Combinatorial identities for binary necklaces from exact ray-splitting trace formulae

Based on an exact trace formula for a one-dimensional ray-splitting system, we derive novel combinatorial identities for cyclic binary sequences (P\'olya necklaces).Comment: 15 page

Growth-mode investigation of epitaxial EuS on InAs(100)

A persistent challenge in the field of spintronics is the search for suitable materials that enable the circumvention of the impedance mismatch preventing efficient spin-injection from metallic ferromagnetic conductors into semiconductors. One promising material is europium sulfide (EuS), a ferromagnetic semiconductor below the Curie temperature of 16.5 K. Investigation and optimization of the conditions required for high-quality growth of epitaxial EuS films on suitable substrates are thus of particular interest for the creation of efficient devices. We present the results of a growth-mode study employing atomic force microscopy and spot-profile analysis low-energy electron diffraction (SPA-LEED) of epitaxial EuS thin films deposited by electron-beam evaporation on InAs(100) substrates with varying combinations of, respectively, growth and annealing temperatures, Tg and Ta, from room temperature to 400 °C. We observed Stranski-Krastanov-like growth featuring low-roughness surfaces with root mean square values between 0.4 – 0.9 nm for all temperature combinations. An increased tendency for nucleation into grains and islands was observed for higher Ta from 300 – 400 °C. The corresponding nucleation mode, defined by varying degrees of 2D and 3D nucleation, was dependent on Tg. A 2D island growth mode was observed for Tg = 150 °C and Ta = 400 °C featuring a sharp and bright SPA-LEED pattern. This suggests the formation of a highly ordered, smooth surface for these growth conditions thereby providing a good starting point for optimization attempts for potential future devices

Weyl formulas for annular ray-splitting billiards

We consider the distribution of eigenvalues for the wave equation in annular (electromagnetic or acoustic) ray-splitting billiards. These systems are interesting in that the derivation of the associated smoothed spectral counting function can be considered as a canonical problem. This is achieved by extending a formalism developed by Berry and Howls for ordinary (without ray-splitting) billiards. Our results are confirmed by numerical computations and permit us to infer a set of rules useful in order to obtain Weyl formulas for more general ray-splitting billiards

Exact trace formulae for a class of one-dimensional ray-splitting systems

Based on quantum graph theory we establish that the ray-splitting trace formula proposed by Couchman {\it et al.} (Phys. Rev. A {\bf 46}, 6193 (1992)) is exact for a class of one-dimensional ray-splitting systems. Important applications in combinatorics are suggested.Comment: 14 pages, 3 figure

Conductance Distribution of a Quantum Dot with Non-Ideal Single-Channel Leads

We have computed the probability distribution of the conductance of a ballistic and chaotic cavity which is connected to two electron reservoirs by leads with a single propagating mode, for arbitrary values of the transmission probability Gamma of the mode, and for all three values of the symmetry index beta. The theory bridges the gap between previous work on ballistic leads (Gamma = 1) and on tunneling point contacts (Gamma << 1). We find that the beta-dependence of the distribution changes drastically in the crossover from the tunneling to the ballistic regime. This is relevant for experiments, which are usually in this crossover regime. ***Submitted to Physical Review B.***Comment: 7 pages, REVTeX-3.0, 4 postscript figures appended as self-extracting archive, INLO-PUB-940607

Diagnostic criterion for crystallized beams

Small ion crystals in a Paul trap are stable even in the absence of laser cooling. Based on this theoretically and experimentally well-established fact we propose the following diagnostic criterion for establishing the presence of a crystallized beam: Absence of heating following the shut-down of all cooling devices. The validity of the criterion is checked with the help of detailed numerical simulations.Comment: REVTeX, 11 pages, 4 figures; submitted to PR

In-vitro resistance of cloned human glioma cells to natural killer activity of allogeneic peripheral lymphocytes.

Cells from an established culture of a human astrocytoma were incubated with normal allogeneic peripheral lymphocytes (PBL) in order to study the natural killer (NK) sensitivity of the in vitro propagated cell line. A proportion of cells in culture formed halos, into which lymphocytes did not penetrate. These cells were successfully cloned and showed a decreased susceptibility to NK cytolysis compared with the parent line. Both cell lines could be transplanted into athymic nude mice. The cloned NK-resistant cells underwent a frequent spontaneous regression in nu/nu mice, despite the fact that when used as targets for nu/nu NK cells in vitro they were only moderately susceptible. Phase-contrast microscopy of the mass-cultured cells co-cultivated with lymphocytes suggested that their morphology and ability to form inpenetrable translucent halos might influence their susceptibility to NK lysis. Experiments performed on this assumption revealed that quiescent and halo forming tumour cells were not the primary targets for NK lysis. Cells in mass culture, although tumorigenic, were thus heterogeneous in respect of susceptibility to NK attack. These findings might be relevant to the mechanism of immune escape and tumour heterogeneity in respect of spontaneous cell-mediated lysis

Reflection Symmetric Ballistic Microstructures: Quantum Transport Properties

We show that reflection symmetry has a strong influence on quantum transport properties. Using a random S-matrix theory approach, we derive the weak-localization correction, the magnitude of the conductance fluctuations, and the distribution of the conductance for three classes of reflection symmetry relevant for experimental ballistic microstructures. The S-matrix ensembles used fall within the general classification scheme introduced by Dyson, but because the conductance couples blocks of the S-matrix of different parity, the resulting conductance properties are highly non-trivial.Comment: 4 pages, includes 3 postscript figs, uses revte