Finite-temperature hole dynamics in the t-J model: Exact results for high dimensions

We discuss the dynamics of a single hole in the t-J model at finite temperature, in the limit of large spatial dimensions. The problem is shown to yield a simple and physically transparent solution, that exemplifies the continuous thermal evolution of the underlying string picture from the T=0 string-pinned limit through to the paramagnetic phase.Comment: 6 pages, including 2 figure

Effects of spin fluctuations in the t-J model

Recent experiments on the Fermi surface and the electronic structure of the cuprate-supercondutors showed the importance of short range antiferromagnetic correlations for the physics in these systems. Theoretically, features like shadow bands were predicted and calculated mainly for the Hubbard model. In our approach we calculate an approximate selfenergy of the $t$-$J$ model. Solving the $U=\infty$ Hubbard model in the Dynamical Mean Field Theory (DMFT) yields a selfenergy that contains most of the local correlations as a starting point. Effects of the nearest neighbor spin interaction $J$ are then included in a heuristical manner. Formally like in $J$-perturbation theory all ring diagrams, with the single bubble assumed to be purely local, are summed to get a correction to the DMFT-self engergy This procedure causes new bands and can furnish strong deformation of quasiparticle bands. % Our results are finally compared with %former approaches to the Hubbard model.Comment: 3 Pages, Latex, 2 Postscript-Figures submitted to Physica

Ferromagnetism in the large-U Hubbard model

We study the Hubbard model on a hypercubic lattice with regard to the possibility of itinerant ferromagnetism. The Dynamical Mean Field theory is used to map the lattice model on an effective local problem, which is treated with help of the Non Crossing Approximation. By investigating spin dependent one-particle Green's functions and the magnetic susceptibility, a region with nonvanishing ferromagnetic polarization is found in the limit $U\to\infty$. The $\delta$-T-phase diagram as well as thermodynamic quantities are discussed. The dependence of the Curie temperature on the Coulomb interaction and the competition between ferromagnetism and antiferromagnetism are studied in the large $U$ limit of the Hubbard model.Comment: 4 pages, 5 figures, accepted for publication in Physical Review B, Rapid Communication

Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory

We present a theory for the spin correlation function of the t-J model in the framework of the dynamical mean-field theory. Using this mapping between the lattice and a local model we are able to obtain an intuitive expression for the non-local spin susceptibility, with the corresponding local correlation function as input. The latter is calculated by means of local Goldstone diagrams following closely the procedures developed and successfully applied for the (single impurity) Anderson model.We present a systematic study of the magnetic susceptibility and compare our results with those of a Hubbard model at large U. Similarities and differences are pointed out and the magnetic phase diagram of the t-J model is discussed.Comment: 28 pages LaTeX, postscript figures as compressed and uuencoded file included fil

Temperature-dependent electronic structure and ferromagnetism in the d=oo Hubbard model studied by a modfied perturbation theory

The infinite-dimensional Hubbard model is studied by means of a modified perturbation theory. The approach reduces to the iterative perturbation theory for weak coupling. It is exact in the atomic limit and correctly reproduces the dispersions and the weights of the Hubbard bands in the strong-coupling regime for arbitrary fillings. Results are presented for the hyper-cubic and an fcc-type lattice. For the latter we find ferromagnetic solutions. The filling-dependent Curie temperature is compared with the results of a recent Quantum Monte Carlo study.Comment: RevTeX, 5 pages, 6 eps figures included, Phys. Rev. B (in press), Ref. 16 correcte

Chain Formation by Spin Pentamers in eta-Na9V14O35

The nature of the gapped ground state in the quasi-one-dimensional compound eta-Na9V14O35 cannot easily be understood, if one takes into account the odd number of spins on each structural element. Combining the results of specific heat, susceptibility and electron spin resonance measurements we show that eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a linear chain. These objects - pentamers - consist of five antiferromagnetically arranged spins with effective spin 1/2. Their spatial extent results in an exchange constant along the chain direction comparable to the one in the high-temperature state.Comment: 6 pages, 5 figure

The NeuroDante Project: Neurometric measurements of participant’s reaction to literary auditory stimuli from dante’s “divina commedia”

Neurodante. Progetto di analisi neurometrica di alcuni brani della Commedi

Numerical Renormalization Group Calculations for the Self-energy of the impurity Anderson model

We present a new method to calculate directly the one-particle self-energy of an impurity Anderson model with Wilson's numerical Renormalization Group method by writing this quantity as the ratio of two correlation functions. This way of calculating Sigma(z) turns out to be considerably more reliable and accurate than via the impurity Green's function alone. We show results for the self-energy for the case of a constant coupling between impurity and conduction band (ImDelta = const) and the effective Delta(z) arising in the Dynamical Mean Field Theory of the Hubbard model. Implications to the problem of the metal-insulator transition in the Hubbard model are also discussed.Comment: 18 pages, 9 figures, submitted to J. Phys.: Condens. Matte

New magnetic phase in metallic V_{2-y}O_3 close to the metal insulator transition

We have observed two spin density wave (SDW) phases in hole doped metallic V_{2-y}O_3, one evolves from the other as a function of doping, pressure or temperature. They differ in their response to an external magnetic field, which can also induce a transition between them. The phase boundary between these two states in the temperature-, doping-, and pressure-dependent phase diagram has been determined by magnetization and magnetotransport measurements. One phase exists at high doping level and has already been described in the literature. The second phase is found in a small parameter range close to the boundary to the antiferromagnetic insulating phase (AFI). The quantum phase transitions between these states as a function of pressure and doping and the respective metamagnetic behavior observed in these phases are discussed in the light of structurally induced changes of the band structure.Comment: REVTeX, 8 pages, 12 EPS figures, submitted to PR

Strong-coupling approach for strongly correlated electron systems

A perturbation theory scheme in terms of electron hopping, which is based on the Wick theorem for Hubbard operators, is developed. Diagrammatic series contain single-site vertices connected by hopping lines and it is shown that for each vertex the problem splits into the subspaces with ``vacuum states'' determined by the diagonal Hubbard operators and only excitations around these vacuum states are allowed. The rules to construct diagrams are proposed. In the limit of infinite spatial dimensions the total auxiliary single-site problem exactly splits into subspaces that allows to build an analytical thermodynamically consistent approach for a Hubbard model. Some analytical results are given for the simple approximations when the two-pole (alloy-analogy solution) and four-pole (Hartree-Fock approximation) structure for Green's function is obtained. Two poles describe contribution from the Fermi-liquid component, which is dominant for small electron and hole concentrations (``overdoped case'' of high-$T_c$'s), whereas other two describe contribution from the non-Fermi liquid and are dominant close to half-filling (``underdoped case'').Comment: 14 pages, revtex, feynmf, 5 EPS figures, two-column PRB style, published in PR