H. Büttner

Entanglement and correlation in anisotropic quantum spin systems

Analytical expressions for the entanglement measures concurrence, i-concurrence, and 3-tangle in terms of spin correlation functions are derived using general symmetries of the quantum spin system. These relations are exploited for the one-dimensional XXZ model, in particular the concurrence and the critical temperature for disentanglement are calculated for finite systems with up to six qubits. A recent NMR quantum error correction experiment is analyzed within the framework of the proposed theoretical approach.

An iterative method for the calculation of narrow solitary excitations on atomic chains

Abstract We discuss the quasicontinuum approximation of Collins and give a general condition for its validity. For the solution of the difference-differential equations of the full, discrete model we develop an iteration procedure in Fourier space. The method is tested for solitary excitations with widths of the order of the lattice constant by a comparison with both exact results and computer simulations. An additional result of the new mwthod is a simple derivation of the Collins approximation which allows several generalizations.

Coulomb interaction and optical spectra in conjugated polymers

Abstract Solitonic excitations in conjugated polymers are studied under the influence of Coulomb interactions. In addition to stabilizing bipolarons these effects change the characteristics of the optical spectra clearly. The results are compared with recent experiments in polythiophene and good agreement is found.

Environment-induced two-mode entanglement in quantum Brownian motion

The time evolution of quantum correlations of entangled two-mode continuous variable states is examined in single-reservoir as well as two-reservoir models, representing noisy correlated or uncorrelated non-Markovian quantum channels. For this purpose the model of quantum Brownian motion is extended. Various separability criteria for Gaussian continuous variable systems are applied. In both types of reservoir models moderate non-Markovian effects prolong the separability time scales. However, in these models the properties of the stationary state may differ. In the two-reservoir model the initial entanglement is completely lost and both modes are finally uncorrelated. In a common reservoir both modes interact indirectly via the coupling to the same bath variables. Below a critical bath temperature entanglement between the two modes is preserved even in the steady state. A separability criterion is derived, which depends on the bath temperature and the response function of the open quantum system. Thus, the extended quantum Brownian motion model of a two-mode continuous variable system in a common reservoir provides an example of environment-induced entanglement.

Information and Entropy in Quantum Brownian Motion

We compare the thermodynamic entropy of a quantum Brownian oscillator derived from the partition function of the subsystem with the von Neumann entropy of its reduced density matrix. At low temperatures we find deviations between these two entropies which are due to the fact that the Brownian particle and its environment are entangled. We give an explanation for these findings and point out that these deviations become important in cases where statements about the information capacity of the subsystem are associated with thermodynamic properties, as it is the case for the Landauer principle.

Fluctuation-induced tunneling and the conduction mechanism in metallic polyacetylene

Abstract Possible conduction mechanisms in metallic polyacetylene are discussed with particular emphasis on recent experiments on a new form of (CH)x, yielding conductivities of order 105(Ωcm)−1. Fluctuation-induced tunneling is examined critically, and it is concluded that it does not allow a consistent description of highly doped (CH)x. Preliminary results of a study based on the Kivelson-Heeger conjecture of a first order transition to a novel polaron-lattice state are summarized. They indicate that progress towards even high conductivities may be possible.

Phonon anomalies in ferroelectrics and superconductors

Many ferroelectrics and superconductors exhibit unusual features in the low-frequency phonon spectrum. In superconductors the microscopic origin of phonon anomalies (“generalized Kohn anomalies”) is known to be an electron-one-phonon potential, while in displacive ferroelectrics different mechanisms for soft modes are presently discussed. In this paper we point out that similar physical mechanisms drive both ferroelectricity and superconductivity in many compounds. An analysis of the electronic band structure in both cubic ferroelectrics and superconductors exhibits high density-of-states near the Fermi level due to nonbonding nonmetal orbitals. Their phonon-induced hybridization with the metal ion states (“dynamical covalency”) is responsible for the strong electron-phonon interactions in both phenomena. This mechanism relates the phonon anomalies for both cases to cluster deformabilities, but with different symmetries. A nonlinear polarizability mechanism is discussed which quantitatively accounts for t...

Dynamic probes of quantum spin chains with the Dzyaloshinskii-Moriya interaction

We consider the spin-

A Peierls-Hubbard model: Exact quantum mechanical results and a variational method

\frac{1}{2}

The phase diagram of high-Tc superconductors in the presence of dynamic stripes

anisotropic