Convergence of Theory and Experiments on Electron-Lattice Interactions in Cuprates: An Overview of SILS

A brief overview of some of main highlights of the Symposium on Localised and Itinerant States (SILS) is presented.     

Magnetization Studies of Mercury-Cuprates and Its Precursors

The mercury-cuprates such as (Hg, Cr)/Sr-1201 and (Hg, Re)/Sr-1201 were synthesized byencapsulation-technique at 880–920°C for 12–16 h, and characterized by power XRD,M(T) andM(H). All the precursors (Sr₂CuO₃, Cr_0.30Sr₂CuO _y or Re_0.10Sr₂CuO_y) were found to be non-superconducting. The samples with optimal composition (Hg_0.70Cr_0.30)Sr₂CuO_4+δ [(Hg, Cr)/Sr-1201] and (Hg_0.90Re_0.10)Sr₂CuO_4+δ [(Hg, Re)/Sr-1201] are superconducting withT _c ^onset ∼ 58 and ∼ 54 K, respectively. At low temperatures (below 10 K), on cooling in D.C.-magnetic field of 4 kOe, the sample (Hg, Re)/Sr-1201 shows a dramatic decrease in diamagnetic signal. Further, this effect is also faintly reflected in theM(H) curve. Such a behaviour was not observed in the (Hg, Cr)/Sr-1201 cuprates. We ascribe this effect to a field enhanced paramagnetic contribution possibly arising from mixed valent (6+, 7+ dominant) Re ions in the (Hg, Re)/Sr-1201 cuprates.     

A Significant Drop in Resistance Around 200 K in Superconducting Hg-1223 and Discussion

Tholence et al. and Yao et al. observed a significant drop in resistance around 250 K and 230 K respectively on superconducting Hg-based compounds. Yao et al. believed that this drop in resistance is due to the presence of the free-state liquid mercury or Hg-Au alloys in the sample. We report here the electrical resistivity measurements on single phase (as determined by X-ray powder technique) Hg-1223, which has shown a partial or total drop in resistivity around 200 K but with no significant observation of diamagnetic signal. On the other hand, an internal friction peak around 203 K was observed in our internal friction experiment. We are sure that the drop in resistance for our Hg-1223 sample around 200 K is not due to the presence of liquid mercury or Hg–Au alloys in the sample since we did not use the gold foil or silver foil to enwrap our sample. We rather believe that the drop in resistivity is due to superconductivity or other mechanisms.     

Relationship between superconductivity and band structures of electrons and phonons

The relationship between superconductivity and band structures of electrons and phonons is established on the basis of a generalized Bardeen-Cooper-Schrieffer model in which the interaction strengths (V ₁₁,V ₁₂,V ₁₂) among and between electron (1) and hole (2) Cooper pairs are differentiated. Elemental superconductors must have local hyperboloidal Fermi surfaces called necks or inverted double caps.     

A possible explanation for the isotope effect in high-Tc and Rb3C60 superconductors

The role of pair breaking effects on the isotope effect coefficient is investigated using the generalized Abrikosov and Gorkov theory of superconductors. It is found that the pair breaking effects enhance the isotope effect coefficient. This provides a plausible explanation for the enhancement of the isotope effect coefficient as a function of impurity concentration in yttrium-, europium-, and lanthanum-based high-T _c superconductors. It may also explain the very large value of the isotope effect coefficient in Rb₃C₆₀ superconductors provided some intrinsic disorder is assumed.     

Exploration of new superconductors and functional materials,and fabrication of superconducting tapes and wires of iron pnictides

Abstract

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ～1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ～700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.     

Coupling of Longitudinal and Transverse Stripe Fluctuations

The interplay of transverse and longitudinal fluctuations of a stripe is analyzed using bosonization techniques. We show the existence of a charge-density-wave instability for quarter-filled stripes if the underlying lattice potential has a zigzag symmetry. Our results are in accordance with the observation of static stripes formation in underdoped La_2–x–y Nd _y Sr _X CuO₄ exactly at the onset of the low-temperature-tetragonal transition.     

Possible Multigap Superconductivity in SmFeAsO0.8F0.2: A Point-contact Andreev-reflection Spectroscopy Study

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO_0.8F_0.2 state-of-the-art polycrystals, with critical temperature T _c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB₂ and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T _c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ₁=6.15±0.45 meV and Δ₂=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T _c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.     

Importance of out-of-plane dispersion of the excitation spectrum of superconducting bilayers with Josephson coupling

We study the effect of Josephson coupling between adjacent superconducting layers on the BCS energy spectrum. We find that the interference between the gap functions of two layers can lead to vanishing condensation energy for perpendicular momenta corresponding to the formation of standing waves. We therefore predict a conventional energy spectrum for large interlayer spacings, if the gap of the single layers has no nodes, and in all cases a gapless spectrum for small spacings. Within the experimental error, our numerical results account for the low-temperature dependence of the penetration depth reported in Nd_1.85Ce_0.15CuO₄ and YBa₂Cu₃O_6.9.     

Vibronic coupling constants in BEDT-TTF and C60

The results of a fullyab-initio determination of the vibronic coupling strengths in molecular and organic superconductors based on C₆₀ and BEDT-TTF are presented, with the vibrational and the electronic structure treated on an equal footing. We obtain accurate vibrational frequencies, and the corresponding coupling constants are obtained without parameters. We examine the implications of the coupling constants in relation to the molecular vibronic coupling theories of superconductivity in these materials. We find that for C ₆₀ ^3– the coupling to both the low- and high-frequency molecular modes are important. For the BEDT-TTF-based organic superconductors the effects of isotope substitution are also examined.     

