The dc conductivity of carbon-black–polymer composites has been measured as a function of carbon-black concentration from just above the percolation threshold ${\mathit{p}}_{\mathit{c}}$ up to 33${\mathit{p}}_{\mathit{c}}$, and in the temperature range from 4 to 300 K. To explain the temperature dependence we have modified the Mott-Deutscher model for variable-range hopping between superlocalized states on a fractal by assuming Coulomb-dominated hopping. Our data then yield the first experimental evidence of electron superlocalization, with a superlocalization exponent ζ=1.94±0.06, close to theoretical estimates. The derived localization length is of the right order of magnitude.

Measurements of the complex ac conductivity are reported for a single crystal of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ for frequencies ${10}^2$≤ν≤${10}^9$ Hz and temperatures 25≤T≤300 K. The conductivity follows a power-law behavior ${\mathrm{\omega }}^{\mathit{s}}$ with the frequency exponent s independent of temperature and independent of frequency. However, the hopping transport is strongly anisotropic, with s≊0.75 within the ${\mathrm{CuO}}_2$ planes and s≊0.25 perpendicular to the planes.

We have directly observed individual hopping events in the flow of curent along a single chain of localized states. These events modulate the conductance of a ballistic constriction formed in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As split-gate transistor, resulting in a time irreversible random telegraph signal. The hopping chain, carrying approximately one electron per second, is situated between the gate and the two-dimensional electron gas, and can be altered by varying the gate voltage.

We report detailed measurements of the low-temperature magnetoconductance in ballistic microstructures in the shape of a ‘‘chaotic’’ stadium and a circle with quantum-point-contact leads. Both structures show large, aperiodic, conductance fluctuations as a function of perpendicular magnetic field, and a zero-field resistance peak indicating geometry-dependent enhanced backscattering. Power spectra of fluctuations are consistent with recent semiclassical analyses based on quantum chaotic scattering, with the circle showing enhanced high-frequency spectral content.

A semiclassical description is given of charge transport through a superconductor-semiconductor interface. As a result of the presence of a potential barrier both Andreev and normal reflection occur. Elastic scatterers in the semiconductor generate multiple reflections at the interface. The constructive quantum interference which results from the phase conjugation between electrons and holes enhances the (differential) conductance above its classical value. This excess conductance is suppressed by a magnetic field, or by a finite energy. The latter can be due to a finite voltage bias or a finite temperature.

We present a detailed study of the dynamic response of overdamped Josephson-junction square arrays driven by a dc plus ac current, including self-induced magnetic-field effects, as a function of a parameter κ. The transition from type-II (κ>1) to type-I (κ<1) behavior is studied in detail. It is found that the IV characteristics show fractional giant Shapiro steps, with and without external magnetic fields, with microscopic coherent vortex dynamics fundamentally different in the two extreme κ limits. Our results in zero external magnetic field are in good agreement with recent experiments in Nb-Au-Nb arrays.

We report resutls of extensive Monte Carlo simulations of a three-dimensional model system containing the essential physics required to describe the melting of an unpinned Abrikosov lattice in an extreme type-II superconductor. A first-order phase transition is found. At the field which we have studied, corresponding very roughly to a field of 10 T along the c axis in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, the latent heat of melting is ≃0.3${\mathit{k}}_{\mathit{B}}$T per vortex per layer.

We attached two current and two voltage contacts on both sides of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ single crystals and performed transport measurements applying the current parallel to the ${\mathrm{CuO}}_2$ layers. In the Ohmic regime, the voltage signal on the side of the current contacts was more than a factor of 100 larger than at the opposite side. The results are interpreted within an anisotropic resistivity model to obtain the true resistivities ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$(B,T) and ${\mathrm{\rho }}_{\mathit{c}}$(B,T). A model is presented based on the movement of pancake vortices involving vortex shear, vortex cutting, and generation of Josephson vortices between the layers.

We examine the influence of a staggered magnetic field on states involving a few holes in the one-dimensional t-J and Hubbard models. We apply exact diagonalizations of small systems, an analytical treatment of an anisotropic model, and a high-field perturbation expansion. Our results suggest that even modest staggered fields, h≪t, induce the formation of bound hole pairs in a broad range of parameters, in particular for J/t<1 in the t-J model and for U/t>1 in the Hubbard model. In the same regions, by studying the density-density correlations and the compressibility, we argue that many holes form a paired state and that phase separation does not occur.

This paper reports the measurements of resistivities near the superconductor-insulator transition for high-${\mathit{T}}_{\mathit{c}}$ oxide superconductor ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ single-crystal thin films. The transition was tuned by magnetic fields and/or introducing disorder. The results of the resistivities are analyzed in line with dynamical scaling theory. The product of the dynamical exponent (${\mathit{z}}_{\mathit{B}}$) and the exponent for the correlation length (${\nu }_{\mathit{B}}$) is extracted. This value is consistent with that expected by the scaling theory. Our results provide the first evidence of the Bose-glass–vortex-glass phase transition in high-${\mathit{T}}_{\mathit{c}}$ ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ single-crystal thin films.

