Maria Daghofer

Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.

Low-temperature Lanczos method for strongly correlated systems

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 introduced by Jaklic andPrelovsek 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 σ r e g (ω) 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, the finite-temperature spectral function is extended to the infinite system, clearly exhibiting the effects of spin-charge separation.

Polaronic aspects of the two-dimensional ferromagnetic Kondo model

The two-dimensional ferromagnetic Kondo model with classical core spins is studied via unbiased Monte Carlo simulations for a hole doping up to x = 12.5%. A canonical algorithm for finite temperatures is developed. We show that, with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation on a two-dimensional lattice but rather stabilizes individual ferromagnetic polarons for this doping range. A detailed analysis of unbiased Monte Carlo results reveals that the polarons can be treated as independent particles for these hole concentrations. It is found that a simple polaron model describes the physics of the ferromagnetic Kondo model amazingly well. The ferromagnetic polaron picture provides an obvious explanation for the pseudogap in the one-particle spectral function Ak(ω) observed in the ferromagnetic Kondo model.

Orbital polarons versus itinerant e g electrons in doped manganites

We study an effective one-dimensional (1D) orbital t-J model derived for strongly correlated e_g electrons in doped manganites. The ferromagnetic spin order at half filling is supported by orbital superexchange prop. to J which stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a doped system it competes with the kinetic energy prop. to t. When a single hole is doped to a half-filled chain, its motion is hindered and a localized orbital polaron is formed. An increasing doping generates either separated polarons or phase separation into hole-rich and hole-poor regions, and eventually polarizes the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals. This crossover, investigated by exact diagonalization at zero temperature, is demonstrated both by the behavior of correlation functions and by spectral properties, showing that the orbital chain with Ising superexchange is more classical and thus radically different from the 1D spin t-J model. At finite temperature we derive and investigate an effective 1D orbital model using a combination of exact diagonalization with classical Monte-Carlo for spin correlations. A competition between the antiferromagnetic and ferromagnetic spin order was established at half filling, and localized polarons were found for antiferromagnetic interactions at low hole doping. Finally, we clarify that the Jahn-Teller alternating potential stabilizes the orbital order with staggered orbitals, inducing the ferromagnetic spin order and enhancing the localized features in the excitation spectra. Implications of these findings for colossal magnetoresistance manganites are discussed.

Single-particle spectrum of the flux phase in the FM Kondo model

Abstract We investigate the 2D ferromagnetic Kondo lattice model for manganites with classical corespins at Hunds rule coupling J H = 6 , with antiferromagnetic superexchange 0.03 ⩽ J ′ ⩽ 0.05 . We employ canonical and grand canonical unbiased Monte Carlo simulations and find paramagnetism, weak ferromagnetism and the flux phase, depending on doping and on J ′ . The observed single-particle spectrum in the flux phase differs from the idealized infinite lattice case, but agrees well with an idealized finite lattice case with thermal fluctuations.

Aspects of the FM Kondo Model: From Unbiased MC Simulations to Back-of-an-Envelope Explanations

Summary. Effective models are derived from the ferromagnetic Kondo lattice model with classical corespins, which greatly reduce the numerical effort. Results for these models are presented. They indicate that double exchange gives the correct order of magnitude and the correct doping dependence of the Curie temperature. Furthermore, we find that the jump in the particle density previously interpreted as phase separation is rather explained by ferromagnetic polarons. Manganites [1] are often described by the ferromagnetic Kondo lattice model, which is considered to explain some of their features, e.g., the transition from antiferromagnetic to ferromagnetic order with doping [2]. The application of the model is motivated by the fact, that crystal field splitting divides the five d-orbitals into two eg and three t2g orbitals, where the latter are energetically favored in the case of manganites. All three t2g orbitals are singly occupied and rather localized. Due to a strong Hund’s rule coupling, these electrons are aligned in parallel and form a core spin with length S = 3/2. The filling of the eg orbitals is determined by doping and these electrons can hop from one Mn ion to the next via the intermediate oxygen. Hund’s rule coupling leads to a ferromagnetic interaction between the itinerant eg electrons and the t2g core spin. The core spins interact through super exchange leading to a weak antiferromagnetic coupling between them. In this chapter, we derive effective models for the ferromagnetic Kondo lattice model and introduce suitable Markov chain Monte Carlo (MC) algorithms. The presented results, were not obtainable by simple analytic considerations, are partly found by this MC method and partly by use of the Wang-Landau algorithm [3].