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Martin Hohenadler

Publications and source records attributed to Martin Hohenadler.

50 records · Page 3Linked to original sources
arXiv2008

Quantum Fluctuations, Temperature and Detuning Effects in Solid-Light Systems

The superfluid to Mott insulator transition in cavity polariton arrays is analyzed using the variational cluster approach, taking into account quantum fluctuations exactly on finite length scales. Phase diagrams in one and two dimensions exhibit important non-mean-field features. Single-particle excitation spectra in the Mott phase are dominated by particle and hole bands separated by a Mott gap. In contrast to Bose-Hubbard models, detuning allows for changing the nature of the bosonic particles from quasi-localized excitons to polaritons to weakly interacting photons. The Mott state with density one exists up to temperatures $T/g\gtrsim0.03$, implying experimentally accessible temperatures for realistic cavity couplings $g$.

cond-mat.other↗
arXiv2007

Lattice exciton-polaron problem by quantum Monte Carlo simulations

Exciton-polaron formation in one-dimensional lattice models with short or long-range carrier-phonon interaction is studied by quantum Monte Carlo simulations. Depending on the relative sign of electron and hole-phonon coupling, the exciton-polaron size increases or decreases with increasing interaction strength. Quantum phonon fluctuations determine the (exciton-)polaron size and yield translation-invariant states at all finite couplings.

cond-mat.str-el↗
arXiv2007

Quantum Monte Carlo results for bipolaron stability in quantum dots

Bipolaron formation in a two-dimensional lattice with harmonic confinement, representing a simplified model for a quantum dot, is investigated by means of quantum Monte Carlo simulations. This method treats all interactions exactly and takes into account quantum lattice fluctuations. Calculations of the bipolaron binding energy reveal that confinement opposes bipolaron formation for weak electron-phonon coupling, but abets a bound state at intermediate to strong coupling. Tuning the system from weak to strong confinement gives rise to a small reduction of the minimum Frohlich coupling parameter for the existence of a bound state.

cond-mat.str-el↗
arXiv2007

Optical conductivity of polaronic charge carriers

The optical conductivity of charge carriers coupled to quantum phonons is studied in the framework of the one-dimensional spinless Holstein model. For one electron, variational diagonalisation yields exact results in the thermodynamic limit, whereas at finite carrier density analytical approximations based on previous work on single-particle spectral functions are obtained. Particular emphasis is put on deviations from weak-coupling, small-polaron or one-electron theories occurring at intermediate coupling and/or finite carrier density. The analytical results are in surprisingly good agreement with exact data, and exhibit the characteristic polaronic excitations observed in experiments on manganites.

cond-mat.str-el↗
arXiv2007

Bipolaron formation in 1D-3D quantum dots: A lattice quantum Monte Carlo approach

Polaron and bipolaron formation in the Holstein-Hubbard model with harmonic confinement potential, relevant to quantum dot structures, is investigated in one to three dimensions by means of unbiased quantum Monte Carlo simulations. The discrete nature of the lattice and quantum phonon effects are fully taken into account. The dependence on phonon frequency, Coulomb repulsion, confinement strength (dot size) and electron-phonon interaction strength is studied over a wide range of parameter values. Confinement is found to reduce the size of (bi-)polarons at a given coupling strength, to reduce the critical coupling for small-(bi-)polaron formation, to increase the polaron binding energy, and to be more important in lower dimensions. The present method also permits to consider models with dispersive phonons, anharmonic confinement, or long-range interactions.

cond-mat.str-el↗
arXiv2006

Carrier-density effects in many-polaron systems

Many-polaron systems with finite charge-carrier density are often encountered experimentally. However, until recently, no satisfactory theoretical description of these systems was available even in the framework of simple models such as the one-dimensional spinless Holstein model considered here. In this work, previous results obtained using numerical as well as analytical approaches are reviewed from a unified perspective, focussing on spectral properties which reveal the nature of the quasiparticles in the system. In the adiabatic regime and for intermediate electron-phonon coupling, a carrier-density driven crossover from a polaronic to a rather metallic system takes place. Further insight into the effects due to changes in density is gained by calculating the phonon spectral function, and the fermion-fermion and fermion-lattice correlation functions. Finally, we provide strong evidence against the possibility of phase separation.

cond-mat.str-el↗
arXiv2006

Many-polaron problem by cluster perturbation theory

The carrier-density dependence of the photoemission spectrum of the Holstein many-polaron model is studied using cluster perturbation theory combined with an improved cluster diagonalization by Chebychev expansion.

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↗