6,022 research outputs found
On electrostatic screening of ions in astrophysical plasmas
There has been some controversy over the expression for the so-called
`interaction energy' due to screening of charged particles in a plasma. Even in
the relatively simple case of weak screening, first discussed in the context of
astrophysical plasmas by Salpeter (1954), there is disagreement. In particular,
Shaviv and Shaviv (1996) have claimed recently that by not considering
explicitly in his calculation the complete screening cloud, Salpeter obtained a
result for the interaction energy between two nuclei separated by a distance
which in the limit is only 2/3 the correct value. It appears that
this claim has arisen from a fundamental misconception concerning the dynamics
of the interaction. We rectify this misconception, and show that Salpeter's
formula is indeed correct.Comment: 17 pages, no figures, AAS Latex, to appear in The Astrophysical
Journa
Neutrino Oscillations for Dummies
The reality of neutrino oscillations has not really sunk in yet. The
phenomenon presents us with purely quantum mechanical effects over macroscopic
time and distance scales (milliseconds and 1000s of km). In order to help with
the pedagogical difficulties this poses, I attempt here to present the physics
in words and pictures rather than math. No disrespect is implied by the title;
I am merely borrowing a term used by a popular series of self-help books
Antiferromagnetic Alignment and Relaxation Rate of Gd Spins in the High Temperature Superconductor GdBa_2Cu_3O_(7-delta)
The complex surface impedance of a number of GdBaCuO
single crystals has been measured at 10, 15 and 21 GHz using a cavity
perturbation technique. At low temperatures a marked increase in the effective
penetration depth and surface resistance is observed associated with the
paramagnetic and antiferromagnetic alignment of the Gd spins. The effective
penetration depth has a sharp change in slope at the N\'eel temperature, ,
and the surface resistance peaks at a frequency dependent temperature below 3K.
The observed temperature and frequency dependence can be described by a model
which assumes a negligibly small interaction between the Gd spins and the
electrons in the superconducting state, with a frequency dependent magnetic
susceptibility and a Gd spin relaxation time being a strong function
of temperature. Above , has a component varying as , while below it increases .Comment: 4 Pages, 4 Figures. Submitted to Phys. Rev.
Time dependence of current-voltage measurements of c-axis quasiparticle conductivity in 2212-BSCCO mesa structures
We report four-point IV measurements of the c-axis conductivity of mesa
structures of 2212-BSCCO, using a system with sub-microsecond resolution along
with multi-level pulses. These allow a test to be made for the presence of
nonequilibrium effects. Our results suggest simple heating alone is important
in measurements of this kind.Comment: to appear in proceedings of LT23; submitted to Physica
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