Coexistence of para and ferromagnetic phases of Fe3+ in undoped CdZnTe (Zn similar to 4%) crystals

The signatures of the coexistence of para and ferromagnetic phases for the Fe3+ charge state of iron have been identified in the low temperature electron spin resonance (ESR) spectra in undoped CdZnTe (Zn similar to 4%) crystals and independently verified by superconducting quantum interference device (SQUID) and AC susceptibility measurements. In the paramagnetic phase the inverse of AC susceptibility follows the Curie-Weiss law. In the ferromagnetic phase the thermal evolution of magnetization follows the well-known Bloch T-3/2 law. This is further supported by the appearance of hysteresis in the SQUID measurements at 2 K below T-c which is expected to lie in between 2 and 2.5 K. (C) 2010 Elsevier Ltd. All rights reserved.

Chemical Shift of the X-Ray K-Absorption Edge of Cu in Some of Its Compounds, Complexes and Superconductors

Chemical shifts, ΔE, of the X-ray K-absorption edge in several compounds, complexes of copper including its superconducting oxides possessing formal oxidation states +1 and +2 have been measured. It has been shown that the chemical shift is primarily governed by the effective ionic charge on the absorbing ion and the nature of the atoms in the first coordination shell around the absorbing ion. The relation between the chemical shift, ΔE , and the effective charge q on the absorbing ion is found to be ΔE=Aq+Bq2+Cq3+Dq4 (A, B, C and D are constants). The effects of electronegativity, atomic number, oxidation state, crystal structure, the valence d-orbital electrons, etc. on the X-ray absorption chemical shift have been discussed. ©1990 The Physical Society of Japan

Magnetic properties of pure, Sr- and Ca-Doped La2NiO4+δ ceramics: Onset of high-Tc superconductivity

We present the results for the temperature and field dependence of the magnetic for ceramic materials of the composition La2−xMxNiO4, with M=Sr or Ca and 0≤x≤0.4. The onset of a strong diamagnetism has been observed at temperatures between 8 and 70 K, depending on sample composition, annealing conditions. and thermal cycling procedures. The results are similar to those obtained earlier for monocrystalline samples and are likewise interpreted as due to the onset of superconductivity in a minority phase. A comparison with the results for superconducting La1.8Sr0.2Cu0.9Ni0.1O4 ceramics is also made; this illustrates some unique features of the nickelate systems, such as the high values of the critical fields Hc1 and Hc2. The differences between monocrystalline and ceramic systems are also discussed.

An automated thermal relaxation calorimeter for operation at low temperature (0.5 K

We describe an automated calorimeter for measurement of specific heat in the temperature range 10 K>T>0.5 K. It uses sample of moderate size (100–1000 mg), has a moderate precision and accuracy (2%–5%), is easy to operate and the measurements can be done quickly with He4 economy. The accuracy of this calorimeter was checked by measurement of specific heat of copper and that of aluminium near its superconducting transition temperature.

Effect of hole localization on Madelung potential of YBa2Cu3O7

The Madelung potential and formation energy of the superconducting compound YBa2Cu3O7 have been computed for hole localization at different sites in the crystal. The cases considered include Cu3+ ion at Cu(1) and Cu(2) sites, O− ion at O(1), O(2), O(3) and O(4) sites and combinations of O− and Cu3+ ions at O(4) and Cu(1) and O(2,3) and Cu(2) sites. The two lowest-energy configurations correspond to Cu3+ ion at Cu(1) site and O− ion at O(4) site. The difference in formation energy between those configurations is relatively small. The next preferred configuration corresponds to simultaneous partial localization of the hole at Cu (1) site and O(1) site. Other configurations are much less stable. The results suggest a resonating or fluctuating valence model for YBa2Cu3O7.

