bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface polaritons in a finite superlattice with a displaced layer in a quantizing magnetic field
Authors: Beletskiı̆ N. N. , Bludov Yu. V.
Published: July 2002

A theoretical study is made of surface polaritons (SPs) in a weakly disordered superlattice consisting of a finite number of infinitely extended two-dimensional electron layers (2DELs) placed in an external static quantizing magnetic field directed perpendicular to the 2DELs. The filling factor of the Landau levels is assumed to be the same in all the 2DELs. The disorder in the superlattice consists in having one of the interior 2DELs displaced from the position of periodicity by a certain distance Δ. The dispersion and energy characteristics of SPs are investigated in the case when a finite superlattice is embedded in a uniform dielectric medium. It is shown that under conditions of the integer quantum Hall effect, all the characteristics of the SPs are represented by quantized values. It is found that in a finite, weakly disordered superlattice there exists a local SP mode whose properties differ substantially from those of the usual SP modes in a finite ordered superlattice. The conditions under which the phase and group velocities of the SPs in a finite, weakly disordered superlattice can be substantially lower than in an isolated 2DEL are determined. It is found that when dissipation is taken into account, a new SP mode arises in a weakly disordered superlattice–an additional SP whose properties depend strongly on the electron momentum relaxation frequency ν and on the displacement Δ.
ESR spectrum of KTm(MoO4)2
Authors: Kobets M. I. , Pashchenko V. A. , Khats’ko E. N.
Published: July 2002

The angular and frequency dependences of the ESR spectrum of single-crystal KTm(MoO4)2 are investigated at helium temperature. It is shown that this compound belongs to the class of highly anisotropic paramagnets. The g factors of the ground state (gx=0.3±0.2; gy=0.3±0.2; gz=13.9±0.1) and the angle of rotation of the local axes in the ac plane (θ=±(7.6±1)°) are determined for two Tm3+ paramagnetic centers.
Spontaneous onset of interaction anisotropy in a strongly correlated quasi-one-dimensional system of electrons
Authors: Apal’kov D. M. , Zvyagin A. A. , Apal’kov D. M. , Zvyagin A. A.
Published: July 2002

Instability of a strongly correlated quasi-one-dimensional electron system in a real three-dimensional crystal is predicted, with the onset of anisotropy of the electron–electron interaction. The influence of an external magnetic field, the occupation of the electron band, and a small nonzero temperature on the predicted instability are investigated.
Growth and dissolution of He3 liquid drops in a He4 crystalline matrix
Authors: Gan’shin A. N. , Grigor’ev V. N. , Maı̆danov V. A. , Penzev A. A. , Rudavskiı̆ É. Ya. , Rybalko A. S.
Published: July 2002

Pressure measurements are used to study the kinetics of phase separation of He3–4He solid solutions under conditions such that after the transition a two-phase system is formed, consisting of liquid drops of the concentrated phase (nearly pure He3) in a crystalline matrix of the dilute phase (nearly pure He4). It is shown that under stepped cooling of the two-phase sample the growth of the liquid drops can be described by a superposition of two exponential functions representing processes with long and short time constants. This is due to the strong influence of the stresses arising in the crystal at the phase transition owing to the large difference in the molar volumes of the two phases and which probably cause plastic deformation of the matrix and the giving rise to nonequilibrium concentrations of He3 in it. The transport of He3 atoms occurs only in connection with stress relaxation. It is established that cyclic processes of growth and dissolution of the liquid drops strongly degrade the quality of the crystal and lead to an increase of the pressure. The possibility that a mixture of the liquid and crystal exists in the drops is also analyzed.
Superconducting transition temperature and isotope exponent in superconductors with low Fermi energies
Authors: Palistrant M. E.
Published: July 2002

A study is made of superconductivity in systems with a variable density of charge carriers, strong electronic correlations (which influence the electron–phonon interaction), and low Fermi energies. This last circumstance leads to violation of the Migdal theorem and makes it necessary to take into account the vertex and crossed diagrams for the electron–phonon interaction (Pv,Pc). Two- and three-dimensional systems are considered. The values of Pv and Pc are found, and the equation for the momentum cutoff Qc of the electron–phonon interaction is derived. The dependence of this quantity on the concentration of charge carriers is demonstrated. Expressions are obtained for the superconducting transition temperature Tc and the isotope exponent α, and their behavior as functions of the carrier concentration is analyzed. The values of the carrier densities for which superconductivity can occur in two- and three-dimensional systems are determined.
Waveguiding properties of two parallel defects under conditions of two-channel scattering
Authors: Kosevich A. M. , Matsokin D. V.
Published: July 2002

It is shown that a pair of parallel planar defects in a crystal can act as a waveguide for a quantum particle or wave having two branches of the dispersion relation. The energies (or frequencies) for which the excitations are localized between the defects and propagate freely along them are determined. In a two-dimensional system there exists a discrete set of energies at which two-dimensional excitations can be “held in” between two parallel linear defects.
Viscous fingering in miscible displacements: Unification of effects of viscosity contrast, anisotropic dispersion, and velocity dependence of dispersion on nonlinear finger propagation
Authors: Zimmerman W. B. , Homsy G. M.
Published: July 2002

