Renormalization-group approach to the stochastic Navier--Stokes equation: Two-loop approximation

The field theoretic renormalization group is applied to the stochastic Navier--Stokes equation that describes fully developed fluid turbulence. The complete two-loop calculation of the renormalization constant, the $\beta$ function, the fixed point and the ultraviolet correction exponent is performed. The Kolmogorov constant and the inertial-range skewness factor, derived to second order of the \eps expansion, are in a good agreement with the experiment. The possibility of the extrapolation of the \eps expansion beyond the threshold where the sweeping effects become important is demonstrated on the example of a Galilean-invariant quantity, the equal-time pair correlation function of the velocity field. The extension to the $d$-dimensional case is briefly discussed.Comment: 20 pages, 3 figure

On the stability problem in the O(N) nonlinear sigma model

The stability problem for the O(N) nonlinear sigma model in the 2+\epsilon dimensions is considered. We present the results of the 1/N^{2} order calculations of the critical exponents (in the 2<d<4 dimensions) of the composite operators relevant for this problem. The arguments in the favor of the scenario with the conventional fixed point are given.Comment: 9 pages, revtex, 1 Postscript figur

O(1/N_f) Corrections to the Thirring Model in 2<d<4

The Thirring model, that is, a relativistic field theory of fermions with a contact interaction between vector currents, is studied for dimensionalities 2<d<4 using the 1/N_f expansion, where N_f is the number of fermion species. The model is found to have no ultraviolet divergences at leading order provided a regularization respecting current conservation is used. Explicit O(1/N_f) corrections are computed, and the model shown to be renormalizable at this order in the massless limit; renormalizability appears to hold to all orders due to a special case of Weinberg's theorem. This implies there is a universal amplitude for four particle scattering in the asymptotic regime. Comparisons are made with both the Gross-Neveu model and QED.Comment: 22 pages in plain TeX, with 7 figs included using psfig.tex (Minor conceptual changes - algebra unaffected

High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV_2O_5

A significant frequency dependence of the ESR line width is found in NaV_2O_5 between 34-100 K and the line width increases as the resonance frequency is increased from 95 GHz to 760 GHz. The observed frequency dependence is qualitatively explained in terms of the dynamical charge disproportionation. The present results show the essential role of the internal charge degree of freedom in a V-O-V bond. We have also proposed the existence of the Dzyaloshinsky-Moriya interaction in the low temperature charge ordered phase considering the breaking of the selection rule of ESR realized as the direct observation of the spin gap excitation.Comment: 9 figures submitted to J. Phys.Soc. Jp

Influence of helicity on scaling regimes in the extended Kraichnan model

We have investigated the advection of a passive scalar quantity by incompressible helical turbulent flow in the frame of extended Kraichnan model. Turbulent fluctuations of velocity field are assumed to have the Gaussian statistics with zero mean and defined noise with finite time-correlation. Actual calculations have been done up to two-loop approximation in the frame of field-theoretic renormalization group approach. It turned out that space parity violation (helicity) of turbulent environment does not affect anomalous scaling which is peculiar attribute of corresponding model without helicity. However, stability of asymptotic regimes, where anomalous scaling takes place, strongly depends on the amount of helicity. Moreover, helicity gives rise to the turbulent diffusivity, which has been calculated in one-loop approximation.Comment: 16 pages, talk given by M. Hnatich at "Renormalization Group 2005", Helsinki, Finland 30 August - 3 September 2005. To apear in J. Phys. A: Math. Ge

Medical Students’ Reactive Anxiety as a Quality Criterion for Distance Learning during the SARS-COV-2 Pandemic

Background: Numerous studies have shown an effect of rapidly spreading SARS-COV-2 on combined anxiety-depressive disorders and maladjustment syndrome occurrences. Objective: To determine the primary medical students’ reactive anxiety level and the final scores of their educational progress in distance learning during the SARS-COV-2 pandemic. Materials and Methods: The study involved 824 students at medical universities in Saratov and Moscow. The assessment of the reactive anxiety level was carried out according to the State-Trait Anxiety Inventory, STAI; the average score of students' academic performance was calculated according to the data of the technological platform "1C: Enterprise" version 8.4.1. The survey was completed during distance learning in May 2020. The quality of distance learning was compared to a similar score for 2019, when students were trained in a traditional way. Statistical analysis was conducted using Statistica 6.0. Results: It was shown the presence of moderately severe disorders among the primary medical students according to the average score of reactive anxiety (43.28 ± 12.85), that expressed more in females s (p0.05) but the practical skills suffered much especially in time trend (p<0.05). Novelty: It was found that a high level of the reactive anxiety negatively affects the students' adaptive capabilities and the education quality. The specificity of programs at different faculties can determine the state of the students' psychological status. Findings:Timely transition to online classes during pandemics helps to preserve the students' mental well-being and the quality of the educational process. The duration of distance learning for a practical course should not exceed 25% of the whole course. Doi: 10.28991/esj-2021-SPER-07 Full Text: PD

Enhancement of the Thermal Conductivity in gapped Quantum Spin Chains

We study mechanism of magnetic energy transport, motivated by recent measurements of the thermal conductivity in low dimensional quantum magnets. We point out a possible mechanism of enhancement of the thermal conductivity in gapped magnetic system, where the magnetic energy transport plays a crucial role. This mechanism gives an interpretation for the recent experiment of CuGeO_3, where the thermal conductivity depends on the crystal direction.Comment: 4 pages, 2 figure

Strong damping of phononic heat current by magnetic excitations in SrCu_2(BO_3)_2

Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu$_2$(BO$_3$)$_2$ are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.Comment: a bit more than 4 pages, 2 figures included; minor changes to improve the clarity of the presentatio

Ultrafast nonlocal control of spontaneous emission

Solid-state cavity quantum electrodynamics systems will form scalable nodes of future quantum networks, allowing the storage, processing and retrieval of quantum bits, where a real-time control of the radiative interaction in the cavity is required to achieve high efficiency. We demonstrate here the dynamic molding of the vacuum field in a coupled-cavity system to achieve the ultrafast nonlocal modulation of spontaneous emission of quantum dots in photonic crystal cavities, on a timescale of ~200 ps, much faster than their natural radiative lifetimes. This opens the way to the ultrafast control of semiconductor-based cavity quantum electrodynamics systems for application in quantum interfaces and to a new class of ultrafast lasers based on nano-photonic cavities.Comment: 15 pages, 4 figure

Investigation of photoneutron reactions close to and above the neutron emission threshold in the rare earth region

We have investigated the photoneutron cross section of the isotopes $^{148,150}$Nd, $^{154}$Sm, and $^{154,160}$Gd close to the neutron emission threshold in photoactivation experiments at the Darmstadt superconducting electron linear accelerator S-DALINAC. Naturally composed targets were activated with a high-intensity bremsstrahlung beam at various energies and the reaction yields have been determined by measuring the activity of the produced radioactive isotopes with HPGe detectors. The results are compared to two different statistical model calculations.Comment: Accepted for publication in Physical Review