A nonadiabatic resonating Hartree-Fock method is applied to the one-dimensional extended Hubbard model with a staggered site-diagonal potential and Su-Schrieffer-Heeger-type electron-phonon coupling to clarify the lattice and electronic structures in the charge-density-wave (CDW) ground state. It is shown that small spin-Peierls-type domains and large dimerized CDW domains appear in the main CDW state having an equidistant lattice. Interference of breathing and translational motions of these domains constitutes quantum fluctuations. The natures of different domains are analyzed by Löwdin-Feshbach partitioning method. The present results, indicating the domains in the CDW ground state as quantum fluctuations, are consistent with the experimentally observed THz electric-field-induced changes in the electronic dipole moment in the neutral phase of tetrathiafulvalene-$p$-chloranil (TTF-CA).

We investigate the physics of a Tomonaga-Luttinger liquid of spin-polarized fermions superimposed on an ion chain. This compound system features (attractive) long-range interspecies interactions. By means of density matrix renormalization group techniques we compute the Tomonaga-Luttinger-liquid parameter and speed of sound as a function of the relative atom/ion density and the two quantum defect parameters, namely, the even and odd short-range phases which characterize the short-range part of the atom-ion polarization potential. The presence of ions is found to allow critical tuning of the atom-atom interaction, and the properties of the system are found to depend significantly on the short-range phases due to the atom-ion interaction. These latter dependencies can be controlled, for instance, by manipulating the ions' internal state. This allows modification of the static properties of the quantum liquid via external driving of the ionic impurities.

Using the Green functions method we study the transport properties of surface electrons of topological insulators in the presence of a correlated random exchange field. Such an exchange field may be induced by a random magnetization with correlated fluctuations. We determine the relaxation times due to scattering from the magnetization fluctuations and also from randomly distributed scalar impurities. The longitudinal charge conductivity is evaluated and discussed while accounting for vertex corrections.

A graphene superlattice is formed by a one-dimensional periodic potential and is characterized by the emergence of new Dirac points in the electronic structure. The group velocity of graphene's massless Dirac fermions at the new points is drastically reduced, resulting in a measurable effect in the conductance spectroscopy. We show here that tunnel spectroscopy using a superconducting hybrid junction is more sensitive to the formation of Dirac points in the spectrum of graphene superlattices due to the additional contribution of Andreev processes. We examine the transport properties of a graphene-based superlattice-superconductor hybrid junction and demonstrate that a superlattice potential can coexist with proximity-induced superconducting correlations. Both effects contribute to change graphene's spectrum for subgap energies, and as a result, the normalized tunneling conductance features sharp changes for voltages proportional to the energy separation between the original and newly generated Dirac points. Consequently, the superconducting differential conductance provides an excellent tool to reveal how the new Dirac points emerge from the original band. This result is robust against asymmetries and finite-size effects in the superlattice potential and is improved by an effective doping comparable to the superconducting gap.

We study dissipation in inhomogeneous two-dimensional electron systems. We predict a relatively strong current-induced spatial asymmetry in the heating of the electron and phonon systems, even if the inhomogeneity responsible for the electrical resistance is symmetric with respect to the current direction. We also show that the heat distributions in the hydrodynamic and impurity-dominated limits are essentially different. In particular, within a wide, experimentally relevant interval of driving fields, the dissipation profile in the hydrodynamic limit turns out to be asymmetric, and the characteristic spatial scale of the temperature distribution can be controlled by the driving field. By contrast, in the same range of parameters, impurity-dominated heating is almost symmetric, with the size of the dissipation region being independent of the field. This allows one to distinguish experimentally the hydrodynamic and impurity-dominated limits. Our results are consistent with recent experimental findings on transport and dissipation in narrow constrictions and quantum point contacts.