The two-dimensional (2-D) isotropic simulations of Tan and Homsy [Phys. Fluids 31, 1330 (1988)] are extended to much broader and longer domains, and the 2-D anisotropic simulations of Zimmerman and Homsy are extended to include a general velocity dependence. The mechanisms of nonlinear interaction of viscous fingers found for the first time in the anisotropic simulations recur in isotropic simulations, but at weaker levels of dispersion. An appropriate scaling to unify the average long time growth of the instability with both anisotropy in geometry and dispersion is provided. The long time growth of the instability from simulations agrees with acoustic measurements in 3-D porous media, Bacri et al. [Phys. Rev. Lett. 67, 2005 (1991)], elucidating the effects of viscosity contrast, anisotropy, and velocity dependence of longitudinal dispersion. The combination of sufficiently high viscosity contrast, weak transverse dispersion, and strong dependence of longitudinal dispersion on velocity results in an augmentation to the long time growth of the instability. The associated critical parameter found by linear stability theory of Yortsos and Zeybek [Phys. Fluids 31, 3511 (1988)] predicts accurately this same long time growth increase.
The Pierce diode with an external circuit. I. Oscillations about nonuniform equilibria
Authors: Lawson William S.
Published: July 2002

The nonuniform (nonlinear) equilibria of the classical (short circuit) Pierce diode and the extended (series RLC external circuit) Pierce diode are described, and the spectrum of oscillations (stable and unstable) about these equilibria are worked out. It is found that only the external capacitance alters the equilibria, though all elements alter the spectrum. In particular, the introduction of an external capacitor destabilizes some equilibria that are marginally stable without the capacitor. Computer simulations are performed to test the theoretical predictions for the case of an external capacitor only. It is found that most equilibria are correctly predicted by theory, but that the continuous set of equilibria of the classical Pierce diode at Pierce parameters (α=ωpL/v0) that are multiples of 2π are not observed. This appears to be a failure of the simulation method under the rather singular conditions rather than a failure of the theory.
Nonlinear analysis of the Cerenkov maser
Authors: Freund H. P. , Ganguly A. K.
Published: July 2002

The nonlinear evolution of the Cerenkov maser amplifier is investigated numerically for a configuration that consists of an energetic electron beam propagating through a dielectric-lined cylindrical waveguide. An axial guide magnetic field is included in the formulation in order to improve beam confinement. A set of coupled nonlinear differential equations is derived in three dimensions that governs the evolution of both the electromagnetic field and the trajectories of an ensemble of electrons. The system is assumed to be azimuthally symmetric, and the electromagnetic field is represented as a superposition of the TM0n modes of the vacuum waveguide. The initial conditions are chosen to model the simultaneous injection of either a solid or annular electron beam, and an electromagnetic wave of arbitrary input power. Thermal effects are treated under the assumption that the beam is initially monoenergetic but exhibits a pitch angle spread; however, the subsequent evolution of the beam is treated in a self-consistent manner. This class of distribution is appropriate to the treatment of diode-produced beams and describes a beam with an initial axial energy spread. This is the crucial determinant in the efficiency, since saturation occurs by means of an axial bunching mechanism that results in the phase trapping of the beam. The specific parameters used in the numerical analysis correspond to experiments conducted at Dartmouth College [J. Appl. Phys. 58, 627 (1985)], and good agreement is found between theory and experiment.
Anomalous absorption of electromagnetic radiation
Authors: Rowland H. L.
Published: July 2002

Transverse structure in a plasma at the critical layer can increase the coupling between electromagnetic and electrostatic waves and lead to enhanced absorption of the radiation. Simulations show cases of nearly 90% absorption of the incident energy. A ripple across the density gradient acts to focus the incoming electromagnetic radiation in the lower density regions coupling it to electrostatic modes through the transverse density gradients. A simple model of the absorption shows good agreement with the scalings observed in the computer simulations. Part of the reflected radiation is side scattered at ±nkt, where kt is the wave number of the transverse structure. This can reduce the directly backscattered radiation to 0.6% of the incident energy. Nonlinear coupling of the electrostatic waves leads to radiation at twice the incident frequency.
Differential forms and canonical variables for drift motion in toroidal geometry
Authors: Littlejohn Robert G.
Published: July 2002

Differential forms provide a simple and powerful computational tool for finding canonical variables for drift motion in toroidal geometry. The method is easily applied to relativistic drift motion.
Quasi-steady monopole and tripole attractors for relaxing vortices
Authors: Rossi Louis F. , Lingevitch Joseph F. , Bernoff Andrew J.
Published: July 2002

Using fully nonlinear simulations of the two-dimensional Navier–Stokes equations at large Reynolds number (Re), we bracket a threshold amplitude above which a perturbed Gaussian monopole will relax to a quasi-steady, rotating tripole, and below which will relax to an axisymmetric monopole. The resulting quasi-steady structures are robust to small perturbations. We propose a means of measuring the decay rate of disturbances to asymptotic vortical structures wherein streamlines and lines of constant vorticity correspond in some rotating or translating frame. These experiments support the hypothesis that small or moderate deviations from asymptotic structures decay through inviscid and viscous mixing.
Toroidal flows in resistive magnetohydrodynamic steady states
Authors: Kamp Leon P. , Montgomery David C. , Bates Jason W.
Published: July 2002

We consider the resistive steady states of a uniformly conducting magnetofluid inside a toroidal boundary. The problem becomes tractable in the limit of slow flow: i.e., low Reynolds number, which may be in turn justified when the viscous Lundquist number is small. Previous calculations are extended to apprehend the toroidal component of the necessary flow. The emerging pattern is one of helical vortices which seem likely to be ubiquitous in toroidal geometry, and which disappear in the “straight-cylinder approximation.”
Energy dissipation in a shear layer with suction
Authors: Doering Charles R. , Spiegel Edward A. , Worthing Rodney A.
Published: July 2002

