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Jul 20, 2013
07/13

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J. Buenemann; F. Gebhard

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We derive the random-phase approximation for spin excitations in general multi-band Hubbard models, starting from a collinear ferromagnetic Hartree-Fock ground state. The results are compared with those of a recently introduced variational many-body approach to spin-waves in itinerant ferromagnets. As we exemplify for Hubbard models with one and two bands, the two approaches lead to qualitatively different results. The discrepancies can be traced back to the fact that the Hartree-Fock theory...

Source: http://arxiv.org/abs/cond-mat/0107050v1

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Sep 20, 2013
09/13

by
R. M. Noack; F. Gebhard

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We calculate the zero-temperature gap and quasiparticle weight of the half-filled Hubbard model with a random dispersion relation. After extrapolation to the thermodynamic limit, we obtain reliable bounds on these quantities for the Hubbard model in infinite dimensions. Our data indicate that the Mott-Hubbard transition is continuous, i.e., that the quasiparticle weight becomes zero at the same critical interaction strength at which the gap opens.

Source: http://arxiv.org/abs/cond-mat/9810222v2

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Sep 23, 2013
09/13

by
F. Gebhard; P. -A. Bares

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We investigate spin-1/2 electrons with local Hubbard interaction and variable range hopping amplitudes which decay like~$\sinh(\kappa)/\sinh(\kappa r)$. Assuming integrability the Asymptotic Bethe Ansatz approach allows us to derive the generalized Lieb-Wu integral equations from the two-particle phase shift. Due to the nesting property there is a metal-to-insulator transition at $U_c(\kappa>0)=0^+$. The charge gap in the singular limit $\kappa=0$ opens when the interaction strength equals...

Source: http://arxiv.org/abs/cond-mat/9406084v3

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Sep 18, 2013
09/13

by
S. Ejima; H. Fehske; F. Gebhard

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We use the density-matrix renormalization group method to investigate ground-state and dynamic properties of the one-dimensional Bose-Hubbard model, the effective model of ultracold bosonic atoms in an optical lattice. For fixed maximum site occupancy $n_b=5$, we calculate the phase boundaries between the Mott insulator and the `superfluid' phase for the lowest two Mott lobes. We extract the Tomonaga-Luttinger parameter from the density-density correlation function and determine accurately the...

Source: http://arxiv.org/abs/1102.2028v1

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Jul 20, 2013
07/13

by
W. Weber; J. Buenemann; F. Gebhard

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Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The exact evaluation of these variational ground states in the limit of large coordination number allows the identification of quasi-particle band structures, and the calculation of a variational spinwave dispersion. The study of a generic two-band model elucidates the co-operation of the Coulomb repulsion and the...

Source: http://arxiv.org/abs/cond-mat/0107033v1

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Jul 20, 2013
07/13

by
J. Bünemann; W. Weber; F. Gebhard

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We introduce Gutzwiller wave functions for multi-band models with general on-site Coulomb interactions. As these wave functions employ correlators for the exact atomic eigenstates they are exact both in the non-interacting and in the atomic limit. We evaluate them in infinite lattice dimensions for all interaction strengths without any restrictions on the structure of the Hamiltonian or the symmetry of the ground state. The results for the ground-state energy allow us to derive an effective...

Source: http://arxiv.org/abs/cond-mat/9712240v1

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Sep 18, 2013
09/13

by
J. Buenemann; F. Gebhard; R. Thul

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We define Landau quasi-particles within the Gutzwiller variational theory, and derive their dispersion relation for general multi-band Hubbard models in the limit of large spatial dimensions D. Thereby we reproduce our previous calculations which were based on a phenomenological effective single-particle Hamiltonian. For the one-band Hubbard model we calculate the first-order corrections in 1/D and find that the corrections to the quasi-particle dispersions are small in three dimensions. They...

Source: http://arxiv.org/abs/cond-mat/0202434v2

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Sep 20, 2013
09/13

by
J. Rissler; E. Jeckelmann; F. Gebhard

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The density-matrix renormalization group (DMRG) is employed to calculate optical properties of the half-filled Hubbard model with nearest-neighbor interactions. In order to model the optical excitations of oligoenes, a Peierls dimerization is included whose strength for the single bonds may fluctuate. Systems with up to 100 electrons are investigated, their wave functions are analyzed, and relevant length-scales for the low-lying optical excitations are identified. The presented approach...

