The level of electronic correlation has been one of the key questions in understanding the nature of superconductivity. Among the iron-based superconductors, the iron chalcogenide family exhibits the strongest electron correlations. To gauge the correlation strength, we performed a systematic angle-resolved photoemission spectroscopy study on the iron chalcogenide series ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_x{\mathrm{Te}}_{1-x}$ $\left(0<x<0.59\right)$, a model system with the simplest structure. Our measurement reveals an incoherent-to-coherent crossover in the electronic structure as the selenium ratio increases and the system evolves from a weakly localized to a more itinerant state. Furthermore, we found that the effective mass of bands dominated by the $d_{xy}$ orbital character significantly decreases with increasing selenium ratio, as compared to the $d_{xz}/d_{yz}$ orbital-dominated bands. The orbital-dependent change in the correlation level agrees with theoretical calculations on the band structure renormalization, and may help to understand the onset of superconductivity in ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_x{\mathrm{Te}}_{1-x}$.

The family of La-based copper oxide sulfides includes LaCuOS, the prototypical transparent conducting oxide sulfide when doped, as well as $\mathrm{L}{\mathrm{a}}_5\mathrm{C}{\mathrm{u}}_6{\mathrm{O}}_4{\mathrm{S}}_7$, a compound with a layered structure analogous to LaCuOS but with a row of O atoms substituted by S atoms in the LaO layer. We propose that $\mathrm{L}{\mathrm{a}}_5\mathrm{C}{\mathrm{u}}_6{\mathrm{O}}_4{\mathrm{S}}_7$ is an instance of an oxide-sulfide intrinsic transparent conductor (i.e., not requiring doping), while, so far, only oxides have been proposed as intrinsic transparent conductors. A one-dimensional (1D) atom chain as that found in ${\mathrm{La}}_5{\mathrm{Cu}}_6{\mathrm{O}}_4{\mathrm{S}}_7$ often promotes in related systems a Peierls distortion with respect to the equispaced chain geometry, whereby adjacent atoms form dimers. Such a distortion is expected to open a band gap and lower the total energy, thus leading to thermodynamic stability. In $\mathrm{L}{\mathrm{a}}_5\mathrm{C}{\mathrm{u}}_6{\mathrm{O}}_4{\mathrm{S}}_7$ we find that the configuration with undimerized 1D chains (the α phase) is both metallic and thermodynamically unstable. The fully dimerized, Peierls configurations (the β phase) exhibits an insulating band gap, however it is energetically unstable. The stable structure has chains with a $\sim 66\%$ degree of dimerization; in this structure the energetically preferred repeating chain motif is one in which two dimers are separated by one S atom, while chain segments with directly adjacent dimers are energetically unfavorable. The stable configuration is metallic with electron depletion in the $\mathrm{C}{\mathrm{u}}_2{\mathrm{S}}_2$ layer with respect to the $\mathrm{Cu}{d}^{10}$ valence configuration that effectively amounts to creating a hole in the Cu bands at the valence maximum. In the stable partially dimerized phase the intraband, plasmonic transitions are weak while the onset of the strong interband transitions is at around 3 eV, which are both properties that enable transparency. In this paper we explain (i) why $\mathrm{L}{\mathrm{a}}_5\mathrm{C}{\mathrm{u}}_6{\mathrm{O}}_4{\mathrm{S}}_7$ has an incomplete Peierls distortion, (ii) how the incomplete Peierls distortion in $\mathrm{L}{\mathrm{a}}_5\mathrm{C}{\mathrm{u}}_6{\mathrm{O}}_4{\mathrm{S}}_7$ creates the coexistence of intrinsic transparency and conductivity which is otherwise unusual in oxide sulfides, and (iii) discuss the agreement between the predicted and the measured structure.

