556 research outputs found
Investigating Habituation to Premonitory Urges in Behavior Therapy for Tic Disorders
Behavior therapy is effective for Persistent Tic Disorders (PTDs), but behavioral processes facilitating tic reduction are not well understood. One process, habituation, is thought to create tic reduction through decreases in premonitory urge severity. The current study tested whether premonitory urges decreased in youth with PTDs (N = 126) and adults with PTDs (N = 122) who participated in parallel randomized clinical trials comparing behavior therapy to psychoeducation and supportive therapy (PST). Trends in premonitory urges, tic severity, and treatment outcome were analyzed according to the predictions of a habituation model, whereby urge severity would be expected to decrease in those who responded to behavior therapy. Although adults who responded to behavior therapy showed a significant trend of declining premonitory urge severity across treatment, results failed to demonstrate that behavior therapy specifically caused changes in premonitory urge severity. In addition, reductions in premonitory urge severity in those who responded to behavior therapy were significant greater than those who did not respond to behavior therapy but no different than those who responded or did not respond to PST. Children with PTDs failed to show any significant changes in premonitory urges. Reductions in premonitory urge severity did not mediate the relationship between treatment and outcome in either adults or children. These results cast doubt on the notion that habituation is the therapeutic process underlying the effectiveness of behavior therapy, which has immediate implications for the psychoeducation and therapeutic rationale presented in clinical practice. Moreover, there may be important developmental changes in premonitory urges in PTDs, and alternative models of therapeutic change warrant investigation
Temperature suppression of STM-induced desorption of hydrogen on Si(100) surfaces
The temperature dependence of hydrogen (H) desorption from Si(100)
H-terminated surfaces by a scanning tunneling microscope (STM) is reported for
negative sample bias. It is found that the STM induced H desorption rate ()
decreases several orders of magnitude when the substrate temperature is
increased from 300 K to 610 K. This is most noticeable at a bias voltage of -7
V where decreases by a factor of ~200 for a temperature change of 80 K,
whilst it only decreases by a factor of ~3 at -5 V upon the same temperature
change. The experimental data can be explained by desorption due to vibrational
heating by inelastic scattering via a hole resonance. This theory predicts a
weak suppression of desorption with increasing temperature due to a decreasing
vibrational lifetime, and a strong bias dependent suppression due to a
temperature dependent lifetime of the hole resonance.Comment: 5 pages, RevTeX, epsf files. Accepted for surface science letter
STM induced hydrogen desorption via a hole resonance
We report STM-induced desorption of H from Si(100)-H(2) at negative
sample bias. The desorption rate exhibits a power-law dependence on current and
a maximum desorption rate at -7 V. The desorption is explained by vibrational
heating of H due to inelastic scattering of tunneling holes with the Si-H
5 hole resonance. The dependence of desorption rate on current and bias
is analyzed using a novel approach for calculating inelastic scattering, which
includes the effect of the electric field between tip and sample. We show that
the maximum desorption rate at -7 V is due to a maximum fraction of
inelastically scattered electrons at the onset of the field emission regime.Comment: 4 pages, 4 figures. To appear in Phys. Rev. Let
Testing of quantum phase in matter wave optics
Various phase concepts may be treated as special cases of the maximum
likelihood estimation. For example the discrete Fourier estimation that
actually coincides with the operational phase of Noh, Fouge`res and Mandel is
obtained for continuous Gaussian signals with phase modulated mean.Since
signals in quantum theory are discrete, a prediction different from that given
by the Gaussian hypothesis should be obtained as the best fit assuming a
discrete Poissonian statistics of the signal. Although the Gaussian estimation
gives a satisfactory approximation for fitting the phase distribution of almost
any state the optimal phase estimation offers in certain cases a measurable
better performance. This has been demonstrated in neutron--optical experiment.Comment: 8 pages, 4 figure
Surface Screening Charge and Effective Charge
The charge on an atom at a metallic surface in an electric field is defined
as the field-derivative of the force on the atom, and this is consistent with
definitions of effective charge and screening charge. This charge can be found
from the shift in the potential outside the surface when the atoms are moved.
This is used to study forces and screening on surface atoms of Ag(001)
c -- Xe as a function of external field. It is found that at low
positive (outward) fields, the Xe with a negative effective charge of -0.093
is pushed into the surface. At a field of 2.3 V \AA the charge
changes sign, and for fields greater than 4.1 V \AA the Xe experiences
an outward force. Field desorption and the Eigler switch are discussed in terms
of these results.Comment: 4 pages, 1 figure, RevTex (accepted by PRL
Mapping the unconventional orbital texture in topological crystalline insulators
The newly discovered topological crystalline insulators (TCIs) harbor a
complex band structure involving multiple Dirac cones. These materials are
potentially highly tunable by external electric field, temperature or strain
and could find future applications in field-effect transistors, photodetectors,
and nano-mechanical systems. Theoretically, it has been predicted that
different Dirac cones, offset in energy and momentum-space, might harbor vastly
different orbital character, a unique property which if experimentally
realized, would present an ideal platform for accomplishing new spintronic
devices. However, the orbital texture of the Dirac cones, which is of immense
importance in determining a variety of materials properties, still remains
elusive in TCIs. Here, we unveil the orbital texture in a prototypical TCI
PbSnSe. By using Fourier-transform (FT) scanning tunneling
spectroscopy (STS) we measure the interference patterns produced by the
scattering of surface state electrons. We discover that the intensity and
energy dependences of FTs show distinct characteristics, which can directly be
attributed to orbital effects. Our experiments reveal the complex band topology
involving two Lifshitz transitions and establish the orbital nature of the
Dirac bands in this new class of topological materials, which could provide a
different pathway towards future quantum applications
Coulomb Explosion and Thermal Spikes
A fast ion penetrating a solid creates a track of excitations. This can
produce displacements seen as an etched track, a process initially used to
detect energetic particles but now used to alter materials. From the seminal
papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C.
Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)],
`Coulomb explosion' and thermal spike models are treated as conflicting models
for describing ion track effects. Here molecular dynamics simulations of
electronic-sputtering, a surface manifestation of ion track formation, show
that `Coulomb explosion' produces a `heat' spike so that these are early and
late aspects of the same process. Therefore, differences in scaling are due to
the use of incomplete spike models.Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see:
http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new
versio
Atomic Tunneling from a STM/AFM tip: Dissipative Quantum Effects from Phonons
We study the effects of phonons on the tunneling of an atom between two
surfaces. In contrast to an atom tunneling in the bulk, the phonons couple very
strongly, and qualitatively change the tunneling behavior. This is the first
example of {\it ohmic} coupling from phonons for a two-state system. We propose
an experiment in which an atom tunnels from the tip of an STM, and show how its
behavior would be similar to the Macroscopic Quantum Coherence behavior
predicted for SQUIDS. The ability to tune and calculate many parameters would
lead to detailed tests of the standard theories. (For a general intro to this
work on the on the World-Wide-Web: http://www.lassp.cornell.edu. Click on
``Entertaining Science Done Here'' and ``Quantum Tunneling of Atoms'')Comment: 12 pages, ReVTex3.0, two figures (postscript). This is a
(substantially) revised version of cond-mat/9406043. More info (+ postscript
text) at : http://www.lassp.cornell.edu/ardlouis/publications.htm
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