1,885 research outputs found
Existence of the Stark-Wannier quantum resonances
In this paper we prove the existence of the Stark-Wannier quantum resonances
for one-dimensional Schrodinger operators with smooth periodic potential and
small external homogeneous electric field. Such a result extends the existence
result previously obtained in the case of periodic potentials with a finite
number of open gaps.Comment: 30 pages, 1 figur
Gradient expansion approach to multiple-band Fermi liquids
Promoted by the recent progress of Berry phase physics in spin
galvanomagnetic communities, we develop a systematic derivation of the reduced
Keldysh equation (RKE) which captures the low-energy dynamics of
quasi-particles constrained within doubly degenerate bands forming a single
Fermi surface. Specifically, we project out the fully occupied/empty band
degrees of freedom perturbatively in the gradient expansion, whose coupling
constant measures how a system is disequilibrated. As for the electron-electron
interactions, however, we only employ the so-called adiabatic assumption of the
Fermi liquid theory, so that the effect of electron correlations onto the
adiabatic transport of quasi-particles, i.e. the hermitian (real) part of the
self-energy, is taken into account in an unbiased manner.Comment: 29 pages, 7 figure
Electron energy spectrum and the Berry phase in graphite bilayer
We emphasize that there exist four Dirac-type points in the electron-energy
spectrum of a graphite bilayer near the point K of its Brillouin zone. One of
the Dirac points is at the point K, and three Dirac points lie nearby. Each of
these three points generates the Berry phase , while the Dirac point at K
gives the phase . It is these four points that determine the Berry phase
in the bilayer. If an electron orbit surrounds all these points, the Berry
phase is equal to .Comment: 4 pages, 2 figures, submitted to Phys. Rev. B ; expande
Immune response in a wild bird is predicted by oxidative status, but does not cause oxidative stress.
The immune system provides vital protection against pathogens, but extensive evidence suggests that mounting immune responses can entail survival and fecundity costs. The physiological mechanisms that underpin these costs remain poorly understood, despite their potentially important role in shaping life-histories. Recent studies involving laboratory models highlight the possibility that oxidative stress could mediate these costs, as immune-activation can increase the production of reactive oxygen species leading to oxidative stress. However, this hypothesis has rarely been tested in free-ranging wild populations, where natural oxidative statuses and compensatory strategies may moderate immune responses and their impacts on oxidative status. Furthermore, the possibility that individuals scale their immune responses according to their oxidative status, conceivably to mitigate such costs, remains virtually unexplored. Here, we experimentally investigate the effects of a phytohaemagglutinin (PHA) immune-challenge on oxidative status in wild male and female white-browed sparrow weavers, Plocepasser mahali. We also establish whether baseline oxidative status prior to challenge predicts the scale of the immune responses. Contrary to previous work on captive animals, our findings suggest that PHA-induced immune-activation does not elicit oxidative stress. Compared with controls (n = 25 birds), PHA-injected birds (n = 27 birds) showed no evidence of a differential change in markers of oxidative damage or enzymatic and non-enzymatic antioxidant protection 24 hours after challenge. We did, however, find that the activity of a key antioxidant enzyme (superoxide dismutase, SOD) prior to immune-activation predicted the scale of the resulting swelling: birds with stronger initial SOD activity subsequently produced smaller swellings. Our findings (i) suggest that wild birds can mount immune responses without suffering from systemic oxidative stress, and (ii) lend support to biomedical evidence that baseline oxidative status can impact the scale of immune responses; a possibility not yet recognised in ecological studies of immunity
Many-body position operator in lattice fermionic systems with periodic boundary conditions
A total position operator in the position representation is derived for
lattice fermionic systems with periodic boundary conditions. The operator is
shown to be Hermitian, the generator of translations in momentum space, and its
time derivative is shown to correspond to the total current operator in a
periodic system. The operator is such that its moments can be calculated up to
any order. To demonstrate its utility finite size scaling is applied to the
Brinkman-Rice transition as well as metallic and insulating Gutzwiller
wavefunctions.Comment: to appear in Journal of Physics A: Mathematical and General
(reference will be added later
A priori Wannier functions from modified Hartree-Fock and Kohn-Sham equations
The Hartree-Fock equations are modified to directly yield Wannier functions
following a proposal of Shukla et al. [Chem. Phys. Lett. 262, 213-218 (1996)].
This approach circumvents the a posteriori application of the Wannier
transformation to Bloch functions. I give a novel and rigorous derivation of
the relevant equations by introducing an orthogonalizing potential to ensure
the orthogonality among the resulting functions. The properties of these,
so-called a priori Wannier functions, are analyzed and the relation of the
modified Hartree-Fock equations to the conventional, Bloch-function-based
equations is elucidated. It is pointed out that the modified equations offer a
different route to maximally localized Wannier functions. Their computational
solution is found to involve an effort that is comparable to the effort for the
solution of the conventional equations. Above all, I show how a priori Wannier
functions can be obtained by a modification of the Kohn-Sham equations of
density-functional theory.Comment: 7 pages, RevTeX4, revise
Choosing a basis that eliminates spurious solutions in k.p theory
A small change of basis in k.p theory yields a Kane-like Hamiltonian for the
conduction and valence bands of narrow-gap semiconductors that has no spurious
solutions, yet provides an accurate fit to all effective masses. The theory is
shown to work in superlattices by direct comparison with first-principles
density-functional calculations of the valence subband structure. A
reinterpretation of the standard data-fitting procedures used in k.p theory is
also proposed.Comment: 15 pages, 2 figures; v3: expanded with much new materia
Topological Insulators with Inversion Symmetry
Topological insulators are materials with a bulk excitation gap generated by
the spin orbit interaction, and which are different from conventional
insulators. This distinction is characterized by Z_2 topological invariants,
which characterize the groundstate. In two dimensions there is a single Z_2
invariant which distinguishes the ordinary insulator from the quantum spin Hall
phase. In three dimensions there are four Z_2 invariants, which distinguish the
ordinary insulator from "weak" and "strong" topological insulators. These
phases are characterized by the presence of gapless surface (or edge) states.
In the 2D quantum spin Hall phase and the 3D strong topological insulator these
states are robust and are insensitive to weak disorder and interactions. In
this paper we show that the presence of inversion symmetry greatly simplifies
the problem of evaluating the Z_2 invariants. We show that the invariants can
be determined from the knowledge of the parity of the occupied Bloch
wavefunctions at the time reversal invariant points in the Brillouin zone.
Using this approach, we predict a number of specific materials are strong
topological insulators, including the semiconducting alloy Bi_{1-x} Sb_x as
well as \alpha-Sn and HgTe under uniaxial strain. This paper also includes an
expanded discussion of our formulation of the topological insulators in both
two and three dimensions, as well as implications for experiments.Comment: 16 pages, 7 figures; published versio
Odd Parity and Line Nodes in Heavy Fermion Superconductors
Group theory arguments have demonstrated that a general odd parity order
parameter cannot have line nodes in the presence of spin-orbit coupling. In
this paper, it is shown that these arguments do not hold on the
zone face of a hexagonal close packed lattice. In particular, three of the six
odd parity representations vanish identically on this face. This has potential
relevance to the heavy fermion superconductor .Comment: 5 pages, revte
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