4,383 research outputs found
Polaron Crystallization and Melting: Effects of the Long-Range Coulomb Forces
On examining the stability of a Wigner crystal in an ionic dielectric, two
competitive effects due to the polaron formation are found to be important: (i)
the screening of the Coulomb force, which destabilizes the crystal, compensated
by (ii) the increase of the carrier mass (polaron mass). The competition
between the two effects is carefully studied, and the quantum melting of the
polaronic Wigner crystal is examined by varying the density at zero
temperature. By calculating the quantum fluctuations of both the electron and
the polarization, we show that there is a competition between the dissociation
of the polarons at the insulator-to-metal transition (IMT), and a melting
towards a polaron liquid. We find that at strong coupling, a liquid state of
dielectric polarons cannot exist, and the IMT is driven by the polaron
dissociation. Next, taking into account the dipolar interactions between
localized carriers, we show that these are responsible for an instability of
the transverse vibrational modes of the polaronic Wigner crystal as the density
increases. This provides a new mechanism for the IMT in doped dielectrics,
which yields interesting dielectric properties below and beyond the transition.
An optical signature of such a mechanism for the IMT is provided.Comment: 10 pages, 3 figures, to be published in Int.J.Mod.Phys.
Ellipsoidal Coulomb Crystals in a Linear Radiofrequency Trap
A static quadrupole potential breaks the cylindrical symmetry of the
effective potential of a linear rf trap. For a one-component fluid plasma at
low temperature, the resulting equilibrium charge distribution is predicted to
be an ellipsoid. We have produced laser-cooled Be ellipsoidal ion crystals
and found good agreement between their shapes and the cold fluid prediction. In
two-species mixtures, containing Be and sympathetically cooled ions of
lower mass, a sufficiently strong static quadrupole potential produces a
spatial separation of the species.Comment: 4 pages, 3 figure
Magnetism and the Weiss Exchange Field - A Theoretical Analysis Inspired by Recent Experiments
The huge spin precession frequency observed in recent experiments with
spin-polarized beams of hot electrons shot through magnetized films is
interpreted as being caused by Zeeman coupling of the electron spins to the
so-called Weiss exchange field in the film. A "Stern-Gerlach experiment" for
electrons moving through an inhomogeneous exchange field is proposed. The
microscopic origin of exchange interactions and of large mean exchange fields,
leading to different types of magnetic order, is elucidated. A microscopic
derivation of the equations of motion of the Weiss exchange field is presented.
Novel proofs of the existence of phase transitions in quantum XY-models and
antiferromagnets, based on an analysis of the statistical distribution of the
exchange field, are outlined.Comment: 36 pages, 3 figure
The Pierre Auger Project and Enhancements
The current status of the scientific results of the Auger Observatory will be
discussed which include spectrum, anisotropy in arrival directions, chemical
composition analyses, and limits on neutrino and photon fluxes. A review of the
Observatory detection systems will be presented. Auger has started the
construction of its second phase which encompasses antennae for radio detection
of cosmic rays, high-elevation telescopes, and surface plus muon detectors.
Details will be presented on the latter, AMIGA (Auger Muons and Infill for the
Ground Array), an Auger project consisting of 85 detector pairs each one
composed of a surface water-Cherenkov detector and a buried muon counter. The
detector pairs are arranged in an array with spacings of 433 and 750 m in order
to perform a detailed study of the 10^17 eV to 10^19 eV spectrum region.
Preliminary results on the performance of the 750 m array of surface detectors
and the first muon counter prototype will be presented.Comment: 10 pages, 8 figures, VIII Latin American Symposium on Nuclear Physics
and Applications December 15-19, 2009, Santiago, Chil
Precision spectroscopy of the molecular ion HD+: control of Zeeman shifts
Precision spectroscopy on cold molecules can potentially enable novel tests
of fundamental laws of physics and alternative determination of some
fundamental constants. Realizing this potential requires a thorough
understanding of the systematic effects that shift the energy levels of
molecules. We have performed a complete ab initio calculation of the magnetic
field effects for a particular system, the heteronuclear molecular hydrogen ion
HD+. Different spectroscopic schemes have been considered, and numerous
transitions, all accessible by modern radiation sources and exhibiting well
controllable or negligible Zeeman shift, have been found to exist. Thus, HD+ is
a perspective candidate for determination of the ratio of electron-to-nuclear
reduced mass, and for tests of its time-independence.Comment: A Table added, references and figures update
Probing Non-Abelian Statistics in nu=12/5 Quantum Hall State
The tunneling current and shot noise of the current between two Fractional
Quantum Hall (FQH) edges in the FQH state in electronic
Mach-Zehnder interferometer are studied. It is shown that the tunneling current
and shot noise can be used to probe the existence of parafermion
statistics in the FQH state. More specifically, the dependence of
the current on the Aharonov-Bohm flux in the Read-Rezayi state is asymmetric
under the change of the sign of the applied voltage. This property is absent in
the Abelian Laughlin states. Moreover the Fano factor can exceed 12.7 electron
charges in the FQH state . This number well exceeds the maximum
possible Fano factor in all Laughlin states and the Moore-Read
state which was shown previously to be and respectively.Comment: 10 pages, 6 figure
Non-demolition measurements of observables with general spectra
It has recently been established that, in a non-demolition measurement of an
observable with a finite point spectrum, the density matrix of
the system approaches an eigenstate of , i.e., it "purifies" over
the spectrum of . We extend this result to observables with
general spectra. It is shown that the spectral density of the state of the
system converges to a delta function exponentially fast, in an appropriate
sense. Furthermore, for observables with absolutely continuous spectra, we show
that the spectral density approaches a Gaussian distribution over the spectrum
of . Our methods highlight the connection between the theory of
non-demolition measurements and classical estimation theory.Comment: 22 page
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