End states, ladder compounds, and domain wall fermions

A magnetic field applied to a cross linked ladder compound can generate isolated electronic states bound to the ends of the chain. After exploring the interference phenomena responsible, I discuss a connection to the domain wall approach to chiral fermions in lattice gauge theory. The robust nature of the states under small variations of the bond strengths is tied to chiral symmetry and the multiplicative renormalization of fermion masses.Comment: 10 pages, 4 figures; final version for Phys. Rev. Let

A multisite microcanonical updating method

We have made a study of several update algorithms using the XY model. We find that sequential local overrelaxation is not ergodic at the scale of typical Monte Carlo simulation time. We have introduced a new multisite microcanonical update method, which yields results compatible with those of random overrelaxation and the microcanonical demon algorithm, which are very much slower, all being incompatible with the sequential overrelaxation results.Comment: 13 pages, 4 figure

Lattice QCD-2+1

We consider a 2+1-dimensional SU(N) lattice gauge theory in an axial gauge with the link field U in the 1-direction set to one. The term in the Hamiltonian containing the square of the electric field in the 1-direction is non-local. Despite this non-locality, we show that weak-coupling perturbation theory in this term gives a finite vacuum-energy density to second order, and suggest that this property holds to all orders. Heavy quarks are confined, the spectrum is gapped, and the space-like Wilson loop has area decay.Comment: Still Latex, 18 pages, no figures, with some further typographical errors corrected. Version to appear in Phys. Rev.

A derivation of Regge trajectories in large-N transverse lattice QCD

Large-N QCD is analysed in light-front coordinates with a transverse lattice at strong coupling. The general formalism can be looked up on as a d+n expansion with a stack of d-dimensional hyperplanes uniformly spaced in n transverse dimensions. It can arise by application of the renormalisation group transformations only in the transverse directions. At leading order in strong coupling, the gauge field dynamics reduces to the constraint that only colour singlet states can jump between the hyperplanes. With d=2, n=2 and large-N, the leading order strong coupling results are simple renormalisations of those for the 't Hooft model. The meson spectrum lies on a set of parallel trajectories labeled by spin. This is the first derivation of the widely anticipated Regge trajectories in a regulated systematic expansion in QCD.Comment: Lattice 2000 (spectrum), 5 pages, to appear in the proceeding

Microcanonical cluster algorithms

I propose a numerical simulation algorithm for statistical systems which combines a microcanonical transfer of energy with global changes in clusters of spins. The advantages of the cluster approach near a critical point augment the speed increases associated with multi-spin coding in the microcanonical approach. The method also provides a limited ability to tune the average cluster size.Comment: 10 page

Abelian Links, Monopoles and Glueballs in SU(2) Lattice Gauge Theory

We investigate the masses of 0+ and 2+ glueballs in SU(2) lattice gauge theory using abelian projection to the maximum abelian gauge. We calculate glueball masses using both abelian links and monopole operators. Both methods reproduce the known full SU(2) results quantitatively. Positivity problems present in the abelian projection are discussed. We study the dependence of the glueball masses on magnetic current loop size, and find that the 0+ state requires a much greater range of sizes than does the 2+ state.Comment: 18 pages, latex, 4 postscript figure

Spatial search and the Dirac equation

We consider the problem of searching a d-dimensional lattice of N sites for a single marked location. We present a Hamiltonian that solves this problem in time of order sqrt(N) for d>2 and of order sqrt(N) log(N) in the critical dimension d=2. This improves upon the performance of our previous quantum walk search algorithm (which has a critical dimension of d=4), and matches the performance of a corresponding discrete-time quantum walk algorithm. The improvement uses a lattice version of the Dirac Hamiltonian, and thus requires the introduction of spin (or coin) degrees of freedom.Comment: 5 pages, 1 figur

Symmetry protected Z2-quantization and quaternionic Berry connection with Kramers degeneracy

As for a generic parameter dependent hamiltonian with the time reversal (TR) invariance, a non Abelian Berry connection with the Kramers (KR) degeneracy are introduced by using a quaternionic Berry connection. This quaternionic structure naturally extends to the many body system with the KR degeneracy. Its topological structure is explicitly discussed in comparison with the one without the KR degeneracy. Natural dimensions to have non trivial topological structures are discussed by presenting explicit gauge fixing. Minimum models to have accidental degeneracies are given with/without the KR degeneracy, which describe the monopoles of Dirac and Yang. We have shown that the Yang monopole is literally a quaternionic Dirac monopole. The generic Berry phases with/without the KR degeneracy are introduced by the complex/quaternionic Berry connections. As for the symmetry protected $\mathbb{Z}_2$ quantization of these general Berry phases, a sufficient condition of the $\mathbb{Z}_2$-quantization is given as the inversion/reflection equivalence. Topological charges of the SO(3) and SO(5) nonlinear $\sigma$-models are discussed in their relation to the Chern numbers of the $CP^1$ and $HP^1$ models as well.Comment: Submitted for New J. Physics, Special issue on Topological Insulators. 18 pages, 2 figure

Topological Modes in Dual Lattice Models

Lattice gauge theory with gauge group $Z_{P}$ is reconsidered in four dimensions on a simplicial complex $K$. One finds that the dual theory, formulated on the dual block complex $\hat{K}$, contains topological modes which are in correspondence with the cohomology group $H^{2}(\hat{K},Z_{P})$, in addition to the usual dynamical link variables. This is a general phenomenon in all models with single plaquette based actions; the action of the dual theory becomes twisted with a field representing the above cohomology class. A similar observation is made about the dual version of the three dimensional Ising model. The importance of distinct topological sectors is confirmed numerically in the two dimensional Ising model where they are parameterized by $H^{1}(\hat{K},Z_{2})$.Comment: 10 pages, DIAS 94-3

Confinement by Monopoles in the Positive Plaquette Model of SU(2) Lattice Gauge Theory

Confinement via 't Hooft-Mandelstam monopoles is studied for the positive plaquette model in SU(2) lattice gauge theory. Positive plaquette model configurations are projected into the maximum abelian gauge and the magnetic current extracted. The resulting magnetic current is used to compute monopole contributions to Wilson loops and extract a monopole contribution to the string tension. As was previously found for the Wilson action, the monopole contribution to the string tension agrees with the string tension calculated directly from the SU(2) links. The fact that the positive plaquette model suppresses Z2 monopoles and vortices is discussed.Comment: 8 pages, one Postscript figure, Latex, uses psfig files: posplaq.tex,posplaq.aux,pp_1_3.ps packaged with uufile