Plasma resonance in layered normal metals and superconductors

A microscopic theory of the plasma resonance in layered metals is presented. It is shown that electron-impurity scattering can suppress the plasma resonance in the normal state and sharpen it in the superconducting state.     

Electronic and vibrational excitations in layered highT c superconductors

In the series of the layered highT _c superconductors (A)_1,2 B₂Ca _n−1Cu _n O_2n+3,4 (A=Tl or Bi;B=Ba or Sr), withn=number of consecutive Cu-O layers, the electron energy states of some of the Tl-based systems have been investigated. The electron and phonon dispersion curves have been obtained. The electronic states near the Fermi level are dominated by the hybridized Cu(d) and the O(p) orbitals. The dispersion curves are highly two-dimensional with very small dispersion alongc-axis. The number of bands is enhanced with increase in the number of the consecutive Cu-O planes (n). The present results agree with those obtained earlier for other superconducting phases. The phonons are overall dominated by the vibrations of the light mass oxygen atom modes both lying in or outside the Cu-O planes. The low-frequency phonons involve the motions of the heavier Tl, Ba atoms etc.     

Some properties of vortices in layered superconducting structures

Some properties of vortices in layered superconductors and superconducting structures with the Josephson interaction between S-layers are studied theoretically. The Josephson vortex lattice is formed in the system if the magnetic fieldH _e is parallel to the layers. Long-wave oscillations of this lattice have a sound-like spectrum; the velocity of “magnetic sound” propagating across the layers is small and diminishes with increase ofH _e . Using an analogy with electron-hole plasma in semiconductors, we investigate also the behaviour of the layered structure in the fieldH _e perpendicular to the layers at temperatures close to the Kosterlitz-Thouless transition temperatureT _KT. The spatial dependence ofH(r) and the impedance of the system are determined atT>T _KT.     

Slip processing of highT c superconductors

Slip-processing technique has been used to fabricate tapes and alumina-supported films of superconducting YBa₂Cu₃O_7−x . Good densification and connectivity are revealed by scanning electron microscopy. Both the tape and film show superconductivity well above the liquid nitrogen temperature with a transition range of 3° and 5°K respectively.     

New effects in highT c superconductors

We show that a proper interpretation of the critical field measurements can distinguish spin fluctuation mechanism from others in the context of the pairing hypothesis. We can predict the existence of a new mode in the mixed state of a superconductor.     

A new look at the highT c superconductors using BCS-theory

Using the conventional phonon-exchange mechanism of superconductivity we have succeeded in reproducing the transition temperatureT _c of a large number of newly discovered highT _c superconductors by introducing a certain modification to the well-known BCS-formula forT _c .     

High resolutionV-I characteristics measurements on highT c superconductors

V-I characteristics of sintered superconducting pellets of YBa₂Cu₃O_7−δ was measured with a resolution one order of magnitude better than the usual dc techniques employing nanovoltmeters. For this purpose software-based lock-in-amplifier technique was developed and used. A square-wave excitation current was used. The voltage signal was digitized into a time series and Fourier-analysed on a computer in this technique. Our results show a very small frequency-dependent resistance in some samples which are nominally superconducting. These results help in checking the material quality and defining the critical current densities better.     

Electron microscopic investigation of high-temperature oxide superconductors

Electron microscopic investigations have been carried out on superconducting YBa₂Cu₃ O_7−δ, NdBa₂Cu₃ O_7−δ and related oxides. All these orthorhombic oxides exhibit twin domains. Based on high resolution electron microscopy, it is shown that there is no significant change in the structure across the twins. Oxides of the La_2−x Sr _x (Ba _x )CuO₄ system do not show twins, but exhibit other types of defects. Twins appear to be characteristic of only the orthorhombic 123 structures. Communication No. 100 from Materials Research Laboratory.     