Source: http://arxiv.org/abs/cond-mat/0405180v2

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Sep 21, 2013
09/13

by
H. Benthien; F. Gebhard; E. Jeckelmann

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We calculate the photoemission spectral function of the one-dimensional Hubbard model away from half filling using the dynamical density matrix renormalization group method. An approach for calculating momentum-dependent quantities in finite open chains is presented. Comparison with exact Bethe Ansatz results demonstrates the unprecedented accuracy of our method. Our results show that the photoemission spectrum of the quasi-one-dimensional conductor TTF-TCNQ provides evidence for spin-charge...

Source: http://arxiv.org/abs/cond-mat/0402664v2

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Sep 23, 2013
09/13

by
S. Ejima; H. Fehske; F. Gebhard

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In order to identify possible experimental signatures of the superfluid to Mott-insulator quantum phase transition we calculate the charge structure factor $S(k,\omega)$ for the one-dimensional Bose-Hubbard model using the dynamical density-matrix renormalisation group (DDMRG) technique. Particularly we analyse the behaviour of $S(k, \omega)$ by varying---at zero temperature---the Coulomb interaction strength within the first Mott lobe. For strong interactions, in the Mott-insulator phase, we...

Source: http://arxiv.org/abs/1110.4498v2

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Sep 22, 2013
09/13

by
S. Ejima; F. Gebhard; R. M. Noack

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We use the Random Dispersion Approximation (RDA) to study the Mott-Hubbard transition in the Hubbard model at half band filling. The RDA becomes exact for the Hubbard model in infinite dimensions. We implement the RDA on finite chains and employ the Lanczos exact diagonalization method in real space to calculate the ground-state energy, the average double occupancy, the charge gap, the momentum distribution, and the quasi-particle weight. We find a satisfactory agreement with perturbative...

Source: http://arxiv.org/abs/0811.4271v1

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Sep 19, 2013
09/13

by
F. Gebhard; A. Girndt; A. E. Ruckenstein

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We discuss the transition from a metal to charge or spin insulating phases characterized by the opening of a gap in the charge or spin excitation spectra, respectively. These transitions are addressed within the context of two exactly solvable Hubbard and tJ chains with long range, $1/r$ hopping. We discuss the specific heat, compressibility, and magnetic susceptibility of these models as a function of temperature, band filling, and interaction strength. We then use conformal field theory...

Source: http://arxiv.org/abs/cond-mat/9309006v1

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Sep 23, 2013
09/13

by
N. Sedlmayr; J. Ren; F. Gebhard; J. Sirker

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We study thermalization in a one-dimensional quantum system consisting of a noninteracting fermionic chain with each site of the chain coupled to an additional bath site. Using a density matrix renormalization group algorithm we investigate the time evolution of observables in the chain after a quantum quench. For low densities we show that the intermediate time dynamics can be quantitatively described by a system of coupled equations of motion. For higher densities our numerical results show a...

Source: http://arxiv.org/abs/1212.0223v2

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Sep 19, 2013
09/13

by
E. Jeckelmann; F. Gebhard; F. H. L. Essler

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We combine well-controlled analytical and numerical methods to determine the optical conductivity of the one-dimensional Mott-Hubbard insulator at zero temperature. A dynamical density-matrix renormalization group method provides the entire absorption spectrum for all but very small coupling strengths. In this limit we calculate the conductivity analytically using exact field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a characteristic square-root increase. For small to...

Source: http://arxiv.org/abs/cond-mat/9911281v2

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Sep 24, 2013
09/13

by
F. H. L. Essler; F. Gebhard; E. Jeckelmann

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We employ dynamical density-matrix renormalization group (DDMRG) and field-theory methods to determine the frequency-dependent optical conductivity in one-dimensional extended, half-filled Hubbard models. The field-theory approach is applicable to the regime of `small' Mott gaps which is the most difficult to access by DDMRG. For very large Mott gaps the DDMRG recovers analytical results obtained previously by means of strong-coupling techniques. We focus on exciton formation at energies below...

Source: http://arxiv.org/abs/cond-mat/0103406v2

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Sep 21, 2013
09/13

by
S. Ejima; F. H. L. Essler; F. Gebhard

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We analyze the Thermodynamic Bethe Ansatz equations of the one-dimensional half-filled Hubbard model in the "spin-disordered regime", which is characterized by the temperature being much larger than the magnetic energy scale but small compared to the Mott-Hubbard gap. In this regime the thermodynamics of the Hubbard model can be thought of in terms of gapped charged excitations with an effective dispersion and spin degrees of freedom that only contribute entropically. In particular,...

