2,858 research outputs found
17beta-oestradiol and Enovid mammary tumorigenesis in C3H/HeJ female mice: counteraction by concurrent 2-bromo-alpha-ergocryptine.
Chronic administration of 17beta-oestradiol (via drinking water) or the oral contraceptive Enovid (norethynodrel and mestranol) (0-1 mg injected s.c. twice weekly) to nulliparous C3H/HeJ female mice, beginning at one month of age and terminating at 20 months (17beta-oestradiol) or 22 months (Enovid), significantly increased the incidence of mammary tumours over solvent-treated controls. Concurrent treatment of the steroid-treated mice with 2-bromo-alpha-ergocryptine (CB-154) (0-1 mg s.c. injected daily) significantly reduced mammary tumour incidence and mammary hyperplastic nodule development to the control level. CB-154 is an efficacious inhibitor of pituitary prolactin secretion. These results demonstrate that steroid-induced mammary gland dysplasias can be sharply reduced by chronic CB-154 treatment, and suggest that some of the mammary tumorigenic activities of oestrogenic steroids in C3H mice are mediated via an increased secretion of pituitary prolactin
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Energy focusing inside a dynamical cavity
We study the exact classical solutions for a real scalar field inside a
cavity with a wall whose motion is self-consistently determined by the pressure
of the field itself. We find that, regardless of the system parameters, the
long-time solution always becomes nonadiabatic and the field's energy
concentrates into narrow peaks, which we explain by means of a simple
mechanical system. We point out implications for the quantized theory.Comment: 5 pages, 6 figures, double column, submitted to P.R.
Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N
The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is
extensively studied over a vast range of Mn concentrations. We calculate the
electronic structures of these materials using density functional theory in
both the local spin density approximation and the LDA+U scheme, that we have
now implemented in the code SIESTA.
For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the
ferromagnetism, with an exchange constant =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong character is found at the
Fermi level in both the LSDA and the LDA+U approaches. In this case the
coupling between the Mn ions decays rapidly with the Mn-Mn separation. This
suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a
robust ferromagnetic phase at large Mn concentration coexisting with a diluted
weak ferromagnetic phase.Comment: 12 pages, 11 figure
The Casimir force on a surface with shallow nanoscale corrugations: Geometry and finite conductivity effects
We measure the Casimir force between a gold sphere and a silicon plate with
nanoscale, rectangular corrugations with depth comparable to the separation
between the surfaces. In the proximity force approximation (PFA), both the top
and bottom surfaces of the corrugations contribute to the force, leading to a
distance dependence that is distinct from a flat surface. The measured Casimir
force is found to deviate from the PFA by up to 15%, in good agreement with
calculations based on scattering theory that includes both geometry effects and
the optical properties of the material
Casimir interaction between a dielectric nanosphere and a metallic plane
We study the Casimir interaction between a dielectric nanosphere and a
metallic plane, using the multiple scattering theory. Exact results are
obtained with the dielectric described by a Sellmeier model and the metal by a
Drude model. Asymptotic forms are discussed for small spheres, large or small
distances. The well-known Casimir-Polder formula is recovered at the limit of
vanishingly small spheres, while an expression better behaved at small
distances is found for any finite value of the radius. The exact results are of
particular interest for the study of quantum states of nanospheres in the
vicinity of surfaces.Comment: 6 pages, 5 figure
Lateral Casimir-Polder force with corrugated surfaces
We derive the lateral Casimir-Polder force on a ground state atom on top of a
corrugated surface, up to first order in the corrugation amplitude. Our
calculation is based on the scattering approach, which takes into account
nonspecular reflections and polarization mixing for electromagnetic quantum
fluctuations impinging on real materials. We compare our first order exact
result with two commonly used approximation methods. We show that the proximity
force approximation (large corrugation wavelengths) overestimates the lateral
force, while the pairwise summation approach underestimates it due to the
non-additivity of dispersion forces. We argue that a frequency shift
measurement for the dipolar lateral oscillations of cold atoms could provide a
striking demonstration of nontrivial geometrical effects on the quantum vacuum.Comment: 12 pages, 6 figures, contribution to QFEXT07 proceeding
Quantum Effects in the Presence of Expanding Semi-Transparent Spherical Mirrors
We study quantum effects in the presence of a spherical semi-transparent
mirror or a system of two concentric mirrors which expand with a constant
acceleration in a flat D-dimensional spacetime. Using the Euclidean approach,
we obtain expressions for fluctuations and the renormalized value of
stress-energy tensor for a scalar non-minimally coupled massless field.
Explicit expressions are obtained for the energy fluxes at the null infinity
generated by such mirrors in the physical spacetime and their properties are
discussed.Comment: 28 pages, Paper is slightly reorganized, additional references are
adde
Ferromagnetic redshift of the optical gap in GdN
We report measurements of the optical gap in a GdN film at temperatures from
300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is
1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split
bands below the Curie temperature. The paramagnetic gap is larger than was
suggested by very early experiments, and has permitted us to refine a
(LSDA+U)-computed band structure. The band structure was computed in the full
translation symmetry of the ferromagnetic ground state, assigning the
paramagnetic-state gap as the average of the majority- and minority-spin gaps
in the ferromagnetic state. That procedure has been further tested by a band
structure in a 32-atom supercell with randomly-oriented spins. After fitting
only the paramagnetic gap the refined band structure then reproduces our
measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure
Casimir energy and geometry : beyond the Proximity Force Approximation
We review the relation between Casimir effect and geometry, emphasizing
deviations from the commonly used Proximity Force Approximation (PFA). We use
to this aim the scattering formalism which is nowadays the best tool available
for accurate and reliable theory-experiment comparisons. We first recall the
main lines of this formalism when the mirrors can be considered to obey
specular reflection. We then discuss the more general case where non planar
mirrors give rise to non-specular reflection with wavevectors and field
polarisations mixed. The general formalism has already been fruitfully used for
evaluating the effect of roughness on the Casimir force as well as the lateral
Casimir force or Casimir torque appearing between corrugated surfaces. In this
short review, we focus our attention on the case of the lateral force which
should make possible in the future an experimental demonstration of the
nontrivial (i.e. beyond PFA) interplay of geometry and Casimir effect.Comment: corrected typos, added references, QFEXT'07 special issue in J. Phys.
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