arXiv subjects

J. Kaupuzs

Publications and source records attributed to J. Kaupuzs.

26 records · Page 2Linked to original sources

arXiv2003

Remarks about an "exact" RG theory of Goldstone modes

A renormalization group (RG) theory of Goldstone mode singularities in the O(n>1)-symmetric phi^4 model is discussed. This perturbative RG theory is claimed to be asymptotically exact, as regards the long-wave limit of the correlation functions, where it predicts a purely Gaussian behavior of the transverse correlation function. However, we show that the results of this theory are incorrect, and the Gaussian behavior originates from a rough error in mathematics. Other relevant perturbative theories are discussed, as well.

cond-mat.stat-mech↗

arXiv2002

Transfer matrix and Monte Carlo tests of critical exponents in lattice models

The corrections to finite-size scaling in the critical two-point correlation function G(r) of 2D Ising model on a square lattice have been studied numerically by means of exact transfer-matrix algorithms. The systems of square geometry with periodic boundaries oriented either along <10> or along <11> direction have been considered, including up to 800 spins. The calculation of G(r) at a distance r equal to the half of the system size L shows the existence of an amplitude correction proportional to 1/L^2. A nontrivial correction proportional to 1/L^0.25 of a very small magnitude also has been detected in agreement with predictions of our recently developed GFD (grouping of Feynman diagrams) theory. A refined analysis of the recent MC data for 3D Ising, phi^4, and XY lattice models has been performed. It includes an analysis of the partition function zeros of 3D Ising model, an estimation of the correction-to-scaling exponent omega from the Binder cumulant data near criticality, as well as a study of the effective critical exponent eta and the effective amplitudes in the asymptotic expansion of susceptibility at the critical point. In all cases a refined analysis is consistent with our (GFD) asymptotic values of the critical exponents (nu=2/3, omega=1/2, eta=1/8 for 3D Ising model and omega=5/9 for 3D XY model), while the actually accepted "conventional" exponents are, in fact, effective exponents which are valid for approximation of the finite--size scaling behavior of not too large systems.

cond-mat.stat-mech↗

arXiv2001

Monte Carlo test of critical exponents and amplitudes in 3D Ising and phi^4 lattice models

We have tested the leading correction-to-scaling exponent omega in O(n)-symmetric models on a three-dimensional lattice by analysing the recent Monte Carlo (MC) data. We have found that the effective critical exponent, estimated at finite sizes of the system L and L/2, decreases remarkably within the range of the simulated L values. This shows the incorrectness of some claims that omega has a very accurate value 0.845(10) at n=1. A selfconsistent infinite volume extrapolation yields row estimates omega=0.547, omega=0.573, and omega=0.625 at n=1, 2, and 3, respectively, in approximate agreement with the corresponding exact values 1/2, 5/9, and 3/5 predicted by our recently developed GFD (grouping of Feynman diagrams) theory. We have fitted the MC data for the susceptibility of 3D Ising model at criticality showing that the effective critical exponent eta tends to increase well above the usually accepted values around 0.036. We have fitted the data within [L;8L], including several terms in the asymptotic expansion with fixed exponents, to obtain the effective amplitudes depending on L. This method clearly demonstrates that the critical exponents of GFD theory are correct (the amplitudes converge to certain asymptotic values at L tending to infinity), whereas those of the perturbative renormalization group (RG) theory are incorrect (the amplitudes diverge). A modification of the standard Ising model by introducing suitable "improved" action (Hamiltonian) does not solve the problem in favour of the perturbative RG theory.

cond-mat.stat-mech↗

arXiv2001

Critical exponents predicted by grouping of Feynman diagrams in phi^4 model

Different perturbation theory treatments of the Ginzburg-Landau phase transition model are discussed. This includes a criticism of the perturbative renormalization group (RG) approach and a proposal of a novel method providing critical exponents consistent with the known exact solutions in two dimensions. The usual perturbation theory is reorganized by appropriate grouping of Feynman diagrams of phi^4 model with O(n) symmetry. As a result, equations for calculation of the two-point correlation function are obtained which allow to predict possible exact values of critical exponents in two and three dimensions by proving relevant scaling properties of the asymptotic solution at (and near) the criticality. The new values of critical exponents are discussed and compared to the results of numerical simulations and experiments.

cond-mat.stat-mech↗

arXiv2001

Monte Carlo test of critical exponents in 3D Heisenberg and Ising models

We have tested the theoretical values of critical exponents, predicted for the three--dimensional Heisenberg model, based on the published Monte Carlo (MC) simulation data for the susceptibility. Two different sets of the critical exponents have been considered - one provided by the usual (perturbative) renormalization group (RG) theory, and another predicted by grouping of Feynman diagrams in phi^4 model (our theory). The test consists of two steps. First we determine the critical coupling by fitting the MC data to the theoretical expression, including both confluent and analytical corrections to scaling, the values of critical exponents being taken from theory. Then we use the obtained value of critical coupling to test the agreement between theory and MC data at criticality. As a result, we have found that predictions of our theory (gamma=19/14, eta=1/10, omega=3/5) are consistent, whereas those of the perturbative RG theory (gamma=1.3895, eta=0.0355, omega=0.782) are inconsistent with the MC data. The seemable agreement between the RG prediction for eta and MC results at criticality, reported in literature, appears due to slightly overestimated value of the critical coupling. Estimation of critical exponents of 3D Ising model from complex zeroth of the partition function is discussed. A refined analysis yields the best estimate 1/nu=1.518. We conclude that the recent MC data can be completely explained within our theory (providing 1/nu=1.5 and omega=0.5) rather than within the conventional RG theory.

cond-mat.stat-mech↗

arXiv2000

Perturbation theory methods applied to critical phenomena

cond-mat.stat-mech↗

arXiv2000

Critical behavior of n-vector model with quenched randomness

We consider the Ginzburg-Landau phase transition model with O(n) symmetry (i.e., the n-vector model) which includes a quenched randomness, i.e., a random temperature disorder. We have proven rigorously that within the diagrammatic perturbation theory the quenched randomness does not change the critical exponents at n tending to 0, which is in contrast to the conventional point of view based on the perturbative renormalization group theory.

cond-mat.stat-mech↗

arXiv2000

Nonexistence of the non-Gaussian fixed point predicted by the RG field theory in 4-epsilon dimensions

The Ginzburg-Landau phase transition model is considered in d=4-epsilon dimensions within the renormalization group (RG) approach. The problem of existence of the non-Gaussian fixed point is discussed. An equation is derived from the first principles of the RG theory (under the assumption that the fixed point exists) for calculation of the correction-to-scaling term in the asymptotic expansion of the two-point correlation (Green's) function. It is demonstrated clearly that, within the framework of the standard methods (well justified in the vicinity of the fixed point) used in the perturbative RG theory, this equation leads to an unremovable contradiction with the known RG results. Thus, in its very basics, the RG field theory in 4-epsilon dimensions is contradictory. To avoid the contradiction, we conclude that such a non-Gaussian fixed point, as predicted by the RG field theory, does not exist. Our consideration does not exclude existence of a different fixed point.

cond-mat.stat-mech↗