158 research outputs found
Childhood predictors and adult life success of adolescent delinquency abstainers
While much is known about adolescent delinquency, considerably less attention has been given to adolescent delinquency abstention. Understanding how or why some adolescents manage to abstain from delinquency during adolescence is informative for understanding and preventing adolescent (minor) delinquency. Using data from the Cambridge Study in Delinquent Development (N = 411 males) to compare abstainers, self-report delinquents and convicted delinquents we found five childhood factors (ages 8-10) that predicted adolescent abstention (ages 10-18). First, we find that adolescent abstainers possess characteristics opposite to those of convicted delinquents (namely, abstainers are high on honesty, conformity and family income). However, we also found that abstainers also share some childhood characteristics with convicted delinquents (namely, low popularity and low school achievement). A latent class analysis indicated that the mixed factors predicting abstention can be accounted for by two groups of abstainers: an adaptive group characterized by high honesty, and a maladaptive group characterized by low popularity and low school achievement. Further, validation of these two types of abstainers using data collected at age 48 suggested that adaptive abstainers outperform all other adolescents in general life success, whereas maladaptive abstainers only fare better than delinquent adolescents in terms of lower substance use and delinquency later in life
Nonlinear Response of a Kondo system: Direct and Alternating Tunneling Currents
Non - equilibrium tunneling current of an Anderson impurity system subject to
both constant and alternating electric fields is studied. A time - dependent
Schrieffer - Wolff transformation maps the time - dependent Anderson
Hamiltonian onto a Kondo one. Perturbation expansion in powers of the Kondo
coupling strength is carried out up to third order, yielding a remarkably
simple analytical expression for the tunneling current. It is found that the
zero - bias anomaly is suppressed by an ac - field. Both dc and the first
harmonic are equally enhanced by the Kondo effect, while the higher harmonics
are relatively small. These results are shown to be valid also below the Kondo
temperature.Comment: 7 pages, RevTeX, 3 PS figures attached, the article has been
significantly developed: time - dependent Schrieffer - Wolff transformation
is presented in the full form, the results are applied to the change in the
direct current induced by an alternating field (2 figures are new
Orbital Kondo behavior from dynamical structural defects
The interaction between an atom moving in a model double-well potential and
the conduction electrons is treated using renormalization group methods in
next-to-leading logarithmic order. A large number of excited states is taken
into account and the Kondo temperature is computed as a function of
barrier parameters. We find that for special parameters can be close to
and it can be of the same order of magnitude as the renormalized
splitting . However, in the perturbative regime we always find that
T_K \alt \Delta with a T_K \alt 1 {\rm K} [Aleiner {\em et al.}, Phys.
Rev. Lett. {\bf 86}, 2629 (2001)]. We also find that remains
unrenormalized at energies above the Debye frequency, .Comment: 9 pages, 9 figures, RevTe
Connective neck evolution and conductance steps in hot point contacts
Dynamic evolution of the connective neck in Al and Pb mechanically
controllable break junctions was studied during continuous approach of
electrodes at bias voltages V_b up to a few hundred mV. A high level of power
dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in
the constriction lead to overheating of the contact area, electromigration and
current-enhanced diffusion of atoms out of the "hot spot". At a low electrode
approach rate (10 - 50 pm/s) the transverse dimension of the neck and the
conductance of the junction depend on V_b and remain nearly constant over the
approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists
of a few atoms only and the quantum nature of conductance manifests itself in
abrupt steps and reversible jumps between two or more levels. These features
are related to an ever changing number of individual conductance channels due
to the continuous rearrangement in atomic configuration of the neck, the
recurring motion of atoms between metastable states, the formation and breaking
of isolated one-atom contacts and the switching between energetically
preferable neck geometries.Comment: 21 pages 10 figure
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples II. Finite Size Dependence in the Kondo Resistivity
The electrical resistivity including the Kondo resistivity increase at low
temperature is calculated for thin films of dilute magnetic alloys. Assuming
that in the non-magnetic host the spin-orbit interaction is strong like in Au
and Cu, the magnetic impurities have a surface anisotropy calculated in part I.
