349 research outputs found
Anyons and Chiral Solitons on a Line
We show that excitations in a recently proposed gauge theory for anyons on a
line in fact do not obey anomalous statistics. On the other hand, the theory
supports novel chiral solitons. Also we construct a field-theoretic description
of lineal anyons, but gauge fields play no role.Comment: 8 pages, revtex, no figure
Phonon Mode Spectroscopy, Electron-Phonon Coupling and the Metal-Insulator Transition in Quasi-One-Dimensional M2Mo6Se6
We present electronic structure calculations, electrical resistivity data and
the first specific heat measurements in the normal and superconducting states
of quasi-one-dimensional M2Mo6Se6 (M = Tl, In, Rb). Rb2Mo6Se6 undergoes a
metal-insulator transition at ~170K: electronic structure calculations indicate
that this is likely to be driven by the formation of a dynamical charge density
wave. However, Tl2Mo6Se6 and In2Mo6Se6 remain metallic down to low temperature,
with superconducting transitions at Tc = 4.2K and 2.85K respectively. The
absence of any metal-insulator transition in these materials is due to a larger
in-plane bandwidth, leading to increased inter-chain hopping which suppresses
the density wave instability. Electronic heat capacity data for the
superconducting compounds reveal an exceptionally low density of states DEF =
0.055 states eV^-1 atom^-1, with BCS fits showing 2Delta/kBTc >= 5 for
Tl2Mo6Se6 and 3.5 for In2Mo6Se6. Modelling the lattice specific heat with a set
of Einstein modes, we obtain the approximate phonon density of states F(w).
Deconvolving the resistivity for the two superconductors then yields their
electron-phonon transport coupling function a^2F(w). In Tl2Mo6Se6 and
In2Mo6Se6, F(w) is dominated by an optical "guest ion" mode at ~5meV and a set
of acoustic modes from ~10-30meV. Rb2Mo6Se6 exhibits a similar spectrum;
however, the optical phonon has a lower intensity and is shifted to ~8meV.
Electrons in Tl2Mo6Se6 couple strongly to both sets of modes, whereas In2Mo6Se6
only displays significant coupling in the 10-18meV range. Although pairing is
clearly not mediated by the guest ion phonon, we believe it has a beneficial
effect on superconductivity in Tl2Mo6Se6, given its extraordinarily large
coupling strength and higher Tc compared to In2Mo6Se6.Comment: 16 pages, 13 figure
Theoretical search for Chevrel phase based thermoelectric materials
We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)MoSe, particularly when Li-rich, and recommend this
system for experimental investigation.Comment: 4 two-column pages, 4 embedded ps figure
Nondegenerate Fermions in the Background of the Sphaleron Barrier
We consider level crossing in the background of the sphaleron barrier for
nondegenerate fermions. The mass splitting within the fermion doublets allows
only for an axially symmetric ansatz for the fermion fields. In the background
of the sphaleron we solve the partial differential equations for the fermion
functions. We find little angular dependence for our choice of ansatz. We
therefore propose a good approximate ansatz with radial functions only. We
generalize this approximate ansatz with radial functions only to fermions in
the background of the sphaleron barrier and argue, that it is a good
approximation there, too.Comment: LATEX, 20 pages, 11 figure
Chiral solitons from dimensional reduction of Chern-Simons gauged non-linear Schr\"odinger model of FQHE: classical and quantum aspects
The soliton structure of a gauge theory recently proposed to describe chiral
excitations in the Fractional Quantum Hall Effect is investigated. A new type
of non-linear derivative Schr\"odinger equation emerges as an effective
description of the system that supports novel chiral solitons. We discuss the
classical properties of solutions with vanishing and non-vanishing boundary
conditions (dark solitons) and we explain their relation to integrable systems.
