Local Coulomb Research Articles

Energy spectrum and metal-insulator transition in system with Anderson-Hubbard centers

Energy spectrum of a model of narrow-band metal into which the periodically spaced Anderson-Hubbard centers are introduced has been studied. Hybridization with conduction band results in the indirect exchange interaction which is different from the interactions between localized magnetic moments and strong on-site Coulomb interaction. To study effect of the lattice deformation under the external pressure (or the self-contraction of the lattice) on electrical properties of the system, the phonon term and elastic energy have been taken into account. The equilibrium values of lattice strain and chemical potential have been calculated self-consistently for non-zero temperatures. Within the Green function approach, the energy spectrum has been calculated as function of model parameters, temperature and external pressure. Our results show that there exists a threshold value of the external pressure above which energy gap value decreases rapidly with temperature and system becomes a correlated metal.

Antiferromagnetism in the Hubbard Model on the Bernal-Stacked Honeycomb Bilayer

Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the threefold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions.

Phase Diagram of Metal-Insulator Transition in System with Anderson-Hubbard Centers

The model of a strongly correlated system in which periodically spaced Anderson Hubbard centers are introduced into narrow-band metal is considered. Besides the interactions between localized magnetic moments and strong on-site Coulomb interaction, the model takes into account the hybridization of localized and band states. To study the e ect of the lattice deformation on the electrical properties of the system, the phonon term and elastic energy have been taken into account. Green functions for band and localized electrons have been found. On this base, the energy spectrum has been investigated as a function of model parameters, temperature and external pressure. The criterion of the metal insulator transition for an integer value of electron concentration has been derived and the phase diagram of the metal insulator transition has been built.

Interplay of Coulomb interaction and spin-orbit effects in multilevel quantum dots

We study electron transport through a multilevel quantum dot with Rashba spin-orbit interaction in the presence of local Coulomb repulsion. We focus on the parameter regime in which the level spacing is larger than the level broadening. Motivated by recent experiments, we compute the level splitting induced by the spin-orbit interaction at finite Zeeman fields $B$, which provides a measure of the renormalized spin-orbit energy. This level splitting is responsible for the suppression of the Kondo ridges at finite $B$ characteristic for the multilevel structure. In addition, the dependence of renormalized $g$ factors on the relative orientation of the applied $B$ field and the spin-orbit direction following two different protocols used in experiments is investigated.

Quench dynamics of correlated quantum dots

We study the relaxation dynamics of a quantum dot with local Coulomb correlations coupled to two noninteracting leads which are held in grandcanonical equilibrium. Only charge degrees of freedom are considered and the dot is described by a model which in the scaling limit becomes equivalent to the interacting resonant level model. The time evolution of the current and dot occupancy resulting out of changes of the dot-lead coupling, the dots onsite energy, or the charging energy are studied. Abrupt and smooth parameter changes as well as setups with and without driving bias voltage are considered. For biased dots we investigate the often studied response after turning on the dot-lead coupling but also the experimentally more relevant case in which the voltage is turned on. We identify and explain a variety of interesting many-body effects and clarify the role of initial correlations.

Effects of spin-dependent quasiparticle renormalization in Fe, Co, and Ni photoemission spectra:An experimental and theoretical study

We have investigated the spin-dependent quasiparticle lifetimes and the strength of electron correlation effects in the ferromagnetic 3$d$ transition metals Fe, Co, and Ni by means of spin- and angle-resolved photoemission spectroscopy. The experimental data are accompanied by state-of-the-art many-body calculations within the dynamical mean-field theory and the three-body scattering approximation, including fully relativistic calculations of the photoemission process within the one-step model. Our quantitative analysis reveals that inclusion of local many-body Coulomb interactions are of ultimate importance for a realistic description of correlation effects in ferromagnetic 3$d$ transition metals. However, we found that more sophisticated many-body calculations with larger modifications in the case of Fe and Co are still needed to improve the quantitative agreement between experiment and theory. In general, it turned out that not only the dispersion behavior of energetic structures should be affected by nonlocal correlations but also the line widths of most of the photoemission peaks are underestimated by the current theoretical approaches. The increasing values of the on-site Coulomb interaction parameter $U$ and the band narrowing of majority spin states obtained when moving from Fe to Ni indicate that the effect of nonlocal correlations becomes weaker with increasing atomic number, whereas correlation effects tend to be stronger.