By a combination of first-principles calculations and semiclassical Boltzmann transport theory, we investigate the effect of epitaxial strain on the electronic structure and transport properties of ${\mathrm{PtCoO}}_2$ and ${\mathrm{PdCoO}}_2$. In contrast to the rather uniform elastic response of both systems, we predict for ${\mathrm{PtCoO}}_2$ a high sensitivity of the out-of-plane transport properties to strain, which is not present in ${\mathrm{PdCoO}}_2$. At ambient temperature we identify a considerable absolute change in the thermopower from $-107\mu \mathrm{V}/\mathrm{K}$ at $-5\%$ compressive strain to $-303\mu \mathrm{V}/\mathrm{K}$ at $+5\%$ tensile strain. This remarkable response is related to distinct changes of the Fermi surface, which involve the crossing of two additional bands at a moderate compressive in-plane strain. Combining our transport results with available experimental data on electrical and lattice thermal conductivity we predict a thermoelectric figure of merit of up to $ZT=0.25$ at $T=600$ K for strained ${\mathrm{PtCoO}}_2$.

We investigate the dynamics of the exciton formation and relaxation on a picosecond time scale following a pulsed photoexcitation of a semiconductor. The study is conducted in the framework of the density matrix theory complemented with the dynamics controlled truncation scheme. We truncate the phonon branch of the resulting hierarchy of equations and propose the form of coupling among single-phonon-assisted and higher-order phonon-assisted density matrices so as to ensure the energy and particle-number conservation in a closed system. Time scales relevant for the exciton formation and relaxation processes are determined from numerical investigations performed on a one-dimensional model for the values of model parameters representative of a typical organic and inorganic semiconductor. The exciton dynamics is examined for different values of central frequency of the exciting field, temperature, and microscopic model parameters, such as the strengths of carrier-carrier and carrier-phonon couplings. We find that for typical organic semiconductor parameters, formation of bound excitons occurs on a several-hundred-femtosecond time scale, while their subsequent relaxation and equilibration take at least several picoseconds. These time scales are consistent with recent experimental studies of the exciton formation and relaxation in conjugated polymer-based materials.

We study valley-dependent anomalous thermomagnetic effects, including the Nernst and Ettingshausen effects, in a graphene monolayer that is subjected to a staggered sublattice potential and off-resonant circularly polarized light. It is found that a topological phase transition in this system can significantly affect the signs of the Nernst conductivity as well as the Ettingshausen thermal conductivity, which provides an alternative method to characterize the phase transition between band and topological insulators. At the topological phase-transition point, pure valley-polarized electric and heat currents are generated. In contrast to traditional thermomagnetism, an anomalous thermomagnetic figure of merit is formulated and used to characterize conversion efficiency. The theoretical approach, including numerical calculations and analytical treatment, can also be used to study the same properties of other graphenelike materials.

We report a glass-ferroic composite (in short “glass-ferroic”) in ferroic materials, an analog of the composite of glassy and crystalline phases (glass-crystal composite, e.g., semicrystalline polymer). The formation of glass-ferroic (i.e., the existence of residual ferroic glass) stems from a time-dependent crystallization of the ferroic glass. Moreover, glass-ferroics show two types of transition characteristics depending on the thermal hysteresis of crystallization transition as exemplified in $\mathrm{T}{\mathrm{i}}_{48.7}\mathrm{N}{\mathrm{i}}_{51.3}$ and $\mathrm{P}{\mathrm{b}}_{0.87}\mathrm{L}{\mathrm{a}}_{0.13}\mathrm{Z}{\mathrm{r}}_{0.4}\mathrm{T}{\mathrm{i}}_{0.6}{\mathrm{O}}_3$. Based on experimental results, a generic phase diagram is established to include all ferroic states, i.e., ferroic crystal, ferroic glass, and glass-ferroic. Being the third class of ferroic materials, glass-ferroics may open a new avenue for achieving novel properties and designing ferroic phase-change memory devices.

We report that the hydrogenation of a single crystal $2H\text{−}{\mathrm{MoS}}_2$ induces a novel-intermediate phase between 2H and 1T phases on its surface, i.e., the large-area, uniform, robust, and surface array of atomic stripes through the intralayer atomic-plane gliding. The total energy calculations confirm that the hydrogenation-induced atomic stripes are energetically most stable on the ${\mathrm{MoS}}_2$ surface between the semiconducting 2H and metallic 1T phase. Furthermore, the electronic states associated with the hydrogen ions, which is bonded to sulfur anions on both sides of the ${\mathrm{MoS}}_2$ surface layer, appear in the vicinity of the Fermi level $\left(E_{\mathrm{F}}\right)$ and reduces the band gap. This is promising in developing the monolayer-based field-effect transistor or vanishing the Schottky barrier for practical applications.

