1,163 research outputs found
Cooperative Jahn-Teller Distortion in PrO2
We report neutron diffraction data on single crystal PrO2 which reveal a
cooperative Jahn-Teller distortion at TD = 120 +/- 2 K. Below this temperature
an internal distortion of the oxygen sublattice causes the unit cell of the
crystallographic structure to become doubled along one crystal axis. We discuss
several possible models for this structure. The antiferromagnetic structure
below TN = 13.5 K is found to consist of two components, one of which shares
the same doubled unit cell as the distorted crystallographic structure. We also
present measurements of the magnetic susceptibility, the specific heat capacity
and the electrical conductivity of PrO2. The susceptibility data show an
anomaly at a temperature close to TD. From the specific heat capacity data we
deduce that the ground state is doubly degenerate, consistent with a distortion
of the cubic local symmetry. We discuss possible mechanisms for this. The
conductivity shows an activated behaviour with an activation energy Ea = 0.262
+/- 0.003 eV.Comment: 12 pages, 14 figures, 2 tables. Additional suggested structure in v
On the equation of state of a dense columnar liquid crystal
An accurate description of a columnar liquid crystal of hard disks at high
packing fractions is presented using an improved free-volume theory. It is
shown that the orientational entropy of the disks in the one-dimensional fluid
direction leads to a different high-density scaling pressure compared to the
prediction from traditional cell theory. Excellent quantitative agreement is
found with recent Monte-Carlo simulation results for various thermodynamic and
structural properties of the columnar state.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Raman and Infra-red properties and layer dependence of the phonon dispersions in multi-layered graphene
The symmetry group analysis is applied to classify the phonon modes of
-stacked graphene layers (NSGL's) with AB- and AA-stacking, particularly
their infra-red and Raman properties. The dispersions of various phonon modes
are calculated in a multi-layer vibrational model, which is generalized from
the lattice vibrational potentials of graphene to including the inter-layer
interactions in NSGL's. The experimentally reported red shift phenomena in the
layer number dependence of the intra-layer optical C-C stretching mode
frequencies are interpreted. An interesting low frequency inter-layer optical
mode is revealed to be Raman or Infra-red active in even or odd NSGL's
respectively. Its frequency shift is sensitive to the layer number and
saturated at about 10 layers.Comment: enlarged versio
Perceived Benefits of Technology Enhanced Language Learning in Beginning Language Classes
This paper examines the perceptions of benefit of Technology EnhancedLanguage Learning (TELL) on students’ language learning, comfort &enjoyment, and increased confidence using technology at a large SouthernCalifornia University during one university term. Through a surveyadministered to 345 beginning language students, 11 tutors and 12instructors, and through selective interviews and classroom observations,several questions were examined: 1.) Perceived confidence, benefits, andcomfort/enjoyment with TELL for instructors, tutors, and students at thebeginning and end of the semester; 2.) Students’ perceived impact ofTELL between pre and post survey measures on second language skills,learning culture, student motivation to learn a language, and preparingstudents for class tests and quizzes; 3.) Whether or not target languageorthography, exposure to TELL, student gender, and instructors’ ortutors’ previous confidence in using TELL, impacted perception of benefitby students; 4.) Positive and negative aspects of incorporating a TELLcomponent in the language classroom for instructors and tutors. Theresults showed that incorporating TELL in a new, but limited, way in allbeginning level classes at one university was a positive experience formany participants, especially in the areas of comfort/enjoyment, andincreased confidence in using technology. However, unless tasks wereclearly tied to learning objectives, students did not recognize theirinstructional value
The confined hydrogen atom with a moving nucleus
We study the hydrogen atom confined to a spherical box with impenetrable
walls but, unlike earlier pedagogical articles on the subject, we assume that
the nucleus also moves. We obtain the ground-state energy approximately by
means of first--order perturbation theory and by a more accurate variational
approach. We show that it is greater than the one for the case in which the
nucleus is clamped at the center of the box. Present approach resembles the
well-known treatment of the helium atom with clamped nucleus
Machine-learned cloud classes from satellite data for process-oriented climate model evaluation
Clouds play a key role in regulating climate change but are difficult to
simulate within Earth system models (ESMs). Improving the representation of
clouds is one of the key tasks towards more robust climate change projections.
