We show that director fluctuations in nematics induce long-range interactions between walls, analogous to van der Waals forces, which are attractive with symmetric boundary conditions, but may become repulsive with mixed ones. They affect the swelling of lamellar phases in a nematic solvent. The same phenomenon takes place in other mesophases, particularly in smectics, where it gives rise to forces with longer range than van der Waals.

We have discovered novel nonlinear dissipative dynamic patterns in nematic liquid crystals under the influence of a continuously rotating magnetic field. We present a state diagram of the soliton structures as a function of field and rotation rate and discuss physical models describing their growths and propagation and transitions.

Using short infrared pulses and sum-frequency up-conversion, the dephasing of the transient polarization of the Si-H adsorbate-substrate vibration is followed in real time. At 120 K, dephasing due to homogeneous linewidth or inhomogeneous broadening are separated.

Experimental evidence for two regimes in the relationship of the defect density to the hydrogenation state of hydrogenated amorphous silicon (a-Si:H) is presented. A prevoius model for the defect density based on the two phases of bound hydrogen in a-Si:H is shown to account for the measured relationship.

We present results of a structural investigation of oxygen ordering in a 0.1-${\mathrm{cm}}^3$ ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.35}$ single crystal. For the first time nonmagnetic superstructure reflections have been observed by neutron diffraction. From the intensities a 2 √2 a×2 √2 a×c oxygen ordering model is derived. In this structure ‘‘half-filled’’ CuO chains alternate with ‘‘quarter-filled’’ chains. Only 15% of the basal-plane oxygen atoms contribute to the superstructure.

Line-shape simulations and resonance-light-scattering experiments of the J band in aggregates of pseudoisocyanine are reported. It is concluded that the J band can be described as a disordered Frenkel-exciton band with states that are pseudolocalized on segments of the aggregate chain. The resonance-light-scattering experiments show that the homogeneous dephasing of the excitonic states is dominated by phonon scattering to thermally accessible band states and that excitonic dephasing occurs in the fast modulation limit.

The Larmor-frequency dependence of the proton spin-lattice relaxation rate in 4-4’ bichlorobiphenylsulphone is found to vary as 1/${\mathit{T}}_1$∝${\mathrm{\omega }}^{\mathrm{-}1/2}$ over more than 5 orders of magnitude of frequency below the phase transition at T=148 K. This result is believed to constitute the first example where the classical damped-harmonic-iscollator dynamic susceptibility, with no additional self-energy terms and a strongly damped, gapless, phason spectrum, is shown to dominate the dynamics of an incommensurate system at frequencies as low as 500 kHz.

The intrinsic dephasing time of an exciton polariton is investigated in the time domain by use of a time-resolved coherent two-photon exciation and probe technique. This method allows discrimination of the elementary interaction processes of the exciton polariton in semiconductors. The results obtained in CuCl show the role of the different phonons in the exciton-polariton dephasing.

Hall-effect measurements of electron mobility in anti-modulation-doped GaAs/AlGaAs quantum wells indicate that the ionized-impurity scattering of a quasi-two-dimensional electron gas immersed in the identical concentration of ionized impurities is greater than that of a bulk electron gas of the same density. This enhancement results from an increase of the overlap of the electronic wave function with the impurities, a decrease in screening, and an increase in large-angle scattering. The enhanced scattering rate may be further increased by confining the dopant ions to a deltalike doping profile in the center of the well.

The distribution of hopping conductances in the strongly localized regime in the presence of both a magnetic field and spin-orbit (SO) scattering is calculated via an analytic independent-directed-path formalism, and a locator expansion which includes all correlations between paths. Both methods lead to a positive magnetoconductance for all strengths of SO scattering, contrary to recent random-matrix theory predictions. Extensive numerical simulations demonstrate that the crossover from negative to positive magnetoconductance occurs as the system size exceeds the localization length.

