Quest for Rare Events in three-dimensional Mesoscopic Disordered Metals

The study reports on the first large statistics numerical experiment searching for rare eigenstates of anomalously high amplitudes in three-dimensional diffusive metallic conductors. Only a small fraction of a huge number of investigated eigenfunctions generates the far asymptotic tail of their amplitude distribution function. The relevance of the relationship between disorder and spectral averaging, as well as of the quantum transport properties of the investigated mesoscopic samples, for the numerical exploration of eigenstate statistics is divulged. The quest provides exact results to serve as a reference point in understanding the limits of approximations employed in different analytical predictions, and thereby the physics (quantum vs semiclassical) behind large deviations from the universal predictions of random matrix theory.Comment: 5 pages, 3 embedded EPS figures, figure 3 replaced with new findings on spectral vs disorder averagin

Composite Spin Waves, Quasi-Particles and Low Temperature resistivity in Double Exchange Systems

We make a quantum description of the electron low temperature properties of double exchange materials. In these systems there is a strong coupling between the core spin and the carriers spin. This large coupling makes the low energy spin waves to be a combination of ion and electron density spin waves. We study the form and dispersion of these composite spin wave excitations. We also analyze the spin up and down spectral functions of the temperature dependent quasi-particles of this system. Finally we obtain that the thermally activated composite spin waves renormalize the carriers effective mass and this gives rise to a low temperature resistivity scaling as T ^{5/2}.Comment: 4 pages, REVTE

Hidden degree of freedom and critical states in a two-dimensional electron gas in the presence of a random magnetic field

We establish the existence of a hidden degree of freedom and the critical states of a spinless electron system in a spatially-correlated random magnetic field with vanishing mean. Whereas the critical states are carried by the zero-field contours of the field landscape, the hidden degree of freedom is recognized as being associated with the formation of vortices in these special contours. It is argued that, as opposed to the coherent backscattering mechanism of weak localization, a new type of scattering processes in the contours controls the underlying physics of localization in the random magnetic field system. In addition, we investigate the role of vortices in governing the metal-insulator transition and propose a renormalization-group diagram for the system under study.Comment: 17 pages, 16 figures; Figs. 1, 7, 9, and 10 have been reduced in quality for e-submissio

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Neurotensin microinjection into the nucleus accumbens antagonizes dopamine-induced increase in locomotion and rearing

Neurotensin is an endogenous neuropeptide with neuronal perikarya or fibers distributed in the vicinity of the mesolimbic dopamine system. This observation, plus behavioral data showing that neurotensin injection into the nucleus accumbens blocks some behavioral effects of amphetamine, indicates that neurotensin may modulate the mesolimbic dopamine system. In this study it was shown that neurotensin given into the nucleus accumbens produces a dose-dependent blockade of locomotion and rearing initiated by dopamine injection into the nucleus accumbens. This effect is not mimicked by inactive neurotensin analogue nor some other endogenous neuropeptides. Since dopamine acts on postsynaptic dopamine receptors in the nucleus accumbens, neurotensin is acting, not on dopamine terminals, but on neurons or neuronal systems which are modulated by the mesolimbic dopamine system. This conclusion is supported by the facts that intra-accumbens injection of neurotensin does not alter accumbens levels of dopamine or its metabolites, nor does it affect the increase in dopamine metabolites produced by injection of neurotensin into the ventral tegmental area. Further, neurotensin was also found to block the dopamine-independent increase in locomotion and rearing produced by the injection of d-Ala 2-Met- 5enkephalinamide into the nucleus accumbens. These data indicate that neurotensin acts on neurons in the nucleus accumbens to counteract the motor stimulant effects of dopamine or enkephalin. Therefore, in the nucleus accumbens, neurotensin is not acting to modulate the mesolimbic dopamine system, but rather appears to antagonize behavioral hyperactivity, regardless of the neurochemical initiation

Behavioral and neurochemical effects of neurotensin microinjection into the ventral tegmental area of the rat

The ventral tegmental area of the rat brain has been shown to possess high densities of neurotensin- and dopamine-containing neuronal perikarya. We recently demonstrated that microinjection of neurotensin into the ventral tegmental area produces behavioral hyperactivity similar to amphetamine-induced increase in exploratory behaviors, but lacking stereotypies. In this study, we report that the threshold dose for neurotensin-induced hyperactivity is 0.10–0.25 μg neurotensin/side. Either intracerebroventricular injection of haloperidol (5.0 μg/lateral ventricle) or destruction of the mesolimbic dopamine system by 6-hydroxydopamine abolishes the behavioral hyperactivity produced by intraventral tegmental injection of neurotensin (2.5 μg/side). Using high pressure liquid chromatography with electrochemical detection, we show that neurotensin injection into the ventral tegmental area increases the concentration of dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the nucleus accumbens and olfactory tubercles, but not in the striatum. This effect is especially profound in the nucleus accumbens where the threshold dose is less than 0.025 μg/side. The ratio of 3,4-dihydroxyphenylacetic acid to dopamine increased in the nucleus accumbens and olfactory tubercles in a dosedependent fashion (0.025 μg–2.50 μg/side). Neurotensin-induced behavioral hyperactivity correlates positively with neurotensin-induced changes in the ratio of 3,4-dihydroxyphenylacetic acid to dopamine. This study indicates that neurotensin acts in the ventral tegmental area to activate the mesolimbic dopamine system. Further, this activation produces behavioral hyperactivity characterized by an increase in exploratory behaviors. The fact that both immunoreactive neurotensin and neurotensin receptors are found in high concentration in the ventral tegmental area supports the possible physiological significance of this peptide-catecholamine interaction