A ferromagnet is shown to exert an exchange field on the spin accumulation localized in the semiconductor interface region of a Fe/MgO/Si tunnel contact. The proximity exchange coupling across the MgO modifies spin precession of the spin accumulation and thereby produces detectable signatures in the inverted Hanle effect: exchange-induced shifts of the inverted Hanle curves, hysteresis, and discontinuities at the coercive field of the ferromagnet, at which the exchange field is reversed. The proximity exchange field is locked antiparallel to the magnetization of the ferromagnet, has values up to 380 Oe, and can be controlled by a bias voltage across the tunnel contact.

We consider clean Dirac metals where the linear band crossing is caused by a strong spin-orbit interaction, and study the quantum phase transitions from the paramagnetic phase to various magnetic phases, including homogeneous ferromagnets, ferrimagnets, canted ferromagnets, and magnetic nematics. We show that in all of these cases the coupling of fermionic soft modes to the order parameter generically renders the quantum phase transition first order, with certain gapless Dirac systems providing a possible exception. These results are surprising since a strong spin-orbit scattering suppresses the mechanism that causes the first order transition in ordinary metals. The important role of chirality in generating a new mechanism for a first-order transition is stressed.

The propagation of edge localized spin waves (E-SWs) in yttrium iron garnet (YIG; ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) microstripes with and without magnetic microstructures in close proximity is investigated by micromagnetic simulations. A splitting of the dispersion curve with the presence of permalloy (Py; ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) stripe is observed. The results show that E-SWs on the two edges of YIG stripe have different wavelengths, group velocities, and decay lengths at the same frequencies. The role of the Py stripe was found to be the source of the inhomogeneous static dipolar field without dynamic coupling with YIG. This work opens new perspectives for the design of innovative spin wave interference-based logic devices.

A method is presented for quantifying quantum-mechanical exchange effects within the dielectric theory of electron scattering, the prevailing model for electron energy losses in condensed-matter systems. The approach utilizes a uniquely symmetric view of direct and exchange scattering events along with a generalized interference phase to account physically for the reduced scattering rates required by the Pauli exclusion of final-state fermions with corresponding spin, energy, and momentum. It is found that existing implementations either neglect or substantially overestimate the impact of exchange interference for condensed materials with broad optical loss spectra due to mathematical approximations or physical assumptions which are largely avoidable. The results suggest that the impact of exchange may alter inelastic scattering cross sections by between 10% and 20% for incident energies below 200 eV, a critical energy range for current investigations in electron and photoelectron spectroscopies.

Motivated by recent prototypes of engineered atomic spin devices, we study a fully connected system of $N$ spins $1/2$, modeled by the Lipkin-Meshkov-Glick (LMG) model of a collective spin $s=N/2$ in the presence of Markovian dissipation processes. We determine and classify the different phases of the dissipative LMG model with Markovian dissipation, including the properties of the steady state and the dynamic behavior in the asymptotic long-time regime. Employing variational methods and a systematic approach based on the Holstein-Primakoff mapping, we determine the phase diagram and the spectral and steady-state properties of the Liouvillian by studying both the infinite-$s$ limit and $1/s$ corrections. Our approach reveals the existence of different kinds of dynamical phases and phase transitions, multistability, and regions where the dynamics is recurrent. We provide a classification of critical and noncritical Liouvillians according to their spectral and steady-state properties.

The order parameters which are thought to detect U(1) gauge symmetry breaking in a superconductor are both nonlocal and gauge dependent. For that reason, they are also ambiguous as a guide to phase structure. We point out that a global subgroup of the local U(1) gauge symmetry may be regarded, in analogy to non-Abelian theories, as a “custodial” symmetry affecting the matter field alone, and construct, along the lines of our previous work, a gauge-invariant criterion for breaking symmetries of this kind. It is shown that spontaneous breaking of custodial symmetry is a necessary condition for the existence of spontaneous symmetry breaking of a global subgroup of the (Abelian or non-Abelian) gauge group in any given gauge, and a sufficient condition for the existence of spontaneous breaking of a global subgroup of the gauge group in some gauge. As an illustration, we compute numerically, in a lattice version of the Ginzburg-Landau model, the phase boundaries of the theory and the order parameters associated with various symmetries in each phase.

