Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder system are studied by the QMC method. A single nonmagnetic impurity induces a localized spin-1/2 moment accompanied by "static" and enhanced AF correlations around it. Small and finite concentration of impurities induces a remarkable change of magnetic and thermodynamic properties with gapless excitations. It also shows rather sharp but continuous crossover around the concentration of about 4%. Above the crossover concentration, all the spins are strongly coupled participating in the enhanced and rather uniform power-law decay of the antiferromagnetic correlation. Below the crossover, each impurity forms an antiferromagnetic cluster only weakly coupled each other. For random distribution of impurities, large Curie-like susceptibility accompanied with small residual entropy is obtained at low temperatures in agreement with recent experimental observation in Zn-doped $SrCu_{2}O_{3}$. Temperature dependence of AF susceptibility shows power-law-like but weaker divergence than the single chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of AF susceptibility are adde

ΔNp73 antisense activates PUMA and induces apoptosis in neuroblastoma cells

The p73 gene codes for various different protein isoforms. They include proteins expressed under the control of the P1 promoter that contain a transactivation domain and are similar in function to p53 (TAp73 isoforms), as well as proteins regulated by the P2 promoter that lack this domain and function as dominant negative inhibitors of TAp73 and p53 (ΔNp73 isoforms). Whereas TAp73 functions as a tumor suppressor with pro-apoptotic function, ΔNp73 is likely to prevent the induction of apoptosis in tumor cells and to participate in oncogenesis. Here we used a loss-of-function strategy to assess the role of ΔNp73 in SH-SY5Y neuroblastoma cells. An antisense oligonucleotide designed to target ΔNp73 mRNA, but not TAp73, was used to effectively downregulate this transcript. ΔNp73 downregulation was accompanied by increased levels of the pro-apoptotic BH3 family member PUMA at the mRNA and protein level, and by conformational activation of BAX which translocated to mitochondria. These ΔNp73 antisense-mediated alterations led to the induction of apoptosis as detected by decreased cell viability, augmented DNA fragmentation and increased caspase-3 activity in cell lysates. Our results demonstrate the cytoprotective role of ΔNp73 in neuroblastoma and suggest its use as a target for molecular intervention therap

Andreev bound states and tunneling characteristics of a non-centrosymmetric superconductor

The tunneling characteristics of planar junctions between a normal metal and a non-centrosymmetric superconductor like CePt3Si are examined. It is shown that the superconducting phase with mixed parity can give rise to characteristic zero-bias anomalies in certain junction directions. The origin of these zero-bias anomalies are Andreev bound states at the interface. The tunneling characteristics for different directions allow to test the structure of the parity-mixed pairing state.Comment: 4 pages, 3 figure

Fano effect in a ring-dot system with tunable coupling

Transport measurements are presented on a quantum ring that is tunnel-coupled to a quantum dot. When the dot is in the Coulomb blockade regime, but strongly coupled to the open ring, Fano line shapes are observed in the current through the ring, when the electron number in the dot changes by one. The symmetry of the Fano resonances is found to depend on the magnetic flux penetrating the area of the ring and on the strength of the ring-dot coupling. At temperatures above T=0.65 K the Fano effect disappears while the Aharonov-Bohm interference in the ring persists up to T=4.2 K. Good agreement is found between these experimental observations and a single channel scattering matrix model including decoherence in the dot.Comment: 9 pages, 6 figure

Heavy fermion superconductivity and magnetic order in non-centrosymmetric CePt3SiCePt_3Si

$\rm CePt_3Si$ is a novel heavy fermion superconductor, crystallising in the $\rm CePt_3B$ structure as a tetragonally distorted low symmetry variant of the $\rm AuCu_3$ structure type. $\rm CePt_3Si$ exhibits antiferromagnetic order at $T_N \approx 2.2$ K and enters into a heavy fermion superconducting state at $T_c \approx 0.75$ K. Large values of $H_{c2}' \approx -8.5$ T/K and $H_{c2}(0) \approx 5$ T refer to heavy quasiparticles forming Cooper pairs. Hitherto, $\rm CePt_3Si$ is the first heavy fermion superconductor without a center of symmetry.Comment: 4 pages, 4 figure

