He-atom scattering and mass-spectrometer flux analysis have been used to study structural changes accompanying photon-stimulated desorption (PSD) by valence electron excitation of NaCl(001) cleavage faces in situ. Within the considered temperature range from 550 to 240 K an incomplete photon-induced layer-by-layer removal is observed. In contrast to previous experiments and models not only neutral Cl and Na atoms but also NaCl molecules have been observed to desorb upon excitation. The rate of PSD is found to be strongly dependent on the temperature but independent of the surface topography.

When an external force acts on an adsorbate structure, the structure may slide or flow relative to the substrate. The mechanism behind this sliding motion is of fundamental importance for the understanding of friction and lubrication between two flat macroscopic surfaces. Here I present a study of the nonlinear (in the external driving force) sliding friction which is found to exhibit hysteresis giving rise to the well-known phenomenon of ‘‘stick-and-slip’’ motion. The theory predicts that for a large class of sliding systems, the ratio ${\mathit{f}}_{\mathit{k}}$/${\mathit{f}}_{\mathit{s}}$ between the kinetic and the static friction coefficients equals 1/2, in good agreement with experimental data.

We use ultrasonic attenuation and light scattering to study spatial correlations in the pores of Vycor on filling and draining with hexane. On filling, the hexane initially adsorbs uniformly, but when capillary condensation occurs, vapor microbubbles are formed and persist until the sample is completely full. However, no long-range correlations of the bubbles are observed. By contrast, on drainage, the empty pores exhibit long-range correlations with a fractal dimension of 2.6. This results from the pore connectivity, suggesting that this behavior can be modeled by invasion percolation.

The purpose of this paper is to point out the steps in conventional many-body perturbation theory which requires separate, additional justification over and above convergence of the pertubation series. The questionable step is the cancellation of the renormalized energy shift by the chemical potential, neglecting any possible modification of the wave functions as well as state-dependent energy shifts.

Reducing the multiplicity of the orthoexciton ground state in ${\mathrm{Cu}}_2$O by applying uniaxial stress greatly increases the quantum degeneracy of the excitonic gas produced by intense pulsed excitation. Simultaneously, the paraexciton luminescence spectrum develops an extra component at low energy which is interpreted as a Bose-Einstein condensate of paraexcitons.

A pseudopotential local-density calculation is performed for ${\mathrm{YH}}_3$ to study the unusual hydrogen displacements previously found in neutron diffraction. These displacements are Peierls distortions associated with (hydrogen) lattice instability in this 3D system. The wave vector of these displacements is close to the vector connecting the electron and hole pockets in the undistorted system. With other electron and hole pockets at Γ that still overlap after distortion, the possibility of the existence of an excitonic insulator phase will be discussed.

We present a complete analytical theory of conductance fluctuations in quantum dots in the regime of chaotic dynamics. Using the supersymmetry method we calculate for the first time exactly the conductance distribution function for a system of noninteracting electrons. Knowledge of this quantity enables us to obtain information about statistical properties of wave functions of a closed dot. The far tail of the distribution function describes fluctuations of resonance conductance in the Coulomb blockade regime.

Optical second-harmonic and sum-frequency spectra of clean and oxidized Si(100) and Si(111) samples reveal a strong resonance band at 3.3 eV photon energy. It is concluded that the resonance arises from direct transitions between valence and conduction band states in a few monolayers of strained silicon at the Si-${\mathrm{SiO}}_2$ interface and at the selvedge of clean reconstructed silicon surfaces.

The resistive upper critical field of overdoped single crystals of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$ has been measured from the zero field transition temperature ${\mathit{T}}_{\mathit{c}}$(0) (approximately 20 K) to temperatures as low as 12 mK, corresponding to less than 0.001${\mathit{T}}_{\mathit{c}}$(0). In sharp contrast to the predictions of standard theories of superconductivity, the critical field is found to rise steeply with positive curvature as the temperature is reduced, and no sign of saturation is observed down to the lowest temperatures reached.

A scaling behavior of the Hall resistivity in a mixed state of high-${\mathit{T}}_{\mathit{c}}$ superconductors is shown to be a general feature of any vortex state with disorder-dominated dynamics (thermally assisted flux flow, vortex glass, etc.). The universal scaling law ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$∝${\mathrm{\rho }}_{\mathit{x}\mathit{x}}^2$ is found. The presented theory agrees with recent experimental data.

Impedance measurements performed on a weakly frustrated triangular array of Josephson junctions over a wide range of frequencies and at temperatures such that vortex pinning is irrelevant reveal that vortex dynamics in an ideal two-dimensional (2D) superconductor does not obey Drude’s classical prediction for a 2D Coulomb gas of free and independent vortex charges. An analysis in terms of a complex vortex dielectric constant implies that the vortex mobility vanishes logarithmically in the limit of small frequencies, thereby pointing to anomalous vortex diffusion.

