All-optical control of skyrmion configuration in CrI 3 monolayer.

The potential for manipulating characteristics of skyrmions in a CrI 3 monolayer using circularly polarised light is explored. The effective skyrmion-light interaction is mediated by bright excitons whose magnetization is selectively influenced by the polarization of photons. The light-induced skyrmion dynamics is illustrated by the dependencies of the skyrmion size and the skyrmion lifetime on the intensity and polarization of the incident light pulse. Two-dimensional magnets hosting excitons thus represent a promising platform for the control of topological magnetic structures by light.

Stability of φ

In most papers, φ^{4}-field theory with the vector (d-component) field φ_{α} is considered as a particular case of the n-component field model for n=d and O(n) symmetry. However, in such a model the symmetry O(d) admits an addition to the action of a term proportional to the squared divergence of the field ∼h(∂_{α}φ_{α})^{2}. From the point of view of renormalization group analysis, it requires a separate consideration, because it may well change the nature of the critical behavior of the system. Therefore, this frequently neglected term in the action requires a detailed and accurate study on the issue of the existence of new fixed points and their stability. It is known that within the lower order of perturbation theory the only infrared stable fixed point with 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 four-loop renormalization group contributions for ω_{h} in d=4-2ɛ within the minimal subtraction scheme, which 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 the corresponding term to be neglected in the action when analyzing the critical behavior of the O(n)-symmetric model. At the same time, the small value of ω_{h} shows that the corresponding corrections to the critical scaling are significant in a wide range.

Critical behavior of the weakly disordered Ising model: Six-loop sqrt[ɛ] expansion study.

The critical behavior of three-dimensional weakly diluted quenched Ising model is examined on the base of six-loop renormalization group expansions obtained within the minimal subtraction scheme in 4-ɛ space dimensions. For this purpose the ϕ^{4} field theory with cubic symmetry was analyzed in the replica limit n→0. Along with renormalization group expansions in terms of renormalized couplings the sqrt[ɛ] expansions of critical exponents are presented. Corresponding numerical estimates for the physical, three-dimensional system are obtained by means of different resummation procedures applied both to the sqrt[ɛ] series and to initial renormalization group expansions. The results given by the latter approach are in a good agreement with their counterparts obtained experimentally and within the Monte Carlo simulations, while resumming of sqrt[ɛ] series themselves turned out to be disappointing.

Anisotropy of a cubic ferromagnet at criticality.

Critical fluctuations change the effective anisotropy of cubic ferromagnet near the Curie point. If the crystal undergoes phase transition into orthorhombic phase and the initial anisotropy is not too strong, reduced anisotropy of nonlinear susceptibility acquires at T_{c} the universal value δ_{4}^{*}=2v^{*}/3(u^{*}+v^{*}) where u^{*} and v^{*} are coordinates of the cubic fixed point on the flow diagram of renormalization group equations. In the paper, the critical value of the reduced anisotropy is estimated within the pseudo-ε expansion approach. The six-loop pseudo-ε expansions for u^{*}, v^{*}, and δ_{4}^{*} are derived for the arbitrary spin dimensionality n. For cubic crystals (n=3) higher-order coefficients of the pseudo-ε expansions obtained turn out to be so small that use of simple Padé approximants yields reliable numerical results. Padé resummation of the pseudo-ε series for u^{*}, v^{*}, and δ_{4}^{*} leads to the estimate δ_{4}^{*}=0.079±0.006, indicating that detection of the anisotropic critical behavior of cubic ferromagnets in physical and computer experiments is certainly possible.