The vibrational properties of NiO(001) films on Ag(001) with thicknesses up to 50 monolayers (ML) are characterized with high-resolution electron energy loss spectroscopy (HREELS). For NiO growth at 300 K, four different coverage regions are distinguished by HREELS. The film-thickness-dependent Fuchs-Kliewer (FK) phonon frequency shifts and intensity changes are identified from the NiO monolayer to bulklike thick films. Characteristic changes of the vibrational properties are analyzed to resolve restructuring processes during annealing and thermal decomposition of NiO films. A quantitative comparison of the experimental data, including a line shape analysis, with the calculated loss function based on dielectric theory reveals an excellent agreement between the bulk and the NiO(001) thin film phonon properties for film thicknesses above 15 ML. In contrast, a strong FK phonon softening is observed for thin films below 5 ML that cannot be explained by dielectric theory nor phonon standing waves. This softening is attributed to the presence of surface stress, which results from the $-2\%$ lattice mismatch between NiO and Ag.

We advocate that in critical spin chains, and possibly in a larger class of one-dimensional critical models, a gap in the momentum-space entanglement spectrum separates the universal part of the spectrum, which is determined by the associated conformal field theory, from the nonuniversal part, which is specific to the model. To this end, we provide affirmative evidence from multicritical spin chains with low-energy sectors described by the ${\mathrm{SU}\left(2\right)}_2$ or the ${\mathrm{SU}\left(3\right)}_1$ Wess-Zumino-Witten model.

We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac electrons with spin-orbit interactions to characterize topological insulators. The imaginary part of the DSS appears as an absorption rate in response to a transverse ac magnetic field, just as in an electron spin resonance experiment for localized spin systems. We found that when the system is in a static magnetic field, the topological state can be identified by an anomalous resonant peak of the imaginary part of the DSS as a function of the frequency of the transverse magnetic field $\omega$. In the absence of a static magnetic field, the imaginary part of the DSS becomes a continuous function of $\omega$ with a threshold frequency ${\omega }_{\mathrm{c}}$. In this case, the topological and the trivial phases can also be distinguished by the values of ${\omega }_{\mathrm{c}}$ and by the line shapes. Thus the DSS is an experimentally observable physical quantity to characterize a topological insulator directly from bulk properties, without observing a topological transition.

We theoretically investigate the incident-photon energy ${\omega }_{\mathrm{i}}$ dependence of resonant inelastic x-ray scattering (RIXS) tuned for the Cu $L$ edge in cuprate superconductors by using the exact diagonalization technique for a single-band Hubbard model. Depending on the value of core-hole Coulomb interaction in the intermediate state, RIXS for non-spin-flip channel shows either a ${\omega }_{\mathrm{i}}$-dependent fluorescencelike or ${\omega }_{\mathrm{i}}$-independent Raman-like behavior for hole doping. An analysis of x-ray absorption suggests that the core-hole Coulomb interaction is larger than on-site Coulomb interaction in the Hubbard model, resulting in a fluorescencelike behavior in RIXS consistent with recent RIXS experiments. A shift on the high-energy side of the center of spectral distribution is also predicted for electron-doped systems though spectral weight is small. Main structures in the spin-flip channel exhibit a Raman-like behavior as expected, accompanied with a fluorescencelike behavior with small intensity.

We elaborate on an operator approach to effective medium theory for homogenization of the periodic multilayered structures composed of nonmagnetic isotropic materials, which is based on equating the spatial evolution operators for the original structure and its effective alternative. We show that the zeroth-, first-, and second-order approximations of the operator effective medium theory correspond to electric dipoles, chirality, and magnetic dipoles plus electric quadrupoles, respectively. We discover that the spatially dispersive bianisotropic effective medium obtained in the second-order approximation perfectly replaces a multilayered composite and does not suffer from the effective medium approximation breakdown that happened near the critical angle of total internal reflection found previously in the conventional effective medium theory. We establish the criterion of the validity of the conventional effective medium theory depending on the ratio of unit-cell length to the wavelength, the number of unit cells, and the angle of incidence. The operator approach to effective medium theory is applicable for periodic and nonperiodic layered systems, being a fruitful tool in the fields of metamaterials and subwavelength nanophotonics.

