9,437 research outputs found
Hierarchical Wigner Crystal at the Edge of Quantum Hall Bar
We show that quasiholes persist near the edge of incompressible Quantum Hall
state forming a Wigner structure. The average density of quasiholes is fixed by
electrostatics and decreases slowly with increasing distance from the edge. As
we see from elementary reasoning, their specific arrangement can not be a
regular Wigner lattice and shows a complex hierarchical structure of
dislocations.Comment: LaTEX file. Ps figures upon reques
Optimal Axes of Siberian Snakes for Polarized Proton Acceleration
Accelerating polarized proton beams and storing them for many turns can lead
to a loss of polarization when accelerating through energies where a spin
rotation frequency is in resonance with orbit oscillation frequencies.
First-order resonance effects can be avoided by installing Siberian Snakes in
the ring, devices which rotate the spin by 180 degrees around the snake axis
while not changing the beam's orbit significantly. For large rings, several
Siberian Snakes are required.
Here a criterion will be derived that allows to find an optimal choice of the
snake axes. Rings with super-period four are analyzed in detail, and the HERA
proton ring is used as an example for approximate four-fold symmetry. The
proposed arrangement of Siberian Snakes matches their effects so that all
spin-orbit coupling integrals vanish at all energies and therefore there is no
first-order spin-orbit coupling at all for this choice, which I call snakes
matching. It will be shown that in general at least eight Siberian Snakes are
needed and that there are exactly four possibilities to arrange their axes.
When the betatron phase advance between snakes is chosen suitably, four
Siberian Snakes can be sufficient.
To show that favorable choice of snakes have been found, polarized protons
are tracked for part of HERA-p's acceleration cycle which shows that
polarization is preserved best for the here proposed arrangement of Siberian
Snakes.Comment: 14 pages, 16 figure
Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb
Scattering of spin-up and spin-down electrons while passing through a
ferromagnetic domain wall leads to an additional resistance for transport
current, usually observed prominently in constricted magnetic structures. In
this report, we use the resistance of the domain wall as a probe to find
indirect signatures of the theoretically predicted spin-singlet supercurrent to
spin-triplet supercurrent conversion effect of ferromagnetic domain walls. Here
we examine the domain wall induced resistance in Ni stripe in a bilayer Ni/Nb
geometry in the normal state and in the superconducting state of Nb. By making
a 6um wide gap in the top Nb layer we routed the transport current through the
Ni layer in the normal state and in the superconducting state of Nb. In the
normal state of Nb, in-field transport measurements showed a clear domain wall
magneto-resistance (DWMR) peak near the coercive field, where the domain wall
density is expected to be maximum. Interestingly, however, below the
superconducting transition temperature of Nb, the DWMR peak of the Ni layer
showed a sharp drop in the field range where the number of domain walls becomes
maximum. This observation may be a possible signature of magnetic domain wall
induced spin-triplet correlations in the Ni layer due to the direct injection
of spin-singlet Cooper pairs from Nb into the magnetic domain walls
Magnetic field-induced spectroscopy of forbidden optical transitions with application to lattice-based optical atomic clocks
We develop a method of spectroscopy that uses a weak static magnetic field to
enable direct optical excitation of forbidden electric-dipole transitions that
are otherwise prohibitively weak. The power of this scheme is demonstrated
using the important application of optical atomic clocks based on neutral atoms
confined to an optical lattice. The simple experimental implementation of this
method -- a single clock laser combined with a DC magnetic field-- relaxes
stringent requirements in current lattice-based clocks (e.g., magnetic field
shielding and light polarization), and could therefore expedite the realization
of the extraordinary performance level predicted for these clocks. We estimate
that a clock using alkaline earth-like atoms such as Yb could achieve a
fractional frequency uncertainty of well below 10^-17 for the metrologically
preferred even isotopes
Optical Lattice Induced Light Shifts in an Yb Atomic Clock
