Spatial quantum coherence between two separated driven-dissipative polariton condensates created nonresonantly and with a different occupation is studied. We identify the regions where the condensates remain coherent with the phase difference continuously changing with the pumping imbalance and the regions where each condensate acquires its own chemical potential with phase differences exhibiting time-dependent oscillations. We show that in the mutual coherence limit the coupling consists of two competing contributions: a symmetric Heisenberg exchange and the Dzyloshinskii-Moriya asymmetric interactions that enable a continuous tuning of the phase relation across the dyad and derive analytic expressions for these types of interactions. The introduction of nonequal pumping increases the complexity of the type of problems that can be solved by polariton condensates arranged in a graph configuration. If equally pumped polaritons condensates arrange their phases to solve the constrained quadratic minimisation problem with a real symmetric matrix, the nonequally pumped condensates solve that problem for a general Hermitian matrix.

We show that a quantum phase transition from ergodic to many-body localized (MBL) phases can be induced via periodic pulsed manipulation of spin systems. Such a transition is enabled by the interplay between weak disorder and slow heating rates. Specifically, we demonstrate that the Hamiltonian of a weakly disordered ergodic spin system can be effectively engineered, by using sufficiently fast coherent controls, to yield a stable MBL phase, which in turn completely suppresses the energy absorption from external control field. Our results imply that a broad class of existing many-body systems can be used to probe nonequilibrium phases of matter for a long time, limited only by coupling to external environment.

The exchange-bias (EB) effect with sign reversal was found in $\mathrm{LuF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ ferrite-chromite, which is a weak ferrimagnet below $T_{\mathrm{N}}=265\mathrm{K}$, exhibiting antiparallel orientation of the ferromagnetic (FM) moments of the Fe and Cr sublattices due to opposite sign of the Fe-Cr Dzyaloshinskii vector, as compared to that of the Fe-Fe and Cr-Cr. The weak FM moments of the studied compound compensate each other at temperature $T_{\mathrm{comp}}=230\mathrm{K}$, leading to the net magnetic moment reversal and to observed negative magnetization, at moderate applied fields, below $T_{\mathrm{comp}}$. Both vertical and horizontal shifts from the origin were gotten in the field-cooled magnetization hysteresis loops. The EB sign was found to be positive below $T_{\mathrm{comp}}$ and negative above $T_{\mathrm{comp}}$, with nonmonotonic dependence on cooling field $H_{\mathrm{cool}}$. It sharply increases at small values of magnetic fields up to $H_{\mathrm{cool}}\sim 1\mathrm{kOe}$, then remains almost unchanged in the range 1–30 kOe and strongly decreases with further increase of $H_{\mathrm{cool}}$. This unusual behavior results from the competition of various Dzyaloshinskii-Moriya interactions between $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{C}{\mathrm{r}}^{3+}$ ions.

The spin-orbital superexchange model is derived for the cubic (perovskite) symmetry of ${\mathrm{LaMnO}}_3$, whereas real crystal structure is strongly deformed. We identify and explain three a priori important physical effects arising from tetragonal deformation: (i) the splitting of $e_g$ orbitals $\propto E_z$, (ii) the directional renormalization of $d\text{−}p$ hybridization $t_{pd}$, and (iii) the directional renormalization of charge excitation energies. Using the example of ${\mathrm{LaMnO}}_3$ crystal we evaluate their magnitude. It is found that the major effects of deformation are an enhanced amplitude of $x^2\text{−}{y}^2$ orbitals induced in the orbital order by $E_z\simeq 300$ meV and anisotropic $t_{pd}\simeq 2.0$ (2.35) eV along the $ab$ ($c$) cubic axis, in very good agreement with Harrison's law. We show that the improved tetragonal model analyzed within mean field approximation provides a surprisingly consistent picture of the ground state. Excellent agreement with the experimental data is obtained simultaneously for: (i) $e_g$ orbital mixing angle, (ii) spin exchange constants, and (iii) the temperatures of spin and orbital phase transition.

