Svetlichny's inequality and genuine tripartite nonlocality in three-qubit pure states

The violation of the Svetlichny's inequality (SI) [Phys. Rev. D 35, 3066 (1987)] is sufficient but not necessary for genuine tripartite nonlocal correlations. Here we quantify the relationship between tripartite entanglement and the maximum expectation value of the Svetlichny operator (which is bounded from above by the inequality) for the two inequivalent subclasses of pure three-qubit states: the Greenberger-Horne-Zeilinger (GHZ) class and the W class. We show that the maximum for the GHZ-class states reduces to Mermin's inequality [Phys. Rev. Lett. 65, 1838 (1990)] modulo a constant factor, and although it is a function of the three tangle and the residual concurrence, large numbers of states do not violate the inequality. We further show that by design SI is more suitable as a measure of genuine tripartite nonlocality between the three qubits in the W-class states,and the maximum is a certain function of the bipartite entanglement (the concurrence) of the three reduced states, and only when their sum attains a certain threshold value do they violate the inequality.

Effect of disorder on the exponent in the coherence region in high temperature superconductors

Effect of disorder on the electrical resistance near the superconducting transition temperature in the paracoherence region of high temperature YBa2CU3O7-delta (YBCO) thin film superconductor is reported. For this, c-axis oriented YBa2Cu3O7-delta thin films having superconducting transition width varying between 0.27 K and 6 K were deposited using laser ablation and high pressure oxygen sputtering techniques. Disorder in these films was further created by using 100 MeV oxygen and 200 MeV silver ions with varying fluences. It is observed that the critical exponent in the paracoherence region for films with high transition temperature and small transition width is in agreement with the theoretically predicted value (gamma = 1.33) and is not affected by disorder, while for films with lower transition temperature and larger transition width the value of exponent is much larger as compared to that theoretically predicted and it varies from sample to sample and usually changes with disorder induced by radiation. This difference in the behaviour of the exponent has been explained on the basis of differences in the strength of weak links and the transition between temperatures T. and T, is interpreted as a percolation like transition with disorder. (c) 2006 Elsevier B.V. All rights reserved.

Novel features of rapidly quenched melts of Bi2(Ca,Sr) 3Cu2O8+δ

By roller quenching and water quenching melts of Bi2(Ca, Sr)3Cu2O8+δ, glasses have been obtained. These glasses exhibit two glass transitions as well as two crystallization transitions. Microwave absorption studies show the glass to be weakly superconducting at 77 K, probably due to the presence of ultramicrocrystallites. The glass on crystallization at 870 K gives the crystalline n=1 member of the homologous series Bi2(Ca, Sr)n+1CunO 2n+4 and the n=2 member on annealing at 1100 K. The glass route provides a unique means of obtaining the n=2 member of the series. On prolonged annealing of the glass at 1120 K, the n=3 member seems to be formed.

New family of thallium cuprate superconductors not containing calcium or barium: TlSrn+1−xLnxCunO2n+3+δ (Ln=La, Pr, or Nd)

The first two members of the new TlSrn+1−xLnxCunO2n+3+δ (Ln=La, Pr, or Nd) series of superconducting cuprates possessing 1021 and 1122 type structures are described. The n=1 (1021) members with Tcs around 40 K have electrons or holes as the majority charge carriers depending on x. The n=2 (1122) cuprate (Ln=Pr or Nd) shows a Tc in the 80–90 K range.

Room temperature ferromagnetism in ZnO (core)/graphite (shell) nanowires fabricated by a one-step method

ZnO (core)/graphitic (shell) nanowires were successfully fabricated by a one-step method. Morphology of the as-grown nanowires was studied in detail by scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDS). High resolution TEM micrographs and selected area electron diffraction patterns reveal the core/shell morphology of the nanowires that grew along the c-axis of ZnO. EDS study of the nanowires confirms that there are no impurities within the detectable limit. Superconducting quantum interference device magnetometer measurements show room temperature ferromagnetic ordering in these core/shell nanowires. (C) 2010 Elsevier Ltd. All rights reserved.

Observation of reduced activation energy and the possible existence of decoupled pancake vortices in superconductor/ferromagnet bilayers

Investigations of different superconducting (S)/ferromagnetic (F) heterostructures grown by pulsed laser deposition reveal that the activation energy (U) for the vortex motion in a high T-c superconductor is reduced remarkably by the presence of F layers. The U exhibits a logarithmic dependence on the applied magnetic field in the S/F bilayers suggesting the existence of decoupled two-dimensional (2D) pancake vortices. This result is discussed in terms of the reduction in the effective S layer thickness and the weakening of the S coherence length due to the presence of F layers. In addition, the U and the superconducting T-c in YBa2Cu3O7-delta/La0.5Sr0.5CoO3 bilayers are observed to be much lower than in the YBa2Cu3O7-delta/La0.7Sr0.3MnO3 ones. This in turn suggests that the degree of spin polarization of the F layer might not play a crucial role for the suppression of superconductivity due to a spin polarized induced pair-breaking effect in S/F bilayers.

