We investigated the variation of the electronic band gap of ZnO bulk and that of bent ZnO nanowires under the influence of uniaxial strain by using density functional theory. By applying a strain of about $\pm 2$% to bulk ZnO in equilibrium, we mimic the recent experimentally determined tensile and compressive strain along the $c$ axis of ZnO microwires which results from the bending of such wires. The slope of band gap size versus tensile-compressive strain at the equilibrium gives a deformation potential parameter, the value of which ranges between $-2.0$ and $-4.0$ eV depending on the exchange correlation treatments applied in order to improve the absolute value of the band gap. We find that the local (local density approximation) and semilocal [generalized gradient approximation (GGA) and the meta-GGA] approximations to the exchange-correlation functionals give a deformation potential, which is in good agreement with experiments. It is shown that the elastic constants derived from bulk ZnO are sufficient to model the strain effects for microwires. On the other hand, nanowires, only a few Å in diameter, respond with stronger changes in the band gap to applied strain. This feature, however, approaches the bulk behavior as the thickness of the nanowire increases.

Theoretical description of Rashba effects in three-dimensional electron gas at electric-dipole spin resonance conditions is presented in the frame of conductivity tensor formalism. The details due to anisotropy of the effective mass tensor, as well as the Lande factor, are considered. The absorbed power is calculated for arbitrary orientation of the sample with respect to external fields: constant magnetic field and rf electric field. The differences between resonance signals in two- and three-dimensional electron gas are pointed out.

The molecular compound $K_6$[V${}_{15}^{\mathrm{IV}}$As${}_6^{\mathrm{III}}{\mathrm{O}}_{42}$(H${}_2$O)] $·$ 8H${}_2$O, in short $V_{15}$, has shown important quantum effects such as coherent spin oscillations. The details of the spin quantum dynamics depend on the exact form of the spin Hamiltonian. In this study, we present a precise analysis of the intramolecular interactions in $V_{15}$. To that purpose, we performed high-field electron spin resonance measurements at 120 GHz and extracted the resonance fields as a function of crystal orientation and temperature. The data are compared against simulations using exact diagonalization to obtain the parameters of the molecular spin Hamiltonian.

Samples of conductive silicon carbide (SiC), which is a promising material due to its excellent properties for devices operating in severe environments, were characterized with the atomic force microscope for roughness, and the optical properties were measured with ellipsometry in a wide range of frequencies. The samples show significant far-infrared absorption due to concentration of charge carriers and a sharp surface phonon-polariton peak. The Casimir interaction of SiC with different materials is calculated and discussed. As a result of the infrared structure and beyond to low frequencies, the Casimir force for SiC-SiC and SiC-Au approaches very slowly the limit of ideal metals, while it saturates significantly below this limit if interaction with insulators takes place (${\text{SiC-SiO}}_2$). At short separations (<10 nm) analysis of the van der Waals force yielded Hamaker constants for SiC-SiC interactions lower but comparable to those of metals, which is of significance to adhesion and surface assembly processes. Finally, bifurcation analysis of microelectromechanical system actuation indicated that SiC can enhance the regime of stable equilibria against stiction.

We study the sensitivity of quantum ring (QR) conductance to potential perturbation introduced by an external charged probe in the context of scanning gate microscopy experiments and analyze its correspondence to the local density of states at the Fermi level for up to four subbands participating in the transport. We find that the conductance of the QR can be weakly sensitive or almost insensitive to the perturbation potential due to magnetic deflection of the wave function. The conductance becomes very sensitive to external perturbations at localized resonances for which the local density of states can be extracted by conductance mapping.

Using the nonequilibrium Green's function method, we investigated theoretically the spin-dependent transport and the current-induced spin transfer torque (CISTT) in a zigzag-graphene-nanoribbon (ZGNR) spin valve in the presence of an applied uniaxial strain to the ZGNR. It is found that, when a longitudinal or transverse strain is applied, the conductance versus the Fermi energy remains unchanged around the Dirac point. However, when the Fermi energy is larger than the molecular field of two ferromagnetic electrodes, the dependence of the conductance on the uniaxial strain exhibits totally different behaviors for parallel and antiparallel configurations for the electrodes' magnetizations, which leads to a transition of magnetoresistance (MR) from a perfect histogramlike behavior to successive cusplike peaks and to a steplike behavior with sharp peaks for the longitudinal and transverse strains, respectively. It is further shown that the CISTT per unit of the bias voltage as a function of the Fermi energy is antisymmetric respective to the Dirac point and exhibits typical successive oscillations composed of broad peaks closely followed by sharp ones.

