3,623 research outputs found
Spin polaron in the J1-J2 Heisenberg model
We have studied the validity of the spin polaron picture in the frustrated
J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral
functions for the Neel, collinear, and disordered phases of this model, by
means of the self-consistent Born approximation and Lanczos exact
diagonalization on finite-size clusters. We have found that the spin polaron
quasiparticle excitation is always well defined for the magnetically ordered
Neel and collinear phases, even in the vicinity of the magnetic quantum
critical points, where the local magnetization vanishes. As a general feature,
the effect of frustration is to increase the amplitude of the multimagnon
states that build up the spin polaron wave function, leading to the reduction
of the quasiparticle coherence. Based on Lanczos results, we discuss the
validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure
Poisson homology of r-matrix type orbits I: example of computation
In this paper we consider the Poisson algebraic structure associated with a
classical -matrix, i.e. with a solution of the modified classical
Yang--Baxter equation. In Section 1 we recall the concept and basic facts of
the -matrix type Poisson orbits. Then we describe the -matrix Poisson
pencil (i.e the pair of compatible Poisson structures) of rank 1 or -type
orbits of . Here we calculate symplectic leaves and the integrable
foliation associated with the pencil. We also describe the algebra of functions
on -type orbits. In Section 2 we calculate the Poisson homology of
Drinfeld--Sklyanin Poisson brackets which belong to the -matrix Poisson
family
Удосконалення хірургічної тактики міні-інвазивного лікування гострих абсцесів печінки
The article adduces the experience of treatment of 48 patients with acute liver abscesses. All patients were operated usingmini-invasive ultrasound technology under control. The results of surgical treatment changed from active to more restrained.Number of complications was 13.8 %, mortality of 2 %. Thus, mini-invasive intervention is not only appropriate, but shouldbe used with the severity of the disease, age of patients, presence of comorbidities; their effectiveness and the advantage overtraditional is evident.У статті наведено досвід лікування 48 хворих із гострими абсцесами печінки. Всі пацієнти були оперовані шляхом ви-користання міні-інвазивних технологій під контролем ультразвуку. Представлено результати зміненої хірургічної так-тики від активної до більш стриманішої. Кількість ускладнень склала 13,8 %, летальність – 2 %. Отже, міні-інвазивнівтручання не тільки доцільно, але й необхідно використовувати з урахуванням тяжкості захворювання, віку хворих,наявності супутньої патології; їх ефективність і перевага перед традиційними є очевидною
Low-energy singlet and triplet excitations in the spin-liquid phase of the two-dimensional J1-J2 model
We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.Comment: 4 pages, 4 figures, small changes, published versio
Magnetic Impurity in the two-dimensional Heisenberg Antiferromagnet
We analyze the ground state properties of the two-dimensional quantum
antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the
strong Kondo coupling limit is developed and the results compared with studies,
based on exact diagonalization of small clusters. We find that at intermediate
coupling the impurity is partially screened and the magnetization locally
suppressed. A local singlet between the impurity and the host spin is formed
asymptotically.Comment: 12 REVTex pages, 4 Postscript figure
Theoretical Aspects of the Fractional Quantum Hall Effect in Graphene
We review the theoretical basis and understanding of electronic interactions
in graphene Landau levels, in the limit of strong correlations. This limit
occurs when inter-Landau-level excitations may be omitted because they belong
to a high-energy sector, whereas the low-energy excitations only involve the
same level, such that the kinetic energy (of the Landau level) is an
unimportant constant. Two prominent effects emerge in this limit of strong
electronic correlations: generalised quantum Hall ferromagnetic states that
profit from the approximate four-fold spin-valley degeneracy of graphene's
Landau levels and the fractional quantum Hall effect. Here, we discuss these
effects in the framework of an SU(4)-symmetric theory, in comparison with
available experimental observations.Comment: 12 pages, 3 figures; review for the proceedings of the Nobel
Symposium on Graphene and Quantum Matte
Active integrated antennas and arrays with field-effect transistors
Electromagnetic wave generation processes in the hybrid log-periodic microstripe antenna on the dielectric substrate integrated
with field-effect transistor are experimentally studied in the frequency range of 6─20 GHz. The possibility of synchronization and power combining in the array consisting of antenna-coupled oscillators placed on the common dielectric substrate is investigated. It is shown that the considerable increasing the generation efficiency and power combining can be obtained by using the external synchronization signal or quasi-optical design of the array
Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration
We have studied the low-energy excitation spectrum of a dimerized and
frustrated antiferromagnetic Heisenberg chain. We use an analytic approach,
based on a description of the excitations as triplets above a strong-coupling
singlet ground state. The quasiparticle spectrum is calculated by treating the
excitations as a dilute Bose gas with infinite on-site repulsion. Additional
singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of
the elementary triplets. We have also calculated the contributions of the
elementary and collective excitations into the spin structure factor. Our
results are in excellent agreement with exact diagonalizations and dimer series
expansions data as long as the dimerization parameter is not too small
(), i.e. while the elementary triplets can be treated as localized
objects.Comment: 18 pages, 13 figure
Excitation spectrum of the S=1/2 quantum spin ladder with frustration: elementary quasiparticles and many-particle bound states
We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin
ladder with additional inter-chain second-neighbor frustrating interactions.
The one and two-particle excitations are analyzed by using a mapping of the
model onto a Bose gas of hard-core triplets. We find that low-lying singlet and
triplet two-particle bound states are present and their binding energy
increases with increasing frustration. In addition, many-particle bound states
are found by a combination of variational and exact diagonalization techniques.
We prove that the larger the number of bound quasiparticles the larger the
binding energy. Thus the excitation spectrum has a complex structure and
consists of elementary triplets and collective many-particle singlet and
triplet excitations which generally mix with the elementary ones.
The model exhibits a quantum phase transition from an antiferromagnetic
ladder phase (small frustration) into Haldane phase (effectively ferromagnetic
ladder for large frustration). We argue that near the transition point the
spectrum in both triplet and singlet channels becomes gapless. The excitation
wave function is dominated by large-size bound states which leads to the
vanishing of the quasiparticle residue.Comment: RevTeX, 23 pages, 12 figure
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