Lifetimes were measured for transitions in the superdeformed band of ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$ with the Doppler-shift attenuation method. The results yield an essentially constant quadrupole moment of 20±2 e b and indicate that the sidefeeding lifetimes are of the same order as the state lifetimes. The data are consistent with calculations using the cranked Woods-Saxon Strutinsky method with pairing.

Driven two-level atoms are shown to exhibit strong, resonantly enhanced, two-photon gain. This gain, unlike that observed in nonlinear mixing processes, is not constrained by a phase-matching condition involving the pump beam. Physically, the gain can be associated with inverted two-photon transitions between dressed states or multiphoton stimulated Raman scattering, and it may be useful in the realization of a two-photon laser. Comparison with the well-known single-photon gain found in the same system is made.

We present experimental evidence for localization of acoustic waves in a novel three-dimensional system. The system consists of a polymer melt solidifying by the growth of spherical semicrystalline nuclei. For sufficiently high excitation frequencies, we find renormalization of sound speed and intense absorption peaks over very narrow ranges of wave number. We generalize theory to include shear modes in the scatterers, and find that localization is consistent with our observations.

Solid solutions of (KI${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$I${)}_{\mathit{x}}$ with concentrations x=0.14, 0.43, and 1 were investigated using dielectric spectroscopy. The high-temperature rocksalt structure of these compounds exhibits a dipole moment of 1.4 D. For x=0.14 and 0.43 the NaCl structure is stable down to the lowest temperatures and the reorienting dipoles undergo a cooperative freezing transition devoid of long-range orientational order.

Field-ion microscopy has been used to map the surface sites visited by an individual Pt adatom migrating at 175 K on the Pt(001) plane and to determine its activation energy for surface diffusion. Atomic displacements are found to occur only in the [100] or [010] directions. A surprisingly low activation energy of 0.47 eV is measured. These results are consistent with a model in which the diffusion occurs by an adatom-substrate exchange mechanism. This mechanism is predicted theoretically for self-diffusion on Al(001).

On the Ir{001} surface an Ir adatom is surprisingly found to jump along the 〈001〉 direction instead of along the smoother closely packed 〈110〉 atomic channel of the substrate, probably by an atomic exchange mechanism. The sites visited by one diffusing adatom therefore form a c(2×2) net of the substrate lattice. The displacement distributions derived are consistent with those expected from a discrete random walk with the atomic jump length of the nearest-neighbor distance of the c(2×2) net.

High-resolution angle-resolved-photoemission results are presented which probe in detail the transition of intrinsic surface bands on clean and hydrogen-covered Ta(011) from pure states to resonances. This is manifested by both a resonant broadening of ∼0.5 eV and a level shift. An unusual avoided crossing between surface and bulk bands is observed in which the surface and bulk characters are interchanged. The resultant hybridization of surface and bulk bands near the crossing has not previously been observed.

A theoretical analysis is made of the bond lengths of small (100–1000 atoms) Cu particles at various temperatures. The interatomic interactions are calculated using the effective-medium theory and the finite-temperature properties obtained from a molecular-dynamics simulation. We find only very small changes in bond length with particle size, but the motion in the small particles is very anharmonic. We use this observation to resolve the current experimental controversy about the existence of bond contraction for small metal particles.

An analytical path-integral method based on the nonequilibrium Green’s function is developed to investigate the linear response of a double-barrier resonant-tunneling system under a small ac signal which is superimposed upon a dc bias. When the dc-bias voltage is zero, our calculated ac conductance reduces back to that of a recent Kubo-formula study. When the system is biased at the negative differential resistance point, both the real (negative) conductance and imaginary part of admittance are derived. Comparisons with previous numerical study and experiments are discussed.

Thermal saturation of band transport in a GaAs/AlGaAs superlattice miniband is observed for the first time. The Drude conductivity of the miniband is measured as a function of temperature. As the temperature is raised so that ${\mathit{k}}_{\mathit{B}}$T is significantly larger than the miniband width the conductivity is strongly quenched. Excellent quantitative agreement is obtained when these results are compared with a semiclassical Boltzmann theory of transport in a narrow band.