Nuclear inelastic scattering studies of lattice dynamics in magnetite with a first- and second-order Verwey transition

Nuclear inelastic-scattering studies were performed to infer temperature evolution of iron atom dynamics in magnetite samples exhibiting Verwey transition of first- and second-order (type-I and type-II materials). The possible difference in this evolution could rationalize the distinct properties of these classes of materials observed in heat capacity and diffuse scattering below the Verwey transition temperature ${T}_{V}$ and could explain the change in transition order triggered by a minute (below 0.3$%$) altering of the iron sublattice. Although we have found the apparent stiffening of the phonon iron spectrum in the low-temperature phase, at the same time, we have shown that these spectra are rather similar for type-I and type-II materials, rendering the lattice vibration-based explanation of the distinct behavior of heat capacities very improbable. The calculation of phonon spectra, aimed at tracing the origin of various features in the phonon density of states (DOS), has shown that the local Coulomb interaction $U$ may have a large effect on phonon DOS. However, the change in the $U$ parameter cannot explain the difference in heat-capacity results for both classes of materials. Thus, an additional factor that differentiates these materials and possibly is responsible for the discontinuous character of the Verwey transition in stoichiometric magnetite still must be found.

Quantum Monte Carlo study on an ionic Hubbard model in one dimension

We study an ionic Hubbard model in one dimension by using a quantum Monte Carlo method. The ionic Hubbard model we study is a strongly correlated model for electrons in alternating potentials with a lattice period of 2. By modifying the Hirsch-Fye algorithm, we incorporate the local variation of the potential in the weight of the partition function at each imaginary-time segment. For a given alternating potential of strength Δ = 1.0, as we change the local Coulomb repulsion, U, between spin-up and spin-down electrons, we calculate various physical quantities, including kinetic energies, correlation energies, and band energies. We try to find the signature of the phase transitions from the behavior of the kinetic energy and that of double occupancies at finite temperatures.

On the role of Coulomb scattering in hafnium-silicate gated silicon n and p-channel metal-oxide-semiconductor-field-effect-transistors

In this work, the impact of the local and remote Coulomb scattering mechanisms on electron and hole mobility are investigated. The effective mobilities in quasi-planar finFETs with TiN/Hf0.4Si0.6O/SiO2 gate stacks have been measured at 300 K and 4 K. At 300 K, electron mobility is degraded below that of bulk MOSFETs in the literature, whereas hole mobility is comparable. The 4 K electron and hole mobilities have been modeled in terms of ionized impurity, local Coulomb, remote Coulomb and local roughness scattering. An existing model for remote Coulomb scattering from a polycrystalline silicon gate has been adapted to model remote Coulomb scattering from a high-κ/SiO2 gate stack. Subsequently, remote charge densities of 8 × 1012 cm−2 at the Hf0.4Si0.6O/SiO2 interface were extracted and shown to be the dominant Coulomb scattering mechanism for both electron and hole mobilities at 4 K. Finally, a Monte Carlo simulation showed remote Coulomb scattering was responsible for the degraded 300 K electron mobility.

Topological Fermi Liquids from Coulomb Interactions in the Doped Honeycomb Lattice

We propose a simple method for obtaining time reversal symmetry (T) broken phases in simple lattice models based on enlarging the unit cell. As an example we study the honeycomb lattice with nearest neighbor hopping and a local nearest neighbor Coulomb interaction V. We show that when the unit cell is enlarged to host six atoms that permits Kekulé distortions, self-consistent currents spontaneously form creating nontrivial magnetic configurations with total zero flux at high electron densities. A very rich phase diagram is obtained within a variational mean field approach that includes metallic phases with broken time reversal symmetry (T). The predominant (T) breaking configuration is an anomalous Hall phase, a realization of a topological Fermi liquid.

Critical states of strongly interacting many-particle systems on a circle

Normally, two scales are considered in multicomponent systems, namely, macro and micro scales. Moreover, it is accepted that macro variables are completely defined by micro variables. We show that in the considered particle system with strong local (Coulomb) interaction this is not always the case. Namely, information concerning some macro variables can be encoded on a scale much finer than the micro scale.

Influence of Ion Induced Local Coulomb Field and Polarity on Charge Generation and Efficiency in Poly(3‐Hexylthiophene)‐Based Solid‐State Dye‐Sensitized Solar Cells

