On a Certain Algebraic Property of Block Ciphers

This is a study on a certain group theoretic property of the set of encryption functions of a block cipher. We have shown how to construct a subset which has this property in a given symmetric group by a computer algebra software GAP4.2 (Groups, Algorithms, and Programming, Version 4.2). These observations on group structures of block ciphers suggest us that we may be able to set a trapdoor based on meet-in-the-middle attack on block ciphers.

on a class of pseudorandom sequences from elliptic curves over finite fields

Following the idea of Xing et al., we investigate a general method for constructing families of pseudorandom sequences with low correlation and large linear complexity from elliptic curves over finite fields in this correspondence. With the help of the tool of exponential sums on elliptic curves, we study their periods, linear complexities, linear complexity profiles, distributions of r-patterns, periodic correlation, partial period distributions, and aperiodic correlation in detail. The results show that they have nice randomness.

Modulation instability of quasi-plane-wave optical beams in biased photorefractive-photovoltaic crystals

We investigate the modulation instability of quasi-plane-wave optical beams in biased photorefractive-photovoltaic crystals by globally treating the space-charge field. The modulation instability growth rate is obtained, which depends on the external bias field, on the bulk photovoltaic effect, and on the ratio of the optical beam's intensity to that of the dark irradiance. Our analysis indicates that this modulation instability growth rate is identical to the modulation instability growth rate studied previously in biased photorefractive-nonphotovoltaic crystals when the bulk photovoltaic effect is negligible for shorted circuits, and predicts the modulation instability growth rate in open- and closed-circuit photorefractive-photovoltaic crystals when the external bias field is absent.

Note: A time-resolved Kerr rotation system with a rotatable in-plane magnetic field

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Electronic states and magnetotransport in quantum waveguides with nonuniform magnetic fields

The electronic states and magnetotransport properties of quantum waveguides (QW's) in the presence of nonuniform magnetic fields perpendicular to the QW plane are investigated theoretically. It is found that the magnetoconductance of those structures as a function of Fermi energy exhibits stepwise variation or square-wave-like oscillations, depending on the specific distributions (both in magnitude and direction) of nonuniform magnetic fields in QW's. We have investigated the dual magnetic strip structures and three magnetic strip structures. The character of the magnetotransport is closely related to the effective magnetic potential and the energy-dispersion spectrum of electron in the structures. It is found that dispersion relations seem to be combined by different sets of dispersion curves that belong to different individual magnetic subwaveguides. The magnetic effective potential leads to the coupling of states and the substantial distortion of the original dispersion curves at the interfaces in which the abrupt change of magnetic fields appears. Magnetic scattering states are created. Only in some three magnetic strip structures, these scattering states produce the dispersion relations with oscillation structures superimposed on the bulk Landau levels. It is the oscillatory behavior in dispersions that leads to the occurrence of square-wave-like modulations in conductance.

INPLANE X-RAY-SCATTERING OF EPITAXIAL STRUCTURES

A new approach for in-plane X-ray scattering from the cleavages of epitaxial films or superlattices, where the scattering vectors are parallel to the interfaces, is proposed. This method can be employed to determine directly the in-plane X-ray strains and other atomic registry along the interfaces of the epitaxial structures.

Calculation of Valence Subband Structures for Strained Quantum-Wells by Plane Wave Expansion Method Within 6 * 6 Luttinger-Kohn Model

The valence subband energies and wave functions of a tensile strained quantum well are calculated by the plane wave expansion method within the 6 * 6 Luttinger-Kohn model. The effect of the number and period of plane-waves used for expansion on the stability of energy eigenvalues is examined. For practical calculation, it should choose the period large sufficiently to ensure the envelope functions vanish at the boundary and the number of plane waves large enough to ensure the energy eigenvalues keep unchanged within a prescribed range.

Effects of polarization field on formation of two-dimensional electron gas in (0001) and (11(2)over-bar0) plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0001) and (11 (2) over bar0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0001) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1120) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0001) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (11 (2) over bar0). The existence of a 2DEG was observed in (0001) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm(2)/V s below 80 K, whereas a much lower mobility was measured in (11 (2) over bar0). These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures. (C) 2004 Elsevier B.V. All rights reserved.

Diffractive microlens with a cascade focal plane fabricated by single mask UV-photolithography and common KOH:H2O etching

A diffractive microlens with a cascade focal plane along the main optical axis of the device is fabricated using a low-cost technique mainly including single mask ultraviolet (UV) photolithography and dual-step KOH:H2O etching. Based on the evolutionary behavior of converse pyramid-shaped microholes (CPSMs) preshaped over a {100}-oriented silicon wafer in KOH etchant, the first-step KOH etching is performed to transfer initial square micro-openings in a SiO2 film grown by plasma enhanced chemical vapor deposition (PECVD) and patterned by single mask UV-photolithography, into CPSMs with needed dimension. After completely removing a thinned SiO2 mask, basic annular phase steps with a relatively steep sidewall and scheduled height can be shaped in the overlapped etching region between the neighboring silicon concave-arc microstructures evolved from CPSMs through the second-step KOH etching. Morphological measurements demonstrate a desirable surface of the silicon microlens with a roughness in nanometer scale and the feature height of the phase steps formed in the submicrometer range. Conventional optics measurements of the plastic diffractive microlens obtained by further hot embossing the fine microrelief phase map over the nickel mask made through a common electrochemical method indicate a highly efficient cascaded focusing performance.