# Seminars at the Landau Institute scientific council

Seminars are held on Fridays in the conference hall of Landau Institute for Theoretical Physics in Chernogolovka, beginning at 11:30.

You can subscribe and receive announcements about ITP seminars. If you have any questions, please contact the scientific secretary Sergey Krashakov.

## Hamiltonian geometry of the associativity equations

22 November, the day after tomorrow in 11:30 (short)

Strizhova Nadezhda

The talk concerns the associativity equations (the WDVV system of equations), which arose in the 1990`s in the papers of Witten, Dijkgraaf, and brothers Verlinde devoted to two-dimensional topological field theories. The complete classification of the associativity equations in the case of 3 primary fields with respect to the existence of a first-order Dubrovin–Novikov Hamiltonian operator will be present at the talk. Also, we consider constructed by us finite-dimensional canonical Hamiltonian reductions of the associativity equations in the cases of 3 and 4 primary fields. The talk is based on joint work with O.I. Mokhov.

## Radiative parton energy loss and baryon stopping in heavy ion collisions

29 November in 11:30 (short)

B.G. Zakharov

We study the radiative energy loss contribution to proton stopping in heavy ion collisions. The radiative parton energy loss is calculated within the light-cone path integral approach to induced gluon emission. We have found that the radiative correction can fill in partly the midrapidity dip in the net proton rapidity distribution in AA collisions at center of mass energy \sqrt{s} about 10 GeV. This energy region is of great interest in connection with the beam energy scan program at RHIC (Brookhaven) and future experiments at collider NICA (Dubna) motivated by searching for the QCD critical point. We show that the net proton fluctuations at midrapidity, that have been proposed to be a good probe of the QCD critical point, may be dominated by the initial fluctuations of the proton flow, which, to a good accuracy, should be binomial, even in the presence of the critical point.

## Relaxation dynamics of nonequilibrium electrons in laser-excited solids

29 November in 11:30

Baerbel Rethfeld (Technische Universitaet Kaiserslautern, Germany)

When an ultrashort laser pulse of visible light is absorbed by a solid, mainly the electrons in the material are excited. In metals, free electrons in the conduction band can directly absorb photons. In semiconductors and dielectrics, on the other hand, a band gap has to be overcome first, as almost no free electrons are present at room temperature in the unexcited material. Due to this excitation, the electronic system, or the so-called electron-hole plasma, is in a nonequilibrium state. A sequence of different relaxation processes transfers the material into a new equilibrium. Depending on the interaction associated with the particular relaxation process, it occurs on a characteristic timescale. On the basis of complete Boltzmann-type collision integrals, we calculate the transient distribution functions of electrons and phonons in different materials. We consider electron-electron interaction, different ionization processes, as well as electron-phonon coupling. By that we trace the relaxation cascade of nonequilibrium electrons after ultrafast heating. Distinct material properties enter through the density of states of the electrons in the conduction band. We study in particular noble metals, dielectrics and ferromagnets. In noble metals and ferromagnets, d-electrons play an important role, whereas in dielectrics two separated bands govern the dynamics and the ionization state may differ from. We show, that the electron distributions deviate from Fermi distributions for timescales up to a few picoseconds. While the initial thermalization within one band has an intrinsic timescale of typically only a few tens of femtoseconds, nonequilibrium occupations of the different bands as well as continous electron-phonon coupling can drive the conduction electrons out of equilibrium for much longer times [1, 2]. We present in detail the mutual influence of different interaction and relaxation processes.
[1] N. Brouwer and B. Rethfeld, Phys. Rev. B 95, 245139 (2017). [2] S.T. Weber and B. Rethfeld, Phys. Rev. B 99, 174314 (2019).

## Color randomization of fast two-parton states in quark-gluon plasma in heavy ion collisions

29 November in 11:30

B.G. Zakharov

We study the color randomization of two-parton states produced after splitting of a primary fast parton in the quark-gluon plasma. We find that the color randomization of the two-parton states in the quark-gluon plasma produced in heavy ion collisions at RHIC and LHC energies is rather slow. At jet energies E= 100 and 500 GeV, for typical jet path length in the plasma in central Pb+Pb collisions, the SU(3)-multiplet averaged color Casimir of the two-parton states differs considerably from its value for the fully color randomized state. We evaluate the energy dependence for generation of the nearly collinear gluon-gluon pairs in the decuplet color state and quark-gluon pairs in the anti-sextet color states, that can lead to an anomalous baryon jet fragmentation, which are forbidden in vacuum for nucleon-nucleon collisions. Our results show that the baryon production via the color anomalous two-parton states can be important in the enhancement of the baryon/meson ratio observed in heavy ion collisions at RHIC and LHC.

## Landau theory for smectic-A–hexatic-B coexistence in smectic films

6 December in 11:30

E. S. Pikina, E. I. Kats, V. V. Lebedev

We explain theoretical peculiarities of the smectic-A–hexatic-B equilibrium phase coexistence in a finite-temperature range recently observed experimentally in free-standing smectic films [I. A. Zaluzhnyy et al., Phys. Rev. E 98, 052703 (2018)]. We quantitatively describe this unexpected phenomenon within Landau phase transitions theory assuming that the film state is close to a tricritical point. We found that the surface hexatic order diminishes the phase coexistence range as the film thickness decreases, shrinking it to zero at some minimal film thickness Lc, of the order of a few hexatic correlation length. We established universal laws for the temperature width of the phase coexistence range in terms of the reduced variables. Our theory is in agreement with the existing experimental data.

