We analyze $2+1d$ and $3+1d$ bosonic symmetry protected topological (SPT) phases of matter protected by onsite symmetry group $G$ by using dual bulk and boundary approaches. In the bulk, we study an effective field theory, which upon coupling to a background flat $G$ gauge field furnishes a purely topological response theory. The response action evaluated on certain manifolds, with appropriate choice of background gauge field, defines a set of SPT topological invariants. Further, SPTs can be gauged by summing over all isomorphism classes of flat $G$ gauge fields to obtain Dijkgraaf-Witten topological $G$ gauge theories. These topological gauge theories can be ungauged by first introducing and then proliferating defects that spoil the gauge symmetry. This mechanism is related to anyon condensation in $2+1d$ and condensing bosonic gauge charges in $3+1d$. In the dual boundary approach, we study $1+1d$ and $2+1d$ quantum field theories that have $G$ 't-Hooft anomalies that can be precisely canceled by (the response theory of) the corresponding bulk SPT. We show how to construct/compute topological invariants for the bulk SPTs directly from the boundary theories. Further, we sum over boundary partition functions with different background gauge fields to construct $G$ characters that generate topological data for the bulk topological gauge theory. Finally, we study a $2+1d$ quantum field theory with a mixed ${\mathbb{Z}}_2^{T/R}\times \text{U}\left(1\right)$ anomaly where ${\mathbb{Z}}_2^{T/R}$ is time-reversal/reflection symmetry, and the $\text{U}\left(1\right)$ could be a 0-form or 1-form symmetry depending on the choice of time reversal/reflection action. We briefly discuss the bulk effective action and topological response for a theory in $3+1d$ that cancels this anomaly. This signals the existence of SPTs in $3+1d$ protected by 0,1-form $\text{U}\left(1\right)\times {\mathbb{Z}}_2^{T,R}$.

We use resonant elastic x-ray scattering to determine the evolution of magnetic order in ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ below $T_{\mathrm{N}}=9.5$ K, as a function of temperature and applied magnetic field. We find an A-type antiferromagnetic structure with in-plane magnetic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our ab initio electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ is also present in ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, but is gapped for $T<T_{\mathrm{N}}$ due to the breaking of $C_3$ symmetry by the magnetic structure.

The antiferromagnetically (AFM) ordered state of ${\mathrm{GdRh}}_2{\mathrm{Si}}_2$ which consists of AFM-stacked ferromagnetic layers is investigated by magnetic resonance spectroscopy. The almost isotropic ${\mathrm{Gd}}^{3+}$ paramagnetic resonance becomes anisotropic in the AFM ordered region below 107 K. The emerging internal anisotropic exchange fields are still small enough to allow an investigation of their magnetization dynamics using a standard microwave-frequency magnetic resonance technique. We could characterize this anisotropy of the excitation at 9 and 34 GHz and found that the material can serve as an interesting illustration of what happens with the dynamics of the order parameter as the anisotropy restrictions become very soft. We have worked out in detail how the magnetic resonance shifts due to a characteristic property of the system, namely, the retardation of the magnetization when the sample is rotated in an external field. To describe the weak in-plane anisotropic behavior, we derived an AFM resonance condition in closed analytical formulas.

We report the synthesis, crystal structure, physical properties, and first-principles calculations of a vanadium-based oxytelluride ${\mathrm{Rb}}_{1-\delta }{\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ ($\delta \approx 0.2$). The crystal structure bears two-dimensional ${\mathrm{V}}_2\mathrm{O}$ square nets sandwiched with tellurium layers, mimicking the structural motifs of cuprate and iron-based superconductors. The material exhibits metallic conductivity with dominant hole-type charge carriers. A weak metal-to-metal transition takes place at $\sim 100$ K, which is conformably characterized by a slight kink/hump in the electrical resistivity, jumps in the Hall and Seebeck coefficients, a minute drop in the magnetic susceptibility, and a small peak in the heat capacity. Neither Bragg-peak splitting nor superlattice reflections can be detected within the resolution of conventional x-ray diffractions. The band-structure calculations show that $\mathrm{V}-3d$ orbitals dominate the electronic states at around Fermi energy where a $d_{yz}/d_{xz}$ orbital polarization shows up. There are three Fermi-surface sheets that seem unfavorable for nesting. Our results suggest an orbital or spin-density-wave order for the low-temperature state and, upon suppression of the competing order, emergence of superconductivity could be expected.

