Theory of superconductivity at high temperatures

Superconductivity is generally explained by an electron-lattice interaction which results in the pairing of electrons and the condensation of these pairs into a state of lower entropy. In high-temperature superconductivity the pairs consist of hybrids in which the top of the oxygen band of the crowded perovskite layer is mixed with the bottom of unoccupiedd- orf-bands from monoxide layers in the crystal. Only electrons and phonons with low quasi-momentum (k) values participate. This makes it possible to localize the lattice perturbation into broad regions in which the van der Waals forces are reduced and the perovskite planes are contracted. The low entropy state associated with superconductivity manifests itself as the formation of a superlattice of lattice distortions which is in actual motion in the current-carrying states. The observability of this superlattice is discussed.     

纤锌矿结构ZnO的准粒子能带结构

理论上对ZnO能带的计算一般采用局域密度近似(LDA),而该方法得到的带隙结果却被严重的低估了.在本文中,我们在密度泛函理论的LDA近似的框架下,通过第一性原理GW近似(GWA)对ZnO的能带进行了修正.在LDA和GWA计算中,将Zn3d电子作为价电子,LDA结果表明ZnO是一种直接带隙半导体,同时讨论了LDA和GWA计算得到的能带之间的差异.     

High-Tc superconductivity and electronic band structure

We review the main results of the van Hove scenario applied to superconducting cuprates. It is based on the assumption that in these materials, the Fermi level lies near a singularity in the density of states (DOS). This hypothesis has recently been confirmed experimentally. We show that this model explains many properties of the high-T _c superconductors. We show that an anaogous model with a peak in the DOS may also be applied to the superconducting doped fullerenes. A general feature of the model is a very short coherence length.     

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.     

Plasmons and phonons in anisotropic systems and their effect on high-Tc superconductivity

The structure of the plasmon and phonon bands in conducting, highly anisotropic layered materials is studied. The observed high superconducting transition temperatures are caused, in our view, by the coexistence of strong electronphonon coupling and the plasmon mechanism. A generalized Eliashberg equation describing the effects of phonons and plasmons on the pairing is presented. Our approach is based on the method of thermodynamic Green's functions. We point out connections to experiments and briefly comment on other models.     

Structure and superconductivity of isotope-enriched boron-doped diamond

Abstract

Superconducting boron-doped diamond samples were synthesized with isotopes of ¹⁰B, ¹¹B, ¹³C and ¹²C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the ‘diamond-carbon’-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm^?1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron. The absence of a superconducting transition (down to 1.6 K) in diamonds synthesized in the Co–C–B system at 1900 K correlates with the small boron concentration deduced from lattice parameters.     

Structure and superconductivity of isotope-enriched boron-doped diamond

Superconducting boron-doped diamond samples were synthesized with isotopes of ¹⁰B, ¹¹B, ¹³C and ¹²C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the ‘diamond-carbon’-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm⁻¹ Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron. The absence of a superconducting transition (down to 1.6 K) in diamonds synthesized in the Co–C–B system at 1900 K correlates with the small boron concentration deduced from lattice parameters.     

Physics of electrons and phonons in low-temperature detectors

An attempt is made to explain on a simple basis how a low-temperature detector works. The different physical processes involved in the production, thermalization and interaction of phonons with electrons, which finally provide the measured electrical signal will be reviewed to give what is needed to determine the basic properties of a low-temperature detector. Spurious signals coming from microphonic noise will be evoked.     

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.     

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi_1·9−x Pb _x Sb_0·1Sr₂Ca₂Cu₃O _y withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withT _c=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.     

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.     

Normal zone propagation within a superconducting magnetic system and cryostabilization of superconductivity

A theoretical study of normal zone propagation along composite conductors in contact with liquid helium which accounts for the resulting rate of heat release and non-uniformity of heat conduction coefficient is described. The Maddock-James-Norris theorem is shown to be true only for a constant heat conduction coefficient. The formulae are given for the speed of the normal zone propagation. Equations have been obtained which describe the collapse dynamics or normal zone growth within the superconducting magnetic systems; time of collapse and normal zone growth being evaluated. Non-linear heat waves of a new type are assumed to exist in a composite in addition to the solution with an interface of normal and superconducting regions.     

c-axis phonons and the enhancement of pairing correlations in high-temperature superconductors

