3,497,276 research outputs found
Coherent manipulation of cold Rydberg atoms near the surface of an atom chip
Coherent superpositions of the 49s and 48s Rydberg states of cold Rb atoms
were studied near the surface of an atom chip. The superpositions were created
and manipulated using microwaves resonant with the two-photon 49s-48s
transition. Coherent behavior was observed using Rabi flopping, Ramsey
sequences, spin-echo and spin-locking. These results are discussed in the
context of Rydberg atoms as electric field noise sensors. We consider the
coherence of systems quadratically coupled to noise fields with 1/f^k power
spectral densities (k \approx 1).Comment: 11 pages, 7 figure
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
A recently developed self-consistent effective medium approximation, for
composites with a columnar microstructure, is applied to such a
three-constituent mixture of isotropic normal conductor, perfect insulator, and
perfect conductor, where a strong magnetic field {\bf B} is present in the
plane perpendicular to the columnar axis. When the insulating and perfectly
conducting constituents do not percolate in that plane, the
microstructure-induced in-plane magnetoresistance is found to saturate for
large {\bf B}, if the volume fraction of the perfect conductor is greater
than that of the perfect insulator . By contrast, if , that
magnetoresistance keeps increasing as without ever saturating. This
abrupt change in the macroscopic response, which occurs when , is a
critical point, with the associated critical exponents and scaling behavior
that are characteristic of such points. The physical reasons for the singular
behavior of the macroscopic response are discussed. A new type of percolation
process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur
Is there a prescribed parameter's space for the adiabatic geometric phase?
The Aharonov-Anandan and Berry phases are determined for the cyclic motions
of a non-relativistic charged spinless particle evolving in the superposition
of the fields produced by a Penning trap and a rotating magnetic field.
Discussion about the selection of the parameter's space and the relationship
between the Berry phase and the symmetry of the binding potential is given.Comment: 7 pages, 2 figure
The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study
We investigate the ground-state magnetic order of the spin-1/2 J1-J2
Heisenberg model on the square lattice with ferromagnetic nearest-neighbor
exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange
J2>0. We use the coupled-cluster method to high orders of approximation and
Lanczos exact diagonalization of finite lattices of up to N=40 sites in order
to calculate the ground-state energy, the spin-spin correlation functions, and
the magnetic order parameter. We find that the transition point at which the
ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact
diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our
results for ferromagnetic J1 with established results for the spin-1/2 J1-J2
Heisenberg model with antiferromagnetic J1. We find that both models (i.e.,
ferro- and antiferromagnetic J1) behave similarly for large J2, although
significant differences between them are observed for J2/|J1| \lesssim 0.6.
Although the semiclassical collinear magnetic long-range order breaks down at
J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar
breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model
with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an
intermediate disordered phase does occur between the phases exhibiting
semiclassical collinear stripe order and ferromagnetic order for ferromagnetic
J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.Comment: 15 pages, 7 figures, 2 table
General study of superscaling in quasielastic and reactions using the relativistic impulse approximation
The phenomenon of superscaling for quasielastic lepton induced reactions at
energies of a few GeV is investigated within the framework of the relativistic
impulse approximation. A global analysis of quasielastic inclusive electron and
charged-current neutrino scattering reactions on nuclei is presented. Scaling
and superscaling properties are shown to emerge from both types of processes.
The crucial role played by final state interactions is evaluated by using
different approaches. The asymmetric shape presented by the experimental
scaling function, with a long tail in the region of positive values of the
scaling variable, is reproduced when the interaction in the final state between
the knockout nucleon and the residual nucleus is described within the
relativistic mean field approach. The impact of gauge ambiguities and off-shell
effects in the scaling function is also analyzed.Comment: 34 pages, 14 figures, accepted in Phys. Rev. C. Section II has been
shortene
Simple Types of Anisotropic Inflation
We display some simple cosmological solutions of gravity theories with
quadratic Ricci curvature terms added to the Einstein-Hilbert lagrangian which
exhibit anisotropic inflation. The Hubble expansion rates are constant and
unequal in three orthogonal directions. We describe the evolution of the
simplest of these homogeneous and anisotropic cosmological models from its
natural initial state and evaluate the deviations they will create from
statistical isotropy in the fluctuations produced during a period of
anisotropic inflation. The anisotropic inflation is not a late-time attractor
in these models but the rate of approach to a final isotropic de Sitter state
is slow and is conducive to the creation of observable anisotropic statistical
effects in the microwave background. The statistical anisotropy would not be
scale invariant and the level of statistical anisotropy will grow with scale.Comment: 8pages, 3 figs v2:refs added, typos fixe
Smoothed Particle Magnetohydrodynamics (some shocking results...)
There have been some issues in the past in attempts to simulate magnetic
fields using the Smoothed Particle Hydrodynamics (SPH) method. SPH is well
suited to star formation problems because of its Lagrangian nature. We present
new, stable and conservative methods for magnetohydrodynamics (MHD) in SPH and
present numerical tests on both waves and shocks in one dimension to show that
it gives robust and accurate results.Comment: Kluwer latex, 6 pages, 3 figures; Proceedings of the International
Workshop "Magnetic Fields and Star Formation: Theory vs Observations",
Madrid, 21-25 April 2003. Revised version accepted to proceedings (exact
solutions added, other minor changes
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