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Wolfgang von der Linden

Publications and source records attributed to Wolfgang von der Linden.

69 records · Page 4Linked to original sources
arXiv2010

Spectral properties of coupled cavity arrays in one dimension

Spectral properties of coupled cavity arrays in one dimension are investigated by means of the variational cluster approach. Coupled cavity arrays consist of two distinct "particles," namely, photons and atomiclike excitations. Spectral functions are evaluated and discussed for both particle types. In addition, densities of states, momentum distributions and spatial correlation functions are presented. Based on this information, polariton "quasiparticles" are introduced as appropriate, wave vector and filling dependent linear combinations of photon and atomiclike particles. Spectral functions and densities of states are evaluated for the polariton quasiparticles, and the weights of their components are analyzed.

cond-mat.other↗
arXiv2010

Spectral properties of strongly correlated bosons in two-dimensional optical lattices

Spectral properties of the two-dimensional Bose-Hubbard model, which emulates ultracold gases of atoms confined in optical lattices, are investigated by means of the variational cluster approach. The phase boundary of the quantum phase transition from Mott phase to superfluid phase is calculated and compared to recent work. Moreover the single-particle spectral functions in both the first and the second Mott lobe are presented and the corresponding densities of states and momentum distributions are evaluated. A qualitatively similar intensity distribution of the spectral weight can be observed for spectral functions in the first and the second Mott lobe.

cond-mat.quant-gas↗
arXiv2009

Annealing Strategies in the Simulation of Fullerene Formation

We investigate the formation of fullerene-like structures from hot Carbon gas using classical molecular dynamics, employing Brenner's potential. In particular we examine the influence of different annealing strategies on fullerene yield, which is characterized by the distribution of coordination numbers and polygon numbers. It will be shown that the fullerene yield strongly depends on the annealing strategy. Furthermore, we observe a close relation between polygon formation and the number of atoms surrounded by three atoms.

cond-mat.mtrl-sci↗
arXiv2009

Effect of the exchange hole on the Gutzwiller approximation in one dimension

The Gutzwiller approximate solution to the Gutzwiller wavefunction yields exact results for the Gutzwiller wavefunction in the infinite dimensional limit. Implicit in the Gutzwiller approximation is an approximate local form of the fermion exchange hole. This approximate form is the same for all dimensions but is incorrect except in infinite dimensions. We implement the correct form for the exchange hole into the Gutzwiller approximation. We perform calculations on the one-dimensional Hubbard model at half-filling. They indicate that the implementation of the exchange hole already brings the Gutzwiller approximation into very close quantitative agreement with the results of the full Gutzwiller wavefunction. Metallicity as well as anti-ferromagnetism are recovered.

cond-mat.str-el↗
arXiv2005

Temperature- and quantum phonon effects on Holstein-Hubbard bipolarons

The one-dimensional Holstein-Hubbard model with two electrons of opposite spin is studied using an extension of a recently developed quantum Monte Carlo method, and a very simple yet rewarding variational approach, both based on a canonically transformed Hamiltonian. The quantum Monte Carlo method yields very accurate results in the regime of small but finite phonon frequencies, characteristic of many strongly correlated materials such as, e.g., the cuprates and the manganites. The influence of electron-electron repulsion, phonon frequency and temperature on the bipolaron state is investigated. Thermal dissociation of the intersite bipolaron is observed at high temperatures, and its relation to an existing theory of the manganites is discussed.

cond-mat.str-el↗
arXiv2004

Single-particle spectral function of the Holstein-Hubbard bipolaron

The one-electron spectral function of the Holstein-Hubbard bipolaron in one dimension is studied using cluster perturbation theory together with the Lanczos method. In contrast to other approaches, this allows one to calculate the spectrum at continuous wave vectors and thereby to study, for the first time, the dispersion and the spectral weight of quasiparticle features. The formation of polarons and bipolarons, and their manifestation in the spectral properties of the system, is studied for the cases of intermediate and large phonon frequencies, with and without Coulomb interaction. A good agreement is found with the most accurate calculations of the bipolaron dispersion available, over a large range of the electron-phonon coupling strength. Additionally, pronounced deviations of the bipolaron dispersion from a simple tight-binding band are found, which can be attributed to next-nearest-neighbor hopping processes.

cond-mat.str-el↗
arXiv2004

Holstein polaron in two and three dimensions by quantum Monte Carlo

A recently developed quantum Monte Carlo approach to the Holstein model with one electron [PRB 69, 024301 (2004)] is extended to two and three dimensional lattices. A moderate sign problem occurs, which is found to diminish with increasing system size in all dimensions, and not to affect simulations significantly. We present an extensive study of the influence of temperature, system size, dimensionality and model parameters on the small-polaron cross over. Results are extrapolated to remove the error due to the Trotter discretization, which significantly improves the accuracy. Comparison with existing work and other quantum Monte Carlo methods is made. The method can be extended to the many-electron case.

