Showing papers in "Journal of Superconductivity in 2002"

Diluted Magnetic Semiconductors in the Low Carrier Density Regime

Abstract: This paper, based on a presentation at the Spintronics 2001 conference, provides a review of our studies on II-VI and III-V Mn-doped Diluted Magnetic Semiconductors. We use simple models appropriate for the low carrier density (insulating) regime, although we believe that some of the unusual features of the magnetization curves should qualitatively be present at larger dopings (metallic regime) as well. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.

Density functional calculations for III-V diluted ferromagnetic semiconductors: A Review

Abstract: In this paper we review the latest achievements of density functional theory in understanding the physics of diluted magnetic semiconductors. We focus on transition-metal-doped III–V semiconductors, which show spontaneous ferromagnetic order at relatively high temperature and good structural compatibility with existing III–V devices. We show that density functional theory is a very powerful tool for (i) studying the effects of local doping defects and disorder on the magnetic properties of these materials, (ii) predicting properties of new materials, and (iii) providing parameters, often not accessible from experiments, for use in model Hamiltonian calculations. Such studies are facilitated by recent advances in numerical implementations of density functional theory, which make the study of systems with a very large number of atoms possible.

Fundamentals of MRAM Technology

Abstract: Developments in portable communication and computing systems are creating a growing demand for nonvolatile random access memory that is dense and fast. None of the existing solid-state memories can provide all the needed attributes in a single memory solution. Therefore, to achieve the required multiple functionality requirements, a number of different memories are being used while compromising performance and adding cost to the system. Magnetoresistive Random Access Memory (MRAM) has the potential to replace these memories in various systems with a single, universal memory solution. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. In addition, MRAM can operate at high-speed and is expected to have competitive densities. In this paper we describe several fundamental technical and scientific aspects of MRAM with emphasis on recent accomplishments that enabled our successful demonstration of a 256 kbit memory chip.

Origin of spin-glass behavior of Zn1-xMnxO

Abstract: Alternating current susceptibility has been studied for polycrystalline Zn1 − x Mn x O. Stoichiometric samples demonstrate Curie–Weiss behavior, which indicates mostly antiferromagnetic interactions. Magnetic susceptibility can be described by a diluted Heisenberg magnet model developed for semimagnetic semiconductors. High-pressure oxygen annealing induces spin-glass like behavior in Zn1 − x Mn x O by precipitation of ZnMnO3 in the paramagnetic matrix.

Static charge and spin inhomogeneity in La2-xSrx CuO4 by NMR

Abstract: NMR experiments on La2 − xSrxCuO4 are shown that characterize the nature of static spatial inhomogeneities in this material. Large amplitude charge density variations are found that are correlated with spin density variations. While the amplitude of the charge variation is temperature-independent, the spin variation increases Curie-like as the temperature varies and shifts toward local antiferromagnetism up to optimal doping. For x = 0.2 the correlation is independent on temperature and agrees with local incommensurability found in neutron scattering. Relations to possible scenarios are discussed.

Spintronics: Sources and Challenge. Personal Perspective

Abstract: Spintronics emerged very recently as a quickly developing interdisciplinary field in the framework of solid-state physics with tempting technological perspectives. This promise is based on the active involvement of the electron spin, side by side with its charge, in the operation of nanometer scale electronic devices. It is a remarkable feature of spintronics that it is growing coherently from several different fields of solid-state physics (semiconductor physics, magnetism, superconductivity, etc.) and involves a multiplicity of rather diversified phenomena. This unique property of spintronics makes it a fascinating field for research and applications but also creates a challenge for researchers. In this short note, I concentrate on the physical background of spintronics and some historical roots of it but avoid making specific prognosis about technological applications.

Optical Properties of Solid Pentacene

Abstract: We have prepared pentacene films on Si, Al2O3, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al2O3 and glass substrates is 1.85 eV.

Degradation of superconducting properties in MgB2 by formation of the MgB4 phase

Abstract: Superconducting MgB 2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB 4 phase. A series of samples was placed in an α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB 2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB 4 phase were formed and the T zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J cmag for the first series of samples was 1.9 x 10 5 A/cm 2 at 10 K and 0 T, and for the second series of samples was 0.7 x 10 4 A/cm 2 at 10 K and 0 T.

Novel Aspects of Spin-Polarized Transport and Spin Dynamics

Abstract: There is a renewed interest to study spin-polarized transport and spin dynamics in various electronic materials. Motivation to examine the spin degrees of freedom (mostly in electrons, but also in holes and nuclei) comes from various sources: ranging from novel applications which are either not feasible or ineffective with conventional electronics, to using the spin-dependent phenomena for exploring fundamental properties of solid-state systems. Taken in a broader context, term spintronics is addressing various aspects of these efforts and stimulating new interactions between different subfields of condensed matter physics. Recent advances in material fabrication made it possible to introduce the nonequilibrium spin in novel class of systems, including ferromagnetic semiconductors, high temperature superconductors, and carbon nanotubes—which leads to a question of how could such a spin be utilized. For this purpose it is important to extend the understanding of spin-polarized transport and spin dynamics to consider inhomogeneous systems, various heterostructures, and the role of interfaces. This article presents some views on novel aspects of spin-polarized transport and spin dynamics (referring also to the topics which were addressed at the conference Spintronics 2001) and suggests possible future research directions.

