674 research outputs found
First and second order transition of frustrated Heisenberg spin systems
Starting from the hypothesis of a second order transition we have studied
modifications of the original Heisenberg antiferromagnet on a stacked
triangular lattice (STA-model) by the Monte Carlo technique. The change is a
local constraint restricting the spins at the corners of selected triangles to
add up to zero without stopping them from moving freely (STAR-model). We have
studied also the closely related dihedral and trihedral models which can be
classified as Stiefel models. We have found indications of a first order
transition for all three modified models instead of a universal critical
behavior. This is in accordance with the renormalization group investigations
but disagrees with the Monte Carlo simulations of the original STA-model
favoring a new universality class. For the corresponding x-y antiferromagnet
studied before, the second order nature of the transition could also not be
confirmed.Comment: 31 pages, 13 figures, to be published in Euro. J. Phys.
A Spin-1/2 Model for CsCuCl_3 in an External Magnetic Field
CsCuCl_3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet.
We discuss models for its zero-temperature magnetization process. The models
range from three antiferromagnetically coupled ferromagnetic chains to the full
three-dimensional situation. The situation with spin-1/2 is treated by
expansions around the Ising limit and exact diagonalization. Further,
weak-coupling perturbation theory is used mainly for three coupled chains which
are also investigated numerically using the density-matrix renormalization
group technique. We find that already the three-chain model gives rise to the
plateau-like feature at one third of the saturation magnetization which is
observed in magnetization experiments on CsCuCl_3 for a magnetic field
perpendicular to the crystal axis. For a magnetic field parallel to the crystal
axis, a jump is observed in the experimental magnetization curve in the region
of again about one third of the saturation magnetization. In contrast to
earlier spinwave computations, we do not find any evidence for such a jump with
the model in the appropriate parameter region.Comment: 13 pages LaTeX2e with EPJ macro package (included), 8 (e)ps figures
included using psfig.sty; this is the final version to appear in Eur. Phys. J
B; a few further explanations and one reference adde
Application of Microcanonical Temperature to the Spin Crossover of Fe-co Compounds
Using the Rugh's microcanonical approach to temperature we study the classical model of three dimensional spin-crossover of Fe-Co compounds. These compounds are characterized by magnetic ions that can be in a high-spin or low-spin state. We consider the case of diamagnetic low-spin state. The values of the magnetization average, and fraction of high-spin/low-spin are studied over a wide range of values for the system size, temperature, magnetic field, energy difference, nearest neighbor coupling and exchange interaction. We also address the metastability according to the relative values of interaction parameters and the phase diagram of the model.
Keywords: phase transition, dynamical temperature, spin crossove
MicroRNAs in Pediatric Acute Lymphoblastic Leukemia: Small players with huge potential
Hematopoiesis is a dynamic balance of cellular proliferation, survival, apoptosis and
differentiation in which the pluripotent hematopoietic stem cell gives rise to lymphoid and
myeloid precursors of blood cells. The B-lymphoid precursor sequentially differentiates from
proB-cells into common/preB-cells and fi nally yields mature B-lymphocytes. The T-lymphoid
precursor generates thymocytes or proT-cells that further differentiate into T-lymphocytes.
The myeloid precursor gives rise to granulocytes, monocytes, platelets and erythrocytes. This
process is under tight surveillance by regulators including transcription factors, cytokines
and growth factors. In case of defects in surveillance and/or oncogenic hits in the maturing
hematopoietic cell, cells may arrest at different levels of maturation. Leukemia cells grow
exponentially with limited differentiation thereby suppressing the development of normal
blood cells in the bone marrow. As a consequence leukemia patients suffer from fatigue
(shortage of erythrocytes or anemia), infections (shortage of mature lymphocytes and
granulocytes) and bleeding (low platelet numbers). Despite the fact that multiple consecutive
events are required for the development of leukemia, the clinical symptoms associated with
acute leukemia occur rather abrupt and the progression of the disease is very fast if not
treated instantly
Quantum shock waves in the Heisenberg XY model
We show the existence of quantum states of the Heisenberg XY chain which
closely follow the motion of the corresponding semi-classical ones, and whose
evolution resemble the propagation of a shock wave in a fluid. These states are
exact solutions of the Schroedinger equation of the XY model and their
classical counterpart are simply domain walls or soliton-like solutions.Comment: 15 pages,6 figure
Fluctuation-induced phase in CsCuCl3 in transverse magnetic field: Theory
CsCuCl3 is a quantum triangular antiferromagnet, ferromagnetically stacked,
with an incommensurate (IC) structure due to a Dzyaloshinskii-Moriya
interaction. Because of the classical degeneracy caused by the frustration,
fluctuations in CsCuCl3 have extraordinarily large effects, such as the phase
transition in longitudinal magnetic field (normal to the planes, parallel to
the IC wavenumber q) and the plateau in q in transverse field (perpendicular to
q). We argue that fluctuations are responsible also for the new IC phase
discovered in transverse field near the Neel temperature T_N, by T. Werner et
al. [Solid State Commun. 102, p.609 (1997)]. We develop and analyse the
corresponding minimal Landau theory; the effects of fluctuations on the
frustration are included phenomenologically, by means of a biquadratic term.
The Landau theory gives two IC phases, one familiar from previous studies;
properties of the new IC phase, which occupies a pocket of the
temperature-field phase diagram near T_N, agree qualitatively with those of the
new phase found experimentally.Comment: 12 pages, revtex, 4 postscript figures, submitted to J. Phys:
Condens. Matte
Dynamical Effective Medium Theory for Quantum Spins and Multipoles
A dynamical effective medium theory is presented for quantum spins and higher
multipoles such as quadrupole moments. The theory is a generalization of the
spherical model approximation for the Ising model, and is accurate up to
O(1/z_n) where z_n is the number of interacting neighbors. The polarization
function is optimized under the condition that it be diagonal in site indices.
With use of auxiliary fields and path integrals, the theory is flexibly applied
to quantum spins and higher multipoles with many interacting neighbors. A
Kondo-type screening of each spin is proposed for systems with extreme quantum
fluctuations but without conduction electrons.Comment: 16 pages, 3 Postscript figure
Oblique triangular antiferromagnetic phase in CsCuCoCl
The spin-1/2 stacked triangular antiferromagnet CsCuCoCl with
undergoes two phase transitions at zero field. The
low-temperature phase is produced by the small amount of Co doping. In
order to investigate the magnetic structures of the two ordered phases, the
neutron elastic scattering experiments have been carried out for the sample
with . It is found that the intermediate phase is identical to
the ordered phase of CsCuCl, and that the low-temperature phase is an
oblique triangular antiferromagnetic phase in which the spins form a triangular
structure in a plane tilted from the basal plane. The tilting angle which is
42 at K decreases with increasing temperature, and becomes
zero at K. An off-diagonal exchange term is proposed as the
origin of the oblique phase.Comment: 6 pages, 7 figure
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