We probe the room temperature photoluminescence of $N$-layer molybdenum ditelluride $\left({\mathrm{MoTe}}_2\right)$ in the continuous wave (cw) regime. The photoluminescence quantum yield of monolayer ${\mathrm{MoTe}}_2$ is three times larger than in bilayer ${\mathrm{MoTe}}_2$ and 40 times greater than in the bulk limit. Mono- and bilayer ${\mathrm{MoTe}}_2$ display almost symmetric emission lines at 1.10 and 1.07 eV, respectively, which predominantly arise from direct radiative recombination of the A exciton. In contrast, $N\ge 3-\mathrm{layer}{\mathrm{MoTe}}_2$ exhibits a much reduced photoluminescence quantum yield and a broader, redshifted, and seemingly bimodal photoluminescence spectrum. The low- and high-energy contributions are attributed to emission from the indirect and direct optical band gaps, respectively. Bulk ${\mathrm{MoTe}}_2$ displays a broad emission line with a dominant contribution at 0.94 eV that is assigned to emission from the indirect optical band gap. As compared to related systems (such as ${\mathrm{MoS}}_2,{\mathrm{MoSe}}_2,{\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$), the smaller energy difference between the monolayer direct optical band gap and the bulk indirect optical band gap leads to a smoother increase of the photoluminescence quantum yield as $N$ decreases. In addition, we study the evolution of the photoluminescence intensity in monolayer ${\mathrm{MoTe}}_2$ as a function of the exciton formation rate $W_{\mathrm{abs}}$ up to $3.6\times {10}^{22}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. The line shape of the photoluminescence spectrum remains largely independent of $W_{\mathrm{abs}}$, whereas the photoluminescence intensity grows sublinearly above $W_{\mathrm{abs}}\sim {10}^{21}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. This behavior is assigned to exciton-exciton annihilation and is well captured by an elementary rate equation model.

The mechanism of pressure-induced amorphous-to-amorphous transitions (AATs) in metallic glasses (MGs) has been a subject of intense research. Most AATs in MGs were found in lanthanide-based alloys and shown to originate from $4f$ orbital delocalization. Recently, evidence of an unexpected AAT in the main-group Ca-Al MGs was reported without a satisfactory explanation. Here, based on the results of first-principles molecular dynamics calculations, the suggested AAT at 12–15 GPa in the $\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ MG is confirmed. Contrary to the common belief that the coordinate of metallic glasses with close packing cannot be increased further, the coordination around Al atoms is found to increase suddenly at the transition as a consequence of atomic migration and the aggregation of Al atoms. This transition originates from pressure-enhanced bonding between Ca $3d$ and Al $3p$ orbitals and is confirmed by the good agreement on the predicted and measured electrical conductivities. The theoretical analysis not only uncovers a mechanism of pressure-induced AAT in main-group MGs, but it can be generalized to establish a different perspective to guide the understanding of transformation phenomena in compressed MGs.

We report the polarization dependence of terahertz magnon absorption in single crystals of ${\mathrm{BiFeO}}_3$ grown by a modified floating zone method. In a (111)${}_{\mathrm{pc}}$-oriented crystal, two major magnon absorption signals were observed for all terahertz polarizations, which indicates that magnetic domains were not aligned in one of the three allowed directions. In contrast, the absorption modes in a (001)${}_{\mathrm{pc}}$-oriented crystal showed significant polarization dependence, which was unchanged even after annealing the crystal at temperatures far above the Néel point to demagnetize it. This polarization dependence coincides with that of $E$ mode phonons. Thus, we conclude that magnon and phonon in ${\mathrm{BiFeO}}_3$ are strongly coupled and the selection rules for magnon absorption are governed by the activity of $E$ mode phonons, namely, the crystalline anisotropy originating from ferroelectric polarization.