The rate of viscous energy dissipation in a shear layer of incompressible Newtonian fluid with injection and suction is studied by means of exact solutions, nonlinear and linearized stability theory, and rigorous upper bounds. The injection and suction rates are maintained constant and equal and this leads to solutions with constant throughput. For strong enough suction, expressed in terms of the entry angle between the injection velocity and the boundaries, a steady laminar flow is nonlinearly stable for all Reynolds numbers. For a narrow range of small but nonzero angles, the laminar flow is linearly unstable at high Reynolds numbers. The upper bound on the energy dissipation rate—valid even for turbulent solutions of the Navier–Stokes equations—scales with viscosity in the same way as the laminar dissipation in the vanishing viscosity limit. For both the laminar and turbulent flows, the energy dissipation rate becomes independent of the viscosity for high Reynolds numbers. Hence the laminar energy dissipation rate and the largest possible turbulent energy dissipation rate for flows in this geometry differ by only a prefactor that depends only on the angle of entry.
Rapid guiding center calculations
Authors: White Roscoe B. , Boozer Allen H.
Published: July 2002

Pseudo-Cartesian coordinates are used to represent the magnetic field and toroidal ripple magnitudes in a Hamiltonian guiding center formalism. The resulting code ORBITX is three to eight times as fast as previous guiding center codes.
Simulations of alpha particle ripple loss from the International Thermonuclear Experimental Reactor
Authors: Redi M. H. , Budny R. V. , McCune D. C. , Miller C. O. , White R. B.
Published: July 2002

Calculations of collisional stochastic ripple loss of alpha particles from the new 20 toroidal field (TF) coil International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] predict small alpha ripple losses, less than 0.4%, close to the loss calculated for the full current operation of the earlier 24 TF coil design. An analytic fit is obtained to the ITER ripple data field demonstrating the nonlinear height dependence of the ripple minimum for D-shaped ripple contours. In contrast to alpha loss simulations for the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)], a simple Goldston, White, and Boozer stochastic loss criterion [Phys. Rev. Lett. 47, 647 (1981)] ripple loss model is found to require an increased renormalization of the stochastic threshold δs/δGWB≳1. Effects of collisions, sawtooth broadening, and reversal of the grad-B drift direction are included in the particle following simulations.
Plasma emission by a nonlinear beam instability in a weakly magnetized plasma
Authors: Yoon Peter H.
Published: July 2002

In a recent series of publications, a new theory of electromagnetic radiation with emission frequency close to the plasma frequency and/or its harmonic (i.e., the plasma emission) was presented. In this theory, the emission of radiation takes place as a result of excitation of long wavelength modes by a nonlinear beam-plasma instability, which are converted to radiative electromagnetic waves by a nonlinear mode conversion process. Unlike standard theories, the new theory predicts high radiation growth rate. In all the previous efforts on this theory, however, effects due to the presence of constant background magnetic field were ignored. The purpose of this article is to generalize the new theory to the case of weakly magnetized plasmas.
Optical properties of very thin GaInAs(P)/InP quantum wells grown by gas source molecular beam epitaxy
Authors: Panish M. B. , Temkin H. , Hamm R. A. , Chu S. N. G.
Published: July 2002

Single quantum wells of GaInAs and GaInAsP isolated by 150-Å-thick InP barriers have been grown by gas source molecular beam epitaxy. The quantum wells ranged in thickness from 5 to 30 Å. Photoluminescence and transmission electron microscopy were used to characterize them. Intense luminescence was observed at low temperature even from the thinnest of the wells. The spectral shifts due to the quantum size effect were 450 meV for the quaternary 5-Å well and 534 meV for the ternary 5-Å well. The latter constitutes approximately 85% of the total band-gap discontinuity. The emission linewidths suggest that on a scale larger than the exciton diameter the interface fluctuates less than one monolayer.
Edge- and surface-emitting distributed Bragg reflector laser with multiquantum well active/passive waveguides
Authors: Kojima Keisuke , Noda Susumu , Mitsunaga Kazumasa , Kyuma Kazuo , Hamanaka Koichi , Nakayama Takashi
Published: July 2002

A novel AlGaAs/GaAs distributed Bragg reflector laser utilizing a multiquantum well structure both as an active waveguide and as a low loss passive waveguide was fabricated. The threshold current was 102 mA at room temperature, and the differential quantum efficiency for the edge- and surface-emitted light was 16% and 3.7%, respectively. No mode hopping was observed in a temperature range of 35 K. A 0.17°×17° far-field pattern was obtained for the surface-emitted light.
Transmission of pulsed laser beams through ‘‘opaque’’ liquids by a cavitation effect
Authors: Sa’ar A. , Gal D. , Wallach R. , Akselrod S. , Katzir A.
Published: July 2002

Stationary cavities were formed in liquids by trains of laser pulses having the following characteristics: high absorption, high repetition rate, and high energy per pulse. Under these conditions laser pulses may be transmitted through several millimeters of ‘‘opaque’’ liquids by a cavitation effect. This facilitates material cutting and laser surgical operation under blood or under water by CO2 or excimer lasers. A simple model is presented to explain the cavitation effect.
Effect of oxide field on hot-carrier-induced degradation of metal-oxide-semiconductor field-effect transistors
Authors: Choi J. Y. , Ko P. K. , Hu C.
Published: July 2002

