We present polarized Raman spectra of ${\mathrm{CaMn}}_2{\mathrm{O}}_4$ (marokite) measured in several exact scattering configurations from oriented single crystals. Most of the 11A${}_g$ + 12B${}_{1g}$ + 10B${}_{2g}$ + 9B${}_{3g}$ Raman-allowed modes are identified and assigned to definite atomic motions by comparison to the predictions of ab initio lattice dynamical calculations. The Raman modes at 474 cm${}^{-1}$ ($A_g$), 478 cm${}^{-1}$ ($B_{1g}$), and 614 cm${}^{-1}$ ($B_{1g}$) exhibit anomalous softening at temperatures below $T_{\mathrm{N}}\approx 220$ K due to a spin-phonon coupling which depends strongly on the phonon eigenvectors. There are no indications for a structural transition in the temperature range between 80 and 850 K. We argue quantitatively that the mechanism of the phonon softening below $T_{\mathrm{N}}$ is similar to that in the undoped manganese perovskites.

The electronic structures of fcc Ce are calculated for large supercells with varying disorder by use of density-functional theory. Thermal disorder induces fluctuations of the amplitude of the magnetic moments and an increase in the average moments in the high-volume phase. The ferromagnetic solutions move towards a lower volume than in calculations for the perfectly ordered lattice. Therefore, disorder contributes via entropy to the stabilization of the $\gamma$ phase at high $T$, and it is important for an understanding of the $\alpha \text{−}\gamma$ transition. Core level spectroscopy would be a means to detect disorder through the spread of Madelung shifts and local exchange splittings.

Pr-based perovskite cobaltites, such as ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ and (${\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y{)}_{1-x}{\mathrm{Ca}}_x\mathrm{Co}{\mathrm{O}}_{3-\delta }$ have recently been discovered to undergo a first-order metal-insulator transition on cooling, thought to arise from an unusual shift in electron occupancy from Pr to hybridized Co-O orbitals. While this transition is known to generally suppress long-range ferromagnetic ordering, the true nature of the magnetic ground state remains unclear. In this work, we have performed structural, magnetic, transport, magnetotransport, and small-angle neutron scattering measurements on (${\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y{)}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ ($0.000\le y\le 0.200$) in order to develop a complete microscopic picture of the ground state and a full appreciation of the evolution of the first-order transition with Y doping. Our magnetization and zero-field resistivity measurements confirm the presence of an abrupt temperature-dependent metal-insulator transition on cooling, accompanied by a sharp drop in magnetization, setting in between $y$ = 0.050 and 0.075. Small-angle neutron scattering measurements suggest the presence of critical ferromagnetic fluctuations above the metal-insulator transition for $y$ = 0.075, indicating that the system is poised to order ferromagnetically. This magnetic scattering is suppressed at the metal-insulator transition but reemerges at lower temperatures (below 40–50 K) due to the formation of a ferromagnetic cluster state. The clusters have a mean correlation length of ∼50 Å at 4 K, although magnetic inhomogeneity occurs across a broad spectrum of length scales, evidencing a highly inhomogeneous ground state, which we relate to the numerous sources of chemical and structural disorder. Interestingly, the magnetically inhomogeneous state manifests an intercluster magnetoresistance effect and a strong field-cooling effect on the low-temperature transport. We interpret these results, quite generally, in terms of the electronic shift from Pr to Co driving the system from the ferromagnetic side of the generic perovskite cobaltite magnetic phase diagram to the insulating side (where ferromagnetic clusters are well known to exist). These results thus shed significant light on the formation of the magnetically inhomogenous ground state of Pr-based cobaltites undergoing a first-order metal-insulator transition and indeed provide clear and direct evidence of such.