In this joint experimental and ab initio study, we focused on the influence of the chemical composition and martensite phase transition on the electronic, magnetic, optical, and magneto-optical properties of the ferromagnetic shape-memory Ni-Mn-Ga alloys. The polar magneto-optical Kerr effect (MOKE) spectra for the polycrystalline sample of the Ni-Mn-Ga alloy of ${\mathrm{Ni}}_{60}{\mathrm{Mn}}_{13}{\mathrm{Ga}}_{27}$ composition were measured by means of the polarization modulation method over the photon energy range $0.8\le h\nu \le 5.8$ eV in magnetic field up to 1.5 T. The optical properties (refractive index $n$ and extinction coefficient $k)$ were measured directly by spectroscopic ellipsometry using the rotating analyzer method. To complement experiments, extensive first-principles calculations were made with two different first-principles approaches combining the advantages of a multiple scattering Green function method and a spin-polarized fully relativistic linear-muffin-tin-orbital method. The electronic, magnetic, and MO properties of Ni-Mn-Ga Heusler alloys were investigated for the cubic austenitic and modulated 7M-like incommensurate martensitic phases in the stoichiometric and off-stoichiometric compositions. The optical and MOKE properties of Ni-Mn-Ga systems are very sensitive to the deviation from the stoichiometry. It was shown that the ab initio calculations reproduce well experimental spectra and allow us to explain the microscopic origin of the ${\mathrm{Ni}}_2\mathrm{MnGa}$ optical and magneto-optical response in terms of interband transitions. The band-by-band decomposition of the ${\mathrm{Ni}}_2\mathrm{MnGa}$ MOKE spectra is presented and the interband transitions responsible for the prominent structures in the spectra are identified.

The $N$-component London $\mathrm{U}\left(1\right)$ superconductor is expressed in terms of integer-valued supercurrents. We show that the inclusion of interband Josephson couplings introduces monopoles in the current fields, which convert the phase transitions of the charge-neutral sector to crossovers. The monopoles only couple to the neutral sector, and leave the phase transition of the charged sector intact. The remnant noncritical fluctuations in the neutral sector influence the one remaining phase transition in the charged sector, and may alter this phase transition from a $3\mathrm{D}XY$ inverted phase transition into a first-order phase transition depending on what the values of the gauge charge and the intercomponent Josephson coupling are. This preemptive effect becomes more pronounced with increasing number of components $N$, since the number of charge-neutral fluctuating modes that can influence the charged sector increases with $N$. We also calculate the gauge-field correlator, and by extension the Higgs mass, in terms of current-current correlators. We show that the onset of the Higgs mass of the photon (Meissner effect) is given in terms of a current loop blowout associated with going into the superconducting state as the temperature of the system is lowered.

We introduce the concept of a reflection quasilattice, the quasiperiodic generalization of a Bravais lattice with irreducible reflection symmetry. Among their applications, reflection quasilattices are the reciprocal (i.e., Bragg diffraction) lattices for quasicrystals and quasicrystal tilings, such as Penrose tilings, with irreducible reflection symmetry and discrete scale invariance. In a follow-up paper, we will show that reflection quasilattices can be used to generate tilings in real space with properties analogous to those in Penrose tilings, but with different symmetries and in various dimensions. Here we explain that reflection quasilattices only exist in dimensions two, three, and four, and we prove that there is a unique reflection quasilattice in dimension four: the “maximal reflection quasilattice” in terms of dimensionality and symmetry. Unlike crystallographic Bravais lattices, all reflection quasilattices are invariant under rescaling by certain discrete scale factors. We tabulate the complete set of scale factors for all reflection quasilattices in dimension $d>2$, and for all those with quadratic irrational scale factors in $d=2$.

