1,721,130 research outputs found
The effects of the electron-phonon interaction on a Mott insulator
No full textUsing the dynamical mean field theory we show that an electron - phonon interaction can revert the energetic balance between the metallic state and the Mott insulator in the density-driven metal-insulator transition at zero temperature. We present results for the Hubbard - Holstein model where the Mott ground state is found for sufficiently high values of the electron - phonon coupling but still not in the polaronic regime. (C) 2003 Elsevier B. V. All rights reserved. RI Capone, Massimo/A-7762-200
Time reversal breaking superconducting state in the phase diagram of the cuprates
No full textWe review and extend a previous study(1) on the symmetry of the superconducting state, stimulated by recent tunneling and Andreev reflection measurements giving robust evidences for the existence of a d(x2-y2) + id(xy) order parameter in the overdoped regime of two different cuprates. Looking for a possible second-order phase transition from a standard d(x2-y2) to a mixed and time reversal breaking state, we confirm the results of our previous analysis on La2-xSrxCuO4. In the case of Y1-yCayBa2Cu3O7-x as well, among all the allowed symmetries, the d(x2-y2) + id(xy) is the most favored one and the unconventional state is likely to occur in a small dome of the phase diagram located in the optimal-overdoped region and at very low temperatures. RI Capone, Massimo/A-7762-200
Normal-pressure hydrocephalus: Twelve years experience
No full textThe authors report on twelve years experience with so-called normal pressure hydrocephalus. If care is taken to make a correct diagnosis and give adequate treatment with a shunt, hydrocephalic dementia has a good prognosis. Such cases should be distinguished from other syndromes, particularly senile dementia and Alzheimer's disease. The occurrence of a syndrome of dementia should compel every neurologist and neurosurgeon to carry out careful investigations in order to determine the favourable time for possible treatment
Pairing and polarization in electron-boson systems with retarded interactions via dynamical mean-field theory
No full textIn a system where a boson (e.g., a phonon) of finite frequency omega(0) is coupled to electrons, two phenomena occur as the coupling is increased: electron pairing and polarization of the boson field. Within a path integral formalism and a dynamical mean-field approach, we introduce ad hoc distribution functions which allow us to pinpoint the two effects. When omega(0) is smaller than the bandwidth D, pairing and polarization occur for fairly similar couplings for all considered temperatures. When omega(0)>D, the two phenomena tend to coincide only for T >omega(0), but are no longer tied for low temperatures so that a state of paired particles with significant fluctuations but zero average polarization is stabilized. RI Capone, Massimo/A-7762-200
Polaron crossover and bipolaronic metal-insulator transition in the half-filled Holstein model
No full textThe formation of a finite density multipolaronic state in the Holstein model is analyzed by means of the Dynamical mean-field theory at zero and finite temperature. The spinless and spinful fermion cases are compared to disentangle the polaron crossover from the bipolaron formation. We show that polaron formation is not associated to a metal-insulator transition, which is instead due to bipolaron formation. In the spinful case a region of bound pairs without significant lattice distortion appears between the weak-coupling regime and the strong-coupling bipolaronic phase. This region shrinks with increasing temperature and with decreasing phonon frequency. (c) 2005 Elsevier B.V. All rights reserved. RI Capone, Massimo/A-7762-200
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Interplay between local response and vertex divergences in many-fermion systems with on-site attraction
No full textWe investigate the divergences appearing in the two-particle-irreducible vertex functions of many-fermion systems with attractive on-site interactions. By means of dynamical mean-field theory calculations, we determine the location of singularity lines in the phase diagram of the attractive Hubbard model at half-filling, where the local Bethe-Salpeter equations are noninvertible. We find that divergences appear both in the magnetic and in the density scattering channels. The former affect a sector of suppressed fluctuations and comply with the mapping of the physical susceptibilities of the repulsive case. At the same time, the appearance of singularities in the density channel of the attractive model demonstrates that vertex divergences can also plague the dominant scattering sectors associated with enhanced local susceptibilities. This constitutes a counterexample to previously proposed interpretations. Eventually, by exploiting the underlying physical symmetries and a spectral representation of the susceptibilities, we clarify the relation between vertex divergences and the local response of the system in different channels
"Proceedings of the International School of Physics "Enrico Fermi" Course CLXI "Polarons in Bulk Materials and Systems with Reduced Dimensionality"
No full textBreakdown of Traditional Many-Body Theories for Correlated Electrons
No full textStarting from the (Hubbard) model of an atom, we demonstrate that the uniqueness of the mapping from the interacting to the noninteracting Green function, G -> G(0), is strongly violated, by providing numerous explicit examples of different G(0) leading to the same physical G. We argue that there are indeed infinitely many such G(0), with numerous crossings with the physical solution. We show that this rich functional structure is directly related to the divergence of certain classes of (irreducible vertex) diagrams, with important consequences for traditional many-body physics based on diagrammatic expansions. Physically, we ascribe the onset of these highly nonperturbative manifestations to the progressive suppression of the charge susceptibility induced by the formation of local magnetic moments and/or resonating valence bond (RVB) states in strongly correlated electron systems
- …
