2,396 research outputs found
Diffusion of Dirac fermions across a topological merging transition in two dimensions
A continuous deformation of a Hamiltonian possessing at low energy two Dirac
points of opposite chiralities can lead to a gap opening by merging of the two
Dirac points. In two dimensions, the critical Hamiltonian possesses a
semi-Dirac spectrum: linear in one direction but quadratic in the other. We
study the transport properties across such a transition, from a Dirac
semi-metal through a semi-Dirac phase towards a gapped phase. Using both a
Boltzmann approach and a diagrammatic Kubo approach, we describe the
conductivity tensor within the diffusive regime. In particular, we show that
both the anisotropy of the Fermi surface and the Dirac nature of the
eigenstates combine to give rise to anisotropic transport times, manifesting
themselves through an unusual matrix self-energy.Comment: 15 pages, 14 figure
Dephasing by a nonstationary classical intermittent noise
We consider a new phenomenological model for a classical
intermittent noise and study its effects on the dephasing of a two-level
system. Within this model, the evolution of the relative phase between the
states is described as a continuous time random walk (CTRW). Using
renewal theory, we find exact expressions for the dephasing factor and identify
the physically relevant various regimes in terms of the coupling to the noise.
In particular, we point out the consequences of the non-stationarity and
pronounced non-Gaussian features of this noise, including some new anomalous
and aging dephasing scenarii.Comment: Submitted to Phys. Rev.
Renormalization of modular invariant Coulomb gas and Sine-Gordon theories, and quantum Hall flow diagram
Using the renormalisation group (RG) we study two dimensional electromagnetic
coulomb gas and extended Sine-Gordon theories invariant under the modular group
SL(2,Z). The flow diagram is established from the scaling equations, and we
derive the critical behaviour at the various transition points of the diagram.
Following proposal for a SL(2,Z) duality between different quantum Hall fluids,
we discuss the analogy between this flow and the global quantum Hall phase
diagram.Comment: 10 pages, 1 EPS figure include
Rational matrix pseudodifferential operators
The skewfield K(d) of rational pseudodifferential operators over a
differential field K is the skewfield of fractions of the algebra of
differential operators K[d]. In our previous paper we showed that any H from
K(d) has a minimal fractional decomposition H=AB^(-1), where A,B are elements
of K[d], B is non-zero, and any common right divisor of A and B is a non-zero
element of K. Moreover, any right fractional decomposition of H is obtained by
multiplying A and B on the right by the same non-zero element of K[d]. In the
present paper we study the ring M_n(K(d)) of nxn matrices over the skewfield
K(d). We show that similarly, any H from M_n(K(d)) has a minimal fractional
decomposition H=AB^(-1), where A,B are elements of M_n(K[d]), B is
non-degenerate, and any common right divisor of A and B is an invertible
element of the ring M_n(K[d]). Moreover, any right fractional decomposition of
H is obtained by multiplying A and B on the right by the same non-degenerate
element of M_n(K [d]). We give several equivalent definitions of the minimal
fractional decomposition. These results are applied to the study of maximal
isotropicity property, used in the theory of Dirac structures.Comment: 20 page
On the relevance of polyynyl-substituted PAHs to astrophysics
We report on the absorption spectra of the polycyclic aromatic hydrocarbon
(PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl
(-C2H) or a butadiynyl (-C4H) group. Measurements were carried out in the mid
infrared at room temperature on grains embedded in CsI pellets and in the near
ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The
infrared measurements show that interstellar populations of
polyynyl-substituted PAHs would give rise to collective features in the same
way non-substituted PAHs give rise to the aromatic infrared bands. The main
features characteristic of the substituted molecules correspond to the
acetylenic CH stretching mode near 3.05 mum and to the almost isoenergetic
acetylenic CCH in- and out-of-plane bending modes near 15.9 mum.
Sub-populations defined by the length of the polyynyl side group cause
collective features which correspond to the various acetylenic CC stretching
modes. The ultraviolet spectra reveal that the addition of an ethynyl group to
a non-substituted PAH molecule results in all its electronic transitions being
redshifted. Due to fast internal energy conversion, the bands at shorter
wavelengths are significantly broadened. Those at longer wavelengths are only
barely affected in this respect. As a consequence, their relative peak
absorption increases. The substitution with the longer butadiynyl chain causes
the same effects with a larger magnitude, resulting in the spectra to show a
prominent if not dominating pi-pi* transition at long wavelength. After
discussing the relevance of polyynyl-substituted PAHs to astrophysics, we
conclude that this class of highly conjugated, unsaturated molecules are valid
candidates for the carriers of the diffuse interstellar bands.Comment: 29 pages, 9 figures, accepted for publication in ApJ 2 April 201
Cross-Over between universality classes in a magnetically disordered metallic wire
In this article we present numerical results of conduction in a disordered
quasi-1D wire in the possible presence of magnetic impurities. Our analysis
leads us to the study of universal properties in different conduction regimes
such as the localized and metallic ones. In particular, we analyse the
cross-over between universality classes occurring when the strength of magnetic
disorder is increased. For this purpose, we use a numerical Landauer approach,
and derive the scattering matrix of the wire from electron's Green's function.Comment: Final version, accepted for publication in New Journ. of Physics, 27
pages, 28 figures. Replaces the earlier shorter preprint arXiv:0910.427
Some algebraic properties of differential operators
First, we study the subskewfield of rational pseudodifferential operators
over a differential field K generated in the skewfield of pseudodifferential
operators over K by the subalgebra of all differential operators.
Second, we show that the Dieudonne' determinant of a matrix
pseudodifferential operator with coefficients in a differential subring A of K
lies in the integral closure of A in K, and we give an example of a 2x2 matrix
differential operator with coefficients in A whose Dieudonne' determiant does
not lie in A.Comment: 15 page
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