Cu chemistry of YBa2Cu3O6+x investigated by nuclear quadrupole resonance

The nuclear quadrupole resonance (NQR) technique has been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa₂Cu₃O_6+x(0 ≤ x ≤ 0.91). Essentially, four distinct NQR lines which correspond to 2, 3,4 oxygen coordinated Cu sites in the Cu-O chains and 5 oxygen coordinated Cu sites in the Cu-O planes have been observed. The zero-field NQR frequencies of these are centered at about 30.1, 24.0, 22.0 and 31.5 MHz for ⁶³Cu, respectively. For the antiferromagnetic ordered state (x ≤ 0.3), antiferromagnetic nuclear resonance (AFNR) has been observed at 90 MHz with quadrupole splittings associated with the moment-bearing Cu sites in the Cu-O planes. The relative intensities of these resonance lines depend on the oxygen content, and this gives us a microscopic understanding of the Cu chemistry of this system.     

Electrooptical effect in YBa2Cu3O6+x films

Optical absorption in YBa₂Cu₃O_6+x metal films carrying a direct current up to 100 mA is investigated. For films with a highly nonlinear I-V curve the current is observed to have a strong influence on interband absorption. The effect is not observed in the region of optical transitions at local levels. The results are analyzed from the standpoint of carrier localization and delocalization processes. Fiz. Tverd. Tela (St. Petersburg) 39, 1747–1749 (October 1997)     

Local magnetism in superconducting YBa2Cu3O6+x

Oxygen-deficient YBa₂Cu₃O_6+x samples were investigated by the muon spin rotation technique in zero external field. In an intermediate oxygen concentration region we find strongly relaxing μSR signals indicating the existence of internal magnetic fields. The strongly relaxing signals were observed only in samples with superconducting transition temperatures below 50 K. As an interpretation of the data we suggest that magnetic ordering and superconductivity coexist in these samples within regions of extensions of a few lattice constants, but not necessarily on the same sites.     

Theoretical study ofμ + site in YBa2Cu3O x site in YBa2Cu3O x

The muon is a useful probe of magnetic fields in superconductors, but knowing the field seen by the muon is often of limited value until we know where the muon is in the crystal lattice. Here we employ two independent theoretical methods to search for candidate muon sites:the potential energy field method, which seeks the minimum of the electrostatic potential of theμ ⁺, and themagnetic dipolar field method, which compares the calculated magnetic field (due to host electronic or nuclear dipolar fields) with the observed local fields at the muon. Work supported by Canadian NRC and NSERC.     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

Investigation of superconductivity and magnetism in ceramic YBa2Cu3O6+x

In this work the magnetic and superconducting properties of YBa₂Cu₃O_6+x , ceramic samples are studied with regard to there preparations technology and oxygen content. It is shown that at 0.41≤x≤0.45 the magnetic and superconducting states coexist.     

Recent progress on neutron studies of YBa2Cu3O6+x

A review is given of recent developments in inelastic neutron scattering experiments on the high temperature superconductor YBa_2Cu_3O_7 and its antiferromagnetic precursor YBa_2Cu_3O_6.2. These include the detection of high energy “optical” spin waves and the determination of the full spin Hamiltonian in YBa_2Cu_3O_6.2, a detailed study of the 40 meV magnetic resonance peak in the superconducting state of YBa_2Cu_3O_7, and experiments on the effect of superconductivity on phonon lifetimes in YBa_2Cu_3O_7. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anomalous proximity effect in underdoped YBa2Cu3O6+x josephson junctions

Josephson junctions were photogenerated in underdoped thin films of the YBa2Cu3O6+x family using a near-field scanning optical microscope. The observation of the Josephson effect for separations as large as 100 nm between two wires indicates the existence of an anomalously large proximity effect and shows that the underdoped insulating material in the gap of the junction is readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson relationship. This result appears to constrain the applicability of SO(5) theory to explain the phase diagram of high critical temperature superconductors.     

Impurities and quantum interference in the chains of YBa2Cu3O6+x

Motivated by recent experiments, we study the electronic structure near impurities in the chains of YBa2Cu3O6+x. Using a model of proximity induced chain superconductivity, we show that a resonance state in the chain density of states is induced only by a magnetic impurity. The spatial form of the resonance reflects the particle-hole nature of chain superconductivity and therefore distinguishes it from other broken symmetry phases. Because of quantum interference effects between impurities, the chains can undergo a quantum phase transition into a polarized state.     

Two-magnon Raman scattering in dielectric and superconducting YBa2Cu3O6+x crystals

Two-magnon Raman scattering in dielectric, as well as superconducting, YBa₂Cu₃O_{6 + x} single crystals with mobile oxygen content x = 0.2–0.7 and superconducting transition temperature T _c = 0–74 K is studied in detail. Doping with oxygen in the range of x = 0.2–0.5 leads to two-magnon scattering peak broadening and a shift in the spectral position of the peak towards lower energies. The most significant qualitative changes in two-magnon scattering in YBa₂Cu₃O_{6 + x} crystals are observed in a narrow oxygen concentration range near x = 0.7. This is explained by a considerable decrease in the correlation length ξ_AF of antiferromagnetic (AF) correlations upon an increase in the concentration of free carriers. For instance, doping is accompanied with a reduction of ξ_AF to values of several lattice constants a for x ≈ 0.7, a transition to the regime of short-range AF order, and local scattering of light from a small AF cluster with a size of 3 × 4 lattice constants. An increase in the free charge carrier concentration destroys the short-range AF order in a narrow range of the stoichiometry index near x = 0.7. Experimental data also indicate heterogeneity of cuprate planes at microscopic level, which leads to coexistence of superconducting and AF regions in YBa₂Cu₃O_{6 + x} super-conducting crystals.     

Anomalies of the electronic spin-lattice relaxation of Gd3+ in YBa2Cu4O8 and YBa2Cu3O6+x near 200 K

A sharp kink in the temperature dependence of the electronic spin-lattice relaxation rate near 200 K was found in the high-temperature superconductors YBa₂Cu₄O₈ and YBa₂Cu₃O_6+x . The effect is correlated with opening of a spin gap and microscopic phase separation. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 567–572 (25 April 1999)