V. Bobrovskii

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.

63Cu NMR study of infinite-layer compound Sr1−xLaxCuO2

Abstract We present the 63 Cu NMR studies on powder samples of an electron-doped infinite-layer high- T c cuprate superconductors Sr 1− x La x CuO 2 ( x =0, 0.07, 0.16). We have found the decrease of magnetic shift due to the superconducting transition. The temperature dependence of the spin-lattice relaxation rate ( T 1 T ) −1 demonstrates non-Korringa Curie–Weiss like behavior in normal state. These results are very similar to the general case of hole-doped high- T c superconductors. The decrease of the quadrupolar frequency with increasing the doping level has been found.

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.

Superposition structure of crystal field spectra in high-Tc superconductors: Overdoped regime

Experiments on inelastic scattering of neutrons show that the crystal field spectra for high-Tc super-conductors R1−yCayBa2Cu3Ox≈7 (R=Ho, Er; 0 > y > 0.25) have two spectral components associated with optimally doped and overdoped clusters, respectively. An increase in the calcium concentration does not affect the local density of charge carriers in clusters, but changes the concentration of clusters themselves and, hence, the spectral weights of the spectral components. In light of such a “two-phase” pattern observed earlier for cuprate-based superconductors with a doping level below optimal, an increase in the charge carrier concentration leads to a smooth transition (crossover) from the underdoped regime to the overdoped one. The obtained results show, however, that these two regions of the phase diagram differ qualitatively in the form of charge distribution in CuO2 planes responsible for superconductivity.

Frustrated phase separation in the overdoped regime of '123' high-Tc superconductors

Recent inelastic neutron scattering measurements revealed the crystal-field (CF) spectra of the overdoped R1 − yCayBa2Cu3O≈7 (R = Ho, Er; 0 < y < 0.25) high-Tc superconductors to consist of two spectral components associated with the optimally doped and the overdoped domains. Increase of the Ca concentration does not affect the local charge density of domains, but changes the spectral weight of the components. In the sense of this “two-phase” picture, which was established earlier for the underdoped region, there is a smooth crossover between the under- and overdoped parts of the phase diagram. The charge origin of the observed superposition effect is confirmed by the systematic analysis of the concentration dependences of the CF parameters for both Ho- and Er-based compounds. Therefore, the local charge inhomogeneity in the CuO2 planes of high-Tc cuprates is a characteristic feature of the doping process, which depends neither on the way to introduce doping nor on the doping level.

Peculiarities of crystal structure and crystal-field excitations in “1212” AlSr2RCu2O7 (R=Y, Er)

Abstract Investigation of substitution effects for various sites in the “1212” perovskite-like systems with a general formula R(Sr,Ba) 2 (Cu 3− y M y )O 7− δ (M=Fe, Co, Hg, Al, Ga, Zn, and Ni and R=rare earth) is of great interest in connection with the problem of high- T c superconductivity. The crystal structure of the “1212” AlSr 2 RCu 2 O 7 (R=Y, Er) compounds has been studied using high-resolution neutron powder and electron microscope diffraction measurements. Inelastic neutron scattering has been employed to search for crystalline-electric-field (CEF) transitions. The structural results yield evidence for superlattice ordering with orthorhombic symmetry ( Pm2a space group). The CEF spectrum turns out to be a superposition of two components, which can be related to a local inhomogeneity of charge states within the CuO 2 planes.

Charge order in high-Tc copper oxides probed by crystalline electric field interaction

Abstract Inelastic neutron scattering has been employed to study the transitions between the lowest levels of the J-multiplet of the Ho3+ ions in Ho1−xCaxBa2Cu3Oy ( x=0–0.3, y=6.3, 7 ) split by crystalline electric filed (CEF). The observed energy spectra exhibit a fine structure which reflects the behavior of the local hole density near the doping center. The results obtained are discussed in terms of the model of the CEF generated by the ordered planar charge structure.

Charge order in high-Tc copper oxides probed by crystal-field interaction

Abstract Inelastic neutron scattering has been employed to search for the transitions between the lowest lying levels of the J -multiplet of the Ho 3+ ions in Ho 1− x Ca x Ba 2 Cu 3 O y ( x =0–0.3; y =6.3–7) split by crystalline electric field (CEF). The observed energy spectra all exhibit a complex fine structure, which reflects the behavior of the local hole density near the doping center. We have found that the obtained results can be explained in terms of the model of the CEF generated by the ordered planar charge structure.

Neutron spectroscopic study of crystalline electric-field in infinite-layer Sr1−xNdxCuO2

Abstract Inelastic neutron scattering has been employed to search for the crystalline-electric-field (CEF) transitions in the infinite-layer (IL) Sr 1−x Nd x O 2 compounds produced by high-pressure technique. For the first time we have been able to observe the complete energy spectrum of the ground state J-multiplet of Nd 3+ in the IL phase and to determine the CEF parameters. Dependence of the CEF splitting on Nd doping is discussed.