The recently synthesized silicene as well as theoretically discussed germanene are examples of buckled honeycomb structures. The buckled structures allow one to manipulate asymmetry between two underlying sublattices of honeycomb structures. Here by taking germanene as a prototype of buckled honeycomb lattices, we explore magnetism induced by breaking sublattice symmetry through saturating chemical bonds on one side of the buckled honeycomb lattice. It is shown that when fractions of chemical bonds on one side are saturated, two narrow bands always exist at half filling. Furthermore, the narrow bands generally support flat band ferromagnetism in the presence of the Hubbard $U$ interaction. The induced magnetization is directly related to the saturation fraction and is thus controllable in magnitude through the saturation fraction. Most importantly, we find that depending on the saturation fraction, the ground state of a one-side-saturated germanene may become a quantum anomalous Hall (QAH) insulator characterized by a Chern number that vanishes for larger magnetization. The nonvanishing Chern number for smaller magnetization implies that the associated quantum Hall effect tends to survive at high temperatures. Our findings provide a potential method to engineer buckled honeycomb structures into high-temperature QAH insulators.

By performing ab initio calculations for one- to four-layer black phosphorus within the $GW$ approximation, we obtain a significant difference in the band gap ($\sim$1.5 eV), which is in line with recent experimental data. The results are analyzed in terms of the constructed four-band tight-binding model, which gives accurate descriptions of the mono- and bilayer band structure near the band gap, and reveal an important role of the interlayer hoppings, which are largely responsible for the obtained gap difference.

We have investigated the positron-stimulated desorption of ions from a ${\text{TiO}}_2\mathrm{(110)}$ surface. Desorbed O${}^{+}$ ions were detected in coincidence with the emission of annihilation $\gamma$ rays. The energy dependence of the ion yields shows that the O${}^{+}$ ions were detected at energies much lower than the previously reported threshold for electron impact desorption corresponding to the excitation energy of Ti($3p$) core electrons. These results provide evidence that core-hole creation by positron annihilation with electrons in the core levels leads to ion desorption.

Electrons in graphene with heavy adatoms (such as In or Tl) have been predicted to form a 2D topological insulator phase with a substantial spectral gap potentially suitable for future practical applications. In order to facilitate the ongoing experimental efforts to identify this phase we perform a theoretical study of its spectral properties in a model graphene system with randomly distributed adatoms. Our extensive modeling shows that random heavy adatoms produce a full spectral gap (as opposed to a mobility gap) accompanied by distinctive quasiparticle interference patterns observable by means of Fourier-transform scanning tunneling spectroscopy.

We theoretically investigated two kinds of density oscillations, the Friedel oscillation and collective excitation in the silicene and germanene within the random phase approximation, and found that the tunable spin-valley coupled band structure could lead to some exotic properties in these two phenomena. Based on an exact analytical and numerical analysis, we demonstrated that the beating of the screened potential as well as the undamped plasmon mode can be taken as fingerprints of a topological phase transition in doped silicene and doped germanene. Thus our proposal here establishes the connection between the topological phase transition and the density oscillations that can be accessed by a variety of experimental techniques.

In this work, we explore the possibility of emergent nonequilibrium steady states arising from the electric-field-driven Mott insulator via the Keldysh-Floquet dynamical mean-field theory (DMFT), which can determine the fully interacting, nonequilibrium steady-state Green's functions with the noninteracting counterparts as an input to the DMFT self-consistency loop. Unlike the retarded component, obtaining the lesser Green's function for the noninteracting system presents an important obstacle since the thermalization of the noninteracting system still requires a precise understanding of the dissipation mechanism. A crucial breakthrough in this work is that the noninteracting lesser Green's function can be determined in terms of the Wannier-Stark ladder (WSL) eigenstates, which are thermalized via the standard canonical ensemble according to the Markovian quantum master equation. As a result, it is shown that the intricate interplay between strong correlation and large electric field can generate a sequence of two dielectric breakdowns with the first induced by a coherent reconstruction of the midgap state within the Mott gap and the second by an incoherent tunneling through the biased Hubbard bands. It is predicted that the reconstructed midgap state generates its own emergent WSL structure with a reduced effective electric field. The two dielectric breakdowns are mediated by a reentrant insulating phase, which is characterized by the population inversion, causing instability toward inhomogeneous current density states at weak electron-impurity scattering.

The electronic properties of quasi-one-dimensional anatase ${\mathrm{TiO}}_2$ nanostructures, in the form of thin nanowires having (101) and (001) facets, have been systematically investigated using the standard, hybrid density functional and quasiparticle calculations. Their visible photoabsorption characteristics have also been studied at these levels of theories. The thin stoichiometric nanowire models are predicted to have larger band gaps relative to their bulk values. The band-gap-related features appear to be better described with the screened Coulomb hybrid density functional method compared to the standard exchange-correlation schemes. Depending on the self-consistency in the perturbative $GW$ methods, even larger energy corrections have been obtained for the band gaps of both (101) and (001) titanium dioxide nanowires.