Dilute magnetic semiconductors (DMS) are nonmagnetic semiconductors doped with magnetic transition metals. The recently discovered DMS material (${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x){\left({\mathrm{Zn}}_{1-y}{\mathrm{Mn}}_y\right)}_2{\mathrm{As}}_2$ offers a unique and versatile control of the Curie temperature $T_{\mathrm{C}}$ by decoupling the spin (${\mathrm{Mn}}^{2+}$, $S=5/2$) and charge (${\mathrm{K}}^{+}$) doping in different crystallographic layers. In an attempt to describe from first-principles calculations the role of hole doping in stabilizing ferromagnetic order, it was recently suggested that the antiferromagnetic exchange coupling $J$ between the nearest-neighbor Mn ions would experience a nearly twofold suppression upon doping 20% of holes by potassium substitution. At the same time, further-neighbor interactions become increasingly ferromagnetic upon doping, leading to a rapid increase of $T_{\mathrm{C}}$. Using inelastic neutron scattering, we have observed a localized magnetic excitation at about 13 meV associated with the destruction of the nearest-neighbor Mn-Mn singlet ground state. Hole doping results in a notable broadening of this peak, evidencing significant particle-hole damping, but with only a minor change in the peak position. We argue that this unexpected result can be explained by a combined effect of superexchange and double-exchange interactions.

We present a study of the low-temperature phases of the antiferromagnetic extended classical Heisenberg model on the kagome lattice, up to third-nearest neighbors. First, we focus on the degenerate lines in the boundaries of the well-known staggered chiral phases. These boundaries have either semiextensive or extensive degeneracy, and we discuss the partial selection of states by thermal fluctuations. Then, we study the model under an external magnetic field on these lines and in the staggered chiral phases. We pay particular attention to the highly frustrated point, where the three exchange couplings are equal. We show that this point can be mapped to a model with spin-liquid behavior and nonzero chirality. Finally, we explore the effect of Dzyaloshinskii-Moriya (DM) interactions in two ways: a homogeneous and a staggered DM interaction. In both cases, there is a rich low-temperature phase diagram, with different spontaneously broken symmetries and nontrivial chiral phases.

The main candidate for the superfluid pathways in solid ${}^4\mathrm{He}$ are dislocations with Burgers vector along the hcp symmetry axis. Here we focus on the quantum behavior of a generic edge dislocation which can perform superclimb; that is, it can climb due to the superflow along its core. The role of the long-range elastic interactions between jogs is addressed by Monte Carlo simulations. It is found that such interactions do not change qualitatively the phase diagram found without accounting for the long-range forces. Their main effect consists of renormalizing the effective scale determining the compressibility of the dislocation in the Tomonaga-Luttinger liquid phase. It is also found that the quantum rough phase of the dislocation can be well described within the Gaussian approximation which features off-diagonal long-range order (ODLRO) in one dimension for the superfluid order parameter along the core.

From theory, we investigate charge localization induced by higher-frequency off-resonance light-pulse excitation in the metallic phase of $\alpha \text{−}(\text{BEDT-}{\mathrm{TTF})}_2{\mathrm{I}}_3$ by numerically solving the time-dependent Schrödinger equation in the quarter-filled extended Hubbard model for the material. Around $ea{A}^{\left(\max \right)}=1$, where $ea{A}^{\left(\max \right)}$ is the maximum amplitude of the dimensionless vector potential of the pump pulse, the charge distribution is significantly changed by photoexcitation, and the light-pulse-induced collective charge oscillations continue after photoexcitation. Furthermore, the charge dynamics depend strongly on the polarization direction of the pump pulse. These results are consistent with experiment. The magnitudes of the effective transfer integrals are reduced by strong photoexcitation, and this precursory phenomenon for dynamical localization is mainly driven by a photoinduced change in the ratio of the effective transfer integrals between the two strongest bonds. For $ea{A}^{\left(\max \right)}\gtrsim 2$, the photoinduced transition to the charge-ordered state, which can be regarded as a light-dressed state, occurs because of dynamical localization. Furthermore, the type of photogenerated charge-ordered state can be controlled by choosing $ea{A}^{\left(\max \right)}$ and the polarization direction.

