5,553 research outputs found
Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet/Semiconductor Heterostructures
We demonstrate methods to locally control the spin rotation of moving
electrons in a GaAs channel. The Larmor frequency of optically-injected spins
is modulated when the spins are dragged through a region of spin-polarized
nuclei created at a MnAs/GaAs interface. The effective field created by the
nuclei is controlled either optically or electrically using the ferromagnetic
proximity polarization effect. Spin rotation is also tuned by controlling the
carrier traverse time through the polarized region. We demonstrate coherent
spin rotations exceeding 4 pi radians during transport.Comment: 15 pages, 4 figure
Observation of coherent electroproduction on deuterons at large momentum transfer
The first experimental results for coherent -electroproduction on the
deuteron, , at large momentum transfer, are reported. The
experiment was performed at Jefferson Laboratory at an incident electron energy
of 4.05 GeV. A large pion production yield has been observed in a kinematical
region for 1.11.8 GeV, from threshold to 200 MeV excitation energy
in the system. The -dependence is compared with theoretical
predictions.Comment: 26 page
Vortices and confinement at weak coupling
We discuss the physical picture of thick vortices as the mechanism
responsible for confinement at arbitrarily weak coupling in SU(2) gauge theory.
By introducing appropriate variables on the lattice we distinguish between
thin, thick and `hybrid' vortices, the latter involving Z(2) monopole loop
boundaries. We present numerical lattice simulation results that demonstrate
that the full SU(2) string tension at weak coupling arises from the presence of
vortices linked to the Wilson loop. Conversely, excluding linked vortices
eliminates the confining potential. The numerical results are stable under
alternate choice of lattice action as well as a smoothing procedure which
removes short distance fluctuations while preserving long distance physics.Comment: 21 pages, LaTe
Combinatorial quantization of the Hamiltonian Chern-Simons theory I
Motivated by a recent paper of Fock and Rosly \cite{FoRo} we describe a
mathematically precise quantization of the Hamiltonian Chern-Simons theory. We
introduce the Chern-Simons theory on the lattice which is expected to reproduce
the results of the continuous theory exactly. The lattice model enjoys the
symmetry with respect to a quantum gauge group. Using this fact we construct
the algebra of observables of the Hamiltonian Chern-Simons theory equipped with
a *-operation and a positive inner product.Comment: 49 pages. Some minor corrections, discussion of positivity improved,
a number of remarks and a reference added
Construction of Field Algebras with Quantum Symmetry from Local Observables
It has been discussed earlier that ( weak quasi-) quantum groups allow for
conventional interpretation as internal symmetries in local quantum theory.
From general arguments and explicit examples their consistency with (braid-)
statistics and locality was established. This work addresses to the
reconstruction of quantum symmetries and algebras of field operators. For every
algebra \A of observables satisfying certain standard assumptions, an
appropriate quantum symmetry is found. Field operators are obtained which act
on a positive definite Hilbert space of states and transform covariantly under
the quantum symmetry. As a substitute for Bose/Fermi (anti-) commutation
relations, these fields are demonstrated to obey local braid relation.Comment: 50 pages, HUTMP 93-B33
Multivalued Fields on the Complex Plane and Conformal Field Theories
In this paper a class of conformal field theories with nonabelian and
discrete group of symmetry is investigated. These theories are realized in
terms of free scalar fields starting from the simple systems and scalar
fields on algebraic curves. The Knizhnik-Zamolodchikov equations for the
conformal blocks can be explicitly solved. Besides of the fact that one obtains
in this way an entire class of theories in which the operators obey a
nonstandard statistics, these systems are interesting in exploring the
connection between statistics and curved space-times, at least in the two
dimensional case.Comment: (revised version), 30 pages + one figure (not included), (requires
harvmac.tex), LMU-TPW 92-1
Variational solution of the Yang-Mills Schr\"odinger equation in Coulomb gauge
The Yang-Mills Schr\"odinger equation is solved in Coulomb gauge for the
vacuum by the variational principle using an ansatz for the wave functional,
which is strongly peaked at the Gribov horizon. A coupled set of
Schwinger-Dyson equations for the gluon and ghost propagators in the Yang-Mills
vacuum as well as for the curvature of gauge orbit space is derived and solved
in one-loop approximation. We find an infrared suppressed gluon propagator, an
infrared singular ghost propagator and a almost linearly rising confinement
potential.Comment: 24 pages, revtex, 13 figure
Conformal fields in the pp-wave limit
The pp-wave (Penrose limit) in conformal field theory can be viewed as a
special contraction of the unitary representations of the conformal group. We
study the kinematics of conformal fields in this limit in a geometric approach
where the effect of the contraction can be visualized as an expansion of
space-time. We discuss the two common models of space-time as carrier spaces
for conformal fields: One is the usual Minkowski space and the other is the
coset of the conformal group over its maximal compact subgroup. We show that
only the latter manifold and the corresponding conformal representation theory
admit a non-singular contraction limit. We also address the issue of
correlation functions of conformal fields in the pp-wave limit. We show that
they have a well-defined contraction limit if their space-time dependence
merges with the dependence on the coordinates of the R symmetry group. This is
a manifestation of the fact that in the limit the space-time and R symmetries
become indistinguishable. Our results might find applications in actual
calculations of correlation functions of composite operators in N=4 super
Yang-Mills theory.Comment: LaTex, 32 pages, 1 figure, discussion of correlation functions
extended; some corrections made; references adde
Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices
We have fabricated electric double-layer field-effect devices to
electrostatically dope our active materials, either =0.015
GaMnAs or =3.2 GaBeAs. The devices
are tailored for interrogation of electric field induced changes to the
frequency dependent conductivity in the accumulation or depletions layers of
the active material via infrared (IR) spectroscopy. The spectra of the
(Ga,Be)As-based device reveal electric field induced changes to the IR
conductivity consistent with an enhancement or reduction of the Drude response
in the accumulation and depletion polarities, respectively. The spectroscopic
features of this device are all indicative of metallic conduction within the
GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show
enhancement of the far-IR itinerant carrier response and broad mid-IR resonance
upon hole accumulation, with a decrease of these features in the depletion
polarity. These later spectral features demonstrate that conduction in
ferromagnetic (FM) GaMnAs is distinct from genuine metallic
behavior due to extended states in the host VB. Furthermore, these data support
the notion that a Mn-induced impurity band plays a vital role in the electron
dynamics of FM GaMnAs. We add, a sum-rule analysis of the spectra
of our devices suggests that the Mn or Be doping does not lead to a substantial
renormalization of the GaAs host VB
Nitric Acid Particles in Cold Thick Ice Clouds Observed at Global Scale: Link with Lightning, Temperature, and Upper Tropospheric Water Vapor
Signatures of nitric acid particles (NAP) in cold thick ice clouds have been derived from satellite observations. Most NAP are detected in the Tropics (9 to 20% of clouds with T less than 202.5 K). Higher occurrences were found in the rare mid-latitudes very cold clouds. NAP occurrence increases as cloud temperature decreases and NAP are more numerous in January than July. Comparisons of NAP and lightning distributions show that lightning is the main source of the NOx, which forms NAP in cold clouds. Qualitative comparisons of NAP with upper tropospheric humidity distributions suggest that NAP play a role in the dehydration of the upper troposphere when the tropopause is colder than 195K
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