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We study the entropy time evolution of a quantum mechanical model, which is frequently used as a prototype for Anderson's localization. Recently Latora and Baranger [V. Latora, M. Baranger, Phys. Rev.Lett. 82, 520(1999)] found that there exist three entropy regimes, a transient regime of passage from dynamics to thermodynamics, a linear in time regime of entropy increase, namely a thermodynamic regime of Kolmogorov kind, and a saturation regime. We use the non-extensive entropic indicator recently advocated by Tsallis [ C. Tsallis, J. Stat. Phys. 52, 479 (1988)] with a mobile entropic index q, and we find that with the adoption of the ``magic'' value q = Q = 1/2 the Kolmogorov regime becomes more extended and more distinct than with the traditional entropic index q = 1. We adopt a two-site model to explain these properties by means of an analytical treatment and we argue that Q =1/2 might be a typical signature of the occurrence of Anderson's localization., Comment: 13 pages, 8 figures submitted to Phys. Rev. E

We study the ground state of the 4-leg spin ladder using a dimer-RVB ansatz and the Lanczos method. Besides the well known resonance mechanism between valence bond configurations we find novel interference effects among nearby resonances., Comment: 4 pages, RevTex, 7 eps figs

We investigate the tunneling process between two symmetric stable islands of a forced pendulum Hamiltonian in the weak chaos regime. We show that when the tunneling doublet is quantized over a classical non-linear resonance the tunneling rate strongly deviates from the semiclassical prediction. This mechanism is responsible for the irregular dependence of the tunneling rate on the system parameters. The weak-chaos condition allows us to make a theoretical prediction which agrees very well with the numerical results. This opens up a possible avenue to a general theory on the dependence of quantum tunneling on classical chaos., Comment: 4 pages, 2 figures

We study the t-J model with the inclusion of the so called three-site term which comes out from the t/U --> 0 expansion of the Hubbard model. We find that this singlet pair hopping term has no qualitative effect on the structure of the pure mean field phase diagram for nonmagnetic states. In accordance with experimental data on high-T_c materials and some numerical studies, we also find wide regions of phase coexistence whenever the coupling J is greater than a critical value J_c. We show that J_c varies linearly with the temperature T, going to zero at T=0., Comment: 10 pages, LaTex, 3 Postscript figures

A diquark model previously formulated to describe exotic mesons is extended to dibaryons. In the model, dibaryons containing only light quarks are unbound. The $H$ dibaryon, consisting of $uuddss$ quarks, is unstable by about 90 MeV or more, and should decay strongly into two $\Lambda$ baryons. A charmed dibaryon $H_c$, composed of $uuddsc$, is unstable by about 60 MeV or more and should decay strongly into $\Lambda + \Lambda_c$. On the other hand, we find that a bottom dibaryon $H_b$, made of $uuddsb$, may be just bound by about 10 MeV with respect to $\Lambda + \Lambda_b$. If so, it should decay weakly into various final states with a charmed hadron. A possible two-body decay would be into $\Lambda + \Lambda_c$., Comment: 4 pages, plaintex, no figures

This is a conference mostly devoted to diquarks. Although an exotic meson can in principle be composed of a diquark and an antidiquark, such an exotic is unlikely to be experimentally observable in the near future. The reason, according to the model, is that diquark-antidiquark exotics have masses well above the threshold for decay into two mesons, and are likely to have widths too large to make them observable. A possible exception is a $ u d \bar b \bar b$ exotic, but it will be a long time before such a state can be observed even if it is stable against strong decay., Comment: 9 pages, plaintex, no figures

We study the so-called nonmagnetic phases (dimer and flux states) in the t-J model below half filling. We present a new phase diagram, at zero and finite temperature, that includes broad areas of phase coexistence (dimer-flux or flux-uniform), in accordance with experimental and numerical data on the possibility of separation into hole-rich and hole-poor regions. We also briefly comment on some techniques used in the literature to discuss phase separation in the t-J model., Comment: 17 pages, Latex, 6 figures

We derive a path-integral expression for the effective action in the continuum limit of an AFM Heisenberg spin ladder with an arbitrary number of legs. The map is onto an $O(3)$ nonlinear $\sigma$-model (NL$\sigma$M) with the addition of a topological term that is effective only for odd-legged ladders and half-odd integer spins. We derive the parameters of the effective NL$\sigma$M and the behaviour of the spin gap for the case of even-legged ladders., Comment: Final version, as published in Phys. Rev. Lett

Regularities in the hadron interaction energies are used to obtain formulas relating the masses of ground-state hadrons, most of which contain heavy quarks. Inputs are the constituent quark model, the Feynman-Hellmann theorem, and the structure of the colormagnetic interaction of QCD. Some of the formulas can also be obtained from heavy quark effective theory or from diquark-antiquark supersymmetry. It is argued that the sum rules are more general than the model from which they are obtained. Where data exist, the formulas agree quite well with experiment, but most of the sum rules proposed provide predictions of heavy baryon masses that will be useful for future measurements., Comment: 13 pages, Plain TeX, no figures

Sum rules relating the masses of ground-state baryons and mesons are obtained in a constituent quark model. The interaction is assumed to be independent of quark spins except for a spin-dependent part that can be treated as a perturbation. Where data are available, the sum rules agree with experiment to better than 1\%., Comment: 3 pages, Plain TeX, no figures

The Feynman-Hellmann theorem and semiempirical mass formulas are used to predict the masses of baryons containing one or two heavy quarks. In particular, the mass of the $\Lambda_b$ is predicted to be $5620 \pm 40$ MeV, a value consistent with measurements., Comment: 11 pages, Plain TeX, 2 figures (not included, available on request from lichten@photon.physics.indiana.edu)

There are striking regularities in the masses and mass differences of known hadrons. Some of these regularities can be understood from known general properties of the interactions of quarks without a need to specify the explicit form of the Hamiltonian. The Feynman--Hellmann theorem is one of the tools providing this understanding. If the mass regularities are exploited, predictions can be made of the masses of as yet undiscovered hadrons. In particular, it is found that the mass of the $B_c^*$ is $6320\pm 20$ MeV. Predictions concerning i) excited vector mesons, ii) pseudoscalar mesons, iii) $P$-wave mesons, and iv) ground-state spin 1/2 and 3/2 baryons are also made., Comment: Plain TeX, 15 pages, 3 figures (not included, available on request from lichten@photon.physics.indiana.edu), IUHET 278

The Feynman--Hellmann theorem is used to show that vector meson energy eigenvalues are monotonically decreasing functions of the reduced masses of their constituent quarks. The experimental meson masses are used to put constraints on the values of quark masses and to predict the masses of some as yet undiscovered mesons. The mass of the $B_c^*$ meson is predicted to be $6320\pm 10$ MeV, and, with less precision, the masses of a number of excited vector mesons are also predicted., Comment: 7 pages in plaintex, two figures not included available from authors (lichten@indiana.edu), IUHET 270