Toward a theory of the integer quantum Hall transition: continuum limit of the Chalker-Coddington model

An N-channel generalization of the network model of Chalker and Coddington is considered. The model for N = 1 is known to describe the critical behavior at the plateau transition in systems exhibiting the integer quantum Hall effect. Using a recently discovered equality of integrals, the network model is transformed into a lattice field theory defined over Efetov's sigma model space with unitary symmetry. The transformation is exact for all N, no saddle-point approximation is made, and no massive modes have to be eliminated. The naive continuum limit of the lattice theory is shown to be a supersymmetric version of Pruisken's nonlinear sigma model with couplings sigma_xx = sigma_xy = N/2 at the symmetric point. It follows that the model for N = 2, which describes a spin degenerate Landau level and the random flux problem, is noncritical. On the basis of symmetry considerations and inspection of the Hamiltonian limit, a modified network model is formulated, which still lies in the quantum Hall universality class. The prospects for deformation to a Yang-Baxter integrable vertex model are briefly discussed.Comment: 25 pages, REVTEX, calculation of sigma_xx correcte

An overview of the focus of the International Gap Junction Conference 2017 and Future Perspectives

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Quantum phase transition in capacitively coupled double quantum dots

We investigate two equivalent, capacitively coupled semiconducting quantum dots, each coupled to its own lead, in a regime where there are two electrons on the double dot. With increasing interdot coupling a rich range of behavior is uncovered: first a crossover from spin- to charge-Kondo physics, via an intermediate SU(4) state with entangled spin and charge degrees of freedom; followed by a quantum phase transition of Kosterlitz-Thouless type to a non-Fermi liquid `charge-ordered' phase with finite residual entropy and anomalous transport properties. Physical arguments and numerical renormalization group methods are employed to obtain a detailed understanding of the problem.Comment: 4 pages, 3 figure

Color identification testing device

Testing device, which determines ability of a technician to identify color-coded electric wires, is superior to standard color blindness tests. It tests speed of wire selection, detects partial color blindness, allows rapid testing, and may be administered by a color blind person

Constraints to the Masses of Brown Dwarf Candidates from the Lithium Test

We present intermediate dispersion (0.7-2.2 \AA ~pix$^{-1}$) optical spectroscopic observations aimed at applying the ``Lithium Test'' to a sample of ten brown dwarf candidates located in the general field, two in young open clusters, and two in close binaries. We find evidence for strong Li depletion in all of them, and thus infer lower mass limits of 0.065~M$_\odot$, depending only slightly ($\pm$0.005~M$_\odot$) on the interior models. None of the field brown dwarf candidates in our sample appears to be a very young (age $<$~10$^8$~yr) substellar object. For one of the faintest proper motion Pleiades members known (V=20.7) the Li test implies a mass greater than $\sim$0.08~M$_\odot$, and therefore it is not a brown dwarf. From our spectra we estimate spectral types for some objects and present measurements of Halpha emission strengths and radial velocities. Finally, we compare the positions in the H-R diagram of our sample of brown dwarf candidates with the theoretical region where Li is expected to be preserved (Substellar Lithium Region). We find that certain combinations of temperature calibrations and evolutionary tracks are consistent with the constraints imposed by the observed Li depletion in brown dwarf candidates, while others are not.Comment: 20 pp.; 4 figs, available under request; plain LaTeX, ApJ in press, OACatania-94-00