Polaron Crossover and Bipolaronic Metal-Insulator Transition in the Holstein model at half-filling

The evolution of the properties of a finite density electronic system as the electron-phonon coupling is increased are investigated in the Holstein model using the Dynamical Mean-Field Theory (DMFT). We compare the spinless fermion case, in which only isolated polarons can be formed, with the spinful model in which the polarons can bind and form bipolarons. In the latter case, the bipolaronic binding occurs through a metal-insulator transition. In the adiabatic regime in which the phonon energy is small with respect to the electron hopping we compare numerically exact DMFT results with an analytical scheme inspired by the Born-Oppenheimer procedure. Within the latter approach,a truncation of the phononic Hilbert space leads to a mapping of the original model onto an Anderson spin-fermion model. In the anti-adiabatic regime (where the phonon energy exceeds the electronic scales) the standard treatment based on Lang-Firsov canonical transformation allows to map the original model on to an attractive Hubbard model in the spinful case. The separate analysis of the two regimes supports the numerical evidence that polaron formation is not necessarily associated to a metal-insulator transition, which is instead due to pairing between the carriers. At the polaron crossover the Born-Oppenheimer approximation is shown to break down due to the entanglement of the electron-phonon state.Comment: 19 pages, 15 figure

Are explicatures cancellable?

Explicatures are not cancellable. Theoretical considerations

Electronic correlations stabilize the antiferromagnetic Mott state in Cs3_3C60_{60}

Cs$_3$C$_{60}$ in the A15 structure is an antiferromagnet at ambient pressure in contrast with other superconducting trivalent fullerides. Superconductivity is recovered under pressure and reaches the highest critical temperature of the family. Comparing density-functional calculations with generalized gradient approximation to the hybrid functional HSE, which includes a suitable component of exchange, we establish that the antiferromagnetic state of Cs$_3$C$_{60}$ is not due to a Slater mechanism, and it is stabilized by electron correlation. HSE also reproduces the pressure-driven metalization. Our findings corroborate previous analyses suggesting that the properties of this compound can be understood as the result of the interplay between electron correlations and Jahn-Teller electron-phonon interaction.Comment: 4 pages, 3 figure

Acoustic detection of UHE neutrinos in the Mediterranean sea: Status and perspective

In recent years the astro-particle community is involved in the realization of experimental apparatuses for the detection of high energy neutrinos originated in cosmic sources or produced in the interaction of Cosmic Rays with the Cosmic Microwave Background. For neutrino energies in the TeV-PeV range, optical Cherenkov detectors, that have been so far positively exploited by Baikal[1], IceCube[2] and ANTARES[3], are considered optimal. For higher energies, three different experimental techniques are under study: the detection of radio pulses produced by showers induced by a neutrino interaction, the detection of air showers initiated by neutrinos interacting with rocks or deep Earth's atmosphere and the detection of acoustic waves produced by deposition of energy following the interaction of neutrinos in an acoustically transparent medium. The potential of the acoustic detection technique, first proposed by Askaryan[4], to build very large neutrino detectors is appealing, thanks to the optimal properties of media such as water or ice as sound propagator. Though the studies on this technique are still in an early stage, acoustic positioning systems used to locate the optical modules in underwater Cherenkov neutrino detectors, give the possibility to study the ambient noise and provide important information for the future analysis of acoustic data. © 2017 The Authors, published by EDP Sciences

Regional competitiveness in tourist local systems

Since the past century, Marshall had described some industrial districts, in England, as agglomerations of small and medium enterprises specialized in a specific production activity. Starting from his contribute, in the last decades an immense literature has been written on industrial districts in Europe and around the World. Italian scholars gave particular attention to this local system of production and extended and developed the concept coined by Marshall. In other countries, different territorial models played a central role as milieu or cluster, for instance. In particular, in the last years these models have been extended to non-industrial fields like culture, rural activities and tourism. In the first part of the paper, a review of the main contributions on the territorial models applied to the tourist industry is proposed like the tourist milieu (Peyrache,-Gadeau, 2003, Bramanti, 2001), tourist cluster (Gordon, Goodall, 2000; Nedlac, 1999; Van Den Berg, Braum, Van Winden, 2001) or tourist district (Becattini et ali., 2001; Aci-Censis, 2001; Antonioli Corigliano, 1999). Thus, we define a model of tourist district and we address how extend to the tourist industry the competitive advantages created from networks of traditional marshallian industrial districts (Marshall, 1966; Becattini, 2000). In the second part of the paper, we perform a methodological exercise of spatial identification through GIS tools. The methodology of identification of industrial districts elaborated from Sforzi (1990) is extended to tourist industry and the adaptability of our model in the Italian territory is verified. The analysis starts from the travel-to-work areas (TTWAs) (Smart, 1974; Combees et al., 1982), which interprets the daily commuting flows due to work reasons defined in Italy by the ISTAT on the 1996 Intermediate Census on Industries and Services. A map of the Italian TLS is presented.