It is known that, in a pure one-dimensional case, the charge-density-wave phase is destroyed by a magnetic field, whereas the spin-density-wave (SDW) phase does not “feel” the field. In reality, the SDW phase is often observed in quasi-one-dimensional (Q1D) conductors due to the so-called “nesting” property of their electron spectra. We show that, in the latter case, a high magnetic field generates some “antinesting” term in a Q1D electron spectrum, which destroys the SDW phase. We suggest performing the corresponding experiments in SDW phases of the real Q1D organic conductors with chemical formula ${\left(\mathrm{TMTSF}\right)}_{2}X$ ($X={\mathrm{PF}}_6$, ${\mathrm{ClO}}_4$, etc.).

Perovskite-structured $\mathrm{AgNb}{\mathrm{O}}_3$ is a promising lead-free ferroelectric material that at room temperature exhibits weak ferroelectric behavior with a large polarization under an applied electric field. Here we report first-principles molecular dynamics (FPMD) simulations of monocrystalline $\mathrm{AgNb}{\mathrm{O}}_3$ over a range of temperatures to examine the microscopic polarization-switching mechanism. Polarization switching is found to occur at temperatures around 200 K and above; regardless of whether the simulations commence from the antiferroelectric Pbcm structure or ferroelectric $Pmc{2}_1$ structure, above 200 K the crystal fluctuates between the two forms. The FPMD are consistent with the coexistence of the two phases at room temperature, which can explain the mixed ferroelectric/antiferroelectric behavior, such as double P-E hysteresis loops, observed experimentally.

We have investigated the structural and magnetic properties of $S=3/2{\mathrm{Cr}}^{3+}$ Heisenberg-like kagome lattice antiferromagnets ${\mathrm{Cs}}_2{\mathrm{K}}_{}{\mathrm{Cr}}_3{\mathrm{F}}_{12}$ and ${\mathrm{Cs}}_2{\mathrm{Na}}_{}{\mathrm{Cr}}_3{\mathrm{F}}_{12}$ using single crystals. Each compound has a slightly distorted kagome lattice, although the space groups differ for the two compounds. The magnetic data suggest that each compound exhibits the formation of antiferromagnetic ordering with in-plane-spin canting approximately below 6 K. The in-plane coplanar structures are clearly different from disordered ground states of the isostructural ${\mathrm{Ti}}^{3+}$ compounds and out-of-plane coplanar structures of the ${\mathrm{V}}^{3+}$ compounds, which is mainly caused by the differences in the strength of quantum fluctuation and the strength of magnetic anisotropy for these three spin systems. In contrast, as well as the ${\mathrm{Ti}}^{3+}$ and ${\mathrm{V}}^{3+}$ compounds, the magnetization curves show 1/3-magnetization-plateaulike anomalies.

The Landau-Ginzburg-Wilson Hamiltonian for the McCoy-Wu strip-random Ising model on a two-dimensional lattice is studied. In the Landau-Ginzburg-Wilson Hamiltonian, the random temperature only depends on one of the coordinates ($x$), the horizontal coordinate. The difference (rather than differential) saddle point equation is solved numerically. In the critical regime, there exist locally ordered regions, where the saddle point solutions are not zero. Due to the translational invariance in the vertical direction, the locally ordered regions are striplike in the vertical direction. In the excited state solutions, there exist horizontal domain walls across the locally ordered regions. It is found that these horizontal domain walls are stabilized by free-energy barriers due to the discreteness. The stability conditions for such domain walls are studied. Usually, the domain wall across multiple locally ordered regions can be stable. These multiple locally ordered regions form an elementary cluster. The criterion for multiple locally ordered regions forming an elementary cluster is obtained. At the saddle point level, we get an effective Hamiltonian in which the elementary clusters behave as Ising spins and are coupled with their neighbors. For the random temperatures with narrow uniform probability distributions, the distribution of elementary cluster size, the horizontal and vertical bonds between elementary clusters are calculated numerically. The vertical bonds are extremely strong and the horizontal bonds are extremely weak. The distribution of vertical bonds is exponential. There is an exponentially small probability of elementary clusters with extremely large size. These elementary clusters with extremely large size will cause the susceptibility divergent in a range of temperature. The random temperature with binary probability distributions is also studied. The qualitative properties are the same. The Griffiths-McCoy singularity in the saddle point solutions does not depend on the distribution of random temperature.

