We have studied the low-frequency conductance fluctuations of very small-area metalinsulator-metal tunnel junctions. We find that the spectral power density at high temperature is typical of $\frac{1}{f}$ noise, but is resolved into a small number of Lorentzian spectra for $T\lesssim 80$ K. The thermally activated Lorentzians are described by a two-rate kinetics and a broad distribution of activation energies and attempt rates.

The range of upper critical fields, as deduced from the field-dependent width of resistivity curves, reflects the structure of inhomogeneities in a superconductor. Within a Gaussian model of conductivity fluctuations the maximum upper critical field as a function of temperature yields both the magnitude of the conductivity fluctuation $\frac{\delta \sigma }{\sigma }$ and the spatial range $l_{\mathrm{inh}}$ of the inhomogeneity. Good fits to the data are achieved from $l_{\mathrm{inh}}\sim \xi \mathrm{}\left(0\right)\mathrm{}$ and $\frac{\delta \sigma }{\sigma }\sim \frac{1}{4}$.

We have discovered phonons active in resonant Raman scattering from GaAs quantum-well heterostructures that are actually the optical vibrations of a thin ionic slab. These modes are revealed by selection rules for the polarizations of incident and scattered light that are different from those of bulk GaAs. Analysis of the Raman tensor indicates new deformation-potential electron-phonon scattering processes which are a direct consequence of the reduced symmetry of the layered system.

The resonant absorption of microwave energy by aqueous solutions containing DNA of known length is experimentally demonstrated. The resonances observed have relaxation times of hundreds of picoseconds. Absorption by linear and supercoiled circular DNA molecules is discussed in terms of a mechanism involving microwave excitation of acoustic modes of the double helix.

We propose a new cluster-growth model for branched polymers which are highly unusual in that they are "chemically linear." The cluster grows by adding a constant number of sites in each successive "chemical shell." We find that these chemically linear clusters (where branches and loops are allowed) are in the same universality class as geometrically linear chains (where loops and branches are not allowed). We argue that this result is general in that for any chemically linear cluster, loops and branches can be neglected.

We consider a class of grand-unified theories in which cosmologically significant axion and neutrino energy densities arise naturally. To obtain large-scale structure we consider (1) an inflationary scenario, (2) inflation followed by string production, and (3) a noninflationary scenario with density fluctuations caused solely by strings. We show that inflation may be compatible with the recent observational indications that $\Omega <1\mathrm{}$ on the scale of superclusters, particularly if strings are present.

We solve for the internal structure of $d$-dimensional percolation clusters. We find that if we randomly choose two points, then the backbone connecting these two points can be described as a randomly constructed "necklace" whose building blocks are $s$-site volatile fractals (they are not stable under a length scale change). We find the blob-size distribution function and calculate the relevant parameters numerically, thereby obtaining the first accurate estimates for the fractal dimension of the backbone in $d=2,3\mathrm{}$.

It may look surprising that direct application of linear system theory to the Laplace transform of the time-dependent Schrödinger equation does not yield the evolution operator or density matrix in a physically reliable form, but requires a further "reduction procedure," earlier introduced in the frame of the so-called determinantal formalsim. It is shown that this procedure can in fact be viewed as a factorization of determinants intended so as to remove unexpected terms from the result.

Assuming that nonperturbative QCD generates the chiral-limiting quark mass and the spinless $\pi$ and $\sigma$ mesons, we obtain a gap equation which can be solved for the scalar mass $m_{\sigma }\approx 2m_{\mathrm{qk}}$ in a manner consistent with the original Nambu-Jona-Lasinio approach. A similar relation is obtained within a hypercolor scenario between the dynamically generated component of the "hyperquark" mass and the mass of the Higgs analog.

The occupation probabilities of shell-model orbits in the lead region are estimated by the addition of random-phase-approximation corrections to nuclear-matter results. The occupation probabilities of single-particle states in shells just below and above the Fermi energy are found to be ∼ 0.7 and 0.1, respectively. It is shown that these estimates explain the quenching of the single-particle contribution observed in many elastic and inelastic electron-scattering experiments.

