4,203 research outputs found
Spectacular Role of Electron Correlation in the Hyperfine Interactions in States in Alkaline Earth Ions
The low-lying n(=3,4,5)d states alkaline earth ions are of vital
importance in a number of different physical applications. The hyperfine
structure constants of these states are characterized by unusually strong
electron correlation effects. Relativistic coupled-cluster theory has been
employed to carry out {\it ab initio} calculations of these constants. The role
of the all order core-polarization effects was found to be decisive in
obtaining good agreement of the results of our calculations with accurate
measurements. The present work is an apt demonstration of the power of the
coupled-cluster method to cope with strongly interacting configurations.Comment: Submitted to Physical Review Letters, 3 figures and 5 table
Origin of the butterfly magnetoresistance in a Dirac nodal-line system
We report a study on the magnetotransport properties and on the Fermi
surfaces (FS) of the ZrSi(Se,Te) semimetals. Density Functional Theory (DFT)
calculations, in absence of spin orbit coupling (SOC), reveal that both the Se
and the Te compounds display Dirac nodal lines (DNL) close to the Fermi level
at symmorphic and non-symmorphic positions, respectively. We
find that the geometry of their FSs agrees well with DFT predictions. ZrSiSe
displays low residual resistivities, pronounced magnetoresistivity, high
carrier mobilities, and a butterfly-like angle-dependent magnetoresistivity
(AMR), although its DNL is not protected against gap opening. As in
CdAs, its transport lifetime is found to be 10 to 10 times
larger than its quantum one. ZrSiTe, which possesses a protected DNL, displays
conventional transport properties. Our evaluation indicates that both compounds
most likely are topologically trivial. Nearly angle-independent effective
masses with strong angle dependent quantum lifetimes lead to the butterfly AMR
in ZrSiSe
Power-law corrections to entanglement entropy of horizons
We re-examine the idea that the origin of black-hole entropy may lie in the
entanglement of quantum fields between inside and outside of the horizon.
Motivated by the observation that certain modes of gravitational fluctuations
in a black-hole background behave as scalar fields, we compute the entanglement
entropy of such a field, by tracing over its degrees of freedom inside a
sphere. We show that while this entropy is proportional to the area of the
sphere when the field is in its ground state, a correction term proportional to
a fractional power of area results when the field is in a superposition of
ground and excited states. The area law is thus recovered for large areas.
Further, we identify location of the degrees of freedom that give rise to the
above entropy.Comment: 16 pages, 6 figures, to appear in Phys. Rev.
Precision spectroscopy with two correlated atoms
We discuss techniques that allow for long coherence times in laser
spectroscopy experiments with two trapped ions. We show that for this purpose
not only entangled ions prepared in decoherence-free subspaces can be used but
also a pair of ions that are not entangled but subject to the same kind of
phase noise. We apply this technique to a measurement of the electric
quadrupole moment of the 3d D5/2 state of 40Ca+ and to a measurement of the
linewidth of an ultrastable laser exciting a pair of 40Ca+ ions
Monge Distance between Quantum States
We define a metric in the space of quantum states taking the Monge distance
between corresponding Husimi distributions (Q--functions). This quantity
fulfills the axioms of a metric and satisfies the following semiclassical
property: the distance between two coherent states is equal to the Euclidean
distance between corresponding points in the classical phase space. We compute
analytically distances between certain states (coherent, squeezed, Fock and
thermal) and discuss a scheme for numerical computation of Monge distance for
two arbitrary quantum states.Comment: 9 pages in LaTex - RevTex + 2 figures in ps. submitted to Phys. Rev.
The Case for Reactive Mass Oral Cholera Vaccinations
Cholera outbreaks have had catastrophic impact on societies for centuries. Despite more than half a century of advocacy for safe water, sanitation and hygiene, approximately 100,000 cholera cases and 5,000 deaths were reported in Zimbabwe between August 2008 and by July 2009. Safe and effective oral cholera vaccines have been licensed and used by affluent tourists for more than a decade to prevent cholera. We asked whether oral cholera vaccines could be used to protect high risk populations at a time of cholera. We calculated how many cholera cases could have been prevented if mass cholera vaccinations would have been implemented in reaction to past cholera outbreaks. We estimate that determined, well organized mass vaccination campaigns could have prevented 34,900 (40%) cholera cases and 1,695 deaths (40%) in Zimbabwe. In the sites with endemic cholera, Kolkata and Zanzibar, a significant number of cases could have been prevented but the impact would have been less dramatic. The barriers which currently prevent the implementation of mass vaccinations, including but not only the cost to purchase the vaccine, seem insurmountable. A concerted effort of donors and key decision makers will be needed to offer better protection to populations at risk
Branching ratios of radiative transitions in O VI
We study the branching ratios of the allowed and forbidden radiative
transitions among the first few (9) fine structure levels of O VI using
relativistic coupled cluster theory. We find irregular patterns for a number of
transitions with in -complexes with . We have used the exisiting
values of the allowed electric dipole () transition as a benchmark of our
theory. Good agreement with the existing values establish accuracies of not
only the theoretical method but the basis function as well. In general the
electric quadrupole () transition probabilities are greater in magnitude
than magnetic dipole () transition probabilities, whereas for medium atomic
transition frequencies they are of the same order of magnitude. On the other
hand if the transitions involved are between two fine structure components of
the same term, then the transition probability is more probable than that
of . We have analyzed these trends with physical arguments and order of
magnitude estimations. The results presented here in tabular and graphical
forms are compared with the available theoretical and observed data. Graphical
analysis helps to understand the trends of electric and magnetic transitions
for the decay channels presented here. Our calculated values of the lifetimes
of the excited states are in very good agreement with the available results.Comment: Submitted to J. Phys. B, March 200
Non-minimal coupling of photons and axions
We establish a new self-consistent system of equations accounting for a
non-minimal interaction of gravitational, electromagnetic and axion fields. The
procedure is based on a non-minimal extension of the standard
Einstein-Maxwell-axion action. The general properties of a ten-parameter family
of non-minimal linear models are discussed. We apply this theory to the models
with pp-wave symmetry and consider propagation of electromagnetic waves
non-minimally coupled to the gravitational and axion fields. We focus on exact
solutions of electrodynamic equations, which describe quasi-minimal and
non-minimal optical activity induced by the axion field. We also discuss
empirical constraints on coupling parameters from astrophysical birefringence
and polarization rotation observations.Comment: 31 pages, 2 Tables; replaced with the final version published in
Classical and Quantum Gravit
A Natural Framework for Solar and 17 keV Neutrinos
Motivated by recent experimental claims for the existence of a 17 keV
neutrino and by the solar neutrino problem, we construct a class of models
which contain in their low-energy spectrum a single light sterile neutrino and
one or more Nambu-Goldstone bosons. In these models the required pattern of
breaking of lepton-number symmetry takes place near the electroweak scale and
all mass heirarchies are technically natural. The models are compatible with
all cosmological and astrophysical constraints, and can solve the solar
neutrino problem via either the MSW effect or vacuum oscillations. The deficit
in atmospheric muon neutrinos seen in the Kamiokande and IMB detectors can also
be explained in these models.Comment: 23 page
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