15,470 research outputs found
Spin correlations among the charge carriers in an ordered stripe phase
We have observed a diffuse component to the low-energy magnetic excitation
spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic
scattering. The diffuse scattering forms a square pattern with sides parallel
and perpendicular to the stripe directions. The signal is dispersive, with a
maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength
with increasing temperature, and is barely visible at 100 K. We argue that the
signal originates from dynamic, quasi- one-dimensional, antiferromagnetic
correlations among the stripe electrons.Comment: 4 pages, 4 figures. To be published in Physical Review Letter
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
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
High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO_0.9F_0.1FeAs_(1-\delta) Superconductor
We report upper critical field B_c2(T) data for disordered
(arsenic-deficient) LaO_0.9F_0.1FeAs_(1-delta) in a wide temperature and
magnetic field range up to 47 T. Because of the large linear slope of Bc2 about
-5.4 T/K to -6.6 T/K near Tc = 28.5 K the T-dependence of the in-plane Bc2(T)
shows a flattening near 23 K above 30 T which points to Pauli-limited behavior
with Bc2(0) about 63-68 T. Our results are discussed in terms of disorder
effects within conventional and unconventional superconducting pairings.Comment: Change of the title as suggested by the Editors, one author added,
typos corrected, references updated, final published versio
Human Substantia Nigra Neurons Encode Unexpected Financial Rewards
The brain's sensitivity to unexpected outcomes plays a fundamental role in an organism's ability to adapt and learn new behaviors. Emerging research suggests that midbrain dopaminergic neurons encode these unexpected outcomes. We used microelectrode recordings during deep brain stimulation surgery to study neuronal activity in the human substantia nigra (SN) while patients with Parkinson's disease engaged in a probabilistic learning task motivated by virtual financial rewards. Based on a model of the participants' expected reward, we divided trial outcomes into expected and unexpected gains and losses. SN neurons exhibited significantly higher firing rates after unexpected gains than unexpected losses. No such differences were observed after expected gains and losses. This result provides critical support for the hypothesized role of the SN in human reinforcement learning
Upper critical field measurements up to 60 T in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields
We report resistivity and upper critical field B_c2(T) data for As deficient
LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to
60 T. These disordered samples exhibit a slightly enhanced superconducting
transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4
T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K
above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB)
with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported
in the literature for clean and disordered samples. Whereas clean samples show
no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of
B_(c2)(T) for applied fields H || ab observed for several disordered closely
related systems is interpreted also as a manifestation of PLB. Consequences of
our results are discussed in terms of disorder effects within the frame of
conventional and unconventional superconductivity.Comment: 4 pages, 3 figures, submitted to RHMF09 (9th International Conference
on the Research in High Magnetic Fields), Dresden, July 22-25, 200
Low-Background In-Trap Decay Spectroscopy with TITAN at TRIUMF
An in-trap decay spectroscopy setup has been developed and constructed for
use with the TITAN facility at TRIUMF. The goal of this device is to observe
weak electron-capture (EC) branching ratios for the odd-odd intermediate nuclei
in the decay process. This apparatus consists of an up-to 6 Tesla,
open-access spectroscopy ion-trap, surrounded radially by up to 7 planar Si(Li)
detectors which are separated from the trap by thin Be windows. This
configuration provides a significant increase in sensitivity for the detection
of low-energy photons by providing backing-free ion storage and eliminating
charged-particle-induced backgrounds. An intense electron beam is also employed
to increase the charge-states of the trapped ions, thus providing storage times
on the order of minutes, allowing for decay-spectroscopy measurements. The
technique of multiple ion-bunch stacking was also recently demonstrated, which
further extends the measurement possibilities of this apparatus. The current
status of the facility and initial results from a In measurement are
presented.Comment: Proceedings for the 2nd International Conference on Advances in
Radioactive Isotope Science (ARIS2014
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