorder strengths. Careful analysis of level statistics of the quasienergy spectrum indicates a “levitation-annihilation” mechanism near these transitions.

The frustration of magnetization and conductivity properties of ultrathin manganite is detrimental to their device performance, preventing their scaling down process. Here we demonstrate that the magnetism of ultrathin $\mathrm{LaMn}{\mathrm{O}}_3$ films can be restored by a $\mathrm{SrTi}{\mathrm{O}}_3$ capping layer, which engineers the surface from a symmetry breaking induced out-of-plane orbital occupancy to the recovered in-plane orbital occupancy. The stabilized in-plane orbital occupancy would strengthen the intralayer double exchange and thus recovers the robust magnetism. This method is proved to be effective for films as thin as 2 unit cells, greatly shrinking the critical thickness of 6 unit cells for ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_3$ as demonstrated previously [Wang et al., Science 349, 716 (2015)]. The achievement made in this work opens up new perspectives to an active control of surface states and thereby tailors the surface functional properties of transition metal oxides.

Recent experiments on ice formed by water under nanoconfinement provide evidence for a two-dimensional (2D) “square ice” phase. However, the interpretation of the experiments has been questioned and the stability of square ice has become a matter of debate. Partially this is because the simulation approaches employed so far (force fields and density functional theory) struggle to accurately describe the very small energy differences between the relevant phases. Here we report a study of 2D ice using an accurate wave-function based electronic structure approach, namely diffusion Monte Carlo (DMC). We find that at relatively high pressure, square ice is indeed the lowest enthalpy phase examined, supporting the initial experimental claim. Moreover, at lower pressures, a “pentagonal ice” phase (not yet observed experimentally) has the lowest enthalpy, and at ambient pressure, the “pentagonal ice” phase is degenerate with a “hexagonal ice” phase. Our DMC results also allow us to evaluate the accuracy of various density functional theory exchange-correlation functionals and force field models, and in doing so we extend the understanding of how such methodologies perform to challenging 2D structures presenting dangling hydrogen bonds.

We propose a spectroscopic method to identify the nodal gap structure in unconventional superconductors. This method is best suited for locating the horizontal line node and for pinpointing the isolated point nodes by measuring polar angle $\left(\theta \right)$ resolved zero-energy density of states $N\left(\theta \right)$. This is measured by specific heat or thermal conductivity at low temperatures under a magnetic field. We examine a variety of uniaxially symmetric nodal structures, including point and/or line nodes with linear and quadratic dispersions, by solving the Eilenberger equation in vortex states. It is found that (a) the maxima of $N\left(\theta \right)$ continuously shift from the antinodal to the nodal direction $\left({\theta }_{\mathrm{n}}\right)$ as a field increases accompanying the oscillation pattern reversal at low and high fields. Furthermore, (b) local minima emerge next to ${\theta }_{\mathrm{n}}$ on both sides, except for the case of the linear point node. These features are robust and detectable experimentally. Experimental results of $N\left(\theta \right)$ performed on several superconductors, ${\mathrm{UPd}}_2{\mathrm{Al}}_3,{\mathrm{URu}}_2{\mathrm{Si}}_2,{\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, and ${\mathrm{UPt}}_3$, are examined and commented on in light of the present theory.

Recent studies show superconductivity provides new perspectives on spintronics. We study a heterostructure composed of an $s$-wave superconductor and a cubic chiral magnet that stabilizes a topological spin texture, a skyrmion. We propose a supercurrent-induced spin torque, which originates from the spin-orbit coupling, and we show that the spin torque can drive a skyrmion in an efficient way that reduces Joule heating. We also study the band structure of Bogoliubov quasiparticles and show the existence of Weyl points, whose positions can be controlled by the magnetization. This results in an effective magnetic field acting on the Weyl quasiparticles in the presence spin textures. Furthermore, the tilt of the Weyl cones can also be tuned by the strength of the spin-orbit coupling, and we propose a possible realization of type-II Weyl points.

We perform quantum Monte Carlo simulations of the itinerant-localized periodic Kondo-Heisenberg model for the underdoped cuprates to calculate the associated spin correlation functions. The strong electron correlations are shown to play a key role in the abrupt destruction of the quasi-long-range antiferromagnetic order in the lightly doped regime.