The paper investigates the coexistence and interplay of spin and mass superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate. Superfluidity is possible only in the presence of uniaxial anisotropy (linear and quadratic Zeeman effect). This follows from the topology of the order-parameter space (vacuum manifold). According to the Landau criterion, the critical phase gradients, both for mass and spin supercurrents, vanish at the phase transition from the easy-plane to the easy-axis anisotropy. However, mass superfluidity is still possible at the phase transition. This is because the Landau criterion signals instability only with respect to nonsingular vortices with special ratio between circulations of mass and spin currents. Phase slips produced by these vortices are not sufficient for complete decay of supercurrents. Full decay of supercurrents requires phase slips with vortices of another topological class and larger energy. These phase slips are suppressed by energetic barriers up to the upper critical velocity (gradient) exceeding the Landau critical velocity. The upper critical velocity does not vanish nor has any anomaly in the critical point at the phase transition from the easy-plane to the easy-axis anisotropy.

In the present work we propose that two-time correlations of Majorana edge localized fermions constitute a novel and versatile toolbox for assessing the topological phases of one-dimensional open lattices. Using analytical and numerical calculations in the Kitaev model, we uncover universal relationships between the decay of the short-time correlations and a particular family of out-of-time-ordered correlators, which provide direct experimental alternatives to the quantitative analysis of the system regime, either normal or topological. Furthermore, we show that the saturation of two-time correlations possesses features of an order parameter. Finally, we find that violations of Leggett-Garg inequalities can indicate the topological-normal phase transition by looking at different qubits formed by pairing local and nonlocal edge Majorana fermions.

Designing novel multiferroic materials with simultaneous ferroelectric and magnetic orders has been one of the focal points over the last decade due to the promising applications and rich physics involved. In this study, using epitaxial strain (up to 3.8%) as tuning knob, we successfully introduce multiferroicity with prominent high-temperature ferroelectricity into the paraelectric $\mathrm{SrMn}{\mathrm{O}}_3$. More interestingly, the experimental temperature-dependent ferroelectric and magnetic studies suggest that the emergent antiferromagnetic order below 100 K greatly enhances the ferroelectric polarization due to the spin-order-induced ionic displacements. We envision that the strain-mediated spin-phonon coupling can be utilized as a pathway to discover functionalities in a wide range of antiferromagnetic insulators with delicate epitaxial manipulations.

The structural phase transitions of different materials, including metal to metal, metal to semiconductor, and semiconductor to semiconductor transitions, were explored using methods based on the random phase approximation. Transition pressures for Si, Ge, SiC, GaAs, ${\mathrm{SiO}}_2$, Pb, C, and BN from their stable low-pressure phases to certain high-pressure phases were computed with several semilocal density functionals and from the adiabatic connection fluctuation-dissipation formulation of density functional theory at zero temperature. In addition to the random phase approximation (RPA), three approximate beyond-RPA methods were also investigated to determine the impact of exchange-correlation kernel corrections. Results at finite temperature were obtained with the inclusion of zero-point energy contributions from the phonon spectra. We find that including temperature effects is most important for systems with nearly degenerate phases such as for boron nitride and carbon. In combination with thermal corrections, the kernel-corrected correlation methods deliver high accuracy compared to experimental data and can serve as a useful benchmark method in place of more expensive correlated calculations.

Band alignment at the interface between evaporated silver films and Zn- or O-terminated polar orientations of ZnO is explored by combining soft and hard x-ray photoemissions on native and hydrogenated surfaces. Ultraviolet photoemission spectroscopy (UPS) is used to track variations of work function, band bending, ionization energy, and Schottky barrier during silver deposition. The absolute values of band bending and the bulk position of the Fermi level are determined on continuous silver films by hard x-ray photoemission spectroscopy (HAXPES) through a dedicated modeling of core levels. Hydrogenation leads to the formation of $\sim 0.3$ monolayer of donorlike hydroxyl groups on both ZnO-O and ZnO-Zn surfaces and to the release of metallic zinc on ZnO-Zn. However, no transition to an accumulation layer is observed. On bare surfaces, silver adsorption is cationic on $\text{ZnO}\left(000\overline{1}\right)\text{-O}$ [anionic on ZnO(0001)-Zn] at the earliest stages of growth as expected from polarity healing before adsorbing as a neutral species. UPS and HAXPES data appear quite consistent. The two surfaces undergo rather similar band bendings for all types of preparation. The downward band bending of $V_{bb,\text{ZnO-O}}=-0.4$ eV and $V_{bb,\text{ZnO-Zn}}=-0.6$ eV found for the bare surfaces is reinforced upon hydrogenation ($V_{bb,\text{ZnO-O}+\mathrm{H}}=-1.1$ eV, $V_{bb,\text{ZnO-Zn}+\mathrm{H}}=-1.2$ eV). At the interface with Ag, a unique value of band bending of $-0.75$ eV is observed. While exposure to atomic hydrogen modulates strongly the energetic positions of the surface levels, a similar Schottky barrier of 0.5–0.7 eV is found for thick silver films on the two surfaces.

