Electron-Phonon Interaction Within the Framework of the Fluctuating Valence of Copper Atoms — a Theoretical Model for High Temperature Superconductivity

Electron-phonon interaction is considered within the framework of the fluctuating valence of Cu atoms. Anderson's lattice Hamiltonian is suitably modified to take this into account. Using Green's function technique tbe possible quasiparticle excitations' are determined. The quantity 2delta k(O)/ kB Tc is calculated for Tc= 40 K. The calculated values are in good agreement with the experimental results.

Studies on development of MgB2 superconductor with improved in-field critical current density

National Institute for Interdisciplinary Science and Technology

Time evolution of Nd:YAG laser-induced plasma from GdBa2Cu3O7 high-Tc superconductor

In order to characterise the laser ablation process from high-Tc superconductors, the time evolution of plasma produced by a Q-switching Nd:YAG laser from a GdBa2Cu3O7 superconducting sample has been studied using spectroscopic and ion-probe techniques. It has been observed that there is a fairly large delay for the onset of the emission from oxide species in comparison with those from atoms and ions of the constituent elements present in the plasma. Faster decay occurs for emission from oxides and ions compared with that from neutral atoms. These observations support the view that oxides are not directly produced from the target, but are formed by the recombination process while the plasma cools down. Plasma parameters such as temperature and velocity are also evaluated.

On the role of acoustic plasmons in high Tc superconductors

The role of acoustic plasmons in the recently discovered high Tc superconductors is discussed. It is shown that the exchange of acoustic plasmons together with the usual phonon exchange between electrons can give rise to a Tc - 100 K.

Studies on development of novel rare earth doped (Bi,Pb)-2212 superconductors with enhanced properties

The objective of the present work is to study the effect of rare-earth (RE) doping on the superconducting properties of (Bi,Pb)-2212 system and to develop novel superconductors in the system with improved properties, especially, the self- and in-field critical current densities so as to use them for practical applications. This dissertation describes a range of findings in Bi-based superconductor using the cationic substitution of rare earth (RE) elements. Most of the experiments reported here take advantage of the difference in the valency and ionic radii of dopant and doping site.

Studies all transport and magnetic properties of nano particle doped MgB2 superconductor for technological applications

This thesis Entitled Studies on transport and magnetic properties of nano particle doped mgb2 superconductor for technological applications.The thesis ahead focuses on the establishment of enhanced superconducting properties in bulk MgB2 via nano particle doping and its conversion into mono/multifilamentary wires. Further, an attempt has also been made to develop prototypes of MgB2 coil and conduction cooled current lead for technological applications. The thesis is configured into 6 chapters. The opening chapter gives an idea on the phenomenon of superconductivity, the various types of superconductors and its applications in different fields. The second chapter is an introduction on MgB2 superconductor and its relevance which includes crystal and electronic structure, superconducting mechanism, basic superconducting properties along with its present international status. The third chapter provides details on the preparation and characterization techniques followed through out the study on MgB2. Fourth chapter discusses the effect of processing temperature and chemical doping using nano sized dopants on the superconducting properties of MgB2• Fifth chapter deals with the optimization of processing parameters and novel preparation techniques for wire fabrication. Sixth chapter furnishes the preparation of multifilamentary wires with various filament configurations, their electromechanical properties and it also incorporates the development of an MgB2 coil and a general purpose conduction cooled current lead.

Properties of Anisotropic Superconductors in SR Model

The present thesis deals with the theoretical investigations on the effect of anisotropy on various properties of magnetically doped superconductors described by fihiba — Rusinov model.Chapter 1 is introductory. It contains a brief account of the current status of theory of superconductivity. In’ chapter 2 we give the formulation of the problem. Chapter 2.1 gives the BCS theory. The effect of magnetic impurities in superconductors as described by A8 theory is given in chapter 2.2A and that described by SR model is discussed in chapter 2.28. Chapter 2.2c deals with Kondo effect. In chapter 2.3 the anisotropy problem is reviewed. Our calculations, results and discussions are given in chapter 3. Chapter 3.1 deals with Josephson tunnel effect. In chapter 3.2 the thermodynamic critical field H62 is described. Chtpter 3.3 deals with the density of states. The ultrasonic attenuation coefficient and ufitlear spin relaxation are given in chapter 3.4 and 3.5 respectively. In chapter 3.6 we give the upper critical field calculations and chapter 3.7 deals with the response function. The Kondo effect is given in chapter 3.8. In chapter 4 we give the sumary of our results

Studies On Preparation And Characterization Of Hts Current Leads Using Multifilamentary Ag And Ag-Alloy Sheathed (Bi,Pb)-2223 Superconducting Tapes

The application vistas of superconductors have widened very much since the discovery of high TC superconductors (HTS) as many of the applications can be realised at 77 K rather than going down to 4.2 K, the liquid He temperature. One such application is the HTS current lead which is used to connect a superconducting system with a room temperature power source. Minimising heat leak to the cryogenic environment is the main advantage of introducing current leads into superconducting systems. The properties of HTSS likes zero resistance (avoiding joule heating) and very low thermal conductivity (minimized conductive heat transfer) make them ideal candidates to be used as current leads. There are two forms of HTS current leads. (i) bulk form (tube or rod) prepared either from YBCO or BSCCO and (ii) tape form prepared from Bi-2223 multifilamentary tapes. The tape form of current leads has many advantages with respect to the mechanical and thermal stability related criteria. Crucial information on various aspects of HTS current lead development are not available in the literature as those are kept proprietary by various companies around the world. The present work has been undertaken to tailor the properties of multifilamentary tapes for the current lead application and to optimise the processing parameters of the same for enhanced critical current density and field tolerance. Also it is the aim of the present investigation is to prepare prototype current leads engineered for operation in conduction cooled mode and test them for operational stability

Studies on High- Tc Superconductors with Inequivalent Conducting Layers

The thesis deals with the study of super conducting properties of layered cuprates within the frame work of a modified Lawrence-Doniach (LD) model. The thesis is organized in seven chapters. Chapter I is a survey of the phenomena and theories of conventional superconductivity which can serve as a springboard for launching the study of the new class of oxide superconductors and it also includes a chronological description of the efforts made to overcome the temperature barrier. Chapter II deals with the structure and properties of the copper oxide superconductors and also the experimental constraints on the theories of high te:::nperature superconductivity. A modified Lawrence-Doniach type of phenomenological model which forms the basis of the presnt study is also discussed. In chapter III~ the temperature dependence of the upper critical field both parallel and perpendicular to the layers is determined and the results are compared with d.c. magnetization measurements on different superconducting compoilllds. The temperature and angular dependence of the lower critical field both parallel and perpendicular to the layers is also discussed. Chapters IV, V and VI deal with thermal fluctuation effects on superconducting properties. Fluctuation specific heat is studied in chapter IV. Paraconductivity both parallel and perpendicular to the layers is discussed in chapter V. Fluctuation diamagnetism is dealt with in chapter VI. Dimensional cross over in the fluctuation regime of all these quantities is also discussed. Chapter VII gives a summary of the results and the conclusions arrived at.