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a $s{p}^3$ diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.

We analyze general properties of the perturbation expansion for two-dimensional quantum critical metals with singular forward scattering, such as metals at an Ising nematic quantum critical point and metals coupled to a $U\left(1\right)$ gauge field. We derive asymptotic properties of fermion loops appearing as subdiagrams of the contributing Feynman diagrams—for large and small momenta. Substantial cancellations are found in important scaling limits, which reduce the degree of divergence of Feynman diagrams with boson legs. Implementing these cancellations, we obtain improved power-counting estimates that yield the true degree of divergence. In particular, we find that perturbative contributions to the boson self-energy are generally ultraviolet convergent for a dynamical critical exponent $z<3$, and divergent beyond three-loop order for $z\ge 3$.

Using density functional theory combined with quasiharmonic approximation, we investigate the thermal expansion behaviors of three different types $(\alpha ,\beta$, and $\gamma )$ of graphyne, which is a two-dimensional carbon allotrope composed of $sp$ and $s{p}^2$ bonds. For each type of graphyne, we obtain the temperature dependent area variation by minimizing its free energy calculated by considering all the phonon modes in the whole Brillouin zone. We find that all three types of graphyne exhibit negative in-plane thermal expansion up to $T\lesssim 1000$ K. The observed in-plane thermal contraction can be attributed partially to the ripple effect, similarly in graphene. The ripple effect itself, however, is not sufficient to explain the anomalously larger thermal contraction found in graphyne than in graphene. Our deliberate analysis on the phonon modes observed in graphyne enables us to reveal another source causing such thermal expansion anomaly. We find that there are particular phonon modes with frequencies of around a few hundreds of ${\mathrm{cm}}^{-1}$ existing exclusively in graphyne that may fill empty spaces resulting in area reduction. These modes are identified as “rigid unit modes” corresponding to the libration of each rigid unit composed of $s{p}^2$ bonds.

Exciton-polariton modes arising from interaction between bound excitons in monolayer thin semiconductor sheets and photons in a Fabry-Perot microcavity are considered theoretically. We calculate the dispersion curves, mode lifetimes, Rabi splitting, and Hopfield coefficients of these structures for two nearly 2D semiconductor materials, ${\text{MoS}}_2$ and ${\text{WS}}_2$, and suggest that they are interesting for studying the rich physics associated with the Bose-Einstein condensation of exciton polaritons. The large exciton binding energy and dipole allowed exciton transitions, in addition to the relatively easily controllable distance between the semiconductor sheets, are the advantages of this system in comparison with traditional GaAs or CdTe based semiconductor microcavities. In particular, in order to mimic the rich physical properties of the quantum degenerate mixture of two bosonic species of dilute atomic gases with tunable interspecies interaction, we put forward a structure containing two semiconductor sheets separated by some atomic-scale distance $\left(l\right)$ using a nearly 2D dielectric (e.g., h-BN), which offers the possibility of tuning the interaction between two exciton-polariton Bose-Enstein condensates. We show that the dynamics of this structure are ruled by two coupled Gross-Pitaevskii equations with the coupling parameter $\sim l^{-1}$.

Inelastic scattering and carrier capture by defects in semiconductors are the primary causes of hot-electron-mediated degradation of power devices, which holds up their commercial development. At the same time, carrier capture is a major issue in the performance of solar cells and light-emitting diodes. A theory of nonradiative (multiphonon) inelastic scattering by defects, however, is nonexistent, while the theory for carrier capture by defects has had a long and arduous history. Here we report the construction of a comprehensive theory of inelastic scattering by defects, with carrier capture being a special case. We distinguish between capture under thermal equilibrium conditions and capture under nonequilibrium conditions, e.g., in the presence of an electrical current or hot carriers where carriers undergo scattering by defects and are described by a mean free path. In the thermal-equilibrium case, capture is mediated by a nonadiabatic perturbation Hamiltonian, originally identified by Huang and Rhys and by Kubo, which is equal to linear electron-phonon coupling to first order. In the nonequilibrium case, we demonstrate that the primary capture mechanism is within the Born-Oppenheimer approximation (adiabatic transitions), with coupling to the defect potential inducing Franck-Condon electronic transitions, followed by multiphonon dissipation of the transition energy, while the nonadiabatic terms are of secondary importance (they scale with the inverse of the mass of typical atoms in the defect complex). We report first-principles density-functional-theory calculations of the capture cross section for a prototype defect using the projector-augmented wave, which allows us to employ all-electron wave functions. We adopt a Monte Carlo scheme to sample multiphonon configurations and obtain converged results. The theory and the results represent a foundation upon which to build engineering-level models for hot-electron degradation of power devices and the performance of solar cells and light-emitting diodes.