Phase-Sensitive Tetracrystal Pairing-Symmetry Measurements and Broken Time-Reversal Symmetry States of High Tc Superconductors

A detailed analysis of the symmetric tetracrystal geometry used in phase-sensitive pairing symmetry experiments on high Tc superconductors is carried out for both bulk and surface time-reversal symmetry-breaking states, such as the d+id' and d+is states. The results depend critically on the substrate geometry. In the general case, for the bulk d+id' (or d+is) state, the measured flux quantization should in general not be too different from that obtained in the pure d-wave case, provided |d'| << |d| (or |s| << |d|). However, in one particular high symmetry geometry, the d+id' state gives results that allow it to be distinguished from the pure d and the d + is states. Results are also given for the cases where surface d+is or d+id' states occur at a [110] surface of a bulk d-wave superconductor. Remarkably, in the highest symmetry geometry, a number of the broken time-reversal symmetry states discussed above give flux quantization conditions usually associated with states not having broken time- reversal symmetry.Comment: 6 page

Nonlinear Magneto-Optical Response of ss- and dd-Wave Superconductors

The nonlinear magneto-optical response of $s$- and $d$-wave superconductors is discussed. We carry out the symmetry analysis of the nonlinear magneto-optical susceptibility in the superconducting state. Due to the surface sensitivity of the nonlinear optical response for systems with bulk inversion symmetry, we perform a group theoretical classification of the superconducting order parameter close to a surface. For the first time, the mixing of singlet and triplet pairing states induced by spin-orbit coupling is systematically taken into account. We show that the interference of singlet and triplet pairing states leads to an observable contribution of the nonlinear magneto-optical Kerr effect. This effect is not only sensitive to the anisotropy of the gap function but also to the symmetry itself. In view of the current discussion of the order parameter symmetry of High-T$_c$ superconductors, results for a tetragonal system with bulk singlet pairing for various pairing symmetries are discussed.Comment: 21 pages (REVTeX) with 8 figures (Postscript

Inhomogeneously doped two-leg ladder systems

A chemical potential difference between the legs of a two-leg ladder is found to be harmful for Cooper pairing. The instability of superconductivity in such systems is analyzed by compairing results of various analytical and numerical methods. Within a strong coupling approach for the t-J model, supplemented by exact numerical diagonalization, hole binding is found unstable beyond a finite, critical chemical potential difference. The spinon-holon mean field theory for the t-J model shows a clear reduction of the the BCS gaps upon increasing the chemical potential difference leading to a breakdown of superconductivity. Based on a renormalization group approach and Abelian bosonization, the doping dependent phase diagram for the weakly interacting Hubbard model with different chemical potentials was determined.Comment: Revtex4, 11 pages, 7 figure

Unc13A and Unc13B contribute to the decoding of distinct sensory information in Drosophila

The physical distance between presynaptic Ca2+ channels and the Ca2+ sensors triggering the release of neurotransmitter-containing vesicles regulates short-term plasticity (STP). While STP is highly diversified across synapse types, the computational and behavioral relevance of this diversity remains unclear. In the Drosophila brain, at nanoscale level, we can distinguish distinct coupling distances between Ca2+ channels and the (m)unc13 family priming factors, Unc13A and Unc13B. Importantly, coupling distance defines release components with distinct STP characteristics. Here, we show that while Unc13A and Unc13B both contribute to synaptic signalling, they play distinct roles in neural decoding of olfactory information at excitatory projection neuron (ePN) output synapses. Unc13A clusters closer to Ca2+ channels than Unc13B, specifically promoting fast phasic signal transfer. Reduction of Unc13A in ePNs attenuates responses to both aversive and appetitive stimuli, while reduction of Unc13B provokes a general shift towards appetitive values. Collectively, we provide direct genetic evidence that release components of distinct nanoscopic coupling distances differentially control STP to play distinct roles in neural decoding of sensory information