3,809 research outputs found
Resolving the Azimuthal Ambiguity in Vector Magnetogram Data with the Divergence-Free Condition: Application to Discrete Data
We investigate how the divergence-free property of magnetic fields can be
exploited to resolve the azimuthal ambiguity present in solar vector
magnetogram data, by using line-of-sight and horizontal heliographic derivative
information as approximated from discrete measurements. Using synthetic data we
test several methods that each make different assumptions about how the
divergence-free property can be used to resolve the ambiguity. We find that the
most robust algorithm involves the minimisation of the absolute value of the
divergence summed over the entire field of view. Away from disk centre this
method requires the sign and magnitude of the line-of-sight derivatives of all
three components of the magnetic field vector.Comment: Solar Physics, in press, 20 pages, 11 figure
Zn-doping effect on the magnetotransport properties of Bi_{2}Sr_{2-x}La_{x}CuO_{6+\delta} single crystals
We report the magnetotransport properties of
Bi_{2}Sr_{2-x}La_{x}Cu_{1-z}Zn_{z}O_{6+\delta} (Zn-doped BSLCO) single crystals
with z of up to 2.2%. Besides the typical Zn-doping effects on the in-plane
resistivity and the Hall angle, we demonstrate that the nature of the
low-temperature normal state in the Zn-doped samples is significantly altered
from that in the pristine samples under high magnetic fields. In particular, we
observe nearly-isotropic negative magnetoresistance as well as an increase in
the Hall coefficient at very low temperatures in non-superconducting Zn-doped
samples, which we propose to be caused by the Kondo scattering from the local
moments induced by Zn impurities.Comment: 4 pages, 4 figures, final version (one reference added), published in
Phys. Rev.
Combination Rules, Charge Symmetry, and Hall Effect in Cuprates
The rule relating the observed Hall coefficient to the spin and charge
responses of the uniform doped Mott insulator is derived. It is essential to
include the contribution of holon and spinon three-current correlations to the
effective action of the gauge field. In the vicinity of the Mott insulating
point the Hall coefficient is holon dominated and weakly temperature dependent.
In the vicinity of a point of charge conjugation symmetry the holon
contribution to the observed Hall coefficient is small: the Hall coefficient
follows the temperature dependence of the diamagnetic susceptibility with a
sign determined by the Fermi surface shape. NOTE: document prepared using
REVTEX. (3 Figs, not included, available on request from: [email protected])Comment: 8 page
Self- generated disorder and structural glass formation in homopolymer globules
We have investigated the interrelation between the spin glasses and the
structural glasses. Spin glasses in this case are random magnets without
reflection symmetry (e.g. - spin interaction spin glasses and Potts
glasses) which contain quenched disorder, whereas the structural glasses are
here exemplified by the homopolymeric globule, which can be viewed as a liquid
of connected molecules on nano scales. It is argued that the homopolymeric
globule problem can be mapped onto a disorder field theoretical model whose
effective Hamiltonian resembles the corresponding one for the spin glass model.
In this sense the disorder in the globule is self - generated (in contrast to
spin glasses) and can be related with competitive interactions (virial
coefficients of different signs) and the chain connectivity. The work is aimed
at giving a quantitative description of this analogy. We have investigated the
phase diagram of the homopolymeric globule where the transition line from the
liquid to glassy globule is treated in terms of the replica symmetry breaking
paradigm. The configurational entropy temperature dependence is also discussed.Comment: 22 pages, 4 figures, submitted to Phys. Rev.
The Evolution of Sunspot Magnetic Fields Associated with a Solar Flare
Solar flares occur due to the sudden release of energy stored in
active-region magnetic fields. To date, the pre-cursors to flaring are still
not fully understood, although there is evidence that flaring is related to
changes in the topology or complexity of an active region's magnetic field.
