590 research outputs found
Strong enhancement of spin fluctuations in the low-temperature-tetragonal phase of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4
Measurements of the static magnetization, susceptibility and ESR of Gd spin
probes have been performed to study the properties of antiferromagnetically
ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature
tetragonal structure. According to the static magnetic measurements the CuO_2
planes are magnetically decoupled in this structural phase. The ESR study
reveals strong magnetic fluctuations at the ESR frequency which are not present
in the orthorhombic phase. It is argued that this drastic enhancement of the
spin fluctuations is due to a considerable weakening of the interlayer exchange
and a pronounced influence of hole motion on the antiferromagnetic properties
of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at
small hole doping is obtained in the present study.Comment: 10 pages, LaTeX, 3 EPS figures; to be published in Journal of
Physics: Condensed Matte
Next-to-next-to-leading order fits to CCFR'97 data and infrared renormalons
We briefly summarize the outcomes of our recent improved fits to the
experimental data of CCFR collaboration for structure function of deep-inelastic scattering at the next-to-next-to-leading order. Special
attention is paid to the extraction of and the parameter of the
infrared renormalon model for -correction at different orders of
perturbation theory. The results can be of interest for planning similar
studies using possible future data of Neutrino Factories.Comment: 3 pages, presented at WG3 of 4th NuFact'02 Workshop, London 1-6 July,
200
Analytical eighth-order light-by-light QED contributions from leptons with heavier masses to the anomalous magnetic moment of electron
The important consequences of the recent results of the numerical evaluations
of eighth and tenth order QED contributions to the anomalous magnetic moment of
electron are commented. The correctness of the results of the numerical
evaluation of new eighth order QED corrections to the electron anomaly are
supported by the demonstration of their consistency with the new analytical
expressions for the QED contributions to from the diagrams with
fourth-order light-by-light scattering muon and tau-lepton loops. The
consistency of the similar results are demonstrated in the case of eighth order
massive dependent contribution to the muon anomalous magnetic moment.Comment: 6 pages, LaTeX, The preliminary version of the longer work with the
same title was registered as CERN Preprint during the visit to Theory
Division of CERN in January of 201
Slow antiferromagnetic dynamics in the low temperature tetragonal phase of La_{2-x}Sr_xCuO_4 as revealed by ESR of Gd spin probes
Measuring the ESR of Gd spin probes we have studied the magnetic properties
of the copper oxide planes in the low temperature tetragonal (LTT) phase of Eu
doped La_{2-x}Sr_xCuO_4. The data give evidence that at particular levels of Sr
and Eu doping the frequency of the antiferromagnetic fluctuations in the LTT
phase dramatically decreases at low temperatures by almost three orders of
magnitude. However, no static magnetic order has been found for T>8K in
contrast to the observation by neutron scattering of stripe ordering of spins
below 50K in a Nd doped La_{2-x}Sr_xCuO_4 single crystal. To our opinion static
order in the Nd doped compound is induced due to the interaction between the Cu
spins with the rare earth magnetic moments. Therefore, a really characteristic
property of the magnetism in the LTT structural phase may be not static
magnetic order at elevated temperatures but rather extremely slow
antiferromagnetic dynamics.Comment: 12 pages RevTex, 2 EPS figures, to appear in Phys.Rev.B, Feb.,9
Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy
Monolayer graphene grown by chemical vapor deposition and transferred to
SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic
energy of 50 eV. The density of defects visible in scanning tunneling
microscopy (STM) is considerably lower than the ion fluence implying that most
of the defects are single vacancies. The vacancies are characterized by
scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak
close to the Fermi level. The peak persists after air exposure up to 180 min,
albeit getting broader. After air exposure for less than 60 min, electron spin
resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a
signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2
spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian
of equal weight exhibiting a width down to 0.17 mT, which, however, depends on
details of the sample preparation. The g-factor anisotropy is about 0.02%.
Temperature dependent measurements reveal antiferromagnetic correlations with a
Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of
graphene is significantly reduced by ion bombardment, the spin resonance
induced change in conductivity is below 10^{-5}.Comment: 10 pages, 5 figures, discussion on STM images in the literature of
defects in graphene adde
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
Reconsidered estimates of the 10th order QED contributions to the muon anomaly
The problem of estimating the 10th order QED corrections to the muon
anomalous magnetic moment is reconsidered. The incorporation of the recently
improved contributions to the and - corrections to
within the renormalization-group inspired scheme-invariant approach
leads to the estimate . It is in good
agreement with the estimate , obtained
by Kinoshita and Nio from the numerical calculations of 2958 10-th order
diagrams, which are considered to be more important than the still uncalculated
6122 10th-order -dependent vertex graphs, and 12672 5-loop
diagrams, responsible for the mass-independent constant contribution both to
and . This confirms Kinoshita and Nio guess about dominance of
the 10-th order diagrams calculated by them. Comparisons with other estimates
of the - contributions to , which exist in the literature,
are presented.Comment: 19 pages, LaTeX, some misprints in the text and literature corrected.
Results unchaged, to appear in Phys.Rev.
Using neural nets for reducing gas concentrations by the data of trass gas analyzer at CO2-laser
Aspects of neutral network construction and its learning for increasing accuracy of gas concentration reduction by measuring data of CO2-laser trass gas analyzer have been considered. Accuracy of reducing atmospheric gas (Н2О, СО2 и О3) concentration by neural network method in comparison with traditionally used method of least square is give
Non-singlet QCD analysis of F_2(x,Q^2) up to NNLO
The significance of NNLO (3-loop) QCD contributions to the flavor non-singlet
sector of F_2^ep and F_2^ed has been studied as compared to uncertainties
(different factorization schemes, higher twist and QED contributions) of
standard NLO (and LO) QCD analyses. The latter effects turn out to be
comparable in size to the NNLO contributions. Therefore the minute NNLO effects
are not observable with presently available data on non-singlet structure
functions.Comment: 14 pages, 2 figure
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