Relative thermodynamic stabilities of constituent oxides of highT c superconductors

A thermodynamic scale of the relative stabilities of constituent oxides can help the choice of stoichiometric multi-component compositions and can provide guidance for judicious selection of heat-treatment conditions for high-T _c superconductors. A thermodynamic analysis was undertaken to study Y₂O₃-BaO-CuO, BaO-K₂O-Bi₂O₃ and La₂O₃-SrO-NiO systems. The relative stability of the oxides was expressed in terms of two ratios,S _v andS _p computed using free-energy of formation and vapour pressure data. CuO, Bi₂O₃ and NiO were taken as reference oxides for YBa₂Cu₃O_7\t-\gd, Ba_0·6K_0·4BiO₃ and La_1·9Sr_0·1NiO₄ respectively. Thermodynamically, the reference oxide was found to be the least stable amongst the constituent oxides in each of these systems.     

CESR in superconductors: Relevance of highT c copper oxide systems

CESR is a highly effective tool to study the interaction among conduction electrons in normal metals. In superconductors belowT _c it can reveal vital information concerning the pairing interactions. A comparative study of highT _c and conventional superconductors is presented and it is shown that the disappearance of CESR in the superconducting state is a feature common to both conventional and highT _c superconductors and establishes the importance of exchange interactions in pairing. This is supported by experimental observations.     

Orbital-dependent effects of electron correlations in microscopic models for iron-based superconductors

The bad metal behavior in the normal state of the iron-based superconductors suggests an intimate connection between the superconductivity and a proximity to a Mott transition. At the same time, there is strong evidence for the multiorbital nature of the electronic excitations. It is then important to understand the orbital-dependent effects of electron correlations. In this paper we review the recent theoretical progresses on the metal-to-insulator transition in multiorbital models for the iron-based superconductors. These include studies of models that contain at least the 3d xy and 3d xz/yz models, using a slave-spin technique. For commensurate filling corresponding to that of the parent iron pnictides and chalcogenideds, a Mott transition generally exists in all these models. Near the Mott transition, a strongly correlated metal exhibiting bad metal features and strong orbital selectivity is stabilized due to the interplay of Hund’s coupling and orbital-degeneracy breaking. Particularly for the alkaline iron selenides, the ordered vacancies effectively reduce the kinetic energy, thereby pushing the system further into the Mott-insulating regime; in the metallic state, there exists an orbital-selective Mott phase in which the iron 3d xy orbital is Mott localized while the other 3d orbitals are still itinerant. An overall phase diagram for the alkaline iron selenides has been proposed, in which the orbital-selective Mott phase connects between the superconducting phase and the Mott-insulating parent state.     

Crystal chemistry of Ruddlesden-Popper type structures in highT c ceramic superconductors

Similar structural patterns have been noticed in the systems La-Cu-O, La-Ni-O and Bi and Tl-containing superconducting oxides. The formation of Ruddlesden-Popper type layers (alternating slabs of rocksalt and perovskite structures) is seen in these oxides which is similar in many respects to what is seen in the system Sr-Ti-O. However, there are some significant differences, for example the rocksalt and perovskite blocks in new superconducting compounds are not necessarily electrically neutral, unlike in the Sr-Ti-O system. It, thus, becomes necessary to create oxygen vacancies in the basic perovskite (figure 1) structure of Bi-containing compounds, when the width of the perovskite slab changes on addition of extra Cu-O planes. Results of our atomistic simulations suggest that these missing oxygen ions allow the Cu-O planes to buckle in Bi-series of compounds. This is also supported by the absence of buckling in the Sr-Ti-O series of compounds and the first member of Bi-containing compounds in which there are no missing oxygen ions. We present additional results on the phase stability of polytypoid structures in La-Cu-O system and defect chemistry of compounds of La-Ni-O system.     

Collective mode dynamics of the vortex lattice in a highT c superconductor

Dispersion relations have been obtained numerically for the collective modes of the flux line lattice of a highT _c superconductor in the London limit and the effect of pinning forces is included. It is found that the waves propagating along the magnetic field the dispersion consists of two branches and that the low frequency branch may interact with other low-frequency excitations in the lattice.     

Study of superconductors by a.c. susceptibility

An apparatus used for a.c. susceptibility measurements is described and the usefulness of a.c. susceptibility measurements for investigating superconductors is discussed. The importance of carefully studying the real and imaginary parts of a.c. susceptibility is pointed out as illustrated by the results obtained on A15 compounds (V₃Si) and on Nb-Ti samples.     