A method, called the adaptive cluster approximation (ACA), for single-impurity Anderson models is proposed. It is based on the reduced density-matrix functional theory, where the one-particle reduced density matrix is used as the basic variable. The adaptive cluster approximation introduces a unitary transformation of the bath states such that the effect of the bath is concentrated to a small cluster around the impurity. For this small effective system, one can then either calculate the reduced density-matrix functional numerically exactly from Levy's constrained-search formalism or approximate it by an implicit approximation of the reduced density-matrix functional. The method is evaluated for single-impurity Anderson models with finite baths. The method converges rapidly to the exact result with the size of the effective bath.

Energy levels of organic molecular films exert paramount influence on the electronic properties of organic semiconductors. Recently the effect of electrostatic energy was highlighted as an origin of such peculiar phenomena as the molecular-orientation dependence and continuous tuning of energy levels. However, the mechanism has been discussed mostly based on theoretical work and has not been adequately supported by experiments. In this work, we propose a procedure to evaluate the electrostatic and electronic polarization energies in organic films solely from the experimental data obtained by ultraviolet photoelectron and low-energy inverse photoelectron spectroscopies. We apply it to the energy levels of thin films of 6,13-pentacenequinone on $\mathrm{Si}{\mathrm{O}}_2$ substrates with three different molecular orientations. The obtained electrostatic energies are fully consistent with the theoretical results at different levels such as the first-principles calculations and the electrostatic energy of charge-multipole interactions. The present work also underlines the importance of the charge-permanent quadrupole interaction which is the leading term of the electrostatic energy in the film of nonpolar molecules.

We theoretically study the spin susceptibility of Dirac semimetals using linear-response theory. The spin susceptibility is decomposed into an intraband contribution and an interband contribution. We obtain analytical expressions for the intraband and interband contributions of massless Dirac fermions. The spin susceptibility is independent of the Fermi energy, whereas it depends on the cutoff energy, which is introduced to regularize the integration. We find that the cutoff energy is appropriately determined by comparing the results for the Wilson-Dirac lattice model, which approximates the massless Dirac Hamiltonian around the Dirac point. We also calculate the spin susceptibility of massive Dirac fermions for the model of topological insulators. We discuss the effect of the band inversion and the strength of spin-orbit coupling.

A time-domain analysis is presented of the transient field excited by a vertical electric dipole on a graphene sheet described through a local scalar conductivity model valid in the intraband regime. The analysis is carried out by means of the double-deformation method of Tsang and Kong [J. Math. Phys. 20, 1170 (1979)], a modified modal technique which allows for defining a causal surface-plasmon-polariton (SPP) contribution to the total transient field that is also computationally very efficient for a graphene sheet. Numerical results are presented and validated against independent numerical full-wave simulations. The proposed approach provides a viable route to the direct time-domain investigation of the role of SPPs in graphene-based structures employed in ultra-wide-band nanosystems.

Modal coupling between frequency-degenerate resonances of an optical ring resonator is a commonly observed phenomenon that results in adverse mode splitting. Traditionally, this coupling is attributed to Rayleigh scattering of a propagating electromagnetic wave into its associated degenerate counterpropagating mode from small perturbations to the dielectric material of the resonator. We have chosen to reframe the problem of intracavity Rayleigh scattering by considering the optical ring resonator as an infinitely long, one-dimensional photonic crystal (PhC) that possesses a lattice constant equal to the perimeter of the ring. Through application of Bloch-Floquet theory, we show that modal coupling between degenerate resonances of a ring can effectively be described as the formation of photonic frequency bands in the dispersion relation of the resonator. We additionally demonstrate that the Bragg planes of the PhC lattice coincide with the phase matching conditions for constructive interference in the ring. Finally, we show that the magnitude of frequency splitting of a particular resonance is proportional to its associated coefficient in the Fourier expansion of the ring's periodic dielectric function. The fine control of the mode splitting that can be obtained in this way provides a straightforward way to obtain ring resonances with anomalous dispersion in arbitrary wavelength ranges.