The isovalent-substituted iron-pnictide superconductor ${\mathrm{SrFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$ ($x=0.35$) has a slightly higher optimum critical temperature than the similar system ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$, and its parent compound ${\mathrm{SrFe}}_2$${\mathrm{As}}_2$ has a much higher Néel temperature than ${\mathrm{BaFe}}_2$${\mathrm{As}}_2$. We have studied the band structure and the Fermi surfaces of optimally doped ${\mathrm{SrFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around $(0,0)$ and two electronlike FSs around $(\pi ,\pi )$ have been observed as in the case of ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$. Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the $k_z$ direction than the corresponding one in BaFe(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$, and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$. A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the $d_{xy}$ and/or $d_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

The interface superconductivity in ${\mathrm{LaAlO}}_3-{\mathrm{SrTiO}}_3$ heterostructures reveals a nonmonotonic behavior of the critical temperature as a function of the two-dimensional density of charge carriers. We develop a theoretical description of interface superconductivity in strongly polar heterostructures, based on the dielectric function formalism. The density dependence of the critical temperature is calculated, accounting for all phonon branches including different types of optical (interface and half-space) and acoustic phonons. The longitudinal-optic- and acoustic-phonon mediated electron-electron interaction is shown to be the dominating mechanism governing the superconducting phase transition in the heterostructure.

We study the Ginzburg-Landau free energy functional for two coupled $U\left(1\right)$ charge order parameters describing two nonequivalent charge orders with wave vector $\mathbf{Q}$ detected in x-ray and STM measurements of underdoped cuprates. We do not rely on a mean-field analysis, but rather utilize a field-theoretical technique suitable to study the interplay between vortex physics and discrete symmetry breaking in two-dimensional systems with $U\left(1\right)$ symmetry. Our calculations support the idea that in the clean systems there are two transitions: from a high-temperature disordered state into a state with a composite charge order which breaks time-reversal symmetry, but leaves $U\left(1\right)$ fields disordered, and then into a state with quasi-long-range order in the $U\left(1\right)$ fields.

We report on a photoinduced transient state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_2$O${}_{6+\delta }$ in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature $T_c$, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low-frequency imaginary conductivity ${\sigma }_2\left(\omega \right)$. Above $T_c$, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive ${\sigma }_2\left(\omega \right)$ that increases with decreasing frequency. These features are observed up to room temperature in ${\mathrm{YBa}}_2{\mathrm{Cu}}_2$O${}_{6.45}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_2$O${}_{6.5.}$ The data above $T_c$ can be fitted by hypothesizing that the light establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10${}^4$ cm${}^2$/V sec at 100 K, with a density of charge carriers similar to the below-$T_c$ superfluid density. Our results are indicative of highly unconventional nonequilibrium physics and open new prospects for optical control of complex solids.

Inspired by the Chern-Simons effective theory description of symmetry protected topological (SPT) phases in two dimensions, we present a projective construction for many-body wave functions of SPT phases. Using this projective construction, we can systematically write down trial wave functions of SPT phases on a lattice. An explicit example of SPT phase with U(1) symmetry is constructed for two types of bosons with filling ${\nu }_{b_1}={\nu }_{b_2}=\frac{1}{2}$ per site on a square lattice. We study continuous phase transitions between different U(1)-SPT phases based on projective construction. The effective theory around the critical point is an emergent ${\mathrm{QED}}_3$ with fermion number $N_f=2$. Such a continuous phase transition, however, needs fine tuning, and in general there are intermediate phases between different U(1)-SPT phases. We show that such an intermediate phase has the same response as an anyon superconductor, and hence dub it “anyon superfluid.” A schematic phase diagram of interacting bosons with U(1) symmetry is depicted.

We investigate dynamical magnetoelectric effect owing to Nambu-Goldstone magnon resonance in helical multiferroic materials. Using the spin wave expansion, we analyzed magneto- and electroactive modes in the $J_1$-$J_2$ Heisenberg model through the spin-current mechanism. Under external magnetic field, the Nambu-Goldstone magnons accompany dynamical toroidal moments, i.e., toroidal magnons, and their resonant absorption shows anomalous features such as nonreciprocal directional dichroism and natural circular dichroism. The estimation indicates that such effects are gigantic in helical magnets.

We implement a multiorbital cluster dynamical mean-field theory (DMFT) by improving a sample update algorithm in the continuous-time quantum Monte Carlo method based on the interaction expansion. The proposed sampling scheme for the spin-flip and pair-hopping interactions in the two-orbital systems mitigates the sign problem, giving an efficient way to deal with these interactions. In particular, in the single-site DMFT, we see that the negative signs vanish. We apply the method to the two-dimensional two-orbital Hubbard model at half-filling, where we take into account the short-range spatial correlation effects within a four-site cluster. We show that, compared to the single-site DMFT results, the critical interaction value for the metal-insulator transition decreases and that the effects of the spin-flip and pair-hopping terms are less significant in the parameter region we have studied. The present method provides a firm starting point for the study of intersite correlations in multiorbital systems. It also has a wide applicable scope in terms of realistic calculations in conjunction with density functional theory.

