Quantum Hall to Insulator Transition in the Bilayer Quantum Hall Ferromagnet

We describe a new phase transition of the bilayer quantum Hall ferromagnet at filling fraction $\nu = 1$. In the presence of static disorder (modeled by a periodic potential), bosonic $S=1/2$ spinons can undergo a superfluid-insulator transition while preserving the ferromagnetic order. The Mott insulating phase has an emergent U(1) photon, and the transition is between Higgs and Coulomb phases of this photon. Physical consequences for charge and counterflow conductivity, and for interlayer tunneling conductance in the presence of quenched disorder are discussed.Comment: 4 pages, no figure

Effective theory of Fermi pockets in fluctuating antiferromagnets

We describe fluctuating two-dimensional metallic antiferromagnets by transforming to a rotating reference frame in which the electron spin polarization is measured by its projections along the local antiferromagnetic order. This leads to a gauge-theoretic description of an `algebraic charge liquid' involving spinless fermions and a spin S=1/2 complex scalar. We propose a phenomenological effective lattice Hamiltonian which describes the binding of these particles into gauge-neutral, electron-like excitations, and describe its implications for the electron spectral function across the entire Brillouin zone. We discuss connections of our results to photoemission experiments in the pseudogap regime of the cuprate superconductors.Comment: 28 pages, 8 figure

Liquid ground state, gap and excited states of a strongly correlated spin chain

We present an exact solution of an experimentally realizable and strongly interacting one-dimensional spin system which is a limiting case of a quantum Ising model with long range interaction in a transverse and longitudinal field. Pronounced quantum fluctuations lead to a strongly correlated liquid ground state. For open boundary conditions the ground state manifold consists of four degenerate sectors whose quantum numbers are determined by the orientation of the edge spins. Explicit expressions for the entanglement properties, the excitation gap as well as the exact wave functions for a couple of excited states are analytically derived and discussed

Quantum phase transitions of the diluted O(3) rotor model

We study the phase diagram and the quantum phase transitions of a site-diluted two-dimensional O(3) quantum rotor model by means of large-scale Monte-Carlo simulations. This system has two quantum phase transitions, a generic one for small dilutions, and a percolation transition across the lattice percolation threshold. We determine the critical behavior for both transitions and for the multicritical point that separates them. In contrast to the exotic scaling scenarios found in other random quantum systems, all these transitions are characterized by finite-disorder fixed points with power-law scaling. We relate our findings to a recent classification of phase transitions with quenched disorder according to the rare region dimensionality, and we discuss experiments in disordered quantum magnets.Comment: 11 pages, 14 eps figures, final version as publishe

Excited state spectra at the superfluid-insulator transition out of paired condensates

We describe gapped single-particle and collective excitations across a superfluid to insulator quantum phase transition of particles (bosons or fermions) in a periodic potential, with an even number of particles per unit cell. We demonstrate that the dynamics is controlled by a quantum impurity problem of a localized particle interacting with the bulk critical modes. Critical exponents are determined by a renormalization group analysis. We discuss applications to spin oscillations of ultracold atoms in optical lattices, and to the electronic phases in the cuprate and related compounds.Comment: 4 pages, 1 figure; fixed referenc

Exotic vs. conventional scaling and universality in a disordered bilayer quantum Heisenberg antiferromagnet

We present large-scale Monte-Carlo simulations of a two-dimensional (2d) bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In contrast to the exotic scaling scenarios found in many other random quantum systems, the quantum phase transition in this system is characterized by a finite-disorder fixed point with power-law scaling. After accounting for strong corrections to scaling, characterized by a leading irrelevant exponent of \omega = 0.48, we find universal, i.e., disorder-independent, critical exponents z=1.310(6) and \nu=1.16(3). We discuss the consequences of these findings and suggest new experiments.Comment: 4 pages, 5eps figures included, final version as publishe

Is the incidence of dementia declining?

