Iranian Journal of Physics Research
Year:2006 | Volume:6 | Issue:3|Papers Count:27

We report systematic investigations of the magnetic superconducting properties of the new superconducting materials (NS): New high temperature superconductors (HTS), Organic superconductors (OS), fullerenes, carbon nanotubes, MgB2 etc. We show that, contrary to conventional superconductors where the superconducting state can be coherent over several tenths of km, the macroscopic coherence range lc of the NS is often as short as 0.1 to 10 mm typically. As a consequence, the magnetic properties are dominated by granular-like effects as well as Josephson coupling between grains. Here, we concentrate on HTS ceramics and organic superconductors exclusively. In the first case we observe three distinct regimes: (i) At very low field (H < 5 Oe to say) all the grains are coupled via Josephson effect and lc can be considered as infinite. (2) At intermediate field (5 <H < 50 Oe, typically) the grains are gradually decoupled by H and/or T. (iii) At higher fields all the grains are decoupled and lc roughly coincides with the diameter of the metallurgical grains. The case of OS is more subtle and is connected with a kind of order-disorder transition that occurs in most of them. For instance, in this study, we exploit quenched disorder (after crossing such a transition) in the k -(BEDTTTF)2 Cu[N(CN)2]Br layered organic superconductor to get new insights on both the superconducting state (T £ 11.6 K) and the glassy transition at Tg, by studying the superconducting properties as functions of annealing time and annealing temperature around the glassy transition. Our main result is that the data can be described by a percolation molecular cluster model in which the topology and the growth of the molecular clusters obey an Ising spin-glass-like model with Tg » 80 K for the hydrogenated compound and Tg » 55 K for the fully deuterated one.

Obtaining the superconductor samples or mainly, structural phase controlling in 123 systems is a matter of special importance. As decreasing of oxygen in this structure has special effects, and mainly causing structural phase transition, by investigating the structure and thermal analysis of the system, tetragonal-orthorhombic structural phase transition was observed and optimum contents of structural phase transition temperature and oxygen were gained (tC630±5oC) and (7-dC @ 6.6), respectively, that are in good agreement with others’ work. In addition by band structure and density of states calculations of YBa2Cu3O7 we gained the equilibrium position, the role and contribution of ions, especially oxygen, in total system’s density of state. It shows that the conductivity of orthorhombic phase is more in compare with tetragonal phase in normal states of 123 systems.

YBa2-xKxCu3O7-8 compound with x = 0, 0.1, 0.15, 0.2, 0.3, 0.5, 0.8, 1 was prepared. The samples were characterized by XRD, Tc, oxygen content and room temperature thermopower measurements. The results shows that by increasing the potassium, the samples go to the underdoped regime. This is due to the depletion of oxygen from the samples. By post annealing of the sample with x = 0.2 and Tc = 78 K in oxygen, the Tc increased up to 93 K which means it is possible to put back the oxygens into the structure.

Doping Pr in 123-systems gives rise to some anomalies, such as critical Temperature suppression. Here, we show that a modification of hole localization theory based on a geometrical modeling by band percolation theory can put forward as a good explanation for the critical temperature suppression for the whole range of Pr-doping value. In this model the key concept of hole clustering is introduced and shown to have a great role near phase transition point, so we can not ignore it in any modeling or calculation. Also we have provided some experimental evidence which manifests an agreement between simulation based on hole clustering effect and the experimental data.

Intensive study of the high temperature superconductors has been ongoing for two decades. A great deal of this effort has been devoted to the underdoped regime, where the new and difficult physics of the doped Mott insulator has met   extra complications including bilayer coupling/splitting, shadow bands, and hot spots. While these complications continue to unfold, in this short overview the focus is moved to the region of actual high-Tc, that of optimal doping. The focus here also is not on the superconducting state itself, but primarily on the characteristics of the normal state from which the superconducting instability arises, and even these can be given only a broad-brush description. A reminder is given of two issues: (i) why the “optimal Tc” varies: for n-layered systems it increases for n up to 3, then decreases; for a given n, Tc increases according to the ‘basis’ atom in the order Bi, Tl, Hg; (ii) how does pressure, or a particular uniaxial strain, increase Tc when the zero-strain system is already optimally doped?

Two decades ago the epoch making discovery of high Tc cuprate superconductivity by Bednorz and Müller shocked the world’s superconductivity community. However, already in 1979 and 1980, the first heavy fermion superconductor CeCu2Si2 and organic superconductor (TMTSF)2PF6 have been discovered respectively. Also we know now that all these superconductors are unconventional and nodal. Further the quasiparticles in the normal state in these systems are Fermi liquids and the super conducting states are described in terms of generalized BCS wave function. Also the pseudogap phase in underdoped high Tc cuprates is described in terms of d-wave density wave. This implies necessarily that the superconductivity in underdoped cuprates is gossamer (i.e. d-wave superconductivity coexists with d-wave density wave). We shall present some quantitative tests of these new concepts, notions and ideas.

