Superconductivity at Tc40 K in MgB2 is thought to origin from the strong electron-phonon (el-ph) coupling of the sigma-bands, whereas the residual interband scattering gives rise to the multigap ...phenomenology. The extremely low charge density of the sigma-bands is reflected in the small Fermi energy EFsigma, a fraction of eV, a common feature which is shared also by cuprates and fullerides. In our contribution we discuss the anomalous effects arising from the small Fermi energy phenomenology, when EFsigma becomes comparable with the other energy scales of the systems. In particular we analyze the nonadiabatic effects arising from the finite adiabatic ratio omegaph/EFsigma; the anharmonic character of the E2g phonon mode, which is shown to be related to the smallness of the Fermi energy with respect to the electron-phonon coupling gE2g: EFsigmagE2g; the anomalous effects of disorder when impurity scattering rate gammaimp is compared with EFsigma. We discuss also the possibility of an enhancement of the nonadiabatic character due to zero point quantum fluctuations.
We apply the dynamic mean field theory to calculate the low-density limit of the small polaron optical conductivity in the Holstein model. Classical schemes of approximation are recovered in weak ...coupling and in the adiabatic and anti-adiabatic strong coupling limits. Such schemes are not suitable for the intermediate coupling regime, where our theory gives an anomalous increase of the spectral weight at low energy as the temperature decreases. This result could be relevant to interpret some features observed in MIR and FIR optical conductivity of the oxide compounds.
Scattering by slowly fluctuating degrees of freedom can cause a transient localization of the current-carrying electrons in metals, driving the system away from normal metallic behavior. We ...illustrate and characterize this general phenomenon by studying how signatures of localization emerge in the optical conductivity of electrons interacting with slow bosonic fluctuations. The buildup of quantum localization corrections manifests itself in the emergence of a displaced Drude peak (DDP), whose existence strongly alters the low frequency optical response and suppresses the d.c. conductivity. We find that for sufficiently strong interactions, many-body renormalization of the fluctuating field induced at metallic densities enhances electron localization and the ensuing DDP phenomenon in comparison with the well-studied low concentration limit. Our results are compatible with the frequent observation of DDPs in electronic systems where slowly fluctuating degrees of freedom couple significantly to the charge carriers.
It has recently been argued that inverse-beta nuclear transmutations might occur at an impressively high rate in a thin layer at a metallic hydride surface under specific conditions. In this note we ...present a calculation of the transmutation rate, which shows that there is little room for such a remarkable effect.
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.
We carefully revisit the electron-boson scattering problem, going beyond popular semi-classical treatments. By providing numerically exact results valid at finite temperatures, we demonstrate the ...existence of a regime of electron-boson scattering where quantum localization processes become relevant despite the absence of extrinsic disorder. Localization in the Anderson sense is caused by the emergent randomness resulting from a large thermal boson population, being effective at transient times before diffusion can set in. Compelling evidence of this transient localization phenomenon is provided by the observation of a distinctive displaced Drude peak (DDP) in the optical absorption and the ensuing suppression of conductivity. Our findings identify a general route for anomalous metallic behavior that can broadly apply in interacting quantum matter.
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