Cuprate/Ferromagnetic Oxide Superlattices

Abstract: YBa2Cu3O7/La0.67Ca0.33MnO3 as well as YBa2Cu3O7/SrRuO3 superlattices have been grown by pulsed laser deposition with individual layer thickness ranging from 4 to 200 unit cells for the YBa2Cu3O7 and 10 to 500 unit cells for the magnetic oxides. Whereas simple heterostructures reproduce the intrinsic properties of the constituent material rather well with reduced critical temperatures for the magnetic and superconducting phase transitions, the critical temperatures systematically vary with the superlattice composition. The introduction of a SrTiO3 spacer layer between the YBCO and LCMO film, respectively, causes a shift of the critical temperatures close to the intrinsic values again. The results are discussed in view of the interaction effects at the electronic level.

Spin Injection and Spin Accumulation in Permalloy–Copper Mesoscopic Spin Valves

Abstract: We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as nonmagnetic metal. Our multiterminal geometry allows us to experimentally distinguish different magnetoresistance signals, being (1) the spin valve effect, (2) the anomalous magnetoresistance (AMR) effect, and (3) Hall effects. We find that the AMR contribution of the Py contacts can be much larger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a “conventional” measurement geometry. However, these “contact” magnetoresistance signals can be used to monitor the magnetization reversal process, of the spin injecting and detecting Py contacts. In a “nonlocal” spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T = 4.2 K as well as at room temperature. We obtain spin diffusion lengths in Cu of 1 μm and 350 nm at T = 4.2 K and room temperature respectively.

A novel approach to the polaronic metallic state of cuprate superconductors and the d-wave pairing mechanism

Abstract: The present novel approach consists of two stages: in the first stage the many-electron states of a CuO6 octahedron or a CuO5 pyramid are calculated by the first-principles variational method, by taking into account the local distortions of a CuO6 octahedron or a CuO5 pyramid In the second stage a metallic state is constructed in the presence of the local AF ordering constructed by the localized spins In this metallic state the local distortions are treated in the mean field approximation, so that a “pseudopolaronic effect” is taken into account Based on this approach, the hole-concentration dependence of Tc and the isotope effect are calculated for LSCO, and compared with experimental results This approach leads to d-wave pairing mechanism

Crystallization activation energy and hole concentration properties of the Bi2Sr2Ca1-xCdxCu2O8+y glass-ceramic superconductor rods

Abstract: The glass rods with the nominal compositions of Bi 2 Sr 2 Ca 1-x Cd x Cu 2 O 8+y , where x = 0.0, 0.4, and 0.8, were prepared by melt-casting method. XRD studies exhibited that the Cd substitution caused a transition from Bi-2212 phase to Bi-2201 phase. The multiphase structure was obtained for higher Cd concentration cases. It was determined that the activation energy for crystallization was decreased with the increase of the Cd concentration. When the Cd concentration was increased in the system the superconducting properties decreased. The hole concentration, p, was calculated using the Presland's method. The increase in the Cd concentration increased the hole concentration and the results showed that the Cd-substituted samples were highly in the overdoped region.

Crossed Andreev Reflections

Abstract: The Andreev reflection mechanism at Superconducting: Normal metal interfaces reverses both the sign of the charge and of the spin of the incoming particle from the N side. In an ideal contact, the conductance at small bias is increased by a factor of 2 compared to that in the normal state. In the presence of a barrier at the interface, the probability of Andreev reflections is diminished, but remains finite, giving a subgap conductance. Spin polarization in the normal metal also reduces the probability of Andreev reflections, which are completely quenched for a full polarization. In that case, the resistance of the contact is infinite, even in the absence of any barrier. Yet, if a second contact is made within a distance of the order of the coherence length, with a metal having an opposite polarization, the full Andreev increased conductance can be recovered when the two contacts are connected in series. Such a device thus has a strong magnetoresistance upon reversal of the magnetization of one of the two contacts. Different consequences of these crossed Andreev reflections are discussed.

