5,295 research outputs found
Friedmann limits of rotating hypersurface-homogeneous dust models
The existence of Friedmann limits is systematically investigated for all the
hypersurface-homogeneous rotating dust models, presented in previous papers by
this author. Limiting transitions that involve a change of the Bianchi type are
included. Except for stationary models that obviously do not allow it, the
Friedmann limit expected for a given Bianchi type exists in all cases. Each of
the 3 Friedmann models has parents in the rotating class; the k = +1 model has
just one parent class, the other two each have several parent classes. The type
IX class is the one investigated in 1951 by Goedel. For each model, the
consecutive limits of zero rotation, zero tilt, zero shear and spatial isotropy
are explicitly calculated.Comment: 39 pages, LaTeX, 1 postscript figure. Subjects: General relativity,
exact solutions, cosmolog
Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations
By use of ultrasound studies we resolved magneto-acoustic quantum oscillation
deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C.
Below the upper critical field, only a very weak additional damping appears
that can be well explained by the field inhomogeneity caused by the flux-line
lattice in the mixed state. This is clear evidence for no or a vanishingly
small gap for one of the bands, namely, the spheroidal alpha band. This
contrasts de Haas--van Alphen data obtained by use of torque magnetometry for
the same sample, with a rapidly vanishing oscillation signal in the mixed
state. This points to a strongly distorted flux-line lattice in the latter case
that, in general, can hamper a reliable extraction of gap parameters by use of
such techniques.Comment: 6 pages, 6 figure
Suppression of ferromagnetism in CeSi_1.81 under temperature and pressure
We have studied the pressure dependence of the magnetization of single
crystalline CeSi_1.81. At ambient pressure ferromagnetism develops below T_C =
9.5 Below ~ 5 K an additional shoulder in low-field hysteresis loops and a
metamagnetic crossover around 4 T suggest the appearance of an additional
magnetic modulation to the ferromagnetic state. The suppression of the magnetic
order in CeSi_1.81 as function of temperature at ambient pressure and as
function of pressure at low temperature are in remarkable qualitative
agreement. The continuous suppression of the ordered moment at p ~ 13.1 kbar
suggests the existence of a ferromagnetic quantum critical point in this
material.Comment: 9 pages, 9 figures, to be published in Physical Review
New insight into the physics of iron pnictides from optical and penetration depth data
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.Comment: 34 pages, 10 figures, submitted to New Journal of Physics
(30.01.2009
Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the Kondo system Ce2Rh3Al9
We have used temperature dependent x-ray absorption at the Ce-L3 edge to
investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic
changes of the spectral lineshape with decreasing temperature are analyzed and
found to be related to a change in the occupation number, n_f, as the
system undergoes a transition into a Kondo state. The temperature dependence of
indicates a characteristic temperature of 150K, which is clearly related
with the high temperature anomaly observed in the magnetic susceptibility of
the same system. The further anomaly observed in the resistivity of this system
at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo
origin.Comment: 7 pages, three figures, submitted to PR
Performance of the EUDET-type beam telescopes
Test beam measurements at the test beam facilities of DESY have been
conducted to characterise the performance of the EUDET-type beam telescopes
originally developed within the EUDET project. The beam telescopes are equipped
with six sensor planes using MIMOSA26 monolithic active pixel devices. A
programmable Trigger Logic Unit provides trigger logic and time stamp
information on particle passage. Both data acquisition framework and offline
reconstruction software packages are available. User devices are easily
integrable into the data acquisition framework via predefined interfaces.
The biased residual distribution is studied as a function of the beam energy,
plane spacing and sensor threshold. Its standard deviation at the two centre
pixel planes using all six planes for tracking in a 6\,GeV
electron/positron-beam is measured to be
(2.88\,\pm\,0.08)\,\upmu\meter.Iterative track fits using the formalism of
General Broken Lines are performed to estimate the intrinsic resolution of the
individual pixel planes. The mean intrinsic resolution over the six sensors
used is found to be (3.24\,\pm\,0.09)\,\upmu\meter.With a 5\,GeV
electron/positron beam, the track resolution halfway between the two inner
pixel planes using an equidistant plane spacing of 20\,mm is estimated to
(1.83\,\pm\,0.03)\,\upmu\meter assuming the measured intrinsic resolution.
Towards lower beam energies the track resolution deteriorates due to increasing
multiple scattering. Threshold studies show an optimal working point of the
MIMOSA26 sensors at a sensor threshold of between five and six times their RMS
noise. Measurements at different plane spacings are used to calibrate the
amount of multiple scattering in the material traversed and allow for
corrections to the predicted angular scattering for electron beams
Pulmonary Hypertension in Patients with Chronic Fibrosing Idiopathic Interstitial Pneumonias
Background Pulmonary hypertension (PH) is a common finding in patients with chronic fibrosing idiopathic interstitial pneumonias (IIP). Little is known about the response to pulmonary vasodilator therapy in this patient population. COMPERA is an international registry that prospectively captures data from patients with various forms of PH receiving pulmonary vasodilator therapies. Methods We retrieved data from COMPERA to compare patient characteristics, treatment patterns, response to therapy and survival in newly diagnosed patients with idiopathic pulmonary arterial hypertension (IPAH) and PH associated with IIP (PH-IIP). Results Compared to patients with IPAH (n = 798), patients with PH-IIP (n = 151) were older and predominantly males. Patients with PH-IIP were treated predominantly with phosphodiesterase-5 inhibitors (88% at entry, 87% after 1 year). From baseline to the first follow-up visit, the median improvement in 6MWD was 30 m in patients with IPAH and 24.5 m in patients with PH-IIP (p = 0.457 for the difference between both groups). Improvements in NYHA functional class were observed in 22.4% and 29.5% of these patients, respectively (p = 0.179 for the difference between both groups). Survival rates were significantly worse in PH-IIP than in IPAH (3-year survival 34.0 versus 68.6%; p<0.001). Total lung capacity, NYHA class IV, and mixed-venous oxygen saturation were independent predictors of survival in patients with PH-IIP. Conclusions Patients with PH-IIP have a dismal prognosis. Our results suggest that pulmonary vasodilator therapy may be associated with short-term functional improvement in some of these patients but it is unclear whether this treatment affects survival
Highly Dispersive Spin Excitations in the Chain Cuprate Li2CuO2
We present an inelastic neutron scattering investigation of Li2CuO2 detecting
the long sought quasi-1D magnetic excitations with a large dispersion along the
CuO2-chains studied up to 25 meV. The total dispersion is governed by a
surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral
J1=-228 K. An anomalous quartic dispersion near the zone center and a
pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral
excitation with a pitch angle about 41 degree point to the vicinity of a 3D
FM-spiral critical point. The leading exchange couplings are obtained applying
standard linear spin-wave theory. The 2nd neighbor inter-chain interaction
suppresses a spiral state and drives the FM in-chain ordering below the Ne'el
temperature. The obtained exchange parameters are in agreement with the results
for a realistic five-band extended Hubbard Cu 3d O 2p model and L(S)DA+U
predictions.Comment: 6 pages, 4 figures, submitted to Europhys. Let
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