Suppression of electron motion under an alternating current (ac) electric field is examined in a one-dimensional Hubbard model. Utilizing three complementary calculation methods, it is found unambiguously that magnitudes of the kinetic-energy suppressions are influenced sensitively by the Coulomb interaction as well as the electron density. The phase and frequency in the ac field do not bring about major effects. The results are interpreted as a combined effect of the Coulomb interaction and the ac field, and provide a guiding principle for the photocontrol of correlated electron motion.

We investigate the behavior of identical dipolar fermions with aligned dipole moments in two-dimensional multilayers at zero temperature. We consider density instabilities that are driven by the attractive part of the dipolar interaction and, for the case of bilayers, we elucidate the properties of the stripe phase recently predicted to exist in this interaction regime. When the number of layers is increased, we find that this “attractive” stripe phase exists for an increasingly larger range of dipole angles, and if the interlayer distance is sufficiently small, the stripe phase eventually spans the full range of angles, including the situation where the dipole moments are aligned perpendicular to the planes. However, in this regime, we expect the behavior to be strongly modified by the binding of dipoles between layers. In the limit of an infinite number of layers, we derive an analytic expression for the mean-field interlayer effects in the density-density response function and, using this result, we find that the stripe phase is replaced by a collapse of the dipolar system.

Using polarization-resolved electronic Raman scattering we study underdoped, optimally doped, and overdoped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping ranges. In the superconducting state, we observe two distinct superconducting pair-breaking peaks corresponding to one large and one small superconducting gap. In addition, we detect a collective mode below the superconducting transition in the $B_{2g}$ channel and determine the evolution of its binding energy with doping. Possible scenarios are proposed to explain the origin of the in-gap collective mode. In the superconducting state of the underdoped regime, we detect a reentrance transition below which the spectral background changes and the collective mode vanishes.

Spin-orbit coupling in organic crystals is responsible for many spin-relaxation phenomena, going from spin diffusion to intersystem crossing. With the goal of constructing effective spin-orbit Hamiltonians to be used in multiscale approaches to the thermodynamical properties of organic crystals, we present a method that combines density functional theory with the construction of Wannier functions. In particular we show that the spin-orbit Hamiltonian constructed over maximally localized Wannier functions can be computed by direct evaluation of the spin-orbit matrix elements over the Wannier functions constructed in absence of spin-orbit interaction. This eliminates the problem of computing the Wannier functions for almost degenerate bands, a problem always present with the spin-orbit-split bands of organic crystals. Examples of the method are presented for isolated organic molecules, for monodimensional chains of Pb and C atoms and for triarylamine-based one-dimensional single crystals.

We study second-harmonic generation in centrosymmetric Weyl semimetal with broken time reversal symmetry. We calculate electric current density at the double frequency of the propagating electromagnetic field in the presence of an applied constant magnetic field, using the method of kinetic equation for electron distribution function. It is shown that the chiral anomaly contribution to second-harmonic generation in the lowest order is linearly proportional to the applied magnetic field. The limit when the chiral anomaly dominates over the Lorentz-type contribution to second-harmonic generation is discussed.

We investigate the $\mathrm{SU}\left(2N\right)$ symmetry effects with $2N>2$ on the two-dimensional interacting Dirac fermions at finite temperatures, including the valence-bond-solid transition, the Pomeranchuk effect, the compressibility, and the uniform spin susceptibility, by performing the determinant quantum Monte Carlo simulations of the half-filled $\mathrm{SU}\left(2N\right)$ Hubbard model on a honeycomb lattice. The columnar valence-bond-solid (cVBS) phase only breaks the threefold discrete symmetry and thus can survive at finite temperatures. The disordered phase in the weak coupling regime is the thermal Dirac semi-metal state, while in the strong coupling regime it is largely a Mott state in which the cVBS order is thermally melted. The calculated entropy-temperature relations for various values of the Hubbard interaction $U$ show that the Pomeranchuk effect occurs when the specific entropy is below a characteristic value of $S^{*}$—the maximal entropy per particle from the spin channel of local moments. The $\mathrm{SU}\left(2N\right)$ symmetry enhances the Pomeranchuk effect, which facilitates the interaction-induced adiabatic cooling. Our work sheds light on future explorations of novel states of matter with ultracold large-spin alkaline fermions.