This study introduces a new machine-learning based framework relying on
satellite observations to improve understanding of the representation of clouds
and their relevant processes in climate models. The proposed method is capable
of assigning distributions of established cloud types to coarse data. It
facilitates a more objective evaluation of clouds in ESMs and improves the
consistency of cloud process analysis. The method is built on satellite data
from the MODIS instrument labelled by deep neural networks with cloud types
defined by the World Meteorological Organization (WMO), using cloud type labels
from CloudSat as ground truth. The method is applicable to datasets with
information about physical cloud variables comparable to MODIS satellite data
and at sufficiently high temporal resolution. We apply the method to
alternative satellite data from the Cloud\_cci project (ESA Climate Change
Initiative), coarse-grained to typical resolutions of climate models. The
resulting cloud type distributions are physically consistent and the horizontal
resolutions typical of ESMs are sufficient to apply our method. We recommend
outputting crucial variables required by our method for future ESM data
evaluation. This will enable the use of labelled satellite data for a more
systematic evaluation of clouds in climate models.Comment: Main Paper 16 pages, 11 figures. Supporting material 7 Pages, 8
figures. This work has been submitted to the IEEE for possible publication.
Copyright may be transferred without notice, after which this version may no
longer be accessibl
Thermo-statistical description of gas mixtures from space partitions
The new mathematical framework based on the free energy of pure classical
fluids presented in [R. D. Rohrmann, Physica A 347, 221 (2005)] is extended to
multi-component systems to determine thermodynamic and structural properties of
chemically complex fluids. Presently, the theory focuses on -dimensional
mixtures in the low-density limit (packing factor ). The formalism
combines the free-energy minimization technique with space partitions that
assign an available volume to each particle. is related to the
closeness of the nearest neighbor and provides an useful tool to evaluate the
perturbations experimented by particles in a fluid. The theory shows a close
relationship between statistical geometry and statistical mechanics. New,
unconventional thermodynamic variables and mathematical identities are derived
as a result of the space division. Thermodynamic potentials ,
conjugate variable of the populations of particles class with the
nearest neighbors of class are defined and their relationships with the
usual chemical potentials are established. Systems of hard spheres are
treated as illustrative examples and their thermodynamics functions are derived
analytically. The low-density expressions obtained agree nicely with those of
scaled-particle theory and Percus-Yevick approximation. Several pair
distribution functions are introduced and evaluated. Analytical expressions are
also presented for hard spheres with attractive forces due to K\^ac-tails and
square-well potentials. Finally, we derive general chemical equilibrium
conditions.Comment: 14 pages, 8 figures. Accepted for publication in Physical Review
On possible skewon effects on light propagation
We start from a local and linear spacetime relation between the
electromagnetic excitation and the field strength. Then we study the generally
covariant Fresnel surfaces for light rays and light waves. The metric and the
connection of spacetime are left unspecified. Accordingly, our framework is
ideally suited for a search of possible violations of the Lorentz symmetry in
the photon sector of the extended standard model. We discuss how the skewon
part of the constitutive tensor, if suitably parametrized, influences the
Fresnel surfaces and disturbs the light cones of vacuum electrodynamics.
Conditions are specified that yield the reduction of the original quartic
Fresnel surface to the double light cone structure (birefringence) and to the
single light cone. Qualitatively, the effects of the real skewon field can be
compared to those in absorbing material media. In contrast, the imaginary
skewon field can be interpreted in terms of non-absorbing media with natural
optical activity and Faraday effects. The astrophysical data on gamma-ray
bursts are used for deriving an upper limit for the magnitude of the skewon
field.Comment: Revtex, 29 pages, 10 figures, references added, text as in the
published versio
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