Lattice approaches to dilute Fermi gases: Legacy of broken Galilean invariance

In the dilute limit, the properties of fermionic lattice models with short-range attractive interactions converge to those of a dilute Fermi gas in continuum space. We investigate this connection using mean-field and we show that the existence of a finite lattice spacing has consequences down to very small densities. In particular we show that the reduced translational invariance associated to the lattice periodicity has a pivotal role in the finite-density corrections to the universal zero-density limit. For a parabolic dispersion with a sharp cut-off, we provide an analytical expression for the leading-order corrections in the whole BCS-BEC crossover. These corrections, which stem only from the unavoidable cut-off, contribute to the leading-order corrections to the relevant observables. In a generic lattice we find a universal power-law behavior $n^{1/3}$ which leads to significant corrections already for small densities. Our results pose strong constraints on lattice extrapolations of dilute Fermi gas properties.Comment: 10 pages, 7 figure

Polaron Crossover and Bipolaronic Metal-Insulator Transition in the half- filled Holstein model

The formation of a finite density multipolaronic state is analyzed in the context of the Holstein model using the Dynamical Mean-Field Theory. The spinless and spinful fermion cases are compared to disentangle the polaron crossover from the bipolaron formation. The exact solution of Dynamical Mean-Field Theory is compared with weak-coupling perturbation theory, non-crossing (Migdal), and vertex correction approximations. We show that polaron formation is not associated to a metal-insulator transition, which is instead due to bipolaron formation.Comment: 4 pages, 5 figure

Antiferromagnetic integer-spin chains in a staggered magnetic field: approaching the thermodynamic limit through the infinite-size DMRG

We investigate the behavior of antiferromagnetic integer-spin chains in a staggered magnetic field, by means of the density-matrix renormalization group, carefully addressing the role of finite-size effects within the Haldane phase at small fields. In the case of spin S=2, we determine the dependence of the groundstate energy and magnetization on the external field, in the thermodynamic limit, and show how the peculiar finite-size behavior can be connected with the crossover in the groundstate from a spin liquid to a polarized N\'eel state.Comment: 7 pages, 5 figure

Cosmic branes and asymptotic structure

Superrotations of asymptotically flat spacetimes in four dimensions can be interpreted in terms of including cosmic strings within the phase space of allowed solutions. In this paper we explore the implications of the inclusion of cosmic branes on the asymptotic structure of vacuum spacetimes in dimension d > 4. We first show that only cosmic (d-3)-branes are Riemann flat in the neighbourhood of the brane, and therefore only branes of such dimension passing through the celestial sphere can respect asymptotic local flatness. We derive the asymptotically locally flat boundary conditions associated with including cosmic branes in the phase space of solutions. We find the asymptotic expansion of vacuum spacetimes in d=5 with such boundary conditions; the expansion is polyhomogenous, with logarithmic terms arising at subleading orders in the expansion. The asymptotically locally flat boundary conditions identified here are associated with an extended asymptotic symmetry group, which may be relevant to soft scattering theorems and memory effects.Comment: 52 pages; v2, minor additions, published versio

A darkless space-time

In cosmology it has become usual to introduce new entities as dark matter and dark energy in order to explain otherwise unexplained observational facts. Here, we propose a different approach treating spacetime as a continuum endowed with properties similar to the ones of ordinary material continua, such as internal viscosity and strain distributions originated by defects in the texture. A Lagrangian modeled on the one valid for simple dissipative phenomena in fluids is built and used for empty spacetime. The internal "viscosity" is shown to correspond to a four-vector field. The vector field is shown to be connected with the displacement vector field induced by a point defect in a four-dimensional continuum. Using the known symmetry of the universe, assuming the vector field to be divergenceless and solving the corresponding Euler-Lagrange equation, we directly obtain inflation and a phase of accelerated expansion of spacetime. The only parameter in the theory is the "strength" of the defect. We show that it is possible to fix it in such a way to also quantitatively reproduce the acceleration of the universe. We have finally verified that the addition of ordinary matter does not change the general behaviour of the model.Comment: 13 pages, 7 figures Typos; section V on Newtonian limit adde