Ultrafast generation and decay of a surface metal.

Band bending at semiconductor surfaces induced by chemical doping or electric fields can create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of such metallic surfaces on ultrafast timescales would be appealing for high-speed electronics. Here, we demonstrate the ultrafast generation of a metal at the (10-10) surface of ZnO upon photoexcitation. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions that occur in the bulk of inorganic semiconductors, the metallization of the ZnO surface is launched by 3-4 orders of magnitude lower photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we show that the phase transition is caused by photoinduced downward surface band bending due to photodepletion of donor-type deep surface defects. The discovered mechanism is in analogy to chemical doping of semiconductor surfaces and presents a general route for controlling surface-confined metallicity on ultrafast timescales.

Uncovering the (un-)occupied electronic structure of a buried hybrid interface.

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p -quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1 0 -1 0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level E F. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above E F, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation, this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces.

Grand unification of quark and lepton flavor changing neutral currents.

In the context of supersymmetric grand unified theories with soft breaking terms arising at the Planck scale, it is generally possible to link flavor changing neutral current and CP violating processes occurring in the leptonic and hadronic sectors. We study the correlation between flavor changing squark and slepton mass insertions in models à la SU(5). We show that the constraints coming from lepton flavor violation exhibit a strong impact on CP-violating B decays.

Decreased heart-period variability in patients with panic disorder: a study of Holter ECG records.

This study investigated cardiac autonomic function in patients with panic disorder and normal controls using Holter ECG records. A decrease in ultra-low frequency power (< 0.0033 Hz) is known to be associated with an increased risk for cardiovascular mortality in humans. Twenty-four-hour ECG was recorded in 29 patients with panic disorder and 23 normal controls using Holter records. Data for 20 h and also 20000 s of awake and 20000 s of sleep periods were analyzed using spectral analysis to quantify absolute and relative heart-period variability in ultra low (ULF: < 0.0033 Hz), very low (VLF: 0.0033-0.04 Hz), low (LF: 0.04-0.15 Hz) and high (HF: 0.15-0.5 Hz) frequency bands. The patients with panic disorder had significantly lower total and absolute ULF power, which was more pronounced during sleep. The patients also had significantly lower relative ULF power and significantly higher relative LF power during sleep. There was a significant increase of relative LF power from awake to sleep period only in the patient group. The decrease in total and ULF power may increase the risk of mortality and sudden arrhythmic death in patients with panic disorder if they experience a cardiac event. The higher relative LF power during sleep also suggests a possible higher sympathetic drive in the patients during sleep.

Fractal dimension of heart rate time series: an effective measure of autonomic function.

Previous studies suggested that heart rate (HR) time series may be more appropriately analyzed by nonlinear techniques because of the nonlinear nature of these data. In this study, we quantified the complexity of the HR time series, using fractal dimension, a previously described measure developed to study axonal growth, which quantifies the space-filling propensity and convolutedness of a waveform, and compared these results with another recently used measure, approximate entropy. Fractal dimension and approximate entropy of HR time series (unfiltered) correlate highly with each other and also with the high-frequency power (0.2-0.5 Hz) and, hence, appear to reflect vagal modulation of HR variability. These measures were also statistically more consistent and effective than measures of spectral analysis. Fractal dimension of the midfrequency time series of HR (filtered with a pass band of 0.05-0.15 Hz) also appears to be a statistically effective measure of relative sympathetic activity, especially in the standing posture.