Negative Magnetoresistance Produced by Hall Fluctuations in a Ferromagnetic Domain Structure

We present a model for a negative magnetoresistance (MR) that would develop in a material with many ferromagnetic domains even if the individual domains have no magnetoresistance and even if there is no boundary resistance. The negative MR is due to a classical current-distortion effect arising from spatial variations in the Hall conductivity, combined with a change in domain structure due to an applied magnetic field. The negative MR can exceed 1000% if the product of the carrier relaxation time and the internal magnetic field due to spontaneous magnetization is sufficiently large.Comment: 3 pages, submitted to Appl. Phys. Let

Fractional vortices in the XY model with π\pi bonds

We define a new set of excitations in the XY model which we call ``fractional vortices''. In the frustrated XY model containing $\pi$ bonds, we make the ansatz that the ground state configurations can be characterized by pairs of oppositely charged fractional vortices. For a chain of $\pi$ bonds, the ground state energy and the phase configurations calculated on the basis of this ansatz agree well with the results from direct numerical simulations. Finally, we discuss the possible connection of these results to some recent experiments by Kirtley {\it et al} [Phys. Rev. B {\bf 51}, R12057 (1995)] on high-T$_c$ superconductors where fractional flux trapping was observed along certain grain boundaries.Comment: 13 pages, 14 figures included (.eps). No essential differences to previous version, however more compact forma

Aliens - Immigration and Nationality Act - Brief Excursion Outside Country\u27s Borders by Resident Alien May Not Subject Him to Consequences of an Entry on His Return (Rosenburg v. Fleuti, United States Supreme Court 1963)

Rosenberg v. Fleuti (United States Supreme Court 1963). Plaintiff, an alien, was originally admitted to the United States for permanent residence in 1952 and has been here continuously except for a visit of about a couple of hours duration to Mexico in 1956. The Immigration and Naturalization Service sought to deport plaintiff on the ground that at the time of his return in 1956, he was afflicted with psychopathic personality. The District Court granted the government\u27s motion for summary judgment. The Ninth Circuit Court of Appeals set aside the deportation order, holding that as applied to plaintiff section 241(a)(1) of the 1952 Immigration and Nationality Act was unconstitutionally void for vagueness as applied to a non-compulsive homosexual. On certiorari, by a 5-4 decision, the United States Supreme Court held: judgment vacated. It was unnecessary to decide the constitutional issue since the threshold question was decided against the government. An innocent, casual, and brief excursion by a resident alien outside this country\u27s borders may not have been intended as a departure disruptive of his resident alien status and therefore may not subject him to the consequences of an entry into this country on his return. Rosenberg v. Fleuti, 374 U.S. 449 (1963)

Superfluid Inhomogeneity and Microwave Absorption in Model High-T_c Superconductors

We investigate the microwave absorption arising from inhomogeneity in the superfluid density of a model high-T_c superconductor. Such inhomogeneities may arise from a wide variety of sources, including quenched random disorder and static charge density waves such as stripes. We show that both mechanisms will inevitably produce additional absorption at finite frequencies. We present simple model calculations for this extra absorption, and discuss applications to other transport properties in high-T_c materials. Finally, we discuss the connection of these predictions to recent measurements by Corson et al. of absorption by the high-temperature superconductor BSCCO in the THz frequency regime.Comment: Presented at ETOPIM-6; latex file requires espcrc2.st

Model for a macroscopically disordered conductor with an exactly linear high-field magnetoresistance

We calculate the effective resistivity of a macroscopically disordered two-dimensional conductor consisting of two components in a perpendicular magnetic field. When the two components have equal area fractions, we use a duality theorem to show that the magnetoresistance is nonsaturating and at high fields varies exactly linearly with the magnetic field. At other compositions, an effective-medium calculation leads to a saturating magnetoresistance. We briefly discuss possible connections between these results and magnetoresistance measurements on heavily disordered chalcogenide semiconductors

Nonsaturating magnetoresistance and Hall coefficient reversal in a model composite semiconductor

We calculate the transverse magnetoresistance (TMR) and Hall coefficient R_H of a three-dimensional composite medium with both positive and negative charge carriers, using the effective-medium approximation. The composite is assumed to be composed of two types of small crystallites, in which the charge carriers are either all electrons or all holes. The conductivity tensors of the two components are assumed to be of the standard free-electron form. At all nonzero concentrations, the composite is found to have a large, nonsaturating TMR. For a given magnetic field h, the TMR is a maximum at the concentration p* where R_H changes sign; at this concentration, the TMR may be a factor of hundreds or thousands for realistic magnetic field. We discuss the relevance of these results to recent experiments on silver chalcogenide semiconductors, where similar behavior has been reported as a function of pressure

Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves

We describe a simple method of including dissipation in the spin wave band structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz equation for the magnetization with the Gilbert damping term. We use this approach to calculate the band structure of square and triangular arrays of Ni nanocylinders embedded in an Fe host. The results show that there are certain bands and special directions in the Brillouin zone where the spin wave lifetime is increased by more than an order of magnitude above its average value. Thus, it may be possible to generate spin waves in such composites decay especially slowly, and propagate especially large distances, for certain frequencies and directions in ${\bf k}$-space.Comment: 13 pages, 4 figures, submitted to Phys Rev

Robust Multi-Partite Multi-Level Quantum Protocols

We present a tripartite three-level state that allows a secret sharing protocol among the three parties, or a quantum key distribution protocol between any two parties. The state used in this scheme contains entanglement even after one system is traced out. We show how to utilize this residual entanglement for quantum key distribution purposes, and propose a realization of the scheme using entanglement of orbital angular momentum states of photons.Comment: 9 pages, 2 figure