Talks

Talk 1

Speaker: Moon Jip Park  

Title : Twisted bilayer magnet and fractionalized skyrmion

Abstract: Recent experiments with twisted bilayer materials have provided a versatile platform for the realization of exotic phases of matter. In this talk, we are going to expand the theory of moire systems to spin systems. Starting from the brief review of twisted bilayer graphene, we develop a concrete theory of twisted bilayer magnetism. Based on the first- principles calculations of two-dimensional honeycomb magnet CrI3, we construct the generic spin models that represent a broad class of twisted bilayer magnetic systems. Using the Monte-Carlo method, we discover a variety of non-collinear magnetic order that has been overlooked in previous theoretical and experimental studies. Finally, we show that the twisted magnets can be a promising candidate for the discovery of topological magnon excitations.

Talk 3

Speaker:  Dong-Hee Kim 

Title: Effect of Flat-band-preserving disorder to superconductivity in the Kagome Hubbard model 

Abstract: We investigate the vulnerability of flat-band superconductivity against disorders in the attractive Hubbard model on the Kagome lattices. We consider two different types of disorders that include a correlated one designed to preserve a flat band and an uncorrelated random onsite potential. In the absence of interactions, the preserved flat-band state under the correlated disorder remains to be extended for an arbitrarily strong disorder strength, which is in contrast to the case of the uncorrelated one where the flat-band degeneracy is destroyed, and the corresponding state become localized at a very weak disorder strength. On the other hand, in the presence of interactions, while the superconductivity survives in the regime of weak interactions and weak disorders for both types, it turns out that increasing the interaction strength make it more vulnerable to the disorders. We discuss these features by employing the mean-field and exact diagonalization calculations on the density imbalance, the superconducting order parameter, and the superfulid weight. 

Talk 5

Speaker: Sang-Jin Sin

Title: Holographic Mean field theory and the Encoding the Lattice.

Abstract: I introduce the holographic mean field theory, which is a MFT working for the strongly interacting system

as a new paradigm for the discussing the strongly correlated electron system.

It is a holographic field theory with Fermion-Order parameter-Fermion interaction.

It utilizes the presence of the dual gravity and differs from the usual mean field theory in that it include the kinetic term of the order parameter.

As a result   the Green function can be obtained approximately but analytically which can tell us the nature of of the electron fluids when it interact strongly.

We can classify the non-fermi liquid and we can classify the symmetry breaking type.

We also introduce a lattice encoding method in the holography which can be considered as a first step of the holographic calculation of the ARPES data.

Talk 7

Speaker:  Sergej Flach 

Title: Flat Bands with Interactions 

Abstract: Certain lattice wave systems in translationally invariant settings have one or more spectral bands that are strictly flat or independent of momentum in the tight binding approximation, arising from either internal symmetries or fine-tuned coupling [1,2]. I will briefly touch on the theoretical design and experimental implementation of flat bands and classification schemes. I will then present recent results on quantum many body interacting systems embedded in flat band environments which lead to controlled currents of charge and heat [3], heat percolation [4], many body localization in translationally invariant systems [5], interaction induced transport in Wannier-Stark flatbands [6], interaction-induced conductance transitions in AB cages [6], and a recent experimental realization with electric circuit networks [8]. [1] Artificial flat band systems: from lattice models to experiments. Daniel Leykam, Alexei Andreanov, Sergej Flach. Adv. Phys.: X 3, 1473052 (2018) [2] Photonic Flat Bands. Daniel Leykam, Sergej Flach. APL PHOTONICS 3, 070901 (2018) [3] Many body flatband localization. Carlo Danieli, Alexei Andreanov, Sergej Flach. Phys. Rev. B 102, 04116(R) (2020) [4] Heat percolation in many-body flatband localizing systems. Ihor Vakulchyk, Carlo Danieli, Alexei Andreanov, Sergej Flach. Phys. Rev. B 104, 144207 (2021) [5] Many-body localization transition from flatband finetuning. Carlo Danieli, Alexei Andreanov, Sergej Flach. Phys. Rev. B 105, L041113 (2022). [6] Correlated metallic two-particle bound states in Wannier-Stark flatbands. Arindam Mallick, Alexei Andreanov, Sergej Flach. Phys. Rev. B 106, 125128 (2022). [7] Conductance transition with interacting bosons in an Aharonov-Bohm cage. A. R. Kolovsky, P. Muraev, S. Flach. Phys. Rev. A 108, L010201 (2023). [8] Compact localized states in electric circuit flatband lattices. Carys Chase-Mayoral, L. Q. English, Yeongjun Kim, Sanghoon Lee, Noah Lape, Alexei Andreanov, P. G. Kevrekidis, Sergej Flach. arXiv:2307.15319 (2023).