Superconductivity and polar charge fluctuation are studied in an organic conductor with the dimer-molecule degree of freedom. The extended Hubbard models, where the intradimer electronic structure and the interdimer Coulomb interactions are taken into account, are analyzed by the random-phase approximation and the fluctuation-exchange approximation. Superconductivity appears in a vicinity of the charge-density wave (CDW) phase where the electronic charge distributions are polarized inside of dimers. The extended $s$-wave type paring is favored and its competitive relations with the superconductivity due to the spin fluctuation depends on the triangular lattice geometry. Comparison between two superconductivities realized near the polar and nonpolar CDW phases are also presented.

Topological properties of antiferromagnetic phases are studied for a correlated topological band insulator by applying the dynamical mean-field theory to an extended Bernevig-Hughes-Zhang model including the Hubbard interaction. The calculation of the magnetic moment and the spin Chern number confirms the existence of a nontrivial antiferromagnetic (AF) phase beyond the Hartree-Fock theory. In particular, we uncover the intriguing fact that the topologically nontrivial AF phase is essentially stabilized by correlation effects but not by the Hartree shifts alone. This counterintuitive effect is demonstrated, through a comparison with the Hartree-Fock results, and should apply for generic topological insulators with strong correlations.

We use a rigorous electromagnetic approach to show the existence of strongly localized modes in the stop band of a linear, two-dimensional, finite photonic crystal near its Dirac point. At normal incidence, the crystal exhibits a Dirac point with 100$\%$ transmission. At angles slightly off the normal, where the crystal is 100$\%$ reflective, instead of exponentially decaying fields as in a photonic stop band, the field becomes strongly localized and enhanced inside the crystal. We explain that this anomalous localization is due to guided mode resonances that are the foundation of the Dirac point itself and also shape its adjacent band gap. Besides shedding new light on the physical origin of Dirac points in finite photonic crystals, our results could have applications in many nonlinear light-matter interaction phenomena in which it is crucial to achieve a high degree of light localization.

In this paper, we study the effect of iron doping in zirconia using both theoretical and experimental approaches. Combining density functional theory (DFT) simulations with the experimental characterization of thin films, we show that iron is in the ${\mathrm{Fe}}^{3+}$ oxidation state and, accordingly, the films are rich in oxygen vacancies ($V_{\mathrm{O}}^{{}^{••}}$). $V_{\mathrm{O}}^{{}^{••}}$ favor the formation of the tetragonal phase in doped zirconia (${\mathrm{ZrO}}_2$:$\mathrm{Fe}$) and affect the density of states at the Fermi level as well as the local magnetization of $\mathrm{Fe}$ atoms. We also show that the $\mathrm{Fe}\left(2p\right)$ and $\mathrm{Fe}\left(3p\right)$ energy levels can be used as a marker for the presence of vacancies in the doped system. In particular, the computed position of the $\mathrm{Fe}\left(3p\right)$ peak is strongly sensitive to the $V_{\mathrm{O}}^{{}^{••}}$ to $\mathrm{Fe}$ atoms ratio. A comparison of the theoretical and experimental $\mathrm{Fe}\left(3p\right)$ peak positions suggests that in our films this ratio is close to $0.5$. Besides the interest in the material by itself, ${\mathrm{ZrO}}_2$:$\mathrm{Fe}$ constitutes a test case for the application of DFT on transition metals embedded in oxides. In ${\mathrm{ZrO}}_2$:$\mathrm{Fe}$, the inclusion of the Hubbard $U$ correction significantly changes the electronic properties of the system. However, the inclusion of this correction, at least for the value $U=3.3$ eV chosen in the present work, worsen the agreement with the measured photoemission valence band spectra.

We study the charge and spin transport under high electric field (up to several kilovolts per centimeter) on the surface of topological insulator Bi${}_2$Se${}_3$, where the electron-surface optical phonon scattering dominates except at very low temperature. Due to the spin mixing of conduction and valence bands, the electric field not only accelerates electrons in each band, but also leads to interband precession. In the presence of the electric field, electrons can transfer from the valence band to the conduction one via the interband precession and interband electron-phonon scattering. The electron density in each band varies with the electric field linearly when the electric field is strong. Due to the spin-momentum locking, a transverse spin polarization, with the magnitude proportional to the momentum scattering time, is induced by the electric field. The induced spin polarization depends on the electric field linearly when the latter is small. Moreover, its magnitude is inversely proportional to the temperature and is insensitive to the electron density at high temperature. Our investigation also reveals that due to the large relative static dielectric constant, the Coulomb scattering is too weak to establish a drifted Fermi distribution with a unified hot-electron temperature in the steady state under the electric field. After turning off the electric field in the steady state, the hot carriers cool down in a time scale of energy relaxation that is very long (of the order of 100–1000 ps), while the spin polarization relaxes in a time scale of momentum scattering that is quite short (of the order of 0.01–0.1 ps).