While the substrate current is a useful tool for extrapolating metal-oxide-semiconductor field-effect transistor lifetime from accelerated stressing data, the substrate current can vary significantly during a constant-voltage stress test. We have studied device degradation using a constant-field method. The critical electron energy for device degradation is found to be 3–6 eV, depending on the oxide electric field. These values are 50% higher than those reported elsewhere.
Quantitative theory of optical properties of HgTe-CdTe superlattices
Authors: Zoryk A. , Jaros M.
Published: July 2002

We have performed full-scale calculations of the optical properties of (001) HgTe-CdTe superlattices, of period ∼60 Å. We present the superlattice band gaps and oscillator strengths for the principal transitions as a function of HgTe layer width, in good agreement with experiment. The valence-band offset is 370 meV. Our results highlight the importance of the degree of localization of interface states in determining the optical properties of the superlattice.
Oxygen removal from Si via reaction with adsorbed Ge
Authors: Morar J. F. , Meyerson B. S. , Karlsson U. O. , Himpsel F. J. , McFeely F. R. , Rieger D. , Taleb-Ibrahimi A. , Yarmoff J. A.
Published: July 2002

Silicon surface optimization prior to film growth is central to the fields of chemical vapor deposition and molecular beam epitaxy. We have examined a method for low-temperature in situ cleaning of the Si (100) surfaces utilizing a submonolayer coverage of germanium. Synchrotron excited x-ray photoemission data indicate that Ge atoms arriving at a Si (100) surface can break silicon–oxygen bonds, thereby producing new chemical species which sublimate at 625 °C. In the absence of Ge, the observed silicon oxide species were stable at temperatures well in excess of 750 °C. These results are used to investigate the mechanisms by which adsorbed Ge can be used to produce oxide-free Si (100) surfaces at 625 °C.
Compact, multipass, single transverse mode CO2 laser
Authors: Xin J. G. , Hall D. R.
Published: July 2002

We report the design and basic operating characteristics of a co-axial radio-frequency discharge carbon dioxide laser which employs a multipass Herriott cell folding system within a linear resonator. A continuous wave laser power output of 65 W was obtained from a device 35 cm in length in a high-quality TEM00 mode without gas flow. The characteristics of transverse rf discharges and the image rotation properties of such resonators indicate that this approach may be scalable to provide much higher laser powers with excellent transverse mode properties.
Complete single longitudinal mode oscillation in a GaAlAs/GaAs distributed feedback laser with a modulated stripe width structure fabricated using reactive ion etching
Authors: Nakano Yoshiaki , Tada Kunio
Published: July 2002

A modulated stripe width structure (MSW) has been applied to a GaAlAs/GaAs distributed feedback buried heterostructure laser to obtain complete single longitudinal mode oscillation. An MSW device having a gradual modulation scheme and antireflecting films on the cleaved facets has been fabricated using reactive ion etching (RIE). Principal results include realization of a reactive ion etched second-order grating with the grooves as deep as 0.15 μm after regrowth and a modulated stripe having extremely fine definition made possible by RIE. The device had a spectrum which agreed markedly well with an analytical result. The effective functioning of the MSW structure has thereby been confirmed experimentally.
High-speed digital modulation of ultralow threshold (<1 mA) GaAs single quantum well lasers w
i
t
h
o
u
t bias
Authors: Lau K. Y. , Bar-Chaim N. , Derry P. L. , Yariv A.
Published: July 2002

GaAlAs buried heterostructure lasers with submilliampere threshold current fabricated from single quantum well wafers can be driven directly with logic level signals without any current bias. The switch-on delay was measured to be <50 ps and no relaxation oscillation ringing was observed. These lasers permit fully on-off multigigabit digital switching while at the same time obviating the need for bias monitoring and feedback control.
Excimer laser i
n s
i
t
u treatment of GaAs surfaces: Electrical properties of tungsten/GaAs diodes
Authors: Maki P. A. , Ehrlich D. J.
Published: July 2002

The effects of low-power irradiation of an n-type GaAs surface by a 193-nm excimer laser during tungsten metallization have been examined. Schottky diode characteristics of surfaces held under ultrahigh vacuum (10−9 Torr) conditions and in oxygen and chlorine ambients of 10−6 Torr show significant shifts in effective barrier height due to irradiation and depending on the ambient. The results of the electrical measurements are interpreted in terms of the possible laser surface chemistry. The applications for in situ surface cleaning for device processing within a vacuum GaAs growth chamber and applications for laser photochemical etching and oxidation of GaAs are discussed.
Doping superlattices grown in channeled GaAs substrates by molecular beam epitaxy through a built-in shadow mask
Authors: Hasnain G. , Mars D. , Döhler G. H. , Ogura M. , Smith J. S.
Published: July 2002

Micron width doping superlattices or n-i-p-i structures with grown-in interdigital highly selective contacts are grown successfully in a channeled GaAs substrate by molecular beam epitaxy (MBE) through a built-in shadow mask. Excellent diode characteristics and efficient lateral injection electroluminescence demonstrate the high quality of the masked MBE growth in the channeled substrate.
Multistable mode locking of InGaAsP semiconductor lasers
Authors: Kuznetsov M. , Tsang D. Z. , Walpole J. N. , Liau Z. L. , Ippen E. P.
Published: July 2002