The recently discovered MgB2 superconductors have a record transition temperature for a BCS superconductor due to the high vibration frequencies associated with its light elements. The transition temperatures in the cuprate family of superconductors are much higher but these do not fit the BCS paradigm. The most promising microscopic origin for their many anomalous properties lies in magnetic pairing described by the RVB (Resonant Valence Bond) ansatz. However a comprehensive theoretical description of the key anomalous properties of the cuprates remains to be an open challenge.

A brief review on some peculiar properties of high temperature superconductors (HTS) is presented. Twenty years after the discovery, it appears more and more clear that the behaviour of this new class of materials is remarkably different from what have been re-classified as “conventional” superconductors. In the following we will focus our attention on the study of two phenomena, namely the Josephson effect and the Meissner effect, where the unconventional nature of superconductivity in HTS offers exciting perspectives both for the understanding of the underlying mechanism, so far still unknown, and for the large potential of applications in different areas of superconducting electronics.

The London free-energy is regeneralized by the Ginsburg-Landau free-energy density in the presence of both d and s order parameters. We have shown that the strength of the s-d coupling, e , makes an important rule to determine the form of the lattice vortex. Appearance of the ratios of the coherence length to penetration depth in the higher order corrections of the free-energy density will truncate these corrections for even large values of e.

show that the first-principles approach is able to describe the effects of chemical doping and pressure on the structural properties, the band structure, the ion charges, and the chemical bonds. We report on the origin of the optimal doping and present results on the inhomegeneity of the charge distribution and the concomitant splitting of the electronic bands and their contributions to the density of states. Due to their individual energy dependence, the role of the intrinsic inhomogeneities for superconductivity strongly depends on the energy and character of the quasiparticle mediating the Cooper pairing. The evolution of the electric field gradients with doping is analyzed and compared to nuclear resonance experiments. The calculated results can explain the origin of doping-induced effects observed either by local or macroscopic experimental probes. From a systematic study of the density of states by varying the doping concentration as well as applying pressure up to 15 GPa, and comparison with the measured critical temperatures, the coupling constant of the quasiparticle has been estimated to be of the order of one. Moreover, we show how density functional theory allows for the calculation of vibrational properties and phonon Raman scattering in the high-Tc cuprates. All results are quantitatively compared to experiment, and have revealed very good agreement.

This paper reviews briefly the development of physical vapour deposition based HTS thin film preparation technologies to today’s state-of-the-art methods. It covers the main trends of in-situ process and growth control. The current activities to fabricate tapes for power applications as well as to tailor interfaces in cuprate are described. Some future trends in HTS thin film research, both for science as well as application driven activities are outlined.

The collapse of antiferromagnetic order as a function of some quantum tuning parameter such as carrier density or hydrostatic pressure is often accompanied by a region of superconductivity. The corresponding phenomenon in the potentially simpler case of itinerant-electron ferromagnetism, however, remains more illusive. In this paper we consider the reasons why this may be so and summaries evidence suggesting that the obstacles to observing the phenomenon are apparently overcome in a few metallic ferromagnets. A new twist to the problem presented by the recent discoveries in ferroelectric symmetric systems and new graphite intercalate superconductors will also be discussed.

The Feshbach shape resonances in the interband pairing in superconducting superlattices of quantum wells or quantum stripes is shown to provide the mechanism for high Tc superconductivity. This mechanism provides the Tc amplification driven by the architecture of material: superlattices of quantum wells (intercalated graphite or diborides) and superlattices of quantum stripes (doped high Tc cuprate perovskites) where the chemical potential is tuned to a Van Hove-Lifshitz singularity (vHs) in the electronic energy spectrum of the superlattice associated with the change of the Fermi surface dimensionality in one of the subbands.

Polycrystalline samples of NdBa2-xLaxCu3O7-3 with x=0, 0.05, 0.10, 0.15, 0.20, and 0.30 were made by standard solid state methods. The transport and superconducting properties have been studied by the  resistivity measurements as a function of temperature and doping concentration. In order to measurements the hole concentration in CuO2 planes, the room temperature thermoelectric power was measured as a function of La doping. The resistance and the room temperature thermoelectric power were increased while the critical temperature was decreased as parabolic-like by increasing doping concentration. The pseudogap temperature (or pseudogap energy) was measured from downturn deviation in a linear dependence of resistance as a function of temperature. The critical temperature, room temperature thermoelectric power S(290 K), resitivity, and pseudogap temperature results were suggested that hole filling was the main reason of suppression of superconductivity in this alloys.

