Russian Academy of Sciences

Landau Institute for Theoretical Physics

In Print

Current induced magnetisation in metal without space-inversion symmetry

15 March in 11:30 (short)

V.P. Mineev

Magneto-electric effect, that is an appearance of magnetisation induced by electric current is allowed by symmetry in metals with crystal structure without space inversion. The microscopic origin of this effect is spin-orbit coupling of electrons with a non-centrosymmetric crystal lattice lifting spin degeneracy of electron energy and mixing spin and orbital degrees of freedom. The presented calculation of magnetisation induced by current based on the application of kinetic equation for the matrix distribution function of electrons occupying the states in two bands split by the spin-orbit interaction.
arXiv:2312.04592

Effective mass and field-reinforced superconductivity in uranium compounds

15 March in 11:30

V.P. Mineev

A theory of strong coupling superconductivity in uranium compounds has been developed, based on electron-electron interaction through magnetic fluctuations described by frequency-dependent magnetic susceptibility. The magnetic field dependence of the electron effective mass is expressed through the field dependence of the magnetic susceptibility components. It is shown that the intensity of triplet pairing, and hence the critical temperature of the transition to the superconducting state, is also determined by the field-dependent susceptibility. The results are discussed in relation to the properties of ferromagnetic uranium compounds URhGe and UCoGe, as well as the recently discovered UTe2.
arXiv:2312.02893

Modeling of nonlinear waves.

15 March in 11:30

A.O. Korotkevich

The talk is based on works included in doctor of science dissertation.

Distortion of a Néel-type magnetic skyrmion in weak nonuniform magnetic and electric fields

16 February in 11:30 (short)

Apostoloff S.S., Buskina A.V., Andriyakhina E.S., Burmistrov I.S.

In this work, we propose a theory for (meta)stable states of Néel-type skyrmions in weak nonuniform magnetic and electrin fields using a novel ansatz for modeling non-symmetric magnetization. Our theory considers changes in skyrmion parameters and deformations from symmetric shapes, simplifying the calculation of skyrmion free energy. By minimizing the free energy in two stages, we can identify stable and metastable states. The theory is employed to study the skyrmion in the stray field of a Pearl vortex. Our methodology reveals how skyrmion parameters depend on the vortex field strength and provides a phase diagram indicating regions with metastable configurations.
arXiv:2311.05578

Mirror symmetry and new approach to constructing orbifolds of Gepner models

19 January in 11:30 (short)

А.А. Belavin, S.E. Parkhomenko

Motivated by the principles of the conformal bootstrap, primarily the principle of Locality, simultaneously with the requirement of space-time supersymmetry, we reconsider constructions of compactified superstring models. Starting from requirements of space-time supersymmetry and mutual locality, we construct a complete set of physical fields of orbifolds of Gepner models. To technically implement this, we use spectral flow generators to construct all physical fields from the chiral primary fields.

Highest weight vectors in Coset Construction

29 December 2023 in 11:30

Mikhail Bershtein

We revisit the classical Goddard Kent Olive coset construction. We find the formulas for the highest weight vectors in coset decomposition and calculate their norms. We also derive formulas for matrix elements of natural vertex operators between these vectors. This leads to relations on conformal blocks. Due to the AGT relation, these relations are equivalent to blowup relations on Nekrasov partition functions with the presence of the surface defect. These relations can be used to prove formulas for the Painlev\'{e} tau-functions (following Nekrasov's method). As another application we prove new Seiberg integral formulas.
Based on joint work with B. Feigin and A. Trufanov

Absorption of inertial waves by geostrophic flow: theory and experiment.