cond-mat.str-el↗
arXiv2004

Quantum Monte Carlo and variational approaches to the Holstein model

Based on the canonical Lang-Firsov transformation of the Hamiltonian we develop a very efficient quantum Monte Carlo algorithm for the Holstein model with one electron. Separation of the fermionic degrees of freedom by a reweighting of the probability distribution leads to a dramatic reduction in computational effort. A principal component representation of the phonon degrees of freedom allows to sample completely uncorrelated phonon configurations. The combination of these elements enables us to perform efficient simulations for a wide range of temperature, phonon frequency and electron-phonon coupling on clusters large enough to avoid finite-size effects. The algorithm is tested in one dimension and the data are compared with exact-diagonalization results and with existing work. Moreover, the ideas presented here can also be applied to the many-electron case. In the one-electron case considered here, the physics of the Holstein model can be described by a simple variational approach.

cond-mat.str-el↗
arXiv2003

Spectral function of electron-phonon models by cluster perturbation theory

Cluster perturbation theory in combination with the Lanczos method is used to compute the one-electron spectral function of the Holstein polaron in one and two dimensions. It is shown that the method allows reliable calculations using relatively small clusters, and at the same time significantly reduces finite-size effects. Results are compared with exact data and the relation to existing work is discussed. We also use a strong-coupling perturbation theory--equivalent to the Hubbard I approximation--to calculate the spectral function of the quarter-filled Holstein model of spinless fermions, starting from the exact atomic-limit Green function. The results agree well with previous calculations within the many-body coherent potential approximation.

cond-mat.str-el↗
arXiv2003

Magnetic Polarons in the 1D FM Kondo Model

The ferromagnetic Kondo model with classical corespins is studied via unbiased Monte-Carlo simulations. We show that with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation in one-dimensional chains but rather stabilizes individual ferromagnetic polarons. Within the ferromagnetic polaron picture, the pseudogap in the one-particle spectral function A_k(ω) can easily be explained. Ferromagnetic polarons also clear up a seeming failure of the double-exchange mechanism in explaining the comparable bandwidths in the ferromagnetic and paramagnetic phase. For our analysis, we extend a simplified model, the finite temperature uniform hopping approach (UHA), to include polarons. It can easily be evaluated numerically and provides a simple quantitative understanding of the physical features of the ferromagnetic Kondo model.

cond-mat.str-el↗
arXiv2002

Uniform hopping approach to the FM Kondo Model at finite temperature

We study the ferromagnetic Kondo model with classical corespins via unbiased Monte-Carlo simulations and derive a simplified model for the treatment of the corespins at any temperature. Our simplified model captures the main aspects of the Kondo model and can easily be evaluated both numerically and analytically. It provides a better qualitative understanding of the physical features of the Kondo model and rationalizes the Monte-Carlo results, including the spectral density A_k(omega) of a 1D chain with nearest neighbor Coulomb repulsion. By calculating the specific heat and the susceptibility of systems up to size 16^3, we determine the Curie temperature of the 3D one-orbital double-exchange model, which agrees with experimental values.

cond-mat.str-el↗
arXiv2002

Effective spinless fermions in the strong coupling Kondo model

Starting from the two-orbital Kondo-lattice model with classical t_2g spins, an effective spinless fermion model is derived for strong Hund coupling J_H with a projection technique. The model is studied by Monte Carlo simulations and analytically using a uniform hopping approximation. The results for the spinless fermion model are in remarkable agreement with those of the original Kondo-lattice model, independent of the carrier concentration, and even for moderate Hund coupling J_H. Phase separation, the phase diagram in uniform hopping approximation, as well as spectral properties including the formation of a pseudo-gap are discussed for both the Kondo-lattice and the effective spinless fermion model in one and three dimensions.

cond-mat.str-el↗
arXiv2002

Low Temperature Lanczos Method

We present a modified finite temperature Lanczos method for the evaluation of dynamical and static quantities of strongly correlated electron systems that complements the finite temperature method (FTLM) introduced by Jaklic and Prelovsek for low temperatures. Together they allow accurate calculations at any temperature with moderate effort. As an example we calculate the static spin correlation function and the regular part of the optical conductivity of the one dimensional Hubbard model at half-filling and show in detail the connection between the ground state and finite temperature method. By using Cluster Perturbation Theory (CPT), the finite temperature spectral function is extended to the infinite system, clearly exhibiting the effects of spin-charge separation.

cond-mat.str-el↗
arXiv2001

Temperature dependence of optical spectral weights in quarter-filled ladder systems

The temperature dependence of the integrated optical conductivity I(T) reflects the changes of the kinetic energy as spin and charge correlations develop. It provides a unique way to explore experimentally the kinetic properties of strongly correlated systems. We calculated I(T) in the frame of a t-J-V model at quarter-filling for ladder systems, like NaV_2O_5, and show that the measured strong T dependence of I(T) for NaV_2O_5 can be explained by the destruction of short range antiferromagnetic correlations. Thus I(T) provides detailed information about super-exchange and magnetic energy scales.

cond-mat.str-el↗