935 research outputs found
Site determination and thermally assisted tunneling in homogenous nucleation
A combined low-temperature scanning tunneling microscopy and density
functional theory study on the binding and diffusion of copper monomers,
dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers
are found in fcc sites only, monomers as well as atoms in dimers can occupy the
stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp
configuration was found to be only 1.3 meV less favorable with respect to the
fcc-fcc configuration. This enables a confined intra-cell dimer motion, which
at temperatures below 5 K is dominated by thermally assisted tunneling.Comment: 4 pages, 4 figure
STM Simulation of Molecules on Ultrathin Insulating Overlayers Using Tight-Binding: Au-Pentacene on NaCl bilayer on Cu
We present a fast and efficient tight-binding (TB) method for simulating
scanning tunneling microscopy (STM) imaging of adsorbate molecules on ultrathin
insulating films. Due to the electronic decoupling of the molecule from the
metal surface caused by the presence of the insulating overlayer, the STM
images of the frontier molecular orbitals can be simulated using a very
efficient scheme, which also enables the analysis of phase shifts in the STM
current. Au-pentacene complex adsorbed on a NaCl bilayer on Cu substrate
provides an intricate model system, which has been previously studied both
experimentally and theoretically. Our calculations indicate that the
complicated shape of the molecular orbitals may cause multivalued constant
current surfaces -- leading to ambiguity of the STM image. The results obtained
using the TB method are found to be consistent with both DFT calculations and
experimental data.Comment: 19 pages, 5 figure
The trap design of PENTATRAP
A novel Penning trap tower consisting of five compensated cylindrical Penning
traps is developed for the PENTATRAP mass spectrometer at the
Max-Planck-Institut f\"ur Kernphysik in Heidelberg, Germany. An analytical
expression for the electrostatic potential inside the trap tower is derived to
calculate standard Penning trap properties like the compensation of
anharmonicities and an orthogonal geometry of the trap electrodes. Since the
PENTATRAP project described in the preceding article aims for ultra
high-precision mass-ratio measurements of highly charged ions up to uranium,
systematic effects for highly charged ions inside the trap tower are considered
for the design process as well. Finally, a limit due to remaining anharmonic
shifts at large amplitudes is estimated for the resulting geometry, which is
important for phase-sensitive measurements of the reduced cyclotron frequency
of the ions
The Clustering of Expressive Timing Within a Phrase in Classical Piano Performances by Gaussian Mixture Models
In computational musicology research, clustering is a common approach to the analysis of expression. Our research uses mathematical model selection criteria to evaluate the performance of clustered and non-clustered models applied to intra-phrase tempo variations in classical piano performances. By engaging different standardisation methods for the tempo variations and engaging different types of covariance matrices, multiple pieces of performances are used for evaluating the performance of candidate models. The results of tests suggest that the clustered models perform better than the non-clustered models and the original tempo data should be standardised by the mean of tempo within a phrase
Cognitive loading affects motor awareness and movement kinematics but not locomotor trajectories during goal-directed walking in a virtual reality environment.
