Recent measurements of proton spin resonance in biphenyl showed a strong relaxation mechanism below 15 K with a surprisingly high activation energy of 172 K. We show quantitatively that this relaxation mode can be the shifting of the modulation boundaries in the metastable incommensurate phase III, and we explain the apparently high energy. The calculations are based on a microscopic theory of both the I-II and II-III phase transitions including the effects of pressure, which makes biphenyl the best understood incommensurate material.

Temporal oscillations in the specular beam of reflection high-energy electron diffraction (RHEED) provide the primary method of monitoring growth by molecular-beam epitaxy. We develop a model to investigate the origin of these oscillations with which, by monitoring the step density of a growing sample, we are able to reproduce all of the principal features of recent RHEED measurements. Our work demonstrates the considerable advantages in adoption of simple monatomic growth models instead of the complex models used for studying the growth of compound semiconductors.

The temperature dependence of the bend elastic constant near the nematic-smectic-$A$-smectic-$C$ multicritical point has been measured directly by use of Fréedericksz deformation. The bend elastic constant in two different binary systems is found to increase monotonically in the nematic phase as the smectic-$C$ phase is approached. The relation between the bend elastic constant obtained in this study and the smectic correlation lengths derived from x-ray scattering is not described by existing theories. This result also raises questions about the interpretation of previous light-scattering data.

It is shown that the continuum percolation of disks can be formulated as a bond percolation problem on a planar triangulated random lattice.

Diffuse scattering experiments of electrons, neutrons, and x rays have been performed on ${\mathrm{Pd}}_3$V. They all give rise to intensity maxima at positions (100), which indicates a local order different from that of the low-temperature $D{0}_{22}$ ordered phase, characterized by (1 ½ 0) diffraction spots. This contradicts the predictions of the usual mean-field theory. By use of the cluster variation method (octahedron-tetrahedron approximation) it is shown that this surprising effect can be explained in a model with first-and second-neighbor interactions $V_1$, $V_2$, of the same sign, with $\frac{V_2}{V_1}\ll 1$.

We present reflectance data for the high-temperature superconductor Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, and the Kramers-Kronig-transformed conductivity from 50 to 900 ${\mathrm{cm}}^{-1\mathrm{}}$. There is a rich phonon structure with peaks at 151, 191, 279, 310, 548, and 609 ${\mathrm{cm}}^{-1\mathrm{}}$. The continuous background is Drude type at high temperature but below the superconducting transition there is a region of suppressed conductivity consistent with a superconducting gap with the weak-coupling BCS value. Also associated with superconductivity are changes to the low-lying phonons and a plasma-type edge at 60 ${\mathrm{cm}}^{-1\mathrm{}}$.

We employ multichannel quantum scattering theory to treat the magnetoconductance tensor of finitesize two-dimensional samples. We find that ${\sigma }_{\mathrm{xy}}\mathrm{}\left(B\right)\mathrm{}$ vanishes identically as a result of unitarity (but the Hall voltage does not vanish), ${\sigma }_{\mathrm{xx}}\mathrm{}\left(B\right)\mathrm{}$ decreases monotonically with magnetic field without any oscillation with period of the quantum flux, and for weak impurity scattering $〈{\sigma }_{\mathrm{xx}}\mathrm{}\left(B\right)\mathrm{}〉$ is smaller (larger) than the conductance of an impurity-free sample for small (large) $B$ values.

We report on the study of large two-level, low-frequency resistance fluctuations in 1-μ${\mathrm{m}}^2$ metal-oxide-silicon tunnel diodes, which are due to the strongly correlated emptying and filling of ensembles or clusters of interacting localized states in the oxide. The interaction mechanism is attributed to ionic forces in the strained oxide. It can give rise to complex structure in the switching noise and, under strong electrical stress, can result in the breaking of oxide bonds by the collective action of localized states.

A drastic energy shift and a growth of new splitting lines were observed in the magnetoabsorption spectra of stacking-fault excitons in Bi${\mathrm{I}}_3$ under high pulsed magnetic fields up to 40 T. The observed magnetic field effects cannot be explained by a conventional effective-mass approximation or the bielectron model, but were successfully understood on the basis of a model of cationic excitons which are perturbed by a specific stacking fault.

Intersubband transitions stimulated by a light wave propagating in the plane of a two-dimensional electron gas are accompanied by a "photon-drag" current due to the momentum imparted by the absorbed photons. In a high-mobility electron gas at low temperatures, a peculiar effect is predicted to occur as a result of the difference in the momentum relaxation times in the ground and the excited subbands. This effect permits the implementation of novel far-infrared detectors, as well as a new type of spectroscopy containing information about the momentum-relaxation kinetics in 2D subbands.

