The atomic structure of the (310) twin in Nb was predicted using interatomic potentials derived from the embedded atom method (EAM), Finnis-Sinclair theory (FS), and the model generalized pseudopotential theory (MGPT). The EAM and FS predicted structures with crystal translations which break mirror symmetry. The MGPT predicted one stable structure which possessed mirror symmetry. This defect was experimentally determined to have mirror symmetry. These findings emphasize that the angular dependent interactions modeled by the MGPT are important for determining defect structures in bcc transition metals.

Monte Carlo calculations have been used to study the mechanical relaxation of a monolayer of ${\mathrm{CH}}_3$(${\mathrm{CH}}_2$${)}_{15}$SH admolecules on a gold substrate when subjected to indentation caused by a nanometer-scale force-microscope tip. An almost elastic response is observed to forces that compress the monolayer by about 25% of its original thickness. Jump to contact on approach of the tip is signaled by an abrupt change in the mean molecular tilt, and the subsequent compression leads to very substantial but reversible changes in the conformations of the molecules. Comparison is made with results obtained by interfacial-force microscopy.

Using a time-of-flight technique, different transport mechanisms, deep trapping, multiple shallow trapping, and ideal itrinsic transport, can be observed in the different temperature and phase regions of the liquid-crystalline photoconductor hexapentyloxytriphenylene. The temperature and field dependences of carrier mobilities up to 1×${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^2$/V s have been determined; this value exceeds considerably the mobilities of the most commonly used organic photoconductors. The experiments reflect a variety of transport phenomena which are novel in the field of liquid-crystalline systems.

We analyze the effect of disorder on the statistics of a ring macromolecule by a computer simulation. We show how the statistics of the ring change from obeying self-avoiding to lattice animal statistics. We find a scaling relationship to characterize this crossover behavior. We further show how this problem maps onto a related study on two-dimensional vesicles with a pressure difference.

A new center related to ${\mathrm{As}}_{\mathrm{Ga}}$ has been found at relatively high concentrations (${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$) in semi-insulating (2×${10}^7$ Ω cm) molecular beam epitaxial GaAs grown at 400 °C. Although the ir photoquenching and thermal recovery characteristics are nearly identical to those of EL2, the thermal activation energy is only 0.65±0.01 eV, much lower than the EL2 value of 0.75±0.01 eV. Other properties which are different include the electron-capture barrier energy, hyperfine constant, and magnetic circular dichroism spectrum.

Light scattering was used to study nucleation in a quenched binary mixture exposed to a uniform shear. Shear deforms and ruptures droplets once they have grown to sufficient size. In the resulting steady state, the droplets are monodisperse in size. A plot of the cloud point line exhibits two segments of different slopes with a crossover near the temperature corresponding to the Becker-Döring limit. An energy-based argument explains a number of our observations.

We measured for the first time simultaneously the ac susceptibility and the static magnetization of high-density bcc solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ through the nuclear-ordering transition. We discovered a sharp peak in the real part of the ac susceptibility and an abrupt depression in the imaginary part near the transition temperature. The transition temperature indicated by ac susceptibility is higher than that indicated by static magnetization. We interpreted the novel behavior in the susceptibility in terms of the spin relaxation between the Zeeman system and the exchange system.

The growth of Ag on Pt(111) has been investigated at submonolayer coverages betwen 80 and 1000 K. Below about 550 K large-scale metastable uniformly strained 2D Ag islands grow pseudomorphically. Heating of the submonolayer film or direct deposition of Ag above this temperature leads to the formation of small, ‘‘strain free’’ 2D equilibrium structures, possibly heptamers.

We have performed numerical calculations of the diagonal and off-diagonal (Hall) components of the conductivity tensor for a system of noninteracting electrons in two dimensions at high magnetic fields (lowest Landau level), in the presence of a random potential. We have considered five different potentials, with varying correlation length, with and without electron-hole symmetry. Our results are consistent, within statistical uncertainties, with a conductivity tensor at the critical energy with both components equal to 0.5${\mathit{e}}^2$/h. This suggests that the conductivity tensor at the lowest-Landau-level integer quantum Hall transition is universal, independent of the details for short-range random potentials.

The electronic states and the magnetoluminescence spectrum of a few correlated electrons in parabolic quantum dots recombining with valence band holes localized by acceptors are investigated. We show that for an ensemble of quantum dots the average energy of emitted photons is a discontinuous function of the magnetic field. The discrete jumps in photon energy correspond to changes in the angular momentum of the ground state with increasing magnetic field. The size and the sign of the discontinuity are a direct measure of electron-electron and final-state interactions.

Far-infrared magneto-optical spectra of shallow donors confined in GaAs quantum wells in the presence of a quasi-two-dimensional electron gas show a characteristic magneto-optical transition whose frequency increases with increasing density of excess electrons, and whose slope exhibits discontinuities versus magnetic field at integer Landau level filling factors. These results demonstrate the importance of electron-electron interactions and possibly dynamical screening in high magnetic fields.