A renormalization-group analysis of the spin-1/2 transverse field Ising model with quenched randomness is presented; it become exact asymptotically near the zero temperature ferromagnetic phase transition. The spontaneous magnetization is found to vanish with an exponent β=1/2(3- √5 ), while in the disordered phase the typical and average spin correlations are found to decay with different correlation lengths, which diverge with exponents ν̃=1 and ν=2, respectively.

We have studied the photoexcitation dynamics in electrochemically polymerized polythiophene thin films using transient photomodulation (PM) spectroscopy measured from 100 fs to 20 ms in the spectral range from 0.25 to 2.2 eV, and by the novel technique of absorption-detected magnetic resonance (ADMR). Our results show that photogenerated exciton polarons are formed within 300 fs; they subsequently decay in the nanosecond time domain by 1D diffusion toward recombination centers The dominant excitations after about 250 ns are charged polarons and bipolarons, which are identified by optical transitions correlated with a spin-1/2 ADMR signal at g=2.003.

Using a low-strain, isotopically pure crystal of ${\mathrm{YLiF}}_4$, we have measured extremely narrow inhomogeneous linewidths (as low as 40 MHz) for the optical transitions of ${\mathrm{Er}}^{3+}$ impurities. Inhomogeneous broadening due to strains and other defects is so low that a new mechanism limits the inhomogeneous linewidth: local magnetic fields due to fluorine nuclear spins. These ultranarrow lines enabled us to make the first direct measurement of the isotope shifts of f-f transitions in a solid, and we find for the isotopes ${}_{}{}^{164}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$, and ${}_{}{}^{170}{}_{}{}^{}\mathrm{Er}$ a shift of 76±2 MHz unit mass on the ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{15/2}$(1${)}^4$${\mathit{F}}_{9/2}$(1) transition. We show that this is consistent with a mechanism of coupling to zero-point vibrations.

It is known that an RNA molecule capable of self-splicing shares a common pattern of Watson-Crick base paris with other RNA species endowed with the same capability. The aim of this work is to introduce a minimal model Hamiltonian which determines a localized strain in the RNA backbone as the search for the molecular conformation is subject to the constraint imposed by the concensus secondary structure. The site where the strain is localized is shown to coincide with the splicing site of the molecule. As justified posteriori, the level of structural complexity of the model is sufficient to account for energy localization in a nontrivial fashion.

Using semiclassical arguments substantiated by numerical results for the quantum kicked rotor, we establish that breaking an antiunitary symmetry in a system with dynamical localization increases the localization length by a factor of 2. The transition between the symmetric and the symmetry broken case is smooth. The semiclassical theory provides an approximate expression for the transition function as well as the critical strength of the symmetry-breaking interaction necessary to achieve the full factor of 2 increase of the localization length.

We describe the results of studies of a random-anisotropy Blume-Emery-Griffiths spin-1 Ising model using mean-field theory, transfer-matrix calculations, and position-space renormalization-group calculations. The interplay between the quenched randomness of the anisotropy and the annealed disorder introduced bythe spin-1 model leads to a rich phase diagram with a variety of phase transitions and reentrant behavior. Our results may be relevant to the study of the phase separation of ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixtures in porous media in the vicinity of the superfluid transition.

The bubble picture for pair-spin correlation functions is applied to two-dimensional Ising-like models, at subcritical temperatures, in an external field h, from which we show, via the fluctuation sum, that the susceptibility χ(h) has an essential singularity at h=0. This is studied further using a solid-on-solid bubble model for (a) a restricted ensemble corresponding to metastability, where h=0 is found to be the limit point of an infinite number of poles of χ along the negative real axis, and (b) an unrestricted ensemble, where a Yang-Lee circle theorem is found.

We show that the low-lying spectrum of three noninteracting anyons in a harmonic potential can be qualitatively and quantitatively understood on the basis of a simple set of analytic wave functions. These wave functions are exact in both the boson and fermion limits and satisfy the anyone wave-function phase condition at all intermediate values of the statistical parameter 0<α<1. The resulting nonlinear spectrum agrees with those of exact numerical calculations to better than a few parts in a thousand.

It is shown that the currently studied ‘‘string-inspired’’ model for gravity on a line can be formulated as a gauge-invariant theory based on the Poincaré group with central extension—a formulation that complements and simplifies H. Verlinde’s construction based on the unextended Poincaré group.

Results that illuminate the physical interpretation of states of nonperturbative quantum gravity are obtained using the recently introduced loop variables. It is shown that (i) while local operators such as the metric at a point may not be well defined, there do exist nonlocal operators, such as the area of a given two-surface, which can be regulated diffeomorphism invariantly and which are finite without renormalization; (ii) there exist quantum states which approximate a given metric at large scales, but such states exhibit a discrete structure at the Planck scale.