Source: http://arxiv.org/abs/cond-mat/0602310v2

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Jul 20, 2013
07/13

by
F. Jansson; S. D. Baranovskii; F. Gebhard; R. Österbacka

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For hopping transport in disordered materials, the mobility of charge carriers is strongly dependent on temperature and the electric field. Our numerical study shows that both the energy distribution and the mobility of charge carriers in systems with a Gaussian density of states, such as organic disordered semiconductors, can be described by a single parameter - effective temperature, dependent on the magnitude of the electric field. Furthermore, this effective temperature does not depend on...

Source: http://arxiv.org/abs/0804.2540v1

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Jul 20, 2013
07/13

by
J. Bünemann; F. Gebhard; T. Ohm; S. Weiser; W. Weber

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We use the Gutzwiller variational theory to investigate the electronic and the magnetic properties of fcc-Nickel. Our particular focus is on the effects of the spin-orbit coupling. Unlike standard relativistic band-structure theories, we reproduce the experimental magnetic moment direction and we explain the change of the Fermi-surface topology that occurs when the magnetic moment direction is rotated by an external magnetic field. The Fermi surface in our calculation deviates from early...

Source: http://arxiv.org/abs/0804.2211v1

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Jul 20, 2013
07/13

by
X. Huang; E. Szirmai; F. Gebhard; J. Sólyom; R. M. Noack

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We investigate the half-filled Hubbard chain with additional nearest- and next-nearest-neighbor spin exchange, J1 and J2, using bosonization and the density-matrix renormalization group. For J2 = 0 we find a spin-density-wave phase for all positive values of the Hubbard interaction U and the Heisenberg exchange J1. A frustrating spin exchange J2 induces a bond-order-wave phase. For some values of J1, J2 and U, we observe a spin-gapped metallic Luther-Emery phase.

Source: http://arxiv.org/abs/0804.4810v1

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Sep 21, 2013
09/13

by
F. Gebhard; K. Bott; M. Scheidler; P. Thomas; S. W. Koch

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In this last of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain we address the dimerized extended Hubbard model in the limit of a large on-site interaction $U$. When the Hubbard interaction is large both compared to the band width $W$ and the nearest neighbor interaction $V$ the charge dynamics is properly described by the Harris-Lange model. This model can be exactly mapped onto a model of free spinless Fermions in parallel (Hubbard-)bands of...

Source: http://arxiv.org/abs/cond-mat/9609091v1

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63

Sep 21, 2013
09/13

by
F. Gebhard; K. Bott; M. Scheidler; P. Thomas; S. W. Koch

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In this second of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain we present exact results for strongly correlated tight-binding electrons. In the limit of a strong on-site interaction $U$ we map the Hubbard model onto the Harris-Lange model which can be solved exactly in one dimension in terms of spinless fermions for the charge excitations. The exact solution allows for an interpretation of the charge dynamics in terms of parallel Hubbard bands...

Source: http://arxiv.org/abs/cond-mat/9609090v1

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83

Sep 21, 2013
09/13

by
F. Gebhard; K. Bott; M. Scheidler; P. Thomas; S. W. Koch

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In this first of three articles on the optical absorption of electrons in half-filled Peierls-distorted chains we present analytical results for non-interacting tight-binding electrons. We carefully derive explicit expressions for the current operator, the dipole transition matrix elements, and the optical absorption for electrons with a cosine dispersion relation of band width $W$ and dimerization parameter $\delta$. New correction (``$\eta$''-)terms to the current operator are identified. A...

Source: http://arxiv.org/abs/cond-mat/9609089v1

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Sep 23, 2013
09/13

by
F. Gebhard; K. zu Muenster; J. Ren; N. Sedlmayr; J. Sirker; B. Ziebarth

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We investigate a model system for the injection of fermionic particles from filled source sites into an empty chain. We study the ensuing dynamics for Hermitian as well as for non-Hermitian time evolution where the particles cannot return to the bath sites (quantum ratchet). A non-homogeneous hybridization between bath and chain sites permits transient currents in the chain. Non-interacting particles show decoherence in the thermodynamic limit: the average particle number and the average...