We have investigated the pulsation behavior of InGaAsP semiconductor lasers with a proton-bombarded segment. These lasers emit picosecond (30–70 ps) pulses at gigahertz (0.6–3.0 GHz) rates. An antireflection-coated diode in an external cavity is passively mode locked and multistable; as many as four co-existing states are observed. Interlocking hysteresis loops are observed in the pulsation frequency, pulse width, and output power as functions of the bias current. A delayed feedback model explains qualitative features of the multistable mode locking. To our knowledge this is the first report of multistability of laser pulsation.
Second harmonic generation in Ge-doped fibers with a mode-locked Kr+ laser
Authors: Valk B. , Kim E. M. , Salour M. M.
Published: July 2002

We report on frequency doubling of the 647.1 nm line from a mode-locked Kr+ laser in a single-mode fiber with pure Ge-doped core. The harmonic light at 323.5 nm builds up after about 20 min of laser irradiation at 647.1 nm. Peak powers as low as 600 W were sufficient to prepare the fibers for second harmonic generation.
Monitoring humidity by polyimide lightguides
Authors: Reuter R. , Franke H.
Published: July 2002

Planar waveguides of the commercially available polyimide ‘‘PI 2566’’ and polyamide-imide ‘‘AI lite’’ are spin coated on glass substrates. TE and TM polarized modes are launched simultaneously into the phase-matched planar lightguide. This polymer guide is sensitive to changes in its birefringence. Polyimide films in the ‘‘as-dried’’ and fast cured state turn out to be very sensitive to moisture. Changes in the vicinity of the waveguide can be monitored instantly.
Threshold voltage drift of InP n-channel enhancement mode metal-insulator-semiconductor field-effect transistors
Authors: Johnson J. G. , Forrest S. R. , Zeisse C. R. , Nguyen R.
Published: July 2002

The drift of drain-to-source current in enhancement mode InP-based metal-insulator-semiconductor field-effect transistors is analyzed. We simultaneously measure the time dependence of both the drain-to-source current (IDS) and the transconductance of these devices when operated in the saturation regime. From the comparison of these long-term data, we conclude that the observed drift is caused by a variation in threshold voltage (VT), whereas the carrier mobility in the channel appears to be independent of time. We interpret these results in terms of charge trapping in the gate insulator and infer a slow surface state density of ≳1×1011/cm2. In spite of this relatively low density of states, large scale drifts in VT and IDS are observed. Apparently, this is the first report of the long-term time dependence of the saturation transconductance and its correlation with IDS drift.
Improvement of the bulk laser damage threshold of potassium dihydrogen phosphate crystals by reducing the organic impurities in growth solution
Authors: Nishida Y. , Yokotani A. , Sasaki T. , Yoshida K. , Yamanaka T. , Yamanaka C.
Published: July 2002

We obtained damage-resistant crystals of potassium dihydrogen phosphate (KDP) by reducing organic impurities in the growth solution. To prevent organic impurity exuded from the wall of the growth vessel, Pyrex glass was used. The residual organic impurities, which mainly derived from KDP raw powder, were reduced by photochemical dissociation using a combination of ultraviolet light and an oxidant (hydrogen peroxide). The resultant total organic carbon of the solution was less than 0.1 ppm. The damage threshold of the crystals grown from this solution was always high (20–22 J/cm2), which is about three times higher than that of crystals grown by conventional methods.
Persistent currents in ceramic and evaporated thin-film toroids of Ba2YCu3O7
Authors: Gyorgy E. M. , Grader G. S. , Johnson D. W. , Feldman L. C. , Murphy D. W. , Rhodes W. W. , Howard R. E. , Mankiewich P. M. , Skocpol W. J.
Published: July 2002

We have demonstrated the existence of circumferential persistent currents in Ba2YCu3O7 ceramic toroids at 77 and 4.2 K and in thin films at 77 K. The field dependence of the critical current densities obtained from these persistent current measurements at 77 K is in fair agreement with that obtained from transport measurements. The persistent critical current densities for fields less than 10 G are 150 A/cm2 for the ceramic at 77 K and 105 A/cm2 for the thin film. A long term experiment showed that the resistivity is less than 3×10−18 Ω cm at 77 K.
Quantum well tunnel triode
Authors: Kastalsky A. , Milshtein M.
Published: July 2002

We demonstrate a novel three-terminal device, the tunnel triode, in which the current within the quantum well is a part of the tunnel current through the p+-n+ junction. A tunnel-diode-like negative differential resistance effect with peak-to-valley ratio as high as 20 was observed, the tunnel current being controlled by the gate voltage. We show that tunneling occurs not in the quantum well, but in the heavily doped n+-Al0.3Ga0.7As layer, and it is preceded by a real-space hot-electron transfer from the quantum well into this layer. Logic operation of a bistable switch was obtained in a circuit comprising a tunnel triode and a series resistance.
Experimental observation of time jitter in semiconductor laser turn-on
Authors: Spano P. , D’Ottavi A. , Mecozzi A. , Daino B.
Published: July 2002

We report an experimental study which shows how time jitter in semiconductor lasers, which arises when such sources are switched from below to above threshold, is related to the actual operating conditions. The effect is observed both in distributed feedback and in Fabry–Perot lasers, even if in the latter case it becomes more and more negligible when the output power of the final state is increased.
rf break-junction SQUID’s at 77 K
Authors: Harvey I. K. , Binks R. A. , Driver R.
Published: July 2002

