Displaying 3505 - 3516 of 5007
Format results
-
-
Beyond the Search for Majorana
Yi-Zhuang You University of California, San Diego
-
Universal driven dynamics near phase transitions : Kibble-Zurek ramps with and without an order parameter
Anushya Chandran Perimeter Institute for Theoretical Physics
-
Fractional quantum Hall effect and tunable interactions
Zlatko Papic University of Leeds
-
-
Dyon condensation in topological Mott insulators
Gil Young Cho Pohang University of Science and Technology
-
-
Chern-Simons Contact Terms
Thomas Dumitrescu University of California, Los Angeles
-
Entanglement and the Fermi surface
Brian Swingle Brandeis University
-
Bootstrapping CFTs with the Extremal Functional Method
Miguel Paulos École Normale Supérieure - PSL
-
Exact Calculations in the 1D Continuum for DFT and Beyond
PIRSA:12110085 -
-
Amplitude mode of the d-density wave state and its relevance to high-Tc cuprates
Ipsita Mandal Shiv Nadar University
We study the spectrum of the amplitude mode, the analog of the Higgs mode in high energy physics, for the d-density wave (DDW) state proposed to describe the anomalous phenomenology of the pseudogap phase of the high Tc cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particle-hole singlet order parameter at the wave vector q = Q = (pi, pi), remarkably, we find that the amplitude mode spectrum can have peaks at both q = (0, 0) and q = Q = (pi , pi). In general, the spectra is non-universal, and, depending on the microscopic parameters, can have one or two peaks in the Brillouin zone, signifying confluence of two kinds of magnetic excitations. In light of the recent unexpected observations of multiple magnetic excitations in the pseudogap phase our theory sheds important light on how multiple inelastic neutron peaks at different wave vectors can arise even with an order parameter that condenses at Q = (pi, pi). [Reference: arXiv:1207.6834] -
Beyond the Search for Majorana
Yi-Zhuang You University of California, San Diego
The search for Majorana zero-modes in condensed matter system has attract increasing research interests recently. Looking for Majorna zero-mode is actually looking for topologically protected ground state degeneracy. The topological degeneracies on closed manifolds have been used to discover/define topological order in many-body systems, which contain excitations with fractional statistics. In this talk, I will present our recent work on new types of topological degeneracy induced by condensing anyons along a line in 2D topological ordered states. Such topological degeneracy can be viewed as carried by each end of the line-defect, which is a generalization of Majorana zero-modes. The ends of line-defects carry projective non-Abelian statistics even though they are produced by condensation of Abelian anyons, and braiding them allow us to perform fault tolerant quantum computations. -
Universal driven dynamics near phase transitions : Kibble-Zurek ramps with and without an order parameter
Anushya Chandran Perimeter Institute for Theoretical Physics
Near a critical point, the equilibrium relaxation time of a system diverges and any change of control parameters leads to non-equilibrium behavior. The Kibble-Zurek (KZ) problem is to determine
the evolution of the system when the change is slow. In this talk, I will introduce a non-equilibrium scaling limit in which these evolutions are universal and define a KZ universality classification with exponents and scaling functions. I will illustrate the physics accessible in this
scaling limit in simple classical and quantum model theories with symmetry-breaking transitions.
I will then turn to the KZ problem near quantum phase transitions without a local order parameter.
First, I will introduce the necessary background through the example of the Ising gauge theory/generalized toric code. Using duality and the scaling theory developed in the first part of the talk, I will then argue that the late time dynamics exhibits a slow coarsening of the string-net
that is condensed in the starting topologically ordered state. I will also discuss a time dependent amplification of the energy splitting between topologically degenerate states on closed manifolds and the dangerous irrelevance of gapped modes. Finally, I will extend these ideas to the non-abelian SU(2)_k ordered phases of the relevant Levin-Wen models.
