Current noise in MgB2 superconducting thin films

In this paper we present some experimental results concerning the current noise produced during the resistive transition in MgB₂ thin films. Preliminary investigations evidenced the presence of electrical noise whose power spectrum has a region of the 1/fⁿ type with n 3. We suggest that the noise may originate from abrupt rearrangement of the current distribution inside the specimen during the percolative process of a diphasic system. Experimental measurements of the spectral components of the current noise taken during the resistive transition will be given and discussed.     

Enhancement of critical current density of YBa2Cu3O7 − δ thin films by self-assembly of Y2O3 nanoparticulates

YBa₂Cu₃O_7 − δ (YBCO) thin films, possessing high critical current density (J_c), have been synthesized by embedding a homogeneous array of Y₂O₃ non-superconducting nanoclusters/nanoparticles using a pulsed laser deposition technique. The size, interparticle spacing, and density of Y₂O₃ nanoparticles in YBCO thin films were tailored by varying the number of laser pulses in order to determine the optimum size for effective immobilization of vortices. Scanning transmission electron microscopy with atomic number contrast and X-ray diffraction techniques were used to determine the size and structure of the nanoparticles. Both techniques indicate that the Y₂O₃ particles are epitaxial with respect to the surrounding YBCO matrix. The information about pinning of vortices by the nanoparticles was obtained by investigating the behavior of critical current density as a function of temperature and applied field, which in turn determines the vortex density in the sample. The superconducting transition temperature (T_c) of YBCO films with the inclusion of nanoparticles was observed to remain almost the same or decrease marginally (1-2 K) with respect to T_c of pure YBCO films deposited under identical conditions. However, J_cs of YBCO films embedded with self-assembled nanoparticles were found to be significantly higher than that of pure YBCO films. The J_c enhancement was up to five times in high magnetic field, which is a key requirement for practical application of high-T_c materials.     

Superconductivity induced by hydrogen anion substitution in 1111-type iron arsenides

Hydrogen is the simplest bipolar element and its valence state can be controlled from +1 to −1. We synthesized the 1111-type iron arsenides CaFeAsH and LnFeAsO_1−xH_x (Ln = lanthanide; 0 ⩽ x ⩽ 0.5) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H⁻), which is isovalent with the fluoride ion (F⁻), in the 1111-type iron arsenides. When some of the O²⁻ ions in LnFeAsO are replaced by H⁻, superconductivity is induced by electron doping to the FeAs-layer to maintain charge neutrality. Since the substitution limit of hydrogen in LnFeAsO (x ≈ 0.5) is much higher than that of fluorine (x ≈ 0.2), the hydrogen substitution technique provides an effective pathway for high-density electron-doping, making it possible to draw the complete electronic phase diagram of LnFeAsO. The x–T diagrams of LnFeAsO_1−xH_x (Ln = La, Ce, Sm, Gd) have a wide superconducting (SC) region spanning the range x = 0.04–0.4, which is far from the parent antiferromagnetic region near x = 0.0. For LaFeAsO_1−xH_x, another SC dome region was found in the range x = ∼0.2 to ∼0.5 with a maximum T_c = 36 K, in addition to a conventional SC dome located at x ∼ 0.08 with maximum T_c = 29 K. Density functional theory calculations performed for LaFeAsO_1−xH_x indicated that the newly observed T_c is correlated with the appearance of degeneration of the Fe 3d bands (d_xy, d_yz and d_zx), which is caused not only by regularization of the tetrahedral shape of FeAs₄ due to chemical pressure effects but also by selective band occupation with doped electrons. In this article, we review the recent progress of superconductivity in 1111-type iron (oxy)arsenides and related compounds induced by hydrogen anion substitution.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

Epitaxial growth of MgO/TiN multilayers on Cu

The growth of epitaxial MgO/TiN multilayer films on (001) Cu has been investigated. In particular, epitaxial structures were grown on (001) Cu layers that were epitaxial on (001) SrTiO₃. X-ray diffraction and reflection high-energy electron diffraction indicate that the multilayer structures are epitaxial on the (001) Cu surface. The motivation is the use of crystalline MgO/TiN multilayers as a diffusion barrier to both copper and oxygen. MgO/TiN multilayers are potentially useful as diffusion barriers for Cu interconnects on semiconductors as well as for superconducting wires based on the epitaxial growth of cuprate superconductors on biaxially textured copper.     

Characterisation of a TES-Based X-ray Microcalorimeter in the Energy Range from 150 to 1800 eV Using an Adiabatic Demagnetisation Refrigerator

We characterised a TES-based X-ray microcalorimeter in an adiabatic demagnetisation refrigerator (ADR) using synchrotron radiation. The detector response and energy resolution was measured at the beam-line in the PTB radiometry laboratory at the electron storage ring BESSY II in the range from 200 to 1800 eV. We present and discuss the results of the energy resolution measurements as a function of energy, beam intensity and detector working point. The measured energy resolution ranges between 1.5 to 2.1 eV in the investigated energy range and is weakly dependent on the detector set point. A first analysis shows a count-rate capability, without considerable loss of performance, of about 500 counts per second.      

An analysis and computer simulation of using elastic strain energy to promote superconductivity

Both a model and a computer simulation are developed for superconducting electrons to move by releasing strain energy over mesoscopic distances. Rotation of a grain provides unique strain energy patterns to facilitate conduction. Numerical estimates are derived for the size of the force an electron could exert on the end of a grain to cause it to rotate. The numerical estimates take into account both electric fields and a quantum mechanical rate of change of momentum of the electrons. The force necessary to produce a torque to oscillate a grain at the same frequency of an electron wave packet is found to be in the same range. It is also shown that the magnetic field associated with a normal conducting current can disappear when a superconducting electron moves in the direction of a gradient in strain energy. A computer simulation in C++ shows that strain energy patterns from granular rotation can allow a high degree of electron movement through the center of a grain that has an aspect ratio from 8/7 to 2 and from 10 to 20. The range of tilt angle of the grain that allows electron movement is narrower and lower for higher aspect ratios.     

Antiferromagnetic,Neutral, and Superconducting Band in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La₂CuO₄ is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomic-like states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomic-like states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which may explain the physical properties of La_2−x Sr _x CuO₄ as a function of the Sr concentrationx.