Resistive transition and upper critical field in underdoped YBa2Cu3O6+x single crystals

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa₂Cu₃O_6+x crystals in the range T _c≲30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T≲13 K it decreased with increasing magnetic field. The transition point T _c(B) was determined by analyzing the fluctuation conductivity. The curves of B _c2(T) measured in the region T/T _c≳0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of T _c. The only difference among the curves of B _c2(T) for different crystal states is the scales of Tand B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov’s model of boson superconductivity. Zh. éksp. Teor. Fiz. 115, 268–284 (January 1999)     

Correlation between superfluid density and T(C) of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.     

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.     

Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa₂Cu₃O_6+x (x≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρ_ab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σ_ab=1/ ρ _ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σ_c=1/ρ_c followed a power law, and neither σ_c(T), nor ρ_c(T) could be fitted by log T. It follows from the ρ_c measurements that the transformation at T=0 is split into two transitions: superconductor-normal-metal and normal-metal-insulator. In our samples, they are separated in oxygen content by Δx≈0.025. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 834–839 (10 June 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magneto-optical evidence for a gapped fermi surface in underdoped YBa2Cu3O6+x

The infrared (900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in the normal state of underdoped High T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples, which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in density wave models.     

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.     

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.     

A study of the upper critical field and anisotropy in YBa2Cu3O7 single crystals

Il Nuovo Cimento D - We report a set of experimental results on second- and third-harmonic generation at microwave frequencies in YBa2Cu3O7 single crystals near T c. The investigation of the...     

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