2,747 research outputs found
Electronic Raman scattering on under-doped Hg-1223 high-Tc superconductors:investigations on the symmetry of the order parameter
In order to obtain high quality, reliable electronic Raman spectra of a
high-Tc superconductor compound, we have studied strongly under-doped
HgBa2Ca2Cu3O8+d. This choice was made such as to i)minimize oxygen disorder in
the Hg-plane generated by oxygen doping ii) avoid the need of phonon background
subtraction from the raw data iii)eliminate traces of parasitic phases
identified and monitored on the crystal surface. Under these experimental
conditions we are able to present the pure electronic Raman response function
in the B2g, B1g, A1g+B2g and A1g+B1g channels. The B2g spectrum exhibits a
linear frequency dependence at low energy whereas the B1g one shows a
cubic-like dependence. The B2g and B1g spectra display two well defined maxima
at 5.6kBTc and 9kBTc respectively. In mixed A1g channels an intense electronic
peak centered around 6.4 kBTc is observed. The low energy parts of the spectra
correspond to the electronic response expected for a pure dx2-y2 gap symmetry
and can be fitted up to the gap energy for the B1g channel. However, in the
upper parts, the relative position of the B1g and B2g peaks needs expanding the
B2g Raman vertex to second order Fermi surface harmonics to fit the data with
the dx2-y2 model. The sharper and more intense A1g peak appears to challenge
the Coulomb screening efficiency present for this channel. As compared to
previous data on more optimally doped, less stoichiometric
Hg-1223 compounds, this work reconciles the electronic Raman spectra of
under- doped Hg-1223 crystals with the dx2-y2 model, provided that the oxygen
doping is not too strong. This apparent extreme sensitivity of the electronic
Raman spectra to the low lying excitations induced by oxygen doping in the
superconducting state is emphasized here and remains an open question.Comment: 12 pages, 5 figure
Property (T) and rigidity for actions on Banach spaces
We study property (T) and the fixed point property for actions on and
other Banach spaces. We show that property (T) holds when is replaced by
(and even a subspace/quotient of ), and that in fact it is
independent of . We show that the fixed point property for
follows from property (T) when 1
. For simple Lie groups and their lattices, we prove that the fixed point property for holds for any if and only if the rank is at least two. Finally, we obtain a superrigidity result for actions of irreducible lattices in products of general groups on superreflexive Banach spaces.Comment: Many minor improvement
Intrinsic Low Temperature Paramagnetism in B-DNA
We present experimental study of magnetization in -DNA in
conjunction with structural measurements. The results show the surprising
interplay between the molecular structures and their magnetic property. In the
B-DNA state, -DNA exhibits paramagnetic behaviour below 20 K that is
non-linear in applied magnetic field whereas in the A-DNA state, remains
diamagnetic down to 2 K. We propose orbital paramagnetism as the origin of the
observed phenomena and discuss its relation to the existence of long range
coherent transport in B-DNA at low temperature.Comment: 5 pages, 4 figures, submitted to Physical Review Letters October 200
Orbit equivalence rigidity for ergodic actions of the mapping class group
We establish orbit equivalence rigidity for any ergodic, essentially free and
measure-preserving action on a standard Borel space with a finite positive
measure of the mapping class group for a compact orientable surface with higher
complexity. We prove similar rigidity results for a finite direct product of
mapping class groups as well.Comment: 11 pages, title changed, a part of contents remove
Fermi Velocity Spectrum and Incipient Magnetism in TiBe2
We address the origin of the incipient magnetism in TiBe through precise
first principles calculations, which overestimate the ferromagnetic tendency
and therefore require correction to account for spin fluctuations. TiBe has
sharp fine structure in its electronic density of states, with a van Hove
singularity only 3 meV above the Fermi level. Similarly to the isovalent weak
ferromagnet ZrZn, it is flat bands along the K-W-U lines of hexagonal face
of the fcc Brillouin zone make the system prone to magnetism, and more so if
electrons are added. We find that the Moriya coefficient (multiplying
in the fluctuation susceptibility )
is divergent when the velocity vanishes at a point on the Fermi surface, which
is very close (3 meV) to occurring in TiBe. In exploring how the FM
instability (the =0 Stoner enhancement is ) might be suppressed
by fluctuations in TiBe, we calculate that the Moriya A coefficient (of
) is negative, so =0 is not the primary instability. Explicit
calculation of shows that its maximum occurs at the X point
; TiBe is thus an incipient {\it anti}ferromagnet
rather than ferromagnet as has been supposed. We further show that simple
temperature smearing of the peak accounts for most of the temperature
dependence of the susceptibility, which previously had been attributed to local
moments (via a Curie-Weiss fit), and that energy dependence of the density of
states also strongly affects the magnetic field variation of
A lattice in more than two Kac--Moody groups is arithmetic
Let be an irreducible lattice in a product of n infinite irreducible
complete Kac-Moody groups of simply laced type over finite fields. We show that
if n is at least 3, then each Kac-Moody groups is in fact a simple algebraic
group over a local field and is an arithmetic lattice. This relies on
the following alternative which is satisfied by any irreducible lattice
provided n is at least 2: either is an S-arithmetic (hence linear)
group, or it is not residually finite. In that case, it is even virtually
simple when the ground field is large enough.
More general CAT(0) groups are also considered throughout.Comment: Subsection 2.B was modified and an example was added ther
Bose-Einstein distribution, condensation transition and multiple stationary states in multiloci evolution of diploid population
The mapping between genotype and phenotype is encoded in the complex web of
epistatic interaction between genetic loci. In this rugged fitness landscape,
recombination processes, which tend to increase variation in the population,
compete with selection processes that tend to reduce genetic variation. Here we
show that the Bose-Einstein distribution describe the multiple stationary
states of a diploid population under this multi-loci evolutionary dynamics.
Moreover, the evolutionary process might undergo an interesting condensation
phase transition in the universality class of a Bose-Einstein condensation when
a finite fraction of pairs of linked loci, is fixed into given allelic states.
Below this phase transition the genetic variation within a species is
significantly reduced and only maintained by the remaining polymorphic loci.Comment: (12 pages, 7 figures
On the distortion of twin building lattices
We show that twin building lattices are undistorted in their ambient group;
equivalently, the orbit map of the lattice to the product of the associated
twin buildings is a quasi-isometric embedding. As a consequence, we provide an
estimate of the quasi-flat rank of these lattices, which implies that there are
infinitely many quasi-isometry classes of finitely presented simple groups. In
an appendix, we describe how non-distortion of lattices is related to the
integrability of the structural cocycle
Ordering in a spin glass under applied magnetic field
Torque, torque relaxation, and magnetization measurements on a AuFe spin
glass sample are reported. The experiments carried out up to 7 T show a
transverse irreversibility line in the (H,T) plane up to high applied fields,
and a distinct strong longitudinal irreversibility line at lower fields. The
data demonstrate for that this type of sample, a Heisenberg spin glass with
moderately strong anisotropy, the spin glass ordered state survives under high
applied fields in contrast to predictions of certain "droplet" type scaling
models. The overall phase diagram closely ressembles those of mean field or
chiral models, which both have replica symmetry breaking transitions.Comment: 4 pages, 3 figures, accepted for PR
- …