We present Auger-electron-detected magnetic resonance (AEDMR) experiments on phosphorus donors in silicon, where the selective optical generation of donor-bound excitons is used for the electrical detection of the electron spin state. Because of the long dephasing times of the electron spins in isotopically purified ${}^{28}\mathrm{Si}$, weak microwave fields are sufficient, which allow one to realize broadband AEDMR in a commercial electron spin resonance resonator. Implementing Auger-electron-detected electron nuclear double resonance (ENDOR), we further demonstrate the optically assisted control of the nuclear spin under conditions where the hyperfine splitting is not resolved in the optical spectrum. Compared to previous studies, this significantly relaxes the requirements on the sample and the experimental setup, e.g., with respect to strain, isotopic purity, and temperature. We show AEDMR of phosphorus donors in silicon with natural isotope composition, and discuss the feasibility of ENDOR measurements also in this system.

We predict that a strong nonreciprocity in the resonance spectra of Dirac quantum dots can be induced by the Berry phase. The nonreciprocity arises in relatively weak magnetic fields and is manifest in anomalously large field-induced splittings of quantum dot resonances which are degenerate at $B=0$ due to time-reversal symmetry. This exotic behavior, which is governed by field-induced jumps in the Berry phase of confined electronic states, is unique to quantum dots in Dirac materials and is absent in conventional quantum dots. The effect is strong for gapless Dirac particles and can overwhelm the $B$-induced orbital and Zeeman splittings. A finite Dirac mass suppresses the effect. The nonreciprocity, predicted for generic two-dimensional Dirac materials, is accessible through Faraday and Kerr optical rotation measurements and scanning tunneling spectroscopy.

We build a tight-binding Hamiltonian describing Co/Ni over graphene, contemplating ATOP (a Co/Ni atom on top of each carbon atom of one graphene sublattice) and HCP (one Co/Ni atom per graphene plaquette) configurations. For the ATOP configuration the orbitals involved, for the Co/Ni, are the $d_{z^2-r^2}$, which most strongly couples to one graphene sublattice and the $d_{xz},d_{yz}$ orbitals that couple directly to the second sublattice site. Such configuration is diagonal in pseudospin and spin space, yielding electron doping of the graphene and antiferromagnetic ordering in the primitive cell in agreement with DFT calculations. The second, HCP, configuration is symmetric in the graphene sublattices and only involves coupling to the $d_{xz},d_{yz}$ orbitals. The register of the lattices in this case allows for a new coupling between nearest-neighbor sites, generating nondiagonal terms in the pseudospin space and novel spin-kinetic couplings mimicking a spin-orbit coupling generated by a magnetic coupling. The resulting proximity effect in this case yields ferromagnetic order in the graphene substrate. We derive the band structure in the vicinity of the $K$ points for both configurations, the Bloch wave functions and their spin polarization.

The Boolean algebra is the natural theoretical framework for a classical information treatment. The basic logical operations are usually performed using logic gates. In this Rapid Communication we demonstrate that not, or, and and gates can be realized exploiting the near-field radiative interaction in $N$-body systems with phase change materials. With the recent development of a photon thermal transistor and thermal memory, this result paves the way for a full information treatment and smart solutions for active thermal management at nanoscale with photons.

We study the electron-electron interaction effects on topological phase transitions by the ab initio quantum Monte Carlo simulation. We analyze two-dimensional class A topological insulators and three-dimensional Weyl semimetals with the long-range Coulomb interaction. The direct computation of the Chern number shows the electron-electron interaction modifies or extinguishes topological phase transitions.

