6 research outputs found
Coulomb Drag of Edge Excitations in the Chern-Simons Theory of the Fractional Quantum Hall Effect
Long range Coulomb interaction between the edges of a Hall bar changes the
nature of the gapless edge excitations. Instead of independent modes
propagating in opposite directions on each edge as expected for a short range
interaction one finds elementary excitations living simultaneously on both
edges, i.e. composed of correlated density waves propagating in the same
direction on opposite edges. We discuss the microscopic features of this
Coulomb drag of excitations in the fractional quantum Hall regime within the
framework of the bosonic Chern-Simons Landau-Ginzburg theory. The dispersion
law of these novel excitations is non linear and depends on the distance
between the edges as well as on the current that flows through the sample. The
latter dependence indicates a possibility of parametric excitation of these
modes. The bulk distributions of the density and currents of the edge
excitations differ significantly for short and long range interactions.Comment: 11 pages, REVTEX, 2 uuencoded postscript figure
Nernst effect as a probe of superconducting fluctuations in disordered thin films
In amorphous superconducting thin films of and ,
a finite Nernst coefficient can be detected in a wide range of temperature and
magnetic field. Due to the negligible contribution of normal quasi-particles,
superconducting fluctuations easily dominate the Nernst response in the entire
range of study. In the vicinity of the critical temperature and in the
zero-field limit, the magnitude of the signal is in quantitative agreement with
what is theoretically expected for the Gaussian fluctuations of the
superconducting order parameter. Even at higher temperatures and finite
magnetic field, the Nernst coefficient is set by the size of superconducting
fluctuations. The Nernst coefficient emerges as a direct probe of the ghost
critical field, the normal-state mirror of the upper critical field. Moreover,
upon leaving the normal state with fluctuating Cooper pairs, we show that the
temperature evolution of the Nernst coefficient is different whether the system
enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting
regime.Comment: Submitted to New. J. Phys. for a focus issue on "Superconductors with
Exotic Symmetries
Composite quasiparticle formation and the low-energy effective Hamiltonians of the one- and two-dimensional Hubbard Model
We investigate the effect of hole doping on the strong-coupling Hubbard model
at half-filling in spatial dimensions . We start with an
antiferromagnetic mean-field description of the insulating state, and show that
doping creates solitons in the antiferromagnetic background. In one dimension,
the soliton is topological, spinless, and decoupled from the background
antiferromagnetic fluctuations at low energies. In two dimensions and above,
the soliton is non-topological, has spin quantum number 1/2, and is strongly
coupled to the antiferromagnetic fluctuations. We derive the effective action
governing the quasiparticle motion, study the properties of a single carrier,
and comment on a possible description at finite concentration.Comment: REVTEX 3.0, 22 pages with 14 figures in the PostScript format
compressed using uufile. Submitted to Phys. Rev. B. The complete PostScript
file including figures can be obtained via ftp at
ftp://serval.berkeley.edu/hubbard.ps . It is also available via www at
http://roemer.fys.ku.dk/recent.ht
Localization and Superconductivity in Doped Semiconductors
Motivated by the discovery of superconductivity in boron-doped (B-doped)
diamond, we investigate the localization and superconductivity in heavily doped
semiconductors. The competition between Anderson localization and s-wave
superconductivity is investigated from the microscopic point of view. The
effect of microscopic inhomogeneity and the thermal fluctuation in
superconductivity are taken into account using the self-consistent 1-loop-order
theory with respect to superconducting fluctuation. The crossover from
superconductivity in the host band to that in the impurity band is described on
the basis of the disordered three-dimensional attractive Hubbard model for
binary alloys. We show that superconductor-insulator transition (SIT)
accompanies the crossover. We point out an enhancement of Cooper pairing in the
crossover regime. Further localization of the electron wave function gives rise
to incoherent Cooper pairs and the pseudogap above T_c. A global phase diagram
is drawn for host band superconductivity, impurity band superconductivity,
Anderson localization, Fermi liquid state, and pseudogap state. A theoretical
interpretation is proposed for superconductivity in the doped diamond, SiC, and
Si.Comment: Final version for publication. To appear in J. Phys. Soc. Jpn. (2009)
No.
The one dimensional Kondo lattice model at partial band filling
The Kondo lattice model introduced in 1977 describes a lattice of localized
magnetic moments interacting with a sea of conduction electrons. It is one of
the most important canonical models in the study of a class of rare earth
compounds, called heavy fermion systems, and as such has been studied
intensively by a wide variety of techniques for more than a quarter of a
century. This review focuses on the one dimensional case at partial band
filling, in which the number of conduction electrons is less than the number of
localized moments. The theoretical understanding, based on the bosonized
solution, of the conventional Kondo lattice model is presented in great detail.
This review divides naturally into two parts, the first relating to the
description of the formalism, and the second to its application. After an
all-inclusive description of the bosonization technique, the bosonized form of
the Kondo lattice hamiltonian is constructed in detail. Next the
double-exchange ordering, Kondo singlet formation, the RKKY interaction and
spin polaron formation are described comprehensively. An in-depth analysis of
the phase diagram follows, with special emphasis on the destruction of the
ferromagnetic phase by spin-flip disorder scattering, and of recent numerical
results. The results are shown to hold for both antiferromagnetic and
ferromagnetic Kondo lattice. The general exposition is pedagogic in tone.Comment: Review, 258 pages, 19 figure
Identification of Disulfides from the Biodegradation of Dibenzothiophene
Several investigations have identified benzothiophene-2,3-dione in the organic solvent extracts of acidified cultures degrading dibenzothiophene via the Kodama pathway. In solution at neutral pH, the 2,3-dione exists as 2-mercaptophenylglyoxylate, which cyclizes upon acidification and is extracted as the 2,3-dione. The fate of these compounds in microbial cultures has never been determined. This study investigated the abiotic reactions of 2-mercaptophenylglyoxylate incubated aerobically in mineral salts medium at neutral pH. Oxidation led to the formation of 2-oxo-2-(2-thiophenyl)ethanoic acid disulfide, formed from two molecules of 2-mercaptophenylglyoxylate. Two sequential abiotic, net losses of both a carbon and an oxygen atom produced two additional disulfides, 2-oxo-2-(2-thiophenyl)ethanoic acid 2-benzoic acid disulfide and 2,2′-dithiosalicylic acid. The methods developed to extract and detect these three disulfides were then used for the analysis of a culture of Pseudomonas sp. strain BT1d grown on dibenzothiophene as its sole carbon and energy source. All three of the disulfides were detected, indicating that 2-mercaptophenylglyoxylate is an important, short-lived intermediate in the breakdown of dibenzothiophene via the Kodama pathway. The disulfides eluded previous investigations because of (i) their high polarity, being dicarboxylic acids; (ii) the need to lower the pH of the aqueous medium to <1 to extract them into an organic solvent such as dichloromethane; (iii) their poor solubility in organic solvents, (iv) their removal from organic extracts of cultures during filtration through the commonly used drying agent anhydrous sodium sulfate; and (v) their high molecular masses (362, 334, and 306 Da) compared to that of dibenzothiophene (184 Da)