AbstractDye‐sensitized solar cells (DSSC) are a realistic option for converting light to electrical energy. Hybrid architectures offer a vast materials library for device optimization, including a variety of metal oxides, organic and inorganic sensitizers, molecular, polymeric and electrolytic hole‐transporter materials. In order to further improve the efficiency of solid‐state dye‐sensitized solar cells, recent attention has focused on using light absorbing polymers such as poly(3‐hexylthiophene) (P3HT), to replace the more commonly used “transparent” 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)9,9′spiro‐bifluorene (spiro‐OMeTAD), in order to enhance the light absorption within thin films. As is the case with spiro‐OMeTAD based solid‐state DSSC, the P3HT‐based devices improve significantly with the addition of lithium bis(trifluoromethylsulfonyl)imide salts (Li‐TFSI), although the precise role of these additives has not yet been clarified in solid‐state DSCs. Here, we present a thorough study on the effect of Li‐TFSI in P3HT based solid‐state DSSC incorporating an indolene‐based organic sensitizer termed D102. Employing ultrafast transient absorption and cw‐emission spectroscopy together with electronic measurements, we demonstrate a fine tuning of the energetic landscape of the active cell components by the local Coulomb field induced by the ions. This increases the charge transfer nature of the excited state on the dye, significantly accelerating electron injection into the TiO2. We demonstrate that this ionic influence on the excited state energy is the primary reason for enhanced charge generation with the addition of ionic additives. The deepening of the relative position of the TiO2 conduction band, which has previously been thought to be the cause for enhanced charge generation in dye sensitized solar cells with the addition of lithium salts, appears to be of minor importance in this system.

Spin-orbit interaction and asymmetry effects on Kondo ridges at finite magnetic field

We study electron transport through a serial double quantum dot with Rashba spin-orbit interaction (SOI) and Zeeman field of amplitude B in presence of local Coulomb repulsion. The linear conductance as a function of a gate voltage Vg equally shifting the levels on both dots shows two B=0 Kondo ridges which are robust against SOI as time-reversal symmetry is preserved. Resulting from the crossing of a spin-up and a spin-down level at vanishing SOI two additional Kondo plateaus appear at finite B. They are not protected by symmetry and rapidly vanish if the SOI is turned on. Left-right asymmetric level-lead couplings and detuned on-site energies lead to a simultaneous breaking of left-right and bonding-anti-bonding state symmetry. In this case the finite-B Kondo ridges in the Vg-B plane are bent with respect to the Vg-axis. For the Kondo ridge to develop different level renormalizations must be compensated by adjusting B.

Qualitative analysis of 3D elastostatic contact problems with orthotropic Coulomb friction and solution-dependent coefficients of friction

This paper analyzes a discrete form of 3D contact problems with local orthotropic Coulomb friction and coefficients of friction which may depend on the solution itself. The analysis is based on the fixed-point reformulation of the original problem. Conditions guaranteeing the existence and uniqueness of discrete solutions are established. Finally, numerical results of a model example are presented.

Approximation and numerical realization of 3D quasistatic contact problems with Coulomb friction

AbstractThis paper deals with the full discretization of quasistatic 3D Signorini problems with local Coulomb friction and a coefficient of friction which may depend on the solution. After a time discretization we obtain a system of static contact problems with Coulomb friction. Each of these problems is solved by the T-FETI domain decomposition method used in auxiliary contact problems with Tresca friction. Numerical experiments show the efficiency of the proposed method.

Valence and origin of metal–insulator transition in Mn doped SrRuO 3 studied by electrical transport, X-ray photoelectron spectroscopy and LSDA+ U calculation

We have studied the valence and electronic properties of Mn doped SrRuO 3 using electrical transport measurement, X-ray photoelectron spectroscopy (XPS) and local (spin) density approximation plus Coulomb interaction strength calculation (LSDA+ U). The resistivity data revealed that the system undergoes transition from metal to insulator at the critical Mn doping level, x∼0.2, which is accompanied by the structural transition from orthorhombic to tetragonal crystal symmetry. Besides, the significant reduction of the spectral weight at the coherent zone (0.8 eV) of the valence band is observed for x>0.2. The core XPS spectra suggest that both the transition elements exist in the mixed ionic pair, Ru +4/Ru +5↔Mn +3/Mn +4. The detail analysis of the results suggests that the Coulomb correlation effect in conjugation with localization of the charge carriers predominate over the mixed ionic pair effect and responsible for the metal–insulator transition in the series.

Quantitative determination of spin-dependent quasiparticle lifetimes and electronic correlations in hcp cobalt

We report on a quantitative investigation of the spin-dependent quasiparticle lifetimes and electron correlation effects in ferromagnetic hcp Co(0001) by means of spin and angle-resolved photoemission spectroscopy. The experimental spectra are compared in detail to state-of-the-art many-body calculations within the dynamical mean field theory and the three-body scattering approximation, including a full calculation of the one-step photoemission process. From this comparison we conclude that although strong local many-body Coulomb interactions are of major importance for the qualitative description of correlation effects in Co, more sophisticated many-body calculations are needed in order to improve the quantitative agreement between theory and experiment, in particular concerning the linewidths. The quality of the overall agreement obtained for Co indicates that the effect of non-local correlations becomes weaker with increasing atomic number.

