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L. Ts. Adzhemyan

Publications and source records attributed to L. Ts. Adzhemyan.

At least 19 recordsLinked to original sources
arXiv2026

Evidence for an Obstruction to Perturbative Renormalization Group in Stochastic Turbulence

We examine whether the perturbative renormalization group (RG) regime of stochastic turbulence can be continued from small $\varepsilon$ to the physical value $\varepsilon_{\mathrm{phys}}=2$, corresponding to large-scale forcing. Exploiting the simplifications of the large-$d$ limit, we calculate the correction exponent $ω$ to five-loop order and find a simple structure suggesting a closed analytic form. This form implies that the fixed point stable near $\varepsilon=0$ ceases to be simultaneously real and infrared stable before reaching $\varepsilon_{\mathrm{phys}}=2$, pointing to a possible change of infrared regime rather than a controlled perturbative RG extrapolation.

nlin.CD↗
arXiv2026

Six-loop renormalization group analysis of the $ϕ^4 + ϕ^6$ model

We investigate the $λ\ph^4+g\ph^6$ model using the renormalization group method and the $\ep$ expansion. This model is used in a situation where the coefficients $λ$, $g$ and the coefficient $τ$ of the term $τ\ph^2$ depend on two parameters $T$ and $P$, and there is a point ($T_c,P_c$) at which $τ$ and $λ$ are zero. This point is named the tricritical point. The description of a system depends on a trajectory that leads to the tricritical point on the plane ($T,P$). In the trajectories, when $λ$ goes to zero fast enough, the description is defined by the $\ph^6$ interaction and then the $\ph^4$ term can be considered as a composite operator. In this case, the logarithmic dimension is $d=3$, and the $\ep$ expansion is carried out in the dimension $d=3-2\ep$. The main exponents of the \textit{tricritical} model have been calculated in the third order of the $\ep$ expansion. Taking into account the $\ph^4$ interaction, we were able to calculate the value of the parameter that determines the required decrease rate in $λ$ to implement the tricritical behavior. The tricritical dimensions of the composite operators $\ph^k$ for $k=1, 2, 4, 6$ have been computed. The resulting values are compared to those known from a conformal field theory and non-perturbative renormalization group.

cond-mat.stat-mech↗
arXiv2023

Stability of $φ^4$-vector model: four-loop $\varepsilon$ expansion study

The stability of $O(n)$-symmetric fixed point regarding the presence of vector-field term ($\sim h p_αp_β$) in the $φ^4$ field theory is analyzed. For this purpose, the four-loop renormalization group expansions in $d=4-2\varepsilon$ within Minimal Subtraction (MS) scheme are obtained. This frequently neglected term in the action requires a detailed and accurate study on the issue of existing of new fixed points and their stability, that can lead to the possible change of the corresponding universality class. We found that within lower order of perturbation theory the only $O(n)$-symmetric fixed point $(g_{\text{H}},h=0)$ exists but the corresponding positive value of stability exponent $ω_h$ is tiny. This led us to analyze this constant in higher orders of perturbation theory by calculating the 4-loop contributions to the $\varepsilon$ expansion for $ω_h$, that should be enough to infer positivity or negativity of this exponent. The value turned out to be undoubtedly positive, although still small even in higher loops: $0.0156(3)$. These results cause that the corresponding vector term should be neglected in the action when analyzing the critical behaviour of $O(n)$-symmetric model. At the same time, the small value of the $ω_h$ shows that the corresponding corrections to the critical scaling are significant in a wide range.

cond-mat.stat-mech↗
arXiv2022

Model A of critical dynamics: 5-loop $\varepsilon$ expansion study

We have calculated the five-loop RG expansions of the $n$-component A model of critical dynamics in dimensions $d=4-\varepsilon$ within the Minimal Subtraction scheme. This is made possible by using the advanced diagram reduction method and the Sector Decomposition technique adapted to the problems of critical dynamics. The $\varepsilon$ expansions for the critical dynamic exponent $z$ for an arbitrary value of the order parameter dimension $n$ are derived. Based on these series, the numerical estimates of $z$ for different universality classes are extracted and compared with the results obtained within different theoretical and experimental methods.

cond-mat.stat-mech↗
arXiv2021

The dynamic critical exponent $z$ for 2d and 3d Ising models from five-loop $ε$ expansion

We calculate the dynamic critical exponent $z$ for 2d and 3d Ising universality classes by means of minimally subtracted five-loop $\varepsilon$ expansion obtained for the one-component model A. This breakthrough turns out to be possible through the successful adaptation of the Sector Decomposition technique to the problems of critical dynamics. The obtained fifth perturbative order accompanied by the use of advanced resummation techniques for asymptotic series allows us to find highly accurate numerical estimates of $z$: for two- and three-dimensional cases we obtain $\boldsymbol{2.14(2)}$ and $\boldsymbol{2.0235(8)}$ respectively. The numbers found are in good agreement with recent results obtained using different approaches.

