During the 2022–23 academic year, the CMSA will be hosting a seminar on Topological Quantum Matter organized by Jie Wang. This seminar will take place on Wednesdays at 9:00 am–10 am (Eastern time). The meetings will take place in Room G10 at the CMSA, 20 Garden Street, Cambridge MA 02138, and some meetings will take place virtually on Zoom or be held in hybrid formats. To learn how to attend, please fill out this form.  

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  • December 07, 2022 10:00 AM
Speaker: Vasil Rokaj
Title: Controlling Quantum Matter with Quantum Cavity Fields
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Vasil Rokaj (Harvard) Title: Controlling Quantum Matter with Quantum Cavity Fields Abstract: Cavity modification of material properties and phenomena is a novel research field motivated by the advances in strong light-matter interactions [1]. For condensed matter systems it has been demonstrated experimentally that the transport properties of 2D materials can be modified via coupling to vacuum fields [2,3]. While in polaritonic chemistry it has been shown that ground state chemical properties can be controlled with cavity fields [4]. In the first part of my talk, I will present how the quantized cavity field can alter the conduction properties of a condensed matter system by focusing on the paradigmatic Sommerfeld model of the free electron…

  • November 23, 2022 09:00 AM
Speaker: Jian Kang
Title: Continuum field theory of graphene bilayer system
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Jian Kang, School of Physical Science and Technology, ShanghaiTech University, Shanghai, China Title: Continuum field theory of graphene bilayer system Abstract: The Bistritzer-MacDonald (BM) model predicted the existence of the narrow bands in the magic-angle twisted bilayer graphene (MATBG), and nowadays is a starting point for most theoretical works. In this talk, I will briefly review the BM model and then present a continuum field theory [1] for graphene bilayer system allowing any smooth lattice deformation including the small twist angle. With the gradient expansion to the second order, the continuum theory for MATBG [2] produces the spectrum that almost perfectly matches the spectrum of the microscopic model, suggesting the validity of…

  • November 16, 2022 10:00 AM
Speaker: Jérôme Faist
Title: Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Jérôme Faist  (ETH Zurich) Title: Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect Abstract: When a collection of electronic excitations are strongly coupled to a single mode cavity, mixed light-matter excitations called polaritons are created. The situation is especially interesting when the strength of the light-matter coupling ΩR is such that the coupling energy becomes close to the one of the bare matter resonance ω0. For this value of parameters, the system enters the so-called ultra-strong coupling regime, in which a number of very interesting physical effects were predicted caused by the counter-rotating and diamagnetic terms of the Hamiltonian. In a microcavity, the strength of…

  • November 02, 2022 09:00 AM
Speaker: Junyeong Ahn
Title: Optical axion electrodynamics
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Junyeong Ahn (Harvard) Title: Optical axion electrodynamics Abstract: Electromagnetic fields in a magneto-electric medium behave in close analogy to photons coupled to the hypothetical elementary particle, the axion. This emergent axion electrodynamics is expected to provide novel ways to detect and control material properties with electromagnetic fields. Despite having been studied intensively for over a decade, its theoretical understanding remains mostly confined to the static limit. Formulating axion electrodynamics at general optical frequencies requires resolving the difficulty of calculating optical magneto-electric coupling in periodic systems and demands a proper generalization of the axion field. In this talk, I will introduce a theory of optical axion electrodynamics that allows for a simple quantitative…

  • October 26, 2022 09:00 AM
Speaker: Bruno Mera
Title: Kähler bands—Chern insulators, holomorphicity and induced quantum geometry
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Bruno Mera, Tohoku University Title: Kähler bands—Chern insulators, holomorphicity and induced quantum geometry Abstract: The notion of topological phases has dramatically changed our understanding of insulators. There is much to learn about a band insulator beyond the assertion that it has a gap separating the valence bands from the conduction bands. In the particular case of two dimensions, the occupied bands may have a nontrivial topological twist determining what is called a Chern insulator. This topological twist is not just a mathematical observation, it has observable consequences—the transverse Hall conductivity is quantized and proportional to the 1st Chern number of the vector bundle of occupied states over the Brillouin zone. Finer properties of…

  • October 19, 2022 04:00 PM
Speaker: Yizhuang You
Title: Symmetric Mass Generation
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Yizhuang You, UC San Diego Title: Symmetric Mass Generation Abstract: Symmetric mass generation (SMG) is a novel mechanism for massless fermions to acquire a mass via a strong-coupling non-perturbative interaction effect. In contrast to the conventional Higgs mechanism for fermion mass generation, the SMG mechanism does not condense any fermion bilinear coupling and preserves the full symmetry. It is connected to a broad range of topics, including anomaly cancellation, topological phase classification, and chiral fermion regularization. In this talk, I will introduce SMG through toy models, and review the current understanding of the SMG transition. I will also mention recent numerical efforts to investigate the SMG phenomenon. I will conclude the talk…

  • October 12, 2022 09:00 AM
Speaker: Jennifer Cano
Title: Engineering topological phases with a superlattice potential
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Jennifer Cano (Stony Brook and Flatiron Institute) Title: Engineering topological phases with a superlattice potential Abstract: We propose an externally imposed superlattice potential as a platform for manipulating topological phases, which has both advantages and disadvantages compared to a moire superlattice. In the first example, we apply the superlattice potential to the 2D surface of a 3D topological insulator. The superlattice potential creates tunable van Hove singularities, which, when combined with strong spin-orbit coupling and Coulomb repulsion give rise to a topological meron lattice spin texture. Thus, the superlattice potential provides a new route to the long sought-after goal of realizing spontaneous magnetic order on the surface of a 3D TI. In the second example,…

  • September 28, 2022 09:00 AM
Speaker: Ruihua Fan
Title: Extracting the quantum Hall conductance from a single bulk wavefunction from the modular flow
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Ruihua Fan, Harvard University Title: Extracting the quantum Hall conductance from a single bulk wavefunction from the modular flow Abstract: One question in the study of topological phases is to identify the topological data from the ground state wavefunction without accessing the Hamiltonian. Since local measurement is not enough, entanglement becomes an indispensable tool. Here, we use modular Hamiltonian (entanglement Hamiltonian) and modular flow to rephrase previous studies on topological entanglement entropy and motivate a natural generalization, which we call the entanglement linear response. We will show how it embraces a previous work by Kim&Shi et al on the chiral central charge, and furthermore, inspires a new formula for the quantum Hall…

  • September 21, 2022 09:00 AM
Speaker: Semyon Klevtsov
Title: Geometric test for topological states of matter
Venue: CMSA Room G10

Topological Quantum Matter Seminar Speaker: Semyon Klevtsov, University of Strasbourg Title: Geometric test for topological states of matter Abstract: We generalize the flux insertion argument due to Laughlin, Niu-Thouless-Tao-Wu, and Avron-Seiler-Zograf to the case of fractional quantum Hall states on a higher-genus surface. We propose this setting as a test to characterise the robustness, or topologicity, of the quantum state of matter and apply our test to the Laughlin states. Laughlin states form a vector bundle, the Laughlin bundle, over the Jacobian – the space of Aharonov-Bohm fluxes through the holes of the surface. The rank of the Laughlin bundle is the degeneracy of Laughlin states or, in presence of quasiholes, the dimension of the corresponding full many-body…