2,314 research outputs found
Robust Adaptive Control of a Class of Nonlinear Strict-feedback Discrete-time Systems with Exact Output Tracking
10.1016/j.automatica.2009.07.025Automatica45112537-2545ATCA
Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering
We use polarized inelastic neutron scattering (INS) to study spin excitations
of optimally hole-doped superconductor BaKFeAs
( K).
In the normal state, the imaginary part of the dynamic susceptibility,
, shows magnetic anisotropy for energies below
7 meV with c-axis polarized spin excitations larger than that of the
in-plane component. Upon entering into the superconducting state, previous
unpolarized INS experiments have shown that spin gaps at 5 and 0.75 meV
open at wave vectors and , respectively, with a
broad neutron spin resonance at meV. Our neutron polarization analysis
reveals that the large difference in spin gaps is purely due to different spin
gaps in the c-axis and in-plane polarized spin excitations, resulting resonance
with different energy widths for the c-axis and in-plane spin excitations. The
observation of spin anisotropy in both opitmally electron and hole-doped
BaFeAs is due to their proximity to the AF ordered BaFeAs where
spin anisotropy exists below .Comment: 5 pages, 4 figure
Scaling of the Fano effect of the in-plane Fe-As phonon and the superconducting critical temperature in BaKFeAs
By means of infrared spectroscopy we determine the temperature-doping phase
diagram of the Fano effect for the in-plane Fe-As stretching mode in
BaKFeAs. The Fano parameter , which is a
measure of the phonon coupling to the electronic particle-hole continuum, shows
a remarkable sensitivity to the magnetic/structural orderings at low
temperatures. More strikingly, at elevated temperatures in the
paramagnetic/tetragonal state we find a linear correlation between and
the superconducting critical temperature . Based on theoretical
calculations and symmetry considerations, we identify the relevant interband
transitions that are coupled to the Fe-As mode. In particular, we show that a
sizable orbital component at the Fermi level is fundamental for the Fano
effect and possibly also for the superconducting pairing.Comment: Supplemental materials are available upon reques
Distinguishing and electron pairing symmetries by neutron spin resonance in superconducting NaFeCoAs
A determination of the superconducting (SC) electron pairing symmetry forms
the basis for establishing a microscopic mechansim for superconductivity. For
iron pnictide superconductors, the -pairing symmetry theory predicts the
presence of a sharp neutron spin resonance at an energy below the sum of hole
and electron SC gap energies () below . On the other hand,
the -pairing symmetry expects a broad spin excitation enhancement at an
energy above below . Although the resonance has been observed in
iron pnictide superconductors at an energy below consistent with the
-pairing symmetry, the mode has also be interpreted as arising from the
-pairing symmetry with due to its broad energy width and
the large uncertainty in determining the SC gaps. Here we use inelastic neutron
scattering to reveal a sharp resonance at E=7 meV in SC
NaFeCoAs ( K). On warming towards , the mode
energy hardly softens while its energy width increases rapidly. By comparing
with calculated spin-excitations spectra within the and
-pairing symmetries, we conclude that the ground-state resonance in
NaFeCoAs is only consistent with the -pairing, and
is inconsistent with the -pairing symmetry.Comment: 9 pages, 8 figures. submitted to PR
Surface waves in photonic crystal slabs
Photonic crystals with a finite size can support surface modes when
appropriately terminated. We calculate the dispersion curves of surface modes
for different terminations using the plane wave expansion method. These
non-radiative surface modes can be excited with the help of attenuated total
reflection technique. We did experiments and simulations to trace the surface
band curve, both in good agreement with the numerical calculations
Experimental Verification of the Quantized Conductance of Photonic Crystal Waveguides
We report experiments that demonstrate the quantization of the conductance of
photonic crystal waveguides. To obtain a diffusive wave, we have added all the
transmitted channels for all the incident angles. The conductance steps have
equal height and a width of one half the wavelength used. Detailed numerical
results agree very well with the novel experimental results.Comment: Phys. Rev. B (submitted
Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction
High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.
Teleportation-based realization of an optical quantum two-qubit entangling gate
In recent years, there has been heightened interest in quantum teleportation,
which allows for the transfer of unknown quantum states over arbitrary
distances. Quantum teleportation not only serves as an essential ingredient in
long-distance quantum communication, but also provides enabling technologies
for practical quantum computation. Of particular interest is the scheme
proposed by Gottesman and Chuang [Nature \textbf{402}, 390 (1999)], showing
that quantum gates can be implemented by teleporting qubits with the help of
some special entangled states. Therefore, the construction of a quantum
computer can be simply based on some multi-particle entangled states, Bell
state measurements and single-qubit operations. The feasibility of this scheme
relaxes experimental constraints on realizing universal quantum computation.
Using two different methods we demonstrate the smallest non-trivial module in
such a scheme---a teleportation-based quantum entangling gate for two different
photonic qubits. One uses a high-fidelity six-photon interferometer to realize
controlled-NOT gates and the other uses four-photon hyper-entanglement to
realize controlled-Phase gates. The results clearly demonstrate the working
principles and the entangling capability of the gates. Our experiment
represents an important step towards the realization of practical quantum
computers and could lead to many further applications in linear optics quantum
information processing.Comment: 10 pages, 6 figure
Doctor-diagnosed sleep apnoea in Hong Kong adolescents: prevalence and associations with night-eating and dinner time
Li Ka Shing Faculty of Medicine Frontiers SeriesSession -: Big Data and Precision Medicine: e-Poster no. 17Symposia Theme: ‘MOOCs in Postmodern Asia’ (Oct 27, 2014) and ‘Big Data and Precision Medicine’ (Oct 28, 2014)published_or_final_versio
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