Locally tunable disorder and entanglement in the one-dimensional plaquette orbital model

We introduce a one-dimensional plaquette orbital model with a topology of a ladder and alternating interactions between $x$ and $z$ pseudospin components along both the ladder legs and on the rungs. We show that it is equivalent to an effective spin model in a magnetic field, with spin dimers that replace plaquettes and are coupled along the chain by three-spin interactions. Using perturbative treatment and mean field approaches with dimer correlations we study the ground state spin configuration and its defects in the lowest excited states. By the exact diagonalization approach we find that the quantum effects in the model are purely short-range and we get estimated values of the ground state energy and the gap in the thermodynamic limit from the system sizes up to $L=12$ dimers. Finally, we study a class of excited states with classical-like defects accumulated in the central region of the chain to find that in this region the quantum entanglement measured by the mutual information of neighboring dimers is locally increased and coincides with disorder and frustration. Such islands of entanglement in otherwise rather classical system may be of interest in the context of quantum computing devices.Comment: 12 pages, 12 figure

Mesoscopic superpositions of states - approach to classicality and diagonalization in coherent state basis

I consider the interaction of a superposition of mesoscopic coherent states and its approach to a mixed state as a result of a suitably controlled environment. I show how the presence of a gain medium in a cavity can lead to diagonalization in coherent state basis in contrast to the standard model of decoherence. I further show how the new model of decoherence can lead to the generation of $s$ ordered quasi distributions.Comment: 10 pages, two figure pages, RevTe

Failure of t-J models in describing doping evolution of spectral weight in x-ray scattering, optical and photoemission spectra of the cuprates

We have analyzed experimental evidence for an anomalous transfer of spectral weight from high to low energy scales in both electron and hole doped cuprates as a function of doping. X-ray scattering, optical and photoemission spectra are all found to show that the high energy spectral weight decreases with increasing doping at a rate much faster than predictions of the large $U-$limit calculations. The observed doping evolution is however well-described by an intermediate coupling scenario where the effective Hubbard $U$ is comparable to the bandwidth. The experimental spectra across various spectroscopies are inconsistent with fixed-$U$ exact diagonalization or quantum Monte Carlo calculations, and suggest a significant doping dependence of the effective $U$ in the cuprates.Comment: Accepted for Phys. Rev. B (2010). 7 pages, 4 figure

Mass Mixing, the Fourth Generation, and the Kinematic Higgs Mechanism

We describe how to construct chiral fermion mass terms using Dirac-Kahler (DK) spinors. Classical massive DK spinors are shown to be equivalent to four generations of Dirac spinors with equal mass coupled to a background U(2,2) gauge field. Quantization breaks U(2,2) to U(2)xU(2), lifts mass spectrum degeneracy, and generates a non-trivial mass mixing matrix.Comment: 12 pages. No figures. Phys Lett B version. Minor typos fixe