Neutron scattering is proved to be a vital probe in unveiling the magnetic properties of high temperature superconductors (HTSC). Detailed information about the energy and momentum dependence of the magnetic dynamics of HTSC have been obtained directly by this technique. Over the past decade by improving the crystal growth methods, large and high quality single crystals of HTSC, which are essential for a neutron scattering experiment, have become available. The results of neutron scattering measurements on such crystals have considerably enhanced our understanding of the magnetism in HTSC both in the superconducting (SC) and normal states. In this review, the neutron scattering results on two main HTSC families, La2-xSrxCuO4 (LSCOx) and YBa2CuO3O6+x (YBCO6+x), are considered with an emphasis on the most prominent properties of these materials that are now widely accepted. These include the presence of strong antiferromagnetic (AF) fluctuations even in optimally doped region of the phase diagram, neutron resonance peak that scales with SC transition temperature, Tc, incommensurate magnetic fluctuations (stripes), and a pseudogap in the normal state of underdoped materials.

Asymmetric features of various physical quantities in the normal and superconducting states between hole- and electron-doped cuprate high-temperature superconductors have been an issue of debate for a long time. Their exploration is very important for the understanding not only of the mechanism of high-Tc superconductivity but also of the nature of doped-Mott insulators. Presented in this review is the present status of theoretical understanding of the electronic states in hole- and electron-doped high- Tc cuprates as well as the origin of the electron-hole asymmetry of the electronic states. In particular, it is shown that numerically exact diagonalization calculations for small clusters in a t-J model with long-range hoppings, t' and t'' nicely reproduce the electron-hole asymmetry observed experimentally in various quantities and thus make it possible to extract the physical origin of the asymmetry. These results give a deep insight on the asymmetric behaviors in hole- and electron-doped high-Tc cuprates and on the nature of doped Mott insulators.

We have studied the structural and electrical properties of Gd(Ba2-xLax) Cu3 O7+d [Gd(BaLa)123], Gd(Ba2-xNdx) Cu3O7+d [Gd(BaNd)123], and Nd(Ba2-xPrx) Cu3O7+d [Nd(BaPr)123] compounds with 0.0£x£0.8 prepared by the standard solidstate reaction. The XRD patterns show that all of the samples with x£0.5 are isosructure 123 phase, but in Gd(BaNd)123 and Nd (BaPr)123 there are several impurity peaks in the XRD patterns for x³0.6. We estimated the xc solubility=1.1, 0.6 and 0.55 in Gd (BaLa)123, Nd (BaPr)123, and Gd (BaNd)123, respectively. The resistivity increases with the increase of doping. The decrease of Tc with the increase of Pr doping is faster than Nd and La doping. The normal-state resistivity is fitted for two and three dimensional variable range hopping (2D&3D-VRH) and Coulomb gap (CG) regimes, separately. Our results indicate that the dominant mechanism for x³xcSIT is 3D-VRH. The broadening of magnetoresistance have been investigated by TAFC and AH models. The pinning energy and Josephson coupling energy, decrease with the increase of applied magnetic field as U~H-b these values also decrease with doping concentration; Pr is more effective than Nd and La.

In the quest for understanding correlated electrons, high-temperature superconductivity remains a formidable challenge and a source of insight. This paper briefly recalls the central achievement by the study of heat transport at low temperatures. At very low temperatures, nodal quasi-particles of the d-wave superconducting gap become the main carriers of heat. Their thermal conductivity is unaffected by disorder and reflects the fine structure of the superconducting gap. This finding had led to new openings in the exploration of other unconventional superconductors.

Resonating valence bond states in a doped Mott insulator was proposed to explain superconductivity in cuprates in January 1987 by Anderson. A challenging task then was proving existence of this unconventional mechanism and a wealth of possibilities, with a rigor acceptable in standard condensed matter physics, in a microscopic theory and develop suitable many body techniques. Shortly, a paper by Anderson, Zou and us (BZA) undertook this task and initiated a program. Three key papers that followed, shortly, essentially completed the program, as far as superconductivity is concerned: i) a gauge theory approach by Anderson and us, that went beyond mean field theory ii) Kotliar’s d-wave solution in BZA theory iii) improvement of a renormalized Hamiltonian in BZA theory, using a Gutzwiller approximation by Zhang, Gros, Rice and Shiba. In this article I shall focus on the merits of BZA and gauge theory papers. They turned out to be a foundation for subsequent developments dealing with more aspects that were unconventional - d-wave order parameter with nodal Bogoliubov quasi particles, Affleck-Marston’s p-flux condensed spin liquid phase, unconventional spin-1 collective mode at (p,p), and other fascinating developments. Kivelson, Rokhsar and Sethna’s idea of holons and their bose condensation found expression in the slave boson formalism and lead to results similar to BZA program. At optimal doping, correlated electrons acquire sufficient fermi sea character, at the same time retain enough superexchange inherited from a Mott insulator parentage, ending in a BCS like situation with superexchange as a glue! Not surprisingly, mean field theory works well at optimal doping, even quantitatively. Further, t-J model is a minimal model only around optimal doping, where RVB superconductivity is also at its best.