Thermodynamic properties of the interacting homogeneous electron gas are calculated using a finite-temperature cumulant Green's function approach over a broad range of densities and temperatures up to the warm dense matter regime $T\sim T_F$, where $T_F$ is the Fermi degeneracy temperature. These properties can be separated into independent particle and exchange-correlation contributions, and our focus here is on the latter. Our approach is based on the Galitskii-Migdal-Koltun and electron number sum rules from the finite temperature many-body Green's function formalism, together with an extension of the cumulant Green's function to finite temperature. Previously this approach yielded exchange-correlation energies and potentials in good agreement with quantum Monte-Carlo calculations. Here the method is extended for various thermodynamic quantities including the chemical potential, total energy, Helmholtz free-energy, electronic equation of state, specific heat, and isothermal compressibility, which optionally include spin dependence. We find that the exchange-correlation contributions are weakly varying at low temperature but exhibit significant temperature dependence in the WDM regime, as well as a crossover from exchange- to correlation-dominated behavior. In contrast to the $T=0^{+}$ limit, we also find that renormalization effects are largely but not completely suppressed at finite temperature. Comparisons with other approaches at various levels of approximation are also discussed.

An efficient electro-optical modulation has been demonstrated here by using an edge-plasmon mode specific for the hybrid plasmonic waveguide. Our approach addresses a major obstacle of the integrated microwave photonics caused by the polarization constraints of both active and passive components. In addition to subwavelength confinement, typical for surface plasmon polaritons, the edge-plasmon modes enable exact matching of the polarization requirements for silicon based input/output grating couplers, waveguides, and electro-optical modulators. A concept of the hybrid waveguide, implemented in a sandwichlike structure, implies a coupling of propagating plasmon modes with a waveguide mode. The vertically arranged sandwich includes a thin layer of epsilon-near-zero material (indium tin oxide) providing an efficient modulation at small length scales. Employed edge plasmons possess a mixed polarization state and can be excited with horizontally polarized waveguide modes. It allows the resulting modulator to work directly with efficient grating couplers and avoid using bulky and lossy polarization converters. A 3D optical model based on Maxwell equations combined with drift-diffusion semiconductor equations is developed. Numerically heavy computations involving the optimization of materials and geometry have been performed. Effective modes, stationary state field distribution, an extinction coefficient, optical losses, and charge transport properties are computed and analyzed. In addition to the polarization matching, the advantages of the proposed model include the compact planar geometry of the silicon waveguide, reduced active electric resistance $R$ and a relatively simple design, attractive for experimental realization.

Inspired by the profound physical connotations and potential application prospects of the valleytronics, we design a two-dimensional (2D) $\mathrm{W}{\mathrm{S}}_2/h-\mathrm{VN}$ magnetic van der Waals (vdW) heterostructure and study the control of valley degrees of freedom through the first-principles calculations. A considerable spin splitting of 627 meV is obtained at the $K$ valley, accompanied with a strong suppression of that at the $K^{\prime }$ valley. An intrinsic large valley splitting of 376 meV is generated in the valence band, which corresponds to an effective Zeeman magnetic field of 2703 T. Besides the valence band, the conduction band of $\mathrm{W}{\mathrm{S}}_2$ possesses a remarkable spin splitting also, leading to the possible formation of valley-labeled dark exciton states at the $K^{\prime }$ valley. The strengths of spin and valley splitting relying on the interfacial orbital hybridization are further tuned continually by the in-plane strain and interlayer spacing. Maximum spin and valley splitting of 654 and 412 meV are finally achieved, respectively, and the effective Zeeman magnetic field can be enhanced to 2989 T with a −3% strain. Time-reversal symmetry breaking and the sizable Berry curvature in the heterostructure lead to a prominent anomalous Hall conductivity at the $K$ and $K^{\prime }$ valleys. Based on these finding, a prototype filter device for both the valley and spin is proposed.

