13,647 research outputs found
A study of topologies and protocols for fiber optic local area network
The emergence of new applications requiring high data traffic necessitates the development of high speed local area networks. Optical fiber is selected as the transmission medium due to its inherent advantages over other possible media and the dual optical bus architecture is shown to be the most suitable topology. Asynchronous access protocols, including token, random, hybrid random/token, and virtual token schemes, are developed and analyzed. Exact expressions for insertion delay and utilization at light and heavy load are derived, and intermediate load behavior is investigated by simulation. A new tokenless adaptive scheme whose control depends only on the detection of activity on the channel is shown to outperform round-robin schemes under uneven loads and multipacket traffic and to perform optimally at light load. An approximate solution to the queueing delay for an oscillating polling scheme under chaining is obtained and results are compared with simulation. Solutions to the problem of building systems with a large number of stations are presented, including maximization of the number of optical couplers, and the use of passive star/bus topologies, bridges and gateways
Coherent control of microwave pulse storage in superconducting circuits
Coherent pulse control for quantum memory is viable in the optical domain but
nascent in microwave quantum circuits. We show how to realize coherent storage
and on-demand pulse retrieval entirely within a superconducting circuit by
exploiting and extending existing electromagnetically induced transparency
technology in superconducting quantum circuits. Our scheme employs a linear
array of superconducting artificial atoms coupled to a microwave transmission
line.Comment: 13 pages, 4 figures and some supplementary materia
Turbine vane coolant flow variations and calculated effects on metal temperatures
Seventy-two air-cooled turbine vanes were tested to determine coolant flow variations among the vanes. Calculations were made to estimate the effect of measured coolant flow variations on local vane metal temperatures. The calculations were based on the following assumed operating conditions: turbine inlet temperature, 1700 K (2600 F); turbine inlet pressure, 31 N/sq cm (45 psia); coolant inlet temperature, 811 K (1000 F); and total coolant to gas flow ratio, 0.065. Variations of total coolant flow were not large (about 10 percent from the arithmetic mean) for all 72 vanes, but variations in local coolant flows were large. The local coolant flow variations ranged from 8 to 75 percent, and calculated metal temperature variations ranged from 8 to 60 K (15 to 180 F)
Heterostructure solar cells
The performance of gallium arsenide solar cells grown on Ge substrates is discussed. In some cases the substrate was thinned to reduce overall cell weight with good ruggedness. The conversion efficiency of 2 by 2 cm cells under AMO reached 17.1 percent with a cell thickness of 6 mils. The work described forms the basis for future cascade cell structures, where similar interconnecting problems between the top cell and the bottom cell must be solved. Applications of the GaAs/Ge solar cell in space and the expected payoffs are discussed
Evidence for Strain-Induced Local Conductance Modulations in Single-Layer Graphene on SiO_2
Graphene has emerged as an electronic material that is promising for device applications and for studying two-dimensional electron gases with relativistic dispersion near two Dirac points. Nonetheless, deviations from Dirac-like spectroscopy have been widely reported with varying interpretations. Here we show evidence for strain-induced spatial modulations in the local conductance of single-layer graphene on SiO_2 substrates from scanning tunneling microscopic (STM) studies. We find that strained graphene exhibits parabolic, U-shaped conductance vs bias voltage spectra rather than the V-shaped spectra expected for Dirac fermions, whereas V-shaped spectra are recovered in regions of relaxed graphene. Strain maps derived from the STM studies further reveal direct correlation with the local tunneling conductance. These results are attributed to a strain-induced frequency increase in the out-of-plane phonon mode that mediates the low-energy inelastic charge tunneling into graphene
Scanning Tunneling Spectroscopic Studies of the Low-Energy Quasiparticle Excitations in Cuprate Superconductors
We report scanning tunneling spectroscopic (STS) studies of the low-energy quasiparticle excitations of cuprate superconductors as a function of magnetic field and doping level. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition T_c is associated with a CO energy Δ_(CO) larger (smaller) than the SC gap Δ_(SC). Moreover, the spatial homogeneity of Δ_(SC) and Δ_(CO) depends on the type of disorder in different cuprates: For optimally and under-doped YBa_2Cu_3O_(7−δ) (Y-123), we find that Δ_(SC) < Δ_(CO) and that both Δ_(SC) and Δ(CO) exhibit long-range spatial homogeneity, in contrast to the highly inhomogeneous STS in Bi_2Sr_2CaCu_2O_(8+x) (Bi-2212). We attribute this contrast to the stoichiometric cations and ordered apical oxygen in Y-123, which differs from the non-stoichiometric Bi-to-Sr ratio in Bi-2212 with disordered Sr and apical oxygen in the SrO planes. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder in the CuO_2 planes, giving rise to increasing inhomogeneity, decreasing Δ_(SC) and Δ_(CO), and a suppressed vortex-solid phase. For electron-type cuprate Sr_(0.9)La_(0.1)CuO_2 (La-112), the homogeneous Δ_(SC) and Δ_(CO) distributions may be attributed to stoichiometric cations and the absence of apical oxygen, with Δ_(CO) < Δ_(SC) revealed only inside the vortex cores. Finally, the vortex-core radius (ξ_(halo)) in electron-type cuprates is comparable to the SC coherence length ξ_(SC), whereas ξ_(halo) ∼ 10ξ_(SC) in hole-type cuprates, suggesting that ξ_(halo) may be correlated with the CO strength. The vortex-state irreversibility line in the magnetic field versus temperature phase diagram also reveals doping dependence, indicating the relevance of competing orders to vortex pinning
Dimensionality of superconductivity in the infinite-layer high-temperature cuprate Sr0.9M0.1CuO2 (M = La, Gd)
The high magnetic field phase diagram of the electron-doped infinite layer
high-temperature superconducting (high-T_c) compound Sr_{0.9}La_{0.1}CuO_2 was
probed by means of penetration depth and magnetization measurements in pulsed
fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical
fields with H parallel (H_{c2}^{ab}) and perpendicular (H_{c2}^c) to the CuO_2
planes, with H_{c2}^{ab} extrapolating to near the Pauli paramagnetic limit of
160 T. The longer superconducting coherence length than the lattice constant
along the c-axis indicates that the orbital degrees of freedom of the pairing
wavefunction are three dimensional. By contrast, low-field magnetization and
specific heat measurements of Sr_{0.9}Gd_{0.1}CuO_2 indicate a coexistence of
bulk s-wave superconductivity with large moment Gd paramagnetism close to the
CuO_2 planes, suggesting a strong confinement of the spin degrees of freedom of
the Cooper pair to the CuO_2 planes. The region between H_{c2}^{ab} and the
irreversibility line in the magnetization, H_{irr}^{ab}, is anomalously large
for an electron-doped high-T_c cuprate, suggesting the existence of additional
quantum fluctuations perhaps due to a competing spin-density wave order.Comment: 4 pages, 4 figures, submitted to Phys. Rev. B, Rapid Communications
(2004). Corresponding author: Nai-Chang Yeh (E-mail: [email protected]
Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy
Scanning tunneling spectroscopic studies of (x =
0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap
superconductivity. These gaps decrease with increasing temperature and vanish
above the superconducting transition . The two-gap nature and the slightly
doping- and energy-dependent quasiparticle scattering interferences near the
wave-vectors and are consistent with
sign-changing -wave superconductivity. The excess zero-bias conductance and
the large gap-to- ratios suggest dominant unitary impurity scattering.Comment: 4 pages, 4 figures. Paper accepted for publication in Physical Review
Letters. Contact author: Nai-Chang Yeh ([email protected]
Effect of Strain Relaxation on Magnetotransport properties of epitaxial La_0.7Ca_0.3MnO_3 films
In this paper, we have studied the effect of strain relaxation on
magneto-transport properties of La_0.7Ca_0.3MnO_3 epitaxial films (200 nm
thick), which were deposited by pulsed laser deposition technique under
identical conditions. All the films are epitaxial and have cubic unit cell. The
amount of strain relaxation has been varied by taking three different single
crystal substrates of SrTiO_3, LaAlO_3 and MgO. It has been found that for
thicker films the strain gets relaxed and produces variable amount of disorder
depending on the strength of strain relaxation. The magnitude of lattice
relaxation has been found to be 0.384, 3.057 and 6.411 percent for film
deposited on SrTiO_3, LaAlO_3 and MgO respectively. The films on LaAlO_3 and
SrTiO_3 show higher T_{IM} of 243 K and 217 K respectively as compared to
T_{IM} of 191 K for the film on MgO. Similarly T_C of the films on SrTiO_3 and
LaAlO_3 is sharper and has value of 245 K and 220 K respectively whereas the TC
of the film on MgO is 175 K. Higher degree of relaxation creates more defects
and hence TIM (T_C) of the film on MgO is significantly lower than of SrTiO_3
and LaAlO_3. We have adopted a different approach to correlate the effect of
strain relaxation on magneto-transport properties of LCMO films by evaluating
the resistivity variation through Mott's VRH model. The variable presence of
disorder in these thick films due to lattice relaxation which have been
analyzed through Mott's VRH model provides a strong additional evidence that
the strength of lattice relaxation produces disorder dominantly by increase in
density of defects such as stacking faults, dislocations, etc. which affect the
magneto-transport properties of thick epitaxial La_0.7Ca_0.3MnO_3 films
Noncommutative D-Brane in Non-Constant NS-NS B Field Background
We show that when the field strength H of the NS-NS B field does not vanish,
the coordinates X and momenta P of an open string endpoints satisfy a set of
mixed commutation relations among themselves. Identifying X and P with the
coordinates and derivatives of the D-brane world volume, we find a new type of
noncommutative spaces which is very different from those associated with a
constant B field background.Comment: 11 pages, Latex, minor modification
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