The magnetic structure of holmium has been studied using neutron diffraction when a magnetic field is applied along the c axis. The field has the effect of suppressing the onset of the commensurate cone phase found at low temperatures in zero field, and instead produces a series of spin-slip structures. In contrast to the zero-field diffraction experiments, where a continuous variation of the magnetic wave vector q was observed, we find that below ≊15 K the wave vector q is always commensurate and forms a devil’s staircase with increasing field.

We have observed a clear series of fine structures superposed on the angular dependence of magnetoresistance in the metallic and magnetic-field-induced spin-density-wave phases in the low-dimensional organic superconductor (TMTSF${)}_2$${\mathrm{ClO}}_4$. They appear at the field angles whose tangent satisfies a fractional condition reflecting the crystal geometry. This new oscillatory phenomenon is considered to result from a resonance effect, i.e., commensurate versus incommensurate, between two periodicities in the electron motion along an open orbit under tilted magnetic fields.

The exact spectrum and thermodynamics of the S=1/2 Heisenberg chain with inverse-square exchange are explicitly obtained in closed form, and a description in terms of semionic spin-1/2 ‘‘spinon’’ excitations is developed. The unexpected degeneracies of the spectrum lead to a new representation of the k=1 SU(2) Wess-Zumino-Witten conformal field theory that exposes an infinite set of SU(2) symmetries of that model.

We present a purely electronic mechanism for superconductivity in a disordered Fermi liquid. It is based on long-time-tail effects in electronic correlations, is strongest in two-dimensional systems, and leads to triplet even-parity pairing. The mechanism has a potential for high transition temperatures and high critical fields. The superconducting state shows interesting features, including a short coherence length, and a gapless tunneling density of states.

We have grown superconducting ${\mathrm{Pr}}_{0.5}$${\mathrm{Ca}}_{0.5}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial thin films that exhibit a superconducting onset temperature of 43 K with ${\mathit{T}}_{\mathit{c}}$(R=0)=34.9 K. This is the first time, to our knowledge, that 1:2:3-phase superconductivity has been achieved by substituting Ca for Pr, without the presence in the alloy of Υ or any other rare-earth element R for which R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is superconducting. This result supports the view that hole localization/filling contributes substantially to the suppression of superconductivity by Pr in ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, and clearly demonstrates that this suppression can be compensated by appropriate hole doping with Ca.

We examine equilibrium phase separation in a grafted polymer layer composed of mutually immiscible chains. Under symmetric melt conditions, we predict that there is a transition to a ‘‘rippled’’ phase–a density wave in composition running along the surface. This transition is expected for a molecular weight 2.27 times that for the same species in a simple blend at its demixing threshold. The ripple wavelength is 1.97 times the chain rms end-to-end distance. The lateral rippling is accompanied by a composition oscillation with depth.

An interaction picture, applied to observables, can be used to introduce in and out constants of the motion even when a collision takes place inside a medium. The localized nature of chemical interactions makes this a practical tool. A computational example (using classical dynamics) is provided.

We study Witten’s (2+1)-dimensional Chern-Simons theory with gauge group SU(2). The partition function in this theory is a topological invariant of three-manifolds. Following the prescription given by Witten, we explicitly calculate this partition function on some three-manifolds and verify many of the predictions made by the path integral. Most strikingly, we check the perturbation theory against the exact solution.

It is suggested that the rotation-spin coupling predicted by Mashhoon may lead, in the case of neutrinos, to a helicity flip that has implications for astrophysical objects such as rotating neutron stars and supernovae. The coupling is generalized here to include gravitational fields and total angular momentum and is derived by solving the covariant Dirac and Maxwell-Proca equations exactly to first order in the metric deviation ${\gamma }_{\mathrm{\mu }\nu }$. For fermions the spin part of the effect also applies to gravitational fields of arbitrary strength.

We propose a new multigrid method for propagators in non-Abelian gauge theory, which preserves the low-energy spectra of the continuum limit and maintains manifest gauge covariance. By introducing a trial wave function on the blocks, an efficient projection is formed using a variational principle. Simulations in 2D SU(2) and 4D U(1) gauge theories demonstrate that the variational-state projection works at least as well as the ground-state projection studied earlier for Abelian gauge theories.