The influence of spin-orbit interaction on the electron-phonon coupling strength at the Fermi level of thin lead films is investigated using first-principles calculations in the density functional perturbation formalism. The calculations both scalar relativistic and including spin-orbit coupling (SOC) have been carried out for free-standing Pb(111) films consisting of four to ten atomic layers. It is shown that the spin-orbit interaction produces a large enhancement of the electron-phonon coupling strength regardless of the film thickness. This partly reflects a strong SOC-induced softening of the film phonon spectra, and partly a SOC-mediated increase in electron-phonon coupling matrix elements. For thin films, quantum size effects result in pronounced oscillations of the average coupling constant with the number of layers, which become damped for thicker films.

The ferroelectric trends of rutile (TiO${}_2$) in bulk and at the (110) surface are investigated by means of ab initio density functional theory. We discuss the underlying mechanism of the incipient ferroelectric behavior of rutile in terms of Born effective charges, which we decompose in individual contributions by means of maximally localized Wannier functions. We show that a ferroelectric phase can be stabilized for a variety of different lattice distortions, which all enlarge the shortest Ti-O distance, even if the longer apical Ti-O bond is simultaneously shortened. At the (110) surface, the ferroelectric trends are modified compared to the bulk, but nevertheless, ferroelectric phases with large polarization even in the topmost surface layer can be stabilized by uniaxial strain.

The structure and electronic phenomena at the 180-degree domain wall in the rhombohedral phase of BaTiO${}_3$ are described using Landau-Ginzburg-Devonshire theory. Dependent on the wall orientation, two types of domain wall behaviors are identified. The low-energy “achiral” phase occurs in the vicinity of the {110} orientation and has odd polarization profile invariant with respect to inversion about the wall center. The “chiral” phase occurs around {211} wall orientations and corresponds to mixed parity domain walls. The temperature-induced transformation between the phases is abrupt and is accompanied with 20$\%$–30$\%$ change of the domain wall thickness. This process gives rise to the significant changes of the electronic structure of the wall. Depending on the temperature and flexoelectric coupling strength, relative conductivity of the wall becomes at least one order of magnitude higher than in the single-domain region. The possible strategies for exploring these transitions based on direct measurements of domain wall width and conductive atomic force microscopy are discussed.

We find, using ab initio atomistic simulations of vacancy-mediated diffusion processes in TiC and ZrC, that a multivacancy self-diffusion mechanism is operative for metal-atom diffusion in substoichiometric carbides. It involves a special type of a stable point defect, a metal vacancy “dressed” in a shell of carbon vacancies. We show that this vacancy cluster is strongly bound and can propagate through the lattice without dissociating.

We show that antilocalization measurements can be used to experimentally study the interactions between InAs surface electrons and local moments of the rare earth ions Sm${}^{3+}$, Gd${}^{3+}$, and Ho${}^{3+}$ on the surface. Magnetic spin-flip scattering and spin-orbit scattering of the accumulation layer electrons are affected by the proximity of the rare earth ions. The spin-flip rate carries information about magnetic interactions. Within the temperature range studied, Sm${}^{3+}$ and Gd${}^{3+}$ yield temperature-independent electron spin-flip rates in proportion to their magnetic moments. In proximity to Ho${}^{3+}$ the InAs electrons however show a spin-flip rate increasing with temperature. We interpret the spin-flip rate due to Ho${}^{3+}$ as resulting from transitions between closely spaced energy levels of the ion on the surface. The experiments also show that the strength of spin-orbit interaction can be modified by the surface species.

Using first-principles density-functional-theory-based calculations, we derive the low-energy spin model of the compound Zn${}_2$VO(PO${}_4$)${}_2$, and $\frac{1}{4}$th Ti-substituted compound Zn${}_8$TiV${}_3$O${}_4$(PO${}_4$)${}_8$ which is yet to be synthesized. We compute the thermodynamics properties of the proposed spin models by the quantum Monte Carlo technique. For the pristine compound Zn${}_2$VO(PO${}_4$)${}_2$ our computed susceptibility is found to be in good agreement with the available experimental data and is in accordance with the earlier proposed spin model of the spin-$\frac{1}{2}$ antiferromagnetic square lattice. Upon Ti substitution, which may be viewed as $\frac{1}{4}$-spin depletion, the two-dimensional antiferromagnetic square-lattice behavior of the parent compound is found to be altered significantly, giving rise to spin-gap behavior. We hope that our work will stimulate further experimental studies on this compound.