That anisotropy hinders the motion of the spin. Including that anisotropy the
effective electron-impurity coupling is calculated by using the second order
renormalization group equations. The amplitude of the Kondo resistivity
contribution is reduced as the position of the impurity approaches the surface
but the increase occurs approximately at the bulk Kondo temperature. Different
proximity effects observed by Giordano are also explained qualitatively where
the films of magnetic alloys are covered by pure second films with different
mean free path. The theory explains the experimental results in those cases
where a considerable amount of impurities is at the surface inside the
ballistic region.Comment: 39 pages, RevTeX (using epsfig), 15 eps figures included, submitted
to PR
Non-linear response of a Kondo system: Perturbation approach to the time dependent Anderson impurity model
Nonlinear tunneling current through a quantum dot
(an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.Comment: 55 latex pages 11 ps figure
Dephasing in Metals by Two-Level Systems in the 2-Channel-Kondo Regime
We point out a novel, non-universal contribution to the dephasing rate
1/\tau_\phi \equiv \gamma_\phi of conduction electrons in metallic systems:
scattering off non-magnetic two-level systems (TLSs) having almost degenerate
Kondo ground states. In the regime \Delta_{ren} < T < T_K (\Delta_{ren} =
renormalized level splitting, T_K = Kondo temperature), such TLSs exhibit
non-Fermi-liquid physics that can cause \gamma_\phi, which generally decreases
with decreasing T, to seemingly saturate in a limited temperature range before
vanishing for T \to 0. This could explain the saturation of dephasing recently
observed in gold wires [Mohanty et al. Phys. Rev. Lett. 78, 3366 (1997)].Comment: Final published version, including minor improvements suggested by
referees. 4 pages, Revtex, 1 figur
Kondo Effect on Mesoscopic Scale (Review)
Following the discovery of the Kondo effect the bulk transport and magnetic
behavior of the dilute magnetic alloys have been successfully described. In the
last fifteen years new directions have been developed as the study of the
systems of reduced dimensions and the artificial atoms so called quantum dots.
In this review the first subject is reviewed starting with the scanning
tunneling microscope (STM) study of a single magnetic impurity. The next
subject is the reduction of the amplitude of the Kondo effect in samples of
reduced dimension which was explained by the surface magnetic anisotropy which
blocks the motion of the integer spin nearby the surface. The electron
dephasing and energy relaxation experiments are discussed with the possible
explanation including the surface anisotropy, where the situation in cases of
integer and half-integer spins is very different. Finally, the present
situation of the theory of dynamical structural defects is briefly presented
which may lead to two-channel Kondo behavior.Comment: 8 pages, submitted to the JPSJ Special Issue "Kondo effect -- 40
years after the Discovery
An anatomically-based masking protocol for the assessment of in-shoe plantar pressure measurement of the forefoot
Background
The area beneath the metatarsal heads is a common location of foot pain, which is often associated with high plantar pressures. Current plantar pressure assessment protocols focus mainly on the gross area of the forefoot with minimal attention paid to specific areas such as the metatarsal heads. The aim of this study was to develop and assess a new anatomically-based masking protocol that is clinically relevant to measure forefoot plantar pressure during shod conditions based on the anatomical positions of the metatarsal heads.
Methods
Initially, we developed a masking protocol to measure forefoot plantar pressure during shod conditions based on the anatomical positions of the metatarsal heads. This new masking protocol divided the forefoot into three sub-areas (proximal, beneath, and distal to the metatarsal heads) as determined by the position of each metatarsal head. Following development of the new masking protocol, we compared the new protocol against a traditional protocol, which defines the forefoot as between 51 and 81% of the foot length. To compare the two masking protocols, we tested two experimental conditions: (i) a control condition (i.e. no metatarsal pad), and (ii) a metatarsal pad condition. We then compared plantar pressure differences between the two experimental conditions for the two masking protocols. Participants for this component of the study included 36 community dwelling older adults (mean age 75.6 years ±5.4) with a history of forefoot pain. Forefoot plantar pressure data were measured while walking using the pedar®-X in-shoe system. Peak pressure, maximum force and contact area at the time of peak pressure were determined and results were compared between the two masking protocols.
Results
The traditional masking protocol showed that the metatarsal pad significantly decreased peak pressure and increased contact area in the forefoot area (i.e. within the entire mask area), but maximum force was not significantly different between the two conditions. In contrast, the newly developed anatomically-based masking protocol indicated that the metatarsal pad decreased peak plantar pressures distal to and beneath the metatarsal heads by increasing force and contact area proximal to the metatarsal heads.
Conclusions
An anatomically-based masking protocol that is clinically relevant was developed to assess forefoot plantar pressure during shod conditions based on the anatomical positions of metatarsal heads. We propose that the new forefoot masking protocol will provide greater interpretability of forefoot plantar pressure data, which will aid clinicians and researchers for diagnostic, prognostic and therapeutic purposes
- …