5,035 research outputs found
Fishtail effect and vortex dynamics in LiFeAs single crystals
We investigate the fishtail effect, critical current density () and
vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak
(SP) in the magnetization loop only with the field c-axis. We calculate a
reasonably high , however, values are lower than in 'Ba-122' and
'1111'-type FeAs-compounds. Magnetic relaxation data imply a strong pinning
which appears not to be due to conventional defects. Instead, its behavior is
similar to that of the triplet superconductor SrRuO. Our data suggest
that the origin of the SP may be related to a vortex lattice phase transition.
We have constructed the vortex phase diagram for LiFeAs on the
field-temperature plane.Comment: 5 pages, 5 figure
Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.Comment: 4 pages, 4 figure
Specific heat of CaNaFeAs single crystals: unconventional s multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
We report a low-temperature specific heat study of high-quality single
crystals of the heavily hole doped superconductor
CaNaFeAs. This compound exhibits bulk
superconductivity with a transition temperature \,K, which is
evident from the magnetization, transport, and specific heat measurements. The
zero field data manifests a significant electronic specific heat in the normal
state with a Sommerfeld coefficient mJ/mol K. Using a
multi-band Eliashberg analysis, we demonstrate that the dependence of the zero
field specific heat in the superconducting state is well described by a
three-band model with an unconventional s pairing symmetry and gap
magnitudes of approximately 2.35, 7.48, and -7.50 meV. Our analysis
indicates a non-negligible attractive intraband coupling,which contributes
significantly to the relatively high value of . The Fermi surface averaged
repulsive and attractive coupling strengths are of comparable size and outside
the strong coupling limit frequently adopted for describing high- iron
pnictide superconductors. We further infer a total mass renormalization of the
order of five, including the effects of correlations and electron-boson
interactions.Comment: 8 Figures, Submitted to PR
DataComm in Flight Deck Surface Trajectory-Based Operations
The purpose of this pilot-in-the-loop aircraft taxi simulation was to evaluate a NextGen concept for surface trajectory-based operations (STBO) in which air traffic control (ATC) issued taxi clearances with a required time of arrival (RTA) by Data Communications (DataComm). Flight deck avionics, driven by an error-nulling algorithm, displayed the speed needed to meet the RTA. To ensure robustness of the algorithm, the ability of 10 two-pilot crews to meet the RTA was tested in nine experimental trials representing a range of realistic conditions including a taxi route change, an RTA change, a departure clearance change, and a crossing traffic hold scenario. In some trials, these DataComm taxi clearances or clearance modifications were accompanied by preview information, in which the airport map display showed a preview of the proposed route changes, including the necessary speed to meet the RTA. Overall, the results of this study show that with the aid of the RTA speed algorithm, pilots were able to meet their RTAs with very little time error in all of the robustness-testing scenarios. Results indicated that when taxi clearance changes were issued by DataComm only, pilots required longer notification distances than with voice communication. However, when the DataComm was accompanied by graphical preview, the notification distance required by pilots was equivalent to that for voice
Gradient-free quantum optimization on NISQ devices
Variational Quantum Eigensolvers (VQEs) have recently attracted considerable
attention. Yet, in practice, they still suffer from the efforts for estimating
cost function gradients for large parameter sets or resource-demanding
reinforcement strategies. Here, we therefore consider recent advances in
weight-agnostic learning and propose a strategy that addresses the trade-off
between finding appropriate circuit architectures and parameter tuning. We
investigate the use of NEAT-inspired algorithms which evaluate circuits via
genetic competition and thus circumvent issues due to exceeding numbers of
parameters. Our methods are tested both via simulation and on real quantum
hardware and are used to solve the transverse Ising Hamiltonian and the
Sherrington-Kirkpatrick spin model.Comment: 13 pages, 6 figures, comments welcome
Chandra and HST observations of gamma-ray blazars: comparing jet emission at small and large scales
We present new Chandra and HST data for four gamma-ray blazars selected on
the basis of radio morphology with the aim of revealing X-ray and optical
emission from their jets at large scales. All the sources have been detected.
Spectral Energy Distributions of the large scale jets are obtained as well as
new X-ray spectra for the blazar cores. Modeling for each object the core
(sub-pc scale) and large-scale (>100 kpc) jet SEDs, we derive the properties of
the same jet at the two scales. The comparison of speeds and powers at
different scales supports a simple scenario for the dynamics and propagation of
high power relativistic jets.Comment: Accepted for publication in Ap
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