A.c. magnetic measurements on superconductors using two-channel dynamic analyser

A.c. magnetic measurements are powerful tools for characterizing superconducting materials. The a.c. susceptibility is the most used measurement and is commonly performed using a two-phase lock-in amplifier. This article describes a different method to carry out this measurement, employing a two-channel dynamic analyser. This instrument also allows us to measure further quantities, such as higher harmonics of the a.c. susceptibility, the so-called wide-band susceptibility and the magnetization loops. A remarkable feature of the dynamic analyser is its wide frequency range, from 244 μHz to 102 kHz, allowing simple, comprehensive and reliable characterization of superconductors.     

Properties of Bi-Sr-Ca-Cu-O highT c superconductor

The effect of sintering in air and annealing in oxygen flow at various temperatures and times on the superconducting transition of Bi₂Sr₂Ca₁Cu₂O₈ ceramic was studied. Samples sintered at 1138 K for 24 h in air and slowly-cooled to room temperature are single-phase with the highest zero resistivity temperature of 88 K. XRD pattern showed that the single-phase was characteristic of tetragonal structure witha=0.5398 nm andc=3.0620 nm. Both a.c. susceptibility and resistivity measurements give similar values for the onset (100 K) of superconducting transition.     

Effect of fluorine on the critical current density of thallium based highT c superconductors

(Tl_0.5, Pb_0.5)Sr_1.6Ba_0.4Ca₂Cu₃O_y/F_x high temperature superconductors are reported with transition temperatures above 125 K. Samples with different fluorine contents (0 x 3) have been prepared. The addition of fluorine has little effect on the transition temperature, but remarkably increases the width of the hysteresis loops (which is proportional to J_c). The width of the hysteresis loops at 4.2 K was found to increase with x reaching an optimum value at x = 2.2. The. maximum width of the hysteresis loops is about 3 times larger than that for fluorine free material. Upon further increases in fluorine content the width of the hysteresis loops starts decreasing.     

A realistic microscopic theory for the high-Tc cuprates

The electronic structure of the high-T_c cuprates is worked out by decomposing the orbitals around the Fermi level into large-U and small-U components. The large-U orbitals are treated by the slave-fermion method, the small-U orbitals by a mean-field approach, and the hybridization between them is then worked out. It turns out that hopping energy binds the spin and the charge of the large-U orbitals. The state so obtained is either antiferromagnetic, or paramagnetic with antiferromagnetic fluctuations and with violation of parity. Two types of charge carriers are predicted: (i) spinless polarons with a very small bandwidth, explaining the anomalous thermoelectric power, the mid-infrared peak, lattice anomalies, etc.; (ii) anomalous carriers of both charge and spin, explaining marginal-Fermi-liquid behavior and deviations from it, anomalous relaxation time, the systematic behavior of the resistivity, the Hall constant, the Hall angle, etc.     

Resputtering-induced chemical inhomogeneity during the growth of highT c superconductor thin film

HighT _c films belonging to Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been fabricated by reactive sputtering of single targets in a planar magnetron and Ar + O₂ sputtering atmosphere. Although it was possible to deposit films of correct composition, resputtering related composition variation was a problem. The key to obtaining correct chemistry was a proper control of the deposition parameters. The pressure and oxygen content of the sputtering gas were found to be the most critical parameters. Results of the variation of these parameters on the cation chemistry are presented in this paper. Results from Monte-Carlo simulation of the sputtering process are also presented showing that low pressure and oxygen content of the sputtering gas result in a higher yield of energetic reflected neutrals which can cause compositional variation in the film mainly due to preferential sputtering of the growing film. The effect was particularly noticeable directly underneath the target. The energetic particle bombardment can be controlled by using moderately high pressures and low oxygen concentration in the gas.     

一种新型功能材料——二维层状磁性锌铝水滑石的制备及表征

采用共沉淀法将磁性基质与锌铝水滑石组装制备二维层状磁性锌铝水滑石,考察了不同n(Zn2 )/n(Fe2 )比及焙烧温度对合成磁性锌铝水滑石结构的影响,并借助XRD、FT-IR、VSM、HRTEM等手段对其晶型结构、磁学性能及结晶度等进行表征,结果表明:锌铝水滑石在赋予磁性后,并没有改变其典型的层状结构,证实了将磁性基质与层状材料组装合成的可行性.