10,189 research outputs found
Noncontact atomic force microscopy simulator with phase-locked-loop controlled frequency detection and excitation
A simulation of an atomic force microscope operating in the constant
amplitude dynamic mode is described. The implementation mimics the electronics
of a real setup including a digital phase-locked loop (PLL). The PLL is not
only used as a very sensitive frequency detector, but also to generate the
time-dependent phase shifted signal driving the cantilever. The optimum
adjustments of individual functional blocks and their joint performance in
typical experiments are determined in detail. Prior to testing the complete
setup, the performances of the numerical PLL and of the amplitude controller
were ascertained to be satisfactory compared to those of the real components.
Attention is also focused on the issue of apparent dissipation, that is, of
spurious variations in the driving amplitude caused by the nonlinear
interaction occurring between the tip and the surface and by the finite
response times of the various controllers. To do so, an estimate of the minimum
dissipated energy that is detectable by the instrument upon operating
conditions is given. This allows us to discuss the relevance of apparent
dissipation that can be conditionally generated with the simulator in
comparison to values reported experimentally. The analysis emphasizes that
apparent dissipation can contribute to the measured dissipation up to 15% of
the intrinsic dissipated energy of the cantilever interacting with the surface,
but can be made negligible when properly adjusting the controllers, the PLL
gains and the scan speed. It is inferred that the experimental values of
dissipation usually reported in the literature cannot only originate in
apparent dissipation, which favors the hypothesis of "physical" channels of
dissipation
Tilt Induced Localization and Delocalization in the Second Landau Level
We have investigated the behavior of electronic phases of the second Landau
level under tilted magnetic fields. The fractional quantum Hall liquids at
2+1/5 and 2+4/5 and the solid phases at 2.30, 2.44, 2.57, and 2.70
are quickly destroyed with tilt. This behavior can be interpreted as a tilt
driven localization of the 2+1/5 and 2+4/5 fractional quantum Hall liquids and
a delocalization through melting of solid phases in the top Landau level,
respectively. The evolution towards the classical Hall gas of the solid phases
is suggestive of antiferromagnetic ordering
Real-space imaging of quantum Hall effect edge strips
We use dynamic scanning capacitance microscopy (DSCM) to image compressible
and incompressible strips at the edge of a Hall bar in a two-dimensional
electron gas (2DEG) in the quantum Hall effect (QHE) regime. This method gives
access to the complex local conductance, Gts, between a sharp metallic tip
scanned across the sample surface and ground, comprising the complex sample
conductance. Near integer filling factors we observe a bright stripe along the
sample edge in the imaginary part of Gts. The simultaneously recorded real part
exhibits a sharp peak at the boundary between the sample interior and the
stripe observed in the imaginary part. The features are periodic in the inverse
magnetic field and consistent with compressible and incompressible strips
forming at the sample edge. For currents larger than the critical current of
the QHE break-down the stripes vanish sharply and a homogeneous signal is
recovered, similar to zero magnetic field. Our experiments directly illustrate
the formation and a variety of properties of the conceptually important QHE
edge states at the physical edge of a 2DEG.Comment: 7 page
Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer
We report on quantum interference between a pair of counterpropagating
quantum Hall edge states that are separated by a high quality tunnel barrier.
Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling
between coupled Luttinger liquids that creates bound electronic states between
pairs of tunnel centers that act like interference slits. We place a lower
bound in the range of 20-40 m for the phase coherence length and directly
confirm the extended phase coherence of quantum Hall edge states.Comment: 4 pages, 3 figures, 1 tabl
Quantization of the diagonal resistance: Density gradients and the empirical resistance rule in a 2D system
We have observed quantization of the diagonal resistance, R_xx, at the edges
of several quantum Hall states. Each quantized R_xx value is close to the
difference between the two adjacent Hall plateaus in the off-diagonal
resistance, R_xy. Peaks in R_xx occur at different positions in positive and
negative magnetic fields. Practically all R_xx features can be explained
quantitatively by a ~1%/cm electron density gradient. Therefore, R_xx is
determined by R_xy and unrelated to the diagonal resistivity rho_xx. Our
findings throw an unexpected light on the empirical resistivity rule for 2D
systems
Optically Pumped NMR Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factor nu=1/3
The Knight shift of Ga-71 nuclei is measured in two different electron-doped
multiple quantum well samples using optically pumped NMR. These data are the
first direct measurements of the electron spin polarization,
P(nu,T)=/max, near nu=1/3. The P(T) data at nu=1/3 probe the
neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In
addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12
Tesla field. The observed depolarization is quite small, consistent with an
average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which
does not appear to be explicable by the current theoretical understanding of
the FQHE near nu=1/3.Comment: 4 pages (REVTEX), 5 eps figures embedded in text; minor changes,
published versio
Emotional abuse interacts with borderline personality in adolescent inpatients with binge-purging eating disorders
Purpose
Childhood abuse is associated with an increased risk of developing eating disorders (EDs) as well as personality disorders (PDs). However, their interaction is still uncertain, particularly in adolescents. This study investigates the correlations between childhood emotional neglect (CEN), childhood emotional abuse (CEA), and obsessive-compulsive and borderline personality styles in female adolescent inpatients with eating disorders (EDs).