We have studied the effect of precursor powder size on the microstructure and intergranular behavior of polycrystalline Bi 2223 superconductors using the XRD, SEM, electrical resistivity and AC susceptibility techniques. Polycrystalline Bi 2223 superconductors were prepared from the powders with different milling times. The XRD results show that by decreasing the precursor powder size the Bi 2223 phase fraction increases. It was found that the grain size and grain connectivity improved by decreasing the precursor powder size. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (Tc) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities (Jc) with temperature indicates that the decreasing of precursor powder size in the Bi2223 system cases an increase in the intergranular critical current density.

The self-energy-functional approach is a powerful many-body tool to investigate different broken symmetry phases of strongly correlated electron systems. We use the variational cluster perturbation theory (also called the variational cluster approximation) to investigate the interplay between the antiferromagnetism and d-wave superconductivity of k-(ET)2 X conductors. These compounds are described by the so-called dimer Hubbard model, with various values of the on-site repulsion U and diagonal hopping amplitude t'. At strong coupling, our zero-temperature calculations show a transition from Néel antiferromagnetism to a spin-liquid phase with no long range order, at around t' ~ 0.9. At lower values of U, we find d-wave superconductivity. Taking into account the point group symmetries of the lattice, we find a transition between n d x2-y2 and dxy pairing symmetries, the latter happening for smaller values of U.

In order to study some theories about nonsuperconductivity of PrBa2Cu3O7, based on the density functional theory and with APW+lo/LAPW method some calculations for PrBa2Cu3O7 (Pr123) and YBa2Cu3O7 (Y123) were performed. The LSDA+U approximation was used for Pr (4f) orbitals and the effect of changing UPr on the band structure, Pr (4f)-DOS, distribution of electrons on the planes and chains, and Pr valence were investigated. Comparison of computational results with some experiments shows that a suitable region for UPr is a number larger than 0.4 Ry. With this selection the band structures of Pr123 and Y123 near Fermi energy are coincident completely. Therefore, the theories that present the reason for nonsuperconductivity of Pr123 corresponds to the difference of holes number or character of holes in Pr123 and Y123 were found to be incorrect.

We explore the important role of the strong electron-phonon interaction in high temperature superconductivity through the study of the results of some important experiments, such as inelastic neutron and X-ray scattering, angle resolved photoemission spectroscopy, and isotope effects. We also present our computational results of the eigenvalues and eigenvectors of the Ag Raman modes, and the ionic displacement dependence of the electronic band structure by density functional theory. It is clearly evident that the role of phonons in the mechanism behind the high-temperature superconducting state should be seriously considered.

In this paper, the effect of the cross-section on the critical current density (Jc) of a sample in ceramic superconductors YBa2Cu3O7-d (YBCO) and Bi1.6Pb0.4Sr2Ca2cu3Oy (BPSCCO) has been studied. Five orthorhombic bar samples of YBCO with cross-sections of 6.25, 7.67, 9.25, 11.76, 14.67 mm2 and also five orthorhombic bar samples of BPSCCO with cross-section of 6.4, 9.01, 11.88, 13.86, 14.98 mm2 with the same synthesis conditions by the solid state reaction method were prepared. After the preparation of the samples, the Meissner effect, the critical temperature (Tc), and the critical current density (Jc) measurements, XRD and SEM have been done on the samples. The results of XRD show that the dominant phase in YBCO and BPSCCO are 123 and 2223, respectively. The results of Jc measurements in 77 K show that in both superconductors, the Jc decreases with increasing of cross-section (A). The type of dissipation obeys a power law with the relation Jc = a A-b . For a given cross-section, Jc of the BPSCCO sample is smaller than the YBCO sample.

In this paper the effect of ammonium hydroxide addition to the solution of metallic oxide on sol gel preparation process of YBCO is studied with differential thermal analysis, thermal graviometry and X-ray diffraction. Two samples with and without ammonium hydroxide. Ammonium hydroxide prevents both barium nitrate precipitate during the gel preparation and also unwanted reaction as well as increasing homogeneous product. After drying the gel, the samples heated up to 1050°C in DTA apparatus in order to find more accurate the type and the temperature of reaction during the preparation process. After the initial reactions in the samples, Y2Cu2O5 and 123 phases are created in the range of 780-840°C and then the 123 phase is strengthened at 900-950°C. As shown in X-ray data, 123 was the only phase after this range. In addition ammonium hydroxide support and increase the creation of 123 phase at lower temperature.