A giant magnetopiezoelectric effect has been revealed in samarium ferroborate. The effective piezomodulus is increased more than twice in the antiferromagnetic phase and it is reduced by a high magnetic field. The nature of the effect is in the joint contribution of both magnetoelectric and magnetoelastic interactions. The evolu-tion of this contribution in the magnetic field is caused by the growth of the magnetic energy including the spin reorientation. Additional data concerning the behavior of the high-frequency magnetocapacitance have been obtained. The parameters of magnetoelectric and magnetoelastic couplings and the magnetic anisotropy constant in the basal plane have been determined.

We show that the magnetic anisotropy in spinel-structure ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ thin films exhibits a strain dependence in which compressive strain induces an out-of-plane magnetic easy axis and tensile strain induces an in-plane easy axis, exactly opposite to the behavior reported for the related compound ${\mathrm{CoFe}}_2{\mathrm{O}}_4$. We use density functional theory calculations within the LSDA+$U$ approximation to reproduce and explain the observed behavior. Using second-order perturbation theory, we analyze the anisotropy tensor of the ${\mathrm{Co}}^{2+}$ ions in both octahedral and tetrahedral coordination, allowing us to extend our results to spinels with general arrangements of ${\mathrm{Co}}^{2+}$ ions.

A common feature of topological phases of matter is the fractionalization of the quantum number in their low-energy excitations. Such information is encoded in their ground state wave functions, but emerges in the bipartite entanglement spectra. The symmetric extensive bipartition is an effective novel method to create deconfined fractionalized edge particles in the reduced subsystem, which lead to quantum critical behavior associated with the transition from the topological phase to its adjacent trivial phase. Here we report the interesting results revealed by applying this method to the one-dimensional SO(5) symmetric valence-bond solid state being a spin-2 symmetry protected topological phase. From the finite-size entanglement spectrum, we find the lowest level to be an SO(5) singlet with a logarithmic entanglement entropy. Surprisingly, the first excited level is also an SO(5) singlet and the spectral gap scales with the subsystem size as $L_A^{-\nu }$ with $\nu \simeq 1.978$. In the thermodynamic limit, a novel quantum criticality emerges with SO(5) spinons and their four-body singlet bound states as elementary excitations, hence ruling out the possibility of being described by a conformal field theory. Moreover, the entanglement Hamiltonian can be determined as an SO(5) symmetric nearest neighbor spin-3/2 quadruple-quadruple interaction with a negative coupling. Our work thus demonstrates the power of this new method in the study of quantum criticality encoded in the topological ground states.

The calculation of finite temperature electron spin resonance (ESR) spectra for concrete specified crystal configurations is a very important issue in the study of quantum spin systems. Although direct evaluation of the Kubo formula by means of numerical diagonalization yields exact results, memory and CPU time restrictions limit the applicability of this approach to small system sizes. Methods based on the time evolution of a single pure quantum state can be used to study larger systems. One such method exploits the property that the expectation value of the autocorrelation function obtained for a few samples of so-called thermal typical states yields a good estimate of the thermal equilibrium value. In this paper, we propose a new method based on a Wiener-Khinchin-like theorem for quantum system. By comparison with exact diagonalization results, it is shown that both methods yield correct results. As the Wiener-Khinchin-based method involves sampling over thermal typical states, we study the statistical properties of the sampling distribution. Effects due to finite observation time are investigated and found to differ for the two methods but it is also found that, for both methods, the effects vanish as the system size increases. We present ESR spectra of the one-dimensional XXZ Heisenberg chain of up to 28 spins and discuss the dependence of separation of double peaks on the chain length.

