Anomalous Stability of nu=1 Bilayer Quantum Hall State

We have studied the fractional and integer quantum Hall (QH) effects in a high-mobility double-layer two-dimensional electron system. We have compared the "stability" of the QH state in balanced and unbalanced double quantum wells. The behavior of the n=1 QH state is found to be strikingly different from all others. It is anomalously stable, though all other states decay, as the electron density is made unbalanced between the two quantum wells. We interpret the peculiar features of the nu=1 state as the consequences of the interlayer quantum coherence developed spontaneously on the basis of the composite-boson picture.Comment: 5 pages, 6 figure

Spin-Pseudospin Coherence and CP3^{3} Skyrmions in Bilayer Quantum Hall Ferromagnets

We analyze bilayer quantum Hall ferromagnets, whose underlying symmetry group is SU(4). Spin-pseudospin coherence develops spontaneously when the total electron density is low enough. Quasiparticles are CP^3 skyrmions. One skyrmion induces charge modulations on both of the two layers. At the filling factor$\nu =2/m$ one elementary excitation consists of a pair of skyrmions and its charge is $2e/m$. Recent experimental data due to Sawada et al. [Phys. Rev. Lett. {\bf 80}, 4534 (1998)] support this conclusion.Comment: 4 pages including 2 figures (published version

Interlayer Coherence in the ν=1\nu=1 and ν=2\nu=2 Bilayer Quantum Hall States

We have measured the Hall-plateau width and the activation energy of the bilayer quantum Hall (BLQH) states at the Landau-level filling factor $\nu=1$ and 2 by tilting the sample and simultaneously changing the electron density in each quantum well. The phase transition between the commensurate and incommensurate states are confirmed at $\nu =1$ and discovered at $\nu =2$. In particular, three different $\nu =2$ BLQH states are identified; the compound state, the coherent commensurate state, and the coherent incommensurate state.Comment: 4 pages including 5 figure

The Role of Monopoles for Color Confinement

We study the role of the monopole for color confinement by using the monopole current system. For the self-energy of the monopole current less than ln$(2d-1)$, long and complicated monopole world-lines appear and the Wilson loop obeys the area law, and therefore the monopole current system almost reproduces essential features of confinement properties in the long-distance physics. In the short-distance physics, however, the monopole-current theory would become nonlocal due to the monopole size effect. This monopole size would provide a critical scale of QCD in terms of the dual Higgs mechanism.Comment: 3 pages LaTeX, 3 figures, uses espcrc2.sty, Talk presented at lattice97, Edinburgh, Scotland, July. 199

Skyrmion ↔\leftrightarrow pseudoSkyrmion Transition in Bilayer Quantum Hall States at ν=1\nu =1

Bilayer quantum Hall states at $\nu =1$ have been demonstrated to possess a distinguished state with interlayer phase coherence. The state has both excitations of Skyrmion with spin and pseudoSkyrmion with pseudospin. We show that Skyrmion $\leftrightarrow$ pseudoSkyrmion transition arises in the state by changing imbalance between electron densities in both layers; PseudoSkyrmion is realized at balance point, while Skyrmion is realized at large imbalance. The transition can be seen by observing the dependence of activation energies on magnetic field parallel to the layers.Comment: 12 pages, no figure

Monopole Current Dynamics and Color Confinement

Color confinement can be understood by the dual Higgs theory, where monopole condensation leads to the exclusion of the electric flux from the QCD vacuum. We study the role of the monopole for color confinement by investigating the monopole current system. When the self-energy of the monopole current is small enough, long and complicated monopole world-lines appear, which is a signal of monopole condensation. In the dense monopole system, the Wilson loop obeys the area-law, and the string tension and the monopole density have similar behavior as the function of the self-energy, which seems that monopole condensation leads to color confinement. On the long-distance physics, the monopole current system almost reproduces essential features of confinement properties in lattice QCD. In the short-distance physics, however, the monopole-current theory would become nonlocal and complicated due to the monopole size effect. This monopole size would provide a critical scale of QCD in terms of the dual Higgs mechanism.Comment: 6 pages LaTeX, 5 figures, uses espcrc1.sty, Talk presented at International Conference on Quark Lepton Nuclear Physics, Osaka, May. 199

Confinement and Topological Charge in the Abelian Gauge of QCD

We study the relation between instantons and monopoles in the abelian gauge. First, we investigate the monopole in the multi-instanton solution in the continuum Yang-Mills theory using the Polyakov gauge. At a large instanton density, the monopole trajectory becomes highly complicated, which can be regarded as a signal of monopole condensation. Second, we study instantons and monopoles in the SU(2) lattice gauge theory both in the maximally abelian (MA) gauge and in the Polyakov gauge. Using the $16^3 \times 4$ lattice, we find monopole dominance for instantons in the confinement phase even at finite temperatures. A linear-type correlation is found between the total monopole-loop length and the integral of the absolute value of the topological density (the total number of instantons and anti-instantons) in the MA gauge. We conjecture that instantons enhance the monopole-loop length and promote monopole condensation.Comment: 3 pages, LaTeX, Talk presented at LATTICE96(topology

Peculiar Width Dependence of the Electronic Property of Carbon Nanoribbons

Nanoribbons (nanographite ribbons) are carbon systems analogous to carbon nanotubes. We characterize a wide class of nanoribbons by a set of two integers $$, and then define the width $w$ in terms of $p$ and $q$. Electronic properties are explored for this class of nanoribbons. Zigzag (armchair) nanoribbons have similar electronic properties to armchair (zigzag) nanotubes. The band gap structure of nanoribbons exhibits a valley structure with stream-like sequences of metallic or almost metallic nanoribbons. These sequences correspond to equi-width curves indexed by $w$. We reveal a peculiar dependence of the electronic property of nanoribbons on the width $w$.Comment: 8 pages, 13 figure