We investigated the electronic structure of $[{\left(\mathrm{SrIr}{\mathrm{O}}_3\right)}_m,(\mathrm{SrTi}{\mathrm{O}}_3\left)\right](m=1,2,\mathrm{and}\infty )$ superlattice (SL) thin films with optical spectroscopy and first principles calculations. Our optical results confirmed the existence of the $J_{\mathrm{eff}}=1/2$ states in SL samples, similar to the bulk Ruddlesden-Popper series $\mathrm{S}{{\mathrm{r}}_n}_{+1}\mathrm{I}{\mathrm{r}}_n{\mathrm{O}}_{3n+1}$ iridates. Apart from this similarity, in the SL samples, we observed red shifts of the characteristic optical excitations in the $J_{\mathrm{eff}}=1/2$ state and an enhancement of the low-energy spectral weight, which implies a reduction in the effective electron correlation for bands near the Fermi energy. The density functional theory plus Coulomb interactions $\left(\mathrm{DFT}+U\right)$ calculations suggested that the $\mathrm{SrTi}{\mathrm{O}}_3$ layer intervened between $\mathrm{SrIr}{\mathrm{O}}_3$ layers in the SLs activated additional hopping channels between the Ir ions, thus increasing the bandwidth and reducing the effective strength of the correlations. This paper demonstrates that fabrication of iridium-based heterostructures can be used to finely tune electronic structures via alteration of their local orbital environments.

We use a nonequilibrium implementation of the dynamical cluster approximation (DCA) to study the effect of short-range correlations on the dynamics of the two-dimensional Falicov-Kimball model after an interaction quench. As in the case of single-site dynamical mean-field theory, thermalization is absent in DCA simulations, and for quenches across the metal-insulator boundary, nearest-neighbor charge correlations in the nonthermal steady state are found to be larger than in the thermal state with identical energy. We investigate to what extent it is possible to define an effective temperature of the trapped state after a quench. Based on the ratio between the lesser and retarded Green's function, we conclude that a roughly thermal distribution is reached within the energy intervals corresponding to the momentum-patch dependent subbands of the spectral function. The effectively different chemical potentials of these distributions, however, lead to a very hot, or even negative, effective temperature in the energy intervals between these subbands.

In this work we put forward a first-principles approach and propose an accurate diagrammatic approximation to calculate the time-resolved (TR) and angle-resolved photoemission spectrum of systems with excitons. We also derive an alternative formula to the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the relaxed regime characterized by the presence of quasistationary excitons and vanishing polarization. The nonequilibrium self-energy diagrams are evaluated using excited Green's functions; since this is not standard, the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated in a first-principles manner. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR and angle-resolved photoemission.

We have studied the role of spin-dependent processes on conductivity in polyfluorene (PFO) thin films by preforming continuous wave (cw) electrically detected magnetic resonance (EDMR) spectroscopy at temperatures between 10 K and room temperature using microwave frequencies between about 1 GHz and 20 GHz, as well as pulsed EDMR at the $X$ band (10 GHz). Variable frequency EDMR allows us to establish the role of spin-orbit coupling in spin-dependent processes whereas pulsed EDMR allows for the observation of coherent spin motion effects. We used PFO for this study in order to allow for the investigation of the effects of microscopic morphological ordering since this material can adopt two distinct intrachain morphologies: an amorphous (glassy) phase, in which monomer units are twisted with respect to each other, and an ordered (β) phase, where all monomers lie within one plane. In thin films of organic light-emitting diodes, the appearance of a particular phase can be controlled by deposition parameters and solvent vapor annealing, and is verified by electroluminescence spectroscopy. Under bipolar charge-carrier injection conditions, we conducted multifrequency cw EDMR, electrically detected Rabi spin-beat experiments, and Hahn echo and inversion-recovery measurements. Coherent echo spectroscopy reveals electrically detected electron-spin-echo envelope modulation due to the coupling of the carrier spins to nearby nuclear spins. Our results demonstrate that, while conformational disorder can influence the observed EDMR signals, including the sign of the current changes on resonance as well as the magnitudes of local hyperfine fields and charge-carrier spin-orbit interactions, it does not qualitatively affect the nature of spin-dependent transitions in this material. In both morphologies, we observe the presence of at least two different spin-dependent recombination processes. At room temperature and 10 K, polaron-pair recombination through weakly spin-spin coupled intermediate charge-carrier pair states is dominant, while at low temperatures, additional signatures of spin-dependent charge transport through the interaction of polarons with triplet excitons are seen in the half-field resonance of a triplet spin-1 species. This additional contribution arises since triplet lifetimes are increased at lower temperatures. We tentatively conclude that spectral broadening induced by hyperfine coupling is slightly weaker in the more ordered β-phase than in the glassy phase since protons are more evenly spaced, whereas broadening effects due to spin-orbit coupling, which impacts the distribution of $g$-factors, appear to be somewhat more significant in the β-phase.