The Hubbard and extended Hubbard models on the honeycomb lattice can be seen as prototype models of a single-layer graphene placed in a high dielectric constant environment that screens the Coulomb interaction. Taking advantage of the absence of a sign problem at half-filling, we study this problem with clusters up to 96 sites with the determinant quantum Monte Carlo method as an impurity solver for the dynamical cluster approximation at finite temperatures. After determining the stability of the semimetallic phase to interaction-induced spin-density wave (SDW), charge-density wave (CDW), and Mott insulating phases, we study the single-particle dynamics of the Dirac fermions. We show that when spontaneous symmetry breaking is avoided, the semimetallic phase is a stable Fermi liquid in the presence of repulsive interactions and that Kondo screening dominates the low temperature regime, even though there is a $\rho \left(\omega \right)=\left|\omega \right|$ type local density of states. We also investigate the impact of the correlation effects on the renormalization of the Fermi velocity $v_F$. We find that $v_F$ is not renormalized when only on-site repulsion $U$ is present, but that near-neighbor repulsion $V$ does renormalize $v_F$. This may explain the variations between different measurements of $v_F$ in graphene.

The nonequilibrium dynamics of a quantum dot with electron-phonon interactions described by a generalized Holstein model is presented. A combination of methodologies, including the reduced density matrix formalism, the multilayer multiconfiguration time-dependent Hartree method, and a time-dependent nonequilibrium Green's function approach, is used to explore the transient behavior on multiple time scales as the system approaches steady state. The dot population dynamics on short to intermediate times is governed by the dot-lead hybridization parameter ($\Gamma$) and by the typical phonon frequency (${\omega }_c$) and depends on the location of the energy level of the dot relative to the bias window. At longer times, the dynamics shows a distinct behavior depending on whether the system is in the adiabatic or nonadiabatic regime, with a quantum dot occupation that may depend on the initial preparation of the phonon degrees of freedom. A “phase” diagram of this effect as a function of the polaron shift ($\lambda$) for various phonon frequencies is derived, suggesting the existence of bistability on experimentally observable time scales.

We study a lattice version of the local density approximation (LDA) based on Bethe ansatz (BALDA). Contrary to what happens in density functional theory in the continuum and despite its name, BALDA displays some very nonlocal features and it has a discontinuous functional derivative. The same features prevent the convergence of the self-consistent Kohn-Sham cycle thus hindering the study of BALDA solutions close to a Mott phase or in the Coulomb blockade regime. Here, we propose a numerical approach which, differently from previous works, does not introduce ad hoc parameters to smear out the singularity. Our results are relevant for all lattice models where BALDA is applied ranging from Kondo systems to harmonically trapped Hubbard fermions. As an example, we apply the method to the study of a one-dimensional lattice model with Hubbard interaction and a staggered potential which can be driven from an ionic to a Mott-insulating state. In the Mott regime, the presence of a “vacuum” allows us to calculate the different contribution to the gap and to highlight an ultranonlocality of BALDA.

We develop a theory of spontaneously magnetized Tomonaga-Luttinger (TLL) liquid in geometrically frustrated quasi-one-dimensional quantum magnets by taking an $S=1/2$ ferrimagnet on a union-jack lattice as an example. We show that a strong frustration leads to a spontaneous magnetization because of the ferrimagnetic nature of lattice. Due to the ferrimagnetic order, the local magnetization has an incommensurate oscillation with the position. We show that the spontaneously magnetized TLL is smoothly connected to the existence of a Nambu-Goldstone boson in the canted ferrimagnetic phase of a two-dimensional frustrated antiferromagnet.

An important loss mechanism in organic electroluminescent devices is exciton quenching by polarons. Gradual electrochemical doping of various conjugated polymer films enabled the determination of the doping density dependence of photoluminescence quenching. Electrochemical doping was achieved by contacting the film with a solid electrochemical gate and an injecting contact. A sharp reduction in photoluminescence was observed for doping densities between 10${}^{18}$ and 10${}^{19}$ cm${}^{-3}$. The doping density dependence is quantitatively modeled by exciton diffusion in a homogeneous density of polarons followed by either Förster resonance energy transfer or charge transfer. Both mechanisms need to be considered to describe polaron-induced exciton quenching. Thus, to reduce exciton-polaron quenching in organic optoelectronic devices, both mechanisms must be prevented by reducing the exciton diffusion, the spectral overlap, the doping density, or a combination thereof.

We present a theory for linear and nonlinear excitations in semiconductor quantum plasmas consisting of electrons and holes. The system is governed by two coupled nonlinear Schrödinger equations for the collective wave functions of the electrons and holes and Poisson's equation for the electrostatic potential. This gives a closed system including the effects of charge separation between the electrons and holes, quantum tunneling, quantum statistics, and exchange correlation due to electron spin. Three typical semiconductors, GaAs, GaSb, and GaN, are studied. For small-amplitude excitations, the dispersion relation reveals the existence of one high-frequency branch due to charge-separation effects and one low-frequency branch due to the balance between pressure and inertia of the electrons and holes. For the fully nonlinear excitations, the profiles of quasistationary soliton solutions are obtained numerically and show depleted electron and hole densities correlated with a localized potential. The simulation results show that the rarefactive solitons are stable and can withstand perturbations and turbulence for a considerable time.