We study a model for the transverse thermoelectric response due to quantum superconducting fluctuations in a two-leg Josephson ladder, subject to a perpendicular magnetic field $B$ and a transverse temperature gradient. Assuming a weak Josephson coupling on the rungs, the off-diagonal Peltier coefficient (${\alpha }_{xy}$) and the Nernst effect are evaluated as functions of $B$ and the temperature $T$. In this regime, the Nernst effect is found to exhibit a prominent peak close to the superconductor–insulator transition, which becomes progressively enhanced at low $T$. In addition, we derive a relation to the diamagnetic response ${\alpha }_{xy}=-M/T_0$, where $M$ is the equilibrium magnetization and $T_0$ a plasma energy in the superconducting legs.

Gapless nodal quasiparticles emerge at a low-energy regime of high-$T_c$ cuprate superconductors due to the $d_{x^2-y^2}$ gap symmetry. We study the unusual renormalizations of the Fermi velocity $v_F$ and gap velocity $v_{\Delta }$ of these quasiparticles close to various quantum critical points in a superconducting dome. Special attention is paid to the behavior of the velocity ratio, $v_{\Delta }/v_F$, since it determines a number of observable quantities. We perform a renormalization-group analysis and show that the velocity ratio may vanish, approach unity, or diverge at different quantum critical points. The corresponding superfluid densities and critical temperatures are suppressed, slightly increased, or significantly enhanced. The effects of three types of static disorders, namely, random mass, random gauge potential, and random chemical potential, on the stability of the system are also addressed. An analogous analysis reveals that both random mass and random gauge potential are irrelevant. This implies that these fixed points of the velocity ratio are stable, and hence observable effects ignited by them are unchanged. However, the random chemical potential is marginal. As a result, these fixed points are broken, and thus, the instabilities of quantum phase transitions are triggered.

Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb${}_{2+x}$Ti${}_{2-x}$O${}_{7+y}$ have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of $x$ from a spin-liquid state ($x<x_{\text{c}}$) to a partly ordered state with a small antiferromagnetic ordering of the order of $0.1{\mu }_{\text{B}}$. Specific heat shows a sharp peak at a phase transition at $T_{\text{c}}=0.5$ K for $x=0.005$. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through $T_{\text{c}}$. The possibility of a Jahn-Teller transition is discussed.

The magnetic properties of Fe${}_{1.03}$Te under hydrostatic pressure up to $p\simeq 5.7$ GPa were investigated by means of muon spin rotation, dc magnetization, and neutron depolarization measurements. All of these methods show consistently that the commensurate antiferromagnetic order is weakened with increasing pressure before the system becomes a low temperature bulk ferromagnet above $p\simeq 2$ GPa.

We analyze work done on a quantum system driven by a control field. The average work depends on the whole dynamics of the system, and is obtained as the integral of the average power operator. As a specific example we focus on a superconducting Cooper-pair box forming a two-level system. We obtain expressions for the average work and work distribution in a closed system, and discuss control field and environment contributions to the average work for an open system.

We have performed systematic studies of the lattice dynamics in $L$FeAsO ($L=\text{La}$, Ce, Pr, Nd, Sm) in the parent and in the $\sim$10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials.

Spin chains have been proposed as wires to transport information between distributed registers in a quantum information processor. Unfortunately, the challenges in manufacturing linear chains with engineered couplings has hindered experimental implementations. Here we present strategies to achieve perfect quantum information transport in arbitrary spin networks. Our proposal is based on the weak coupling limit for pure state transport, where information is transferred between two end spins that are only weakly coupled to the rest of the network. This regime allows ignoring the complex, internal dynamics of the bulk network and relying on virtual transitions or on the coupling to a single bulk eigenmode. We further introduce control methods capable of tuning the transport process and achieve perfect fidelity with limited resources, involving only manipulation of the end qubits. These strategies could be thus applied not only to engineered systems with relaxed fabrication precision, but also to naturally occurring networks; specifically, we discuss the practical implementation of quantum state transfer between two separated nitrogen vacancy (NV) centers through a network of nitrogen substitutional impurities.

Vortex cores in Landau-domain patterns are resonantly excitable by alternating magnetic fields in the sub-gigahertz regime. We present a highly sensitive method to detect the vortex gyration in single micrometer-sized elements spectroscopically by measuring spectra of induction voltages caused by the stray fields of a single ferromagnetic square exposed to an alternating Oersted field. A distinct change of the induction voltage is observed around the resonance frequency of the vortex core. The shape of the measured spectra deviates from Lorentzian profiles due to voltages induced by magnetic fringing fields of the exciting currents. An analytical description of the measured spectra is given. A characteristic frequency shift in external magnetic fields proves that signals detected by the induction sensor originate from the dynamically excited Landau-domain pattern. The measurements on a single square are compared with measurements on an ensemble of uniform squares.

The spin waves in the multi-k antiferromagnet USb soften and become quasielastic well below the antiferromagnetic ordering temperature $T_N$. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to dephasing of the different multi-k components: a switch from 3-k to 1-k behavior. In this work, we use inelastic neutron scattering with tridirectional polarization analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not dephase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above $T_N$ (up to at least $1.4T_N$).