Donor spins in silicon are some of the most promising qubits for upcoming solid-state quantum technologies. The nuclear spins of phosphorus donors in enriched silicon have among the longest coherence times of any solid-state system as well as simultaneous high fidelity qubit initialization, manipulation, and readout. Here we characterize the phosphorus in silicon system in the regime of “zero” magnetic field, where a singlet-triplet spin clock transition can be accessed, using laser spectroscopy and magnetic resonance methods. We show the system can be optically hyperpolarized and has $\sim 10$ s Hahn echo coherence times, even for applied static magnetic fields below Earth's field.

We demonstrate that the intensity of optical second and third harmonics generated in a one-dimensional photonic crystal–thin metal film sample is greatly enhanced in the presence of Tamm plasmons. The emergence of a localized optical Tamm state means that the local density of photonic states is resonantly enhanced at the interface of photonic crystal and metal within the vicinity of the excitation wavelength. When the fundamental radiation spectrum overlaps with the Tamm mode, the efficiency of the second-harmonic generation was enhanced by two orders of magnitude as compared with a bare metal film. In the case of the off-resonant fundamental radiation and the second-harmonic wavelength overlapping with the Tamm plasmon mode, the conversion efficiency increased by one order of magnitude. The overall intensity of the third-harmonic generation was enhanced by almost five orders of magnitude in the phase-matching conditions. In this case the fundamental radiation resonates with the fundamental Tamm plasmon, while the third-harmonic radiation resonates with the third-order Tamm plasmon.

We have studied the electronic structure of SnSe and Na-doped SnSe by means of angle-resolved photoemission spectroscopy. The valence-band top reaches the Fermi level by the Na doping, indicating that Na-doped SnSe can be viewed as a degenerate semiconductor. However, in the Na-doped system, the chemical potential shift with temperature is unexpectedly large and is apparently inconsistent with the degenerate semiconductor picture. The large chemical potential shift and anomalous spectral shape are key ingredients for an understanding of the novel metallic state with the large thermoelectric performance in Na-doped SnSe.

The valence-to-core (V2C) portion of x-ray emission spectroscopy (XES) measures the electron states close to the Fermi level. These states are involved in bonding, thus providing a measure of the chemistry of the material. In this article, we show the V2C XES spectra for several niobium compounds. The $K{\beta }^{″}$ peak in the V2C XES results from the transition of a ligand $2s$ electron into the $1s$ core-hole of the niobium, a transition allowed by hybridization with the niobium $4p$. This location in energy of this weak peak shows a strong ligand dependence, thus providing a sensitive probe of the ligand environment about the niobium.

Magneto-optical effects have been investigated over the infrared visible spectral range in $A{\mathrm{Cr}}_2{\mathrm{O}}_4$ ($A=\text{Fe},\text{Co}$) spinel oxides with noncollinear spin orders in their ground states. We found large magneto-optical Kerr rotation and ellipticity at the on-site $d-d$ transitions of the $A^{2+}$ ions located within the charge gap. The magneto-optical Kerr rotation of ${\vartheta }_{\mathrm{Kerr}}\approx {12}^{\circ }$ observed in ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ is unprecedentedly large among magnetic semiconductors and points toward the uniqueness of tetrahedrally coordinated ${\mathrm{Co}}^{2+}$ ions in generating strong magneto-optical response. Criteria of strong magneto-optical effects emerging at on-site $d-d$ transitions of transition metal ions are discussed.

The Luttinger-Ward functional (LWF) has been a starting point for conserving approximations in many-body physics for 50 years. The recent discoveries of its multivaluedness and the associated divergence of the two-particle irreducible vertex function $\mathrm{\Gamma }$ have revealed an inherent limitation of this approach. Here we demonstrate how these undesirable properties of the LWF can lead to a failure of computational methods based on an approximation of the LWF. We apply the nested cluster scheme (NCS) to the Hubbard model and observe the existence of an additional stationary point of the self-consistent equations, associated with an unphysical branch of the LWF. In the strongly correlated regime, starting with the first divergence of $\mathrm{\Gamma }$, this unphysical stationary point becomes attractive in the standard iterative technique used to solve DMFT. This leads to an incorrect solution, even in the large cluster size limit, for which we discuss diagnostics.