The quantum analysis is also addressed in the framework of a semiclassical
approximation improved by Renormalization Group arguments.Comment: 39 page, RevTeX, 6 figure
Wear of human teeth: a tribological perspective
The four main types of wear in teeth are attrition (enamel-on-enamel contact), abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking in enamel and subsequent material loss), and erosion (chemical decomposition of the tooth). They occur as a result of a number of mechanisms including thegosis (sliding of teeth into their lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, tooth flexure, and chemical effects. In this paper the current understanding of wear of enamel and dentine in teeth is reviewed in terms of these mechanisms and the major influencing factors are examined. In vitro tooth wear simulation and in vivo wear measurement and ranking are also discussed
Level Crossing Along Sphaleron Barriers
In the electroweak sector of the standard model topologically inequivalent
vacua are separated by finite energy barriers, whose height is given by the
sphale\-ron. For large values of the Higgs mass there exist several sphaleron
solutions and the barriers are no longer symmetric. We construct paths of
classical configurations from one vacuum to a neighbouring one and solve the
fermion equations in the background field configurations along such paths,
choosing the fermions of a doublet degenerate in mass. As in the case of light
Higgs masses we observe the level crossing phenomenon also for large Higgs
masses.Comment: 17 pages, latex, 10 figures in uuencoded postscript files. THU-94/0
Sphalerons, spectral flow, and anomalies
The topology of configuration space may be responsible in part for the
existence of sphalerons. Here, sphalerons are defined to be static but unstable
finite-energy solutions of the classical field equations. Another manifestation
of the nontrivial topology of configuration space is the phenomenon of spectral
flow for the eigenvalues of the Dirac Hamiltonian. The spectral flow, in turn,
is related to the possible existence of anomalies. In this review, the
interconnection of these topics is illustrated for three particular sphalerons
of SU(2) Yang-Mills-Higgs theory.Comment: 35 pages with revtex4; invited paper for the August special issue of
JMP on "Integrability, topological solitons and beyond
The Cyclostratigraphy Intercomparison Project (CIP): consistency, merits and pitfalls
Cyclostratigraphy is an important tool for understanding astronomical climate forcing and reading geological time in sedimentary sequences, provided that an imprint of insolation variations caused by Earthâs orbital eccentricity, obliquity and/or precession is preserved (Milankovitch forcing). Numerous stratigraphic and paleoclimate studies have applied cyclostratigraphy, but the robustness of the methodology and its dependence on the investigator have not been systematically evaluated. We developed the Cyclostratigraphy Intercomparison Project (CIP) to assess the robustness of cyclostratigraphic methods using an experimental design of three artificial cyclostratigraphic case studies with known input parameters. Each case study is designed to address specific challenges that are relevant to cyclostratigraphy. Case 1 represents an offshore research vessel environment, as only a drill-core photo and the approximate position of a late Miocene stage boundary are available for analysis. In Case 2, the Pleistocene proxy record displays clear nonlinear cyclical patterns and the interpretation is complicated by the presence of a hiatus. Case 3 represents a Late Devonian proxy record with a low signal-to-noise ratio with no specific theoretical astronomical solution available for this age. Each case was analyzed by a test group of 17-20 participants, with varying experience levels, methodological preferences and dedicated analysis time. During the CIP 2018 meeting in Brussels, Belgium, the ensuing analyses and discussion demonstrated that most participants did not arrive at a perfect solution, which may be partly explained by the limited amount of time spent on the exercises (âŒ4.5 hours per case). However, in all three cases, the median solution of all submitted analyses accurately approached the correct result and several participants obtained the exact correct answers. Interestingly, systematically better performances were obtained for cases that represented the data type and stratigraphic age that were closest to the individual participantsâ experience. This experiment demonstrates that cyclostratigraphy is a powerful tool for deciphering time in sedimentary successions and, importantly, that it is a trainable skill. Finally, we emphasize the importance of an integrated stratigraphic approach and provide flexible guidelines on what good practices in cyclostratigraphy should include. Our case studies provide valuable insight into current common practices in cyclostratigraphy, their potential merits and pitfalls. Our work does not provide a quantitative measure of reliability and uncertainty of cyclostratigraphy, but rather constitutes a starting point for further discussions on how to move the maturing field of cyclostratigraphy forward
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