EPL

Alder, B. J.; Peters, D. S.

doi: 10.1209/0295-5075/10/1/001pmid: N/A

The superconducting region of the high-temperature superconductors can be semi-quantitatively modeled by holes paired in real space to form bosons which interact mutually via a long-range Coulomb potential with a short-range hard-core cut-off. The bosons are localized at low carrier density by Wigner crystallization and by ordinary crystallization at high carrier densities although dissociation is also predicted. At intermediate densities the superfluid/superconducting transition follows the Kosterlitz-Thouless criterion with a reasonable effective mass.

Yan, H.; Li, G.; Sander, L. M.

doi: 10.1209/0295-5075/10/1/002pmid: N/A

We present numerical simulations for a stochastic model for fracture growth taking noncentral forces into account. The noncentral forces are included using the Born model to describe the elastic energy. The simulations show that the fracture patterns retain fractal geometry with noncentral forces, but the effective fractal dimension D is larger than with central forces only. The geometry of the pattern is not sensitive to the relative strength of the noncentral forces. The results are interpreted in the context of a constraint counting argument.

Petrillo, C.; Sacchetti, F.

doi: 10.1209/0295-5075/10/1/003pmid: N/A

A simple phenomenological model for the high fusion rate recently observed in transition metal deuterides is presented. It is shown that the existence of localized vibrational modes of deuterium atoms, like those actually found in NbD and TaD, can account for the experimental fusion rates.

Scharf, G.; Sonnenmoser, K.; Wreszinski, W. F.

doi: 10.1209/0295-5075/10/1/004pmid: N/A

A one-dimensional model of an atom with one electron in an attractive delta-function potential and an external sinusoidal electric field is solved exactly and numerically calculated. The ionization probability is found to display a rich structure of erratic oscillations as a function of field strength. This effect increases as the ratio of frequency of the external field to binding energy decreases, showing clearly the important role played by multiphoton transitions in the phenomenon.

Troian, S. M.; Herbolzheimer, E.; Safran, S. A.; Joanny, J. F.

doi: 10.1209/0295-5075/10/1/005pmid: N/A

We show that a thin film with small dynamic contact angle and driven by an external body force is unstable to the formation of fingers in the direction perpendicular to the main flow. The instability is largest in the capillary region near the contact line, where the force due to surface tension is comparable to the viscous and gravitational forces. The fastest growing wavelength is calculated in the limit of small-amplitude disturbances. These instabilities may be related to finger patterns observed in gravitational flows and spinning drops.

Coe, S. E.; Afshar-Rad, T.; Desselberger, M.; Khattak, F.; Willi, O.; Giulietti, A.; Lin, Z. Q.; Yu, W.; Danson, C.

doi: 10.1209/0295-5075/10/1/006pmid: N/A

We report on measurements made of the levels of stimulated Raman and Brillouin scattering in a long-scalelength preformed plasma. The effectiveness of a random phase plate and an induced spatial incoherence system to suppress these instabilities by reducing the coherence of the laser beam have been studied. It has been found that both these methods give rise to a significant reduction in the levels of the Raman and Brillouin instabilities under these experimental conditions.

Egelstaff, P. A.; Kearley, G.; Suck, J.-B.; Youden, J. P. A.

doi: 10.1209/0295-5075/10/1/007pmid: N/A

Computer simulations and theoretical calculations show that the van Hove scattering function for 0.3 < Q < 3 nm-1 is an important test case for theories of dense fluids. To study this region experimentally we have assembled a new neutron spectrometer system, which operates efficiently over this range. With this system we have measured the scattering from dense nitrogen gas at wave numbers from 0.5 to 1.5 nm-1. These results are compared to the Navier-Stokes and generalized hydrodynamics theories. At our lowest momentum transfers these two theories merge with one another and agree with the data. As the momentum transfer is increased the peaks broaden in frequency, the theories diverge from one another and this divergence can be studied experimentally for the first time.

Pansu, B.; Dubois-Violette, E.

doi: 10.1209/0295-5075/10/1/008pmid: N/A

In the geometrical models of blue phases, the axes of double-twist cylinders are directed along the skeleton of some infinite periodic minimal surfaces which can therefore be seen as separating the two networks of cylinders. We show that the director field which minimizes the double-twist energy on these minimal surfaces follows the asymptotic directions (with no curvature).

Herget, G.; Müllner, M.; Suck, J. B.; Schmidt, R.; Wipf, H.

doi: 10.1209/0295-5075/10/1/009pmid: N/A

The generalized phonon density of states (PDOS) of the shape memory alloy NiTi was measured in three structural phases by means of inelastic neutron scattering for the first time. The PDOS are dominated by two pronounced peaks near 17 meV and at 33 meV. Within experimental accuracy the PDOS of the austenitic and the intermediate phase are identical, but a strong hardening of predominantly acoustic phonons in the martensitic phase is observed. In contrast to model calculations, based on fits to the measured dispersion of acoustic phonons, we find no pronounced gap between the acoustic and the optic modes in the PDOS. Data for the lattice vibrational part of the specific heat as derived from the PDOS are presented.

Burki, Y.; Czaja, W.

doi: 10.1209/0295-5075/10/1/010pmid: N/A

Excitation spectra of the iodine bound exciton in AgBr are performed and a new interpretation is given for the fine structure observed. The high-energy range of the excitation spectra is compared and discussed with other impurity spectra.