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Biomedical subjects

H Berger

Publications and source records attributed to H Berger.

At least 19 recordsLinked to original sources

Early endothelial and haematological response to cryoplasty compared with balloon angioplasty of the superficial femoral artery--a pilot study.

The purpose of the present study was to assess the course of adhesion molecules (intercellular adhesion molecule (ICAM), vascular cell adhesion molecule (VCAM), e-selectin, p-selectin and monocyte chemoatlractant protein 1 (MCP-1)), growth factors (transforming growth factor beta (TGFbeta) and basic fibroblast growth factor (bFGF)) and the cytokine tumour necrosis factor alpha (TNFalpha) after both angioplasty and cryoplasty. Recently cryoplasty has been suggested as a new method to oppose neointimal hyperplasia resulting in restenosis formation. While in vitro models have shown that the application of cryothermal energy to the endothelium during angioplasty leads to apoptosis induction and reduced proliferation rates, no human in vivo proof for an inhibition of neointimal hyperplasia exists. For restenosis initiation adhesion molecules, growth factors and cytokines play an important role. One possibility to investigate the endothelial response to angioplasty is the measurement of the soluble forms of adhesion molecules, growth factors and cytokines that are released into the circulation after denuding the vessel wall. In the present study we assessed the distribution pattern of the soluble forms of e-selectin, p-selectin, ICAM, VCAM, MCP-1, TGFbeta, bFGF and TNFalpha after angiography, angioplasty and cryoplasty of the femoropopliteal artery in the early course of 4 weeks in 29 patients with peripheral arterial occlusive disease. During the 4 weeks after intervention levels of e-selectin, ICAM, VCAM and MCP-1 increased after both angioplasty and cryoplasty. The course of the screened biomarkers was similar between angioplasty and cryoplasty. P-selectin and TGFbeta both decreased after cryoplasty, but not significantly. The present results show that the release of adhesion molecules, growth factors and cytokines is similar between balloon angioplasty and cryoplasty.

Aged↗

Evidence that corticotropin-releasing factor receptor type 1 couples to Gs- and Gi-proteins through different conformations of its J-domain.

BACKGROUND AND PURPOSE: According to the two-domain model for the corticotropin-releasing factor receptor type 1 (CRF(1)), peptide antagonists bind to the N-terminal domain (N-domain), non-peptide antagonists to the transmembrane region (J-domain), whereas peptide agonists attach to both the N- and J-domain of the receptor to express activity. The aim of this study was to search for possible differences in the antagonism of the Gs- and Gi-protein coupling of CRF(1) by a peptide (alpha-helical CRF(9-41)) and non-peptide antagonist (antalarmin), to determine whether the conformational requirements of the activated CRF(1) states for Gs and Gi coupling are similar or different. EXPERIMENTAL APPROACH: We studied the inhibitory effect of alpha-helical CRF(9-41) and antalarmin on the coupling of CRF(1) to Gs- and Gi-protein in human embryonic kidney cells, using the [(35)S]-GTPgammaS binding stimulation assay. KEY RESULTS: The non-peptide antagonized the receptor coupling to Gs competitively but that to Gi noncompetitively, and its antagonistic potency was different for urocortin- and sauvagine-evoked G-protein activation. In contrast, the peptide antagonist exhibited uniformly competitive antagonism. CONCLUSIONS AND IMPLICATIONS: The results allow us to extend the two-domain model of CRF(1) activation by assuming that CRF(1) agonists activate the receptor by binding to at least two ensembles of J-domain configurations which couple to Gs or Gi, that are in turn antagonized by a non-peptide antagonist competitively and allosterically, respectively. It is further concluded that the allosteric mechanism of non-peptide antagonism is not valid for the Gs-mediated physiological activities of CRF(1).

Allosteric Regulation↗

Stripe correlations in Na(0.75)CoO2.

We present a combined high-energy x-ray diffraction and local-density approximation study of the sodium ordering in Na(0.75)CoO2. The obtained results rule out previously proposed Na-ordering models and provide strong evidence for the formation of sodium-density stripes in this material. The local-density approximation calculations prove that the sodium-density stripes lead to a sizable dip in the density of the Co states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripelike charge correlations within the CoO2 layers, leading to an astonishing similarity between the doped cuprates and the NaxCoO2 compounds.

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Time evolution of the electronic structure of 1T-TaS2 through the insulator-metal transition.

Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.

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Probing the superfluid velocity with a superconducting tip: the Doppler shift effect.

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing scanning tunneling spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle (QP) spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe(2), reveals both the vortex cores, on the scale of the SC coherence length xi, and the supercurrents, on the scale of the London penetration length lambda. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.

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Constituents of the quasiparticle spectrum along the nodal direction of high-Tc cuprates.

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in detail the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects such as bilayer splitting, diffraction replicas, etc. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below some doping dependent temperature and sharpening below Tc, and is mainly responsible for the formation of the kink in question.

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Separation of orbital contributions to the optical conductivity of BaVS(3).

