121 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
Magnetic properties of the spin-1 two-dimensional Heisenberg model on a triangular lattice
Motivated by the recent experiment in NiGaS, the spin-1 Heisenberg
model on a triangular lattice with the ferromagnetic nearest- and
antiferromagnetic third-nearest-neighbor exchange interactions,
and , is studied in the range of the parameter . Mori's projection operator technique is used as a method, which retains the
rotation symmetry of spin components and does not anticipate any magnetic
ordering. For zero temperature several phase transitions are observed. At the ground state is transformed from the ferromagnetic order into
a disordered state, which in its turn is changed to an antiferromagnetic
long-range ordered state with the incommensurate ordering vector at . With growing the ordering vector moves along the line to the
commensurate point , which is reached at . The
final state with the antiferromagnetic long-range order can be conceived as
four interpenetrating sublattices with the spin structure on each of
them. Obtained results offer a satisfactory explanation for the experimental
data in NiGaS.Comment: 2 pages, 3 figure
Magnetic incommensurability and fluctuating charge density waves in the repulsive Hubbard model
Magnetic and charge susceptibilities of the two-dimensional repulsive Hubbard
model are investigated applying a strong coupling diagram technique in which
the expansion in powers of the hopping constants is used. For small lattices
and high temperatures results are in agreement with Monte Carlo simulations.
With the departure from half-filling the low-frequency magnetic
susceptibility becomes incommensurate and the incommensurability parameter
grows with . The incommensurability, its dependence on frequency and on
resemble experimental results in lanthanum cuprates. Also for finite sharp
maxima appear in the static charge susceptibility. The maxima are finite which
points to the absence of the long-range charge ordering (static stripes).
However, for the maxima are located near the momenta
, . In this case an interaction of carriers with
tetragonal distortions can stabilize stripes with the wavelength of four
lattice spacings, as observed in the low-temperature tetragonal phase of
cuprates. As follows from the obtained results, the magnetic incommensurability
is not a consequence of the stripes.Comment: 4 pages, 3 figures, manuscript for proceefings of LT2
Spin dynamics in cuprate perovskites
Results obtained with the use of the t–J model of Cu–O planes and Mori’s projection operator
formalism are compared with data of neutron scattering experiments in lanthanum and yttrium
cuprates. This comparison allows us to interpret the intensive peak at the antiferromagnetic wave
vector observed in yttrium cuprates as a manifestation of excitations of localized Cu spins. The
high-frequency incommensurability detected both in lanthanum and yttrium cuprates is connected
with the dispersion of these excitations, while the low-frequency incommensurability arises due to
a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping
the dip stems from the weakness of the interaction between the spin excitations and holes near hot
spots. It is conjectured that the dissimilarity of the susceptibility frequency dependencies in yttrium
and lanthanum cuprates may be connected with different values of the hole bandwidth and
damping in these crystals
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