2,948 research outputs found
Nutation versus angular dependent NQR spectroscopy and the impact of underdoping on charge inhomogeneities in YBaCuO
We describe two different nuclear quadrupole resonance (NQR) based
techniques, designed to measure the local asymmetry of the internal electric
field gradient, and the tilt angle of the main NQR principal axis z from the
crystallographic axis c. These techniques use the dependence of the NQR signal
on the duration of the radio frequency (rf) pulse and on the direction of the
rf field H1 with respect to the crystal axis. The techniques are applied to
oriented powder of YBaCuO fully enriched with 63Cu.
Measurements were performed at different frequencies, corresponding to
different in-plane copper sites with respect to the dopant. Combining the
results from both techniques, we conclude that oxygen deficiency in the chain
layer lead to a rotation of the NQR main principal axis at the nearby Cu on the
CuO2 planes by 20+-degrees. This occurs with no change to the asymmetry. The
axis rotation associated with oxygen deficiency means that there must be
electric field inhomogeneities in the CuO2 planes only in the vicinity of the
missing oxygen.Comment: 9 pages, 10 figure
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
A quest for frustration driven distortion in Y2Mo2O7
We investigated the nature of the freezing in the geometrically frustrated
Heisenberg spin-glass Y2Mo2O7 by measuring the temperature dependence of the
static internal magnetic field distribution above the spin-glass temperature,
Tg, using the muSR technique. The evolution of the field distribution cannot be
explained by changes in the spin susceptibility alone and suggests a lattice
deformation. This possibility is addressed by numerical simulations of the
Heisenberg Hamiltonian with magneto-elastic coupling at T>0.Comment: 5 pages 4 figures. Accepted for publication in PR
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
Stellar Populations of Highly Magnified Lensed Galaxies: Young Starbursts at z~2
We present a comprehensive analysis of the rest-frame UV to near-IR spectral
energy distributions and rest-frame optical spectra of four of the brightest
gravitationally lensed galaxies in the literature: RCSGA 032727-132609 at
z=1.70, MS1512-cB58 at z=2.73, SGAS J152745.1+065219 at z=2.76 and SGAS
J122651.3+215220 at z=2.92. This includes new Spitzer imaging for RCSGA0327 as
well as new spectra, near-IR imaging and Spitzer imaging for SGAS1527 and
SGAS1226. Lensing magnifications of 3-4 magnitudes allow a detailed study of
the stellar populations and physical conditions. We compare star formation
rates as measured from the SED fit, the H-alpha and [OII] emission lines, and
the UV+IR bolometric luminosity where 24 micron photometry is available. The
SFR estimate from the SED fit is consistently higher than the other indicators,
which suggests that the Calzetti dust extinction law used in the SED fitting is
too flat for young star-forming galaxies at z~2. Our analysis finds similar
stellar population parameters for all four lensed galaxies: stellar masses
3-7*10^9 M_sun, young ages ~ 100 Myr, little dust content E(B-V)=0.10-0.25, and
star formation rates around 20-100 M_sun/yr. Compared to typical values for the
galaxy population at z~2, this suggests we are looking at newly formed,
starbursting systems that have only recently started the build-up of stellar
mass. These results constitute the first detailed, uniform analysis of a sample
of the growing number of strongly lensed galaxies known at z~2.Comment: 13 pages, 8 figures, Accepted to Ap
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
Evolution from a nodeless gap to d(x2-y2) form in underdoped La(2-x)SrxCuO4
Using angle-resolved photoemission (ARPES), it is revealed that the
low-energy electronic excitation spectra of highly underdoped superconducting
and non-superconducting La(2-x)SrxCuO4 cuprates are gapped along the entire
underlying Fermi surface at low temperatures. We show how the gap function
evolves to a d(x2-y2) form as increasing temperature or doping, consistent with
the vast majority of ARPES studies of cuprates. Our results provide essential
information for uncovering the symmetry of the order parameter(s) in strongly
underdoped cuprates, which is a prerequisite for understanding the pairing
mechanism and how superconductivity emerges from a Mott insulator.Comment: 5 pages, 4 figure
A Biohybrid Setup for Coupling Biological and Neuromorphic Neural Networks
Developing technologies for coupling neural activity and artificial neural components, is key for advancing neural interfaces and neuroprosthetics. We present a biohybrid experimental setting, where the activity of a biological neural network is coupled to a biomimetic hardware network. The implementation of the hardware network (denoted NeuroSoC) exhibits complex dynamics with a multiplicity of time-scales, emulating 2880 neurons and 12.7 M synapses, designed on a VLSI chip. This network is coupled to a neural network in vitro, where the activities of both the biological and the hardware networks can be recorded, processed, and integrated bidirectionally in real-time. This experimental setup enables an adjustable and well-monitored coupling, while providing access to key functional features of neural networks. We demonstrate the feasibility to functionally couple the two networks and to implement control circuits to modify the biohybrid activity. Overall, we provide an experimental model for neuromorphic-neural interfaces, hopefully to advance the capability to interface with neural activity, and with its irregularities in pathology
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