4,073 research outputs found
Phonons in intrinsic Josephson systems with parallel magnetic field
Subgap resonances in the I-V curves of layered superconductors are explained
by the coupling between Josephson oscillations and phonons with dispersion in
c-direction. In the presence of a magnetic field applied parallel to the layers
additional structures due to fluxon motion appear. Their coupling with phonons
is investigated theoretically and a shift of the phonon resonances in strong
magnetic fields is predicted.Comment: Invited Paper to the "2nd International Symposium on Intrinsic
Josephson Effects and Plasma Oscillations in High-Tc Superconductors", 22-24
August 2000, Sendai, Japan, to be published in Physica
Microscopic theory of the coupling of intrinsic Josephson oscillations and phonons
A microscopic theory for the coupling of intrinsic Josephson oscillations and
dispersive phonon branches in layered superconductors is developed. Thereby the
effect of phonons on the electronic c-axis transport enters through an
effective longitudinal dielectric function. This coupling provides an
explanation of recently observed subgap resonances in the --
curve of anisotropic cuprate superconductors forming a stack of short Josephson
junctions. Due to the finite dispersion these resonances can appear at
van-Hove-singularities of both optical and acoustical phonon branches,
explaining low-voltage structures in the I-V-characteristic, which are not
understood in phonon models without dispersion. In long junctions the
dispersion of collective electron-phonon modes parallel to the layers is
investigated.Comment: 4 pages, 3 figures, 1 table, espcrc2.sty, invited contribution to
"Materials and Mechanisms of Superconductivity and High Temperature
Superconductors VI - M2S-HTSC-VI", Houston, Texas, 20-25 Feb 2000, to appear
in Physica
Optical Resonances in Reflectivity near Crystal Modes with Spatial Dispersion
We study the effect of spatial dispersion of crystal modes on optical
properties such as the reflectivity . As an example for isotropic media, we
investigate the simplest model for phonons in ionic crystals and compare with
previous results for highly anisotropic plasmons, which are now understood from
a more general point of view. As a consequence of the wave vector dependence of
the dielectric function small changes in the lineshape are predicted. Beyond
that, if the frequency of minimal is near a pole of the dispersionless
dielectric function, the relative amplitude of dips in with normal and
anomalous dispersion differ significantly, if dissipation and disorder are low.Comment: 4 pages, 7 eps figures, minor change
Magnetism in Graphene Induced by Single-Atom Defects
We study from first principles the magnetism in graphene induced by single
carbon atom defects. For two types of defects considered in our study, the
hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism
due to the defect-induced extended states has been observed. Calculated
magnetic moments are equal to 1 per hydrogen chemisorption defect and
1.121.53 per vacancy defect depending on the defect concentration.
The coupling between the magnetic moments is either ferromagnetic or
antiferromagnetic, depending on whether the defects correspond to the same or
to different hexagonal sublattices of the graphene lattice, respectively. The
relevance of itinerant magnetism in graphene to the high- magnetic
ordering is discussed.Comment: 5 pages, 6 figure
Absence of ferromagnetism in V-implanted ZnO single crystals
The structural and magnetic properties of V doped ZnO are presented. V ions
were introduced into hydrothermal ZnO single crystals by ion implantation with
fluences of 1.2*10^16 to 6*10^16 cm^-2. Post-implantation annealing was
performed in high vacuum from 823 K to 1023 K. The ZnO host material still
partly remains in a crystalline state after irradiation, and is partly
recovered by annealing. The V ions show a thermal mobility as revealed by depth
profile Auger electron spectroscopy. Synchrotron radiation x-ray diffraction
revealed no secondary phase formation which indicates the substitution of V
onto Zn site. However in all samples no pronounced ferromagnetism was observed
down to 5 K by a superconducting quantum interference device magnetometer.Comment: 13 pages, 4 figs, MMM conference 2007, accepted by J. Appl. Phy
Optical Properties of Crystals with Spatial Dispersion: Josephson Plasma Resonance in Layered Superconductors
We derive the transmission coefficient, , for grazing incidence of
crystals with spatial dispersion accounting for the excitation of multiple
modes with different wave vectors for a given frequency . The
generalization of the Fresnel formulas contains the refraction indices of these
modes as determined by the dielectric function . Near
frequencies , where the group velocity vanishes, depends
also on an additional parameter determined by the crystal microstructure. The
transmission is significantly suppressed, if one of the excited modes is
decaying into the crystal. We derive these features microscopically for the
Josephson plasma resonance in layered superconductors.Comment: 4 pages, 2 figures, epl.cls style file, minor change
MnSi nanoparticles embedded in Si: Superparamagnetism with a collective behavior
The doping of Mn in Si is attracting research attentions due to the
possibility to fabricate Si-based diluted magnetic semiconductors. However, the
low solubility of Mn in Si favors the precipitation of Mn ions even at
non-equilibrium growth conditions. MnSi nanoparticles are the common
precipitates, which show exotic magnetic properties in comparison with the
MnSi bulk phase. In this paper we present the static and dynamic
magnetic properties of MnSi nanoparticles. Using the Preisach model, we
derive the magnetic parameters, such as the magnetization of individual
particles, the distribution of coercive fields and the inter-particle
interaction field. Time-dependent magnetization measurements reveal a
spin-glass behavior of the system.Comment: 11 pages, 6 figures, submitted to PR
When the sun never sets: diverse activity rhythms under continuous daylight in free-living arctic-breeding birds
Circadian clocks are centrally involved in the regulation of daily behavioural and physiological processes. These clocks are synchronized to the 24-hour day by external cues (Zeitgeber), the most important of which is the light-dark cycle. In polar environments, however, the strength of the Zeitgeber is greatly reduced around the summer and winter solstices (continuous daylight or continuous darkness). How animals time their behaviour under such conditions has rarely been studied in the wild. Using a radio-telemetry-based system, we investigated daily activity rhythms under continuous daylight in Barrow, Alaska, throughout the breeding season in four bird species that differ in mating system and parental behaviour. We find substantial diversity in daily activity rhythms depending on species, sex and breeding stage. Individuals exhibited either robust, entrained 24-hour activity cycles, were continuously active (arrhythmic), or showed “free-running” activity cycles. In semipalmated sandpipers, a shorebird with biparental incubation, we show that the free-running rhythm is synchronized between pair mates. The diversity of diel time-keeping under continuous daylight emphasizes the plasticity of the circadian system and the importance of the social and life-history context. Our results support the idea that circadian behaviour can be adaptively modified to enable species-specific time-keeping under polar conditions
- …