251 research outputs found
Studies of spin-orbit scattering in noble-metal nanoparticles using energy level tunneling spectroscopy
The effects of spin-orbit scattering on discrete electronic energy levels are
studied in copper, silver, and gold nanoparticles. Level-to-level fluctuations
of the effective -factor for Zeeman splitting are characterized, and the
statistics are found to be well-described by random matrix theory predictions.
The strength of spin-orbit scattering increases with atomic number and also
varies between nanoparticles made of the same metal. The spin-orbit scattering
rates in the nanoparticles are in order-of-magnitude agreement with bulk
measurements on disordered samples.Comment: 4 pages, 3 figures, 1 in colo
Flicker Noise Induced by Dynamic Impurities in a Quantum Point Contact
We calculate low-frequency noise (LFN) in a quantum point contact (QPC) which
is electrostatically defined in a 2D electron gas of a GaAs-AlGaAs
heterostructure. The conventional source of LFN in such systems are scattering
potentials fluctuating in time acting upon injected electrons. One can
discriminate between potentials of different origin -- noise may be caused by
the externally applied gate- and source-drain voltages, the motion of defects
with internal degrees of freedom close to the channel, electrons hopping
between localized states in the doped region, etc. In the present study we
propose a model of LFN based upon the assumption that there are many dynamic
defects in the surrounding of a QPC. A general expression for the
time-dependent current-current correlation function is derived and applied to a
QPC with quantized conductance. It is shown that the level of LFN is
significantly different at and between the steps in a plot of the conductance
vs. gate voltage. On the plateaus, the level of noise is found to be low and
strongly model-dependent. At the steps, LFN is much larger and only weakly
model-dependent. As long as the system is biased to be at a fixed position
relative the conductance step,Comment: 26 revtex APR 94-4
Dynamics of a tunneling magnetic impurity: Kondo effect induced incoherence
We study how the formation of the Kondo compensation cloud influences the
dynamical properties of a magnetic impurity that tunnels between two positions
in a metal. The Kondo effect dynamically generates a strong tunneling
impurity-conduction electron coupling, changes the temperature dependence of
the tunneling rate, and may ultimately result in the destruction of the
coherent motion of the particle at zero temperature. We find an interesting
two-channel Kondo fixed point as well for a vanishing overlap between the
electronic states that screen the magnetic impurity. We propose a number of
systems where the predicted features could be observed.Comment: 4 pages, 3 figures, ReVTe
Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects
We study effect of the adiabatic electron renormalization on the parameters
of the dynamical defects in the ballistic metallic point contact. The upper
energy states of the ``dressed'' defect are shown to give a smaller
contribution to a resistance of the contact than the lower energy ones. This
holds both for the "classical" renormalization related to defect coupling with
average local electron density and for the "mesoscopic" renormalization caused
by the mesoscopic fluctuations of electronic density the dynamical defects are
coupled with. In the case of mesoscopic renormalization one may treat the
dynamical defect as coupled with Friedel oscillations originated by the other
defects, both static and mobile. Such coupling lifts the energy degeneracy of
the states of the dynamical defects giving different mesoscopic contribution to
resistance, and provides a new model for the fluctuator as for the object
originated by the electronic mesoscopic disorder rather than by the structural
one. The correlation between the defect energy and the defect contribution to
the resistance leads to zero-temperature and zero-bias anomalies of the point
contact resistance.
A comparison of these anomalies with those predicted by the Two Channel Kondo
Model (TCKM) is made. It is shown, that although the proposed model is based on
a completely different from TCKM physical background, it leads to a zero-bias
anomalies of the point contact resistance, which are qualitatively similar to
TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.
Ballistic electron transport through magnetic domain walls
Electron transport limited by the rotating exchange-potential of domain walls
is calculated in the ballistic limit for the itinerant ferromagnets Fe, Co, and
Ni. When realistic band structures are used, the domain wall magnetoresistance
is enhanced by orders of magnitude compared to the results for previously
studied two-band models. Increasing the pitch of a domain wall by confinement
in a nano-structured point contact is predicted to give rise to a strongly
enhanced magnetoresistance.Comment: 4 pages, 2 figures; to appear in PRB as a brief repor
Sex-biased parental care and sexual size dimorphism in a provisioning arthropod
The diverse selection pressures driving the evolution of sexual size dimorphism (SSD) have long been debated. While the balance between fecundity selection and sexual selection has received much attention, explanations based on sex-specific ecology have proven harder to test. In ectotherms, females are typically larger than males, and this is frequently thought to be because size constrains female fecundity more than it constrains male mating success. However, SSD could additionally reflect maternal care strategies. Under this hypothesis, females are relatively larger where reproduction requires greater maximum maternal effort – for example where mothers transport heavy provisions to nests.
To test this hypothesis we focussed on digger wasps (Hymenoptera: Ammophilini), a relatively homogeneous group in which only females provision offspring. In some species, a single large prey item, up to 10 times the mother’s weight, must be carried to each burrow on foot; other species provide many small prey, each flown individually to the nest.
We found more pronounced female-biased SSD in species where females carry single, heavy prey. More generally, SSD was negatively correlated with numbers of prey provided per offspring. Females provisioning multiple small items had longer wings and thoraxes, probably because smaller prey are carried in flight.
Despite much theorising, few empirical studies have tested how sex-biased parental care can affect SSD. Our study reveals that such costs can be associated with the evolution of dimorphism, and this should be investigated in other clades where parental care costs differ between sexes and species
Thermodynamics of the dissipative two-state system: a Bethe Ansatz study
The thermodynamics of the dissipative two-state system is calculated exactly
for all temperatures and level asymmetries for the case of Ohmic dissipation.
We exploit the equivalence of the two-state system to the anisotropic Kondo
model and extract the thermodynamics of the former by solving the thermodynamic
Bethe Ansatz equations of the latter. The universal scaling functions for the
specific heat and static dielectric susceptibility
are extracted for all dissipation strengths for
both symmetric and asymmetric two-state systems. The logarithmic corrections to
these quantities at high temperatures are found in the Kondo limit , whereas for we find the expected power law temperature
dependences with the powers being functions of the dissipative coupling
. The low temperature behaviour is always that of a Fermi liquid.Comment: 24 pages, 32 PS figures. Typos corrected, final versio
Vocal Learning and Auditory-Vocal Feedback
Vocal learning is usually studied in songbirds and humans, species that can form auditory templates by listening to acoustic models and then learn to vocalize to match the template. Most other species are thought to develop vocalizations without auditory feedback. However, auditory input influences the acoustic structure of vocalizations in a broad distribution of birds and mammals. Vocalizations are dened here as sounds generated by forcing air past vibrating membranes. A vocal motor program may generate vocalizations such as crying or laughter, but auditory feedback may be required for matching precise acoustic features of vocalizations. This chapter discriminates limited vocal learning, which uses auditory input to fine-tune acoustic features of an inherited auditory template, from complex vocal learning, in which novel sounds are learned by matching a learned auditory template. Two or three songbird taxa and four or ve mammalian taxa are known for complex vocal learning. A broader range of mammals converge in the acoustic structure of vocalizations when in socially interacting groups, which qualifies as limited vocal learning. All birds and mammals tested use auditory-vocal feedback to adjust their vocalizations to compensate for the effects of noise, and many species modulate their signals as the costs and benefits of communicating vary. This chapter asks whether some auditory-vocal feedback may have provided neural substrates for the evolution of vocal learning. Progress will require more precise definitions of different forms of vocal learning, broad comparative review of their presence and absence, and behavioral and neurobiological investigations into the mechanisms underlying the skills.PostprintPeer reviewe
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