112 research outputs found
Az alvásidő felmérése Magyarországon és Romániában élő iskoláskorú gyermekek körében
Introduction: Children's sleep duration is decreasing in the last decade. Despite of the well known negative consequences, there are no data on children's sleep duration in Hungary and Romania. Aim: The aim of the authors was to assess sleep duration of school-age children in Hungary and Romania. Method: A self-edited questionnaire was used for the study. 2446 children were enrolled. All elementary and secondary schools in a Hungarian city, and one elementary and secondary school in a Romanian city took part in the study. Results: Mean sleep duration was 8.3+/-1.2 hours on weekdays. There was a significant difference between the two countries (Hungary vs. Romania, 8.5+/-1.2 hours vs. 7.8+/-0.9 hours, p = 0.001). Age correlated with sleep duration on weekdays (r= -0.605, p = 0.001), but not during weekend. Conclusions: this is the first study on children's sleep duration in Hungary and Romania. The difference between countries may be due to the difference in mean age or cultural and/or geographical differences. Orv. Hetil., 2013, 154, 1592-1596
Momentum-Resolved Charge Excitations in a Prototype One Dimensional Mott Insulator
We report momentum resolved charge excitations in a one dimensional (1-D)
Mott insulator studied using high resolution (~ 325 meV) inelastic x-ray
scattering over the entire Brillouin zone for the first time. Excitations at
the insulating gap edge are found to be highly dispersive (momentum
dependent)compared to excitations observed in two dimensional Mott insulators.
The observed dispersion in 1-D is consistent with charge excitations involving
holons which is unique to spin-1/2 quantum chain systems. These results point
to the potential utility of inelastic x-ray scattering in providing valuable
information about electronic structure of strongly correlated insulators.Comment: 3 pages, 2 figures, Revised with minor change
Spectral properties of one dimensional insulators and superconductors
Conformal field theory and Bethe ansatz are used to investigate the low
energy features of the spectral function in one dimensional models which
exhibit a gap in the spin or in the charge excitation spectrum. Exotic behavior
is found in the superconducting case, where the Green function displays
momentum dependent Luttinger Liquid exponents. The predictions of the formalism
are confirmed by Lanczos diagonalizations in the model up to 32 sites.
These results may be relevant in connection to photoemission experiments in
quasi one dimensional insulators or superconductors.Comment: 11 pages , RevTeX , 3 uuencoded picture
Superconductivity in a Two-Orbital Hubbard Model with Electron and Hole Fermi Pockets: Application in Iron Oxypnictide Superconductors
We investigate the electronic states of a one-dimensional two-orbital Hubbard
model with band splitting by the exact diagonalization method. The Luttinger
liquid parameter is calculated to obtain superconducting (SC) phase
diagram as a function of on-site interactions: the intra- and inter-orbital
Coulomb and , the Hund coupling , and the pair transfer . In
this model, electron and hole Fermi pockets are produced when the Fermi level
crosses both the upper and lower orbital bands. We find that the system shows
two types of SC phases, the SC \Roman{u'-large} for and the SC
\Roman{u-large} for , in the wide parameter region including both weak
and strong correlation regimes. Pairing correlation functions indicate that the
most dominant pairing for the SC \Roman{u'-large} (SC \Roman{u-large}) is the
intersite (on-site) intraorbital spin-singlet with (without) sign reversal of
the order parameters between two Fermi pockets. The result of the SC
\Roman{u'-large} is consistent with the sign-reversing s-wave pairing that has
recently been proposed for iron oxypnictide superconductors.Comment: 5 pages, 8 figures, to appear in J. Phys. Soc. Jpn., Vol.78, No.12,
p.12470
Angle-resolved photoemission study of untwinned PrBaCuO: undoped CuO plane and doped CuO chain
We have performed an angle-resolved photoemission study on untwinned
PrBaCuO, which has low resistivity but does not show
superconductivity. We have observed a dispersive feature with a band maximum
around (/2,/2), indicating that this band is derived from the undoped
CuO plane. We have observed another dispersive band exhibiting
one-dimensional character, which we attribute to signals from the doped CuO
chain. The overall band dispersion of the one-dimensional band agrees with the
prediction of model calculation with parameters relevant to cuprates
except that the intensity near the Fermi level is considerably suppressed in
the experiment.Comment: 6 pages, 10 figure
Angle-resolved photoemission study of insulating and metallic Cu-O chains in PrBaCuO and PrBaCuO
We compare the angle-resolved photoemission spectra of the hole-doped Cu-O
chains in PrBaCuO (Pr123) and in PrBaCuO (Pr124).
