Институт теоретической физики им. Л.Д. Ландау

Российской академии наук

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Some Aspects of Diquarks as seen by String Theory

20 апреля в 11:30

Oleg Andreev

I will discuss a few aspects of diquarks in QCD from the viewpoint of a 5-dimensional effective string theory.

Magnetic oscillations of in-plane conductivity in quasi-two-dimensional metals

13 апреля в 11:30

Павел Григорьев

We develop the theory of transverse magnetoresistance in layered quasi-two-dimensional metals. Using the Kubo formula and harmonic expansion we calculate intralayer conductivity in a magnetic field perpendicular to conducting layers. The analytical expressions for the amplitudes and phases of magnetic quantum oscillations (MQO) and of the so-called slow oscillations (SlO) 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 SlO of intralayer and interlayer conductivity have approximately opposite phase in weak magnetic field and the same phase in strong field. The amplitude of SlO of intralayer conductivity changes sign at $\omega_c\tau\approx\sqrt{3}$. There are several other qualitative differences between magnetic oscillations of in-plane and out-of-plane conductivity. The results obtained are useful to analyze experimental data on magnetoresistance oscillations in various strongly anisotropic quasi-2D metals.

Long-lived quantum vortex knots

6 апреля в 11:30

V.P. Ruban

In the bulk of a superfluid, besides well-known and experimentally observed quantum vortex rings, theoretically there can exist (developing in time) also solitary topologically non-trivial excitations as vortex knots [1-3]. The simplest of them are torus knots ${\cal T}_{p,q}$, where $p$ and $q$ are co-prime integers, while parameters of torus are the toroidal (large) radius $R_0$ and the poloidal (small) radius $r_0$, both sizes being large in comparison with a width of quantum vortex core $\xi$. It was believed on the basis of previously obtained numerical results that such knots are unstable and they reconnect during just a few typical times, traveling a distance of several $R_0$ (the lifetime is somewhat longer for smaller ratios $B_0=r_0/R_0$). The mentioned results were obtained for not too large ratios $R_0/\xi\lesssim 20$, and with a very coarse step (about 0.1) on parameter $B_0$. In this work it was numerically found that actually the situation is much more complicated and interesting. The dynamics of trefoil knot ${\cal T}_{2,3}$ was accurately simulated within a regularized Biot-Savart law using a small step on $B_0$. At fixed values of parameter $\Lambda=\log(R_0/\xi)$, the dependence of knot lifetime on parameter $B_0$ turned out to be drastically non-monotonic on sufficiently small $B_0\lesssim 0.2$. Moreover, at $\Lambda\gtrsim 3$ quasi-stability bands appear, where vortex knot remains nearly unchanged for many dozens and even hundreds of typical times [4]. Qualitatively similar results take place also for ${\cal T}_{3,2}$ [4], for some other knots (${\cal T}_{2,5}$, ${\cal T}_{2,7}$, ${\cal T}_{3,4}$, ${\cal T}_{3,5}$, ${\cal T}_{3,7}$), and for unknots ${\cal U}_{2,1}$ [5]. These observations essentially enrich our knowledge about dynamics of vortex filaments. References: [1] D. Proment, M. Onorato, and C. F. Barenghi, "Vortex knots in a Bose-Einstein condensate", Phys. Rev. E 85, 036306 (2012). [2] D. Proment, M. Onorato, and C. F. Barenghi, "Torus quantum vortex knots in the Gross-Pitaevskii model for Bose-Einstein condensates", J. Phys.: Conf. Ser. 544, 012022, (2014). [3] D. Kleckner, L. H. Kauffman, and W. T. M. Irvine, "How superfluid vortex knots untie", Nature Physics 12, 650 (2016). [4] Рубан В.П., "Долгоживущие квантовые вихревые узлы", Письма в ЖЭТФ, 107 (5), 325-328 (2018). [5] V. P. Ruban, unpublished.