Here, the evolution of the magnetic field in active region NOAA 10953 was
examined using Hinode/SOT-SP data, over a period of 12 hours leading up to and
after a GOES B1.0 flare. A number of magnetic-field properties and low-order
aspects of magnetic-field topology were extracted from two flux regions that
exhibited increased Ca II H emission during the flare. Pre-flare increases in
vertical field strength, vertical current density, and inclination angle of ~
8degrees towards the vertical were observed in flux elements surrounding the
primary sunspot. The vertical field strength and current density subsequently
decreased in the post-flare state, with the inclination becoming more
horizontal by ~7degrees. This behaviour of the field vector may provide a
physical basis for future flare forecasting efforts.Comment: Accepted for Publication in Solar Physics. 16 pages, 4 figure
Non-Universal Power Law of the "Hall Scattering Rate" in a Single-Layer Cuprate Bi_{2}Sr_{2-x}La_{x}CuO_{6}
In-plane resistivity \rho_{ab}, Hall coefficient, and magnetoresistance (MR)
are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals
with various carrier concentrations, from underdope to overdope. Our crystals
show the highest T_c (33 K) and the smallest residual resistivity ever reported
for Bi-2201 at optimum doping. It is found that the temperature dependence of
the Hall angle obeys a power law T^n with n systematically decreasing with
increasing doping, which questions the universality of the Fermi-liquid-like
T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of
the optimally-doped sample changes as T^{1.7}, not as T^2, while \rho_{ab}
shows a good T-linear behavior. The systematics of the MR indicates an
increasing role of spin scattering in underdoped samples.Comment: 4 pages, 5 figure
On the Thermodynamic Geometry of BTZ Black Holes
We investigate the Ruppeiner geometry of the thermodynamic state space of a
general class of BTZ black holes. It is shown that the thermodynamic geometry
is flat for both the rotating BTZ and the BTZ Chern Simons black holes in the
canonical ensemble. We further investigate the inclusion of thermal
fluctuations to the canonical entropy of the BTZ Chern Simons black holes and
show that the leading logartithmic correction due to Carlip is reproduced. We
establish that the inclusion of thermal fluctuations induces a non zero scalar
curvature to the thermodynamic geometry.Comment: 1+17 pages, LaTeX, 4 eps figure
Universal Scaling of Strong-Field Localization in an Integer Quantum Hall Liquid
We study the Landau level localization and scaling properties of a disordered
two-dimensional electron gas in the presence of a strong external magnetic
field. The impurities are treated as random distributed scattering centers with
parameterized potentials. Using a transfer matrix for a finite-width strip
geometry, we calculate the localization length as a function of system size and
electron energy. The finite-size localization length is determined by
calculating the Lyapunov exponents of the transfer matrix. A detailed
finite-size scaling analysis is used to study the critical behavior near the
center of the Landau bands. The influence of varying the impurity
concentration, the scattering potential range and its nature, and the Landau
level index on the scaling behavior and on the critical exponent is
systematically investigated. Particular emphasis is put on studying the effects
of finite range of the disorder potential and Landau level coupling on the
quantum localization behavior. Our numerical results, which are carried out on
systems much larger than those studied before, indicate that pure
-function disorder in the absence of any Landau level coupling gives
rise to non-universal localization properties with the critical exponents in
the lowest two Landau levels being substantially different. Inclusion of a
finite potential range and/or Landau level mixing may be essential in producing
universality in the localization.Comment: 28 pages, Latex, 17 figures (available upon request), #phd0
Effect of a magnetic field on the spin- and charge-density wave order in La1.45Nd0.4Sr0.15CuO4
The spin-density wave (SDW) and charge-density wave (CDW) order in
superconducting La1.45Nd0.4Sr0.15CuO4 were studied under an applied magnetic
field using neutron and X-ray diffraction techniques. In zero field,
incommensurate (IC) SDW order appears below ~ 40 K, which is characterized by
neutron diffraction peaks at (1/2 +/- 0.134, 1/2 +/- 0.134, 0). The intensity
of these IC peaks increases rapidly below T_Nd ~ 8 K due to an ordering of the
Nd^3+ spins. The application of a 1 T magnetic field parallel to the c-axis
markedly diminishes the intensity below T_Nd, while only a slight decrease in
intensity is observed at higher temperatures for fields up to 7 T. Our
interpretation is that the c-axis field suppresses the parasitic Nd^3+ spin
order at the incommensurate wave vector without disturbing the stripe order of
Cu^2+ spins. Consistent with this picture, the CDW order, which appears below
60 K, shows no change for magnetic fields up to 4 T. These results stand in
contrast to the significant field-induced enhancement of the SDW order observed
in superconducting La2-xSrxCuO4 with x ~ 0.12 and stage-4 La2CuO4+y. The
differences can be understood in terms of the relative volume fraction
exhibiting stripe order in zero field, and the collective results are
consistent with the idea that suppression of superconductivity by vortices
nucleates local patches of stripe order.Comment: 7 pages, 5 figure
Scalar Field Theory on Fuzzy S^4
Scalar fields are studied on fuzzy and a solution is found for the
elimination of the unwanted degrees of freedom that occur in the model. The
resulting theory can be interpreted as a Kaluza-Klein reduction of CP^3 to S^4
in the fuzzy context.Comment: 16 pages, LaTe
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