30,007 research outputs found
The Role of Crystal Symmetry in the Magnetic Instabilities of -YbAlB and -YbAlB
Density functional theory methods are applied to investigate the properties
of the new superconductor -YbAlB and its polymorph
-YbAlB. We utilize the generalized gradient approximation + Hubbard
U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of
the strong correlations due to the open shell of Yb. We examine closely
the differences in crystal bonding and symmetry of -YbAlB and
-YbAlB. The in-plane bonding structure amongst the dominant
itinerant electrons in the boron sheets is shown to differ significantly. Our
calculations indicate that, in both polymorphs, the localized 4 electrons
hybridize strongly with the conduction sea when compared to the related
materials YbRhSi and YbB. Comparing -YbAlB to the
electronic structure of related crystal structures indicates a key role of the
7-member boron coordination of the Yb ion in -YbAlB in producing its
enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend
strongly on the angle between the B neighbors and the Yb ion, relative to the
plane, which relates some of the physical behavior to structural
characteristics.Comment: 9 pages, 9 figures, 2 table
Spectral functions of the Falicov-Kimball model with electronic ferroelectricity
We calculate the angular resolved photoemission spectrum of the
Falicov-Kimball model with electronic ferroelectricity where - and
-electrons have different hoppings. In mix-valence regimes, the presence of
strong scattering processes between - excitons and a hole, created by
emission of an electron, leads to the formation of pseudospin polarons and
novel electronic structures with bandwidth scaling with that of -
excitons. Especially, in the two-dimensional case, we find that flat regions
exist near the bottom of the quasiparticle band in a wide range of the - and
-level energy difference.Comment: 5 pages, 5 figure
Universal Tomonaga-Luttinger liquid phases in one-dimensional strongly attractive SU(N) fermionic cold atoms
A simple set of algebraic equations is derived for the exact low-temperature
thermodynamics of one-dimensional multi-component strongly attractive fermionic
atoms with enlarged SU(N) spin symmetry and Zeeman splitting. Universal
multi-component Tomonaga-Luttinger liquid (TLL) phases are thus determined. For
linear Zeeman splitting, the physics of the gapless phase at low temperatures
belongs to the universality class of a two-component asymmetric TLL
corresponding to spin-neutral N-atom composites and spin-(N-1)/2 single atoms.
The equation of states is also obtained to open up the study of multi-component
TLL phases in 1D systems of N-component Fermi gases with population imbalance.Comment: 12 pages, 3 figure
In situ XRF and gamma ray spectrometer for Mars sample return mission
A combined in situ X-ray fluorescence (XRF) and passive gamma ray spectrometer instrument is proposed for the chemical elemental analysis of various Martian surfaces and samples. The combined instrument can be carried on board a rover. The passive gamma ray or the neutron excited gamma ray system would be used to determine the elemental composition of the Martian surface while the rover is in motion. The XRF system would be used to perform analysis either on the Martian surface or on collected samples when the rover is stationary. The latter function is important both in cataloging the collected samples and in the selection of samples to be returned to earth. For both systems, data accumulation time would be on the order of 30 minutes. No sample preparation would be necessary
Charge-to-spin conversion of electron entanglement states and spin-interaction-free solid-state quantum computation
Without resorting to spin-spin coupling, we propose a scalable spin quantum
computing scheme assisted with a semiconductor multiple-quantum-dot structure.