Action on preventative health could lower the risk of dementia for future generations, argues this report. Executive summary The world-wide projections of the prevalence of dementia in the coming decades have been a source of great concern to health systems and societies around the world. The World Alzheimer Report 2010 estimated that there were 36 million people with dementia in 2010, with an expected doubling every 20 years to nearly 115 million in 2050. These sobering figures are based on assumptions that the age-adjusted prevalence of dementia would remain constant and the population would continue to age at the current rate. The assumption that the incidence of dementia will remain stable is now being put into question. There is emerging evidence to suggest that the incidence of dementia in older individuals may be declining. It appears that this change may be recent and has possibly occurred only in the last one to two decades. It may also be restricted so far to high income countries, although data from low and middle income countries are lacking. The reasons for this change are not understood, but education, more stimulating environments and better control of vascular risk factors may have contributed. The data are still preliminary and more studies are needed to establish the extent of this change and understand its causes. It should be noted that the decline is not large enough to offset the increase in prevalence of dementia due to the ageing of the population and therefore investment and efforts to develop better treatments and care for people with dementia need to continue. The fact that dementia rates are malleable is an encouraging finding but the reduction cannot be taken for granted as gains in population health can easily be lost if societies do not remain vigilant and continually proactive. These preliminary findings provide a strong argument for large scale Government investment in dementia-prevention strategies, which should start from early life

What can gauge-gravity duality teach us about condensed matter physics?

I discuss the impact of gauge-gravity duality on our understanding of two classes of systems: conformal quantum matter and compressible quantum matter. The first conformal class includes systems, such as the boson Hubbard model in two spatial dimensions, which display quantum critical points described by conformal field theories. Questions associated with non-zero temperature dynamics and transport are difficult to answer using conventional field theoretic methods. I argue that many of these can be addressed systematically using gauge-gravity duality, and discuss the prospects for reliable computation of low frequency correlations. Compressible quantum matter is characterized by the smooth dependence of the charge density, associated with a global U(1) symmetry, upon a chemical potential. Familiar examples are solids, superfluids, and Fermi liquids, but there are more exotic possibilities involving deconfined phases of gauge fields in the presence of Fermi surfaces. I survey the compressible systems studied using gauge-gravity duality, and discuss their relationship to the condensed matter classification of such states. The gravity methods offer hope of a deeper understanding of exotic and strongly-coupled compressible quantum states.Comment: 34 pages, 11 figures + 16 pages of Supplementary Material with 4 figures; to appear in Annual Reviews of Condensed Matter Physics; (v2) add a figure, and clarifications; (v3) final version; (v4) small correction

Valence bond solid order near impurities in two-dimensional quantum antiferromagnets

Recent scanning tunnelling microscopy (STM) experiments on underdoped cuprates have displayed modulations in the local electronic density of states which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a paradigm of the pinning of such bond-centered ordering in strongly correlated systems, we present the theory of valence bond solid (VBS) correlations near a single impurity in a square lattice antiferromagnet. The antiferromagnet is assumed to be in the vicinity of a quantum transition from a magnetically ordered Neel state to a spin-gap state with long-range VBS order. We identify two distinct classes of impurities: i) local modulation in the exchange constants, and ii) a missing or additional spin, for which the impurity perturbation is represented by an uncompensated Berry phase. The `boundary' critical theory for these classes is developed: in the second class we find a `VBS pinwheel' around the impurity, accompanied by a suppression in the VBS susceptibility. Implications for numerical studies of quantum antiferromagnets and for STM experiments on the cuprates are noted.Comment: 41 pages, 6 figures; (v2) Minor changes in terminology, added reference

Fluctuating spin density waves in metals

Recent work has used a U(1) gauge theory to describe the physics of Fermi pockets in the presence of fluctuating spin density wave order. We generalize this theory to an arbitrary band structure and ordering wavevector. The transition to the large Fermi surface state, without pockets induced by local spin density wave order, is described by embedding the U(1) gauge theory in a SU(2) gauge theory. The phase diagram of the SU(2) gauge theory shows that the onset of spin density wave order in the Fermi liquid occurs either directly, in the framework discussed by Hertz, or via intermediate non-Fermi liquid phases with Fermi surfaces of fractionalized excitations. We discuss application of our results to the phase diagram of the cuprates.Comment: 15 pages, 2 figures; (v2) Improved figure