563 research outputs found
Quantum Magnetic Impurities in Magnetically Ordered Systems
We discuss the problem of a spin 1/2 impurity immersed in a spin S
magnetically ordered background. We show that the problem maps onto a
generalization of the dissipative two level system (DTLS) with two independent
heat baths, associated with the Goldstone modes of the magnet, that couple to
different components of the impurity spin operator. Using analytical
perturbative renormalization group (RG) methods and accurate numerical
renormalization group (NRG) we show that contrary to other dissipative models
there is quantum frustration of decoherence and quasi-scaling even in the
strong coupling regime. We make predictions for the behavior of the impurity
magnetic susceptibility that can be measured in nuclear magnetic resonance
(NMR) experiments. Our results may also have relevance to quantum computation.Comment: 4 pages, 3 figure
Assisted hopping and interaction effects in impurity models
We study, using Numerical Renormalization Group methods, the generalization
of the Anderson impurity model where the hopping depends on the filling of the
impurity. We show that the model, for sufficiently large values of the assisted
hopping term, shows a regime where local pairing correlations are enhanced.
These correlations involve pairs fluctuating between on site and nearest
neighbor positions
Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study
We systematically study the influence of ferromagnetic leads on the Kondo
resonance in a quantum dot tuned to the local moment regime. We employ Wilson's
numerical renormalization group method, extended to handle leads with a spin
asymmetric density of states, to identify the effects of (i) a finite spin
polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in
the bands (governed by the band edges) and (iii) an arbitrary shape of the
leads density of states. For a generic lead density of states the quantum dot
favors being occupied by a particular spin-species due to exchange interaction
with ferromagnetic leads leading to a suppression and splitting of the Kondo
resonance. The application of a magnetic field can compensate this asymmetry
restoring the Kondo effect. We study both the gate-voltage dependence (for a
fixed band structure in the leads) and the spin polarization dependence (for
fixed gate voltage) of this compensation field for various types of bands.
Interestingly, we find that the full recovery of the Kondo resonance of a
quantum dot in presence of leads with an energy dependent density of states is
not only possible by an appropriately tuned external magnetic field but also
via an appropriately tuned gate voltage. For flat bands simple formulas for the
splitting of the local level as a function of the spin polarization and gate
voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR
Frustration of Decoherence in Open Quantum Systems
We study a model of frustration of decoherence in an open quantum system.
Contrary to other dissipative ohmic impurity models, such as the Kondo model or
the dissipative two-level system, the impurity model discussed here never
presents overdamped dynamics even for strong coupling to the environment. We
show that this unusual effect has its origins in the quantum mechanical nature
of the coupling between the quantum impurity and the environment. We study the
problem using analytic and numerical renormalization group methods and obtain
expressions for the frequency and temperature dependence of the impurity
susceptibility in different regimes.Comment: 14 pages, 5 figure
Orbital Kondo effect in carbon nanotubes
Progress in the fabrication of nanometer-scale electronic devices is opening
new opportunities to uncover the deepest aspects of the Kondo effect, one of
the paradigmatic phenomena in the physics of strongly correlated electrons.
Artificial single-impurity Kondo systems have been realized in various
nanostructures, including semiconductor quantum dots, carbon nanotubes and
individual molecules. The Kondo effect is usually regarded as a spin-related
phenomenon, namely the coherent exchange of the spin between a localized state
and a Fermi sea of electrons. In principle, however, the role of the spin could
be replaced by other degrees of freedom, such as an orbital quantum number.
Here we demonstrate that the unique electronic structure of carbon nanotubes
enables the observation of a purely orbital Kondo effect. We use a magnetic
field to tune spin-polarized states into orbital degeneracy and conclude that
the orbital quantum number is conserved during tunneling. When orbital and spin
degeneracies are simultaneously present, we observe a strongly enhanced Kondo
effect, with a multiple splitting of the Kondo resonance at finite field and
predicted to obey a so-called SU(4) symmetry.Comment: 26 pages, including 4+2 figure
Tunable Kondo effect in a single donor atom
The Kondo effect has been observed in a single gate-tunable atom. The
measurement device consists of a single As dopant incorporated in a Silicon
nanostructure. The atomic orbitals of the dopant are tunable by the gate
electric field. When they are tuned such that the ground state of the atomic
system becomes a (nearly) degenerate superposition of two of the Silicon
valleys, an exotic and hitherto unobserved valley Kondo effect appears.
Together with the regular spin Kondo, the tunable valley Kondo effect allows
for reversible electrical control over the symmetry of the Kondo ground state
from an SU(2)- to an SU(4) -configuration.Comment: 10 pages, 8 figure
Nanocomposite Bienzymatic Sensor for Monitoring Xanthine in Wound Diagnostics
This work reports a biosensor for monitoring xanthine for potential wound healing assessment. Active substrate of the biosensor has xanthine oxidase (XO) and horseradish peroxidase (HRP) physisorbed on a nanocomposite of multiwalled carbon nanotubes (MWCNT) decorated with gold nanoparticles (AuNP). The presence of HRP provided a two-fold increase in response to xanthine, and a three-fold increase in response to the nanocomposite. With a sensitivity of 155.71 nA μM−1 cm−2 the biosensor offers a detection limit of 1.3 μM, with linear response between 22 μM and 0.4 mM. Clinical sample analyses showed the feasibility of xanthine detection from biofluids in a lesion site due to diffusion of the analyte into surrounding biofluids. Higher concentrations by three-fold were observed from wound proximity, than away from injury, with an average recovery of 110%. Results show the feasibility of monitoring wound severity through longitudinal measurements of xanthine from injured vicinity
Proposals for evaluating the regularity of a scientist'sresearch output
Evaluating the career of individual scientists according to their scientific output is a common bibliometric problem. Two aspects are classically taken into account: overall productivity and overall diffusion/impact, which can be measured by a plethora of indicators that consider publications and/or citations separately or synthesise these two quantities into a single number (e.g. h-index). A secondary aspect, which is sometimes mentioned in the rules of competitive examinations for research position/promotion, is time regularity of one researcher's scientific output. Despite the fact that it is sometimes invoked, a clear definition of regularity is still lacking. We define it as the ability of generating an active and stable research output over time, in terms of both publications/ quantity and citations/diffusion. The goal of this paper is introducing three analysis tools to perform qualitative/quantitative evaluations on the regularity of one scientist's output in a simple and organic way. These tools are respectively (1) the PY/CY diagram, (2) the publication/citation Ferrers diagram and (3) a simplified procedure for comparing the research output of several scientists according to their publication and citation temporal distributions (Borda's ranking). Description of these tools is supported by several examples
Statistical mechanics of voting
Decision procedures aggregating the preferences of multiple agents can
produce cycles and hence outcomes which have been described heuristically as
`chaotic'. We make this description precise by constructing an explicit
dynamical system from the agents' preferences and a voting rule. The dynamics
form a one dimensional statistical mechanics model; this suggests the use of
the topological entropy to quantify the complexity of the system. We formulate
natural political/social questions about the expected complexity of a voting
rule and degree of cohesion/diversity among agents in terms of random matrix
models---ensembles of statistical mechanics models---and compute quantitative
answers in some representative cases.Comment: 9 pages, plain TeX, 2 PostScript figures included with epsf.tex
(ignore the under/overfull \vbox error messages
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