We have conducted molecular dynamics (MD) simulations of quasi-isentropic ramp-wave compression to very high pressures over a range of strain rates from ${10}^{11}$ down to ${10}^8$ 1/s. Using scaling methods, we collapse wave profiles from various strain rates to a master profile curve, which shows deviations when material response is strain-rate dependent. Thus, we can show with precision where, and how, strain-rate dependence affects the ramp wave. We find that strain rate affects the stress-strain material response most dramatically at strains below 20%, and that above 30% strain the material response is largely independent of strain rate. We show good overall agreement with experimental stress-strain curves up to approximately 30% strain, above which simulated response is somewhat too stiff. We postulate that this could be due to our interatomic potential or to differences in grain structure and/or size between simulation and experiment. Strength is directly measured from per-atom stress tensor and shows significantly enhanced elastic response at the highest strain rates. This enhanced elastic response is less pronounced at higher pressures and at lower strain rates.

Driven dynamics across a quantum critical point is usually described by the Kibble-Zurek scaling. Although the original Kibble-Zurek scaling requires an adiabatic initial state, it has been shown that scaling behaviors exist even when the driven dynamics is triggered from a thermal equilibrium state exactly at the critical point, in spite of the breakdown of the initial adiabaticity. In this paper, we show that the existence of the scaling behavior can be generalized to the case of the initial state being a thermal equilibrium state near the critical point. We propose a scaling theory in which the initial parameters are included as additional scaling variables due to the breakdown of the initial adiabaticity. In particular, we demonstrate that for the driven critical dynamics in a closed system, the nontrivial thermal effects are closely related to the initial distance to the critical point. We numerically confirm the scaling theory by simulating the real-time dynamics of the one-dimensional quantum Ising model at both zero and finite temperatures.

Phase transitions in a classical Heisenberg spin model of a chiral helimagnet with the Dzyaloshinskii-Moriya interaction in three dimensions are numerically studied. By using the event-chain Monte Carlo algorithm recently developed for particle and continuous spin systems, we perform equilibrium Monte Carlo simulations for large systems up to about ${10}^6$ spins. Without magnetic fields, the system undergoes a continuous phase transition with critical exponents of the three-dimensional XY model, and a uniaxial periodic helical structure emerges in the low-temperature region. In the presence of a magnetic field perpendicular to the axis of the helical structure, it is found that there exists a critical point on the temperature and magnetic-field phase diagram and that above the critical point the system exhibits a phase transition with strong divergence of the specific heat and the uniform magnetic susceptibility.

Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential of the ion cores. Based on this premise, we develop a methodology for evaluating electronic thermal conductivity of metals by combining the free electron model and nonequilibrium ab initio molecular dynamics simulations. We confirm that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation, which is induced by the thermal motion of ion cores. This method directly predicts the electronic thermal conductivity of pure metals with a high degree of accuracy, without explicitly addressing any complicated scattering processes of free electrons. Our methodology offers a route to understand the physics of heat transfer by electrons at the atomistic level. The methodology can be further extended to the study of similar electron-involved problems in materials, such as electron-phonon coupling, which is underway currently.

The electronic structure and magnetic properties of the strongly correlated material ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$ are studied by using both the density-functional theory plus $U$ $\left(\mathrm{DFT}+U\right)$ method and the DFT plus Gutzwiller (DFT + G) variational method. The ground-state magnetic structure of this material obtained with $\mathrm{DFT}+U$ is consistent with recent experiments with an appropriate $U$ parameter, but its band gap is significantly overestimated by $\mathrm{DFT}+U$, even with a small Hubbard $U$ value. In contrast, the DFT + G method yields a band gap of 0.1–0.2 eV, in excellent agreement with experiment. Detailed analysis shows that the electronic and magnetic properties of ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$ are strongly affected by charge and spin fluctuations which are missing in the $\mathrm{DFT}+U$ method.

