17,762 research outputs found
Blackbody radiation shift in 87Rb frequency standard
The operation of atomic clocks is generally carried out at room temperature,
whereas the definition of the second refers to the clock transition in an atom
at absolute zero. This implies that the clock transition frequency should be
corrected in practice for the effect of finite temperature of which the leading
contributor is the blackbody radiation (BBR) shift. Experimental measurements
of the BBR shifts are difficult. In this work, we have calculated the blackbody
radiation shift of the ground-state hyperfine microwave transition in 87Rb
using the relativistic all-order method and carried out detailed evaluation of
the accuracy of our final value. Particular care is taken to accurately account
for the contributions from highly-excited states. Our predicted value for the
Stark coefficient, k_S=-1.240(4)\times 10^{-10}\text{Hz/(V/m)}^{2} is three
times more accurate than the previous calculation [1].Comment: 7 page
Information Use by PhD Students in Agriculture and Biology: A Dissertation Citation Analysis
This article reports the findings of a study conducted to examine the types of information used by graduate students in the fields of biological and agricultural sciences at Iowa State University (ISU). The citations of doctoral dissertations submitted in nine agriculture and biological science subject fields (crop production and physiology; molecular, cellular, and developmental biology; entomology; genetics; microbiology; plant breeding; plant pathology; plant physiology; and soil science) at ISU from 1997–2006 were analyzed. The article discusses the types and ages of resources cited in the different subject fields studied. The most cited journals in each discipline were identified, and the journal title dispersion was examined
Foldy-Wouthuysen Transformation in Strong Magnetic Fields and Relativistic Corrections for Quantum Cyclotron Energy Levels
We carry out a direct, iterative Foldy--Wouthuysen transformation of a
general Dirac Hamiltonian coupled to an electromagnetic field, including the
anomalous magnetic moment. The transformation is carried out through an
iterative disentangling of the particle and antiparticle Hamiltonians, in the
expansion for higher orders of the momenta. The time-derivative term from the
unitary transformation is found to be crucial in supplementing the transverse
component of the electric field in higher orders. Final expressions are
obtained for general combined electric and magnetic fields, including strong
magnetic fields. The time-derivative of the electric field is shown to enter
only in the seventh order of the fine-structure constant if the transformation
is carried out in the standard fashion. We put special emphasis on the case of
strong fields, which are important for a number of applications, such as
electrons bound in Penning traps.Comment: 12 pages; RevTe
Intraoperative Organ Motion Models with an Ensemble of Conditional Generative Adversarial Networks
In this paper, we describe how a patient-specific, ultrasound-probe-induced
prostate motion model can be directly generated from a single preoperative MR
image. Our motion model allows for sampling from the conditional distribution
of dense displacement fields, is encoded by a generative neural network
conditioned on a medical image, and accepts random noise as additional input.
The generative network is trained by a minimax optimisation with a second
discriminative neural network, tasked to distinguish generated samples from
training motion data. In this work, we propose that 1) jointly optimising a
third conditioning neural network that pre-processes the input image, can
effectively extract patient-specific features for conditioning; and 2)
combining multiple generative models trained separately with heuristically
pre-disjointed training data sets can adequately mitigate the problem of mode
collapse. Trained with diagnostic T2-weighted MR images from 143 real patients
and 73,216 3D dense displacement fields from finite element simulations of
intraoperative prostate motion due to transrectal ultrasound probe pressure,
the proposed models produced physically-plausible patient-specific motion of
prostate glands. The ability to capture biomechanically simulated motion was
evaluated using two errors representing generalisability and specificity of the
model. The median values, calculated from a 10-fold cross-validation, were
2.8+/-0.3 mm and 1.7+/-0.1 mm, respectively. We conclude that the introduced
approach demonstrates the feasibility of applying state-of-the-art machine
learning algorithms to generate organ motion models from patient images, and
shows significant promise for future research.Comment: Accepted to MICCAI 201
Excitation energies, hyperfine constants, E1, E2, M1 transition rates, and lifetimes of (6s2)nl states in Tl I and Pb II
Energies of np (n=6-9), ns (n=7-9), nd (n=6-8), and nf (n=5-6) states in Tl I
and Pb II are obtained using relativistic many-body perturbation theory.
Reduced matrix elements, oscillator strengths, transition rates, and lifetimes
are determined for the 72 possible electric-dipole transitions.
Electric-quadrupole and magnetic-dipole matrix elements are evaluated to obtain
np(3/2) - mp(1/2) (n,m=6,7) transition rates. Hyperfine constants A are
evaluated for a number of states in 205Tl. First-, second-, third-, and
all-order corrections to the energies and matrix elements and first- and
second-order Breit corrections to energies are calculated. In our
implementation of the all-order method, single and double excitations of
Dirac-Fock wave functions are included to all orders in perturbation theory.
