[Landau ITP Seminars] семинары ВШЭ 26 декабря -- Сербин (11:00) и Островский (14:00)

[Pavel Ioselevich]

Дорогие коллеги,

Международная лаборатория физики конденсированного состояния НИУ ВШЭ приглашает на доклады М. Сербина и П. Островского 26 декабря 2018. 
Семинары пройдут по адресу: Ст.Басманная д.21/4, стр.5, ауд. Б-807 (8 этаж)

в 11:00
Maxim Serbyn (Institute of Science and Technology, Austria)
Weak ergodicity breaking from quantum many-body scars

The statistical mechanics description of many-particle systems rests on the assumption of ergodicity, the ability of a system to explore all allowed configurations in the phase space. For quantum many-body systems statistical mechanics predicts the equilibration of highly excited non-equilibrium state towards a featureless thermal state. Hence, it is highly desirable to explore possible ways to avoid ergodicity in quantum systems. Many-body localization presents one generic mechanism for a strong violation of ergodicity relying on the presence of quenched disorder. In my talk I will discuss a different mechanism of the weak ergodicity breaking relevant for the experimentally realized Rydberg-atom quantum simulator [1]. This mechanism arises from the presence of special eigenstates in the many-body spectrum that are reminiscent of quantum scars in chaotic non-interacting systems [2]. In the single-particle case, quantum scars correspond to wave functions concentrated in the vicinity of unstable periodic classical trajectories. I will demonstrate that many-body scars appear in the Fibonacci chain, a model with a constrained local Hilbert space which can be realized by a Rydberg chain. The quantum scarred eigenstates are embedded throughout the otherwise thermalizing many-body spectrum but lead to direct experimental signatures, as I show for periodic recurrences that reproduce those observed in the experiment [1]. Finally, I will construct the weak deformation of the Rydberg chain Hamiltonian that makes revivals virtually perfect [3]. I will conclude with discussing a new opportunities for the creation of novel states with long-lived coherence in systems that are now experimentally realizable and possible generalizations of these results. 
[1] Bernien, H. et al., Nature 551, 579–584 (2017), arXiv:1707.04344
[2] C. J. Turner, A. A. Michailidis, D. A. Abanin, M. Serbyn, Z. Papić, Nature Physics (May 2018), arXiv:1711.03528 and Phys. Rev. B 98, 155134 (2018) arXiv:1806.10933 
[3] S. Choi, C. J. Turner, et al.  arXiv:1812.05561

в 14:00
Pavel Ostrovsky (Max-Planck-Institut Stuttgart, Germany)
Interplay between Anderson localization and proximity effect

When a normal metal is connected to a superconductor, Cooper pairs penetrate in the normal region and induce superconducting correlations there. This phenomenon is known as the proximity effect. Its main manifestation is a suppression of the local density of states in the normal part of the junction. Standard description of the proximity effect is based on the quasiclassical Usadel equation, which neglects possible electron interference. The latter is responsible for Anderson localization in the disordered metal and in general competes with the proximity effect reducing the superconducting correlations. In this work we develop a fully quantum approach based on the nonlinear sigma model to describe both proximity effect and localization within the same framework. We obtain exact results for the global (spatially integrated) density of states in the normal part of the junction applicable at all energies. At low energies, interference effects are strongest and considerably modify the quasiclassical result. In this limit we also calculate local (spatially resolved) density of states. We find that localization limits the spatial extent of the proximity effect but at the same time, and quite counterintuitively, enhances it at short distances.

При необходимости заказа пропуска в НИУ ВШЭ обращайтесь к Ирине Аванесовой, iavanesova at hse.ru <https://e.mail.ru/compose/?mailto=mailto%3aiavanesova@hse.ru>
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