Table of contents

We study the dynamics of a directed manifold of internal dimension D in a d-dimensional quenched random force field. We obtain an exact solution for d → ∞ and a Hartree approximation for finite d. They yield a Flory-like roughness exponent ζ and a nontrivial anomalous diffusion exponent ν continuously dependent on the ratio g_T/g_L of divergence-free (g_T) to potential (g_L) disorder strength. For the particle (D = 0) our results agree with previous order ² RG calculations. The time-translational invariant dynamics for g_T > 0 smoothly crosses over to the previously studied ultrametric aging solution in the potential case.

With a new simulation method, involving viscous lubricating forces, we identify the exact list of active contacts in a model two-dimensional frictionless granular packing at equilibrium under an isotropic load. Within a well-justified approximation of infinitesimal displacements, efforts are localized on a minimal central-force structure that satisfies a geometric optimization criterion. Size and boundary effects are analysed. The maximum packing density of discs is evaluated to first order in their small polydispersity parameter. We find no diverging correlation length in the thermodynamic limit, and accurately compute the distribution of contact forces in large systems.

We present a new kind of stochastic resonance effect in a system without periodic external force, but perturbed by a multiplicative colored noise. The phenomenon, obtained as a function of the correlation time of the noise instead of the usually considered noise intensity, occurs in a discrete system with a Hopf bifurcation, but could also be observed in a wide range of discrete and continuous models with similar bifurcation behavior.

We consider the 2D J₁ − J₂ classical XY model on a square lattice. In the frustrated phase corresponding to J₂ > J₁/2, an Ising order parameter emerges by an "order due to disorder" effect. This leads to a discrete symmetry plus the O(2) global one. We formulate the problem in a Coulomb gas language and show by a renormalization group analysis that only two phases are still possible: a locked phase at low temperature and a disordered one at high temperature. The transition is characterized by the loss of Ising and XY order at the same point. This analysis suggests that the 2D J₁ − J₂ XY model is in the same universality class as XY-Ising models.

Dynamical correlations of the spin and the energy density are investigated in the critical region of the random transverse-field Ising chain by numerically exact calculations in large finite systems (L ⩽ 128). The spin-spin autocorrelation function is found to decay proportionally to (log t)^−2x_m and (log t)^−2x_ms in the bulk and on the surface, respectively, with x_m and x_m^s the bulk and surface magnetization exponents, respectively. On the other hand, the critical energy-energy autocorrelation functions have a power law decay, which are characterized by novel critical exponents η_e ≈ 2.2 in the bulk and η_e^s ≈ 2.5 at the surface, respectively. The numerical results are compared with the predictions of a scaling theory.

We show that it is possible to achieve one-step gauge coupling unification in a general class of non-supersymmetric models which at low energies have only the standard particle content and extra Higgs fields doublets. The constraints are the experimental values of α_em, α_s and sin ²θ_W at 10² GeV s, and the lower bounds for FCNC and proton decay rates. Specific examples are pointed out.

New experimental results for the optical collision process Na(3s) + Kr + hν → Na(3p) + Kr are presented, which were obtained in a crossed-beam experiment with differential detection. The oscillatory structure in the differential cross-sections caused by Stueckelberg interferences is investigated in detail for relative velocities between 450 and 1500 m/s and for laser detunings Δν/c from 80 to 360 cm⁻¹ on the blue-wing side of the Na resonance lines. The data are used to determine the potential of the excited repulsive B²Σ state of NaKr with an estimated accuracy of ±24 cm⁻¹ (3 meV).

The resonance frequency and the damping behaviour of a vibrating cantilever as used in dynamic Scanning Force Microscopy (SFM) is investigated with the frequency modulation (FM) technique. The properties of a silicon-cantilever/tip vibrating in front of a silicon surface were studied under UHV conditions as a function of the tip-sample distance. The experimental results are compared with a computer simulation. The different characteristic parts of the experimental curves can be interpreted in terms of attractive and repulsive forces acting on the cantilever as well as the behaviour of the amplitude control electronics. Using the computer simulation noncontact regimes can clearly be distinguished from intermittent contact regimes.

Scanning tunneling microscopy images of ultrathin Gd films on Y(0001) show a strong chemical contrast depending on the tunneling voltage. For small voltages the tunneling spectra for Gd reveal pronounced features which are attributed to occupied and unoccupied electronic surface states as found previously in photoemission experiments. These Gd surface states give rise to a large tunneling current and the image contrast between Gd and Y is enhanced by a factor of two because low-energy surface states are absent in Y. On the other hand, for a high positive bias of + 2 V the contrast is reversed due to tunneling into unoccupied Y electronic states presumably originating from the Y 5d band. The corresponding current increase with respect to Gd exceeds the current increase induced by a monolayer step.

We have used dynamic-mechanical measurements to probe the long-time dynamics of linear, almost monodisperse polybutadiene chains in two media: a) their own melts and b) as trapped in our recently prepared elastomeric networks. The results are compared with the classical reptation-in-a-tube model of de Gennes and Doi and Edwards. Within experimental accuracy, the obtained experimental scaling of the longest relaxation time τ_L relative to the chain mass M (up to 9·10⁵ g/mol) is the same in both media: τ_M ∼ M^x with x = 3.33 ± 0.10 as against x = 3 predicted by the "pure" reptation-in-a-tube model. However, the network data are well described in the range of molar masses 3.4·10⁴ − 5.5·10⁵ g/mol, after correcting the reptation model for chain-end effects (in the form of contour length fluctuations). The τ_L for the chains trapped in a network were found to be 1.9–3.2 times longer than the τ_L of the respective melts. We present data on ten polybutadienes confined in two kinds of networks and covering the range of weight average molar masses 3.4·10⁴–1.5·10⁶ g/mol.