In this paper, we elaborate on the operator effective medium approximation developed recently in Popov et al. [Phys. Rev. B 94, 085428 (2016)] to get insight into the surface polariton excitation at the interface of a multilayer hyperbolic metamaterial (HMM). In particular, we find that HMMs with bilayer unit cells support the TE- and TM-polarized surface waves beyond the Maxwell Garnett approximation due to the spatial dispersion interpreted as effective magnetoelectric coupling. The latter is also responsible for the dependence of surface wave propagation on the order of layers in the unit cell. Elimination of the magnetoelectric coupling in three-layer unit cells complying with inversion symmetry restores the qualitative regularity of the Maxwell Garnett approximation, as well as strongly suppresses the influence of the order of layers in the unit cell.

Ferroelectrics have long been recognized as one of the candidate class of materials for applications in photovoltaic devices. Recently, ferroelectric perovskite $(\text{K},\text{Ba})(\text{Ni},\text{Nb}){\mathrm{O}}_{3-\delta }$ has been successfully synthesized and demonstrated to have a near-optimal band gap (1.39 eV), exhibiting good photovoltaic performance. However, the connection between the structural order-disorder transformation, electronic structure, bulk photovoltaic, and photocatalytic properties remains not well understood. Here, we investigate the phase transition evolutions of lead-free ${\left[{\mathrm{KNbO}}_3\right]}_{1-x}{\left[{\mathrm{BaNi}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_{3-\delta }\right]}_x$ ($\mathrm{KBNNO}x$, $x=0–0.5$) ceramics via x-ray diffraction (XRD), Raman scattering, and computational evidences. The lattice dynamics and the origin of the successive rhombohedral$\to$orthorhombic$\to$tetragonal$\to$cubic phase transitions have been systemically explored based on temperature-dependent XRD peak positions and phonon modes under different geometries. Moreover, the differences in the phase transition temperature and interior structure between the solid solution $x=0.2$ and the end member $x=0$ highlight local and nonlocal characteristics, which are helpful for understanding the photovoltaic mechanisms. Additionally, the robust photocatalytic decoloration effect on methylene blue can further confirm the photon-generated carrier behavior in the partly structural disordered orthorhombic phase. This identification of structural phases, combined with the ability to perform photocatalytic decoloration, give some insights on promising oxide applications as semiconducting ferroelectric absorbers and carrier-separating layers in photocatalytic or photovoltaic devices.

The paper reports results of the ${}^{57}\mathrm{Fe}$ Mössbauer measurements on an $\mathrm{F}{\mathrm{e}}_3\mathrm{P}{\mathrm{O}}_4{\mathrm{O}}_3$ powder sample recorded at various temperatures, including the point of magnetic phase transition $T_{\mathrm{N}}\approx 163\mathrm{K}$. The spectra measured above $T_{\mathrm{N}}$ consist of a quadrupole doublet with high quadrupole splitting of ${\mathrm{\Delta }}_{300\mathrm{K}}\approx 1.10\mathrm{mm}/\mathrm{s}$, emphasizing that $\mathrm{F}{\mathrm{e}}^{3+}$ ions are located in crystal positions with a strong electric-field gradient (EFG). To predict the sign and orientation of the main components of the EFG tensor, we calculated the EFG using the density-functional-theory approach. In the temperature range $T<T_{\mathrm{N}}$, the experimental spectra were fitted assuming that the electric hyperfine interactions are modulated when the $\mathrm{F}{\mathrm{e}}^{3+}$ spin $\left(\mathit{S}\right)$ rotates with respect to the EFG axis, and with the emergence of spatial anisotropy of the hyperfine field $H_{\mathrm{hf}}\propto \tilde{\mathit{A}}·\mathit{S}$ at ${}^{57}\mathrm{Fe}$ nuclei. These data were analyzed to estimate the components of the anisotropic hyperfine coupling tensor $\left(\tilde{\mathit{A}}\right)$. The large anharmonicity parameter, $m\approx 0.94$, of the spiral spin structure results from easy-axis anisotropy in the plane of the iron spin rotation. The temperature evolution of the hyperfine field $H_{\mathrm{hf}}\left(T\right)$ was described by the Bean-Rodbell model, which takes into account that the exchange magnetic interactions are a strong function of the lattice spacing. The obtained Mössbauer data are in qualitative agreement with previous neutron-diffraction data for a modulated helical magnetic structure in strongly frustrated $\mathrm{F}{\mathrm{e}}_3\mathrm{P}{\mathrm{O}}_4{\mathrm{O}}_3$.

