45,675 research outputs found
Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase transition of point defects
A uniform distribution of point defects in an otherwise perfect
crystallographic structure usually describes a unique pseudo phase of that
state of a non-stoichiometric material. With off-stoichiometric uranium dioxide
as a prototype, we show that analogous to a conventional phase transition,
these pseudo phases also will transform from one state into another via
changing the predominant defect species when external conditions of pressure,
temperature, or chemical composition are varied. This exotic transition is
numerically observed along shock Hugoniots and isothermal compression curves in
UO2 with first-principles calculations. At low temperatures, it leads to
anomalies (or quasi-discontinuities) in thermodynamic properties and electronic
structures. In particular, the anomaly is pronounced in both shock temperature
and the specific heat at constant pressure. With increasing of the temperature,
however, it transforms gradually to a smooth cross-over, and becomes less
discernible. The underlying physical mechanism and characteristics of this type
of transition are encoded in the Gibbs free energy, and are elucidated clearly
by analyzing the correlation with the variation of defect populations as a
function of pressure and temperature. The opportunities and challenges for a
possible experimental observation of this phase change are also discussed.Comment: 11 pages, 5 figure
model with Hopf term and fractional spin statistics
We reconsider the model with the Hopf term by using the
Batalin-Fradkin-Tyutin (BFT) scheme, which is an improved version of the Dirac
quantization method. We also perform a semi-classical quantization of the
topological charge Q sector by exploiting the collective coordinates to
explicitly show the fractional spin statistics.Comment: 15 page
Helical motions in the jet of blazar 1156+295
The blazar 1156+295 was observed by VLBA and EVN + MERLIN at 5 GHz in June
1996 and February 1997 respectively. The results show that the jet of the
source has structural oscillations on the milliarcsecond scale and turns
through a large angle to the direction of the arcsecond-scale extension. A
helical jet model can explain most of the observed properties of the radio
structure in 1156+295.Comment: 6 pages, 2 figures, to appear in New Astronomy Reviews (EVN/JIVE
Symposium No. 4, special issue
Reconsideration of Second Harmonic Generation from neat Air/Water Interface: Broken of Kleinman Symmetry from Dipolar Contribution
It has been generally accepted that there are significant quadrupolar and
bulk contributions to the second harmonic generation (SHG) reflected from the
neat air/water interface, as well as common liquid interfaces. Because there
has been no general methodology to determine the quadrupolar and bulk
contributions to the SHG signal from a liquid interface, this conclusion was
reached based on the following two experimental phenomena. Namely, the broken
of the macroscopic Kleinman symmetry, and the significant temperature
dependence of the SHG signal from the neat air/water interface. However,
because sum frequency generation vibrational spectroscopy (SFG-VS) measurement
of the neat air/water interface observed no apparent temperature dependence,
the temperature dependence in the SHG measurement has been reexamined and
proven to be an experimental artifact. Here we present a complete microscopic
analysis of the susceptibility tensors of the air/water interface, and show
that dipolar contribution alone can be used to address the issue of broken of
the macroscopic Kleinman symmetry at the neat air/water interface. Using this
analysis, the orientation of the water molecules at the interface can be
obtained, and it is consistent with the measurement from SFG-VS. Therefore, the
key rationales to conclude significantly quadrupolar and bulk contributions to
the SHG signal of the neat air/water interface can no longer be considered as
valid as before. This new understanding of the air/water interface can shed
light on our understanding of the nonlinear optical responses from other
molecular interfaces as well
Ion confinement and transport in a toroidal plasma with externally imposed radial electric fields
Strong electric fields were imposed along the minor radius of the toroidal plasma by biasing it with electrodes maintained at kilovolt potentials. Coherent, low-frequency disturbances characteristic of various magnetohydrodynamic instabilities were absent in the high-density, well-confined regime. High, direct-current radial electric fields with magnitudes up to 135 volts per centimeter penetrated inward to at least one-half the plasma radius. When the electric field pointed radially toward, the ion transport was inward against a strong local density gradient; and the plasma density and confinement time were significantly enhanced. The radial transport along the electric field appeared to be consistent with fluctuation-induced transport. With negative electrode polarity the particle confinement was consistent with a balance of two processes: a radial infusion of ions, in those sectors of the plasma not containing electrodes, that resulted from the radially inward fields; and ion losses to the electrodes, each of the which acted as a sink and drew ions out of the plasma. A simple model of particle confinement was proposed in which the particle confinement time is proportional to the plasma volume. The scaling predicted by this model was consistent with experimental measurements
Quakes in Solid Quark Stars
A starquake mechanism for pulsar glitches is developed in the solid quark
star model. It is found that the general glitch natures (i.e., the glitch
amplitudes and the time intervals) could be reproduced if solid quark matter,
with high baryon density but low temperature, has properties of shear modulus
\mu = 10^{30~34} erg/cm^3 and critical stress \sigma_c = 10^{18~24} erg/cm^3.
The post-glitch behavior may represent a kind of damped oscillations.Comment: 11 pages, 4 figures (but Fig.3 is lost), a complete version can be
obtained by http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm, a new
version to be published on Astroparticle Physic
A fluctuation-induced plasma transport diagnostic based upon fast-Fourier transform spectral analysis
A diagnostic, based on fast Fourier-transform spectral analysis techniques, that provides experimental insight into the relationship between the experimentally observable spectral characteristics of the fluctuations and the fluctuation-induced plasma transport is described. The model upon which the diagnostic technique is based and its experimental implementation is discussed. Some characteristic results obtained during the course of an experimental study of fluctuation-induced transport in the electric field dominated NASA Lewis bumpy torus plasma are presented
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