Source: http://arxiv.org/abs/1110.5245v1

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400

Oct 24, 2008
10/08

by
Gebhard i. e. F . Gebhard Eckler, J. K ., Eduard Ferdinand Angerstein, Gebhard Eckler

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Book digitized by Google from the library of Harvard University and uploaded to the Internet Archive by user tpb.

Source: http://books.google.com/books?id=JhkAAAAAYAAJ&oe=UTF-8

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Sep 21, 2013
09/13

by
A. V. Nenashev; F. Jansson; S. D. Baranovskii; R. Österbacka; A. V. Dvurechenskii; F. Gebhard

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Effects of strong electric fields on hopping conductivity are studied theoretically. Monte-Carlo computer simulations show that the analytical theory of Nguyen and Shklovskii [Solid State Commun. 38, 99 (1981)] provides an accurate description of hopping transport in the limit of very high electric fields and low concentrations of charge carriers as compared to the concentration of localization sites and also at the relative concentration of carriers equal to 0.5. At intermediate concentrations...

Source: http://arxiv.org/abs/0808.2430v2

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112

Sep 17, 2013
09/13

by
A. V. Nenashev; F. Jansson; S. D. Baranovskii; R. Österbacka; A. V. Dvurechenskii; F. Gebhard

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An exact analytical theory is developed for calculating the diffusion coefficient of charge carriers in strongly anisotropic disordered solids with one-dimensional hopping transport mode for any dependence of the hopping rates on space and energy. So far such a theory existed only for calculating the carrier mobility. The dependence of the diffusion coefficient on the electric field evidences a linear, non-analytic behavior at low fields for all considered models of disorder. The mobility, on...

Source: http://arxiv.org/abs/0912.3161v1

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63

Sep 17, 2013
09/13

by
A. V. Nenashev; F. Jansson; S. D. Baranovskii; R. Österbacka; A. V. Dvurechenskii; F. Gebhard

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In the previous paper [Nenashev et al., arXiv:0912.3161] an analytical theory confirmed by numerical simulations has been developed for the field-dependent hopping diffusion coefficient D(F) in one-dimensional systems with Gaussian disorder. The main result of that paper is the linear, non-analytic field dependence of the diffusion coefficient at low electric fields. In the current paper, an analytical theory is developed for the field-dependent diffusion coefficient in three- and...

Source: http://arxiv.org/abs/0912.3169v1

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Sep 22, 2013
09/13

by
A. V. Nenashev; F. Jansson; S. D. Baranovskii; R. Österbacka; A. V. Dvurechenskii; F. Gebhard

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Experiments on carrier recombination in two-dimensional organic structures are often interpreted in the frame of the Langevin model with taking into account only the drift of the charge carriers in their mutual electric field. While this approach is well justified for three-dimensional systems, it is in general not valid for two-dimensional structures, where the contribution of diffusion can play a dominant role. We study the two-dimensional Langevin recombination theoretically and find the...

Source: http://arxiv.org/abs/1004.0821v1

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Sep 23, 2013
09/13

by
A. Gliesche; C. Michel; V. Rajevac; K. Lips; S. D. Baranovskii; F. Gebhard; C. Boehme

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The effect of exchange interactions within spin pairs on spin-dependent transport and recombination rates through localized states in semiconductors during coherent electron spin resonant excitation is studied theoretically. It is shown that for identical spin systems, significant quantitative differences are to be expected between the results of pEDMR/pODMR experiments were permutation symmetry is the observable as compared to pESR experiments with polarization as the observable. It is...

Source: http://arxiv.org/abs/0801.1304v2

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Sep 20, 2013
09/13

by
J. Buenemann; F. Gebhard; T. Ohm; R. Umstaetter; S. Weiser; W. Weber; R. Claessen; D. Ehm; A. Harasawa; A. Kakizaki; A. Kimura; G. Nicolay; S. Shin; V. N. Strocov

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We measure the band structure of nickel along various high-symmetry lines of the bulk Brillouin zone with angle-resolved photoelectron spectroscopy. The Gutzwiller theory for a nine-band Hubbard model whose tight-binding parameters are obtained from non-magnetic density-functional theory resolves most of the long-standing discrepancies between experiment and theory on nickel. Thereby we support the view of itinerant ferromagnetism as induced by atomic correlations.

Source: http://arxiv.org/abs/cond-mat/0204142v1