Exact ground states for the four-electron problem in a two-dimensional finite Hubbard square system

We present exact explicit analytical results describing the exact ground state of four electrons in a two dimensional square Hubbard cluster containing 16 sites taken with periodic boundary conditions. The presented procedure, which works for arbitrary even particle number and lattice sites, is based on explicitly given symmetry adapted base vectors constructed in r-space. The Hamiltonian acting on these states generates a closed system of 85 linear equations providing by its minimum eigenvalue the exact ground state of the system. The presented results, described with the aim to generate further creative developments, not only show how the ground state can be exactly obtained and what kind of contributions enter in its construction, but emphasize further characteristics of the spectrum. On this line i) possible explications are found regarding why weak coupling expansions often provide a good approximation for the Hubbard model at intermediate couplings, or ii) explicitly given low lying energy states of the kinetic energy, avoiding double occupancy, suggest new roots for pairing mechanism attracting decrease in the kinetic energy, as emphasized by kinetic energy driven superconductivity theories.Comment: 37 pages, 18 figure

Grain boundary effects on magnetotransport in bi-epitaxial films of La0.7_{0.7}Sr0.3_{0.3}MnO3_3

The low field magnetotransport of La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) films grown on SrTiO$_3$ substrates has been investigated. A high qualtity LSMO film exhibits anisotropic magnetoresistance (AMR) and a peak in the magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films prepared using a seed layer of MgO and a buffer layer of CeO$_2$ display a resistance dominated by grain boundaries. One film was prepared with seed and buffer layers intact, while a second sample was prepared as a 2D square array of grain boundaries. These films exhibit i) a low temperature tail in the low field magnetoresistance; ii) a magnetoconductance with a constant high field slope; and iii) a comparably large AMR effect. A model based on a two-step tunneling process, including spin-flip tunneling, is discussed and shown to be consistent with the experimental findings of the bi-epitaxial films.Comment: REVTeX style; 14 pages, 9 figures. Figure 1 included in jpeg format (zdf1.jpg); the eps was huge. Accepted to Phys. Rev.

Sigma E Regulators Control Hemolytic Activity and Virulence in a Shrimp Pathogenic Vibrio harveyi

Members of the genus Vibrio are important marine and aquaculture pathogens. Hemolytic activity has been identified as a virulence factor in many pathogenic vibrios including V. cholerae, V. parahaemolyticus, V. alginolyticus, V. harveyi and V. vulnificus. We have used transposon mutagenesis to identify genes involved in the hemolytic activity of shrimp-pathogenic V. harveyi strain PSU3316. Out of 1,764 mutants screened, five mutants showed reduced hemolytic activity on sheep blood agar and exhibited virulence attenuation in shrimp (Litopenaeus vannamei). Mutants were identified by comparing transposon junction sequences to a draft of assembly of the PSU3316 genome. Surprisingly none of the disrupted open reading frames or gene neighborhoods contained genes annotated as hemolysins. The gene encoding RseB, a negative regulator of the sigma factor (σE), was interrupted in 2 out of 5 transposon mutants, in addition, the transcription factor CytR, a threonine synthetase, and an efflux-associated cytoplasmic protein were also identified. Knockout mutations introduced into the rpoE operon at the rseB gene exhibited low hemolytic activity in sheep blood agar, and were 3-to 7-fold attenuated for colonization in shrimp. Comparison of whole cell extracted proteins in the rseB mutant (PSU4030) to the wild-type by 2-D gel electrophoresis revealed 6 differentially expressed proteins, including two down-regulated porins (OmpC-like and OmpN) and an upregulated protease (DegQ) which have been associated with σE in other organisms. Our study is the first report linking hemolytic activity to the σE regulators in pathogenic Vibrio species and suggests expression of this virulence-linked phenotype is governed by multiple regulatory pathways within the V. harveyi

Extrinsic Magnetotransport Phenomena in Ferromagnetic Oxides

This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surfacial and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin-polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.Comment: 133 pages, 47 figures, submitted to Rep. Prog. Phy

