Chiral exponents in O(N) x O(m) spin models at O(1/N^2)

The critical exponents corresponding to chirality are computed at O(1/N^2) in d-dimensions at the stable chiral fixed point of a scalar field theory with an O(N) x O(m) symmetry. Pade-Borel estimates for the exponents are given in three dimensions for the Landau-Ginzburg-Wilson model at m = 2.Comment: 8 latex page

Enhanced longitudinal mode spacing in blue-violet InGaN semiconductor laser

A novel explanation of observed enhanced longitudinal mode spacing in InGaN semiconductor lasers has been proposed. It has been demonstrated that e-h plasma oscillations, which can exist in the laser active layer at certain driving conditions, are responsible for mode clustering effect. The resonant excitation of the plasma oscillations occurs due to longitudinal mode beating. The separation of mode clusters is typically by an order of magnitude larger that the individual mode spacing.Comment: 3 pages, 2 figure

Description of paramagnetic--spin glass transition in Edwards-Anderson model in terms of critical dynamics

Possibility of description of the glass transition in terms of critical dynamics considering a hierarchy of the intermodal relaxation time is shown. The generalized Vogel-Fulcher law for the system relaxation time is derived in terms of this approach. It is shown that the system satisfies the fluctuating--dissipative theorem in case of the absence of the intermodal relaxation time hierarchy.Comment: 10 pages, 6 figure

Self-consistent theory of turbulence

A new approach to the stochastic theory of turbulence is suggested. The coloured noise that is present in the stochastic Navier-Stokes equation is generated from the delta-correlated noise allowing us to avoid the nonlocal field theory as it is the case in the conventional theory. A feed-back mechanism is introduced in order to control the noise intensity.Comment: submitted to J.Tech. Phys.Letters (St. Petersburg

Coherent interaction of laser pulses in a resonant optically dense extended medium under the regime of strong field-matter coupling

Nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. A special attention is paid to the case, where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser-fields can be coherently absorbed and reemitted by the medium. Thus, the field of medium reaction plays a key role in the interaction processes, which leads to the collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in the fast excitation interchanges between the field and a medium in the form of the optical ringing, which is analogous to polariton beating in the solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave-packets of different group velocities, is shown to significantly affect propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to the resonance in the broadband probe-spectrum, exceed the absorption-line width, since, under the strong-coupling regime, the frequency of the optical ringing exceeds the rate of incoherent relaxation. Contrary to the stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of the collective oscillations and cannot be obtained in the framework of the single-atom model.Comment: 10 pages, 8 figures, to be published in Phys. Rev.

Leading infrared logarithms for sigma-model with fields on arbitrary Riemann manifold

We derive non-linear recursion equation for the leading infrared logarithms (LL) in four dimensional sigma-model with fields on an arbitrary Riemann manifold. The derived equation allows one to compute leading infrared logarithms to essentially unlimited loop order in terms of geometric characteristics of the Riemann manifold. We reduce the solution of the SU(oo) principal chiral field in arbitrary number of dimensions in the LL approximation to the solution of very simple recursive equation. This result paves a way to the solution of the model in arbitrary number of dimensions at N-->ooComment: Talk given by MVP at the conference devoted to memory of A.N. Vasilie

A multiloop improvement of non-singlet QCD evolution equations

An approach is elaborated for calculation of "all loop" contributions to the non-singlet evolution kernels from the diagrams with renormalon chain insertions. Closed expressions are obtained for sums of contributions to kernels $P(z)$ for the DGLAP equation and $V(x,y)$ for the "nonforward" ER-BL equation from these diagrams that dominate for a large value of $b_0$, the first $\beta$-function coefficient. Calculations are performed in the covariant $\xi$-gauge in a MS-like scheme. It is established that a special choice of the gauge parameter $\xi=-3$ generalizes the standard "naive nonabelianization" approximation. The solutions are obtained to the ER-BL evolution equation (taken at the "all loop" improved kernel), which are in form similar to one-loop solutions. A consequence for QCD descriptions of hard processes and the benefits and incompleteness of the approach are briefly discussed.Comment: 13 pages, revtex, 2 figures are enclosed as eps-file, the text style and figures are corrected following version, accepted for publication to Phys. Rev.

Magnetocaloric effect and magnetization in a Ni-Mn-Ga Heusler alloy in the vicinity of magnetostructural transition

The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with coupled magnetic and structural (martensitic) phase transitions were studied experimentally and theoretically. The magnetocaloric effect was measured by a direct method in magnetic fields 0-26 kOe at temperatures close to the magnetostructural transition temperature. For theoretical description of the alloy properties near the magnetostructural transition a statistical model is suggested, that takes into account the coexistence of martensite and austenite domains in the vicinity of martensite transformation point.Comment: presented at ICM-2003, to appear in JMM

An improved \eps expansion for three-dimensional turbulence: two-loop renormalization near two dimensions

An improved \eps expansion in the $d$-dimensional ($d > 2$) stochastic theory of turbulence is constructed at two-loop order which incorporates the effect of pole singularities at $d \to 2$ in coefficients of the \eps expansion of universal quantities. For a proper account of the effect of these singularities two different approaches to the renormalization of the powerlike correlation function of the random force are analyzed near two dimensions. By direct calculation it is shown that the approach based on the mere renormalization of the nonlocal correlation function leads to contradictions at two-loop order. On the other hand, a two-loop calculation in the renormalization scheme with the addition to the force correlation function of a local term to be renormalized instead of the nonlocal one yields consistent results in accordance with the UV renormalization theory. The latter renormalization prescription is used for the two-loop renormalization-group analysis amended with partial resummation of the pole singularities near two dimensions leading to a significant improvement of the agreement with experimental results for the Kolmogorov constant.Comment: 23 pages, 2 figure

Causal signal transmission by quantum fields. IV: The causal Wick theorem

Wick's theorem in the Schwinger-Perel-Keldysh closed-time-loop formalism is written in a form where the place of contractions is taken by the linear response function of the field. This result demonstrates that the physical information supplied by Wick's theorem for operators is propagation of the free field in space and time.Comment: Final version, to appear in Phys Rev