Dynamic phase transition in rare events statistics of 1D KPZ problem

30 марта в 11:30

Alex Kamenev (University of Minnesota)

I will review the concept of non-equilibrium phase transitions in rare events statistics as well as a recent dramatic progress in studies of 1D KPZ. The focus of my talk is on the reflection symmetry breaking phase transition recently found stationary KPZ problem: https://arxiv.org/abs/1606.08738

Chiral magnetic crystals

23 марта в 11:30

Markus Garst (TU Dresden)

The weak Dzyaloshinskii-Moriya interaction in chiral cubic magnets like MnSi, FeGe or Cu2OSeO3 twists the magnetization on long length scales resulting in spatially periodic magnetic textures — magnetic crystals. There exist especially magnetic crystals with a one- and two-dimensional periodicity corresponding to the magnetic helix and the topologically non-trivial skyrmion lattice, respectively. In this talk, we provide an overview of their properties. In particular, we discuss the crystallization process of these magnetic crystals that is characterized by strongly correlated chiral paramagnons that drive the transition first-order [1,2]. This fluctuation-induced first-order transition is well described by a theory put forward by Brazovskii. We will introduce the magnon band structure and their non-reciprocal properties in the presence of a magnetic field [3,4]. For the skyrmion lattice, this band structure is topological and characterized by finite Chern numbers that can be attributed to the formation of magnon Landau levels due to an emergent orbital magnetic field [5,6,7]. Finally, we will discuss domain walls of helimagnets that share similarities with grain boundaries consisting of disclination and dislocation defects of the helimagnetic order [8]. References: [1] M. Janoschek et al. Phys. Rev. B 87, 134407 (2013). [2] A. Bauer, M. Garst and C. Pfleiderer, Phys. Rev. Lett. 110, 177207 (2013). [3] M. Kugler et al. Phys. Rev. Lett. 115, 097203 (2015) [4] T. Weber et al. arXiv:1708.02098 [5] C. Schütte and M. Garst, Phys. Rev. B 90, 094423 (2014). [6] T. Schwarze, J. Waizner, M. Garst, A. Bauer, I. Stasinopoulos, H. Berger, C. Pfleiderer, and D. Grundler, Nat. Mater. 14, 478 (2015). [7] M. Garst J. Waizner, and D. Grundler, J. Phys. D: Appl. Phys. 50, 293002 (2017) [8] P. Schoenherr et al. Nat. Phys. in press, arXiv:1704.06288

Universality in statistics of Stokes flow over no-slip wall with random roughness

16 марта в 11:30

V. Parfenyev, S. Belan, and V. Lebedev

Stochastic roughness is widespread feature of natural surfaces and is an inherent by-product of most fabrication techniques. In view of rapid development of microfluidics, the important question is how this inevitable evil affects the low-Reynolds flows which are common for micro-devices. Moreover, one could potentially turn the flaw into a virtue and control the flow properties by means of specially "tuned" random roughness. We investigate theoretically the statistics of fluctuations in fluid velocity produced by the waviness irregularities at the surface of a no-slip wall. Particular emphasis is laid on the issue of the universality of our findings.

Спиновое эхо в накопительных кольцах

2 марта в 11:30

Н. Николаев, Ф. Ратманн, Ф. Салеев

Синхротронные (продольные) колебания в банчированных накопленных пучках приводят к разбросу частот прецессии спина в накопителе. При усреднении по ансамблю частиц это приводит к затухании вынужденных резонансным радиочастотным ротатором спина колебаний поляризации частиц. Будет показано, что при слабой, но конечной отстройке частоты ротатора (мы рассмотрим радиочастотный фильтр Вина) от частоты свободной прецессии спина в удерживающем магнитном поле накопителя, возникает режим своеобразного спинового эха: огибающая вынужденных колебаний убывает вначале по степенному закону, затем амплитуда полностью восстанавливается, и процесс повторяется периодически. Это курьёзное наблюдение может иметь практическое приложение к поискам резонансного вращения ЭДМ заряженных частиц в накопителях.

Матрицы Картана в теории цепочек Тода-Дарбу

2 марта в 11:30

А. Шабат, В. Адлер

Обсуждается взаимно однозначное соответствие полиномиальных первых интегралов гамильтоновых систем с экспоненциальным взаимодействием и “гипер-интегралов” двумеризованной цепочки Тоды. Установлены формулы пересчёта соответствующих многочленов и некоторые общие свойства их алгебраической структуры.

Повторное появление аномальных волн в оптике.

16 февраля в 11:30

П.Г. Гриневич, P.M. Santini

Я хотел бы рассказать о нашей работе с экспериментаторами из Римского университета о наблюдении повторной генерации аномальной волны в оптическом кристалле.