Break-junction rf superconducting quantum interference devices (SQUID’s) have been developed using bulk YBa2Cu3O7 ceramic samples which have been fabricated in the laboratory. The SQUID’s have been produced in both single, and two-hole topologies and their noise characteristics have been measured at 77 K. The equivalent flux noise of the two-hole SQUID at a frequency of 200 Hz was found to be 6.8×10−4 flux quantum/(Hz)1/2.
Crystalline to amorphous transformation in Laves phase GdFe2 induced by hydrogen absorption
Authors: Aoki K. , Yanagitani A. , Masumoto T.
Published: July 2002

The hydrogen-induced transformation from the crystalline to the amorphous state in GdFe2 has been studied by means of differential thermal analysis. GdFe2 absorbs hydrogen in the crystalline state above 420 K by an exothermic reaction. The resultant crystalline c-GdFe2H3.8 alloy transforms to the amorphous a-GdFe2H2.8 alloy above 470 K exothermally, which crystallizes to the stable phases α-Fe and GdH2 above about 700 K. The driving force for the hydrogen-induced transformation is suggested to be the difference in the enthalpy resulting from the different hydrogen occupation sites in both states of alloy.
Bistable switching in nonlinear Al0.06Ga0.94As étalons
Authors: Sahlén Olof , Masseboeuf Eric , Rask Michael , Nordell Nils , Landgren Gunnar
Published: July 2002

Optical bistability at 5 mW input power at 840 nm is reported in Al0.06Ga0.94As étalons grown by metalorganic vapor phase epitaxy, having an epitaxially grown high-reflecting back mirror. The devices have been switched from the high-reflecting to the low-reflecting state with pulses from a laser diode operating at 778 nm with 10 pJ switching energy. The devices could be thermally stable for as long as 200 μs. The experimental results are compared with a simple theoretical model.
Electric-field-dependent photoconductivity in GaInAs-InP quantum wells
Authors: Mowbray D. J. , Skolnick M. S. , Lee D. , Claxton P. A. , Roberts J. S.
Published: July 2002

We report a study of the effects of an electric field on the excitonic, band-to-band photoconductivity spectra of a Ga0.47In0.53As-InP quantum well structure. In a sample with five wells of widths 10–110 Å we show that for carrier motion normal to the quantum well layers a photoconductivity signal is only observed from those wells which are in a region of nonzero electric field. The spectral line shapes of the transitions in the narrowest two wells show strong variations with field, which we attribute to exciton dissociation at high field, and possibly exciton screening by free carriers trapped in the wells at low fields. The results are compared with photoconductivity spectra of the same structure, but for carrier motion in the plane of the layers. The latter geometry is found to reflect more accurately the sample absorption.
Stable “zeta” peptides that act as potent antagonists of the high-affinity IgE receptor
Authors: Nakamura Gerald R. , Reynolds Mark E. , Chen Yvonne M. , Starovasnik Melissa A. , Lowman Henry B.
Published: July 2002

Recently we described a family of peptides, unrelated in sequence to IgE, that form stable β-hairpins in solution and inhibit IgE activity in the μM range [Nakamura, G. R., Starovasnik, M. A., Reynolds, M. E. & Lowman, H. B. (2001)Biochemistry40, 9828–9835]. Using an expanded set of peptide–phage libraries, we found a simpler motif, X2CPX2CYX, for binding to the high-affinity IgE receptor. In solution, one of these peptides spontaneously formed a covalent antiparallel dimer. We subsequently linked these monomers in a single-chain construct on phage and optimized receptor binding. Ultimately, peptides with 30 nM affinity were produced. NMR studies showed that the peptide adopts a stable fold consisting of two “zeta” (ζ)-shaped moieties. Structure–activity analyses reveal a single binding site created by the zeta-dimer, with two tyrosine residues important for structural stability and two proline residues important for FcɛRI binding. The peptides inhibit histamine release from cultured cells and are extremely stable in biological fluids. The zeta peptides appear to act as competitive IgE inhibitors and suggest possibilities for design of novel IgE antagonists.
Ubiquitin-dependent mechanism regulates rapid turnover of AU-rich cytokine mRNAs
Authors: Laroia Gaurav , Sarkar Bedabrata , Schneider Robert J.
Published: July 2002

An AU rich element (ARE) in the 3′ noncoding region promotes the rapid degradation of mammalian cytokine and proto-oncogene mRNAs, such as tumor necrosis factor-α, granulocyte–macrophage colony-stimulating factor (GM-CSF) and c-fos. Destabilization of ARE-mRNAs involves the association of ARE-binding proteins tristetraprolin or AUF1 and proteasome activity, of which the latter has not been characterized. Here, we show that the stability of a model short-lived mRNA containing the GM-CSF ARE was regulated by the level of ubiquitin-conjugating activity in the cell, which links ARE-mRNA decay to proteasome activity. Increased expression of a cytokine-inducible deubiquitinating protein (DUB) that impairs addition of ubiquitin to proteins fully blocked ARE-mRNA decay, whereas increased expression of a DUB that promotes ubiquitin addition to proteins strongly accelerated ARE-mRNA decay. ARE-mRNA turnover was found to be activated by the ubiquitin-addition reaction and blocked by the ubiquitin-removal reaction. Saturation of the ARE-mRNA decay machinery by high levels of ARE-mRNA, which is well established but not understood, was found to be relieved by increased expression of a DUB that promotes ubiquitin addition to proteins. Finally, inhibition of proteasome activity also blocked accelerated ARE-mRNA decay that is mediated by increased ubiquitin recycling. These results demonstrate that both ubiquitinating activity and proteasome activity are essential for rapid turnover of a model cytokine ARE-mRNA containing the GM-CSF ARE.
The cAMP-specific phosphodiesterase TbPDE2C is an essential enzyme in bloodstream formTrypanosoma brucei
Authors: Zoraghi Roya , Seebeck Thomas
Published: July 2002

