1,260 research outputs found
Substitution induced pinning in MgB_2 superconductor doped with SiC nano-particles
By doping MgB_2 superconductor with SiC nano-particles, we have successfully
introduced pinning sites directly into the crystal lattice of MgB_2 grains
(intra-grain pinning). It became possible due to the combination of
counter-balanced Si and C co-substitution for B, leading to a large number of
intra-granular dislocations and the dispersed nano-size impurities induced by
the substitution. The magnetic field dependence of the critical current density
was significantly improved in a wide temperature range, whereas the transition
temperature in the sample MgB_2(SiC)_x having x = 0.34, the highest doping
level prepared, dropped only by 2.6 K.Comment: 4 pages, 6 figure
Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2
Doping effect of diamond nanoparticles on the superconducting properties of
MgB2 bulk material has been studied. It is found that the superconducting
transition temperature Tc of MgB2 is suppressed by the diamond-doping, however,
the irreversibility field Hirr and the critical current density Jc are
systematically enhanced. Microstructural analysis shows that the diamond-doped
MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm)
with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20
nm) inside the grains. High density of dislocations and diamond nanoparticles
may take the responsibility for the enhanced flux pinning in the diamond-doped
MgB2.Comment: 16 pages, 6 figure
Maximum likelihood estimation of photon number distribution from homodyne statistics
We present a method for reconstructing the photon number distribution from
the homodyne statistics based on maximization of the likelihood function
derived from the exact statistical description of a homodyne experiment. This
method incorporates in a natural way the physical constraints on the
reconstructed quantities, and the compensation for the nonunit detection
efficiency.Comment: 3 pages REVTeX. Final version, to appear in Phys. Rev. A as a Brief
Repor
Operational Theory of Homodyne Detection
We discuss a balanced homodyne detection scheme with imperfect detectors in
the framework of the operational approach to quantum measurement. We show that
a realistic homodyne measurement is described by a family of operational
observables that depends on the experimental setup, rather than a single field
quadrature operator. We find an explicit form of this family, which fully
characterizes the experimental device and is independent of a specific state of
the measured system. We also derive operational homodyne observables for the
setup with a random phase, which has been recently applied in an ultrafast
measurement of the photon statistics of a pulsed diode laser. The operational
formulation directly gives the relation between the detected noise and the
intrinsic quantum fluctuations of the measured field. We demonstrate this on
two examples: the operational uncertainty relation for the field quadratures,
and the homodyne detection of suppressed fluctuations in photon statistics.Comment: 7 pages, REVTe
Superconductivity, critical current density, and flux pinning in MgB_{2-x}(SiC)_{x/2} superconductor after SiC nanoparticle doping
We investigated the effect of SiC nano-particle doping on the crystal lattice
structure, critical temperature T_c, critical current density J_c, and flux
pinning in MgB_2 superconductor. A series of MgB_{2-x}(SiC)_{x/2} samples with
x = 0 to 1.0 were fabricated using in-situ reaction process. The contraction of
the lattice and depression of T_c with increasing SiC doping level remained
rather small due to the counter-balanced effect of Si and C co-doping. The high
level Si and C co-doping allowed the creation of intra-grain defects and highly
dispersed nano-inclusions within the grains which can act as effective pinning
centers for vortices, improving J_c behavior as a function of the applied
magnetic field. The enhanced pinning is mainly attributable to the
substitution-induced defects and a local structure fluctuations within grains.