29 December 2023 in 11:30 (short)

S.S. Vergeles

We consider the interaction of inertial waves with geostrophic flow in a fluid rapidly rotating as a whole. In accordance with the experimental conditions [1], we believe that the inertial waves are excited by a source localized in space near the flow boundary and then propagate into the region where a vortex geostrophic flow is present. First, in the approximation of quasi-homogeneity of the geostrophic flow, we consider an evolution of a wave packet of inertial waves. We show that the Doppler effect and the dispersion law of inertial waves lead to the possibility for the wave number of the packet to turn to infinity at a certain point in space. In order to study this process through the analysis of the wave equation, we consider the problem of the propagation of an inertial wave in a time-constant shear flow with straight streamlines. In this problem, there is homogeneity in coordinates along the axis of rotation and along the streamlines of the shear flow, and also in time, which makes it possible to derive a one-dimensional wave equation along the spanwise direction of the shear flow. We show that the absorption of the wave by the shear flow is possible if the spatical wariation of the shear flow exceeds some threshold which is of order of the phase velocity of the wave. The mathematical description of the wave absorption process is equivalent to the quantum mechanical problem of the one-dimensional fall of a particle in a quadratic potential -1/r^2. Based on the constructed analytical picture, we interpret the recently obtained experimental results.

Development of magnetoresistance theory in layered quasi-2D metals

22 December 2023 in 11:30

P.D. Grigoriev, T.I. Mogilyuk

We develop the theory of transverse magnetoresistance in layered quasi-two-dimensional metals. Using the Kubo-Streda formula, we calculate Hall intralayer conductivity in a magnetic field perpendicular to conducting layers. We neglect the interelectronic interaction and the influence of phonons. The analytical expressions for the amplitudes and phases of magnetic quantum oscillations (MQO) and of the difference or the so-called slow oscillations (SO) are derived and applied to analyze their behavior as a function of several parameters: magnetic field strength, interlayer transfer integral and the Landau-level width. Both the MQO and SO of intralayer and interlayer conductivities have approximately opposite phase in a weak magnetic field and the same phase in a strong field. The amplitude of SO of intralayer conductivity changes sign at $\omega_{c}\tau_{0} = 1/\sqrt{3}$. We also find the magnetoresistance tensor. The results obtained are useful to analyze experimental data on magnetoresistance oscillations in various strongly anisotropic quasi-2D metals.

We also study the behavior of the interlayer magnetoresistance $R_{zz}$ in layered quasi-two-dimensional metals at the Yamaji angles, that is, the magnetic field inclination angles at which a minimum of interlayer conductivity is observed. The cases of the Lorentz shape of the Landau levels and the shape corresponding to the self-consistent Born approximation are considered. The dependence $R_{zz}\propto B^{3/2}$ in a strong field is theoretically predicted, which is consistent with experimental data.

Dynamic flexoexoelectric instabilities in nematic liquid crystals

15 December 2023 in 11:30

E.S. Pikina, A.R. Muratov, E.I. Kats and V.V. Lebedev

Наш доклад посвящен рассмотрению флексоэлектрических неустойчивостей в нематических жидких кристаллах. Сначала нами была построена полная система точных нелинейных динамических уравнений для нематической и смектической А мезофаз помещенных в переменное электрическое поле, с учетом флексоэлектрического эффекта. Построение такой системы является обязательным шагом для изучения любых нелинейных эффектов в жидких кристаллах, которые обсуждаются, начиная примерно с 50-летней давности и до совсем недавнего времени. Наше теоретическое описание учитывает все мягкие (гидродинамические) степени свободы системы, связанные с общими законами сохранения и нарушениями непрерывной симметрии, а также возможность наличия гидродинамической скорости у вещества в области локализованного возмущения. На следующем этапе мы сформулировали линейное приближение для построенной нелинейной системы в случае нематического жидкого кристалла, линеаризовав ее по малым отклонениям полей от начального равновесного однородного состояния и исследовали устойчивость начального однородного распределения нематического директора в переменном электрическом поле. Главным результатом данного исследования явилось обнаружение нового типа неустойчивости (никогда не обсуждаемой ранее), а именно возможности возникновения двумерной модулированной структуры нематического директора, осциллирующей со временем. Этот эффект особенно замечателен тем, что представляет собой волновое движение в диссипативной системе, которой и является нематический жидкий кристалл, помещенной в переменное электрическое поле. Доклад основан на материалах публикации: «Nonlinear electrohydrodynamics of liquid crystals.» E.S. Pikina, A.R. Muratov, E.I. Kats and V.V. Lebedev, (JETP, vol. 137, pp. 114–124 (2023)) и готовящегося к отправке в печать манускрипта «Dynamic flexoelectric instabilities in nematic liquid crystals.» E.S. Pikina, A.R. Muratov, E.I. Kats and V.V. Lebedev (to be published).