У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 15. априла 2022. године у 11 часова путем BigBlueButton платформе, Душан Ђорђевић (Физички факултет у Београду), одржаће предавање:

Noncommutative D=5 Chern-Simons Gravity: Kaluza-Klein Reduction and Chiral Gravitational Anomaly

САЖЕТАК:

Chern-Simons (CS) theory has greatly influenced physics and physicists during the past half-century. In this seminar, we will review some basic facts about Chern-Simons Lagrangians defined on some (2n-1)-dimensional manifold. We will then show how CS theory can be connected with gravity. Our focus will be on an SO(4,2) CS gravity in five dimensions and its connections with four-dimensional gravitation. Furthermore, we will analyse the noncommutative deformation of SO(4,2) CS gravity using the geometric Siberg-Witten map. This theory has a nontrivial first-order correction in \theta. By performing Kalutza-Klein dimensional reduction and suitable symmetry breaking, we obtain an effective, four-dimensional gravity theory that has Einstein-Hilbert action with (negative) cosmological constant as its classical (commutative) limit. We then proceed to analyse some concrete solutions of the field equations. We show that AdS space-time remains a solution after including the first-order corrections. On the other hand, AdS-Schwarzschild black hole solution develops a torsion that we explicitly compute. Moreover, we will see that this modified solution implies the existence of a chiral gravitational anomaly if one couples massless Dirac fermions to this NC background.

This seminar is based on arXiv:2203.05020

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У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 14. априла 2022. године у 14 часова путем Zoom платформе, др Лук Радемакер (Департман за теоријску физику, Универзитет у Женеви), одржаће предавање:

Symmetry breaking and Chern insulators in twisted graphene structures

САЖЕТАК:

Twisted bilayer graphene (tBG) and variants like twisted monolayer-bilayer graphene (tMBG) were proposed to be a platform for strongly correlated physics akin to the cuprate family. However, we will show that many of the observed interacting phenomena can be explained in terms of breaking of spin/valley symmetry. To do so, we will first describe how flat bands in tBG can be directly observed using ARPES [1]. An effective description of the flat bands is hampered by band renormalization due to charge transfer [2] and band topology. The combination of interactions and topology naturally leads to spin/valley symmetry breaking with a quantum anomalous Hall effect in tMBG [3]. In tBG in large magnetic fields, similar spin/valley symmetry breaking creates a series of Chern insulator states [4]. We will conclude with a brief discussion of the possibility of genuine strong correlations in Moiré structures.

[1] S. Lisi, X. Lu, T. Benschop, et al., Nature Phys. 17, 189 (2021).
[2] L. Rademaker, D. A. Abanin, P. Mellado, Phys. Rev. B 100, 205114 (2019).
[3] L. Rademaker, I. V. Protopopov, D. A. Abanin, Phys. Rev. Research 2, 033150 (2020).
[4] Y. Saito, J. Ge, L. Rademaker, et al., Nat. Phys. 17, 478 (2021).

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Meeting ID: 897 1269 4555
Passcode: 580212

У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 01. априла 2022. године у 11 часова путем BigBlueButton платформе, др Бранислав Цветковић (Институт за физику у Београду), одржаће други део предавања:

Entropy in Poincaré gauge theory: Hamiltonian approach

САЖЕТАК:

Canonical generator G of local symmetries in Poincaré gauge theory is constructed as an integral over a spatial section Σ of spacetime. Its regularity (differentiability) on the phase space is ensured by adding a suitable surface term, an integral over the boundary of Σ at infinity, which represents the asymptotic canonical charge. For black hole solutions, Σ has two boundaries, one at infinity and the other at horizon. It is shown that the canonical charge at horizon defines entropy, whereas the regularity of G implies the first law of black hole thermodynamics.

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У оквиру семинара Групе за гравитацију, честице и поља Института за физику у Београду, у петак, 25. марта 2022. године у 11 часова путем BigBlueButton платформе, др Бранислав Цветковић (Институт за физику у Београду), одржаће предавање:

Entropy in Poincaré gauge theory: Hamiltonian approach

САЖЕТАК:

Canonical generator G of local symmetries in Poincaré gauge theory is constructed as an integral over a spatial section Σ of spacetime. Its regularity (differentiability) on the phase space is ensured by adding a suitable surface term, an integral over the boundary of Σ at infinity, which represents the asymptotic canonical charge. For black hole solutions, Σ has two boundaries, one at infinity and the other at horizon. It is shown that the canonical charge at horizon defines entropy, whereas the regularity of G implies the first law of black hole thermodynamics.

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