Using a simple variational ansatz, we estimate the pair wave function of a superconductor obtained by doping a Mott insulator with short-range antiferromagnetic spin correlations. The pairing symmetry is generically d wave. The so-called ‘‘anyon superconductors’’ are related to traditional BCS states with d-wave order parameters that break parity and time-reversal invariance. The transition between a tetragonal normal metal and a superconductor of this type generically proceeds through an intermediate phase with either superconducting order or broken time reversal, but not both.

The full spectrum of a single hole in a quantum antiferromagnetic background (t-${\mathit{J}}_{\mathit{z}}$-${\mathit{J}}_{\mathrm{\perp }}$ model) is obtained by complete exact diagonalization of small two-dimensional clusters. Various statistical properties of the spectrum are investigated. On a very wide range of the parameters the level-spacing distribution follows Gaussian-orthogonal-ensemble Wigner law characteristic of chaotic spectra. At small separation, the spectral rigidity follows the universal behavior described by random-matrix theory and presents deviations at higher energies. We argue that quantum chaos is a generic feature of complex (i.e., nonintegrable) strongly correlated fermion systems. Our results suggest that random-matrix theory might be useful to investigate the incoherent part of dynamical correlation functions (such as the hole spectral density or the spin structure factor).

Thermal expansion and magnetostriction measurements on single crystalline ${\mathrm{UPd}}_2$${\mathrm{Al}}_3$, a very clean, strongly Pauli-limited, heavy-fermion superconductor (${\mathit{T}}_{\mathit{c}}$≃1.82 K), have been utilized to establish a first-order transition at T≤T̃≃1.5 K and B<${\mathit{B}}_{\mathit{c}2}$(T). This is ascribed to the hitherto unobserved formation of a nonuniform superconducting state, as pedicted theoretically in 1963.

Measurments of the ac penetration depth of a-MoGe films in the presence of a perpendicular magnetic field reveal an anomaly in the ac response of the vortex lattice at a characteristic temperature below the ${\mathit{H}}_{\mathit{c}2}$(T) line. The field and frequency dependence of this anomaly is found to be consistent with a Kosterlitz-Thouless-type melting of the two-dimensional vortex lattice. Moreover, we observe a crossover in the frequency dependence which suggests that the vortex lattice remains disordered on long length scales below the melting temperature.

Trilayer sandwiches of Pd/Pd(1.2 at. % Fe)/Pd have been prepared by sputtering onto Si(111) substrates. The dc magnetization of a 2 nm thick film has been studied as a function of sample temperature and applied magnetic field. It is found that for 6.5 ≤T≤15 K the 2 nm film has a uniform susceptibility which increases as exp(53/T) in qualitative agreement with predictions for a 2d Heisenberg ferromagnet and that the field dependence of M(H,T) follows predicted behavior. At 6.5 K there appears either a phase transition or a rather sharp crossover from 2d Heisenberg model behavior to that of another model.

Light scattering experiments on solutions of bovine CO-hemoglobin have revealed that a substantial component of the interactions responsible for the protein stability versus tetramer-to-dimer dissociation is due to electrostatic effects. Their magnitude has been estimated to reach ≃4 kcal/mole (≃0.15 eV) at pH=5 compared with the nonelectrostatic term which is of the order of 7 kcal/mole (≃0.3 eV).

We report on a a new measurement of the relativistic Doppler shift in neon. We measured the frequency difference between a two-photon transition in a fast beam and a slow beam. The result is compared to the prediction of special relativity. The experiment represents a more than tenfold improvement over other Doppler shift measurements and verifies the time dilation effect at an accuracy level of 2.3 ppm.

Point-to-point correlation functions of hadron currents in the QCD vacuum are calculated on a lattice and analyzed using dispersion relations, providing physical information down to small spatial separations. Qualitative agreement with phenomenological results is obtained in channels for which experimental data are available, and these correlation functions are shown to be useful in exploring approximations based on sum rules and interacting instantons.

Experimental and theoretical evidence is presented that the proton exchange is strongly enhanced by a mixing of single-particle configurations in ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ (in the system ${}_{}{}^{36}{}_{}{}^{}\mathrm{S}$${+}^{37}$Cl, which is shown to be the clearest example of hybridization in nuclear physics. The experimental data on elastic and inelastic transfer are only reproduced if the complete set of single-particle states (${\mathit{d}}_{3/2}$,${\mathit{s}}_{1/2}$,${\mathit{f}}_{7/2}$,${\mathit{p}}_{3/2}$,${\mathit{f}}_{5/2}$, and ${\mathit{p}}_{1/2}$) is included in a coupled-reaction-channel calculation. The strong enhancement is explained by the hybridization of orbits of different parity. In a two-center molecular-orbital approach the density of the proton orbitals in the lowest state is shown to be concentrated at the center between the two ${}_{}{}^{36}{}_{}{}^{}\mathrm{S}$ cores.