We report first-principles calculations for one of the few materials that is believed to be a ferroelectric ferromagnet, ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$. Our calculations show that, contrary to what has been reported so far, bulk ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$ does not have a polarization. Instead, like ${\mathrm{BiMnO}}_3$, it crystallizes into a centrosymmetric structure with space group $C2/c$. We also predict that ${\mathrm{Bi}}_2{\mathrm{NiMnO}}_6$ will indeed be a ferroelectric ferromagnet if it is grown as an epitaxial film on a substrate with in-plane square symmetry and a lattice constant around 4 Å, such as ${\mathrm{BaTiO}}_3$ or ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$.

The coupling between the electrical transport properties of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) thin films and structural phase transitions of $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrates at $T_{\mathrm{s}}=105\mathrm{K}$ has been investigated. We found that the electrical resistivity of LSMO films exhibit a “cusp” at $T_{\mathrm{s}}$, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate competing electronic states. Our results demonstrate that small amounts of strain can tip the subtle balance of competing interactions and tune the electronic properties in correlated electron materials.

Charge ordering in the quasi-one-dimensional organic conductors ${\left(\mathrm{TMTTF}\right)}_{2}X\left(X=\mathrm{Sb}{\mathrm{F}}_6,\mathrm{As}{\mathrm{F}}_6,\mathrm{P}{\mathrm{F}}_6\right)$ was studied by using Raman spectroscopy. In the charge-ordered phase three vibrational features related to the ring breathing mode ${\nu }_{10}\left(a_g\right)$ of neutral ($\mathrm{TMTT}{\mathrm{F}}^0$) and ionized ($\mathrm{TMTT}{\mathrm{F}}^{+0.5}$ and $\mathrm{TMTT}{\mathrm{F}}^{+1}$) are observed at about 503, 507, and $526\mathrm{c}{\mathrm{m}}^{-1}$, respectively. The bands of donor molecules with charge $+0.5e$ are assigned to ferroelectric domains while the bands of neutral and fully ionized molecules to domain walls. The shape of the band at about $526\mathrm{c}{\mathrm{m}}^{-1}$, attributed to the charged domain walls (molecules with charge $+1e$), reveals important differences between salts, i.e., indicates the presence of relaxor ferroelectricity which is well seen in ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$.

Quantum corrections to charge transport can give rise to an oscillatory magnetoconductance, typically observed in mesoscopic samples with a length shorter than or comparable to the phase coherence length. Here, we report the observation of magnetoconductance oscillations periodic in magnetic field with an amplitude of the order of $e^2/h$ in macroscopic samples of highly oriented pyrolytic graphite (HOPG). The observed effect emerges when all carriers are confined to their lowest Landau levels. We argue that this quantum interference phenomenon can be explained by invoking moiré superlattices with a discrete distribution in periodicity. According to our results, when the magnetic length ${\ell }_B$, the Fermi wavelength ${\lambda }_F$, and the length scale of fluctuations in local chemical potential are comparable in a layered conductor, quantum corrections can be detected over centimetric length scales.

We study a system of interacting matter quasiparticles strongly coupled to photons inside an optomechanical cavity. The resulting normal modes of the system are represented by hybrid polaritonic quasiparticles, which acquire effective nonlinearity. Its strength is influenced by the presence of a mechanical mode and depends on the resonance frequency of the cavity. This leads to an interactive type of optomechanical coupling, which is distinct from previously studied dispersive and dissipative couplings in optomechanical systems. The emergent interactive coupling is shown to generate effective optical nonlinearity terms of high order, which are quartic in the polariton number. We consider particular systems of exciton polaritons and dipolaritons, and show that the induced effective optical nonlinearity due to interactive coupling can exceed in magnitude the strength of Kerr nonlinear terms, such as those arising from polariton-polariton interactions. As applications, we show that the higher-order terms give rise to localized bright flattop solitons, which may form spontaneously in polariton condensates.

