Ways of interpreting urban regeneration: Hamburg, London, Brussels and Rome

Over the coming decades all cities throughout and beyond Europe, be they large or small, will face the great challenge of regeneration. European Commission has promoted a “regeneration agenda” focused on an integrated sustainable approach. But, while the European Commission draws the path, European cities provide a variety of ways to transform drafts in deeds. The four case studies described below – Hamburg, London, Brussels, Rome – give evidence that, in the last decades, every city had drawn its own “regeneration way”, with a different level of sensitiveness regarding the European principles. However, all the case studies deliver at least one action attuned to the principles of a sustainable regeneration, and it’s possible to select from every experience the “good” that has been realized

Is the Quantum Melting of a Polaron Wigner Crystal an Insulator-to-Superconductor transition ?

On examining the stability of a Wigner Crystal (WC) in an ionic dielectric, two competitive effects due to Polaron formation are found to be important: (i) the screening of the Coulomb forces which destabilizes the crystal, compensated by (ii) the increase of the carrier mass (polaron mass). The quantum melting of the Polaron Wigner Crystal (PWC) is examined. By calculating the quantum fluctuations of both the electrons and the polarization, we show that there is a competition between the dissociation of the Polarons at the insulator-to-metal transition (IMT), and a melting towards a polaron liquid. We find that at strong coupling ($\alpha > \alpha^*$), a liquid state of polarons cannot exist, and the IMT is driven by polaron dissociation. Next, we show that the dipolar interactions between localized polarons are responsible for a phonon instability of the PWC as the density increases. This provides a new mechanism for the IMT in doped dielectrics. Examining the sign of the dielectric constant of the PWC, we conjecture that such an instability could yield an Insulator-to-Superconductor transition.Comment: 4 Pages, 2 Figures included, Int. Conf. M2S-HTSC-VI (Houston 2000) to be published in Physica

Band dispersion and electronic lifetimes in crystalline organic semiconductors

The consequences of several microscopic interactions on the photoemission spectra of crystalline organic semiconductors (OSC) are studied theoretically. It is argued that their relative roles can be disentangled by analyzing both their temperature and their momentum/energy dependence. Our analysis shows that the polaronic thermal band narrowing, that is the foundation of most theories of electrical transport in OSC, is inconsistent in the range of microscopic parameters appropriate for these materials. An alternative scenario is proposed to explain the experimental trends.Comment: 4+ pages, revised conclusions; accepted for publication in Phys. Rev. Let

On dynamical localization corrections to band transport

Bloch-Boltzmann transport theory fails to describe the carrier diffusion in current crystalline organic semiconductors, where the presence of large-amplitude thermal molecular motions causes substantial dynamical disorder. The charge transport mechanism in this original situation is now understood in terms of a transient localization of the carriers' wavefunctions, whose applicability is however limited to the strong disorder regime. In order to deal with the ever-improving performances of new materials, we develop here a unified theoretical framework that includes transient localization theory as a limiting case, and smoothly connects with the standard band description when molecular disorder is weak. The theory, which specifically adresses the emergence of dynamical localization corrections to semiclassical transport, is used to determine a "transport phase diagram" of high-mobility organic semiconductors.Comment: 14 pages, 6 figures completely revised versio

Pairing and condensation in a resonant Bose-Fermi mixture

We study by diagrammatic methods a mixture of single-component bosons and fermions, with boson-fermion coupling tuned by a Fano-Feshbach resonance. For increasing coupling, the growing boson-fermion pairing correlations progressively reduce the boson condensation temperature and make it eventually vanish at a critical coupling. Such quantum critical point depends very weakly on the population imbalance and for vanishing boson densities coincides with that found for the polaron-molecule transition in a strongly imbalanced Fermi gas, thus bridging two quite distinct physical systems.Comment: 4 pages, 4 figure

Hopping dynamics of interacting polarons

We derive an effective cluster model to address the transport properties of mutually interacting small polarons. We propose a decoupling scheme where the hopping dynamics of any given particle is determined by separating out explicitly the degrees of freedom of its environment, which are treated as a statistical bath. The general cavity method developed here shows that the long-range Coulomb repulsion between the carriers leads to a net increase of the thermal activation barrier for electrical transport, and hence to a sizable reduction of the carrier mobility. A mean-field calculation of this effect is provided, based on the known correlation functions of the interacting liquid in two and three dimensions. The present theory gives a natural explanation of recent experiments performed in organic field-effect transistors with highly polarizable gate dielectrics, and might well find application in other classes of polaronic systems such as doped transition-metal oxides

Electronic transport and quantum localization effects in organic semiconductors

We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula, we develop a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity. The electron mobility is then calculated through a relaxation time approximation that accounts for quantum localization corrections beyond Boltzmann theory, and allows us to efficiently address the interplay between highly conducting states in the band range and localized states induced by disorder in the band tails. The emergence of a "transient localization" phenomenon is shown to be a general feature of organic semiconductors, that is compatible with the bandlike temperature dependence of the mobility observed in pure compounds. Carrier trapping by extrinsic disorder causes a crossover to a thermally activated behavior at low temperature, which is progressively suppressed upon increasing the carrier concentration, as is commonly observed in organic field-effect transistors. Our results establish a direct connection between the localization of the electronic states and their conductive properties, formalizing phenomenological considerations that are commonly used in the literature