We report an experimental investigation that shows how magnetic vortices are generated and cross a current carrying superconducting strip when illuminated by a bright (∼MeV) and fast (<500 ps duration) infrared light pulse. The work has been carried out using a strike-and-probe electro-optic technique on a device consisting of a parallel superconducting strip configuration, with wide spacing between the strips to allow the interaction of the photons with a single strip. We find that photons hitting one strip induce a collective current redistribution in the parallel strips, which we can quantitatively account for in the framework of the London model by including the effect of generated and trapped magnetic vortices in the superconducting loops formed by the two adjacent slots. The amount of trapped vorticity and its dependence on increasing current density flowing in the illuminated strip is in good agreement with the photon-assisted unbinding of vortex-antivortex pairs. This work allows us to gain a deeper understanding of the interaction between photons and current carrying superconducting strips.

We develop the technique to study the spin-wave dynamics of the full-polarized state of the Dzyaloshinskii-Moriya helimagnets by polarized small-angle neutron scattering. We have experimentally proven that the spin-waves dispersion in this state has the anisotropic form. We show that the neutron scattering image displays a circle with a certain radius which is centered at the momentum transfer corresponding to the helix wave vector in helimagnetic phase $k_s$, which is oriented along the applied magnetic field $\mathbf{H}$. The radius of this circle is directly related to the spin-wave stiffness of this system. This scattering depends on the neutron polarization showing the one-handed nature of the spin waves in Dzyaloshinskii-Moriya helimagnets in the full-polarized phase. We show that the spin-wave stiffness $A$ for MnSi helimagnet decreased twice as the temperature increases from zero to the critical temperature $T_c$.

Inspired by recent developments in constructing novel Dirac liquid boundary states of a three-dimensional (3D) topological insulator, we propose one possible two-dimensional boundary state of a 3D bosonic symmetry protected topological state with $U{\left(1\right)}_e⋊Z_2^T\times U{\left(1\right)}_s$ symmetry. This boundary theory is described by a $(2+1)$-dimensional quantum electrodynamics $\left({\mathrm{QED}}_3\right)$ with two flavors of Dirac fermions $\left(N_f=2\right)$ coupled with a noncompact U(1) gauge field, $\mathcal{L}={\sum }_{j=1}^2{\overline{\psi }}_j{\gamma }_{\mu }({\partial }_{\mu }-i{a}_{\mu }){\psi }_j-i{A}_{\mu }^s\overline{{\psi }_i}{\gamma }_{\mu }{\tau }_{ij}^z{\psi }_j+\frac{i}{2\pi }{\epsilon }_{\mu \nu \rho }{a}_{\mu }{\partial }_{\nu }{A}_{\rho }^e$, where $a_{\mu }$ is the internal noncompact U(1) gauge field, and $A_{\mu }^s$ and $A_{\mu }^e$ are two external gauge fields that couple to $U{\left(1\right)}_s$ and $U{\left(1\right)}_e$ global symmetries, respectively. We demonstrate that this theory has a “self-dual” structure, which is a fermionic analog of the self-duality of the noncompact ${\mathrm{CP}}^1$ theory with easy plane anisotropy. Under the self-duality, the boundary action takes exactly the same form except for an exchange between $A_{\mu }^s$ and $A_{\mu }^e$. The self-duality may still hold after we break one of the U(1) symmetries (which makes the system a bosonic topological insulator), with some subtleties that will be discussed.

The identification of defect levels from photoluminescence spectroscopy is a useful but challenging task. Density-functional theory (DFT) is a highly valuable tool to this aim. However, the semilocal approximations of DFT that are affected by a band gap underestimation are not reliable to evaluate defect properties, such as charge transition levels. It is now established that hybrid functional approximations to DFT improve the defect description in semiconductors. Here we demonstrate that the use of hybrid functionals systematically stabilizes donor defect states in the lower part of the band gap for many defects, impurities or vacancies, in III-V and in II-VI semiconductors, even though these defects are usually considered as acceptors. These donor defect states are a very general feature and, to the best of our knowledge, have been overlooked in previous studies. The states we identify here may challenge the older assignments to photoluminescent peaks. Though appealing to screen quickly through the possible stable charge states of a defect, semilocal approximations should not be trusted for that purpose.