Systematic X-ray absorption study of hole doping in BSCCO - phases

X-ray absorption spectroscopy (XAS) on the O 1s threshold was applied to Bi-based, single crystalline high temperature superconductors (HTc's), whose hole densities in the CuO2 planes was varied by different methods. XAS gives the intensity of the so-called pre-peak of the O 1s line due to the unoccupied part of the Zhang-Rice (ZR) singlet state. The effects of variation of the number n of CuO2 - planes per unit cell (n = 1,2,3) and the effect of La-substitution for Sr for the n = 1 and n = 2 phase were studied systematically. Furthermore the symmetry of the states could be probed by the polarization of the impinging radiation.Comment: 4 pages, 2 figures, to appear in the proceedings of SCES2001, Ann Arbor, August 6-10, 200

Analysis of the serotonergic system in a mouse model of Rett syndrome reveals unusual upregulation of serotonin receptor 5b

Mutations in the transcription factor methyl-CpG-binding-protein 2 (MeCP2) cause a delayed-onset neurodevelopmental disorder known as Rett syndrome (RTT). Although alteration in serotonin levels have been reported in RTT patients, the molecular mechanisms underlying these defects are not well understood. Therefore, we chose to investigate the serotonergic system in hippocampus and brainstem of male Mecp2(-/y) knock-out mice in the B6.129P2(C)-Mecp2(tm1.1Bird) mouse model of RTT. The serotonergic system in mouse is comprised of 16 genes, whose mRNA expression profile was analyzed by quantitative RT-PCR. Mecp2(-/y) mice are an established animal model for RTT displaying most of the cognitive and physical impairments of human patients and the selected areas receive significant modulation through serotonin. Using anatomically and functional characterized areas, we found region-specific differential expression between wild type and Mecp2(-/y) mice at post-natal day 40. In brainstem, we found five genes to be dysregulated, while in hippocampus, two genes were dysregulated. The one gene dysregulated in both brain regions was dopamine decarboxylase, but of special interest is the serotonin receptor 5b (5-ht(5b)), which showed 75-fold dysregulation in brainstem of Mecp2(-/y) mice. This dysregulation was not due to upregulation, but due to failure of down-regulation in Mecp2(-/y) mice during development. Detailed analysis of 5-ht(5b) revealed a receptor that localizes to endosomes and interacts with G(αi) proteins

Remnant Fermi surface in the presence of an underlying instability in layered 1T-TaS_2

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS_2 in the quasi commensurate (QC) and the commensurate (C) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS even when passing from the QC-phase to the C-phase. Possibly, this Mott localization induced transition represents the underlying instability responsible for the pseudogapped FS

Novel Fine-Structure in the Low-Energy Excitation Spectrum of a High-Tc Superconductor by Polarization Dependent Photoemission

Angle-resolved photoemission spectroscopy is performed on single crystals of the single-layer high-Tc superconductor Bi(2)Sr(2-x)La(x)CuO(6+d) at optimal doping (x=0.4) in order to study in great detail the Zhang-Rice (ZR) singlet band at the Fermi level. Besides the high crystal quality the advantages of a single-layer material are the absence of bilayer effects and the distinct reduction of thermal broadening. Due to the high energy and angle resolution and, most important, due to the controlled variation of the polarization vector of the synchrotron radiation the emission from the ZR singlet band reveals a distinct fine-structure. It consists of two maxima, the first showing only weak and the second at EF extremely strong polarization dependence. However, our observation has enormous consequences for line shape analyses and the determination of pseudo gaps by photoemission.Comment: 10 pages, 2 figures. to appear in PRB (Rapid Comm.