These calculations provide a theoretical benchmark for comparison with
experiment and theory.Comment: twelve tables, no figure
Quantum walks on Cayley graphs
We address the problem of the construction of quantum walks on Cayley graphs.
Our main motivation is the relationship between quantum algorithms and quantum
walks. In particular, we discuss the choice of the dimension of the local
Hilbert space and consider various classes of graphs on which the structure of
quantum walks may differ. We completely characterise quantum walks on free
groups and present partial results on more general cases. Some examples are
given, including a family of quantum walks on the hypercube involving a
Clifford Algebra.Comment: J. Phys. A (accepted for publication
Intrachromosomal excision of a hybrid Ds element induces large genomic deletions in Arabidopsis
Transposon activity is known to cause chromosome rearrangements in the host genome. Surprisingly, extremely little is known about Dissociation (Ds)-induced chromosome rearrangements in Arabidopsis, where Ds is intensively used for insertional mutagenesis. Here, we describe three Arabidopsis mutants with reduced fertility and propose that excision of a hybrid Ds element induced a large genomic deletion flanking Ds. In the mutants anat and haumea, the deletion mechanism consists of a local Ds transposition from replicated into unreplicated DNA followed by Ds excision, where one end of the newly transposed element and one end of the Ds transposon at the donor site served as substrate for transposase. Excision of this hybrid element reminiscent of a macrotransposon leads to loss of the chromosomal piece located between the two ends, including one full Ds element and the flanking genomic sequence. This mechanism was found to be responsible for several other deletions and occurs at a genetically trackable frequency. Thus, it could be applied to efficiently generate deletions of various sizes in the vicinity of any existing Ds element present in the genome. In the mutant tons missing, a mechanism that involves endogenous repetitive sequences caused a large flanking deletion at a position unlinked to the starter locus. Our study of Ds transposition in Arabidopsis revealed previously undescribed mechanisms that lead to large genomic deletions flanking Ds elements, which may contribute to genome dynamics and evolution
Relativistic coupled-cluster single-double method applied to alkali-metal atoms
A relativistic version of the coupled-cluster single-double (CCSD) method is
developed for atoms with a single valence electron. In earlier work, a
linearized version of the CCSD method (with extensions to include a dominant
class of triple excitations) led to accurate predictions for energies,
transition amplitudes, hyperfine constants, and other properties of monovalent
atoms. Further progress in high-precision atomic structure calculations for
heavy atoms calls for improvement of the linearized coupled-cluster
methodology. In the present work, equations for the single and double
excitation coefficients of the Dirac-Fock wave function, including all
non-linear coupled-cluster terms that contribute at the single-double level are
worked out. Contributions of the non-linear terms to energies, electric-dipole
matrix elements, and hyperfine constants of low-lying states in alkali-metal
atoms from Li to Cs are evaluated and the results are compared with other
calculations and with precise experiments.Comment: 12 page
Non-equilibrium tunneling into general quantum Hall edge states
In this paper we formulate the theory of tunneling into general Abelian
fractional quantum Hall edge states. In contrast to the simple Laughlin states,
a number of charge transfer processes must be accounted for. Nonetheless, it is
possible to identify a unique value corresponding to dissipationless transport
as the asymptotic large- conductance through a tunneling junction, and find
fixed points (CFT boundary conditions) corresponding to this value. The
symmetries of a given edge tunneling problem determine the appropriate boundary
condition, and the boundary condition determines the strong-coupling operator
content and current noise.Comment: 6 pages, 3 figures; published versio
Rare manifestation of a c.290 C\u3eT, p.Gly97Glu VCP mutation
Introduction. The valosin-containing protein (VCP) regulates several distinct cellular processes. Consistent with this, VCP mutations manifest variable clinical phenotypes among and within families and are a diagnostic challenge. Methods. A 60-year-old man who played ice hockey into his 50’s was evaluated by electrodiagnostics, muscle biopsy, and molecular genetics. Results. With long-standing pes cavus and toe walking, our patient developed progressive weakness, cramps, memory loss, and paresthesias at age 52. An axonal sensorimotor neuropathy was found upon repeated testing at age 58. Neuropathic histopathology was present in the quadriceps, and exome sequencing revealed the VCP mutation c.290 C>T, p.Gly97Glu. Conclusions. Our patient reflects the clinical heterogeneity of VCP mutations, as his neurological localization is a spectrum between a lower motor neuron disorder and a hereditary axonal peripheral neuropathy such as CMT2. Our case demonstrates a rare manifestation of the c.290 C>T, pGly97Glu VCP mutation
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