A pinning mechanism is proposed to account for different contributions of
different defects in MgB_{2-x}(SiC)_{x/2} superconductors.Comment: 7 pages, 8 figure
Perfectionism, achievement motives, and attribution of success and failure in female soccer players
While some researchers have identified adaptive perfectionism as a key characteristic to achieving elite performance in sport, others see perfectionism as a maladaptive characteristic that undermines, rather than helps, athletic performance. Arguing that perfectionism in sport contains both adaptive and maladaptive facets, the present article presents a study of N 5 74 female soccer players investigating how two facets of perfectionismâperfectionistic strivings and negative reactions to imperfection (Stoeber, Otto, Pescheck, Becker, & Stoll, 2007)âare related to achievement motives and attributions of success and failure. Results show that striving for perfection was related to hope of success and self-serving attributions (internal attribution of success). Moreover, once overlap between the two facets of perfectionism was controlled for, striving for perfection was inversely related to fear of failure and self-depreciating attributions (internal attribution of failure). In contrast,
negative reactions to imperfection were positively related to fear of failure and self-depreciating attributions (external attribution of success) and inversely related to self-serving attributions (internal attribution of success and external attribution of failure). It is concluded that striving for perfection in sport is associated with an adaptive pattern of positive motivational orientations and self-serving attributions of success and failure, which
may help athletic performance. In contrast, negative reactions to imperfection are associated with a maladaptive
pattern of negative motivational orientations and self-depreciating attributions, which is likely to undermine athletic performance. Consequently, perfectionism in sport may be adaptive in those athletes who strive for perfection, but can control their negative reactions when performance is less than perfect
Bitwise Bell inequality violations for an entangled state involving 2N ions
Following on from previous work [J. A. Larsson, Phys. Rev. A 67, 022108
(2003)], Bell inequalities based on correlations between binary digits are
considered for a particular entangled state involving 2N trapped ions. These
inequalities involve applying displacement operations to half of the ions and
then measuring correlations between pairs of corresponding bits in the binary
representations of the number of centre-of-mass phonons of N particular ions.
It is shown that the state violates the inequalities and thus displays
nonclassical correlations. It is also demonstrated that it violates a Bell
inequality when the displacements are replaced by squeezing operations.Comment: 12 pages, 5 figures, accepted for publication in Phys. Rev.
Quantum homodyne tomography with a priori constraints
I present a novel algorithm for reconstructing the Wigner function from
homodyne statistics. The proposed method, based on maximum-likelihood
estimation, is capable of compensating for detection losses in a numerically
stable way.Comment: 4 pages, REVTeX, 2 figure
Magnetic field processing to enhance critical current densities of MgB2 superconductors
Magnetic field of up to 12 T was applied during the sintering process of pure
MgB2 and carbon nanotube (CNT) doped MgB2 wires. We have demonstrated that
magnetic field processing results in grain refinement, homogeneity and
significant enhancement in Jc(H) and Hirr. The Jc of pure MgB2 wire increased
by up to a factor of 3 to 4 and CNT doped MgB2 by up to an order of magnitude
in high field region respectively, compared to that of the non-field processed
samples. Hirr for CNT doped sample reached 7.7 T at 20 K. Magnetic field
processing reduces the resistivity in CNT doped MgB2, straightens the entangled
CNT and improves the adherence between CNTs and MgB2 matrix. No crystalline
alignment of MgB2 was observed. This method can be easily scalable for a
continuous production and represents a new milestone in the development of MgB2
superconductors and related systems
Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping
Doping of MgB2 by nano-SiC and its potential for improvement of flux pinning
was studied for MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10wt% nano-SiC
doped MgB2 samples. Co-substitution of B by Si and C counterbalanced the
effects of single-element doping, decreasing Tc by only 1.5K, introducing
pinning centres effective at high fields and temperatures and enhancing Jc and
Hirr significantly. Compared to the non-doped sample, Jc for the 10wt% doped
sample increased by a factor of 32 at 5K and 8T, 42 at 20K and 5T, and 14 at
30K and 2T. At 20K, which is considered to be a benchmark operating temperature
for MgB2, the best Jc for the doped sample was 2.4x10^5A/cm2 at 2T, which is
comparable to Jc of the best Ag/Bi-2223 tapes. At 20K and 4T, Jc was
36,000A/cm2, which was twice as high as for the best MgB2 thin films and an
order of magnitude higher than for the best Fe/MgB2 tapes. Because of such high
performance, it is anticipated that the future MgB2 conductors will be made
using the formula of MgBxSiyCz instead of the pure MgB2
- âŠ