Chemotherapy of human sleeping sickness, a fatal disease caused by the protozoan parasiteTrypanosoma brucei, is in a dismal state, and the identification and characterization of new drug targets is an urgent prerequisite for an improvement of the dramatic situation in the field. Over the last several years, inhibitors of cyclic nucleotide-specific phosphodiesterases have proven to be highly successful drug candidates for an assortment of clinical conditions. Their potential as antiparasitic drugs has not been explored so far. This study reports the characterization of a cAMP-specific phosphodiesterase fromT. brucei, TbPDE2C. This enzyme is a class I phosphodiesterase, and it is a member of a small enzyme family inT. brucei,TbPDE2. Inhibitors of this enzyme block the proliferation of bloodstream form trypanosomes in culture. RNA interference experiments demonstrated that the TbPDE2 family, and in particular TbPDE2C, are essential for maintaining intracellular cAMP concentrations within a physiological range. Bloodstream form trypanosomes are exquisitely sensitive to elevated concentrations of intracellular cAMP, and a disruption of TbPDE2C function quickly leads to the disruption of nuclear and cellular cell division, and to cell death. TbPDE2C might represent a novel drug target for the development of new and effective trypanocidal drugs.
Visualization of biochemical networks in living cells
Authors: Remy Ingrid , Michnick Stephen W.
Published: July 2002

Functional annotation of novel genes can be achieved by detection of interactions of their encoded proteins with known proteins followed by assays to validate that the gene participates in a specific cellular function. We report an experimental strategy that allows for detection of protein interactions and functional assays with a single reporter system. Interactions among biochemical network component proteins are detected and probed with stimulators and inhibitors of the network. In addition, the cellular location of the interacting proteins is determined. We used this strategy to map a signal transduction network that controls initiation of translation in eukaryotes. We analyzed 35 different pairs of full-length proteins and identified 14 interactions, of which five have not been observed previously, suggesting that the organization of the pathway is more ramified and integrated than previously shown. Our results demonstrate the feasibility of using this strategy in efforts of genomewide functional annotation.
Modulation of CTP:phosphocholine cytidylyltransferase by membrane curvature elastic stress
Authors: Attard George S. , Templer Richard H. , Smith Wendy S. , Hunt Alan N. , Jackowski Suzanne
Published: July 2002

The activity of CTP:phosphocholine cytidylyltransferase, a rate-limiting enzyme in phosphatidylcholine biosynthesis, is modulated by its interaction with lipid bilayers [Kent, C. (1997)
Biochim. Biophys. Acta
1348, 79–90]. Its regulation is of central importance in the maintenance of membrane lipid homeostasis. Here we show evidence that the stored curvature elastic stress in the lipid membrane's monolayer modulates the activity of CTP:phosphocholine cytidylyltransferase. Our results show how a purely physical feedback signal could play a key role in the control of membrane lipid synthesis.
Anterograde flow of cargo across the Golgi stack potentially mediated via bidirectional “percolating” COPI vesicles
Authors: Orci Lelio , Ravazzola Mariella , Volchuk Allen , Engel Thomas , Gmachl Michael , Amherdt Mylène , Perrelet Alain , Söllner Thomas H. , Rothman James E.
Published: July 2002

How do secretory proteins and other cargo targeted to post-Golgi locations traverse the Golgi stack? We report immunoelectron microscopy experiments establishing that a Golgi-restricted SNARE, GOS 28, is present in the same population of COPI vesicles as anterograde cargo marked by vesicular stomatitis virus glycoprotein, but is excluded from the COPI vesicles containing retrograde-targeted cargo (marked by KDEL receptor). We also report that GOS 28 and its partnering t-SNARE heavy chain, syntaxin 5, reside together in every cisterna of the stack. Taken together, these data raise the possibility that the anterograde cargo-laden COPI vesicles, retained locally by means of tethers, are inherently capable of fusing with neighboring cisternae on either side. If so, quanta of exported proteins would transit the stack in GOS 28–COPI vesicles via a bidirectional random walk, entering at the cis face and leaving at the trans face and percolating up and down the stack in between. Percolating vesicles carrying both post-Golgi cargo and Golgi residents up and down the stack would reconcile disparate observations on Golgi transport in cells and in cell-free systems.
Induction of tolerance to immunogenic tumor antigens associated with lymphomagenesis in HOX11 transgenic mice
Authors: Rosic-Kablar Suzana , Chan Kin , Reis Marciano D. , Dubé Ian D. , Hough Margaret R.
Published: July 2002