Topological entanglement entropy has been regarded as a smoking-gun signature of topological order in two dimensions, capturing the total quantum dimension of the topological particle content. An extrapolation method on cylinders has been used frequently to measure the topological entanglement entropy. Here, we show that a class of short-range entangled 2D states, when put on an infinite cylinder of circumference $L$, exhibits the entanglement Rényi entropy of any integer index $\alpha \ge 2$ that obeys $S_{\alpha }=aL-\gamma$, where $a,\gamma >0$. Under the extrapolation method, the subleading term $\gamma$ would be identified as the topological entanglement entropy, which is spurious. A nonzero $\gamma$ is always present if the 2D state reduces to a certain symmetry-protected topological 1D state, upon disentangling spins that are far from the entanglement cut. The internal symmetry that stabilizes $\gamma >0$ is not necessarily a symmetry of the 2D state, but should be present after the disentangling reduction. If the symmetry is absent, $\gamma$ decays exponentially in $L$ with a characteristic length, termed as a replica correlation length, which can be arbitrarily large compared to the two-point correlation length of the 2D state. We propose a simple numerical procedure to measure the replica correlation length through replica correlation functions. We also calculate the replica correlation functions for representative wave functions of Abelian discrete gauge theories and the double semion theory in 2D, to show that they decay abruptly to zero. This supports a conjecture that the replica correlation length being small implies that the subleading term from the extrapolation method determines the total quantum dimension.

By hard x-ray photoemission spectroscopy we have studied the electronic states of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$(100) and ${\mathrm{LaAlO}}_3/\mathrm{Nb}$ : ${\mathrm{SrTiO}}_3$(100) heterostructures, where the polar $\mathrm{LaAl}{\mathrm{O}}_3$ layer is grown on the insulating and conducting substrates, respectively. The analysis of the valence band has shown that the electron affinity is as small as $+0.4\mathrm{eV}$ in both the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures. The interface is “staggered type” (type II) in both of the heterostructures with the valence band maximum of $\mathrm{LaAl}{\mathrm{O}}_3$ above the valence band maximum of $\mathrm{SrTi}{\mathrm{O}}_3$. The valence band offset of 0.42 eV in the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure has increased to 0.61 eV in the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ as the $\mathrm{LaAl}{\mathrm{O}}_3$ component shifts towards the lower binding energy side. The electronic states of $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ have been merged with each other in the lower energy part of the valence band of the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures.

Quantum phases characterized by surfaces of gapless excitations are known to violate the otherwise ubiquitous boundary law of entanglement entropy in the form of a multiplicative log correction: $S\sim L^{d-1}logL$. Using variational Monte Carlo, we calculate the second Rényi entropy for a model wave function of the $\nu =1/2$ composite Fermi liquid (CFL) state defined on the two-dimensional triangular lattice. By carefully studying the scaling of the total Rényi entropy and, crucially, its contributions from the modulus and sign of the wave function on various finite-size geometries, we argue that the prefactor of the leading $LlogL$ term is equivalent to that in the analogous free fermion wave function. In contrast to the recent results of Shao et al. [Phys. Rev. Lett. 114, 206402 (2015)], we thus conclude that the “Widom formula” holds even in this non-Fermi liquid CFL state. More generally, our results further elucidate—and place on a more quantitative footing—the relationship between nontrivial wave function sign structure and $S\sim LlogL$ entanglement scaling in such highly entangled gapless phases.

${\mathrm{SmB}}_6$ is a mixed valence Kondo system resulting from the hybridization between localized $f$ electrons and delocalized $d$ electrons. We have investigated its out-of-equilibrium electron dynamics by means of time- and angle-resolved photoelectron spectroscopy. The transient electronic population above the Fermi level can be described by a time-dependent Fermi-Dirac distribution. By solving a two-temperature model that well reproduces the relaxation dynamics of the effective electronic temperature, we estimate the electron-phonon coupling constant $\lambda$ to range from $0.13\pm 0.03$ to $0.04\pm 0.01$. These extremes are obtained assuming a coupling of the electrons with either a phonon mode at 10 or 19 meV. A realistic value of the average phonon energy will give an actual value of $\lambda$ within this range. Our results provide an experimental report on the material electron-phonon coupling, contributing to both the electronic transport and the macroscopic thermodynamic properties of ${\mathrm{SmB}}_6$.