We consider the two-dimensional Hubbard model including electron-phonon interaction. Strong local correlations (U limit) are taken into account within the mean-field approximation for auxiliary boson fields. Phonon-assisted transitions between intraand interlayer states are introduced as the source of coupling between two-dimensional CuO₂ layers. This type of processes effectively leads to the nonlinear (quadratic) interaction of intralayer electrons withc-axis phonons. We construct the Eliashberg equations for the resulting Hamiltonian and evaluate the superconducting transition temperatureT _c. Our model calculation demonstrates that a pronounced enhancement ofT _c in thed-wave channel is possible. The largest enhancement ofT _c tends to take place for small hole concentrations. This means that the coupling toc-axis phonons could compete with two-dimensional correlations responsible for the onset of antiferromagnetic order. It is remarkable that the two-dimensional features in the normal state are hardly affected by this specific interlayer interaction. Therefore,c-axis two-phonon-mediated interlayer coupling can cooperate with interlayer pair tunneling and substantially contribute to an increased pairing.     

Effect of heat treatment and composition on the superconductivity in Pb-Sr-R-Ca-Cu-O oxide system

The effect of compositions, sintering temperatures and furnace atmospheres on the superconducting transition temperatures of several members of the series Pb₂Sr₂R_1−x Ca _x Cu₃O _z (0.0⩽x⩽0.7, R=Y, Gd) has been studied. The effect of partial replacement of Pb, Ca and Y by In has also been studied. The shapes of the resistivity-temperature curves and the zero resistance temperatures are found to be extremely sensitive to the synthesis parameters employed. Superconductivity is not observed in samples treated in oxidizing atmosphere or heated at very high temperatures. Samples sintered in flowing nitrogen showed incomplete transition in resistivity starting at about 75 K and extending down to 15 K. A new and comparatively simple synthesis procedure involving treatment of the samples in vacuum has been employed to obtain nearly single-phase materials showing metallic behaviour. Using this procedure, samples of the above composition, but containing no Ca(i.e. Pb₂Sr₂RCu₃O_z, R=Y and Dy), are also found to be superconducting with zero resistance up to 48 K.     

Band structure and electron-phonon interaction of LaAgO3

A comparison is made between the band structure of LaAgO₃ and LaCuO₃, which is the cubic counterpart of the high-T _c superconducting oxides. The electron-phonon parameter is also calculated and is found to be larger for Ag and O in LaAgO₃ than for Cu and O in LaCuO₃.     

The frozen-field approximation and the Ginzburg-Landau equations of superconductivity

The Ginzburg-Landau (GL) equations of superconductivity provide a computational model for the study of magnetic flux vortices in type-II superconductors. In this article it is shown through numerical examples and rigorous mathematical analysis that the GL model reduces to the frozen-field model when the charge of the Cooper pairs (the superconducting charge carriers) goes to zero while the applied field stays near the upper critical field.     

Effect of the fluctuation range of formation temperature on the preparation and superconductivity of Bi(Pb)2223 single-phase

Effects of the fluctuation range of formation temperature on preparation and superconducting properties of the Bi(Pb)2223 phase have been investigated in detail. Our results show that knowledge about the temperature distribution and fluctuation in the muffle furnace is necessary for preparation of the Bi(Pb)2223 single-phase. Single-phase samples from Bi_0.17Pb_0.3Sr₂Ca₂Cu₃O_y were obtained by a solid-state reaction in air at 835±5C. The best superconducting property of the as-prepared samples shows a one-step transition of a.c. susceptibility withT _c=109 K. Using this optimum sintering temperature and keeping the temperature fluctuation less than 5C, we have prepared pure Bi(Pb)2223 single-phase samples with good reproducibility.     

Unconventional pairing in doped band insulators on a honeycomb lattice: the role of the disconnected Fermi surface and a possible application to superconducting β-MNCl (M=Hf,Zr)

Abstract

We investigate the possibility of realizing unconventional superconductivity in doped band insulators on the square and honeycomb lattices. The latter lattice is found to be a good candidate due to the disconnectivity of the Fermi surface. We propose applying the theory to the superconductivity in doped layered nitride β-MNCl (M= Hf, Zr). Finally, we compare two groups of superconductors with disconnected Fermi surface, β-MNCl and the iron pnictides, which have high critical temperature T_c, despite some faults against superconductivity are present.