We present indications for a metal-insulator transition in the icosahedral AlPdRe phase of high structural quality. We follow the conductivity through the transition and its dependence on temperature and magnetic field in a series of i-AlPdRe samples. Features typical for systems on both sides of the metal-insulator transition are found. Moreover, the variation of conductivity with temperature is very unusual with σ(T) varying like T^α (1 ⩽ α ⩽ 1.5) in a wide temperature range from 7 K to 700 K.

The crossover temperature between the mean-field and the full-critical regions for the thermal fluctuations around the superconducting transition is calculated for the first time in single-layered and bilayered (with two different distances and tunneling couplings between adjacent layers) superconductors at zero applied magnetic field. These results show, in particular, that the width of the full-critical region is somewhat smaller in layered superconductors than in equivalent bulk materials. These results are then discussed in connection with the superconducting transition of the high-T_c copper oxide superconductors.

In this letter we investigate the nature of relaxation in a model system which consists of a chain of particles that interact with two periodic potentials which represent two confining plates, one of which is externally driven. We find that the relaxation of the "slip" part between two "stick" events in the stick-slip time series may display more than one characteristic time scale. Similar relaxation patterns are observed also in the case of a single particle located between the plates. The different relaxation times are shown to be related to a velocity-dependent effective frictional force felt by the driven plate, and their actual observation is dictated by the experimental conditions.

We extend previous results concerning the statistical properties of step meandering by including random pinning sites. Relationships between the amount of step wandering, the strength of step-step interactions and the random pinning potential are derived. We compute in particular the broadening of the terrace width distribution induced by randomness.

The equivalence between the Lawrence-Doniach model for films of extreme type-II layered superconductors and a generalization of the back-scattering model for spin-(1/2) electrons in one dimension is demonstrated. This fermion analogy is then exploited to obtain an anomalous H_∥⁻¹ tail for the parallel equilibrium magnetization of the minimal double-layer case in the limit of high parallel magnetic fields H_∥ for temperatures in the critical regime.

We consider the effect of fluctuations of the superconducting d_{x² − y²}-wave gap above T_c for the 2D Hubbard model. The order parameter fluctuations (OPFL) lead to a decrease of T_c and to a suppression of the dynamical spin susceptibility below a crossover temperature T_* which can be much larger than T_c. The temperature T_* increases while T_c decreases with increasing strength of OPFL, or decreasing doping. The resulting neutron scattering intensity and NMR relaxation rates decrease below T_* for decreasing T. This agrees qualitatively with the observed spin gap behavior in the underdoped high-T_c cuprates.

Pure electronic Raman spectra with no phonon structures superimposed to the electronic continuum are reported, in optimally doped HgBa₂CaCu₂O_{6 + δ} single crystals (T_c = 126 K). As a consequence, the spectra in the A_1g, B_1g and B_2g symmetries, including the crucial low-energy frequency dependence of the electronic scattering, are directly and reliably measured. The B_2g and, most strikingly, the B_1g spectra exhibit a strong intrinsic linear term, which suggests that the nodes are shifted from the [110] and [ 10] directions, a result inconsistent with a pure d_{x² − y²} model.

We report the first observation of a strong change in the phase (up to 360 degrees) of the interlayer magnetic exchange coupling in Co/Ru/Co and Co/Cu/Co trilayer system which is controlled by the addition of small amounts of Ag (up to 8%) to the Co layers. The phase shifts may be due to modifications of magnetic-film spin-dependent potentials which control conduction electron contributions to the interlayer coupling. In addition, a strong additive antiparallel bias to the interlayer magnetic exchange coupling in Co/Ru/Co trilayers is observed, indicating a possible additive superexchange interaction.

We present a quantum-statistical theory of electron tunneling in systems of the form MPO(P)M', where M and M' represent ferromagnetic transition metal layers, P are metallic non-magnetic layers and O is an oxide tunnel barrier one to a few nanometers thick. This system is a model of the magnetic tunnel junctions CoFe/Al/Al₂O₃/Co recently studied by Moodera (Moodera J. S. et al., Phys. Rev. Lett., 74 (1995) 3273; J. Appl. Phys., 79 (1996) 4724). We show that the presence of a thin non-magnetic layer between the magnetic electrodes and the barrier can lead to the formation of quantum well states adjacent to the barrier. For particular thicknesses, resonance effect can occur leading to much higher tunnel conductance and magnetoresistance than without the non-magnetic layer. The unidimensional character induced by the tunneling process increases significantly the sharpness of these resonances.

We discuss the phase diagram of fourth-order membrane elastic theories for systems with internal degrees of freedom. The presence of scalar or more generally tensorial fields coupled to the membrane curvature renormalizes the elastic constants and destabilizes flat membranes. A simple analysis of the second-order curvature elasticity indicates that spherical or saddle-like membranes shapes will be favored. By taking into account the fourth-order stabilizing terms, we show that tubular shapes can form the most stable state. Hence non-chiral tubular vesicles can be explained on the basis of simple mean-field elasticity. Tubules may appear when an in-plane tensorial (or vectorial) field is coupled to the difference J ∝ c₁ − c₂ between the membrane principal curvatures.

It is shown that a recent claim, according to which the spontaneous-emission process could be altered by continuously monitoring the emitting system by means of a spherical counter of quantum efficiency η < 1 placed at a finite distance L, is unfounded. The error arises from an incorrect reasoning.

We confute the criticism contained in a paper appearing in this issue of Europhys. Lett., in which the author claims that the main result of a series of our papers concerning the influence of a distant detector on spontaneous emission is unfounded.