The first observation of scattering-controlled transmission resonances in superlattices is reported. These originate from states supported by subsets of the superlattice of thickness equal to the electron coherence length. At lower electric fields a broad region of negative differential conductance due to progressive field-induced wave-function localization is also observed. This phenomenon is shown to be physically equivalent to the Bragg-diffraction-induced negative differential conductance predicted by Esaki and Tsu in 1970.

Systematic line-shape studies of the ‘‘anomalous’’ cyclotron resonance of quasi-two-dimensional electrons in Si metal-oxide-semiconductor devices at low temperature and low densities have revealed a multiple-line structure. The ratio of electron density to magnetic field (Landau-level filling factor) at which the anomalies appear is systematically correlated with the inverse of the electron peak effective mobility, demonstrating the importance of localization. Several features cannot be explained by single-particle localization, and it is argued that electron-electron correlations also play an importnat role.

Vector-spin models with competing interactions have in general nontrivial ground states which completely break rotational invariance. We study the prototypical stacked triangular Heisenberg antiferromagnet by means of a O(3)×O(2)/O(2) nonlinear σ model in a 2+ε expansion. We find a dynamically generated O(4) symmetry. We propose that such systems in three dimensions have a first-order transition or a second-order one with either N=4 or tricritical mean-field exponents. We argue that this view is supported by experimental and numerical data.

The magnetization in ultrathin Fe layers (2.5–3.5 atomic layers) on Cu(100) reversibly switches between perpendicular (at low temperature) and in-plane magnetization (at higher temperature). The switching temperature decreases with increasing film thickness. The switching transition is attributed to the temperature dependence of the perpendicular anisotropy. The transition is accompanied by a loss of magnetization over a temperature range of 20–30 K and shows evidence for a canted-spin configuration.

We investigate the effect of quantum fluctuations on the Coulomb blockade in a single tunnel junction coupled to its environment by a transmission line of arbitrary impedance Z(ω). The quantized oscillation modes of the transmission line are suddenly displaced when an electron tunnels through the junction. For small Z (relative to the quantum of resitance), a weak power-law zero-bias anomaly occurs associated with the infrared-divergent shakeup of low-frequency transmission-line modes. For large Z, the full blockade is recovered. Comparison with recent experiments is made.

We discuss thermally activated resistivity of strongly layered high-temperature superconductors such as ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ in the limit in which the dominant pinning mechanism is the intrinsic pinning due to the layered structure. From a comparison of our theory with the experimental data we conclude that the vortex state of such superconductors must be disordered over a substantial range of parameters—a melted liquid. The magnetic field, the temperature, and the current dependence of the calculated activation energy are consistent with experiments.

Heteroepitaxial growth of strained multilayer thin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/${\mathrm{Nd}}_{1.83}$${\mathrm{Ce}}_{0.17}$${\mathrm{CuO}}_{\mathit{x}}$ by pulsed-laser deposition is reported. The coherency strain results in biaxial compression of the tetragonal ${\mathrm{Nd}}_{1.83}$${\mathrm{Ce}}_{0.17}$${\mathrm{CuO}}_{\mathit{x}}$ layers, whereas the biaxial tension in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ layers removes the orthorhombic distortion and makes the unit cell isotropic in the basal plane (a=b). Depending on their oxygen content, either the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ or the ${\mathrm{Nd}}_{1.83}$${\mathrm{Ce}}_{0.17}$${\mathrm{CuO}}_{\mathit{x}}$ layers are superconducting in these multilayers. The strain-induced structural modification has a significant influence on the superconducting transition temperature of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ layers.

Studies of flux motion due to the thermal force resulting in the Nernst effect are complementary to Lorentz-force experiments, because the thermal force is not subjected to spatial redistribution. We have measured the Nernst effect in superconducting epitaxial and polycrystalline Y-Ba-Cu-O films. From our experimental results the transport entropy per unit length of flux line is calculated. Its order of magnitude agrees with the theoretical prediction based on the time-dependent Ginzburg-Landau theory.

A method to obtain a superposition of time evolutions of a quantum system which correspond to different Hamiltonians as well as to different periods of time is derived. Its application to amplification of an effect due to the action of weak forces is considered. A quantum time-translation machine based on the same principle, utilizing the gravitational field, is also considered.

A gauged, nonlinear Schrödinger equation in two spatial dimensions is considered. This equation describes nonrelativistic matter interacting with Chern-Simons gauge fields. We find explicit static, self-dual solutions that satisfy the Liouville equation.