Thermodynamics and the elastic moduli of Pu

Abstract: Pu exhibits at least five structures below its melting point of 914 K. The lower-temperature phases have unusually low symmetry for an elemental metal, but at higher temperatures, Pu behaves like many other metals, going from fcc (face-centered cubic) to bcc (body-centered cubic) before melting. Electronic structure calculations appear to explain the lowest temperature monoclinic phase and the changes it undergoes with pressure (O. Eriksson, J. D. Baker, A. V. Balatsky, and J. M. Wills, J. Alloys Comp.287, 1 (1999)), but the much simpler fcc phase of Pu remains difficult to understand ab initio. We show how the unusually small values of elastic moduli require that the effects of temperature and phonon entropy be included in any theory of fcc Pu, and we present new high-accuracy data for elastic moduli, that show that the elastic moduli of Ga-stabilized fcc Pu do not depend on Ga at 300 K, and that polycrystal and monocrystal elastic modulus data for Pu are in agreement.

Structural Investigations of La-2125 Mixed Oxide Superconducting System

Abstract: Crystallographic studies on the (La2 − xNdx)CayBa2Cu4 + yOz′ [y = 2x; 0.0 ≤ x ≤ 0.5] samples are carried out using neutron diffraction technique. The analysis of the neutron diffraction patterns for the series of samples with 0.0 ≤ x ≤ 0.5 was done using Rietveld method. The La2Ba2Cu4Oz′ (La-224, x = 0.0) system exhibits tetragonal phase and is a nonsuperconductor, which on addition of a “CaCuO2” rock salt like layer becomes superconducting. It is important to note the significant role played by Ca-doping in improvising the superconducting properties. Analysis of the structural data reveals that, as Ca concentration increases, the unit cell volume decreases while Tc increases with a maximum value of 78 K for y = 1.0. The detailed crystal structure for LaNdCaBCO series of samples has been studied in the light of changes in bond lengths with increase in Ca concentration and its role in “turning on” the superconductivity.

Superconductivity in high-pressure synthesized pure and doped MgB2 compounds

Abstract: Dense pure and doped (Mg 1-x A x )B 2 samples with A = Na, Ca, Cu, Ag, Zn, and Al were synthesized at high pressure-high temperature in a multi-anvils press (3.5-6 GPa, 900-1000°C) for 0 < x ≤ 0.20. They were studied by X-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by AC susceptibility, magnetization, and transport measurements. Only Al is really substituted on the Mg site. The other elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties T c , j c critical current, H irr , and H c2 .

Evidence for a Local Structural Change in La2CuO4.1 Across the Superconducting Transition

Abstract: Polarized Cu—K edge XAFS (X-ray absorption fine structure) on La2CuO4.1 indicate that the radial distribution function of the copper in plane oxygen pairs is a two-site distribution, in agreement with the results found by Bianconi et al. for temperatures below the appearance of a pseudogap. Additionally we find evidence of a change in this distribution across the superconducting transition, suggesting coupling between the local lattice structure and the charged particles involved in the superconductivity.

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures

Abstract: Quasiparticle nonequilibrium due to spin-polarized current injection in perovskite superconductor/ferromagnet (S/F) thin-film heterostructures has been studied with the technique of cryogenic scanning tunneling spectroscopy. The spin-injection heterostructures consisted of epitaxial bilayers of the high-Tc superconductor YBa2Cu3O7 − δ and the half-metallic ferromagnet La0.7Ca0.3MnO3, with a spin-polarized quasiparticle current injected into the cuprate layer from the manganite layer. The tunneling conductance measured at 4.2 K on the cuprate layer showed a distinctly nonequilibrium quasiparticle spectrum as a result of the spin injection. Quantitative analysis of the tunneling spectral evolution versus the injection current yielded an estimate of the quasiparticle spin-diffusion depth and the spin-relaxation time in the superconducting cuprate.

On the magnetism of the normal state in MgB2

Abstract: The experimentally observed ferromagnetism in MgB2 in the normal state is attributed to microphase-separated inclusions of iron. This magnetic character is also observed when the iron content of the samples is reduced below 20 μg/g, however in these samples the diamagnetism of MgB2 is apparent and is measured. It is found experimentally that the diamagnetic susceptibility at room temperature of B element, MgB2, and MgB4 is close to the ratio 1:2:4, suggesting that the diamagnetism in these borides is confined to the boron atoms. This observation supports a picture in which the two electrons of Mg are donated to B in MgB2.

Study on the Physical and Electrical Properties of Bi2 − δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

Abstract: We have fabricated a series of glass-ceramic (Bi 2-δ Zn δ ) Sr 2 Ca 2 Cu 3 O 10+y , where δ = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the δ = 0 sample, the T c and T zero was obtained at 110 K and 107 K, respectively. The J c values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis-Bennett. The calculated activation energy, E a , of the system was found to be in the range of 258-336 kJ/mol.

