3,777 research outputs found
A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: the role of spin polarised tunnelling at the grain boundaries
It has been observed that in bulk and polycrystalline thin films of collossal
magnetoresistive (CMR) materials the magnetoresistance follows a different
behaviour compared to single crystals or single crystalline films below the
ferromagnetic transition temperature Tc. In this paper we develop a
phenomenological model to explain the magnetic field dependence of resistance
in granular CMR materials taking into account the spin polarised tunnelling at
the grain boundaries. The model has been fitted to two systems, namely,
La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have
separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the
intergranular contribution to the magnetoresistance coming from spin polarised
tunnelling at the grain boundaries. It is observed that the temperature
dependence of the intrinsic contribution to the magnetoresistance in
La0.55Ho0.15Sr0.3MnO3 follows the prediction of double exchange model for all
values of field.Comment: 14 pages + 5 figures, postscript (to appear in Journal of Applied
Physics
The Role of in Two-pion Exchange Three-nucleon Potential
In this paper we have studied the two-pion exchange three-nucleon potential
using an approximate chiral symmetry of the
strong interaction. The off-shell pion-nucleon scattering amplitudes obtained
from the Weinberg Lagangian are supplemented with contributions from the
well-known -term and the exchange. It is the role of the
-resonance in , which we have investigated in detail in the
framework of the Lagrangian field theory. The -contribution is quite
appreciable and, more significantly, it is dependent on a parameter Z which is
arbitrary but has the empirical bounds . We find that the
-contribution to the important parameters of the depends
on the choice of a value for Z, although the correction to the binding energy
of triton is not expected to be very sensitive to the variation of Z within its
bounds.Comment: 14 pages, LaTe
CP Violation and Dark Matter
A brief review is given of the effects of CP violation on the direct
detection of neutralinos in dark matter detectors. We first summarize the
current developments using the cancellation mechanism which allows for the
existence of large CP violating phases consistent with experimental limits on
the electron and on the neutron electric dipole moments in a broad class of
SUSY, string and D brane models. We then discuss their effects on the
scattering of neutralinos from quarks and on the event rates. It is found that
while CP effects on the event rates can be enormous such effects are reduced
significantly with the imposition of the EDM constraints. However, even with
the inclusion of the EDM constraints the effects are still very significant and
should be included in a precision prediction of event rates in any SUSY, string
or D brane model.Comment: Based on an invited talk at the conference "Sources and Detection of
Dark Matter in the Universe", at Marina del Rey, CA, Feb. 23-25, 2000; 12
pages, Latex including 2 figure
Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite : Evidence of surface spin freezing of manganite nanoparticles
We have investigated the effect of nanometric grain size on magnetic
properties of single phase, nanocrystalline, granular La0.7Ca0.3MnO3 (LCMO)
sample. We have considered core-shell structure of our LCMO nanoparticles,
which can explain its magnetic properties. From the temperature dependence of
field cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), the
magnetic properties could be distinguished into two regimes: a relatively high
temperature regime T > 40 K where the broad maximum of ZFC curve (at T = Tmax)
is associated with the blocking of core particle moments, whereas the sharp
maximum (at T = TS) is related to the freezing of surface (shell) spins. The
unusual shape of M (H) loop at T = 1.5 K, temperature dependent feature of
coercive field and remanent magnetization give a strong support of surface spin
freezing that are occurring at lower temperature regime (T < 40 K) in this LCMO
nanoparticles. Additionally, waiting time (tw) dependence of ZFC relaxation
measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on tw
and dM/dln(t) following a logarithmic variation on time. Both of these features
strongly support the high temperature regime to be associated with the blocking
of core moments. At T = 20 K, ZFC relaxation measurements indicates the
existence of two different types of relaxation processes in the sample with
S(t) attaining a maximum at the elapsed time very close to the wait time tw =
1000 sec, which is an unequivocal sign of glassy behavior. This age-dependent
effect convincingly establish the surface spin freezing of our LCMO
nanoparticles associated with a background of superparamagnetic (SPM) phase of
core moments.Comment: 41 pages, 10 figure
An Automated Home Made Low Cost Vibrating Sample Magnetometer
The design and operation of a homemade low cost vibrating sample magnetometer
is described here. The sensitivity of this instrument is better than 10-2 emu
and found to be very efficient for the measurement of magnetization of most of
the ferromagnetic and other magnetic materials as a function of temperature
down to 77 K and magnetic field upto 800 Oe. Both M(H) and M(T) data
acquisition are fully automated employing computer and Labview softwar
On the Thermodynamic Geometry of Hot QCD
We study the nature of the covariant thermodynamic geometry arising from the
free energy of hot QCD. We systematically analyze the underlying equilibrium
thermodynamic configurations of the free energy of 2- and 3-flavor hot QCD with
or without including thermal fluctuations in the neighborhood of the QCD
transition temperature. We show that there exists a well-defined thermodynamic
geometric notion for QCD thermodynamics. The geometry thus obtained has no
singularity as an intrinsic Riemannian manifold. We further show that there is
a close connection of this geometric approach with the existing studies of
correlations and quark number susceptibilities in hot QCD.Comment: 15 pages, 12 figures, Keywords: Thermodynamic Geometry, Hot QCD,
Quasi-particles, PACS: 12.38.-t; 05.70.Fh; 02.40.Ky; 12.40.E
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