Condensed Matter

Spatial symmetries can enrich the topological classification of interacting quantum matter and endow additional ``weak” topological indices upon systems with non-trivial strong topological invariants (protected by internal symmetries). In this talk, I will discuss the boundary physics of charge conserving systems with a non-zero shift invariant, which is protected by either a continuous U(1) or discrete CN rotation symmetry. In particular, I will discuss an interface between two systems with the same Chern number but distinct shift invariants and show that the interface hosts protected gapless edge modes. For general Abelian topological orders in 2D, I will prove sufficient conditions for gapless edge states protected by continuous rotation symmetry. For the case of discrete rotation symmetries, I will show that the Chern-Simons field theory for systems with gappable edges predicts fractional corner charges. These can also be computed when the system is placed on the two-dimensional surface of a Platonic solid, which relates to the fractional charge bound at disclination defects. Time permitting, I will discuss recent results regarding the relation between the shift and many-body real-space invariants for 2D systems with crystalline symmetry.

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Weak measurements can be viewed as a soft projection that interpolates between an identity operator and a projection operator, and can induce an effective central charge distinct from the unmeasured CFT. In the first part, I will discuss the effect of measurement and postselection on the critical ground state of a Luttinger liquid theory. Depending on the Luttinger parameter K, the effect of measurement is irrelevant, marginal, or relevant, respectively. When the measurement is marginal, and we find a critical state whose entanglement entropy exhibits a logarithmic behavior with a continuous effective central charge as a function of measurement strength. Inspired by this result, in the second part, I will discuss a holographic description of the weak measurement. The weak measurement is modeled by an interface brane, separating different geometries dual to the post-measurement state and the unmeasured CFT. In an infinite system, the weak measurement is related to ICFT via a spacetime rotation. We find that the holographic entanglement entropy with twist operators located on the defect is consistent in both calculations for ICFT and weak measurements. In a finite system, the weak measurement can lead to a rich phase diagram, in which the post-measurement geometry can realize a Python’s lunch.

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The Levin-Wen model is known to produce a vast array of topological phases of matter. Among these theories are gauge theories such as the twisted quantum double. In this talk, we will show that the Levin-Wen model is itself a gauge theory. In particular, given a unitary fusion category C, we construct a globally tube algebra (Tube(C)) symmetric lattice model and gauge this symmetry to produce the Levin-Wen model with anyons described by the Drinfeld center Z(C). This construction endows the terms of the Levin-Wen Hamiltonian with the interpretation of flux and charge operators for the Tube(C) gauge symmetry. Furthermore, this construction gives a gauge theoretic interpretation to the mathematical fact that the category of representations of Tube(C) is equivalent to Z(C). To demonstrate this new class of Tube(C) symmetric theories, we will explicitly explore the case where C is the Fibonacci category Fib. We will write down the ungauged Tube(Fib) symmetric theory, compute the symmetry action, and show how to gauge the Tube(Fib) global symmetry to produce the double Fibonacci Levin-Wen model.

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In this talk I will use matrix product states (MPS) to study topological families of gapped ground states in one spatial dimension. To such families I will describe how to associate a gerbe, a mathematical structure which generalizes the line bundle associated to gapped ground states in 0d. Nontriviality of the gerbe represents an obstruction to representing the family of ground states with an MPS tensor that is continuous everywhere over parameter space. I will illustrate these constructions using an exactly solvable topological family which exhibits the key physics in a simple manner.

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Zoom link https://pitp.zoom.us/j/91497524520?pwd=MkFHSW9PeGlMb2lFOTR3Qmo5clU0dz09

It is known that the classical O(N) model in dimension d > 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extraordinary and the special. The extraordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3, it is less clear what happens to the extraordinary and special fixed points when d = 3 and N is greater or equal to 2. I'll show that formally treating N as a continuous parameter, there exists a critical value Nc > 2 separating two distinct regimes. For N < Nc the extra-ordinary fixed point survives in d = 3, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of log r. For N > Nc there is no fixed point with order parameter correlations decaying slower than power law. I'll discuss how these findings compare to recent Monte-Carlo studies of classical and quantum spin models.

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Zoom link https://pitp.zoom.us/j/97209122334?pwd=UHQ2OXR4bnVZREV0SlJOYXphWjh0QT09