While, in Pr123, a dispersive feature from the chain takes a band maximum at
(momentum along the chain) and loses its spectral weight
around the Fermi level, it reaches the Fermi level at in
Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled,
they show contrasting behaviors: While the chains in Pr123 have an instability
to charge ordering, those in Pr124 avoid it and show an interesting spectral
feature of a metallic coupled-chain system.Comment: 4 pages, 5 figures, to be published in PR
On the 3-particle scattering continuum in quasi one dimensional integer spin Heisenberg magnets
We analyse the three-particle scattering continuum in quasi one dimensional
integer spin Heisenberg antiferromagnets within a low-energy effective field
theory framework. We exactly determine the zero temperature dynamical structure
factor in the O(3) nonlinear sigma model and in Tsvelik's Majorana fermion
theory. We study the effects of interchain coupling in a Random Phase
Approximation. We discuss the application of our results to recent
neutron-scattering experiments on the Haldane-gap material .Comment: 8 pages of revtex, 5 figures, small changes, to appear in PR
Parquet solution for a flat Fermi surface
We study instabilities occurring in the electron system whose Fermi surface
has flat regions on its opposite sides. Such a Fermi surface resembles Fermi
surfaces of some high- superconductors. In the framework of the parquet
approximation, we classify possible instabilities and derive
renormalization-group equations that determine the evolution of corresponding
susceptibilities with decreasing temperature. Numerical solutions of the
parquet equations are found to be in qualitative agreement with a ladder
approximation. For the repulsive Hubbard interaction, the antiferromagnetic
(spin-density-wave) instability dominates, but when the Fermi surface is not
perfectly flat, the -wave superconducting instability takes over.Comment: REVTeX, 36 pages, 20 ps figures inserted via psfig. Submitted to
Phys. Rev.
Statistical Theory of Spin Relaxation and Diffusion in Solids
A comprehensive theoretical description is given for the spin relaxation and
diffusion in solids. The formulation is made in a general
statistical-mechanical way. The method of the nonequilibrium statistical
operator (NSO) developed by D. N. Zubarev is employed to analyze a relaxation
dynamics of a spin subsystem. Perturbation of this subsystem in solids may
produce a nonequilibrium state which is then relaxed to an equilibrium state
due to the interaction between the particles or with a thermal bath (lattice).
The generalized kinetic equations were derived previously for a system weakly
coupled to a thermal bath to elucidate the nature of transport and relaxation
processes. In this paper, these results are used to describe the relaxation and
diffusion of nuclear spins in solids. The aim is to formulate a successive and
coherent microscopic description of the nuclear magnetic relaxation and
diffusion in solids. The nuclear spin-lattice relaxation is considered and the
Gorter relation is derived. As an example, a theory of spin diffusion of the
nuclear magnetic moment in dilute alloys (like Cu-Mn) is developed. It is shown
that due to the dipolar interaction between host nuclear spins and impurity
spins, a nonuniform distribution in the host nuclear spin system will occur and
consequently the macroscopic relaxation time will be strongly determined by the
spin diffusion. The explicit expressions for the relaxation time in certain
physically relevant cases are given.Comment: 41 pages, 119 Refs. Corrected typos, added reference
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