735 research outputs found
Short note on the density of states in 3D Weyl semimetals
The average density of states in a disordered three-dimensional Weyl system
is discussed in the case of a continuous distribution of random scattering. Our
result clearly indicate that the average density of states does not vanish,
reflecting the absence of a critical point for a metal-insulator transition.
This calculation supports recent suggestions of an avoided quantum critical
point in the disordered three-dimensional Weyl semimetal. However, the
effective density of states can be very small such that the
saddle-approximation with a vanishing density of states might be valid for
practical cases.Comment: 5 pages, 2 figures, minor changes, additional supplemen
Lattice symmetries, spectral topology and opto-electronic properties of graphene-like materials
The topology of the band structure, which is determined by the lattice
symmetries, has a strong influence on the transport properties. Here we
consider an anisotropic honeycomb lattice and study the effect of a
continuously deformed band structure on the optical conductivity and on
diffusion due to quantum fluctuations. In contrast to the behavior at an
isotropic node we find super- and subdiffusion for the anisotropic node. The
spectral saddle points create van Hove singularities in the optical
conductivity, which could be used to characterize the spectral properties
experimentally.Comment: 9 pages, 6 figures. Slightly extended version, e.g. Eq.(12) include
Interplay of topology and geometry in frustrated 2d Heisenberg magnets
We investigate two-dimensional frustrated Heisenberg magnets using
non-perturbative renormalization group techniques. These magnets allow for
point-like topological defects which are believed to unbind and drive either a
crossover or a phase transition which separates a low temperature, spin-wave
dominated regime from a high temperature regime where defects are abundant. Our
approach can account for the crossover qualitatively and both the temperature
dependence of the correlation length as well as a broad but well defined peak
in the specific heat are reproduced. We find no signatures of a finite
temperature transition and an accompanying diverging length scale. Our analysis
is consistent with a rapid crossover driven by topological defects.Comment: 12 pages, 8 figures, final version to appear in Physical Review
Optical conductivity of graphene in the presence of random lattice deformations
We study the influence of lattice deformations on the optical conductivity of
a two-dimensional electron gas. Lattice deformations are taken into account by
introducing a non-abelian gauge field into the Eucledian action of
two-dimensional Dirac electrons. This is in analogy to the introduction of the
gravitation in the four-dimensional quantum field theory. We examine the effect
of these deformations on the averaged optical conductivity. Within the
perturbative theory up to second order we show that corrections of the
conductivity due to the deformations cancel each other exactly. We argue that
these corrections vanish to any order in perturbative expansion.Comment: 9 pages, 9 figure
Valley symmetry breaking and gap tuning in graphene by spin doping
We study graphene with an adsorbed spin texture, where the localized spins
create a periodic magnetic flux. The latter produces gaps in the graphene
spectrum and breaks the valley symmetry. The resulting effective electronic
model, which is similar to Haldane's periodic flux model, allows us to tune the
gap of one valley independently from that of the other valley. This leads to
the formation of two Hall plateaux and a quantum Hall transition. We discuss
the density of states, optical longitudinal and Hall conductivities for nonzero
frequencies and nonzero temperatures. A robust logarithmic singularity appears
in the Hall conductivity when the frequency of the external field agrees with
the width of the gap.Comment: 14 pages, 7 figure
There\u27s a New World Coming by George A. Sinner, Winter Commencement: December 20, 1970
Text of speech delivered by George Sinner at the UND Winter Commencement on December 20, 1970. Sinner was a North Dakota state senator from 1962 to 1966 and was later elected governor of North Dakota in 1984. He entitled his remarks: There\u27s a New World Coming
Functional renormalization group in the broken symmetry phase: momentum dependence and two-parameter scaling of the self-energy
We include spontaneous symmetry breaking into the functional renormalization
group (RG) equations for the irreducible vertices of Ginzburg-Landau theories
by augmenting these equations by a flow equation for the order parameter, which
is determined from the requirement that at each RG step the vertex with one
external leg vanishes identically. Using this strategy, we propose a simple
truncation of the coupled RG flow equations for the vertices in the broken
symmetry phase of the Ising universality class in D dimensions. Our truncation
yields the full momentum dependence of the self-energy Sigma (k) and
interpolates between lowest order perturbation theory at large momenta k and
the critical scaling regime for small k. Close to the critical point, our
method yields the self-energy in the scaling form Sigma (k) = k_c^2 sigma^{-}
(k | xi, k / k_c), where xi is the order parameter correlation length, k_c is
the Ginzburg scale, and sigma^{-} (x, y) is a dimensionless two-parameter
scaling function for the broken symmetry phase which we explicitly calculate
within our truncation.Comment: 9 pages, 4 figures, puplished versio
Spectral function and quasi-particle damping of interacting bosons in two dimensions
We employ the functional renormalization group to study dynamical properties
of the two-dimensional Bose gas. Our approach is free of infrared divergences,
which plague the usual diagrammatic approaches, and is consistent with the
exact Nepomnyashchy identity, which states that the anomalous self-energy
vanishes at zero frequency and momentum. We recover the correct infrared
behavior of the propagators and present explicit results for the spectral
line-shape, from which we extract the quasi-particle dispersion and damping.Comment: 4 pages, 3 figures, revisited version, to appear as Phys. Rev. Lette
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