Experimental and theoretical studies of manganite and magnetite compounds Transition oxide compounds

In the recent years interest in the transition oxide compounds has renewed among researchers in the field of condensed matter physics. This thesis presents the studies of the two families of the transition oxides, the manganite and magnetite compounds. Manganite has regained the interest since the discovery of the large magnetoresistance around its Curie temperature in 1990s. Magnetite on the other hand is the oldest magnetic material known to man however some of its physical properties are still controversial. The experimental works address some basic properties of these compounds when fabricated in the form of thin films. These include the resistivity measurements and magnetic measurements as well as the Hall effect. The various models of transport mechanism have been compared. The magnetic field and the temperature dependence of magnetoresistance have also been studied. Simple devices such as an artificial grain boundary and bilayers thin film have been investigated. The second part of this thesis concentrates on the theoretical aspects of the fundamental physics behind these two compounds. The problem of electrons tunnelling between the magnetite electrodes has been addressed taking into account the surface effect with distortion. The last chapter presents a theoretical study of the spinless-Hubbard model which is the simplest approximation of the conduction electrons in magnetite and manganite. The results are obtained from the Hartree-Fock and the Hubbard-I approximations as well as the exact diagonalisation method

Extraordinary Hall effect in

The Hall effect of La0.7Ba0.3MnO3 and La0.7Ca0.3MnO3 thin films was measured in magnetic fields $\mu_0 H \leq 1$ T and for temperatures 10 K $\leq T \leq$ 340 K. In the ferromagnetic phase the slope of the Hall resistivity at zero magnetic induction rapidly increases with temperature until a maximum is reached at the Curie temperature TC. A detailed comparison of the Hall resistivity with the magnetization facilitates the separation of the ordinary and extraordinary Hall effect. The ordinary Hall constant RH shows a jump below TC. The extraordinary Hall constant RA scales with the zero-field resistivity. At low temperatures we find a quadratic dependence of RA on the resistivity crossing over to a linear dependence at higher temperatures. Whereas the quadratic scaling law in the ferromagnetic phase can be explained by a conventional side-jump mechanism, the linear dependence near TC and in the hopping regime is inconsistent with conventional skew scattering. The data are compared to a recent theory of the Hall effect in double-exchange ferromagnets

Characterization and CRISPR-based genotyping of clinical trh-positive Vibrio parahaemolyticus

Abstract Background Vibrio parahaemolyticus is a causative agent of gastroenteritis. Most of the clinical isolates carry either tdh and/or trh genes which are considered as the major virulence genes of this pathogen. In this study, the clinical isolates of V. parahaemolyticus carrying trh gene (n = 73) obtained from 1886 to 2012 from various countries were investigated for the urease production, haemolytic activity, and biofilm formation. In addition, the potential of clustered regularly interspaced short palindromic repeats (CRISPR)-based genotyping among these isolates was investigated. Results In this study, no significant differences were observed in the urease production between tdh + trh1+ and tdh + trh2+ isolates (p = 0.063) and between the tdh − trh1+ and tdh − trh2+ isolates (p = 0.788). The isolates carrying only the trh gene showed variation in their haemolytic activity. The ratio of urease production and haemolytic activity between the trh1+ and trh2+ isolates and biofilm formation of trh + V. parahaemolyticus isolates were not significantly different. Sixteen of thirty-four tested isolates (47.0%) of trh + V. parahaemolyticus were positive for CRISPR detection. The discriminatory power index (DI) of CRISPR-virulence typing was higher than the DI obtained by CRISPR typing alone. Conclusion The tdh and trh genes were not involved in urease production in the trh + V. parahaemolyticus, and variation of haemolytic activity detected in V. parahaemolyticus carrying only the trh gene might be correlated to the sequence variation within trh1 and trh2 genes. Additionally, biofilm production of V. parahaemolyticus was not associated with harboring of virulence genes. For genotyping, CRISPR sequences combined with virulence genes can be used as genetic markers to differentiate trh + V. parahaemolyticus strains