112 research outputs found
Phase diagrams of a classical two-dimensional Heisenberg antiferromagnet with single-ion anisotropy
A classical variant of the two-dimensional anisotropic Heisenberg model
reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41
[M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly
Monte Carlo techniques. Phase diagrams with external fields parallel and
perpendicular to the easy axis of the anisotropic interactions are determined,
including antiferromagnetic and spin-flop phases. Mobile spinless defects, or
holes, are found to form stripes which bunch, debunch and break up at a phase
transition. A parallel field can lead to a spin-flop phase.Comment: 9 pages, 9 figures; final version as accepted by Phys. Rev. B (Fig. 5
replaced, added remarks in Secs. I, III, and V
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Theory of the density fluctuation spectrum of strongly correlated electrons
The density response function of the two-dimensional
model is studied starting from a mixed gauge formulation of the slave boson
approach. Our results for are in remarkable agreement with exact
diagonalization studies, and provide a natural explanation of the anomalous
features in the density response in terms of the spin polaron nature of the
charge carriers. In particular we have identified unexplained low energy
structures in the diagonalization data as arising from the coherent polaron
motion of holes in a spin liquid.Comment: 4 pages with 4 figures, to be published in Physical Review B (RC
The unusual electronic structure of the "pseudo-ladder" compound CaCu2O3
Experimental and theoretical studies of the unoccupied electronic structure
of CaCu2O3 single crystals have been performed using polarization-dependent
x-ray absorption spectroscopy and band structure calculations. The measured
hole distribution shows an unusual large number of holes in orbitals parallel
to the interlayer direction which is in agreement with the theoretical
analysis. CaCu2O3 deviates significantly from the standard pd-sigma cuprate
picture. The corresponding strong interlayer exchange is responsible for the
missing spin gap generic for other two-leg ladder cuprates.Comment: 4 pages, 3 figures include
Hole distribution for (Sr,Ca,Y,La)_14 Cu_24 O_41 ladder compounds studied by x-ray absorption spectroscopy
The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg
ladder compounds was investigated for different partial substitutions of Sr^2+
by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+,
reducing the nominal hole count from its full value of 6 per formula unit.
Using polarization-dependent x-ray absorption spectroscopy on single crystals,
hole states on both the chain and ladder sites could be studied. While for
intermediate hole counts all holes reside on O sites of the chains, a partial
hole occupation on the ladder sites in orbitals oriented along the legs is
observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for
Sr orbitals within the ladder planes but perpendicular to the legs receive some
hole occupation as well.Comment: 10 pages RevTeX style with 7 embedded figures + 1 table; accepted by
Phys. Rev.
Разработка системы автоматизации установки термической доподготовки тяжелой нефти
Объектом исследования является установка термической доподготовки нефти. Цель работы – разработка автоматизированной системы управления установки термической доподготовки нефти с использованием ПЛК, на основе выбранной SCADA-системы. В данном проекте была разработана система контроля и управления технологическим процессом на базе промышленных контроллеров ПЛК AllenBradley Micro 850, с применением SCADA-системы Master Scada Разработанная система может применяться в системах контроля, управления и сбора данных на различных промышленных предприятиях. Данная система позволит увеличить производительность, повысить точность и надежность измерений, сократить число аварий.The object of the study is the installation of thermal oil additional training. The work purpose – development of automated control system installation, additional training of thermal oil by using a PLC based on the selected SCADA-system. This project developed a system of monitoring and technological process control based on industrial PLC controllers Allen-Bradley Micro 850, with the use of SCADA Master Scada system Developed can be applied in systems monitoring, control and data acquisition in various industrial enterprises. This system will allow to increase productivity, to improve the accuracy and reliability of measurements to reduce the number of accidents
Tight-binding modelling of the electronic band structure of layered superconducting perovskites
A detailed tight-binding analysis of the electron band structure of the CuO_2
plane of layered cuprates is performed within a sigma-band Hamiltonian
including four orbitals - Cu3d_x^2-y^2, Cu4s, O2p_x, and O2p_y. Both the
experimental and theoretical hints in favor of Fermi level located in a Cu or O
band, respectively, are considered. For these two alternatives analytical
expressions are obtained for the LCAO electron wave functions suitable for the
treatment of electron superexchange. Simple formulae for the Fermi surface and
electron dispersions are derived by applying the Loewdin down-fold procedure to
set up the effective copper and oxygen Hamiltonians. They are used to fit the
experimental ARUPS Fermi surface of Pb_0.42Bi_1.73Sr_1.94Ca_1.3Cu_1.92O_8+x and
both the ARPES and LDA Fermi surface of Nd_2-xCe_xCuO_4-delta. The value of
presenting the hopping amplitudes as surface integrals of ab initio atomic wave
functions is demonstrated as well. The same approach is applied to the RuO_2
plane of the ruthenate Sr_2RuO_4. The LCAO Hamiltonians including the three
in-plane pi-orbitals Ru4d_xy, O_a 2p_y, O_b 2p_x and the four transversal
pi-orbitals Ru4d_zx, Ru4d_yz, O_a 2p_z, O_b 2p_z, are separately considered. It
is shown that the equation for the constant energy curves and the Fermi
contours has the same canonical form as the one for the layered cuprates.Comment: 21 pages, 10 figures, published in J. Phys.: Condens. Matter
(complete and corrected References section
Magnon Heat Transport in (Sr,La)_14Cu_24O_41
We have measured the thermal heat conductivity kappa of the compounds
Sr_14Cu_24O_41 and Ca_9La_5Cu_24O_41 containing doped and undoped spin ladders,
respectively. We find a huge anisotropy of both, the size and the temperature
dependence of kappa which we interpret in terms of a very large heat
conductivity due to the magnetic excitations of the one-dimensional spin
ladders. This magnon heat conductivity decreases with increasing hole doping of
the ladders. The magnon heat transport is analyzed theoretically using a simple
kinetic model. From this analysis we determine the spin gap and the temperature
dependent mean free path of the magnons which ranges by several thousand
angstroms at low temperature. The relevance of several scattering channels for
the magnon transport is discussed.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41
Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2
two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the
electrical field parallel to the legs and the rungs, respectively. Two peaks at
about 2140 and 2800 1/cm reflect van-Hove singularities in the density of
states of the strongly dispersing 2-magnon singlet bound state, and a broad
peak at about 4000 1/cm is identified with the 2-magnon continuum. Two
different theoretical approaches (Jordan-Wigner fermions and perturbation
theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and
J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy
continuum absorption of the ladders and of the undoped high T_c cuprates is
observed.Comment: 4 pages, 3 figures, Revte
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