We have studied the electronic structure of ${\mathrm{BaCoO}}_3$ using soft x-ray absorption spectroscopy at the Co $L_{2,3}$ and O $K$ edges, magnetic circular dichroism at the Co $L_{2,3}$ edges, and valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent $3d^5$ state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling also plays an important role for the magnetism of the system. At the same time, a trigonal crystal field is present with sufficient strength to bring the $3d^5$ ion away from the $J_{\mathrm{eff}}=1/2$ state. The sign of this crystal field is such that the $a_{1g}$ orbital is doubly occupied, explaining the absence of a Peierls transition in this system, which consists of chains of face-sharing ${\mathrm{CoO}}_6$ octahedra. Moreover, with one hole residing in $e_g^{\pi }$, the presence of an orbital moment and strong magnetocrystalline anisotropy can be understood. Yet we also infer that crystal fields with lower symmetry must be present to reproduce the measured orbital moment quantitatively, thereby suggesting the possibility for orbital ordering to occur in ${\mathrm{BaCoO}}_3$.

We present new experimental low-temperature heat capacity and detailed dynamical spin-structure factor data for the quantum spin-liquid candidate material ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$. The measured heat capacity shows an almost-perfect linear temperature dependence in the range $0.1\mathrm{K}\lesssim T\lesssim 0.5\mathrm{K}$, reminiscent of fermionic spinon degrees of freedom. The spin-structure factor exhibits two energy regimes of strong signal which display rather different but solely diffuse scattering features. We theoretically describe these findings by an effective spinon-hopping model which crucially relies on the existence of strong ferromagnetically coupled triangles in the system. Our spinon theory is shown to naturally reproduce the overall weight distribution of the measured spin-structure factor. Particularly, we argue that various different observed characteristic properties of the spin-structure factor and the heat capacity consistently indicate the existence of a spinon Fermi surface. A closer analysis of the heat capacity at the lowest accessible temperatures hints toward the presence of weak $f$-wave spinon-pairing terms inducing a small partial gap along the Fermi surface (except for discrete nodal Dirac points) and suggesting an overall ${\mathbb{Z}}_2$ quantum spin-liquid scenario for ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$.

We employ metadynamics simulations to calculate the free-energy landscape of thin ferromagnetic films and perform a systematic study of the temperature dependence of magnetic anisotropy and of the spin-reorientation transitions. By using a simple spin model we recover the well-known power-law behavior of the magnetic anisotropy energy against magnetization and present a rather detailed analysis of the spin-reorientation transitions in ultrathin films. Based on tensorial exchange interactions and anisotropy parameters derived from first-principles calculations, we perform simulations for Fe double layers deposited on Au(001) and W(110). In the case of ${\mathrm{Fe}}_2\mathrm{W}\left(110\right)$ our simulations display an out-of-plane to in-plane spin-reorientation transition in agreement with experiments.

The effects of a local magnetic field on nonequilibrium stationary states (NESSs) of the open quantum XXZ spin chain are investigated with a Lindblad master equation approach in the limit of strong dissipation. The local magnetic field is applied to a single bulk spin of the chain while the ends are kept at fixed polarizations by dissipation. We show that suitable changes of the local magnetic field permit us to invert the spin current by switching pure NESS with opposite chiralities while preserving the state purity and achieving optimal transport.

Low-dimensional materials, such as ultrathin films, nanoislands, and wires, are actively being studied due to their interesting magnetic properties and possible technological applications for example in high density data storage. Results of density functional theory calculations within the generalized gradient approximation of an Fe nanoisland on a W(110) surface are presented here with particular focus on the effect of hydrogen adsorption on magnetic properties. The adsorption is found to strongly decrease the magnetic moment of the Fe atoms the H atoms are bound to, down to less than a half in some cases as compared with the clean Fe island. This is an important consideration since hydrogen can relatively easily be introduced and removed from the system, thus providing a way of tuning magnetic properties, and it can also be unintentionally present even under ultrahigh vacuum conditions, especially at low temperature.