# 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.

## Coalescence of isotropic droplets in overheated free standing smectic films

2 October, today in 11:30

__Elena S. Pikina__, Boris I. Ostrovskii and Sergey A. Pikin

A theoretical study of the interaction and coalescence of isotropic droplets in overheated free-standing smectic films (FSSF) is presented. Experimentally it is clear that merging of such droplets is extremely rare. On the basis of the general thermodynamic approach to the stability of FSSF, we determined the energy gains and losses involved in the coalescence process. The main contributions to the critical work
of drop coalescence are due to the gain related to the decrease of the surface energy of the merging drops, which is opposed by the entropic repulsions of elementary steps at the smectic interface between them. To quantify the evolution of the merging drops, we use a simple geometrical model in which the volume of the smectic material, rearranged in the process of coalescence, is described by an asymmetrical pyramid at the intersection of two drops. In this way, the critical work for drop coalescence and the corresponding energy barrier have been calculated. The probability of the thermal
activation of the coalescence process was found to be negligibly small, indicating that droplet merging can be initiated by only an external stimulus. The dynamics of drop merging was calculated by equating the capillary force driving the coalescence, and the Stokes viscous force slowing it down. For the latter, an approximation of moving oblate spheroids permitting exact calculations was used. The time evolution of the height of the neck between the coalescing drops and that of their lateral size are in good agreement with experiments.

E.S. Pikina, B.I. Ostrovskii, S.A. Pikin, Coalescence of isotropic droplets in overheated free standing smectic films, Soft Matter, 16(19), 4591-4606 (2020)

E.S. Pikina, B.I. Ostrovskii, S.A. Pikin, Coalescence of isotropic droplets in overheated free standing smectic films, Soft Matter, 16(19), 4591-4606 (2020)

## Critical behavior at the localization transition on random regular graphs

9 October in 11:30

Konstantin Tikhonov

We study numerically the critical behavior at the localization transition in the Anderson model on infinite Bethe lattice and on random regular graphs. As a first step, we carry out a precise determination of the critical disorder. After this, we determine the dependence of the correlation volume on disorder on the delocalized side of the transition by means of population dynamics. The asymptotic critical behavior is found to be in agreement with analytical prediction based on the stability analysis of the symmetry-broken solution near criticality.

## Conductivity of superconductors in the flux flow regime

9 October in 11:30

M. Smith, A.V. Andreev, M.V. Feigel'man, and B.Z.Spivak

We develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude Δ(r) and the diffusion coefficient D(r). We find a contribution to the conductivity that is proportional to the inelastic relaxation time τin, which is much longer than the elastic relaxation time. This new contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than Hc2.

## Interaction-induced metallicity in a two-dimensional disordered non-Fermi liquid

16 October in 11:30

Pavel Nosov, Igor Burmistrov, Srinivas Raghu

The interplay of interactions and disorder in two-dimensional (2D) electron systems has actively been studied for decades. The paradigmatic approach involves starting with a clean Fermi liquid and perturbing the system with both disorder and interactions. We instead start with a clean non-Fermi liquid near a 2D ferromagnetic quantum critical point and consider the effects of disorder. In contrast with the disordered Fermi liquid, we find that our model does not suffer from runaway flows to strong coupling and the system has a marginally stable fixed point with perfect conduction.

## Spectroscopic evidence for strong correlations between local resistance and superconducting gap in ultrathin NbN films

16 October in 11:30 (short)

M.A.Skvortsov and M.V.Feigel'man

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap Δ and of the critical temperature Tc, the ratio Δ/kBTc remaining roughly constant. In addition, in most films, an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature Tc and the energy gap Δ starts decoupling from Tc. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of an NbN film with scanning tunneling spectroscopy (STS), we show that the inhomogeneous spatial distribution of Δ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modeling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of Δ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.