Transgenic mice expressing humanHOX11in B lymphocytes die prematurely from lymphomas that initiate in the spleen and frequently disseminate to distant sites. Preneoplastic hematopoiesis in these mice is unperturbed. We now report that expression of theHOX11transgene does not affect the ability of dendritic cells (DCs) to process and present foreign peptides and activate antigen-specific T cell responses. We also show that nontransgenic DCs presenting peptides derived from the human HOX11 protein are highly efficient stimulators of autologous T cells, whereas transgenic T cells are nonresponsive to peptides derived from theHOX11transgene and the murine Meis1 protein. HOX11 transgenic mice thus show normal development of tolerance to immunogenic antigens expressed throughout B cell maturation. DCs pulsed with cell lysates prepared from lymphomas, obtained from HOX11 transgenic mice with terminal lymphoma, activate T cells from nontransgenic and premalignant transgenic mice, whereas T cells isolated from lymphomatous transgenic mice are nonresponsive to autologous tumor cell antigens. These data indicate that HOX11 lymphoma cells express tumor-rejection antigens that are recognized as foreign in healthy transgenic mice and that lymphomagenesis is associated with the induction of anergy to tumor antigen-specific T cells. These findings are highly relevant for the development of immunotherapeutic protocols for the treatment of lymphoma.
Doppel-induced cerebellar degeneration in transgenic mice
Authors: Moore Richard C. , Mastrangelo Peter , Bouzamondo Essia , Heinrich Cornelia , Legname Giuseppe , Prusiner Stanley B. , Hood Leroy , Westaway David , DeArmond Stephen J. , Tremblay Patrick
Published: July 2002

Doppel (Dpl) is a paralog of the mammalian prion protein
(PrP); it is abundant in testes but expressed at low levels in the
adult central nervous system. In two
Prnp
-deficient
(
Prnp
0/0
) mouse lines (Ngsk and Rcm0), Dpl
overexpression correlated with ataxia and death of cerebellar neurons.
To determine whether Dpl overexpression, rather than the dysregulation
of genes neighboring the
Prn
gene complex, was responsible
for the ataxic syndrome, we placed the mouse Dpl coding sequence under
the control of the
Prnp
promoter and produced transgenic
(Tg) mice on the
Prnp
0/0
-ZrchI background
(hereafter referred to as ZrchI). ZrchI mice exhibit neither Dpl
overexpression nor cerebellar degeneration. In contrast,
Tg(Dpl)ZrchI mice showed cerebellar granule and Purkinje cell loss;
the age of onset of ataxia was inversely proportional to the levels of
Dpl protein. Crosses of Tg mice overexpressing wild-type PrP with two
lines of Tg(Dpl)ZrchI mice resulted in a phenotypic rescue of the
ataxic syndrome, while Dpl overexpression was unchanged. Restoration of
PrP expression also rendered the Tg(Dpl) mice susceptible to prion
infection, with incubation times indistinguishable from non-Tg
controls. Whereas the rescue of Dpl-induced neurotoxicity by
coexpression of PrP argues for an interaction between the PrP and Dpl
proteins
in vivo
, the unaltered incubation times in Tg mice
overexpressing Dpl in the central nervous system suggest that Dpl is
unlikely to be involved in prion formation.
Loss of tramtrack gene activity results in ectopic R7 cell formation, even in a sina mutant background.
Authors: Lai Z C , Harrison S D , Karim F , Li Y , Rubin G M
Published: July 2002

We have screened a collection of transposable-element-induced mutations for those which dominantly modify the extra R7 phenotype of a hypomorphic yan mutation. The members of one of the identified complementation groups correspond to disruptions of the tramtrack (ttk) gene. As heterozygotes, ttk alleles increase the percentage of R7 cells in yan mutant eyes. Just as yan mutations increase ectopic R7 cell formation, homozygous ttk mutant eye clones also contain supernumerary R7 cells. However, in contrast to yan, the formation of these cells in ttk mutant eye tissue is not necessarily dependent on the activity of the sina gene. Furthermore, although yan mutations dominantly interact with mutations in the Ras1, Draf, Dsor1, and rolled (rl) genes to influence R7 cell development, ttk mutations only interact with yan and rl gene mutations to affect this signaling pathway. Our data suggest that yan and ttk both function to repress inappropriate R7 cell development but that their mechanisms of action differ. In particular, TTK activity appears to be autonomously required to regulate a sina-independent mechanism of R7 determination.
Mutant LexA proteins with specific defects in autodigestion.
Authors: Shepley D P , Little J W
Published: July 2002

In self-processing biochemical reactions, a protein or RNA molecule specifically modifies its own structure. Many such reactions are regulated in response to the needs of the cell by an interaction with another effector molecule. In the system we study here, specific cleavage of the Escherichia coli LexA repressor, LexA cleaves itself in vitro at a slow rate, but in vivo cleavage requires interaction with an activated form of RecA protein. RecA acts indirectly as a coprotease to stimulate LexA autodigestion. We describe here a new class of lexA mutants, lexA (Adg-; for autodigestion-defective) mutants, termed Adg- for brevity. Adg- mutants specifically interfered with the ability of LexA to autodigest but left intact its ability to undergo RecA-mediated cleavage. The data are consistent with a conformational model in which RecA favors a reactive conformation capable of undergoing cleavage. To our knowledge, this is the first example of a mutation in a regulated self-processing reaction that impairs the rate of self-processing without markedly affecting the stimulated reaction. Had wild-type lexA carried such a substitution, discovery of its self-processing would have been difficult; we suggest that, in other systems, a slow rate of self-processing has prevented recognition that a reaction is of this nature.