19,034 research outputs found
Magnetic properties of the doped two-dimensional antiferromagnet
The variety of the normal-state magnetic properties of cuprate high-Tc
superconductors is interpreted based on the self-consistent solution of the
self-energy equations for the two-dimensional t-J model. The observed
variations of the spin correlation length with the hole concentration x, of the
spin susceptibility with x and temperature T and the scaling of the static
uniform susceptibility are well reproduced by the calculated results. The
nonmonotonic temperature dependence of the Cu spin-lattice relaxation rate is
connected with two competing tendencies in the low-frequency susceptibility:
its temperature decrease due to the increasing spin gap and the growth of the
susceptibility in this frequency region with the temperature broadening of the
maximum in the susceptibility.Comment: 6 pages, 5 figures, Proc. Int. Conf. "Modern Problems of
Superconductivity", 9-14 Sept. 2002, Yalta, Ukrain
Two-dimensional t-J model at moderate doping
Using the method which retains the rotation symmetry of spin components in
the paramagnetic state and has no preset magnetic ordering, spectral and
magnetic properties of the two-dimensional t-J model in the normal state are
investigated for the ranges of hole concentrations 0 <= x <= 0.16 and
temperatures 0.01t <= T <= 0.2t. The used hopping t and exchange J parameters
of the model correspond to hole-doped cuprates. The obtained solutions are
homogeneous which indicates that stripes and other types of phase separation
are not connected with the strong electron correlations described by the model.
A series of nearly equidistant maxima in the hole spectral function calculated
for low T and x is connected with hole vibrations in the region of the
perturbed short-range antiferromagnetic order. The hole spectrum has a
pseudogap in the vicinity of (0,\pi) and (\pi,0). For x \approx 0.05 the shape
of the hole Fermi surface is transformed from four small ellipses around
(\pm\pi/2,\pm\pi/2) to two large rhombuses centered at (0,0) and (\pi,\pi). The
calculated temperature and concentration dependencies of the spin correlation
length and the magnetic susceptibility are close to those observed in cuprate
perovskites. These results offer explanations for the observed scaling of the
static uniform susceptibility and for the changes in the spin-lattice
relaxation and spin-echo decay rates in terms of the temperature and doping
variations in the spin excitation spectrum of the model.Comment: 12 pages, 14 figure
A 1.82 m^2 ring laser gyroscope for nano-rotational motion sensing
We present a fully active-controlled He-Ne ring laser gyroscope, operating in
square cavity 1.35 m in side. The apparatus is designed to provide a very low
mechanical and thermal drift of the ring cavity geometry and is conceived to be
operative in two different orientations of the laser plane, in order to detect
rotations around the vertical or the horizontal direction. Since June 2010 the
system is active inside the Virgo interferometer central area with the aim of
performing high sensitivity measurements of environmental rotational noise. So
far, continuous not attempted operation of the gyroscope has been longer than
30 days. The main characteristics of the laser, the active remote-controlled
stabilization systems and the data acquisition techniques are presented. An
off-line data processing, supported by a simple model of the sensor, is shown
to improve the effective long term stability. A rotational sensitivity at the
level of ten nanoradiants per squareroot of Hz below 1 Hz, very close to the
required specification for the improvement of the Virgo suspension control
system, is demonstrated for the configuration where the laser plane is
horizontal
Numerical investigations of scaling at the Anderson transition
At low temperature T, a significant difference between the behavior of
crystals on the one hand and disordered solids on the other is seen:
sufficiently strong disorder can give rise to a transition of the transport
properties from conducting behavior with finite resistance R to insulating
behavior with R=infinity as T -> 0. This well-studied phenomenon is called the
disorder-driven metal-insulator transition and it is characteristic to
non-crystalline solids. In this review of recent advances, we have presented
results of transport studies in disordered systems, ranging from modifications
of the standard Anderson model of localization to effects of a two-body
interaction. Of paramount importance in these studies was always the highest
possible accuracy of the raw data combined with the careful subsequent
application of the finite-size scaling technique. In fact, it is this scaling
method that has allowed numerical studies to move beyond simple extrapolations
and reliably construct estimates of quantities as if one were studying an
infinite system.Comment: 18 pages, 6 figures, "The Anderson Transition and its
Ramifications-Localisation, Quantum Interference, and Interactions", 'Lecture
Notes in Physics' series, ed. T. Brandes and S. Kettemann, Springer Verlag,
to be publishe
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