We examine the pairing structure of holes injected into two distinct spin backgrounds: a short-range antiferromagnetic phase versus a symmetry protected topological phase. Based on density matrix renormalization group (DMRG) simulation, we find that although there is a strong binding between two holes in both phases, phase fluctuations can significantly influence the pair-pair correlation depending on the spin-spin correlation in the background. Here the phase fluctuation is identified as an intrinsic string operator nonlocally controlled by the spins. We show that while the pairing amplitude is generally large, the coherent Cooper pairing can be substantially weakened by the phase fluctuation in the symmetry-protected topological phase, in contrast to the short-range antiferromagnetic phase. It provides an example of a non-BCS mechanism for pairing, in which the paring phase coherence is determined by the underlying spin state self-consistently, bearing an interesting resemblance to the pseudogap physics in the cuprate.

The electron-phonon interaction causes thermal and zero-point motion shifts of electron quasiparticle (QP) energies ${\epsilon }_k\left(T\right)$. Other consequences of interactions, visible in angle-resolved photoemission spectroscopy (ARPES) experiments, are broadening of QP peaks and appearance of sidebands, contained in the electron spectral function $A(k,\omega )=-\Im m{G}_R(k,\omega )/\pi$, where $G_R$ is the retarded Green's function. Electronic structure codes (e.g., using density-functional theory) are now available that compute the shifts and start to address broadening and sidebands. Here we consider MgO and LiF, and determine their nonadiabatic Migdal self-energy. The spectral function obtained from the Dyson equation makes errors in the weight and energy of the QP peak and the position and weight of the phonon-induced sidebands. Only one phonon satellite appears, with an unphysically large energy difference (larger than the highest phonon energy) with respect to the QP peak. By contrast, the spectral function from a cumulant treatment of the same self-energy is physically better, giving a quite accurate QP energy and several satellites approximately spaced by the LO phonon energy. In particular, the positions of the QP peak and first satellite agree closely with those found for the Fröhlich Hamiltonian by Mishchenko et al. [Phys. Rev. B 62, 6317 (2000)] using diagrammatic Monte Carlo. We provide a detailed comparison between the first-principles MgO and LiF results and those of the Fröhlich Hamiltonian. Such an analysis applies widely to materials with infrared(IR)-active phonons.

We explore the effects of random and short-period crystal-phase intermixing in InAs nanowires (NW) on the carrier trapping and thermal activation behavior using correlated optical and electrical transport spectroscopy. The polytypic InAs NWs are grown by catalyst-free molecular beam epitaxy under different temperatures, resulting in different fractions of wurtzite (WZ) and zincblende (ZB) and variable short-period (∼1–4 nm) WZ/ZB stacking sequences. Temperature-dependent microphotoluminescence $\left(\mu \text{PL}\right)$ studies reveal that variations in the WZ/ZB stacking govern the emission energy and carrier confinement properties. The optical transition energies are modeled for a wide range of WZ/ZB stacking sequences using a Kronig-Penney type effective mass approximation, while comparison with experimental results suggests that polarization sheet charges on the order of ∼0.0016–0.08 C/m along the WZ/ZB interfaces need to be considered to best describe the data. The thermal activation characteristics of carriers trapped inside the short-period WZ/ZB structure are directly reproduced in the temperature-dependent carrier density evolution (4–300 K) probed by four-terminal (4T) NW-field effect transistor measurements. In particular, we find that activation of carriers in-between $\sim {10}^{16}-{10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ follows a two-step process, with activation at low temperature attributed to WZ/ZB traps and activation at high temperature being linked to surface states and electron accumulation at the InAs NW surface.