Current trends in the development and applications of superconducting materials

The discovery of the phenomenon of superconductivity by Kamerlingh Onnes in 1911 was the first indication of the possibility of electrical conduction without any associated Joule loss. The technological application of the property (which was essentially manifested at liquid helium temperatures) had to await the development of stable superconducting materials capable of withstanding high currents and large magnetic fields. Although many materials — elements, alloys, ternary chalcogenides, and recently oxides — have been found to be superconducting, only a few of them have received attention for significant applications. This is based on three important parameters namelyT _c , the transition temperature,H _c2, the upper critical field andJ _c , the critical current density.T _c andH _c2 are considered intrinsic to the material, whileJ _c is influenced by the microstructure, and has to be optimised during fabrication of the material in the useful form. On these considerations, Nb-Ti, Nb₃Sn and V₃Ga have emerged as proven materials for significant applications while PbMo₆S₈ is still under development. Despite the fact that all these materials have to be used only at liquid helium temperatures on account of their lowT _c , major developments have taken place in harnessing particularly the niobium alloys to produce superconducting magnets. Towards the end of 1986, a break-through has been achieved in the direction of raising theT _c . Many ceramic oxides, notably Y₁Ba₂Cu₃O₇, have exhibitedT _c in the vicinity of 100 K. These materials have also been shown to have highH _c2, about 180 Tesla. Attempts are now being made to realise a highJ _c . It is too early to say whether such materials can be fabricated in suitable forms capable of carrying high currents. Among the major areas in which superconducting materials have so far been used, mention should be made of superconducting magnets for high energy particle accelerators, magnetohydrodynamic power generation, magnetic resonance imaging, and fusion research programmes. In other potential applications such as motors and magnetically levitated transportation, economic break-even has not been achieved, mostly on account of the need to use liquid helium. The discovery of the high temperature superconductors capable of operating at liquid nitrogen temperatures thus promises a revolution in electrical technology. The paper reviews the development and applications of superconducting materials, with reference to work being done in India. Talk delivered at the International Conference on Recent Advances in Materials and Processes, Varanasi, November–December 1987.     

铁磷族化合物及其超导电性(英文)

氟掺杂的LaFeAsO中26K超导电性的发现极大地促进了凝聚态物理的发展。在短短的两年时间中,已经有过千篇文章发表,已经有七种新结构被发现,超导转变温度也被迅速提升到了55～57K。此外,在超导机理方面这类化合物也已经被认定属于非常规,而反铁磁自旋涨落被认为是导致超导配对的媒介。从材料和物理学的角度对这类化合物的性质和超导机理给出了简单的综述。还对未来的研究及应用给出了一些展望。     

Magnetothermal transition in YBCO melt textured materials

Conductors cut from large YBCO single domains textured by a combination of Melt Texturing Growth and Top Seeding techniques were studied for Fault Current Limiter applications. In self field, homogeneous transport critical current densities as large as 4 10⁴ and 9 10⁴ A/cm² were measured in shaped YBCO single domains up to 5 cm in length. Above J_c, experiments focused on magnetothermal transitions showed that the silver coating should have an adequate thickness so as to prevent the conductors from burning. Capability of textured materials to support high instantaneous power (40 GW/m³) and energy dissipation (190 MJ/m³) during the transition was demonstrated in a 4 T magnetic field.     

Mössbauer studies of Gd-doped bismuth-based ceramic superconductors

The results of¹⁵⁵Gd Mössbauer studies (above 1·5K) of the oxides, Bi_3·5Gd_0·5Ca₃Sr₃Cu₄O _y and Bi₂Sr₂Ca_1?x Gd _x Cu₂O _y (x=0·25 and 0·5) are reported. The observed electric quadrupolar split resonance patterns suggest that Gd ions prefer to occupy Ca site in both series. The spectral features give evidence for paramagnetic relaxation of isolated Gd ions at low temperatures.     

Metallic materials for superconductor stabilization with very high specific heat and good thermal conductivity

In a search for materials useful for enthalpy stabilization of technical superconductors we have performed specific-heat measurements on PrCu₂, PrB₆, NdSn₃ and Nd_0.9Pr_0.1Sn₃ between 3 and 10 K and in magnetic fields up to 7 T. Due to different types of phase transitions these compounds exhibit large peaks in the heat-capacity curves. In these materials the high specific heat values are not substantially reduced the the external magnetic fields, which makes them attractive for technical application.     

The Study on AC Susceptibility and Magnetization of High-Tc Superconductor Bi-2223 Using a Critical-State Model

The Anderson–Kim model for a granular superconductor was employed to calculate both temperature and magnetic field dependencies of the AC susceptibility of a Bi-2223 superconductor. Moreover, similar calculations were performed for the magnetization. The prediction of the model, including the intergranular and intragranular contribution, for susceptibilities and magnetizations was consistent with experimental data very well by considering the temperature-dependent effective volume fraction. The temperature-dependence of fitting parameters was shown to obey almost quadratic power relation (1–T/T _c) with 2.     

Positron annihilation spectroscopy in materials science

The application of positron annihilation spectroscopy to the study of defects in materials is illustrated through several examples drawn from our work. These include the study of vacancy clustering in metals, clustering of He atoms to form bubbles, and solute clustering in alloys. Results of studies on novel materials such as quasicrystals, cuprate superconductors and fullerenes are also presented.