We present a numerical scheme to reconstruct a subset of the entanglement spectrum of quantum many body systems using quantum Monte Carlo. The approach builds on the replica trick to evaluate particle number resolved traces of the first $n$ of powers of a reduced density matrix. From this information we reconstruct $n$ entanglement spectrum levels using a polynomial root solver. We illustrate the power and limitations of the method by an application to the extended Bose-Hubbard model in one dimension where we are able to resolve the quasidegeneracy of the entanglement spectrum in the Haldane-insulator phase. In general, the method is able to reconstruct the largest few eigenvalues in each symmetry sector and typically performs better when the eigenvalues are not too different.

We propose a model for the two-dimensional electron gas formed at the interface of oxide heterostructures that includes a Rashba spin-orbit coupling proportional to an electric field oriented perpendicularly to the interface. Taking into account the electron density dependence of this electric field confining the electron gas at the interface, we report the occurrence of a phase separation instability (signaled by a negative compressibility) for realistic values of the spin-orbit coupling and of the electronic band-structure parameters at zero temperature. We extend the analysis to finite temperatures and in the presence of an in-plane magnetic field, thereby obtaining two phase diagrams that exhibit a phase separation dome. By varying the gating potential, the phase separation dome may shrink and vanish at zero temperature into a quantum critical point where the charge fluctuates dynamically. Similarly, the phase separation may be spoiled by a planar magnetic field even at zero temperature leading to a line of quantum critical points.

Optical resonators offer an excellent analog of quantum two-level systems. Studying optical analogs of the electromagnetically induced transparency and the Fano interference has greatly stimulated the field of nanophotonics. In this context, the theoretical analysis is developed for the optical analog of superradiance in multiple nanoresonators. Unique superradiant effects are found for light absorption in nanoresonators, such as superradiance-induced transparency.

We present high resolution angle resolved photoemission spectroscopy (ARPES) data for ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ (blue bronze) and propose a theoretical description of these results based primarily on electron-electron interactions. The observed Fermi surface, with two quasi-one-dimensional sheets, is consistent with a ladder material with a weak interladder coupling. Hence, we base our description on spectral properties of one-dimensional ladders. The marked broadening of the ARPES line shape, a significant fraction of an eV, is interpreted in terms of spin-charge separation. A high-energy feature, which is revealed in the spectra near the Fermi momentum thanks to improved energy resolution, is seen as a signature of a higher-energy bound state of soliton excitations on a ladder.

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, in which the superconductivity of the BiS${}_2$ layer and the ferromagnetism of the ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x$ layer are induced by the F doping, in order to investigate the impact of the F doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS${}_2$ exhibits coexistence of $4f^1$ (Ce${}^{3+}$) and $4f^0$ (Ce${}^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of the Ce-S-Bi coupling channel by the F doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS${}_2$ layer and the ferromagnetic ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x$ layer.

The role of quasicylindrical waves and surface plasmon polaritons in beam shaping with resonant nanogratings is investigated. It is shown that the field on the grating surface can be strongly influenced by plasmons and quasicylindrical waves in the infrared. A method that combines far-field measurements with the fast Fourier transform to map the field amplitude at the grating surface is demonstrated. For samples with a small degree of geometric asymmetry, it is shown that the imaginary part of the transform (with null zeroth-order component) can better map the amplitude of the resonant surface waves than the full complex-valued transform. Our results will impact the study, design, and footprint of resonant nanogratings.

We introduce an integrable model of spin-polarized interacting electrons subject to a spin-conserving spin-orbit interaction. Using the Bethe ansatz and conformal field theory, we calculate the exact large-time single-electron and density correlations and find that while the spin-orbit interaction enhances the single-electron Green's function, the density correlations get suppressed. Adding a localized impurity and coupling it to the electrons so that integrability is preserved, the dynamic correlations are found to change significantly after a quantum quench with the impurity interaction switched on suddenly. When the electrons are confined to a periodic structure, the correlations are indifferent to the location of the impurity and only carry an imprint of its intrinsic properties. We conjecture that this unusual feature originates from the integrability of the model.