182 research outputs found
Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres
The bulk composition of an exoplanet is commonly inferred from its average
density. For small planets, however, the average density is not unique within
the range of compositions. Variations of a number of important planetary
parameters--which are difficult or impossible to constrain from measurements
alone--produce planets with the same average densities but widely varying bulk
compositions. We find that adding a gas envelope equivalent to 0.1%-10% of the
mass of a solid planet causes the radius to increase 5-60% above its gas-free
value. A planet with a given mass and radius might have substantial water ice
content (a so-called ocean planet) or alternatively a large rocky-iron core and
some H and/or He. For example, a wide variety of compositions can explain the
observed radius of GJ 436b, although all models require some H/He. We conclude
that the identification of water worlds based on the mass-radius relationship
alone is impossible unless a significant gas layer can be ruled out by other
means.Comment: 5 pages, 3 figures, accepted to Ap
Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems
We consider the absorption of light with emission of one S(tot)=1 magnetic
excitation in systems with a spin gap induced by quantum fluctuations. We argue
that an electric dipole transition is allowed on the condition that a virtual
phonon instantaneously breaks the inversion symmetry. We derive an effective
operator for the transition and argue that the proposed theory explains the
polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).Comment: 9 pages, 4 figure
Versatile helimagnetic phases under magnetic fields in cubic perovskite SrFeO3
A helical spin texture is of great current interest for a host of novel
spin-dependent transport phenomena. We report a rich variety of nontrivial,
helimagnetic phases in the simple cubic perovskite SrFeO3 under magnetic fields
up to 42 T. Magnetic and resistivity measurements revealed that the
proper-screw spin phase proposed for SrFeO3 can be subdivided into at least
five kinds of ordered phases. Near the multicritical point, an unconventional
anomalous Hall effect was found to show up and was interpreted as due to a
possible long-period noncoplanar spin texture with scalar spin chirality.Comment: 5 pages, 5 figures, Physical Review B in pres
Incommensurate spin correlations induced by magnetic Fe ions substituted into overdoped Bi1.75Pb0.35Sr1.90CuO6+z
Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have
been explored with Fe-doped single crystals characterized by neutron
scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility
measurements. Static incommensurate spin correlations induced by the Fe spins
are revealed by elastic neutron scattering. The resultant incommensurability
delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in
overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this
overdoped region is close to the hole concentration p. This result is
reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4;
however, it is inconsistent with the saturation of delta in the latter compound
in the overdoped regime. While our findings in Fe-doped
Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin
correlations in high-Tc cuprate superconductors, they also suggest that the
magnetic response might be dominated by a distinct mechanism in the overdoped
region.Comment: 4 pages, 5 figures. Revision in introduction, discussion, and
conclusion
Effects of hole-doping on the magnetic ground state and excitations in the edge-sharing CuO chains of CaYCuO
Neutron scattering experiments were performed on the undoped and hole-doped
CaYCuO, which consists of ferromagnetic edge-sharing
CuO chains. It was previously reported that in the undoped
CaYCuO there is an anomalous broadening of spin-wave
excitations along the chain, which is caused mainly by the antiferromagnetic
interchain interactions [Matsuda , Phys. Rev. B 63, 180403(R)
(2001)]. A systematic study of temperature and hole concentration dependencies
of the magnetic excitations shows that the magnetic excitations are softened
and broadened with increasing temperature or doping holes irrespective of
direction. The broadening is larger at higher . A characteristic feature is
that hole-doping is much more effective to broaden the excitations along the
chain. It is also suggested that the intrachain interaction does not change so
much with increasing temperature or doping although the anisotropic interaction
and the interchain interaction are reduced. In the spin-glass phase (=1.5)
and nearly disordered phase (=1.67) the magnetic excitations are much
broadened in energy and . It is suggested that the spin-glass phase
originates from the antiferromagnetic clusters, which are caused by the hole
disproportionation.Comment: 8 pages, submitted to Phys. Rev.
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