Manufacturing of holes in supported films: transition from beam dependent to shock dependent radius of hole as absorbed energy increases

9 февраля в 11:30

N. Inogamov, V. Shepelev, P. Danilov, A. Kuchmizhak

Thin films on supporting substrates are important class of laser targets for surface nanomodification for, e.g., plasmonic or sensoric applications. There are many papers devoted to this problem. But all of them are concentrated on dynamics of a film, paying small attention to substrate. In those papers the substrate is just an object absorbing the first shock. Here we present another point of view directed namely onto dynamics of a substrate. We consider (i) generation of a shock wave (SW) in a supporting substrate, (this si generation by impact of a film/support contact on supporting condensed medium); (ii) transition from 1D to 2D propagation of SW; (iii) we analyze lateral propagation of the SW along a film/support contact; and (iv) we calculate pressure in the compressed layer behind the SW decaying with time. This positive pressure acting from substrate to the film accelerates the film in direction to vacuum. Above some threshold, velocity of accelerated film is enough to separate the film from support. In the cases with large energy absorbed by a film, the circle of separation is significantly wider than the circle of high heating around the focal laser spot on film surface. Absorbed laser heat exponentially decays around an irradiated spot $F = Fc\, exp(-r^2/RL^2)$, where RL is radius of laser Gaussian beam. While the law of decay for the 2D SW in substrate is the power law. Therefore in the mentioned cases of powerful laser action, the edge of a separation circle is driven by SW in support.
Illustrative materials are posted on youtube:

Video 1
This movie shows the map of evolution of density field. The gold film is the narrow horizontal strip, "vacuum" is above, supporting substrate is below the strip

Video 2
This is the pressure map. We see two shocks: one above and second below the film. Don't pay attention to the shock above, i.e. to shock in "vacuum", because in our simulation we cannot use real vacuum rho=0, p=0. Therefore we use low density media in place of vacuum. Pay attention to the left and right wings of crescent type shock propagating down. These wings pass along the film. Shock pressure in the wings accelerate the film up thus separating it from the silica substrate.


Дифференциальный коэффициент Пуассона в двумерных кристаллических мембранах

26 января в 11:30

И.С. Бурмистров

Анализируется дифференциальный коэффициент Пуассона двумерной кристаллической мембраны, вложенной в пространство большой размерности $ d \ gg 1 $. Показано, что в режиме аномального закона Гука дифференциальный коэффициент Пуассона $\nu$ приближается к универсальному значению, определяемому исключительно пространственной размерностью $ d_c $, с степенным разложением $ \ nu = -1/3 + 0.016 / d_c + O (1 / d_c ^ 2) $, где $ d_c = d-2 $. Таким образом, значение $ -1 / 3 $, известное в предыдущей литературе, имеет место только в пределе $ d_c \ to \ infty $.

Влияние туннелирование в резервуар на мезоскопическую стонеровскую неустойчивость

19 января в 11:30

И.С. Бурмистров

Выведено обобщение действия Амбегаокар-Эккерн-Шона, которое описывает диссипативную динамику зарядовой и спиновой степеней свободы для квантовой точки, соединенной туннельным контактом с резервуаром. В отличие от предыдущих работ, это диссипативное действие выведено без следующих предположений (i) абсолютное значение спина на квантовой точке не меняется при учете туннелирования (ii) спин медленно вращается так, что выполняется адиабатическое приближение. Полученное диссипативное действие использовано для анализа устойчивости мезоскопического явления Стонера относительно туннелирования электронов в резервуар. Мы пришли к выводу, что при конечной температуре электронное туннелирование подавляет мезоскопическую неустойчивость Стонера при туннельной проводимости, которая зависит от температура. При нулевой температуре предсказывается существование квантового фазового перехода между мезоскопической фазой Стонера и парамагнитной фазой.

Exact solutions for nonlinear development of Kelvin-Helmholtz instability for counterflow of superfluid and normal components of Helium II

22 декабря 2017 в 11:30

Pavel M. Lushnikov and Nikolay M. Zubarev

A relative motion of the normal and superfluid components of Helium II results in Kelvin-Helmholtz instability (KHI) at their common free surface. We found the exact solutions for the nonlinear stage of the development of that instability. Contrary to the usual KHI of the interface between two fluids, the dynamics of Helium II free surface allows decoupling of the governing equations with their reduction to the Laplace growth equation which has the infinite number of exact solutions including the formation of sharp cusps at free surface in a finite time.