We study the effect of the electron-phonon coupling on vibrational eigenmodes of nano- and micromechanical systems made of semiconductors with equivalent energy valleys. We show that the coupling can lead to a strong mode nonlinearity. The mechanism is the lifting of the valley degeneracy by the strain. The redistribution of the electrons between the valleys is controlled by a large ratio of the electron-phonon coupling constant to the electron chemical potential or temperature. We find the quartic in the strain terms in the electron free energy, which determine the amplitude dependence of the mode frequencies. This dependence is calculated for silicon microsystems. It is significantly different for different modes and the crystal orientation, and can vary nonmonotonously with the electron density and temperature.

We address the statistics of continuous weak linear measurement on a few-state quantum system that is subject to a conditioned quantum evolution. For a conditioned evolution, both the initial and final states of the system are fixed: the latter is achieved by the postselection in the end of the evolution. The statistics may drastically differ from the nonconditioned case, and the interference between initial and final states can be observed in the probability distributions of measurement outcomes as well as in the average values exceeding the conventional range of nonconditioned averages. We develop a proper formalism to compute the distributions of measurement outcomes, and evaluate and discuss the distributions in experimentally relevant setups. We demonstrate the manifestations of the interference between initial and final states in various regimes. We consider analytically simple examples of nontrivial probability distributions. We reveal peaks (or dips) at half-quantized values of the measurement outputs. We discuss in detail the case of zero overlap between initial and final states demonstrating anomalously big average outputs and sudden jump in time-integrated output. We present and discuss the numerical evaluation of the probability distribution aiming at extending the analytical results and describing a realistic experimental situation of a qubit in the regime of resonant fluorescence.

We develop a theory for the optical conductivity of doped ABC-stacked multilayer graphene including the effects of electron-electron interactions. Applying the quantum kinetic formalism, we formulate a set of pseudospin Bloch equations that govern the dynamics of the nonequilibrium density matrix driven by an external ac electric field under the influence of Coulomb interactions. These equations reveal a dynamical mechanism that couples the Drude and interband responses arising from the chirality of pseudospin textures in multilayer graphene systems. We demonstrate that this results in an interaction-induced enhancement of the Drude weight and plasmon frequency strongly dependent on the pseudospin winding number. Using bilayer graphene as an example, we also study the influence of higher-energy bands and find that they contribute considerable renormalization effects not captured by a low-energy two-band description. We argue that this enhancement of Drude weight and plasmon frequency occurs generally in materials characterized by electronic chirality.

The observation of spin-related phenomena of microcavity polaritons has been limited due to the weak Zeeman effect of nonmagnetic semiconductors. We demonstrate that the incorporation of magnetic ions into quantum wells placed in a nonmagnetic microcavity results in enhanced effects of magnetic field on exciton-polaritons. We show that in such a structure the Zeeman splitting of exciton-polaritons strongly depends on the photon-exciton detuning and polariton wave vector. Our experimental data are explained by a model where the impact of magnetic field on the lower polariton state is directly inherited from the excitonic component, and the coupling strength to the cavity photon is modified by an external magnetic field.

The phase diagram of uranium has been explored up to 100 GPa and 2000 K by means of ab initio molecular dynamics (AIMD) simulations. The lattice dynamics and energetics of the stable phases observed experimentally in this range of pressure and temperature are studied in this work. The phonon spectra of the $\alpha$ and $\gamma$ phases are shown to evolve strongly as a function of temperature, unveiling the huge anharmonic effects present in this material. If the elastics constants and the bulk and shear moduli of the $\gamma$ phase do not disclose any temperature effects, the shear modulus of the $\alpha$ phase decreases strongly as a function of temperature. Using the pressure- and temperature-dependent vibrational density of states and the Gibbs free energy of these two structures, we found a line transition between the $\alpha$ and $\gamma$ phases which slightly underestimates the experimental one. Coherently with experiments, the bct structure is never found stable between 0 and 100 GPa.

