A. Mirmelstein

Neutron-diffraction study of anisotropy of crystal-lattice expansion in La2−xSrxCuO4 under doping with Sr and radiation-induced disorder

Abstract The anisotropy of the La 2−x Sr x CuO 4 crystal-lattice expansion as a function of chemical content and radiation-induced disorder has been investigated by the neutron powder diffraction technique. The results obtained as well as the comparison with the anisotropy of the 1-2-3 crystal-lattice expansion give evidence for different couplings between the CuO 2 planes and charge reservoir in the copper oxide superconductors with different structures and of T c being higher the less this coupling is.

Neutron spectroscopic studies of crystalline electric fields in high-Tc ErBa2Cu3O7 doped with zn and Ni

Abstract The crystalline electric field (CEF) splitting of the ER 3+ 4f-shell in the high- T c superconductor ErBa 2 Cu 3 O 7 doped with Zn and Ni has been investigated by the inelastic neutron scattering tecnique. the observed exhibit a large number of well resolved CEF transitions between 8 and 80 meV, so that we have been able to determine all nine CEF parameters required for the orthorhombic symmetry. The results are used to derive information on the effective charge associated with the superconducting CuO 2 planes.

Specific heat of high temperature superconductors in high magnetic fields

Abstract We review investigations of the mixed-state specific heat (C) of extreme type-II superconductors in magnetic fields B≤16 T, from 1 K to Tc. In YBa2Cu3Ox (Y123) at low temperature T, a scaling law Δ C ∞( TB 1 2 )ƒ( T/B 1 2 ) , which is a signature of d-wave symmetry, is observed for the background-free anisotropic component ΔCC(B//c)-C(B//ab). At intermediate T, a C(B)C(0)∞TnBlnB behaviour, which may be explained by the London model, smoothly sets in. Near Tc, superconducting fluctuations are investigated using the background-free derivative ∂(C/T)/∂B. 3D-XY scaling properties are verified up to the overdoped regime. Fluctuations obey different laws in Bi2Sr2CaCu2Ox (Bi2212), which is strongly affected by its 2D character. Finally, the melting line of the vortex solid, which is now believed to be the only true phase transition above Hc1 in the presence of fluctuations, is observed in Y123 either as a 1st- or 2nd-order transition, depending on pinning.

D-wave specific heat in Y-123 and Bi-2212 single crystals: new data, alternative explanation

The low temperature specific heat of Bi-2212 and Y-123 is measured for 1.2≤T≤7K and 0≤B≤14 T. We find that the field-induced “linear” term λ*(B)∔λ*(0) near 1–3K saturates for B≥8 T, decreases as a function of temeprature, and has nearly the same amplitude for both compounds. This is not easily reconciled with the predicted d-wave contribution C∝γT(B/Bc2)1/2. Oscillations of 2D vortices offer an alternative explanation.

Crystal-field spectrum in RBa2Cu3Ox (R = Er, Ho) high-Tc superconductors: evidence for charge order in CuO2 planes

A model of the crystal field generated by a periodic array of charged tapes is developed to analyse the crystal-field interaction in RBa2Cu3Ox (R = Er, Ho) high-Tc copper oxides observed by the inelastic neutron scattering technique. The explicit calculation of the parameters of the Stevens Hamiltonian describing crystalline electric field effects in solids is performed for a specific charge density distribution uniformly extended in a certain direction of the crystal lattice. The model accounts for the x-dependence of the crystal-field parameters and allows us to determine the hole concentration in the CuO2 planes as a function of oxygen stoichiometry. The model of the periodic array of charged tapes suggests a charge order induced in the CuO2 planes by doping.

Neutron scattering studies of crystal structure and crystalline electric field in high-Tc ErBa2Cu3Ox disordered by fast neutron irradiation

Abstract The crystal structure and the crystalline-electric-field splitting of the Er 3+ 4f-shell in the high- T c superconductor ErBa 2 Cu 3 O x ( x = 6.95, 6.18) disordered with fast neutron irradiation has been investigated by elastic and inelastic neutron scattering techniques, respectively. The results yield evidence for the absence of a charge transfer between the planes and chains under disorder and point to the atomic displacements being the main radiation-induced defects in the CuO 2 planes.

Relation between structural distortions and Tc in high-Tc superconductors

Abstract Structural distortions in high-T c 1-2-3 compounds under low temperature fast neutron irradiation are studied with neutron powder diffraction and compared with oxygen deficient samples. The results show the absence of essential charge transfer between the chains and planes for radiation-induced disorder and point to random defects to be responsible for the behavior of physical properties of high-T c superconductors.

Synthesis, crystal structure and inelastic neutron scattering in the infinite-layer compounds Sr1−xNdxCuO2

Abstract For the first time large samples of the nominal composition Sr 1− x Nd x CuO 2 ( x =0.07, 0.15) 10 g in mass each have been prepared using a high-pressure technique. Neutron-diffraction and X-ray measurements have shown that the main phase in the samples obtained is of the infinite-layer structure which has been refined in the tetragonal P4/mmm space group. Both samples are not superconducting. Inelastic neutron scattering has been employed to search for crystalline-electric-field transitions in these compounds. The observed low-energy spectra exhibit one inelastic line of magnetic origin at 18 meV, comparable in energy with a crystalline-electric-field excitation in the high- T c superconductor NdBa 2 Cu 3 O x .

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T1−1)α and the echo-decay rate (T2−1) of 63Cu were measured for the electron-doped infinite-layer superconductor Sr0.93La0.07CuO2/Tconset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-Tc material. Above Tc the 63Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the 63Cu Knight shift in the electron-doped CuO2 layer can be understood as a “compensation effect” between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4pz) and/or Cu(3dz2) orbitals.

The effect of Ca and Th substitution on the crystal-field spectrum of the high-Tc superconductor HoBa2Cu3Ox

The inelastic neutron scattering technique was employed to study the transitions between the low-lying levels of the ground-state J-multiplet of the Ho3+ ions in Ho1-yCayBa2Cu3Ox (x≈6.3, 7.0; y≈0.1, 0.2) and Ho1-yThyBa2Cu3Ox (x≈6.3; y≈0.07, 0.13) split by the crystalline electric field. For oxygen-rich Ca-doped samples, the observed energy spectra exhibit a fine structure, which reflects the behaviour of the local hole density near the doping centre. For the first time this experimental observation unambiguously confirms that the charge inhomogeneity, which was found to be a characteristic feature of the underdoped cuprates, still exists in the deeply overdoped regime with Tc = 56 K. On the other hand, the modification of the crystal-field spectra observed for the oxygen-deficient samples with both Ca2+ and Th4+ substitutions for Ho3+ is found to result from distortions of the copper-oxygen polyhedron around the Ho3+ ion in the direct neighbourhood of the implanted ions, rather than from the doping-induced variation of the carrier concentration in the CuO2 planes.