2,573 research outputs found
Critical slowing down in the geometrically frustrated pyrochlore antiferromagnet Gd_2Ti_2O_7
Longitudinal-field muon spin relaxation experiments have been carried out in
the paramagnetic state of single-crystal Gd_2Ti_2O_7 just above the phase
transition at T_m = 1.0 K. At high applied fields the exponential relaxation
time T_1 is proportional to field, whereas T_1 saturates below a crossover
field B_c that is ~2.5 T at 1.5 K and decreases as T_m is approached. At low
fields the relaxation rate increases markedly as the freezing temperature is
approached, as expected for critical slowing down of the spin fluctuations, but
the increase is suppressed by applied field. This behavior is consistent with
the very long autocorrelation function cutoff time implied by the low value of
B_c.Comment: 4 pages, 2 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals
Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc
and ac magnetic measurements. These crystals have rectangular needle-like
shapes with the long needle axis parallel to the crystallographic c axis
(c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the
temperature dependence of the upper critical fields (HC2) and the surface
critical field (HC3) were measured. The H-T phase diagram is presented. Close
to TC =35 K, for the c-crystal, {\gamma}c = / = 1.80(2), whereas for the
a-crystal the {\gamma}a = / =4.0(2) obtained, is much higher than the
theoretical value 1.69. At low applied dc fields, positive field-cooled
branches known as the "paramagnetic Meissner effect" (PME) are observed, their
magnitude is inversely proportional to H. The anisotropic PME is observed in
both a- and c-crystals, only when the applied field is along the basal planes.
It is speculated that the high {\gamma}a and the PME are connected to each
other.Comment: 10 pages, 7 figuer
Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects
We present the first Ga(4f) NMR study of the Cr susceptibility in the
archetype of Kagome based frustrated antiferromagnets,
SrCrGaO. Our major finding is that the susceptibility of the
frustrated lattice goes through a maximum around 50 K. Our data also supports
the existence of paramagnetic ``clusters'' of spins, responsible for the Curie
behavior observed in the macroscopic susceptibility at low T. These results set
novel features for the constantly debated physics of geometrically frustrated
magnets.Comment: 4 pages, 5 figures Submitted to Phys. Rev. Let
No-fault (strict liability) for injuries from innovative treatments: fairness or also efficiency?
Innovative treatments (ITs) have a distinct SIROT pattern: they often show, and are expected to show, significantly improved results over time. Of the four IT categories discussed, two stand out: SIROT treatments which are currently not in the patient's best interest (BI) but will become superior treatment over time (category 3), and treatments which are already arguably BI but will clearly become the superior option as they improve with time (category 2).
There is a strong fairness argument to compensate patients injured from ITs because their injury led to improved knowledge benefitting future patients. By analogy to private necessity, IT patients should be considered ‘rescuers’ entitled to costs reimbursement, since future patients receive an incontrovertible benefit inextricably linked to their loss. Crucially, category 2 patients also deserve compensation, notwithstanding that their treatment was BI.
From an efficiency perspective, patients should avoid only irresponsible ITs (category 4) but they are not well-placed to identify such treatments. Patients’ incentives to submit to ITs are no worse, and perhaps slightly improve under strict liability (SL): SL might incentivise patients to undergo a treatment whose prospect as BI is in doubt but is likely to be SIROT. Finally, while under SL (but not negligence) category 2 cases yield liability, which may deter physicians from offering ITs, reputation loss under SL is lessened, so physicians’ incentives may improve (and they also have strong non-legal incentives to offer ITs)
Evidence of magnetic mechanism for cuprate superconductivity
A proper understanding of the mechanism for cuprate superconductivity can
emerge only by comparing materials in which physical parameters vary one at a
time. Here we present a variety of bulk, resonance, and scattering measurements
on the (Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_y high temperature
superconductors, in which this can be done. We determine the superconducting,
Neel, glass, and pseudopage critical temperatures. In addition, we clarify
which physical parameter varies, and, equally important, which does not, with
each chemical modification. This allows us to demonstrate that a single energy
scale, set by the superexchange interaction J, controls all the critical
temperatures of the system. J, in-turn, is determined by the in plane Cu-O-Cu
buckling angle.Comment: 17 pages, 13 figure
The Herbertsmithite Hamiltonian: SR measurements on single crystals
We present transverse field muon spin rotation/relaxation measurements on
single crystals of the spin-1/2 kagome antiferromagnet Herbertsmithite. We find
that the spins are more easily polarized when the field is perpendicular to the
kagome plane. We demonstrate that the difference in magnetization between the
different directions cannot be accounted for by Dzyaloshinksii-Moriya type
interactions alone, and that anisotropic axial interaction is present.Comment: 8 pages, 3 figures, accepted to JPCM special issue on geometrically
frustrated magnetis
Gravitational lensing reveals ionizing ultraviolet photons escaping from a distant galaxy
During the epoch of reionisation, neutral gas in the early Universe was
ionized by hard ultraviolet radiation emitted by young stars in the first
galaxies. To do so, ionizing ultraviolet photons must escape from the host
galaxy. We present Hubble Space Telescope observations of the gravitationally
lensed galaxy PSZ1-ARC G311.6602-18.4624, revealing bright, multiply-imaged
ionizing photon escape from a compact star-forming region through a narrow
channel in an optically thick gas. The gravitational lensing magnification
shows how ionizing photons escape this galaxy, contributing to the
re-ionization of the Universe. The multiple sight lines to the source probe
absorption by intergalactic neutral hydrogen on scales of no more than a few
hundred, perhaps even less than ten, parsec.Comment: 17 pages, 9 figures. Published in Scienc
Star Formation at z=2.481 in the Lensed Galaxy SDSS J1110+6459, I: Lens Modeling and Source Reconstruction
Using the combined resolving power of the Hubble Space Telescope and
gravitational lensing, we resolve star-forming structures in a z~2.5 galaxy on
scales much smaller than the usual kiloparsec diffraction limit of HST. SGAS
J111020.0+645950.8 is a clumpy, star forming galaxy lensed by the galaxy
cluster SDSS J1110+6459 at z = 0.659, with a total magnification ~30x across
the entire arc. We use a hybrid parametric/non-parametric strong lensing mass
model to compute the deflection and magnification of this giant arc,
reconstruct the light distribution of the lensed galaxy in the source plane,
and resolve the star formation into two dozen clumps. We develop a
forward-modeling technique to model each clump in the source plane. We ray
trace the model to the image plane, convolve with the instrumental point spread
function (PSF), and compare with the GALFIT model of the clumps in the image
plane, which decomposes clump structure from more extended emission. This
technique has the advantage, over ray tracing, by accounting for the asymmetric
lensing shear of the galaxy in the image plane and the instrument PSF. At this
resolution, we can begin to study star formation on a clump-by-clump basis,
toward the goal of understanding feedback mechanisms and the buildup of
exponential disks at high redshift.Comment: 19 pages, 12 figures, accepted to Ap
- …