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# Emergence in Complex Systems

Emergence in Complex Systems

## Many-body localization with dipoles

Friday Feb 14, 2014
Speaker(s):

Statistical mechanics is the framework that connects thermodynamics to the microscopic world. It hinges on the assumption of equilibration; when equilibration fails, so does much of our understanding. In isolated quantum systems, this breakdown is captured by the phenomenon known as many-body localization. This breakdown manifests in a variety of ways, as elucidated by much recent theoretical and numerical work.

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## Low Spin State Mott Insulators and Emergence of novel Quantum Spin Liquids

Friday Feb 14, 2014
Speaker(s):

In a conventional Mott insulator, magnitude of local spin moments remain fixed. They are fixed spin Mott insulators'. We suggest that, in a multi orbital Hubbard model, when local Hund coupling is won over by inter-orbital superexchange couplings between neighboring sites, local spin moment can decrease its value in a cooperative fashion, through a first order phase transition, These are Low spin state Mott insulators' (LSSMI).

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## A unification of symmetric Z2 spin liquids on kagome lattice

Friday Feb 14, 2014
Speaker(s):

While there is mounting numerical evidence for a gapped Z2 spin liquid in the kagome Heisenberg model, a complete characterization of this topological phase remains to be accomplished. A defining property, the projective symmetry group (PSG) which fixes how the emergent excitations of the spin liquid phase transform under symmetry, remains to be determined. Following a Chern-Simons field theory, we show how PSG determines measurable properties of a Z2 spin liquid, such as the existence of symmetry protected gapless edge states.

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## How universal is the entanglement spectrum?

Friday Feb 14, 2014
Speaker(s):

It is now commonly believed that the ground state entanglement spectrum (ES) exhibits universal features characteristic of a given phase. In this talk, I will present evidence to the contrary. I will show that the entanglement Hamiltonian can undergo quantum phase transitions in which its ground state and low energy spectrum exhibit singular changes, even when the physical system remains in the same phase.

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## Are non-Fermi-liquids stable to pairing?

Thursday Feb 13, 2014
Speaker(s):

States of matter with a sharp Fermi-surface but no well-defined Landau
quasiparticles are expected to arise in a number of physical systems.
Examples include i) quantum critical points associated with the onset
of order in metals, ii) the spinon Fermi-surface (U(1) spin-liquid)
state of a Mott insulator and iii) the Halperin-Lee-Read composite
fermion charge liquid state of a half-filled Landau level. In this
talk, I will use renormalization group techniques to investigate

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## Gauging symmetry of 2D topological phases

Thursday Feb 13, 2014
Speaker(s):

I will discuss the mathematical framework for gauging a local unitary finite group symmetry of a 2D topological phase of matter.

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## Incoherent metals

Thursday Feb 13, 2014
Speaker(s):

I'll talk about some work in progress concerning the topic of metals which have no coherent quasiparticles. In particular, I'll compare and contrast the ubiquitous near horizon AdS2 region appearing in holographic models with a phase of matter called the spin incoherent luttinger liquid. By analyzing the structure of entanglement and correlations, we will find many similarities between these two states of matter.

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## Interacting electronic topological insulators in three dimensions

Thursday Feb 13, 2014
Speaker(s):

I will review recent progress in describing interacting electronic topological insulators/superconductors in three dimensions. The focus will be on Symmetry Protected Topological (SPT) phases of electronic systems with charge conservation and time reversal. I will argue that the well known Z2 classification of free fermion insulators with this important symmetry generalizes to a Z2^3 classification in the presence of interactions. I will describe the experimental fingerprints and other physical properties of these states.

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## Chiral spin liquid and emergent anyons in a Kagome lattice Mott insulator

Thursday Feb 13, 2014
Speaker(s):

Topological phases in frustrated quantum spin systems have fascinated researchers for decades. One of the earliest proposals for such a phase was the chiral spin liquid put forward by Kalmeyer and Laughlin in 1987 as the bosonic analogue of the fractional quantum Hall effect. Elusive for many years, recent times have finally seen a number of models that realize this phase. However, these models are somewhat artificial and unlikely to be found in realistic materials.

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## Quantum Tapestries

Wednesday Feb 12, 2014
Speaker(s):

Within each of nature's crystals is an exotic quantum world of electrons weaving to and fro. Each crystal has it's own unique tapestry, as varied as the crystals themselves. In some crystals, the electrons weave an orderly quilt. Within others, the electrons are seemingly entwined in an entangled web of quantum motion. In this talk, I will describe the ongoing efforts to disentangle even nature's most intricate quantum embroidery.

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