Non-resonant microwave absorption evidence for intrinsic Josephson coupling in Tl2Ba2CaCu2O8 single crystals

We report the results of our non-resonant microwave absorption (NRMA) studies on single crystals of Tl2Ba2CaCu2O8 (Tl 2212) which reflect the occurrence of intrinsic Josephson coupling in these crystals. We have studied the magnetic field induced microwave absorption at various temperatures from 4.2K to T-c (similar to 104K) using a standard CW EPR spectrometer (H-dc parallel to c). We observe the appearance of a characteristic feature in the NRMA signals similar to the ones observed earlier by us in Bi2Sr2CaCu2O8 (Bi 2212) starting a few degrees below T-c, which on cooling passes through a maximum in intensity before disappearing at a further low temperature. This behaviour is attributed to the appearance, strengthening and disappearance of Josephson response consequent to the temperature dependence of the viscosity of the Josephson medium between the CuO2 superconducting sheets.

Study of the electronic structures of high Tc cuprate superconductors by electron energy loss and secondary electron emission spectroscopies

Energy loss spectra of superconducting YBa2Cu3O6.9' Bi1.5Pb0.5Ca2.5Sr1.5Cu3O10+δ and Tl2CaBa2Cu3O8 obtained at primary electron energies in the 170–310 eV range show features reflecting the commonalities in their electronic structures. The relative intensity of the plasmon peak shows a marked drop across the transition temperature. Secondary electron emission spectra of the cuprates also reveal some features of the electronic structure.

Electronegativities of constituent atoms and Tc of superconductors

In the superconducting state of high Tc oxides, it is possible to conceive that the mobility of the charge carrier pairs is a consequence of the absence of a net chemical force on them. On this assumption, we have examined a heuristic relation between Tc and a simple function of electronegativities of constituent atoms. We find that Tc varies approximately linearly with the fractional electronegativity of all cations considered together.

Investigation of novel cuprates of the TlCa1-xLnxSr2Cu2O7- δ (Ln=rare earth) series showing electron- or hole-superconductivity depending on the composition

Superconductivity in cuprates of the general formula TlCa1-xLnxSr2Cu2O7+ delta has been investigated as a function of Ln and x. Compositions with 0.25superconducting for all Ln with Tc in the range 60-90 K (except when Ln=Yb where x=1 is also superconducting); the value of the Tc varies with x as well as Ln. When x approximately=0.25, electrons are the majority charge carriers, while holes are the majority carrier when x >0.25. Thus, these cuprates exhibit a composition-dependent electron- or hole-superconductivity. In the normal state, most of the members of the series traverse compositionally determined metal-insulator transitions. High-energy spectroscopies show the presence of Cu in the 1+ and 2+ states. The Raman frequency around 525 cm-1 characteristic of the Tl-O2-Cu linkage is sensitive to both Ln and x, indicating a possible involvement in the mechanism of superconductivity.

A core-level photoemission spectroscopic study of the electron-doped superconductor, Nd2-xCexCuO4-δ

A photoemission study of superconducting Nd1.85Ce0.15CuO4-δ shows that Ce in the cuprate is essentially in the 4+ state. While the electron donated by Ce does not appear to affect the Cu 3d band, we still find evidence for the presence of considerable Cu1+ - related configurations due to covalency effects. A role for oxygen holes and Cu1+ species is indicated just as in other cuprate superconductors.

Reduction in magnetic field induced broadening of the resistive transition in laser‐deposited YBa2Cu3O7−x thin films on MgO

The magnetic field induced broadening of the normal to superconducting resistive transition of YBa2Cu3O7−x thin films laser deposited on (100) MgO substrates for field oriented parallel to the c axis is found to be significantly reduced in comparison with that found previously in single crystals and in films deposited on SrTiO3. This reduction in broadening is associated with a high density of defects which, while causing a slight decrease in Tc and an increase in the zero‐field transition width, seems to provide strong vortex pinning centers that reduce flux creep

Bi2−xPbx(Ca, Sr)n+1CunO2n+4+δ (n = 1, 2, 3, and 4) family of superconductors

Superconducting oxides of the Bi1.5Pb0.5(Ca, Sr)n+1CunO2n+4+δ series with n = 1, 2, 3 and 4 have been characterized. The superconducting transition temperature increases markedly with n up to n = 3, but the Tc of the n = 4 member is not much higher than that of the n = 3 member. The Tc does not change significantly in Bi2−xPbxCaSr2Cu2O8+δ with x (0.1 < x ≤ 0.5).

Electronic structure of high-Tc Ba0.6K0.4BiO3 by x-ray photoelectron spectroscopy

We have investigated the electronic structure of Ba1-xKxBiO3 (0superconducting composition a finite density of O(2p) states is observed near EF. Similarly, the Ba(5p) binding energies decrease with potassium doping, indicating increased metallicity. The behavior of the O(1s), Ba(5p), and the valence band resembles that of all the cuprate superconductors and we conclude that in all these oxide superconductors, a hole in the (filled) O(2p) band is the carrier responsible for superconductivity, which predicts hole conduction in the Ba-K-Bi-O and Ba-Pb-Bi-O systems.

An investigation of the PrBa2Cu3O7-δ system

Members of the PrBa2Cu3O7–delta system possessing the orthorhombic structure over a wide range of oxygen stoichiometry (delta=–0.5 to +0.5) have been prepared and characterized. Similar compositions with a tetragonal structure have also been prepared. None of the oxides is superconducting, independent of the structure or stoichiometry. Praseodymium seems to be present to a small extent in the 4+ state in oxygen-excess (negative delta) samples. Orthorhombic PrBa2Cu3O7–delta samples show the presence of twins suggesting that twins arise from orthorhombicity and have no relation to the superconductivity. A neutron diffraction study of a near-stoichiometric sample has shown a disordered orthorhombic structure with 72% occupancy of the 01 (chain) sites and with no interchange between Pr and Ba sites.