Methods
One hundred and twenty-eight inpatients (ages 14-18) were assessed, 54 were diagnosed with restricting-type anorexia nervosa (AN-R) and 33 with a binge-purging ED [BP-ED; comprising patients with binge-purging type anorexia nervosa (AN-BP), n = 15, and bulimia nervosa (BN), n = 18]. Fifty healthy participants made up the control group (CG). CEN and CEA were assessed with the Childhood Trauma Questionnaire, while the Personality Style and Disorder Inventory was implemented to determine personality styles.
Results
A MANOVA revealed a significant main effect of CEA on spontaneous-borderline personality style [F(8,119) = 17.1, p < 0.001, η2 = 0.126], as well as a main effect of ED group on spontaneous-borderline [F(2,119) = 3.1, p = 0.048, η2 = 0.050]. A significant interaction between ED group, CEA, and spontaneous-borderline was found [F(2,119) = 3.5, p = 0.034, η2 = 0.055] with BP-ED showing significantly higher scores in CEA (9.3 ± 4.0) and in spontaneous-borderline (14.2 ± 6.2).
Conclusions
Considering CEA and borderline personality style in adolescent inpatients with BN or AN-BP may help improve the understanding of the etiology and maintenance of BP-ED and provide more effective treatment targets.
Level of evidence
Level III, case–control analytic study
Effects of High Charge Densities in Multi-GEM Detectors
A comprehensive study, supported by systematic measurements and numerical
computations, of the intrinsic limits of multi-GEM detectors when exposed to
very high particle fluxes or operated at very large gains is presented. The
observed variations of the gain, of the ion back-flow, and of the pulse height
spectra are explained in terms of the effects of the spatial distribution of
positive ions and their movement throughout the amplification structure. The
intrinsic dynamic character of the processes involved imposes the use of a
non-standard simulation tool for the interpretation of the measurements.
Computations done with a Finite Element Analysis software reproduce the
observed behaviour of the detector. The impact of this detailed description of
the detector in extreme conditions is multiple: it clarifies some detector
behaviours already observed, it helps in defining intrinsic limits of the GEM
technology, and it suggests ways to extend them.Comment: 5 pages, 6 figures, 2015 IEEE Nuclear Science Symposiu
Warping and Precession of Accretion Disks Around Magnetic Stars: Nonlinear Evolution
The inner region of the accretion disk around a magnetized star (T Tauri
star, white dwarf or neutron star) is subjected to magnetic torques that induce
warping and precession of the disk. These torques arise from the interaction
between the stellar field and the induced electric currents in the disk. We
carry out numerical simulations of the nonlinear evolution of warped, viscous
accretion disks driven by the magnetic torques. We show that the disk can
develop into a highly warped steady state in which the disk attains a fixed
(warped) shape and precesses rigidly. The warp is most pronounced at the disk
inner radius (near the magnetosphere boundary). As the system parameters (such
as accretion rate) change, the disk can switch between a completely flat state
(warping stable) and a highly warped state. The precession of warped disks may
be responsible for a variety of quasi-periodic oscillations or radiation flux
variabilities observed in many different systems, including young stellar
objects and X-ray binaries.Comment: 16 pages, 7 figures; extended parameter searches, changes in
discussion; accepted for publication in Ap
Initial data for Einstein's equations with superposed gravitational waves
A method is presented to construct initial data for Einstein's equations as a
superposition of a gravitational wave perturbation on an arbitrary stationary
background spacetime. The method combines the conformal thin sandwich formalism
with linear gravitational waves, and allows detailed control over
characteristics of the superposed gravitational wave like shape, location and
propagation direction. It is furthermore fully covariant with respect to
spatial coordinate changes and allows for very large amplitude of the
gravitational wave.Comment: Version accepted by PRD; added convergence plots, expanded
discussion. 9 pages, 9 figure
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