Neel order in square and triangular lattice Heisenberg models

Using examples of the square- and triangular-lattice Heisenberg models we demonstrate that the density matrix renormalization group method (DMRG) can be effectively used to study magnetic ordering in two-dimensional lattice spin models. We show that local quantities in DMRG calculations, such as the on-site magnetization M, should be extrapolated with the truncation error, not with its square root, as previously assumed. We also introduce convenient sequences of clusters, using cylindrical boundary conditions and pinning magnetic fields, which provide for rapidly converging finite-size scaling. This scaling behavior on our clusters is clarified using finite-size analysis of the effective sigma-model and finite-size spin-wave theory. The resulting greatly improved extrapolations allow us to determine the thermodynamic limit of M for the square lattice with an error comparable to quantum Monte Carlo. For the triangular lattice, we verify the existence of three-sublattice magnetic order, and estimate the order parameter to be M = 0.205(15).Comment: 4 pages, 5 figures, typo fixed, reference adde

Spin polarons and high-Tc superconductivity

In this short review article, we consider some aspects of the spin-polaron concept as they relate to high-Tc superconductivity. The article is intended to be an overview in which a flavor of the research on spin polarons is given.Comment: 47 pages, 16 figures, pdf-onl

Signatures of a staggered-flux phase in the t-J model with two holes on a 32-site lattice

We study the relevance of the staggered-flux phase in the t-J model using a system with two holes on a 32-site lattice with periodic boundary conditions. We find a staggered-flux pattern in the current-current correlation in the lowest energy d-wave state where there is mutual attraction between the holes. This staggered correlation decays faster with distance when the hole binding becomes stronger. This is in complete agreement with a recent study by Ivanov, Lee and Wen (Phys. Rev. Lett. 84, 3958, (2000)) based on the SU(2) theory, and strongly suggests that the staggered-flux phase is a key ingredient in the t-J model. We further show that this staggered-flux pattern does not exist in a state where the holes repel each other. Correlations of the chirality operator S_1.(S_2xS_3) show that the staggered pattern of the chirality is closely tied to the holes.Comment: To appear in Phys. Rev. B (Rapid Communications

Acoustic instability of a circular vortex with a smoothed vorticity profile

It is known that a localized vortex can have two specific mechanisms of interaction with the ambient flow. The first mechanism is associated with acoustic radiation, which is accompanied by a loss of energy and causes instability in the case of negative energy of vortex disturbances. The second is a Miles mechanism of interaction of the vortex core oscillations with disturbances in the vicinity of the critical layer (where the phase velocity of the disturbances coincides with the velocity of the mean flow), accompanied by an energy flux from the critical layer vicinity, which leads to damping in the case of negative energy of the oscillations. For the first time, the flow with both of these mechanisms is considered. The problem is solved from the first principles. It is shown that the Miles mechanism can completely suppress acoustic instability, however, in the case of a stronger loss of energy due to acoustic radiation, acoustic instability will dominate. The role of various parameters is analyzed and a quantitative criterion for the acoustic instability of a vortex with a smoothed vorticity profile is obtained

Holes in the t-J_z model: a thorough study

The t-J_z model is the strongly anisotropic limit of the t-J model which captures some general properties of the doped antiferromagnets (AF). The absence of spin fluctuations simplifies the analytical treatment of hole motion in an AF background and allows us to calculate the single- and two-hole spectra with high accuracy using regular diagram technique combined with real-space approach. At the same time, numerical studies of this model via exact diagonalization (ED) on small clusters show negligible finite size effects for a number of quantities, thus allowing a direct comparison between analytical and numerical results. Both approaches demonstrate that the holes have tendency to pair in the p- and d-wave channels at realistic values of t/J. The interactions leading to pairing and effects selecting p and d waves are thoroughly investigated. The role of transverse spin fluctuations is considered using perturbation theory. Based on the results of the present study, we discuss the pairing problem in the realistic t-J-like model. Possible implications for preformed pairs formation and phase separation are drawn.Comment: 21 pages, 15 figure

About inefficiency of personnel policy in health care of russia

The shortage of medical personnel, primarily in the district service and rural health care, the insufficient effectiveness of measures taken by the state to solve the personnel problem («presidential» allowance for the district service, the program «Zemsky doctor», etc.) indicate the insolvency of personnel policy in the industry. The state spending huge budgetary funds for training specialists deprived itself of the right to use them where they are urgently needed. The desire to improve the quality of training of medical students on the basis of accession to the Bologna agreement did not give the desired result. The availability of medical care is also limited by the fact that in the general structure of medical specialties, the number of doctors of clinical specialties, i.e. those who directly work with patients is significantly lower than in the EU and their share continues to decline. All of the above suggests that the Ministry of Health has no clear idea about the issue of personnel, is not formed the system able to solve it, which must include: career counseling -training - public distribution trained on a budgetary basis - professional development - career growth. The state and the leadership of the regions should create favorable conditions for work and life, attracting specialists, as well as interesting them in long-term work at the place of distribution. On the basis of the Soviet experience in the organization of personnel work in health care, best practice of other countries, the authors propose measures in the medical personnel training improvement, in attracting and long-term interest in their work in «problem» positions (district service, rural health care, etc.)

MORBIDITY WITH TEMPORARY DISABILITY IN SIBERIAN FEDERAL DISTRICT

The aim of the article was analysis of morbidity with temporary disability in general and those of working age in Siberian federal district. To evaluate the morbidity with temporary disability of the population of the Siberian federal district analyzed the statistical data of annual reports on Form 16 BH for 2010-2011 (number of cases, and disability days per 100 employees). Analysis of the overall incidence of the working population in the Siberian federal district is made on the basis of new statistical form N 12 for 2011. In 2011 compared with 2010 in Siberian federal district decreased the duration of a single case of the disease with temporary disability from 15,1 to 14,8 days, it is in direct proportion to the incidence in the general population and those of working age. Morbidity with temporary disability in Siberian federal district is lower than the average for Russia, but there are significant variations in different regions. The level of overall morbidity in the working population of the Siberian federal district remains as high as possible over the years in the Altai region. Omsk region is in second place. The lowest figure of morbidity with temporary disability is in Buryat Republic and Transbaikal Territory. 2/3 of cases of temporary disability occur in women, in men the duration of one case is 17 % higher than in women. Number of days with temporary disability remains like on the maximal level in Novosiberskaya oblast (865,4 in Transbaikal Territory), Tomsk (858,2 per 100 employees) and Kemerovo regions (856,9 per 100 employees), and minimal level is in Transbaikal Territory (651,5 per 100 employees) and Tyva Republic (617,3 per 100 employees). These facts require to study the reasons caused this difference in terms of effective management and decision-making in the regions of Siberian federal district