## Effect of small dissipation on the NLS anomalous waves recurrence.

13 December in 11:30

P.G. Grinevich, P.M. Santini

We provide analytic formulas decribing the effect of small loss/gain on the recurrence of anomalous waves in the focusing Nonlinear Schrodinger equation. We show that very small loss or gain essentially affects the statistics and the character of the recurrence. In particular, our formulas explains the results of numerical simulations from the paper by O. Kimmoun, H.C. Hsu, H. Branger, M.S. Li, Y.Y. Chen, C. Kharif, M. Onorato, E.J.R. Kelleher, B. Kibler, N. Akhmediev, A. Chabchoub (2016).

## KP-2 solutions in the form of soliton sequences.

13 December in 11:30 (short)

P.G. Grinevich, S. Abenda

Some photos of the ocean surface demonstrate waves looking like lattices formed by solitons. They can be modelled by finite-gap Kadomtsev-Petviashvili 2 solutions corresponding to almost dgenerate spectral curves. We show how to construct such solutions in the first non-trivial case corresponding to GR(4,2).

## Ordered fluctuations: about vestigial order in quantum materials

20 December in 11:30

Joerg Schmalian (Karlsruhe Institute of Technology)

A hallmark of the phase diagrams of quantum materials is the existence of multiple electronic ordered states. In many cases those are not independent, competing phases, but instead display a complex intertwinement. In this talk, we focus on a realization of intertwined orders with a fluctuation-driven vestigial phase characterized by a composite order parameter. In other words, we are investigating the condensation of fluctuations.
We demonstrate that this concept naturally explains the nematic state in iron-based superconductors and nematic superconductivity in doped topological insulators. In addition we propose a natural mechanism for charge 4e superconductivity with half flux quanta. We present a formalism that provides a framework to understand the complexity of quantum materials based on symmetry, largely without resorting to microscopic models.

## Non-ergodic delocalized states for efficient population transfer within a narrow band of the energy landscape

20 December in 15:00

Vadim Smelyanskiy (Google, Los Angeles)

We will review the advances and challenges in the field of quantum combinatorial optimization and closely related problem of low-energy eigenstates and coherent dynamics in transverse field quantum spin glass models. We will discuss the role of collective spin tunneling that gives rise to bands of delocalized non-ergodic quantum states providing the coherent pathway for the population transfer (PT) algorithm: the quantum evolution under a constant transverse field that starts at a low-energy spin configuration and ends up in a superposition of spin configurations inside a narrow energy window. We study the transverse field induced quantum dynamics of the following spin model: zero energy of all spin configurations except for a small fraction of spin configuration that form a narrow band at large negative energy. We use the cavity method for heavy-tailed random matrices to obtain the statistical properties of the low-energy eigenstates in an explicit analytical form. In a broad interval of transverse fields, they are non-ergodic, albeit extended giving rise to a qualitatively new type of quantum dynamics. For large transverse fields »1 the typical runtime of PT algorithm $\sim \sqrt{2^n / \Omega e^r}$ scales with n and Ω as that of the Grover’s quantum search, except for the small correction to the exponent θ ≈ 1/(2). The model we consider is non-integrable. As a result, our PT protocol does not require any fine-tuning of and may be initialized in a computational basis state. We argue that our approach can be applied to study PT protocol in other optimization problems with the potential quantum advantage over classical algorithms.

## Disorder and interaction in chiral chains: Majoranas versus complex fermions.

27 December in 11:30

Mirlin A.D.

We study the low-energy physics of a chain of Majorana fermions in the presence of interaction and disorder, emphasizing the difference between Majoranas and conventional (complex) fermions. While in the noninteracting limit both models are equivalent (in particular, belong to the same symmetry class BDI and flow towards the same infinite-randomness critical fixed point), their behavior differs drastically once interaction is added. Our density-matrix renormalization group calculations show that the complex-fermion chain remains at the noninteracting fixed point. On the other hand, the Majorana fermion chain experiences a spontaneous symmetry breaking and localizes for repulsive interaction. To explain the instability of the critical Majorana chain with respect to a combined effect of interaction and disorder, we consider interaction as perturbation to the infinite-randomness fixed point and explore correlations of wave functions that enter interaction matrix elements. Our numerical and analytical results exhibit a rich structure of critical eigenstate correlations. This allows us to identify a relevant interaction operator that drives the Majorana chain away from the infinite-randomness fixed point. For the case of complex fermions, the interaction is irrelevant.

## Dual description of $\eta$-deformed OSP sigma-models

17 January 2020 in 11:30

Alexey Litvinov

My talk will be based on joint work with M. Alfimov, B. Fegin and B. Hoare. We propose a system of fermionic screening fields depending on a continuous parameter $b$, which defines $\eta$-deformed $OSP(n|2m)$ sigma-model in the limit $b\rightarrow\infty$ and a super-renormalizable QFT in $b\rightarrow0$. In the sigma-model regime we show that leading UV asymptotic of the RG group flow equations coincides with perturbation around Gaussian theory. In perturbative regime $b\rightarrow0$ we show that the tree level two-particle scattering matrix matches the expansion of the trigonometric $OSP(n|2m)$ $R$-matrix.