R.J. Singh

EPR STUDIES OF HIGH-Tc SUPERCONDUCTORS AND RELATED SYSTEMS

A detailed and systematic review of the EPR studies of high-Tc superconductors and their related and parent systems are presented. The various EPR signals reported from the high-Tc oxides and from their related cuprates are summarized. The experimental evidences highlighted by various workers claiming the EPR silence of copper in the high-Tc materials are also reviewed. Various theoretical models suggested by several workers to explain the observed unusual EPR silence of copper in the high-Tc superconductors are critically discussed.

EPR observation of Cu2+Cu2+ pairs in cupric oxide powders

Abstract We report the observation of an EPR spectrum attributable to Cu2+Cu2+ pairs with resultant electronic spin S = 1, from cupric oxide annealed in the temperature range from 700 to 900°C. The spectrum consists of only a single line at g = 3.12 with seven hyperfine components. From the weight loss and oxygen content measurements, it was found that the observation of such a signal strongly depends on the oxygen content of the sample.

Copper tetramers in high-temperature superconductors

Abstract Clusters of four copper ions coupled by an exchange interaction are shown by EPR in the constituents of high- T c superconductors and in an oxygen deficient 123-superconductor. The spin-Hamiltonian parameters of the EPR spectra have been reported. Copper tetramers may be at the heart of the superconductivity mechanism.

EPR study of deoxygenated La2CuO4

Abstract The EPR spectra of deoxygenated La 2 CuO 4 were studied. On deoxygenation, various species are formed as copper monomers and copper tetramers. Their spin-Hamiltonian parameters have been given. The ferromagnetic coupling of spins of the 4 Cu 2+ ions in the tetramers may be due to superexchange interactions mediated by intervening oxygen ions. The importance of this study has been discussed.

EPR and optical studies of VO2+ ions in sodium formate single crystal

Abstract Results of EPR and optical studies of vanadyl ion (VO2+) doped in single crystals of sodium formate are reported. Impurity sites in the unit cell and the molecular structure of the vanadyl complexes are discussed. The spin Hamiltonian parameters are determined. From optical and EPR data, crystal field parameters are calculated and discussed. Two complexes at substitutional sites at two different levels of doping have been identified.

EPR spectra of deoxygenated high Tc superconductors

EPR spectra of deoxygenated Bi-2201, Bi-2212, Bi-2223, Y-123, Tl-2223 and Hg-1223 have been investigated. Different types of Cu-clusters as octamers, tetramers, dimers and monomers have been identified. EPR spectra were recorded for more than 100 samples of these materials, from different batches of preparations and from different portions of the same batch with the aim of recording all varieties of the spectra in these materials. Different samples of the same compound gave different spectra, some of which were repetitive but some spectra of the samples of different compounds were also similar. In different samples of the same compounds, different combinations of the spectra of octamers, tetramers, dimers and monomers appeared. In all 20 different types of spectra could be observed. The role of Cu-tetramers in the mechanism of high temperature superconductivity has been hinted at.

EPR study of La1.854Sr0.146CuO4

Abstract EPR spectra of La1.854Sr0.146CuO4 and its aging effect have been studied. Temperature variation of the EPR spectra of La1.854Sr0.146CuO4 (from room temperature to liquid nitrogen temperature, LNT) along with CaCuO2 for comparison has been carried out. In all the materials, after quenching the most abundant species, the ferromagnetically coupled Cu-tetramer (CuO)4 was obtained. The (CuO)4 square, being the unit cell of all important two-dimensional CuO2 planes superconductors, may reflect the properties of the superconductors. At about 100 K, in La1.854Sr0.146CuO4, the tetramer signal was destabilized, disappeared and was immediately followed by a new signal, which may be seen as a signature of formation of quantum stripes.

EPR study of La2-xMxCuO4 (M = Sr, Ba)

Abstract EPR spectra of deoxygenated La1.81Sr0.19CuO4 and La1.94Ba0.06CuO4 have been investigated. In both of them 4 Cu2+ ions are found to combine ferromagnetically to give S = 2 spectra. At low Sr Ba content the spins probably start getting polarised in the ab plane. Spin-Hamiltonian parameters for all the spectra have been determined. Dependence of Tc on the content of Sr or Ba has been attempted to be explained. It seems that holes which are generated on the substitution of La3+ by Sr2+ or Ba2+ and are current carriers first get uniformly distributed over the LaO plane but on increasing Sr2+ or Ba2+ content, they may combine together to form O22− peroxide ions and become unable to carry current. On further increasing the Sr2+ or Ba2+ content, the same process is repeated, until the substance becomes metallic or until further amounts of the substituents can bot be dissolved in the parent compound.

EPR and optical absorption studies of VO2+ and Cu2+ ions doped in LiHSO4 single crystals

Abstract Electron paramagnetic resonance and optical absorption studies of LiHSO 4 single crystal doped with VO 2+ and Cu 2+ have been carried out at room temperature. Two magnetically equivalent but differently oriented VO 2+ complexes are found and molecular bonding coefficients ( β 2 , ϵ 2 ), Fermi contact term ( k ) and dipolar hyperfine coupling parameter (ϱ) are reported. Some interesting but unusual results are obtained in the EPR and optical absorption studies of the Cu 2− ion doped LiHSO 4 .

EPR study of Cu2+ ion doped in potassium oxalate monoperhydrate single crystal

Abstract The EPR study of Cu 2+ doped potassium oxalate monoperhydrate is carried out at room temperature. Two magnetically inequivalent interstitial sites are found embedded in orthorhombic crystal fields. The ground state wave functions are constructed. The obtained results are compared with that of isotypic crystal Cu 2+ doped potassium oxalate monohydrate to explore the difference arising due to the presence of H 2 O 2 in the former and H 2 O in the latter crystal. The spin-Hamiltonian parameters (g- and A-values) covalency parameter (α′ 2 ), the mixing coefficients (α, β) and Fermi contact term ( k ) are calculated for both the complexes.