It is shown in perturbation theory that if a dc current is set up in a quasicrystal (or Fibonacci lattice), and then the electric field turned off, the current will decay slowly to zero with an anomalous logarithmic dependence on time. This is most likely due to the extremely long-wavelength Fourier components possessed by a quasiperiodic potential. The calculated electrical conductivity approaches infinity as ${\mathrm{}\left(\mathrm{frequency}\right)\mathrm{}}^{-1\mathrm{}}$.

It is shown theoretically that the conversion of spin-up into spin-down current near a ferromagnetic-nonferromagnetic metal interface gives rise to an electrochemical potential difference of spin-up and spin-down electrons. The effect shows up as an additional boundary resistance. The situation is analogous to the normal-metal-superconductor interface. Experimental methods that may be used to observe the effects are discussed.

Circle maps are used to study the "spectrum of singularities" of trajectories on two-tori with golden-mean winding numbers at the onset of chaos. Within a renormalization-group approach, the effect of a deviation from criticality is understood as a crossover from a strong- to a weak-coupling fixed point.

We study the effects of Wilson fermions in SU(3) lattice gauge theory by an algorithm considering the fermion determinant exactly. We use lattices of size $4^3$×2 and $4^4$. A comparison is made with the outcomes of fast approximate algorithms of the bush-factorized type. Our results point to the occurrence of a phase transition near $\beta =5.3\mathrm{}$ and $\kappa =0.14\mathrm{}$. We discuss the relation of this phase transition to that observed recently with the Kogut-Susskind-type fermionic action.

We introduce the minijet cross section, computed from QCD, together with a standard soft component, into a unitarization scheme (eikonal model) and show that most of the increase of the inelastic cross section between CERN ISR and SPS collider energies is due to the soft component. We also show that the main properties of minijet production, observed by the UA1 collaboration, can be understood by the introduction of semihard scattering in the dual parton model.

The data from the spectrum measurement of the tritium decay in a valine molecule carried out in a wide energy range (3.4 keV) with the Institute of Theoretical and Experimental Physics spectrometer are analyzed. The combined analysis of both these data and the data of the previous cycle gives the neutrino mass ${30.3}_{-8\mathrm{}}^{+2\mathrm{}}$ eV. The model-independent mass interval $17<\mathrm{}{M}_{\nu }<40\mathrm{}$ eV is derived from the mass difference of the doublet ${}_{}{}^3{}_{}{}^{}\mathrm{H}$-${}_{}{}^3{}_{}{}^{}\mathrm{He}$.

The beta spectrum of free molecular tritium has been measured in order to search for a finite electron-antineutrino mass. The final-state effects in molecular tritium are accurately known and the data thus yield an essentially model-independent upper limit of 27 eV on the ${\nu }_e$ mass at the 95% confidence level.

Pion-induced $\eta$-meson production populating a discrete nuclear state is observed for the first time in the reaction ${\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to \eta +t$. At 680 MeV/c ${\pi }^{-}$ momentum, this reaction is identified by measurement of the recoiling tritons. The measured cross sections are approximately 3 orders of magnitude greater than the $p+d\to \eta +{}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the $d+d\to \eta +{}_{}{}^4{}_{}{}^{}\mathrm{He}$ cross sections. Distorted-wave impulse-approximation calculations underpredict the observed cross sections by a factor of 2-4.

Hyperfine structure (hfs) has been observed in the $P$, $Q$, and $R$ branches of the fundamental rotation-vibration spectrum of the $X{}_{}{}^2{}_{}{}^{}\Pi$ electronic state of the molecular anion N${\mathrm{H}}^{-}$. A coaxial ion-beam-laser-beam spectrometer is employed in which rotation-vibration transitions are induced by a color-center laser operating near 3000 ${\mathrm{cm}}^{-1\mathrm{}}$. The vibrationally excited ions autodetach, and the fast neutral NH is detected. The structure has been analyzed and the magnetic dipole hfs parameters obtained in both ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}^{-}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}^{-}$. This is the first analyzed case of hfs in any molecular anion.

Thanks to an acoustic technique, we have measured the longitudinal dispersion of miscible fluids in three disordered porous media over a wide range of Péclet number. The hydrodynamic dispersion is always Gaussian whatever the flow rate, the sample, or the length of the sample is. The low-flow-rate regime gives the tortuosity of the medium. At higher flow rate, two media having the same permeability and tortuosity but very different pore-size distributions exhibit linear (glass beads) and nonlinear dispersion (foamlike fireproof brick) in accordance with recent numerical simulations.