6,539 research outputs found
Strain-Modified RKKY Interaction in Carbon Nanotubes
For low-dimensional metallic structures, such as nanotubes, the exchange
coupling between localized magnetic dopants is predicted to decay slowly with
separation. The long-range character of this interaction plays a significant
role in determining the magnetic order of the system. It has previously been
shown that the interaction range depends on the conformation of the magnetic
dopants in both graphene and nanotubes. Here we examine the RKKY interaction in
carbon nanotubes in the presence of uniaxial strain for a range of different
impurity configurations. We show that strain is capable of amplifying or
attenuating the RKKY interaction, significantly increasing certain interaction
ranges, and acting as a switch: effectively turning on or off the interaction.
We argue that uniaxial strain can be employed to significantly manipulate
magnetic interactions in carbon nanotubes, allowing an interplay between
mechanical and magnetic properties in future spintronic devices. We also
examine the dimensional relationship between graphene and nanotubes with
regards to the decay rate of the RKKY interaction.Comment: 7 pages, 6 figures, submitte
Reflexivity of the translation-dilation algebras on L^2(R)
The hyperbolic algebra A_h, studied recently by Katavolos and Power, is the
weak star closed operator algebra on L^2(R) generated by H^\infty(R), as
multiplication operators, and by the dilation operators V_t, t \geq 0, given by
V_t f(x) = e^{t/2} f(e^t x). We show that A_h is a reflexive operator algebra
and that the four dimensional manifold Lat A_h (with the natural topology) is
the reflexive hull of a natural two dimensional subspace.Comment: 10 pages, no figures To appear in the International Journal of
Mathematic
Strain-induced modulation of magnetic interactions in graphene
The ease with which the physical properties of graphene can be tuned suggests
a wide range of possible applications. Recently, strain engineering of these
properties has been of particular interest. Possible spintronic applications of
magnetically doped graphene systems have motivated recent theoretical
investigations of the so-called Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction between localized moments in graphene. In this work a combination
of analytic and numerical techniques are used to examine the effects of
uniaxial strain on such an interaction. A range of interesting features are
uncovered depending on the separation and strain directions. Amplification,
suppression, and oscillatory behavior are reported as a function of the strain
and mathematically transparent expressions predicting these features are
derived. Since a wide range of effects, including overall moment formation and
magnetotransport response, are underpinned by such interactions we predict that
the ability to manipulate the coupling by applying strain may lead to
interesting spintronic applications.Comment: 6 pages, 3 figure
Dark Matter Halo Profiles in Scale-Free Cosmologies
We explore the dependence of the central logarithmic slope of dark matter
halo density profiles on the spectral index of the linear matter
power spectrum using cosmological -body simulations of scale-free
models (i.e. ). For each of our simulations we identify
samples of well resolved haloes in dynamical equilibrium and we analyse their
mass profiles. By parameterising the mass profile using a ``generalised''
Navarro, Frenk & White profile in which the central logarithmic slope
is allowed to vary while preserving the asymptotic form at large
radii, we obtain preferred central slopes for haloes in each of our models.
There is a strong correlation between and , such that
becomes shallower as becomes steeper. However, if we normalise our mass
profiles by , the radius at which the logarithmic slope of the density
profile is -2, we find that these differences are no longer present. We
conclude that there is no evidence for convergence to a unique central
asymptotic slope, at least on the scales that we can resolve.Comment: 9 pages, 4 figures. Accepted for publication in MNRA
Rendering an Account: An Open-State Archive in Postgraduate Supervision
The paper begins with a brief account of the transformation of research degree studies under the pressures of global capitalism and neo-liberal governmentality. A parallel transformation is occurring in the conduct of research through the use of information and communication technologies. Yet the potential of ICTs to shape practices of surveillance or to produce new student-supervisor relations and enhance the processes of developing the dissertation has received almost no critical attention. As doctoral supervisor and student, we then describe the features and uses of a web-based open state archive of the student's work-in-progress, developed by the student and accessible to his supervisor. Our intention was to encourage more open conversations between data and theorising, student and supervisor, and ultimately between the student and professional community. However, we recognise that relations of accountability, as these have developed within a contemporary "audit revolution" (Power, 1994, 1997) in universities, create particular "lines of visibility" (Munro, 1996). Thus while the open-state archive may help to redefine in less managerial terms notions of quality, transparency, flexibility and accountability, it might also make possible greater supervisory surveillance. How should we think about the panoptical potential of this archive? We argue that the diverse kinds of interactional patterns and pedagogical intervention it encourages help to create shifting subjectivities. Moreover, the archive itself is multiple, in bringing together an array of diverse materials that can be read in various ways, by following multiple paths. It therefore constitutes a collage, which we identify as a mode of cognition and of accounting distinct from but related to argument and narrative. As a more "open" text (Iser, 1978) it has an indeterminacy which may render it less open to abuse for the technologies of managerial accountability
Noncovariant gauge fixing in the quantum Dirac field theory of atoms and molecules
Starting from the Weyl gauge formulation of quantum electrodynamics (QED),
the formalism of quantum-mechanical gauge fixing is extended using techniques
from nonrelativistic QED. This involves expressing the redundant gauge degrees
of freedom through an arbitrary functional of the gauge-invariant transverse
degrees of freedom. Particular choices of functional can be made to yield the
Coulomb gauge and Poincar\'{e} gauge representations. The Hamiltonian we derive
therefore serves as a good starting point for the description of atoms and
molecules by means of a relativistic Dirac field. We discuss important
implications for the ontology of noncovariant canonical QED due to the gauge
freedom that remains present in our formulation.Comment: 8 pages, 0 figure
Benchmarking quantum control methods on a 12-qubit system
In this letter, we present an experimental benchmark of operational control
methods in quantum information processors extended up to 12 qubits. We
implement universal control of this large Hilbert space using two complementary
approaches and discuss their accuracy and scalability. Despite decoherence, we
were able to reach a 12-coherence state (or 12-qubits pseudo-pure cat state),
and decode it into an 11 qubit plus one qutrit labeled observable pseudo-pure
state using liquid state nuclear magnetic resonance quantum information
processors.Comment: 11 pages, 4 figures, to be published in PR
RKKY interaction between adsorbed magnetic impurities in graphene: Symmetry and strain effects
The growing interest in carbon-based spintronics has stimulated a number of
recent theoretical studies on the RKKY interaction in graphene, with the aim of
determining the most energetically favourable alignments between embedded
magnetic moments. The RKKY interaction in undoped graphene decays faster than
expected for conventional two-dimensional materials and recent studies suggest
that the adsorption configurations favoured by many transition-metal impurities
may lead to even shorter ranged decays and possible sign-changing oscillations.
Here we show that these features emerge in a mathematically transparent manner
when the symmetry of the configurations is included in the calculation.
Furthermore, we show that by breaking the symmetry of the graphene lattice, via
uniaxial strain, the decay rate, and hence the range, of the RKKY interaction
can be significantly altered. Our results suggest that magnetic interactions
between adsorbed impurities in graphene can be manipulated by careful strain
engineering of such systems.Comment: 12 pages, 6 figures, submitte
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