Large-bipolaron superconductivity is plausible with carrier densities well below those of conventional metals. Bipolarons form when carriers self-trap in pairs. Coherently moving large-bipolarons require extremely large ratios of static to optical dielectric-constants. The mutual Coulomb repulsion of a planar large-bipolaron’s paired carriers drives it to a four-lobed shape. A phonon-mediated attraction among large-bipolarons propels their condensation into a liquid. This liquid’s excitations move slowly with a huge effective mass. Excitations’ concomitant weak scattering by phonons produces a moderate low-temperature dc resistivity that increases linearly with rising temperature. With falling temperature an energy gap opens between large-bipolarons’ excitations and those of their self-trapped electronic carriers.
Under appropriate conditions absorption of light by a solid can initiate a process by which it is cooled. Here, two
schemes for laser cooling via localized electrons are addressed. The first scheme utilizes two states of a localized center.
In this two-level scheme, the cooling process is initiated with photon absorption in the low-energy tail of a localized
state's strain-broadened absorption spectrum. The subsequent atomic relaxation transfers energy of especially large
vibratory atomic strains into electrical energy that is then extracted via photon emission. Cooling can occur at elevated
temperatures but is suppressed as the temperature is lowered. The second scheme involves three energy levels of a
localized center. Cooling is facilitated when i) the photo-excitation of an electron promotes it to the lower of the two
upper levels followed by ii) its electron-phonon-induced promotion to the upper-most level and the subsequent iii) return
of the electron to its initial state via emission of a photon of higher energy than that of the absorbed photon. However,
competing relaxation processes contribute to heating. The net cooling power is calculated. Heating predominates at low
temperatures. Significant cooling at elevated temperatures requires satisfying very restrictive conditions. Among these:
i) the energy separation between two highest states must be very small; ii) the degeneracy of the highest state must
exceed that of the state below it, and; iii) the effective electron-phonon interaction, responsible for energy levels' Stokes
shifts, must be exceptionally weak. Different avenues to promising systems to achieve laser cooling are identified.
Three issues concerning phonon-assisted hopping in molecularly doped polymers are considered. The first issue is whether Arrhenius jump rates in the vicinity of room temperature arise from single-phonon or small-polaronic hopping. It is concluded that Arrhenius hopping only occurs above low temperatures through small-polaronic hopping. Second, hopping in molecularly doped polymers is compared with small-polaronic hopping of other systems. Small- polaronic hopping typically occurs between similar chemical structures whose energies are relatively insensitive to their surroundings. Thus, disorder energies experienced by carriers are often modest, values of several hundredths of an eV are common. Nonetheless, the effects of large electric fields on carrier mobilities differ significantly among disordered systems. Data reported for molecularly doped polymers is unlike that for either transition-metal-oxide or chalcogenide glasses. In no case is high-field transport well understood. Finally, I stress that steady-state flow is driven by differences in sites' quasielectrochemical potentials (QECPs). With disorder, differences of QECPs are not simply related to the driving emf. Solution of the (nonlinear) stochastic equations for the QECPs shows that bottlenecks produced by disorder result in nonohmic conduction. Solving the linearized stochastic (disordered resistor network) equations underestimates bottleneck effects. Linearization is inappropriate when intersite differences in the QECPs exceed (kappa) T.