Phase diagram ofLaVO3under epitaxial strain: Implications for thin films grown onSrTiO3andLaAlO3substrates

Various exotic phenomena have been observed in epitaxially grown films and superlattices of transition-metal oxides. In these systems, not only the interface properties but also the strain-induced modification in the bulk properties play important roles. With the recent experimental activities [Y. Hotta, T. Susaki, and H. Y. Hwang, Phys. Rev. Lett. 99, 236805 (2007)] in mind, we have studied the epitaxial strain effects on the electronic structure of Mott insulator ${\text{LaVO}}_{3}$. The present work is based on the calculations using density-functional theory supplemented by adding local Coulomb repulsion $U$ for $\text{V}\text{ }d$ orbitals. The range of strain studied here extends from $c/a=0.98$ (bulk ${\text{LaVO}}_{3}$ case) to $c/a=1.107$ (${\text{LaAlO}}_{3}$ substrate case). In this range of the strain, we have found the following three different antiferromagnetic spin-ordering (SO) phases. For $0.98<c/a<1.005$, the combination of $C$-type SO and $G$-type orbital ordering (OO) is the most stable. The bulk ${\text{LaVO}}_{3}$ belongs to this range. For $1.005<c/a<1.095$, the ground state has $A$-type SO and $G$-type OO. ${\text{LaVO}}_{3}$ epitaxially grown on ${\text{SrTiO}}_{3}$ is in this range. When $c/a>1.095$, $G$-type SO with ferromagnetic OO becomes the ground state. This range includes the case of ${\text{LaAlO}}_{3}$ substrate. The implications of these results with regard to the experimental data for thin films of ${\text{LaVO}}_{3}$ on ${\text{SrTiO}}_{3}$ and ${\text{LaAlO}}_{3}$ substrates will be described. Detailed discussion is given on the mechanisms of stabilizing particular combination of SO and OO in each of three phases.

Competition between antiferromagnetic and charge order in the Hubbard-Holstein model

We study the competition between an instantaneous local Coulomb repulsion and a boson mediated retarded attraction, as described by the Hubbard-Holstein model. Restricting to the case of half filling, the ground-state phase diagram and the transitions from antiferromagnetically ordered states to charge ordered states are analyzed. The calculations are based on the model in large dimensions, so that dynamical mean field theory can be applied, and the associated impurity problem is solved using the numerical renormalization group method. The transition is found to occur when electron-electron coupling strength $U$ and the induced interaction $\lambda$ due to electron-phonon coupling approximately coincide, $U\simeq \lambda$. We find a continuous transition for small coupling and large $\omega_0$, and a discontinuous one for large coupling and/or small $\omega_0$. We present results for the order parameters, the static expectation values for the electrons and phonons, and the corresponding spectral functions. They illustrate the different types of behavior to be seen near the transitions. Additionally, the quasiparticle properties are calculated in the normal state, which leads to a consistent interpretation of the low energy excitations.

Aftershock Sequences Modeled with 3-D Stress Heterogeneity and Rate-State Seismicity Equations: Implications for Crustal Stress Estimation

In this paper, we present a model for studying aftershock sequences that integrates Coulomb static stress change analysis, seismicity equations based on rate-state friction nucleation of earthquakes, slip of geometrically complex faults, and fractal-like, spatially heterogeneous models of crustal stress. In addition to modeling instantaneous aftershock seismicity rate patterns with initial clustering on the Coulomb stress increase areas and an approximately 1/t diffusion back to the pre-mainshock background seismicity, the simulations capture previously unmodeled effects. These include production of a significant number of aftershocks in the traditional Coulomb stress shadow zones and temporal changes in aftershock focal mechanism statistics. The occurrence of aftershock stress shadow zones arises from two sources. The first source is spatially heterogeneous initial crustal stress, and the second is slip on geometrically rough faults, which produces localized positive Coulomb stress changes within the traditional stress shadow zones. Temporal changes in simulated aftershock focal mechanisms result in inferred stress rotations that greatly exceed the true stress rotations due to the main shock, even for a moderately strong crust (mean stress 50 MPa) when stress is spatially heterogeneous. This arises from biased sampling of the crustal stress by the synthetic aftershocks due to the non-linear dependence of seismicity rates on stress changes. The model indicates that one cannot use focal mechanism inversion rotations to conclusively demonstrate low crustal strength (≤10 MPa); therefore, studies of crustal strength following a stress perturbation may significantly underestimate the mean crustal stress state for regions with spatially heterogeneous stress.