Creation of nonlocal spin-entangled electrons via Andreev tunneling, Coulomb blockade, and resonant transport

Abstract: We discuss several scenarios for the creation of nonlocal spin-entangled electrons which provide a source of electronic Einstein–Podolsky–Rosen (EPR) pairs. Such EPR pairs can be used to test nonlocality of electrons in solid state systems, and they form the basic resources for quantum information processing. The central idea is to exploit the spin correlations naturally present in superconductors in form of Cooper pairs possessing spin-singlet wavefunctions. We show that nonlocal spin-entanglement in form of an effective Heisenberg spin interaction is induced between electron spins residing on two quantum dots with no direct coupling between them, but each of them being tunnel-coupled to the same superconductor. We then discuss a nonequilibrium setup with an applied bias where mobile and nonlocal spin-entanglement can be created by coherent injection of two electrons, in a pair (Andreev) tunneling process, into two spatially separated quantum dots and subsequently into two Fermi liquid leads. The current for injecting two spin-entangled electrons into different leads shows a resonance and allows the injection of electrons at the same orbital energy, which is a crucial requirement for the detection of spin-entanglement via the current noise. On the other hand, tunneling via the same dot into the same lead is suppressed by the Coulomb blockade effect of the quantum dots. We discuss Aharonov–Bohm oscillations in the current and show that they contain h/e and h/2e periods, which provides an experimental means to test the nonlocality of the entangled pair. Finally, we discuss a structure consisting of a superconductor weakly coupled to two separate one-dimensional leads with Luttinger liquid properties. We show that strong correlations again suppress the coherent subsequent tunneling of two electrons into the same lead, thus generating again nonlocal spin-entangled electrons in the Luttinger liquid leads.

Levitation force between a horizontally oriented point magnetic dipole and a superconducting sphere

Abstract: The magnetic interaction between a point dipole of horizontally oriented moment and a superconducting sphere in the complete Meissner state is investigated in detail. It is demonstrated that the levitation force for this configuration is precisely one half the value of the configuration with vertically oriented point dipole. An extension to other applied fields, still assuming the perfect flux exclusion state, is presented. The results have application to models for magnetic levitation and magnetic force microscopy.

Fermi Surface and Superconducting Gap of Triple-Layered Bi2Sr2Ca2Cu3O10 + δ

Abstract: We present a comprehensive study performed with high-resolution angle-resolved photoemission spectroscopy on triple-layered Bi2Sr2Ca2Cu3O10 + δ single crystals. By measurements above T C the Fermi surface topology defined by the Fermi level crossings of the CuO2-derived band was determined. A hole-like Fermi surface as for single and double-CuO2 layered Bi-based cuprates is found, giving new input to the current debate of the general Fermi surface topology of the high-T C superconductors. Furthermore, we present measurements of the superconducting gap of Bi-2223 and show that there are clear indications for a strong anisotropy of the superconducting gap. The universal properties of this phase in comparison to the other Bi-based cuprates will be discussed.

Surface Energy of Bismuth Cuprate

Abstract: The parameters of Bi2CuO4 crystal lattice versus the temperature are measured. The thermal expansion coefficient of Bi2CuO4 is determined. The mean values of grain boundaries and grain-melt interface energies are estimated. A relationship between the surface tension and the thermal expansion coefficient are established.

Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods

Abstract: We report on the results of a comparative investigation of highly dense bulk MgB2 samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by AC susceptibility, DC magnetization, and resistivity measurements in magnetic fields up to μ0 H = 160 kOe. A small but distinct anisotropy of the upper critical field $$\psi {\rm H}_{c2}^{\alpha ,b} /H_{c2}^c \sim 1.2$$ connected with some texture of MgB2 grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields H c2 (μ0 H c2 (0) ∼ 23 T), irreversibility fields H irr which are strongly shifted towards higher values H irr(T) ∼ 0.8 H c2(T) and high critical current J c (J c = 105 A/cm2 at 20 K and 1 T).

Superlight Bipolarons Explain the High Superconducting Temperature

Abstract: Over the last decade, several competing models of high-temperature superconductivity (HTSC) were proposed, none of which have succeeded to explain high values of the superconducting critical temperature T c without adjustable parameters. Most of the proposed models are based on the short-range electron-electron interaction or/and on a short-range electron-phonon interaction. Here we propose a model of HTSC which explicitly takes into account the long-range origin of both types of interaction. Remarkably, the long-range electron-phonon interaction binds carriers into superlight real-space pairs-bipolarons-with a relatively low mass but sufficient binding energy. The model quantitatively explains high T c values without any fitting parameters and other key features of the cuprates.

Absence of a competition between magnetism and superconductivity in layered nickel borocarbides: Common correlations of Tc with crystal chemical parameters for magnetic and nonmagnetic compounds

Abstract: In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi 2 B 2 C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (T c ) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of T c on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the T c of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and T c of compounds at various substitutions R are calculated.

Charge and Sodium Ordering in β-Na0.33V2O3

Abstract: Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate β-Na0.33V2O3 are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.