Two-Fermi liquid model for high Tc superconductivity involving suppression of tunnelling by decoherence

We consider a model system of two interacting Fermi-liquids, one of which is light and the other much heavier. In the normal state the lighter component provides a quantum mechanical bath coupled 'ohmically' to the heavier component in the sense of Caldeira and Leggett, suppressing thereby the band (tunnelling) matrix elements of the heavier component. Thus we lose the energy of delocalization. On the other hand, a superconducting ordering stiffens the bath spectral function at low energies and so restores the tunnelling. The resulting regain of the delocalization energy bootstraps so as to stabilize the superconducting order that caused it. It is conceivable that the motions parallel to the easy ab-plane and along the hard c-axis may also effectively correspond to the light and the heavy Fermi-liquids, respectively.

Epitaxial metallic LaNiO3 thin films grown by pulsed laser deposition

Epitaxial LaNiO3(LNO) thin films on LaAlO3(LAO), SrTiO3(STO), and YSZ are grown by pulsed laser deposition method at 350 mTorr oxygen partial pressure and 700 °C substrate temperature. As‐deposited LNO films are metallic down to 10 K. c‐axis oriented YBa2Cu3O7 (YBCO) films were grown on LNO/LAO as well as LNO/STO surfaces without affecting superconducting transition temperature of YBCO. Textured LNO thin films were grown on c‐axis oriented YBCO/STO and YBCO/YSZ . Transport measurements of these bilayer films showed that LNO is a good metallic contact material for YBCO.

Plasma diagnostics of the high pressure oxygen-sputtering process

The characteristics of the high pressure oxygen-sputtering plasma in the pressure range 0.8–2.4 mbar have been studied using the Langmuir probe technique. The variation in plasma parameters such as positive ion density, electron density, mean electron energy and floating potential with pressure and temperature has been investigated. It has been observed that the positive ion density increases at high substrate temperatures whereas the negative ion density decreases. The study of the variation in mean electron energy and floating potential also indicated the possibility that the number of negative ions is less when the substrates are at elevated temperatures. Since the negative ions are supposed to cause re-sputtering and make the films off-stoichiometric, the reduction in the negative ion density as observed at elevated substrate temperatures is better suited for depositing stoichiometric YBa2Cu3O7−δ superconducting thin films.

Structure and properties of Tl0.5Pb0.5Sr2Gd2−xCexCu2O9−δ

A systematic study of the Tl0.5Pb0.5Sr2Gd2−xCexCu2O9−δ system has revealed the existence of a pure phase in the compositional. range 0.0≤x≤0.6 crystalizzing in the 1222 structure. It has an intersheet distance of approximately 6 Å, a value much higher than those found in other cuprates with double CuO2 sheets interleaved by a single fluorite layer. Superconductivity has been observed in the range 0.1≤x≤0.4 with a Tc of 45 K and a superconductive volume fraction up to 20% for the optimal composition. An interesting variation of the superconducting properties of the above system with the composition, i.e. cerium content, has also been noticed. A possible dependence of superconductivity on the coupling between CuO2 sheets in the layered cuprates has been pointed out to bring out a correlation between structure and properties.

Evidence for decoupled two-dimensional vortex behavior of YBa2Cu3O7-delta in La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 trilayer

We investigate the vortex behavior of YBa2Cu3O7-delta thin films sandwiched between two ferromagnetic layers (La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3). The magnetization study on La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 trilayers conspicuously shows the presence of both ferromagnetic and diamagnetic phases. The magnetotransport study on the trilayers reveals a significant reduction in the activation energy (U) for the vortex motion in YBa2Cu3O7-delta. Besides, the ``U'' exhibits a logarithmic dependence on the applied magnetic field which directly indicates the existence of decoupled two-dimensional (2D) pancake vortices present in the CuO2 layers. The evidence of 2D decoupled vortex behavior in La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3 is believed to arise from (a) the weakening of superconducting coherence length along the c-axis and (b) enhanced intraplane vortex-vortex interaction due to the presence of ferromagnetic layers. (C) 2010 American Institute of Physics. doi: 10.1063/1.3524545]