-
Fractional quantum Hall effect and tunable interactions
Zlatko Papic University of Leeds
In this talk I will review some existing experimental methods, as well as a few recent theoretical proposals, to tune the interactions in a number of low-dimensional systems exhibiting the fractional quantum Hall effect (FQHE). The materials in question include GaAs wide quantum wells and multilayer graphene, where the tunability of the electron-electron interactions can be achieved via modifying the band structure, dielectric environment of the sample, by tilting the magnetic field or varying the mass tensor, and by mixing of electronic subbands and Landau levels. Because the interesting topological (and in particular, non-Abelian) states arise solely due to strong interactions, the ability to tune them is essential for ``designing" more robust FQHE states. Furthermore, I will argue that some of these mechanisms can also be used to probe the subtle aspects of FQHE physics, such as the breaking of particle-hole symmetry between the Moore-Read Pfaffian and anti-Pfaffian states, and the transition between FQHE fluids and broken-symmetry states due to the fluctuation of the intrinsic geometric degree of freedom. -
Fractional Chern Insulators
Titus Neupert ETH Zurich
Fractional Chern insulators (FCIs) are topologically ordered states of interacting fermions that share their universal properties with fractional quantum Hall states in Landau levels. FCIs have been found numerically in a variety of two-dimensional lattice models upon partially filling an almost dispersionless band with nontrivial topological character with repulsively interacting fermions. I will show how FCIs emerge in bands with Chern number C=1 and C=2 and in Z_2 topological insulators, where the latter are accompanied by a spontaneous breaking of time-reversal symmetry. Further, I will discuss the relevance of the noncommutative quantum geometry of the flat topological band to the stability of FCIs and how they can be distinguished from phases of spontaneously broken point group symmetry, such as charge density waves. -
Dyon condensation in topological Mott insulators
Gil Young Cho Pohang University of Science and Technology
We consider quantum phase transitions out of topological Mott insulators in which the ground state of the fractionalized excitations (fermionic spinons) is topologically non-trivial. The spinons in topological Mott insulators are coupled to an emergent compact U(1) gauge field with a so-called "axion" term. We study the confinement transitions from the topological Mott insulator to broken symmetry phases, which may occur via the condensation of dyons. Dyons carry both "electric" and "magnetic" charges, and arise naturally in this system because the monopoles of the emergent U(1) gauge theory acquires gauge charge due to the axion term. It is shown that the dyon condensate, in general, induces simultaneous current and bond orders. When the magnetic transition is driven by dyon condensation, we identify the bond order as valence bond solid order and the current order as scalar spin chirality order. Hence, the confined phase of the topological Mott insulator is an exotic phase where the scalar spin chirality and the valence bond order coexist and appear via a single transition. If time allows, I will also discuss our recent work on the proximate symmetry-broken phases of Z2 spin liquid on Kagome lattice. -
Wiring up Quantum Systems: Fun with Artificial Atoms and Microwave Photons
Steve Girvin Yale University
A revolution is underway in the construction of ‘artificial atoms’ out of superconducting electrical circuits. These macroscopic ‘atoms’ have quantized energy levels and can emit and absorb quanta of light (in this case microwave photons), just like ordinary atoms. Unlike ‘real’ atoms, the properties of these artificial atoms can be engineered to suit various particular applications, and they can be connected together by wires to form quantum ‘computer chips.’ This so-called ‘circuit QED’ architecture has given us the ability to do non-linear quantum optics in electrical circuits at the single photon level. It is now possible to entangle multiple qubits, count individual microwave photons, create large ‘Schrödinger cat’ photon states and perform quantum feedback. This talk will present an elementary introduction to the field. -
Chern-Simons Contact Terms
Thomas Dumitrescu University of California, Los Angeles
Chern-Simons contact terms constitute new observables in three-dimensional quantum field theory. In N=2 supersymmetric theories with an R-symmetry, they lead to a superconformal anomaly. This understanding clarifies several puzzles surrounding the S3 partition function of these theories. In particular, it leads to a proof of the F-maximization principle. Chern-Simons contact terms can be computed exactly using localization and lead to new tests of proposed dualities. -
Entanglement and the Fermi surface
Brian Swingle Brandeis University
In this talk I will describe my work characterizing quantum entanglement in systems with a Fermi surface. This class includes everything from Fermi liquids to exotic spin liquids in frustrated magnets and perhaps even holographic systems. I review my original scaling argument and then describe in detail a number of new precise results on entanglement in Fermi liquids. I will also discuss recent quantum Monte Carlo calculations of Renyi entropies and will argue that we now have a rather complete agreement between theory and numerics for Fermi liquid entanglement. I will also discuss universal crossovers between thermal and entanglement entropy and a class of solvable interacting models where we can prove the universality of the Widom formula for Fermi surface entanglement. If there is time, I will comment on several other topics including fluctuations of conserved quantities and connections to holography. -
Bootstrapping CFTs with the Extremal Functional Method
Miguel Paulos École Normale Supérieure - PSL
The existence of a positive linear functional acting on the space of (differences between) conformal blocks has been shown to rule out regions in the parameter space of conformal field theories (CFTs). We argue that at the boundary of the allowed region the extremal functional contains, in principle, enough information to determine the dimensions and OPE coefficients of an infinite number of operators appearing in the correlator under analysis. Based on this idea we develop the Extremal Functional Method (EFM), a numerical procedure for deriving the spectrum and OPE coefficients of CFTs lying on the boundary (of solution space). We test the EFM by using it to rederive the low lying spectrum and OPE coefficients of the 2d Ising model based solely on the dimension of a single scalar quasi-primary -- no Virasoro algebra required. Our work serves as a benchmark for applications to more interesting, less known CFTs in the near future, such as the 3d Ising model.
-
Exact Calculations in the 1D Continuum for DFT and Beyond
PIRSA:12110085Most applications of the density matrix renormalization group (DMRG) have been to lattice models with short range interactions. But recent developments in DMRG technology open the door to studying continuum systems with long-range interactions in one dimension (1d). One key motivation is simulating cold atom experiments, where it is possible to engineer Hamiltonians of precisely this type. We have been applying DMRG in the 1d continuum with another motivation: to investigate and improve density functional theory (DFT). DFT has exact mathematical foundations, but in practice one must use approximations. These approximations work incredibly well for weakly correlated systems yet fail when correlations are strong. Improving DFT directly for realistic 3d systems is hard because few systems can be solved exactly. By working in the 1d continuum instead, we can use the power of DMRG to study DFT. We can implement both the exact DFT formalism and standard DFT approximations. After showing how to overcome the challenges in performing these calculations, I will discuss some of the key questions we are investigating, for example, the ability of DFT to predict gaps of insulating systems. -