cond-mat.stat-mech↗
arXiv2021

Six-loop $\varepsilon$ expansion of three-dimensional $\text{U}(n)\times \text{U}(m)$ models

We analyze the Landau-Wilson field theory with $\text{U}(n)\times\text{U}(m)$ symmetry which describes the finite-temperature phase transition in QCD in the limit of vanishing quark masses with $n=m=N_f$ flavors and unbroken anomaly at the critical temperature. The six-loop expansions of the renormalization group functions are calculated within the Minimal Subtraction scheme in $4 - \varepsilon$ dimensions. The $\varepsilon$ series for the upper marginal dimensionality $n^{+}(m,4-\varepsilon)$ -- the key quantity of the theory -- are obtained and resummed by means of different approaches. The numbers found are compared with their counterparts obtained earlier within lower perturbative orders and the pseudo-$\varepsilon$ analysis of massive six-loop three-dimensional expansions. In particular, using an increase in the accuracy of numerical results for $n^{+}(m,3)$ by one order of magnitude, we strengthen the conclusions obtained within previous order in perturbation theory about fairness of the inequality $n^{+}(m,3)>m$. This, in turn, indicates the absence of a stable three-dimensional fixed point for $n=m$, and as a consequence a first-order kind of finite-temperature phase transition in light QCD.

hep-th↗
arXiv2019

Six-loop $\varepsilon$ expansion study of three-dimensional $n$-vector model with cubic anisotropy

The six-loop expansions of the renormalization-group functions of $φ^4$ $n$-vector model with cubic anisotropy are calculated within the minimal subtraction (MS) scheme in $4 - \varepsilon$ dimensions. The $\varepsilon$ expansions for the cubic fixed point coordinates, critical exponents corresponding to the cubic universality class and marginal order parameter dimensionality $n_c$ separating different regimes of critical behavior are presented. Since the $\varepsilon$ expansions are divergent numerical estimates of the quantities of interest are obtained employing proper resummation techniques. The numbers found are compared with their counterparts obtained earlier within various field-theoretical approaches and by lattice calculations. In particular, our analysis of $n_c$ strengthens the existing arguments in favor of stability of the cubic fixed point in the physical case $n = 3$.

cond-mat.stat-mech↗
arXiv2017

Diagram Reduction in Problem of Critical Dynamics of Ferromagnets: 4-Loop Approximation

Within the framework of the renormalization group approach to the models of critical dynamics, we propose a method for a considerable reduction of the number of integrals needed to calculate the critical exponents. With this method we perform a calculation of the critical exponent $z$ of model A at 4-loop level, where our method allows to reduce number of integrals from 66 to 17. The way of constructing the integrand in Feynman representation of such diagrams is discussed. Integrals were estimated numerically with Sector Decomposition technique.

cond-mat.stat-mech↗
arXiv2016

Numerical calculation of scaling exponents of percolation process in the framework of renormalization group approach

We use the renormalization group theory to study the directed bond percolation (Gribov process) near its second-order phase transition between absorbing and active state. We present a numerical calculation of the renormalization group functions in the $ε$-expansion where $ε$ is a deviation from the upper critical dimension $d_c = 4$. Within this procedure anomalous dimensions $γ$ are expressed in terms of irreducible renormalized Feynman diagrams and thus the calculation of renormalization constants could be entirely skipped. The renormalization group is included by means of the $R$ operation, and for computational purposes we choose the null momentum subtraction scheme.

cond-mat.stat-mech↗
arXiv2016

Critical exponent $η$ in 2D $O(N)$-symmetric $φ^4$-model up to 6~loops

Critical exponent $η$ (Fisher exponent) in $O(N)$-symmetric $φ^4$-model was calculated using renormalization group approach in the space of fixed dimension $D=2$ up to 6~loops. The calculation of the renormalization constants was performed with the use of $R'$-operation and specific values for diagrams were calculated in Feynman representation using sector decomposition method. Presented approach allows easy automation and generalization for the case of complex symmetries. Also a summation of the perturbation series was obtained by Borel transformation with conformal mapping. The contribution of the 6-th term of the series led to the increase of the Fisher exponent in $O(1)$ model up to $8\%$.

cond-mat.stat-mech↗
arXiv2013

Five-loop numerical evaluation of critical exponents of the $φ^4$ theory

We present a new approach to calculation of anomalous dimensions in the framework of $ε$-expansion and renormalization group method. This approach allows one to skip the calculation of renormalization constants and express anomalous dimensions in terms of renormalized diagrams, which are presented in a form suitable for numerical calculations. This approach can be easily automated and extended to a wide range of models. The power of this approach is illustrated on 5 loop calculations of beta-function and anomalous dimensions in $ϕ^4$ model.

