2,824 research outputs found
The Effects of Amplification Bias in Gravitational Microlensing Experiments
Although a source star is fainter than the detection limit imposed by
crowding, it is still possible to detect an event if the star is located in the
seeing disk of a bright star is and gravitationally amplified: amplification
bias. Using a well-constrained luminosity function, I show that of
events detected toward the Galactic bulge are affected by amplification bias
and the optical depth might be overestimated by a factor . In
addition, I show that if one takes amplification bias into consideration, the
observed time scale distribution matches significantly better, especially in
the short time-scale region, with the distribution expected from a
mass-spectrum model in which lenses are composed of the known stellar
population plus an additional population of brown dwarfs than it is without the
effect of the amplification bias.Comment: 16 pages including 4 figures, ApJ, submitte
Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite
We report Cl NMR, ESR, SR and specific heat measurements on the
frustrated kagom\'e magnet kapellasite,
CuZn(OH)Cl, where a gapless spin liquid phase is
stabilized by a set of competing exchange interactions. Our measurements
confirm the ferromagnetic character of the nearest-neighbour exchange
interaction and give an energy scale for the competing interactions K. The study of the temperature-dependent ESR lineshift reveals a
moderate symmetric exchange anisotropy term , with %. These
findings validate a posteriori the use of the Heisenberg
model to describe the magnetic properties of kapellasite [Bernu et al., Phys.
Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this
model is the severe random depletion of the magnetic kagom\'e lattice by 27%,
due to Cu/Zn site mixing, and specifically address the effect of this disorder
by Cl NMR, performed on an oriented polycrystalline sample.
Surprisingly, while being very sensitive to local structural deformations, our
NMR measurements demonstrate that the system remains homogeneous with a unique
spin susceptibility at high temperature, despite a variety of magnetic
environments. Unconventional spin dynamics is further revealed by NMR and
SR in the low-, correlated, spin liquid regime, where a broad
distribution of spin-lattice relaxation times is observed. We ascribe this to
the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.
Thermal properties of MgB2: the effect of disorder on gap amplitudes and relaxation times of p and s bands
We present thermal conductivity and specific heat measurements on MgB2 and
Mg-AlB2 samples. Thermal properties have been analysed by using a two-gap model
in order to estimate the gap amplitudes, D(0)p and D(0)s and the intra-band
scattering rates, Gss and Gpp. As a function of Al doping and disorder D(0)s
rapidly decreases, while D(0)p is rather constant. Gss and Gpp are increased by
the disorder, being Gpp more affected than Gss.Comment: 2 pages, 3 figures, presented to the conference M2S-HTSC, 25-30 May
2003, Rio de Janeir
Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Specific heat of heavy fermion CePd2Si2 in high magnetic fields
We report specific heat measurements on the heavy fermion compound CePd2Si2
in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp
peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3
K in zero field. The transition is found to shift to lower temperatures when a
magnetic field is applied along the crystallographic a-axis, while a field
applied parallel to the tetragonal c-axis does not affect the transition. The
magnetic contribution to the specific heat below T_N is well described by a sum
of a linear electronic term and an antiferromagnetic spin wave contribution.
Just below T_N, an additional positive curvature, especially at high fields,
arises most probably due to thermal fluctuations. The field dependence of the
coefficient of the low temperature linear term, gamma_0, extracted from the
fits shows a maximum at about 6 T, at the point where an anomaly was detected
in susceptibility measurements. The relative field dependence of both T_N and
the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the
disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of
the antiferromagnetic transition temperature to zero makes the existence of a
magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte
Noether symmetries for two-dimensional charged particle motion
We find the Noether point symmetries for non-relativistic two-dimensional
charged particle motion. These symmetries are composed of a quasi-invariance
transformation, a time-dependent rotation and a time-dependent spatial
translation. The associated electromagnetic field satisfy a system of
first-order linear partial differential equations. This system is solved
exactly, yielding three classes of electromagnetic fields compatible with
Noether point symmetries. The corresponding Noether invariants are derived and
interpreted
A New Method of Probing the Phonon Mechanism in Superconductors including MgB
Weak localization has a strong influence on both the normal and
superconducting properties of metals. In particular, since weak localization
leads to the decoupling of electrons and phonons, the temperature dependence of
resistance (i.e., ) is decreasing with increasing disorder, as
manifested by Mooij's empirical rule. In addition, Testardi's universal
correlation of (i.e., ) and the resistance ratio (i.e.,
) follows. This understanding provides a new means to probe the
phonon mechanism in superconductors including MgB. The merits of this
method are its applicability to any superconductors and its reliability because
the McMillan's electron-phonon coupling constant and
change in a broad range, from finite values to zero, due to weak localization.
Karkin et al's preliminary data of irradiated MgB show the Testardi
correlation, indicating that the dominant pairing mechanism in MgB is the
phonon-mediated interaction.Comment: 9 pages, latex, 3 figure
Searching for TeV dark matter by atmospheric Cerenkov techniques
There is a growing interest in the possibility that dark matter could be
formed of weakly interacting particles with a mass in the 100 GeV - 2 TeV
range, and supersymmetric particles are favorite candidates. If they constitute
the dark halo of our Galaxy, their mutual annihilations produce energetic gamma
rays that could be detected using existing atmospheric \u{C}erenkov techniques.Comment: 10 pp, LaTex (3 figures available by e-mail) PAR-LPTHE 92X
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