4,121 research outputs found
Upper critical fields and thermally-activated transport of Nd(O_0.7F_0.3)FeAs single crystal
We present measurements of the resistivity and the upper critical field H_c2
of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields
up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is
comparable to ~100 T of high T_c cuprates. H_c2(T) parallel to the c-axis
exhibits a pronounced upward curvature similar to what was extracted from
earlier measurements on polycrystalline samples. Thus this behavior is indeed
an intrinsic feature of oxypnictides, rather than manifestation of vortex
lattice melting or granularity. The orientational dependence of H_c2 shows
deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy
decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent
magnetoresistance and nonlinearities in the Hall coefficient suggest
contribution to the conductivity from electron-electron interactions modified
by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic
resistivity measured below T_c but above the irreversibility field exhibits a
clear Arrhenius thermally activated behavior over 4-5 decades. The activation
energy has very different field dependencies for H||ab and H\perp ab. We
discuss to what extent different pairing scenarios can manifest themselves in
the observed behavior of H_{c2}, using the two-band model of superconductivity.
The results indicate the importance of paramagnetic effects on H_c2(T),which
may significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based on
extrapolations of H_c2(T) near T_c down to low temperatures.Comment: 11 pages, 16 figure
A balanced homodyne detector for high-rate Gaussian-modulated coherent-state quantum key distribution
We discuss excess noise contributions of a practical balanced homodyne
detector in Gaussian-modulated coherent-state (GMCS) quantum key distribution
(QKD). We point out the key generated from the original realistic model of GMCS
QKD may not be secure. In our refined realistic model, we take into account
excess noise due to the finite bandwidth of the homodyne detector and the
fluctuation of the local oscillator. A high speed balanced homodyne detector
suitable for GMCS QKD in the telecommunication wavelength region is built and
experimentally tested. The 3dB bandwidth of the balanced homodyne detector is
found to be 104MHz and its electronic noise level is 13dB below the shot noise
at a local oscillator level of 8.5*10^8 photon per pulse. The secure key rate
of a GMCS QKD experiment with this homodyne detector is expected to reach
Mbits/s over a few kilometers.Comment: 22 pages, 11 figure
Fluctuation-driven capacity distribution in complex networks
Maximizing robustness and minimizing cost are common objectives in the design
of infrastructure networks. However, most infrastructure networks evolve and
operate in a highly decentralized fashion, which may significantly impact the
allocation of resources across the system. Here, we investigate this question
by focusing on the relation between capacity and load in different types of
real-world communication and transportation networks. We find strong empirical
evidence that the actual capacity of the network elements tends to be similar
to the maximum available capacity, if the cost is not strongly constraining. As
more weight is given to the cost, however, the capacity approaches the load
nonlinearly. In particular, all systems analyzed show larger unoccupied
portions of the capacities on network elements subjected to smaller loads,
which is in sharp contrast with the assumptions involved in (linear) models
proposed in previous theoretical studies. We describe the observed behavior of
the capacity-load relation as a function of the relative importance of the cost
by using a model that optimizes capacities to cope with network traffic
fluctuations. These results suggest that infrastructure systems have evolved
under pressure to minimize local failures, but not necessarily global failures
that can be caused by the spread of local damage through cascading processes
The falling chain of Hopkins, Tait, Steele and Cayley
A uniform, flexible and frictionless chain falling link by link from a heap
by the edge of a table falls with an acceleration if the motion is
nonconservative, but if the motion is conservative, being the
acceleration due to gravity. Unable to construct such a falling chain, we use
instead higher-dimensional versions of it. A home camcorder is used to measure
the fall of a three-dimensional version called an -slider. After
frictional effects are corrected for, its vertical falling acceleration is
found to be . This result agrees with the theoretical
value of for an ideal energy-conserving -slider.Comment: 17 pages, 5 figure
Feasibility of quantum key distribution through dense wavelength division multiplexing network
In this paper, we study the feasibility of conducting quantum key
distribution (QKD) together with classical communication through the same
optical fiber by employing dense-wavelength-division-multiplexing (DWDM)
technology at telecom wavelength. The impact of the classical channels to the
quantum channel has been investigated for both QKD based on single photon
detection and QKD based on homodyne detection. Our studies show that the latter
can tolerate a much higher level of contamination from the classical channels
than the former. This is because the local oscillator used in the homodyne
detector acts as a "mode selector" which can suppress noise photons
effectively. We have performed simulations based on both the decoy BB84 QKD
protocol and the Gaussian modulated coherent state (GMCS) QKD protocol. While
the former cannot tolerate even one classical channel (with a power of 0dBm),
the latter can be multiplexed with 38 classical channels (0dBm power each
channel) and still has a secure distance around 10km. Preliminary experiment
has been conducted based on a 100MHz bandwidth homodyne detector.Comment: 18 pages, 5 figure
Half-metallic antiferromagnets in double perovskites: LaAVRuO (A=Ca, Sr, and Ba)
Based on the theoretical exploration of electronic structures, we propose
that the ordered double perovskites LaAVRuO and LaVO/ARuO (001)
superlattice (A = Ca, Sr and Ba) are strong candidates for half-metallic (HM)
antiferromagnets (AFMs). %LaAVRuO and LaVO/ARuO have the %100% spin
polarizations at the Fermi level but with zero %total magnetic moments. We have
shown that the HM-AFM nature in LaAVRuO is very robust regardless of (i)
divalent ion replacement at A-sites, (ii) oxygen site relaxation, (iii) the
inclusion of the Coulomb correlation, and (iv) cation disorder. A type of the
double exchange interaction is expected to be responsible for the
half-metallicity and the antiferromagnetism in these systems.Comment: 4 pages, 4 figure
The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules
Beams of atoms and molecules are stalwart tools for spectroscopy and studies
of collisional processes. The supersonic expansion technique can create cold
beams of many species of atoms and molecules. However, the resulting beam is
typically moving at a speed of 300-600 m/s in the lab frame, and for a large
class of species has insufficient flux (i.e. brightness) for important
applications. In contrast, buffer gas beams can be a superior method in many
cases, producing cold and relatively slow molecules in the lab frame with high
brightness and great versatility. There are basic differences between
supersonic and buffer gas cooled beams regarding particular technological
advantages and constraints. At present, it is clear that not all of the
possible variations on the buffer gas method have been studied. In this review,
we will present a survey of the current state of the art in buffer gas beams,
and explore some of the possible future directions that these new methods might
take
Journal Staff
We present the first measurements of the differential cross section d sigma/dp(T)(gamma) for the production of an isolated photon in association with at least two b-quark jets. The measurements consider photons with rapidities vertical bar y(gamma)vertical bar < 1.0 and transverse momenta 30 < p(T)(gamma) < 200 GeV. The b-quark jets are required to have p(T)(jet) > 15 GeVand vertical bar y(jet)vertical bar < 1.5. The ratio of differential production cross sections for gamma + 2 b-jets to gamma + b-jet as a function of p(T)(gamma) is also presented. The results are based on the proton-antiproton collision data at root s = 1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The measured cross sections and their ratios are compared to the next- to- leading order perturbative QCD calculations as well as predictions based on the k(T)- factorization approach and those from the sherpa and pythia Monte Carlo event generators
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