cond-mat.stat-mech↗
arXiv2013

Anomalous scaling of a passive vector field in $d$ dimensions: Higher-order structure functions

The problem of anomalous scaling in the model of a transverse vector field $θ_{i}(t,x)$ passively advected by the non-Gaussian, correlated in time turbulent velocity field governed by the Navier--Stokes equation, is studied by means of the field-theoretic renormalization group and operator product expansion. The anomalous exponents of the $2n$-th order structure function $S_{2n}(r) = <[θ(t,x) - θ(t,x+ r)]^{2n}>$, where $θ$ is the component of the vector field parallel to the separation $r$, are determined by the critical dimensions of the family of composite fields (operators) of the form $(\partialθ\partialθ)^{2n}$, which mix heavily in renormalization. The daunting task of the calculation of the matrices of their critical dimensions (whose eigenvalues determine the anomalous exponents) simplifies drastically in the limit of high spatial dimension, $d\to\infty$. This allowed us to find the leading and correction anomalous exponents for the structure functions up to the order $S_{56}$. They reveal intriguing regularities, which suggest for the anomalous exponents simple "empiric" formulae that become practically exact for $n$ large enough. Along with the explicit results for modest $n$, they provide the full description of the anomalous scaling in the model. Key words: passive vector field, turbulent advection, anomalous scaling, renormalization group, operator product expansion.

cond-mat.stat-mech↗
arXiv2008

Renormalization group in the infinite-dimensional turbulence: third-order results

The field theoretic renormalization group is applied to the stochastic Navier-Stokes equation with the stirring force correlator of the form k^(4-d-2ε) in the d-dimensional space, in connection with the problem of construction of the 1/d expansion for the fully developed fluid turbulence beyond the scope of the standard epsilon expansion. It is shown that in the large-d limit the number of the Feynman diagrams for the Green function (linear response function) decreases drastically, and the technique of their analytical calculation is developed. The main ingredients of the renormalization group approach -- the renormalization constant, beta function and the ultraviolet correction exponent omega, are calculated to order epsilon^3 (three-loop approximation). The two-point velocity-velocity correlation function, the Kolmogorov constant C_K in the spectrum of turbulent energy and the inertial-range skewness factor S are calculated in the large-d limit to third order of the epsilon expansion. Surprisingly enough, our results for C_K are in a reasonable agreement with the existing experimental estimates.

nlin.CD↗
arXiv2008

Calculation of the dynamical critical exponent in the model A of critical dynamics to order ε^4

A new method based on the R'-operation of the renormalization theory is proposed for the numerical calculation of the renormalization constants in the theory of critical behaviour. The problem of finding residues of the poles of the Green's functions at ε= 0, where ε= 4 - d, is reduced to the evaluation of multiple UV-finite integrals, which can be performed by means of standard integration programs. The method is used to calculate the renormalization group functions of the model A of critical dynamics in four-loop approximation. Dynamical exponent z of the model A is calculated in the fourth order of the ε-expansion.

cond-mat.stat-mech↗
arXiv2004

Two-loop calculation of the turbulent Prandtl number

The turbulent Prandtl number has been calculated in the two-loop approximation of the $\eps$ expansion of the stochastic theory of turbulence. The strikingly small value obtained for the two-loop correction explains the good agreement of the earlier one-loop result with the experiment. This situation is drastically different from other available nontrivial two-loop results, which exhibit corrections of the magnitude of the one-loop term. The reason is traced to the mutual cancellation of additional divergences appearing in two dimensions which have had a major effect on the results of previous calculations of other quantities.

physics.flu-dyn↗
arXiv2004

Anomalous scaling of a passive scalar advected by the Navier--Stokes velocity field: Two-loop approximation

The field theoretic renormalization group and operator product expansion are applied to the model of a passive scalar quantity advected by a non-Gaussian velocity field with finite correlation time. The velocity is governed by the Navier--Stokes equation, subject to an external random stirring force with the correlation function $\propto δ(t-t') k^{4-d-2ε}$. It is shown that the scalar field is intermittent already for small $ε$, its structure functions display anomalous scaling behavior, and the corresponding exponents can be systematically calculated as series in $ε$. The practical calculation is accomplished to order $ε^{2}$ (two-loop approximation), including anisotropic sectors. Like for the well-known Kraichnan's rapid-change model, the anomalous scaling results from the existence in the model of composite fields (operators) with negative scaling dimensions, identified with the anomalous exponents. Thus the mechanism of the origin of anomalous scaling appears similar for the Gaussian model with zero correlation time and non-Gaussian model with finite correlation time. It should be emphasized that, in contrast to Gaussian velocity ensembles with finite correlation time, the model and the perturbation theory discussed here are manifestly Galilean covariant. The relevance of these results for the real passive advection, comparison with the Gaussian models and experiments are briefly discussed.

nlin.CD↗
arXiv2004

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.

nlin.CD↗