High rates for mesomolecular processes were found in a study of neutron spectra from muon-catalyzed $\mathrm{dt}$ fusion in low-density D/T mixtures. An interpretation is given in terms of a reaction-kinetics model which includes hyperfine effects. The hyperfine components of the $d\mu t$ formation rates, first separated in this experiment, are large from 30 to 300 K. An unexpected temperature dependence for the transition rate between $\mu t$ hyperfine states is found.

Fine-structure intervals separating the $n=10\mathrm{}$, $L=4, 5, 6, \mathrm{and} 7$ manifolds of helium have been measured in a fast atomic beam with precision sufficient to resolve predicted vacuum-fluctuation corrections to the structure. We find the intervals (in the absence of spin) to be as follows: $10G-H$, 490.990(10) MHz; $10H-I$, 157.068(13) MHz; $10I-K$, 60.818(10) MHz. The results set an experimental limit on the contributions of retarded long-range interactions that is comparable to their expected size.

We calculate the energy shifts, in Rydberg states of helium, due to retardation corrections to the two-photon exchange potential. The correction to the level spacings is at the accuracy of existing measurements in the $n=10\mathrm{}$ states of neutral He. It may soon be possible for the first time to detect the effect of these corrections in atomic systems. Estimates of the energy shifts for high-$Z$ He-like ions yield effects of several percent of the total spacing.

It is observed that all solutions to the quasilinear ideal fluid flow equations ${\left[\right(\rho +c^{-2\mathrm{}}p\left)u_{\mu }{u}_{\nu }\right],}_{\nu }=-p_{,\mu }$ with $p=p\left(\rho \right)$ and $u_{\nu }{u}_{\nu }\equiv -c^2$ can be classified algebraically in terms of three basic types of solutions and combinations thereof. This classification engenders the definition of symmetric flows, a broad class for which a general intermediate integral is derived here by application of the Frobenius integration theorem. Representative exact analytical solutions are presented.

A direct ion heating process which is resonant with the wave at the cyclotron subharmonic frequency, $\omega =\frac{3}{2}{\Omega }_i$, is discovered through the particle-simulation investigation of the ion Bernstein-wave heating. The particle trapping in phase space due to the wave of an arbitrary cyclotron subharmonic frequency is studied theoretically and numerically confirmed.

Cross-correlation intensity-fluctuation spectroscopy is used to exhibit the space-time behavior of fluctuations in the director orientation difference between a pair of points in the two-dimensional nematiclike director field of a freely suspended tilted smectic film. The Landau-Peierls divergence expected with increasing time is observed directly.

An exactly solvable generalized dimer model is proposed. In one limit this model is the isotropic Ising model and in another limit it is the anisotropic Kasteleyn model with radically different critical behavior. The value of the crossover exponent $\phi$ near the multicritical point is determined to be ½. The proposed model is isomorphic to a domain-wall model with dislocations, thereby providing an example of dislocation-mediated crossover in uniaxial commensurate-incommensurate transitions.

We have measured the nuclear susceptibility of liquid ${\mathrm{H}}_2$ with different ortho concentrations, and of liquid HD. In ${\mathrm{H}}_2$ the susceptibility is larger than the Curie value, whereas in HD the susceptibility is less. The signs and magnitudes of the differences in susceptibility can be understood qualitatively on the basis of the statistics and the expected magnitude of the exchange interactions between molecules.

Observations by cw NMR of standing spin-wave resonances in normal liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ are found to agree well with Leggett's theory. In the superfluid $A$ phase the resonances show an alternating pattern of frequency shifts, which disappears in the $B$ phase.

We demonstrate the feasibility of x-ray scattering studies of freely suspended strands of discotic liquid crystals (of triphenylene hexa-$n$-dodecanoate). We are able to grow strands, which are stable for days, of diameter > 50 μm with a few single-crystal domains. Unexpectedly, we find that the quasi two-dimensional structural phase transition from columnar $D2$ to columnar $D1$ corresponds to the orientational ordering of columns of molecules with the molecules tilted at finite angles to the column axis in both phases.