Elastic scattering of metastable $2^3$S He has been used for the first time to determine the long-range spin ordering on the surface of an antiferromagnetic insulator. The diffraction pattern obtained from a NiO(100) surface is interpreted as a frozen spin-wave structure in the 〈10〉 direction with a period of 6×a. The diffraction arises from a spatial variation of the beam attenuation due to scattering from the periodic local spin ordering on the surface and is not caused by a spin-spin interaction. The results have been modeled with a modified version of the eikonal approximation using a dissipative, hard, corrugated wall.

We report measurements of the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin echo decay rate ${}_{}{}^{63}{}_{}{}^{}1$/${\mathit{T}}_{2\mathit{G}}$ in the paramagnetic state of a quasi-two-dimensional antiferromagnet ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ up to 900 K, and from it deduce the temperature dependence of the spin-spin correlation length ξ. ξ shows a crossover from ξ∼exp(J/${\mathit{k}}_{\mathit{B}}$T) in the renormalized classical regime (T≲600 K) to ξ∼J/${\mathit{k}}_{\mathit{B}}$T in the quantum critical regime (T≳600 K), where J is exchange interaction. Our finding that the ratio ${}_{}{}^{63}{}_{}{}^{}$${\mathit{T}}_1$T${/}^{63}$${\mathit{T}}_{2\mathit{G}}$ is temperature independent, where ${}_{}{}^{63}{}_{}{}^{}$${\mathit{T}}_1$ is the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation time, gives clear evidence for quantum critical scaling.

Anharmonic effects in crystals typically result in the symmetric broadening of the single-phonon Lorentzian line shape. However, a simple model, based on consideration of a highly anharmonic potential, shows that anharmonicity can lead not only to asymmetric line shapes but also to line shapes composed of multiple subpeaks. Experimental Raman data on the soft ${\mathit{A}}_1$(TO) phonon in ferroelectric ${\mathrm{PbTiO}}_3$ indicates that this mode does indeed consist of a series of subpeaks whose intensities are accurately predicted (with no fitting parameters) by our model.

The leading contribution to the polarization dependent four-wave-mixing signal is shown to result from density induced dephasing processes. Experimental observations are in qualitative agreement with theoretical calculations based on the semiconductor Bloch equations where dephasing due to excitonic screening has been taken into account.

In the present work energies of photoluminescent (PL) peaks of porous Si were measured on as-etched samples. It was found that the PL peak energies occur only at or nearly at a series of discrete values and exhibit a ‘‘pinning’’ characteristic. A tight-binding calculation was carried out to study the variation of gap energies of Si nanostructures with the size of the clusters. If Si clusters took stable forms of closed shells with their sizes in the range of 1–2 nm, the change of gap energies gave obvious discontinuity. Good agreement was found between the ‘‘pinning’’ energies of PL and the energy gaps of subsequent closed-shell Si clusters. This might be considered as evidence for the quantum size effect of porous Si.

We consider fluctuations in both the shattering and normal (nonshattering) fragmentation processes. The continuum shattering process is shown to lack the self-averaging property: the rms fluctuation in the total number of the finite-size fragements is larger than the mean value. The discrete shattering process exhibits strong fluctuations at the intermediate times when the total number of fragments is much greater than 1 but smaller than the number of the monomers contained in the primary particle. For the normal process, the fluctuations are small and self-averaging exists.

We report the experimental observation of an endogeneous antisymmetric wave source in a quasi-one-dimensional chemical system. Substantiated by numerical simulations, a theoretical interpretation relying on the interaction between Turing and Hopf modes is proposed.

We report the first simulation of a surface chemical reaction performed within the ab initio molecular dynamics approach. A set of trajectories with different initial conditions has been generated for single ${\mathrm{Cl}}_2$ molecules impinging on the Si(111)-2×1 surface with incident translational energy of 1 eV. We observe a high probability of dissociation, triggered by active sites on the π-bonded chains, and accompanied by a large surface response and local rehybridization effects.

Systems of globally coupled oscillators often display states of full synchrony in which all oscillators are phase locked. It is shown that for globally coupled oscillators with neuronlike pulse interactions, the phase-locked state is unstable to inhomogeneity in the local frequency. For weak inhomogeneity the system breaks into two subpopulations: one that is phase locked and another one that consists of aperiodic oscillators. The fraction of the unlocked population remains finite in the limit of vanishing inhomogeneity.