Detectability of motions in AAA with ECG-gated CTA: A quantitative study

Purpose: ECG-gated CT enables the visualization of motions caused by the beating of the heart. Although ECG gating is frequently used in cardiac CT imaging, this technique is also very promising for evaluating vessel wall motion of the aortic artery and the motions of (stent grafts inside) abdominal aortic aneurysms (AAA). Late stent graft failure is a serious complication in endovascular repair of aortic aneurysms. Better understanding of the motion characteristics of stent grafts will be beneficial for designing future devices. In addition, these data can be valuable in predicting stent graft failure in patients. To be able to reliably quantify the motion, however, it is of importance to know the performance and limitations of ECG gating, especially when the motions are small, as is the case in AAA. Since the details of the reconstruction algorithms are proprietary information on the CT manufacturers and not in the public domain, empirical experiments are required. The goal of this study is to investigate as to what extent the motions in AAA can be measured using ECG-gated CT. The authors quantitatively investigate four aspects of motion in ECG-gated CT: The detectability of the motion of objects at different amplitudes and different periodic motions, the temporal resolution, and the volume gaps that occur as a function of heart rate.\ud \ud Methods: They designed an experiment on a standard static phantom to empirically determine temporal resolution. To investigate motion amplitude and frequency, as well as patient heart rate, they designed dynamic experiments in which a home-made phantom driven by a motion unit moves in a predetermined pattern.\ud \ud Results: The duration of each ECG-gated phase was found to be 185 ms, which corresponds to half of the rotation time and is thus in accordance with half scan reconstruction applied by the scanner. By using subpixel localization, motions become detectable from amplitudes of as small as 0.4 mm in the x direction and 0.7 mm in the z direction. With the rotation time used in this study, motions up to 2.7 Hz can be reliably detected. The reconstruction algorithm fills volume gaps with noisy data using interpolation, but objects within these gaps remain hidden.\ud \ud Conclusions: This study gives insight into the possibilities and limitations for measuring small motions using ECG-gated CT. Application of the experimental method is not restricted to the CT scanner of a single manufacturer. From the results, they conclude that ECG-gated CTA is a suitable technique for studying the expected motions of the stent graft and vessel wall in AAA.\u

Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe2_2

High-resolution angle-resolved photoemission (ARPES) data show that a metal-insulator Mott transition occurs at the surface of the quasi-two dimensional compound TaSe$_2$. The transition is driven by the narrowing of the Ta $5d$ band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.Comment: 4 pages, 4 figures; one modified figure, added referenc

Energetic and spatial bonding properties from angular distributions of ultraviolet photoelectrons: application to the GaAs(110) surface

Angle-resolved ultraviolet photoemission spectra are interpreted by combining the energetics and spatial properties of the contributing states. One-step calculations are in excellent agreement with new azimuthal experimental data for GaAs(110). Strong variations caused by the dispersion of the surface bands permit an accurate mapping of the electronic structure. The delocalization of the valence states is discussed analogous to photoelectron diffraction. The spatial origin of the electrons is determined, and found to be strongly energy dependent, with uv excitation probing the bonding region.Comment: 5 pages, 3 figures, submitted for publicatio

Condensation of Excitons in Cu2O at Ultracold Temperatures: Experiment and Theory

We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under cw-excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose-Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose-Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.Comment: 41 pages, 23 figure

Luttinger liquid phenomenology and angle resolved photoemission for single layer \chem{Bi_2Sr_{2-x}La_xCuO_{6+\delta}} high--temperature superconductor

Recently observed splitting in angular resolved photoemission spectroscopy (ARPES) on \chem{Bi_2Sr_{2-x}La_xCuO_{6+\delta}} high--temperature superconductor (Janowitz C. {\it et al.}, {\it Europhys. Lett.}, {\bf 60} (2002) 615) is interpreted within the phenomenological Luttinger--liquid framework, in which both the non--Fermi liquid scaling exponent of the spectral function and the spin--charge separation are introduced. The anomalous Green function with adjustable parameters fits very well to the Fermi edge and the low--energy part of ARPES along the $\Gamma-M$ line in the Brillouin zone. In contrast to one--dimensional models with Luttinger--liquid behavior we find that both the anomalous scaling $\alpha$ and the parameter $\delta$ describing the spin--charge separation are momentum dependent. The higher--energy part of the spectra is not accounted for by this simple Luttinger--liquid form of the Green function. In this energy regime additional scattering processes are plausible to produce the experimentally observed wide incoherent background, which diminishes as the inverse of the energy.Comment: 7 pages, 3 figures, EPL styl