1,721,077 research outputs found
Squeezing phenomena in interacting electron-phonon systems
No full textThe molecular crystal model of electrons coupled to Einstein phonons is
studied as a function of the two parameters: and the ratio of
the electron-phonon coupling energy denoted by . Both the one
electron and the many-electron models are studied starting (for the
former) from the adiabatic limit and (for the latter) from the
ant-adiabatic one. In the "multiphonon" regime , the sharp
crossover between quasi-free electrons () and small
polarons () is investigated emphasizing the anomalous
lattice fluctuations which occur in the intermediate regime (). These fluctuations are due to the band motion of electrons
strongly coupled to the lattice and are shown in turn to weaken the
electron mass renormalization inherent to self-trapping. In a relevant
part of the intermediate region the electron mass slowly increases with
, due to the competition between the phonon dressing effect
and the reduction of lattice momentum fluctuations. This reduction is
reminiscent of squeezing phenomena occurring in quantum optics. In a
gaussian approximation squeezed states imply a dynamical phonon
softening
Crucial role of Jahn-Teller distortions in LaMnO(3)
No full textThe crucial role of the cooperative Jahn-Teller deformations in determining the magnetic properties of stoechiometric LaMnO(3) is studied by means of exact diagonalization. The layered antiferromagnetic order mainly results from the strong cooperative Jahn-Teller deformations, stabilizing a certain type of orbital ordering. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) call be understood only if the Jahn-Teller energy is much larger than the superexchange couplings, which is consistent with experiments. The crucial role of the deformation anisotropy Q(2)/Q(3) is also emphasized: realistic values of Q(2)/Q(3) are needed to stabilize the layered order. The experimental values of the superxchange couplings can be recovered using reasonable sets of parameters. RI Grilli, Marco/C-6309-200
Crucial role of Jahn-Teller distortions in stabilizing magnetic ordering in insulating manganite phases
No full textThe phase diagram of manganites combines magnetic, orbital, lattice and charge ordering. We underline the essential role of strong cooperative Jahn-Teller interactions in selecting the magnetic structure and fine tuning by external parameters. First, the realistic Jahn-Teller distortions is LaMnO(3) controls the orbital order and consistently explain the layered antiferromagnetic ordering. Secondly, the phase diagram of half-doped manganites of the family La(0.5)Ca(0.5)MnO(3) can be reproduced, with the observed ferromagnetic, charge-ordered (CE) antiferromagnetic and paramagnetic insulating phases. RI Grilli, Marco/C-6309-2009; Fratini, Simone/A-4692-200
Role of electron-lattice interactions in determining the magnetic structure of insulating manganites
No full textTogether with the strong electron-electron interactions, the role of the lattice distorsions turns out to be relevant in determining the specific magnetic structure of some insulating manganites. In particular, while a substantial Jahn-Teller distorsion is needed to account for the layered antiferromagnetic structure of LaMnO3, the observed collective shear of alternate ions in La0.5Ca0.5MnO3 is required to stabilize the CE magnetic structure of this compounds. The specific action of the lattice in stabilizing these peculiar magnetic orderings is thus clarified. RI Grilli, Marco/C-6309-2009; 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
Dynamical mean-field theory of the small polaron
No full textA dynamical mean-field theory of the small polaron problem is presented, which becomes exact in the limit of infinite dimensions. The ground-state properties and the one-electron spectral function are obtained for a single electron interacting with Einstein phonons by a mapping of the lattice problem onto a polaronic impurity model. The one-electron propagator of the impurity model is calculated through a continued fraction expansion, at both zero and finite temperature, for any electron-phonon coupling and phonon energy. In contrast to the ground-state properties, such as the effective polaron mass, which show a continuous behavior as the coupling is increased, spectral properties exhibit a sharp qualitative change at low enough phonon frequency: beyond a critical coupling, one energy gap and then more open in the density of states at low energy, while the high-energy part of the spectrum is broad and can be qualitatively explained by a strong coupling adiabatic approximation. As a consequence, narrow and coherent low-energy subbands coexist with an incoherent featureless structure at high energy. The subbands denote the formation of quasiparticle polaron states. Also, divergencies of the self-energy may occur in the gaps. At finite temperature such an effect triggers an important damping and broadening of the polaron subbands. On the other hand, in the large phonon frequency regime such a separation of energy scales does not exist and the spectrum always has a multipeaked structure
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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