The primary purpose of this study was to investigate the effects of cognitive loading on movement kinematics and trajectory formation during goal-directed walking in a virtual reality (VR) environment. The secondary objective was to measure how participants corrected their trajectories for perturbed feedback and how participants' awareness of such perturbations changed under cognitive loading. We asked 14 healthy young adults to walk towards four different target locations in a VR environment while their movements were tracked and played back in real-time on a large projection screen. In 75% of all trials we introduced angular deviations of ±5° to ±30° between the veridical walking trajectory and the visual feedback. Participants performed a second experimental block under cognitive load (serial-7 subtraction, counter-balanced across participants). We measured walking kinematics (joint-angles, velocity profiles) and motor performance (end-point-compensation, trajectory-deviations). Motor awareness was determined by asking participants to rate the veracity of the feedback after every trial. In-line with previous findings in natural settings, participants displayed stereotypical walking trajectories in a VR environment. Our results extend these findings as they demonstrate that taxing cognitive resources did not affect trajectory formation and deviations although it interfered with the participants' movement kinematics, in particular walking velocity. Additionally, we report that motor awareness was selectively impaired by the secondary task in trials with high perceptual uncertainty. Compared with data on eye and arm movements our findings lend support to the hypothesis that the central nervous system (CNS) uses common mechanisms to govern goal-directed movements, including locomotion. We discuss our results with respect to the use of VR methods in gait control and rehabilitation
PENTATRAP: A novel cryogenic multi-Penning trap experiment for high-precision mass measurements on highly charged ions
The novel five-Penning trap mass spectrometer PENTATRAP is developed at the
Max-Planck-Institut f\"ur Kernphysik (MPIK), Heidelberg. Ions of interest are
long-lived highly charged nuclides up to bare uranium. PENTATRAP aims for an
accuracy of a few parts in 10^12 for mass ratios of mass doublets. A physics
program for PENTATRAP includes Q-values measurements of \beta-transitions
relevant for neutrino physics, stringent tests of quantum electrodynamics in
the regime of extreme electric fields, and a test of special relativity. Main
features of PENTATRAP are an access to a source of highly charged ions, a
multi-trap configuration, simultaneous measurements of frequencies, a
continuous precise monitoring of magnetic field fluctuations, a fast exchange
between different ions, and a highly sensitive cryogenic non-destructive
detection system. This paper gives a motivation for the new mass spectrometer
PENTATRAP, presents its experimental setup, and describes the present status
Atomic Hole Doping of Graphene
Graphene is an excellent candidate for the next generation of electronic
materials due to the strict two-dimensionality of its electronic structure as
well as the extremely high carrier mobility. A prerequisite for the development
of graphene based electronics is the reliable control of the type and density
of the charge carriers by external (gate) and internal (doping) means. While
gating has been successfully demonstrated for graphene flakes and epitaxial
graphene on silicon carbide, the development of reliable chemical doping
methods turns out to be a real challenge. In particular hole doping is an
unsolved issue. So far it has only been achieved with reactive molecular
adsorbates, which are largely incompatible with any device technology. Here we
show by angle-resolved photoemission spectroscopy that atomic doping of an
epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony
or gold presents effective means of p-type doping. Not only is the atomic
doping the method of choice for the internal control of the carrier density. In
combination with the intrinsic n-type character of epitaxial graphene on SiC,
the charge carriers can be tuned from electrons to holes, without affecting the
conical band structure
Strong polarization-induced reduction of addition energies in single-molecule nanojunctions
We address polarization-induced renormalization of molecular levels in
solid-state based single-molecule transistors and focus on an organic conjugate
molecule where a surprisingly large reduction of the addition energy has been
observed. We have developed a scheme that combines a self-consistent solution
of a quantum chemical calculation with a realistic description of the screening
environment. Our results indeed show a large reduction, and we explain this to
be a consequence of both (a) a reduction of the electrostatic molecular
charging energy and (b) polarization induced level shifts of the HOMO and LUMO
levels. Finally, we calculate the charge stability diagram and explain at a
qualitative level general features observed experimentally.Comment: 9 pages, 5 figure
Population redistribution in optically trapped polar molecules
We investigate the rovibrational population redistribution of polar molecules
in the electronic ground state induced by spontaneous emission and blackbody
radiation. As a model system we use optically trapped LiCs molecules formed by
photoassociation in an ultracold two-species gas. The population dynamics of
vibrational and rotational states is modeled using an ab-initio electric dipole
moment function and experimental potential energy curves. Comparison with the
evolution of the v"=3 electronic ground state yields good qualitative
agreement. The analysis provides important input to assess applications of
ultracold LiCs molecules in quantum simulation and ultracold chemistry.Comment: 6 pages, 5 figures, EPJD Topical issue on Cold Quantum Matter -
Achievements and Prospect
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