Electrical transport through a mechanically gated molecular wire

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.Comment: 31 pages, 10 figure

Quantum transport through STM-lifted single PTCDA molecules

Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip-molecule-surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip substrate distances show a clear shift and sharpening of the molecular chemisorption level on increasing the STM-surface distance, in agreement with experiment.Comment: accepted for publication in Applied Physics

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Effects of body position on autonomic regulation of cardiovascular function in young, healthy adults

Background: Analysis of rhythmic patterns embedded within beat-to-beat variations in heart rate (heart rate variability) is a tool used to assess the balance of cardiac autonomic nervous activity and may be predictive for prognosis of some medical conditions, such as myocardial infarction. It has also been used to evaluate the impact of manipulative therapeutics and body position on autonomic regulation of the cardiovascular system. However, few have compared cardiac autonomic activity in supine and prone positions, postures commonly assumed by patients in manual therapy. We intend to redress this deficiency. Methods: Heart rate, heart rate variability, and beat-to-beat blood pressure were measured in young, healthy non-smokers, during prone, supine, and sitting postures and with breathing paced at 0.25 Hz. Data were recorded for 5 minutes in each posture: Day 1 - prone and supine; Day 2 - prone and sitting. Paired t-tests or Wilcoxon signed-rank tests were used to evaluate posture-related differences in blood pressure, heart rate, and heart rate variability. Results: Prone versus supine: blood pressure and heart rate were significantly higher in the prone posture (p < 0.001). Prone versus sitting: blood pressure was higher and heart rate was lower in the prone posture (p < 0.05) and significant differences were found in some components of heart rate variability. Conclusion: Cardiac autonomic activity was not measurably different in prone and supine postures, but heart rate and blood pressure were. Although heart rate variability parameters indicated sympathetic dominance during sitting (supporting work of others), blood pressure was higher in the prone posture. These differences should be considered when autonomic regulation of cardiovascular function is studied in different postures

Chondrosarkom des Felsenbeins 25 Jahre nach Strahlentherapie

Einleitung: Das Auftreten einer malignen Neoplasie des Felsenbeins wird als Spätfolge der Radiotherapie eines malignen Tumors im Kinder-/Jugendalter angesehen.Methoden: Ein 42-jähriger Patient stellte sich mit einer progredienten rechtsseitigen Hörminderung sowie leichtem Schwindel vor. Ohrmikroskopisch imponierte hinter intaktem Trommelfell eine rötliche tumoröse Raumforderung aus dem Epitympanon in die Paukenhöhle hineinreichend. Der Patient wurde im Alter von 13 Jahren an einem Corpus pineale Tumor operiert und nachfolgend strahlentherapiert. In einer präoperativ angefertigten CT und MRT des Felsenbeins konnte eine ausgedehnte tumorassoziierte Veränderung des rechten Felsenbeins mit Ausdehnung in die mittlere und hintere Schädelgrube dargestellt werden. Die Raumforderung im Mittelohr wurde unter Lokalanästhesie biopsiert und nach histologischer Diagnosesicherung eines Sarkoms in einer interdisziplinären Operation durch die HNO und Neurochirurgie reseziert.Ergebnisse: Histopathologisch konnte das Resektat als Manifestation eines spindelzelligen, Vimentin-positiven Sarkoms, einem Chondrosarkom entsprechend, mit einer Proliferationsrate von circa 60% klassifiziert werden.Schlussfolgerung: Das Auftreten einer malignen Neoplasie im ehemaligen Strahlengebiet des Felsenbeins ist mit sehr hoher Wahrscheinlichkeit als Langzeitfolge der Strahlentherapie im Kinder- oder Jugendalter anzusehen. Das Risiko, eine derartige maligne Erkrankung zu entwickeln, ist in dieser Patientengruppe etwa fünffach höher als bei der Normalbevölkerung. In der Literatur findet man als häufigste Folgen von Strahlentherapie das Auftreten maligner Prozesse im ZNS gefolgt von Sarkomen und Leukämien.Der Erstautor gibt keinen Interessenkonflikt an