The correlation-driven metal-insulator transition (MIT) of BaVS(3) was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: the quasi-1D metallic transport of A(1g) electrons is superimposed on the isotropic hopping conduction of localized E(g) electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass m(eff) approximately 7 m(e) and scattering rate Gamma > or = 160 meV of the quasiparticles in the A(1g) band. There is a pseudogap above T(MI) = 69 K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with Delta(ch) approximately 42 meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.

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Experimental proof of a structural origin for the shadow fermi surface of Bi2Sr2CaCu2O8+delta.

By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi(2)Sr(2)CaCu(2)O(8+delta) with structural information from LEED and ab initio one-step photoemission simulations, we prove that the shadow Fermi surface in these systems is of structural origin, being due to orthorhombic distortions from tetragonal symmetry present both in surface and bulk. Consequently, one of the longest standing open issues in the investigation of the Fermi surface of these widely studied systems finally meets its resolution.

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Parity of the pairing bosons in a high-temperature Pb-Bi2Sr2CaCu2O8 bilayer superconductor by angle-resolved photoemission spectroscopy.

We report the observation of a novel effect in the bilayer Pb-Bi2Sr2CaCu2O8 (Pb-Bi2212) high-T(c) superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favors the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the k space, suggesting a magnetic nature of the pairing mediator.

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Effect of Zn and Ni impurities on the quasiparticle renormalization of superconducting Bi-2212.

The Cu substitution by Zn and Ni impurities and its influence on the mass renormalization effects in angle-resolved photoelectron spectra (ARPES) of Bi2Sr2CaCu2O8-delta is addressed. We show that the nonmagnetic Zn atoms have a much stronger effect in both the nodal and antinodal parts of the Brillouin zone than magnetic Ni. The observed changes are consistent with the behavior of the spin resonance mode as seen by inelastic neutron scattering in YBCO. This strongly suggests that the "peak-dip-hump" and the kink in ARPES on the one side and neutron resonance on the other are closely related features.

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Prolonged exposure to angiotensin-converting enzyme inhibitors during pregnancy. Fetal toxicity could be reversible.

QUESTION: I read in a Motherisk Update that angiotensin-converting enzyme (ACE) inhibitors are contraindicated during pregnancy. Many women, however, do not know they are pregnant for quite some time after conception. One of my patients was taking ACE inhibitors for 3 to 4 months while she was pregnant. How should I advise her? ANSWER: The deleterious effects ACE inhibitors have on fetuses were seen only after exposure during the second and third trimesters and were mostly secondary to renal damage. These effects can be reversed, as described in this Motherisk Update.

Adult↗

Incommensurate magnetic ordering in Cu2Te2O5X2 (X = Cl,Br) studied by neutron diffraction.

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.

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Manifestation of the magnetic resonance mode in the nodal quasiparticle lifetime of the superconducting cuprates.

Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on applicability of a phonon-mediated pairing mechanism to high-temperature superconductors.

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Circular dichroism in angle-resolved photoemission spectra of under- and overdoped Pb-Bi2212.

We use angle-resolved photoemission with circularly polarized excitation to demonstrate that in the 5 x 1 superstructure-free (Pb,Bi)(2)Sr(2)CaCu(2)O(8+delta) (Pb-Bi2212) material there are no signatures of time-reversal symmetry breaking in the sense of the criteria developed earlier [Nature (London) 416, 610 (2002)]]. The dichroic signal retains reflection antisymmetry as a function of temperature and doping and in all mirror planes, precisely defined by the experimental dispersion at low energies. The obtained results demonstrate that the signatures of time-reversal symmetry violation in pristine Bi2212, as determined by angle-resolved photoemission spectroscopy, are not a universal feature of all cuprate superconductors.

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Low energy electronic excitations in the layered cuprates studied by copper L3 resonant inelastic x-ray scattering.

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.

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Quasiparticle spectra, charge-density waves, superconductivity, and electron-phonon coupling in 2H-NbSe2.

High-resolution photoemission has been used to study the electronic structure of the charge-density wave (CDW) and superconducting dichalcogenide, 2H-NbSe2. From the extracted self-energies, important components of the quasiparticle interactions have been identified. In contrast to previously studied TaSe2, the CDW transition does not affect the electronic properties significantly. The electron-phonon coupling is identified as a dominant contribution to the quasiparticle self-energy and is shown to be very anisotropic (k dependent) and much stronger than in TaSe2.

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Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2.

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.

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Doping dependence of the mass enhancement in (Pb,Bi)2Sr2CaCu2O8 at the antinodal point in the superconducting and normal states.

Angle-resolved photoemission spectroscopy is used to study the mass renormalization of the charge carriers in the high-T(c) superconductor (Pb,Bi)2Sr2CaCu2O8 in the vicinity of the (pi,0) point in the superconducting and the normal states. Using matrix element effects at different photon energies and due to a high momentum and energy resolution the bonding and the antibonding bands could be separated in the whole dopant range. A huge coupling to a bosonic collective mode is observed below T(c) for both bands, in particular, for the underdoped case. Above T(c), a weaker coupling to a continuous spectrum of modes is detected.

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