We present an experimental study of the lattice induced light shifts on the
1S_0-3P_0 optical clock transition (v_clock~518 THz) in neutral ytterbium. The
``magic'' frequency, v_magic, for the 174Yb isotope was determined to be 394
799 475(35)MHz, which leads to a first order light shift uncertainty of 0.38 Hz
on the 518 THz clock transition. Also investigated were the hyperpolarizability
shifts due to the nearby 6s6p 3P_0 - 6s8p 3P_0, 6s8p 3P_2, and 6s5f 3F_2
two-photon resonances at 759.708 nm, 754.23 nm, and 764.95 nm respectively. By
tuning the lattice frequency over the two-photon resonances and measuring the
corresponding clock transition shifts, the hyperpolarizability shift was
estimated to be 170(33) mHz for a linear polarized, 50 uK deep, lattice at the
magic wavelength. In addition, we have confirmed that a circularly polarized
lattice eliminates the J=0 - J=0 two-photon resonance. These results indicate
that the differential polarizability and hyperpolarizability frequency shift
uncertainties in a Yb lattice clock could be held to well below 10^-17.Comment: Accepted to PR
A Study of an Inverted Wing with Endplates in Ground Effect
The aerodynamics of an inverted wing with endplates in ground effect has been investigated using experimental methods. A wind tunnel containing a moving belt was used to reproduce the effect of the ground on the flow field and the measurements were made using Laser Doppler Anemometry (LDA). Flow visualization showed that the implementation of an endplate on a wing yields the formation of two corotating vortices originating from the top and the bottom of the endplate. Initially the effect of the size of the endplate and the height of the wing above the ground on the vortices behaviour was studied. Then, the most relevant configuration of the wing was studied using LDA measurements
Phase structure and monopoles in U(1) gauge theory
We investigate the phase structure of pure compact U(1) lattice gauge theory
in 4 dimensions with the Wilson action supplemented by a monopole term. To
overcome the suppression of transitions between the phases in the simulations
we make the monopole coupling a dynamical variable. We determine the phase
diagram and find that the strength of the first order transition decreases with
increasing weight of the monopole term, the transition thus ultimately getting
of second order. After outlining the appropriate topological characterization
of networks of currents lines, we present an analysis of the occurring monopole
currents which shows that the phases are related to topological properties.Comment: 22 pages (latex), 14 figures (available upon request), BU-HEP 94-
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
Scaling of geometric phases close to quantum phase transition in the XY chain
We show that geometric phase of the ground state in the XY model obeys
scaling behavior in the vicinity of a quantum phase transition. In particular
we find that geometric phase is non-analytical and its derivative with respect
to the field strength diverges at the critical magnetic field. Furthermore,
universality in the critical properties of the geometric phase in a family of
models is verified. In addition, since quantum phase transition occurs at a
level crossing or avoided level crossing and these level structures can be
captured by Berry curvature, the established relation between geometric phase
and quantum phase transitions is not a specific property of the XY model, but a
very general result of many-body systems.Comment: 4 page
Direct excitation of the forbidden clock transition in neutral 174Yb atoms confined to an optical lattice
We report direct single-laser excitation of the strictly forbidden
(6s^2)^1S_0 -(6s6p)^3P_0 clock transition in the even 174Yb isotope confined to
a 1D optical lattice. A small (~1.2 mT) static magnetic field was used to
induce a nonzero electric dipole transition probability between the clock
states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FHWM) with high
contrast were observed, demonstrating a record neutral-atom resonance quality
factor of 2.6x10^13. The previously unknown ac Stark shift-canceling (magic)
wavelength was determined to be 759.35+/-0.02 nm. This method for using the
metrologically superior even isotope can be easily implemented in current Yb
and Sr lattice clocks, and can create new clock possibilities in other alkaline
earth-like atoms such as Mg and Ca.Comment: Submitted to Physics Review Letter
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