Superconductivity in rapidly quenched metallic systems with nanoscale structure

Nanocomposites of Al?In, Al?Pb, and Zn?Pb have been prepared and characterized using rapid quenching techniques and the nature of superconducting transitions in them has been studied by resistivity measurements. The precipitated second phases (In and Pb) have particle sizes (d) of a few tens of nanometers such that ?0?d?dmin, where ?0 is the superconducting zero temperature coherence length and dmin is the minimum particle size that supports superconductivity. The onset of superconductivity generally starts in samples with d??0 and progressively other grains with d??0 become superconducting. We suggest that the proximity effect of the matrix plays a significant role. In an Al?In system, even with 40?wt.% In, the zero resistivity state is obtained at T?1.33 times the Tc of Al. But in Al?Pb and Zn?Pb, the zero resistivity state is obtained at T?4 and 5 times the Tc of Al and Zn with only 10�15 wt?% Pb, respectively.

Phenomenological Ginzburg-Landau-like theory for superconductivity in the cuprates

We propose and develop here a phenomenological Ginzburg-Landau-like theory of cuprate high-temperature superconductivity. The free energy of a cuprate superconductor is expressed as a functional F of the complex spin-singlet pair amplitude psi(ij) equivalent to psi(m) = Delta(m) exp(i phi(m)), where i and j are nearest-neighbor sites of the square planar Cu lattice in which the superconductivity is believed to primarily reside, and m labels the site located at the center of the bond between i and j. The system is modeled as a weakly coupled stack of such planes. We hypothesize a simple form FDelta, phi] = Sigma(m)A Delta(2)(m) + (B/2)Delta(4)(m)] + C Sigma(< mn >) Delta(m) Delta(n) cos(phi(m) - phi(n)) for the functional, where m and n are nearest-neighbor sites on the bond-center lattice. This form is analogous to the original continuum Ginzburg-Landau free-energy functional; the coefficients A, B, and C are determined from comparison with experiments. A combination of analytic approximations, numerical minimization, and Monte Carlo simulations is used to work out a number of consequences of the proposed functional for specific choices of A, B, and C as functions of hole density x and temperature T. There can be a rapid crossover of from small to large values as A changes sign from positive to negative on lowering T; this crossover temperature T-ms(x) is identified with the observed pseudogap temperature T*(x). The thermodynamic superconducting phase-coherence transition occurs at a lower temperature T-c(x), and describes superconductivity with d-wave symmetry for positive C. The calculated T-c(x) curve has the observed parabolic shape. The results for the superfluid density rho(s)(x, T), the local gap magnitude , the specific heat C-v(x, T) (with and without a magnetic field), as well as vortex properties, all obtained using the proposed functional, are compared successfully with experiments. We also obtain the electron spectral density as influenced by the coupling between the electrons and the correlation function of the pair amplitude calculated from the functional, and compare the results successfully with the electronic spectrum measured through angle resolved photoemission spectroscopy (ARPES). For the specific heat, vortex structure, and electron spectral density, only some of the final results are reported here; the details are presented in subsequent papers.

Experimental Test of the Quantum No-Hiding Theorem

The no-hiding theorem says that if any physical process leads to bleaching of quantum information from the original system, then it must reside in the rest of the Universe with no information being hidden in the correlation between these two subsystems. Here, we report an experimental test of the no-hiding theorem with the technique of nuclear magnetic resonance. We use the quantum state randomization of a qubit as one example of the bleaching process and show that the missing information can be fully recovered up to local unitary transformations in the ancilla qubits.

Fluorination of La2NiO4 and La2CuO4+?

We have carried out fluorination on La2NiO4 and La2CuO4+?. Only a small fraction of fluorine enters the bulk; the rest resides on the surface. The cuprate after fluorination exhibits greater departures from tetragonal symmetry than the parent material and transforms at not, vert, similar40 K to the superconducting state. By contrast, the nickelate deviates less from tetragonal symmetry upon fluorination; no superconductivity was observed down to 4.2 K.

A New Era for Chemical Superconductors

A review of the development of ceramic superconductors and the steady increase in the superconducting transition temperature which currently stands at 135 K or about 164 K under pressure.