13,562 research outputs found
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
The spin-1 two-dimensional J1-J2 Heisenberg antiferromagnet on a triangular lattice
The spin-1 Heisenberg antiferromagnet on a triangular lattice with the
nearest- and next-nearest-neighbor couplings, and , ,
is studied in the entire 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
four second-order phase transitions are observed. At the
ground state is transformed from the long-range ordered spin
structure into a state with short-range ordering, which in its turn is changed
to a long-range ordered state with the ordering vector at . For
a new transition to a state with a short-range order occurs.
This state has a large correlation length which continuously grows with
until the establishment of a long-range order happens at . In
the range , the ordering vector is incommensurate. With growing
it moves along the line to the point which is reached at . The obtained state with a long-range order can be conceived as three
interpenetrating sublattices with the spin structure on each of
them.Comment: 13 pages, 5 figures, accepted for publication in Physics Letters
Stirling cycle cryogenic cooler
A long lifetime Stirling cycle cryogenic cooler particularly adapted for space applications is described. It consists of a compressor section centrally aligned end to end with an expansion section, and respectively includes a reciprocating compressor piston and displacer radially suspended in interconnecting cylindrical housings by active magnetic bearings and has adjacent reduced clearance regions so as to be in noncontacting relationship therewith and wherein one or more of these regions operate as clearance seals. The piston and displacer are reciprocated in their housings by linear drive motors to vary the volume of respectively adjacent compression and expansion spaces which contain a gaseous working fluid and a thermal regenerator to effect Stirling cycle cryogenic cooling
Ferron-like states in YBa_2Cu_3O_(6+x)
With the use of the Hubbard model bound hole states in YBa_2Cu_3O_(6+x) are
studied. For the parameters of this crystal the exchange interaction between
the spin-carrying chain ion O^- and Cu-O plane sites is shown to ensure the
formation of a large ferromagnetically ordered clusters around holes in the
plane.Comment: 8 pages, 2 figure
Magnetic properties of the two-dimensional Heisenberg model on a triangular lattice
The spin Green's function of the antiferromagnetic Heisenberg model on a
triangular lattice is calculated using Mori's projection operator technique. At
T=0 the spin excitation spectrum is shown to have gaps at the wave vectors of
the classical Neel ordering. This points to the absence of the
antiferromagnetic long-range order in the ground state. The calculated spin
correlation on the neighboring sites of the same sublattice is in good
agreement with the value derived from exact diagonalization. The temperature
dependencies of the spin correlations and the gaps are calculated.Comment: 5 pages, 3 figure
Magnetic phase diagram of the spin-1 two-dimensional J1-J3 Heisenberg model on a triangular lattice
The spin-1 Heisenberg model on a triangular lattice with the ferromagnetic
nearest, , and antiferromagnetic third-nearest-neighbor,
, exchange interactions 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 spin structure into a disordered state, which in its
turn is changed to an antiferromagnetic long-range ordered state with the
incommensurate ordering vector at
. With the further growth of the ordering vector moves along
the line to the commensurate point , which is reached at . The final state with an
antiferromagnetic long-range order can be conceived as four interpenetrating
sublattices with the spin structure on each of them. Obtained
results are used for interpretation of the incommensurate magnetic ordering
observed in NiGaS.Comment: 18 pages, 6 figures, accepted for publication in Physics Letters
Resonance peak in underdoped cuprates
The magnetic susceptibility measured in neutron scattering experiments in
underdoped YBaCuO is interpreted based on the self-consistent
solution of the t-J model of a Cu-O plane. The calculations reproduce correctly
the frequency and momentum dependencies of the susceptibility and its variation
with doping and temperature in the normal and superconducting states. This
allows us to interpret the maximum in the frequency dependence -- the resonance
peak -- as a manifestation of the excitation branch of localized Cu spins and
to relate the frequency of the maximum to the size of the spin gap. The
low-frequency shoulder well resolved in the susceptibility of superconducting
crystals is connected with a pronounced maximum in the damping of the spin
excitations. This maximum is caused by intense quasiparticle peaks in the hole
spectral function for momenta near the Fermi surface and by the nesting.Comment: 9 pages, 6 figure
Aquila X--1: a low inclination soft X-ray transient
We have obtained I-band photometry of the neutron star X-ray transient Aql
X--1 during quiescence. We find a periodicity at 2.487 cd-1, which we interpret
as twice the orbital frequency (19.30+/-0.05 h). Folding the data on the
orbital period, we model the light curve variations as the ellipsoidal
modulation of the secondary star. We determine the binary inclination to be
20--31 degrees (90 per cent confidence) and also 95 per cent upper limits to
the radial velocity semi-amplitude and rotational broadening of the secondary
star to be 117 kms-1 and 50 kms-1 respectively.Comment: 4 pages text, 3 figures, to appear in MNRA
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