Condensed Matter

Utilizing the Baym-Kadanoff formalism with the polarization function calculated in the random phase approximation, the dynamics of the ν=0, ±1, ±2, ±3, ±4 quantum Hall states in bilayer graphene is analyzed. In particular, in the undoped graphene, corresponding to the ν =0 state, two phases with nonzero energy gap, the ferromagnetic and layer asymmetric ones, are found. The phase diagram in the plane (Δ0,B), where Δ0 is a top-bottom gates voltage imbalance, is described. It is shown that the energy gaps in these phases scale linearly, ΔE~10 B [T] K, with magnetic field. The ground states of the doped states, with ν=±1, ±2, ±3, ±4, are also described. The comparison of these results with recent experiments in bilayer graphene is presented.

The nature of the pairing mechanism in the recently discoverediron-pnictide family of superconductors remains an outstanding issue. To answer this question, it is instructive to know the symmetry of the superconducting energy gap. Low temperature thermal conductivity measurements provide a robust test of the presence or absence of low energy electronic quasiparticles that in turn can be used to characterise the symmetry of the gap function. In this talk, I will review what has been learnt so far from such measurements across several families of iron-pnictide superconductors, including our own recent results on the stoichiometric material LaFePO.

Direct visualization of the electronic structure within each crystalline unit cell of a solid is a new frontier in condensed matter physics (M.J. Lawler et al, Nature 466, 347 (2010)). In this talk, I will introduce the techniques of spectroscopic imaging scanning tunneling microscopy (SI-­‐STM) and then explain how our new application of this technique allows visualization of the intra-­‐unit-­‐cell electronic structure. We use this approach to study the pseudogap phase of cuprate high temperature superconductors. Recent experiments provide evidence that this phase may be associated with spontaneously broken electronic symmetries. By studying the Bragg peaks in Fourier transforms of SI-­-STM images, and in particular by resolving both the real and imaginary components of these Bragg amplitudes (as opposed to the Bragg intensities without phase information which are the observables in scattering experiments), we reveal the intra-­‐unit-­‐cell broken electronic symmetries of the cuprate pseudogap phase (J.P.Hinton et al, Science (2011)).

Recently resonant elastic soft x-ray scattering (RSXS) has emerged as a powerful new tool to study electronic ordering in materials like cuprates and manganites. The power of this technique is to combine xray scattering, which is sensitive to spatial order, with x-ray spectroscopy, which is sensitive to the valence, spin and orbital symmetry of specific atoms. This combination allows one to probe very directly and considerable detail a variety of exotic spin, charge, orbital or structural ordering phenomena. I will discuss the application of this technique to an investigation of charge density wave (CDW) order in the cuprate superconductor La1.475Nd0.4Sr0.125CuO4. Analysis of the photon energy dependence of the resonant scattering intensity provides unique insight into the microscopic details of the CDW ordering. New results from the REIXS beamline at the Canadian Light Source will be presented.

Many of the most interesting electronic behaviors arise in materials with strong electron-electron correlations. Many of these same materials are disordered either intrinsically or due to doping. The combination of disorder and interactions generally gives rise to a feature in the density of states at the Fermi level, with two of the most influential examples being the Altshuler-Aronov anomaly and the Efros-Shklovskii Coulomb gap. Experiments on strongly correlated materials and recent numerical results on the Anderson-Hubbard model, however, show behavior which is inconsistent with both of these frameworks. This talk will present some of the features of the zero bias anomaly in strongly correlated systems, both in the case of a purely on-site interaction and in the presence of nearest-neighbor interactions, and it will describe the physical origin of some of these features.

It is a generic feature of strongly correlated electronic systems that several mechanisms (broadly represented by interactions) compete with each other. This competition often leads to the phenomenon of frustration. In strongly correlated systems such as doped Mott insulators kinetic energy and Coulomb repulsion frustrate the tendency of doped holes to phase separate. The result is the onset of a set of novel phases, which we dubbed Electronic Liquid Crystal (ELC) states, with varying degrees of complexity. Much like classical liquid crystals, electronic liquid crystal phases break translation and rotational invariance to varying degrees. In this talk I will focus on the experimental evidence and theory of two these phases, stripes and nematics, in several different systems including two-dimensional electron gases in large magnetic fields, ruthenate oxides, heavy fermions, and cuprate and pnictide superconductors.

In this talk, I will first give an overview of holographic entanglement entropy. Next I will introduce recent our results on its applications to the quantum quenches and a holography for flat spacetime.

We will discuss the effect of non-equilibrium drive near a quantum critical point in itinerant electron systems. Non-equilibrium field theory is formulated in terms of the Keldysh functional integral. The renormalization group approach is used to study the universality class of the non-equilibrium phase transition in the steady state system. The role of the non-equilibrium drive in the quantum-classical crossover will be discussed using the example of the Hertz-Millis theory and the generalization thereof.

We propose a mechanism where high entanglement between very distant boundary spins is generated by suddenly connecting two long Kondo spin chains. We show that this procedure provides an efficient way to route entanglement between multiple distant sites useful for quantum computation and multi-party quantum communication. We observe that the key features of the entanglement dynamics of the composite spin chain are remarkably well described using a simple model of two singlets, each formed by two spins. The proposed entanglement routing mechanism is a footprint of the emergence of a Kondo cloud in a Kondo system and can be engineered and observed in varied physical settings.

In the last many years a number of metallic solids have been studied that defy understanding within the principles of conventional textbook solid state physics. The most famous are the cuprate high temperature superconductors though many other examples have been found. In this talk I will argue that the mysterious properties of many such materials arises from an imminent `death' of their Fermi surfaces. I will discuss some theoretical ideas on how to kill a Fermi surface, and their implications for experiments.

The standard method to study nonperturbative properties of quantum field theories is to Wick rotate the theory to Euclidean space and regulate it on a Euclidean Lattice. An alternative is "fuzzy field theory". This involves replacing the lattice field theory by a matrix model that approximates the field theory of interest, with the approximation becoming better as the matrix size is increased. The regulated field theory is one on a background noncommutative space. I will describe how this method works and present recent progress and surprises.

Traditional condensed matter physics is based on two theories: symmetry breaking theory for phases and phase transitions, and Fermi liquid theory for metals. Mean-field theory is a powerful method to describe symmetry breaking phases and phase transitions by assuming the ground state wavefunctions for many-body systems can be approximately described by direct product states. The Fermi liquid theory is another powerful method to study electron systems by assuming that the ground state wavefunctions for the electrons can be approximately described by Slater determinants. From the encoding point of view, both methods only use a polynomial amount of information to approximately encode many-body ground state wavefunctions which contain an exponentially large amount of information. Moreover, another nice property of both approaches is that all the physical quantities (energy, correlation functions, etc.) can be efficiently calculated (polynomially hard). In this talk, I'll introduce a new class of states: (Grassmann-number) tensor-net states. These states only need polynomial amount of information to approximately encode many-body ground states. Many classes of states, such as Slater determinant states, projective states, string-net states and their generalizations, etc., are subclasses of (Grassmann-number) tensor-net states. However, calculating the physical quantities for these state can be exponentially hard in general. To solve this difficulty, we develop the Tensor-Entanglement Renormalization Group (TERG) method to efficiently calculate the physical quantities. We demonstrate our algorithm by studying several interesting boson/fermion models, including t-J model on a honeycomb lattice.