Phase-change memories are usually associated with GeTe-${\mathrm{Sb}}_2{\mathrm{Te}}_3$ quasibinary alloys, where the large optical contrast between the crystalline and amorphous phases is attributed to the formation of resonant bonds in the crystalline phase, which has a rocksalt-like structure. The recent findings that tetrahedrally bonded ${\mathrm{Ga}}_2{\mathrm{Te}}_3$ possesses a similarly large property contrast and very low thermal conductivity in the crystalline phase and undergoes low-energy switching [H. Zhu et al., Appl. Phys. Lett. 97, 083504 (2010); K. Kurosaki et al., Appl. Phys. Lett. 93, 012101 (2008)] challenge the existing paradigm. In this work we report on the local structure of the crystalline and amorphous phases of ${\mathrm{Ga}}_2{\mathrm{Te}}_3$ obtained from x-ray absorption measurements and ab initio simulations. Based on the obtained results, a model of phase change in ${\mathrm{Ga}}_2{\mathrm{Te}}_3$ is proposed. We argue that efficient switching in ${\mathrm{Ga}}_2{\mathrm{Te}}_3$ is due to the presence of primary and secondary bonding in the crystalline phase originating from the high concentration of Ga vacancies, whereas the structural stability of both phases is ensured by polyvalency of Te atoms due to the presence of lone-pair electrons and the formation of like-atom bonds in the amorphous phase.

The structural and magnetic properties of multiferroic CuO have been studied by means of neutron and x-ray powder diffraction at pressures up to 11 and 38 GPa, respectively, and by first-principles theoretical calculations. Anomalous lattice compression is observed, with enlargement of the lattice parameter $a$, reaching a maximum at $P=13\mathrm{GPa}$, followed by its reduction at higher pressures. The lattice distortion of the monoclinic structure at high pressures is accompanied by a progressive change of the oxygen coordination around Cu atoms from the square fourfold towards the octahedral sixfold coordination. The pressure-induced evolution of the structural properties and electronic structure of CuO was successfully elucidated in the framework of full-electronic density functional theory calculations with range-separated HSE06, and meta–generalized gradient approximation hybrid M06 functionals. The antiferromagnetic (AFM) ground state with a propagation vector $q=(0.5,0,-0.5)$ remains stable in the studied pressure range. From the obtained structural parameters, the pressure dependencies of the principal superexchange magnetic interactions were analyzed, and the pressure behavior of the Néel temperature as well as the magnetic transition temperature from the intermediate incommensurate AFM multiferroic state to the commensurate AFM ground state were evaluated. The estimated upper limit of the Néel temperature at $P=38\mathrm{GPa}$ is about 260 K, not supporting the previously predicted existence of the multiferroic phase at room temperature and high pressure.

Organic semiconductors find increasing importance in spin transport devices due to the modulation and control of their properties through chemical synthetic versatility. The organic materials have been used as interlayers between two ferromagnet (FM) electrodes in organic spin valves, as well as for magnetic spin manipulation of metal-organic complexes at the molecular level. In the latter, the substrate-induced magnetic switching in a paramagnetic molecule has been evoked extensively but studied by delicate surface spectroscopies. Here we present evidence of the substantial magnetic switching in a thin film of the paramagnetic molecule, tris(8-hydroxyquinoline)iron(III) ($\mathrm{Fe}{\mathrm{q}}_3$) deposited on a FM substrate, using the magnetoresistance response of electrical spin-injection in an organic spin valve structure, as well as the inverse-spin-Hall effect induced by state-of-art pulsed microwave spin-pumping. We show that interfacial spin control at the molecular level may lead to a macroscopic organic spin transport device, thus bridging the gap between organic spintronics and molecular spintronics.