1,114 research outputs found
The formation of pitted features on the international simple glass during dynamic experiments at alkaline pH
The forward rate of dissolution of the International Simple Glass (ISG) was determined under alkaline conditions at 40 °C using the Single Pass Flow Through (SPFT) method. Forward rates were consistent with those obtained in the literature for this glass composition. The formation of altered gel layers and surface pits was observed on the surface of glass particles, especially at the very highest pH values, despite the application of high flow rates to prevent the build-up of solubility limiting phases. These features could be attributed to preferential localized dissolution at sites with a higher alkali concentration or from a separate, less durable, vitreous phase. These results may indicate that surface pit and altered gel formation occurs under the forward rate of dissolution as imposed by the SPFT method, particularly for simplified borosilicate glass materials
Contacts and Edge State Equilibration in the Fractional Quantum Hall Effect
We develop a simple kinetic equation description of edge state dynamics in
the fractional quantum Hall effect (FQHE), which allows us to examine in detail
equilibration processes between multiple edge modes. As in the integer quantum
Hall effect (IQHE), inter-mode equilibration is a prerequisite for quantization
of the Hall conductance. Two sources for such equilibration are considered:
Edge impurity scattering and equilibration by the electrical contacts. Several
specific models for electrical contacts are introduced and analyzed. For FQHE
states in which edge channels move in both directions, such as , these
models for the electrical contacts {\it do not} equilibrate the edge modes,
resulting in a non-quantized Hall conductance, even in a four-terminal
measurement. Inclusion of edge-impurity scattering, which {\it directly}
transfers charge between channels, is shown to restore the four-terminal
quantized conductance. For specific filling factors, notably and
, the equilibration length due to impurity scattering diverges in the
zero temperature limit, which should lead to a breakdown of quantization for
small samples at low temperatures. Experimental implications are discussed.Comment: 14 pages REVTeX, 6 postscript figures (uuencoded and compressed
Quantized Thermal Transport in the Fractional Quantum Hall Effect
We analyze thermal transport in the fractional quantum Hall effect (FQHE),
employing a Luttinger liquid model of edge states. Impurity mediated
inter-channel scattering events are incorporated in a hydrodynamic description
of heat and charge transport. The thermal Hall conductance, , is shown to
provide a new and universal characterization of the FQHE state, and reveals
non-trivial information about the edge structure. The Lorenz ratio between
thermal and electrical Hall conductances {\it violates} the free-electron
Wiedemann-Franz law, and for some fractional states is predicted to be {\it
negative}. We argue that thermal transport may provide a unique way to detect
the presence of the elusive upstream propagating modes, predicted for fractions
such as and .Comment: 6 pages REVTeX, 2 postscript figures (uuencoded and compressed
Soluble `Supersymmetric' Quantum XY Model
We present a `supersymmetric' modification of the -dimensional quantum
rotor model whose ground state is exactly soluble. The model undergoes a
vortex-binding transition from insulator to metal as the rotor coupling is
varied. The Hamiltonian contains three-site terms which are relevant: they
change the universality class of the transition from that of the ()--- to
the -dimensional classical XY model. The metallic phase has algebraic ODLRO
but the superfluid density is identically zero. Variational wave functions for
single-particle and collective excitations are presented.Comment: 12 pages, REVTEX 3.0, IUCM93-00
Resonant Tunneling Between Quantum Hall Edge States
Resonant tunneling between fractional quantum Hall edge states is studied in
the Luttinger liquid picture. For the Laughlin parent states, the resonance
line shape is a universal function whose width scales to zero at zero
temperature. Extensive quantum Monte Carlo simulations are presented for which confirm this picture and provide a parameter-free prediction for the
line shape.Comment: 14 pages , revtex , IUCM93-00
Transport of Surface States in the Bulk Quantum Hall Effect
The two-dimensional surface of a coupled multilayer integer quantum Hall
system consists of an anisotropic chiral metal. This unusual metal is
characterized by ballistic motion transverse and diffusive motion parallel
(\hat{z}) to the magnetic field. Employing a network model, we calculate
numerically the phase coherent two-terminal z-axis conductance and its
mesoscopic fluctuations. Quasi-1d localization effects are evident in the limit
of many layers. We consider the role of inelastic de-phasing effects in
modifying the transport of the chiral surface sheath, discussing their
importance in the recent experiments of Druist et al.Comment: 9 pages LaTex, 9 postscript figures included using eps
Synthesis of simulant ‘lava-like’ fuel containing materials (LFCM) from the Chernobyl reactor Unit 4 meltdown
A preliminary investigation of the synthesis and characterization of simulant ‘lava-like’ fuel containing materials (LFCM), as low activity analogues of LFCM produced by the melt down of Chernobyl Unit 4. Simulant materials were synthesized by melting batched reagents in a tube furnace at 1500 °C, under reducing atmosphere with controlled cooling to room temperature, to simulate conditions of lava formation. Characterization using XRD and SEM-EDX identified several crystalline phases including ZrO2, UOx and solid solutions with spherical metal particles encapsulated by a glassy matrix. The UOX and ZrO2 phase morphology was very diverse comprising of fused crystals to dendritic crystallites from the crystallization of uranium initially dissolved in the glass phase. This project aims to develop simulant LFCM to assess the durability of Chernobyl lavas and to determine the rate of dissolution, behavior and evolution of these materials under shelter conditions
Weyl group multiple Dirichlet series constructed from quadratic characters
We construct multiple Dirichlet series in several complex variables whose
coefficients involve quadratic residue symbols. The series are shown to have an
analytic continuation and satisfy a certain group of functional equations.
These are the first examples of an infinite collection of unstable Weyl group
multiple Dirichlet series in greater than two variables.Comment: incorporated referee's comment
SUE: A Special Purpose Computer for Spin Glass Models
The use of last generation Programmable Electronic Components makes possible
the construction of very powerful and competitive special purpose computers. We
have designed, constructed and tested a three-dimensional Spin Glass model
dedicated machine, which consists of 12 identical boards. Each single board can
simulate 8 different systems, updating all the systems at every clock cycle.
The update speed of the whole machine is 217ps/spin with 48 MHz clock
frequency. A device devoted to fast random number generation has been developed
and included in every board. The on-board reprogrammability permits us to
change easily the lattice size, or even the update algorithm or the action. We
present here a detailed description of the machine and the first runs using the
Heat Bath algorithm.Comment: Submitted to Computer Physics Communications, 19 pages, 5 figures,
references adde
Phase diagram of d=4 Ising Model with two couplings
We study the phase diagram of the four dimensional Ising model with first and
second neighbour couplings, specially in the antiferromagnetic region, by using
Mean Field and Monte Carlo methods. From the later, all the transition lines
seem to be first order except that between ferromagnetic and disordered phases
in a region including the first-neighbour Ising transition point.Comment: Latex file and 4 figures (epsfig required). It replaces the preprint
entitled "Non-classical exponents in the d=4 Ising Model with two couplings".
New analysis with more statistical data is performed. Final version to appear
in Phys. Lett.
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