The techniques of single electron transitions and the nanostructure of
quantum-dot cellular automata (QCA) are used to generate charge entangled
states of two electrons, which are then converted into spin entanglement states
using single-spin rotations only. Deterministic two-qubit quantum gates are
also manipulated using only single-spin rotations with the help of QCA. A
single-shot readout of spin states can be carried out by coupling the multiple
dot structure to a quantum point contact. As a result, deterministic
spin-interaction-free quantum computing can be implemented in semiconductor
nanostructure.Comment: 5 pages, 4 figures, the revised version of quant-ph/0502002 for
publication in Phys. Rev. B (to be appear on the issue of Oct. 15, 2007
High-energy kink in high-temperature superconductors
In conventional metals, electron-phonon coupling, or the phonon-mediated
interaction between electrons, has long been known to be the pairing
interaction responsible for the superconductivity. The strength of this
interaction essentially determines the superconducting transition temperature
TC. One manifestation of electron-phonon coupling is a mass renormalization of
the electronic dispersion at the energy scale associated with the phonons. This
renormalization is directly observable in photoemission experiments. In
contrast, there remains little consensus on the pairing mechanism in cuprate
high temperature superconductors. The recent observation of similar
renormalization effects in cuprates has raised the hope that the mechanism of
high temperature superconductivity may finally be resolved. The focus has been
on the low energy renormalization and associated "kink" in the dispersion at
around 50 meV. However at that energy scale, there are multiple candidates
including phonon branches, structure in the spin-fluctuation spectrum, and the
superconducting gap itself, making the unique identification of the excitation
responsible for the kink difficult. Here we show that the low-energy
renormalization at ~50 meV is only a small component of the total
renormalization, the majority of which occurs at an order of magnitude higher
energy (~350 meV). This high energy kink poses a new challenge for the physics
of the cuprates. Its role in superconductivity and relation to the low-energy
kink remains to be determined.Comment: 13 pages, 4 figure
Potential Models and Lattice Gauge Current-Current Correlators
We compare current-current correlators in lattice gauge calculations with
correlators in different potential models, for a pseudoscalar charmonium in the
quark-gluon plasma. An important ingredient in the evaluation of the
current-current correlator in the potential model is the basic principle that
out of the set of continuum states, only resonance states and Gamow states with
lifetimes of sufficient magnitudes can propagate as composite objects and can
contribute to the current-current correlator. When the contributions from the
bound states and continuum states are properly treated, the potential model
current-current correlators obtained with the potential proposed in Ref. [11]
are consistent with the lattice gauge correlators. The proposed potential model
thus gains support to be a useful tool to complement lattice gauge calculations
for the study of states at high temperatures.Comment: 18 pages, 4 figures, to be published in Physcial Review
Bayesian Identification of Elastic Constants in Multi-Directional Laminate from Moir\'e Interferometry Displacement Fields
The ply elastic constants needed for classical lamination theory analysis of
multi-directional laminates may differ from those obtained from unidirectional
laminates because of three dimensional effects. In addition, the unidirectional
laminates may not be available for testing. In such cases, full-field
displacement measurements offer the potential of identifying several material
properties simultaneously. For that, it is desirable to create complex
displacement fields that are strongly influenced by all the elastic constants.
In this work, we explore the potential of using a laminated plate with an
open-hole under traction loading to achieve that and identify all four ply
elastic constants (E 1, E 2, 12, G 12) at once. However, the accuracy of the
identified properties may not be as good as properties measured from individual
tests due to the complexity of the experiment, the relative insensitivity of
the measured quantities to some of the properties and the various possible
sources of uncertainty. It is thus important to quantify the uncertainty (or
confidence) with which these properties are identified. Here, Bayesian
identification is used for this purpose, because it can readily model all the
uncertainties in the analysis and measurements, and because it provides the
full coupled probability distribution of the identified material properties. In
addition, it offers the potential to combine properties identified based on
substantially different experiments. The full-field measurement is obtained by
moir\'e interferometry. For computational efficiency the Bayesian approach was
applied to a proper orthogonal decomposition (POD) of the displacement fields.
The analysis showed that the four orthotropic elastic constants are determined
with quite different confidence levels as well as with significant correlation.
Comparison with manufacturing specifications showed substantial difference in
one constant, and this conclusion agreed with earlier measurement of that
constant by a traditional four-point bending test. It is possible that the POD
approach did not take full advantage of the copious data provided by the full
field measurements, and for that reason that data is provided for others to use
(as on line material attached to the article)
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