6,459 research outputs found
Combination Rules, Charge Symmetry, and Hall Effect in Cuprates
The rule relating the observed Hall coefficient to the spin and charge
responses of the uniform doped Mott insulator is derived. It is essential to
include the contribution of holon and spinon three-current correlations to the
effective action of the gauge field. In the vicinity of the Mott insulating
point the Hall coefficient is holon dominated and weakly temperature dependent.
In the vicinity of a point of charge conjugation symmetry the holon
contribution to the observed Hall coefficient is small: the Hall coefficient
follows the temperature dependence of the diamagnetic susceptibility with a
sign determined by the Fermi surface shape. NOTE: document prepared using
REVTEX. (3 Figs, not included, available on request from: [email protected])Comment: 8 page
Proximity effects and triplet correlations in Ferromagnet/Ferromagnet/Superconductor nanostructures
We report the results of a study of superconducting proximity effects in
clean Ferromagnet/Ferromagnet/Superconductor ()
heterostructures, where the pairing state in S is a conventional singlet
s-wave. We numerically find the self-consistent solutions of the Bogoliubov-de
Gennes (BdG) equations and use these solutions to calculate the relevant
physical quantities. By linearizing the BdG equations, we obtain the
superconducting transition temperatures as a function of the angle
between the exchange fields in and . We find that
the results for in systems are clearly different
from those in systems, where monotonically increases
with and is highest for antiparallel magnetizations. Here,
is in general a non-monotonic function, and often has a minimum
near . For certain values of the exchange field and
layer thicknesses, the system exhibits reentrant superconductivity with
: it transitions from superconducting to normal, and then returns to a
superconducting state again with increasing . This phenomenon is
substantiated by a calculation of the condensation energy. We compute, in
addition to the ordinary singlet pair amplitude, the induced odd triplet
pairing amplitudes. The results indicate a connection between equal-spin
triplet pairing and the singlet pairing state that characterizes . We find
also that the induced triplet amplitudes can be very long-ranged in both the S
and F sides and characterize their range. We discuss the average density of
states for both the magnetic and the S regions, and its relation to the pairing
amplitudes and . The local magnetization vector, which exhibits reverse
proximity effects, is also investigated.Comment: 14 pages including 11 figure
Interaction induced ferro-electricity in the rotational states of polar molecules
We show that a ferro-electric quantum phase transition can be driven by the
dipolar interaction of polar molecules in the presence a micro-wave field. The
obtained ferro-electricity crucially depends on the harmonic confinement
potential, and the resulting dipole moment persists even when the external
field is turned off adiabatically. The transition is shown to be second order
for fermions and for bosons of a smaller permanent dipole moment, but is first
order for bosons of a larger moment. Our results suggest the possibility of
manipulating the microscopic rotational state of polar molecules by tuning the
trap's aspect ratio (and other mesoscopic parameters), even though the later's
energy scale is smaller than the former's by six orders of magnitude.Comment: 4 pages and 4 figure
Spectacular Shells in the Host Galaxy of the QSO MC2 1635+119
We present deep HST/ACS images and Keck spectroscopy of MC2 1635+119, a QSO
hosted by a galaxy previously classified as an undisturbed elliptical. Our new
images reveal dramatic shell structure indicative of a merger event in the
relatively recent past. The brightest shells in the central regions of the host
are distributed alternately in radius, with at least two distinct shells on one
side of the nucleus and three on the other, out to a distance of ~13 kpc. The
light within the five shells comprises ~6% of the total galaxy light. Lower
surface brightness ripples or tails and other debris extend out to a distance
of ~65 kpc. A simple N-body model for a merger reproduces the inner shell
structure and gives an estimate for the age of the merger between ~30 Myr and
~1.7 Gyr, depending on a range of reasonable assumptions. While the inner shell
structure is suggestive of a minor merger, the total light contribution from
the shells and extended structures are more indicative of a major merger. The
spectrum of the host galaxy is dominated by a population of intermediate age
(~1.4 Gyr), indicating a strong starburst episode that may have occurred at the
time of the merger event. We speculate that the current QSO activity may have
been triggered in the recent past by either a minor merger, or by debris from
an older (~Gyr) major merger that is currently ``raining'' back into the
central regions of the merger remnant.Comment: 14 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
Feynman Rules in the Type III Natural Flavour-Conserving Two-Higgs Doublet Model
We consider a two Higgs-doublet model with symmetry, which implies a
rather than 0 relative phase between the vacuum expectation
values . The corresponding Feynman rules are derived
accordingly and the transformation of the Higgs fields from the weak to the
mass eigenstates includes not only an angle rotation but also a phase
transformation. In this model, both doublets couple to the same type of
fermions and the flavour-changing neutral currents are naturally suppressed. We
also demonstrate that the Type III natural flavour-conserving model is valid at
tree-level even when an explicit symmetry breaking perturbation is
introduced to get a reasonable CKM matrix. In the special case , as the ratio runs from 0 to ,
the dominant Yukawa coupling will change from the first two generations to the
third generation. In the Feynman rules, we also find that the charged Higgs
currents are explicitly left-right asymmetric. The ratios between the left- and
right-handed currents for the quarks in the same generations are estimated.Comment: 16 pages (figures not included), NCKU-HEP/93-1
An Investigation of Student Performance, Student Satisfaction , and Learner Characteristics in Online Versus Face-to-Face Classes--RESEARCH
The purpose of this study was to investigate student performance, student satisfaction, and learner characteristics in online versus face-to-face delivery formats in accounting. Forty-four students, enrolled in either the face-to-face or online section of an accounting course, completed a survey assessing satisfaction, learning outcomes, and learner characteristics. Significant differences were found regarding satisfaction with learning and perception of the instructor. However, online learners displayed similar traits as their face-to-face counterparts. In addition, researchers compared pre-test / post-test, homework, quiz, and exam scores to determine if delivery method affected learning. A significant difference was found between face-to-face and online students in post-test performance. Overall, the results of this study suggest that students perform as well in an online course as they do in a traditional face-to-face classroom and learner characteristics are similar for both online students and face-to-face classroom students. Post-test results indicate that students in an online class do not retain the information as well as those in a face-to-face class. These results, like others, show that online instructors continue to face challenges in promoting student participation and interaction in the course and overall satisfaction with learning. There is a need to replicate this study in other online courses in different disciplines
The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
Since the discovery of superconductivity in the cuprates two decades ago, it
has been firmly established that the CuO_2 plane is consequential for high T_C
superconductivity and a host of other very unusual properties. A new family of
superconductors with the general composition of LaFeAsO_(1-x)F_x has recently
been discovered but with the conspicuous lacking of the CuO_2 planes, thus
raising the tantalizing questions of the different pairing mechanisms in these
oxypnictide superconductors. Intimately related to pairing in a superconductor
are the superconducting gap, its value, structure, and temperature dependence.
Here we report the observation of a single gap in the superconductor
SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap
value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS
prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a
manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but
dramatically different from that of the pseudogap behavior in the cuprate
superconductors. Our results clearly indicate a nodeless gap order parameter,
which is nearly isotropic in size across different sections of the Fermi
surface, and are not compatible with models involving antiferromagnetic
fluctuations, strong correlations, t-J model, and the like, originally designed
for cuprates.Comment: 8 pages, 3 figure
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