A. Kebede

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa2(Cu1−xZnx)3O7−y withy∼0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa2Cu3O7−y causes a rapid and nearly linear depression of the superconducting transition temperature,Tc, withTc going to 0 K forx≥ 0.10. YBa2(Cu1−xZnx)3O7−y retains the YBa2Cu3O7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.

Magnetic and thermodynamic properties of nonsuperconducting (Y,Pr)Ba2Cu3O6

The concentration dependence of the room‐temperature lattice constants and the concentration and temperature dependence of the specific heat C(T) and the magnetic susceptibility have been measured on the tetragonal, nonsuperconducting phase of Y1−xPrxBa2Cu3O6. The effective paramagnetic moment μeff is independent of x and has an average value of 2.7±0.2 μB. For PrBa2Cu3O6, the Neel temperature TN for Pr moment ordering is 10.5 K as compared to TN=17 K for the orthorhombic PrBa2Cu3O7 phase. The TN is depressed rapidly with Y doping in the oxygen‐depleted compound and goes to zero at x≊0.4–0.5. At low temperatures and x≤0.5, C(T)/T vs T2 is nonlinear and shows a rapid decrease of C(T)/T indicative of strong magnetic correlations.

Magnetic properties of Pr in non-superconducting PrBa2Cu3O7

The magnetic order and spin fluctuations of Pr in non-superconducting PrBa,Cu,O, have been studied by specific heat, susceptibility and neutron scattering measurements. The neutron data show that the basic ordering consists of a simple antiferromagnetic arrangement, with an ordering temperature of -17 K, while the specific heat data reveal a large value of the electronic specific heat coefficient y, comparable to heavy fermion like materials. The observed magnetic inelastic scattering shows a broad quasi-elastic response as a function of energy, similar to mixed valent-like systems. These results suggest that there is substantial f-electron character at the Fermi level in this material. The superconducting properties of the Y, _,%!ilxBa,Cu,O, (2 = rare earth) system are not affected significantly by the concentration x of heavy rare earth elements [l]. The existence of these rare earth superconductors suggests that the 4f electrons are well localized, and the rare earth and copper sublattices are electronically decoupled for all practical purposes in these systems, similar to conventional “magnetic-superconductor” systems. Exceptions to this basic behavior are provided by Ce, Pr and Tb. In particular, Pr forms the same orthorhombic structure as for the heavy rare earths [2], but the superconducting state is found to be strongly suppressed as a function of Pr concentration [3-61, and superconductivity is lost for Pr concentrations x 3 0.6. Moreover, the observed suppression is consistent with the classical Abrikosov-Gorkov depairing theory [7]. In the present paper we report both elastic and inelastic neutron scattering measurements of the magnetic properties of Pr in PrBa,Cu,O,. A simple antiferromagnetic order, with a moment of 0.74pg, is observed below a Neel temperature TN = 17 K [8]. This NCel temperature is two orders of magnitude higher than would be expected if one scales the T, for the heavy rare earth materials, assuming either purely dipolar interactions, or Ruderman-Kittel-KasuyaYosida (RKKY) exchange. The small ordered moment, the large value of electronic specific heat coefficient which we have observed [8], and the broad spectrum of inelastic magnetic scattering indicate that the Pr f-electrons are strongly hybridized in this material.

Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation

Using the zero‐field‐muon‐spin‐relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Neel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast‐relaxing component of the muon polarization in addition to a long‐time tail, reminiscent of spin‐glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr‐ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Neel temperature is ∼325 K.

μSR investigation of magnetism and superconductivity in (Y1−xPrx)Ba2Cu3O7

Abstract Muon spin rotation and relaxation techniques have been used to study the superconductivity and magnetism in (Y 1− x P x )Ba 2 Cu 3 O 7 (0⩽ x ⩽1) and PrBa 2 Cu 3 O 6 . Clear evidence for magnetic ordering of the Cu moments within the Cu-O planes is seen. Additionally, a lower magnetic transition is observed which, based upon previous work, has been associated with the ordering of Pr moments on the Y sublattice of the YBa 2 Cu 3 O 7 structure. For x = 1, the upper Neel temperature T N1 is ∼270 K and the magnitude of the fully developed local magnetic field is ∼16 mT. Below the lower Neel temperature T N2 = 17 K, the magnitude of the static field is reduced to ∼ 12mT. For 0.4⩽ x ⩽0.54, there appears to be a coexistence region of long range magnetism and superconductivity.

Preparation of YBa 2 Cu 3 O 7− x using barium hydroxide flux

High quality bulk YBa 2 Cu 3 O 7− x was synthesized by fusing stoichiometric amounts of yttrium and copper nitrates and barium hydroxide in air, using an ordinary Bunsen burner. The starting materials go through a short-lived liquid phase yielding a solid black product which was subsequently heat treated (900 °C, 18–24 h in air, followed by 500 °C, 5 h in O 2 ). These materials were greater than 99% phase pure with CuO as the only other phase and they exhibited a transition temperature of 92 K, a 15.5% perfect diamagnet response (field cooled). This synthesis represents an improvement over the much more labor and time intensive conventional methods in that it allows high quality materials of various compositions to be prepared quickly.

Normal‐state 63Cu Knight shift and hole‐band modification in Y1−xPrxBa2Cu3O7

The 63Cu NMR Knight shift K has been measured in oriented powder samples of normal‐state Y1−xPrxBa2Cu3O7, x=0.2 and 0.4. At plane Cu sites K is found to decrease with Pr doping, and a temperature dependence (dK/dT>0) develops. The variation of K with bulk susceptibility χ is considerably greater than can be explained using RKKY coupling between Pr spins and Cu nuclei, and the temperature variation of K resembles that of χ in high‐Tc systems with reduced conduction‐hole concentration. These results are consistent with the view that Pr impurities significantly modify conduction‐band properties, rather than acting as a ‘‘conventional’’ pair‐breaking mechanism.

Superconducting and magnetic phase boundaries for the heavy fermion system (Y, Pr)Ba2Cu3O7−y

Abstract A series of measurements of the structural, electrical, magnetic, and thermal properties are presented for the (Y 1−x Pr x )Ba 2 Cu 3 O 7−y system. Orthorhombic samples for 0⩽ x ⩽1.0 were prepared for all the experiments performed. Unlike nearly all other systems where rare earth (R) are substituted for the Y cation in superconducting YBa 2 Cu 3 O 7−y the substitution of Pr depresses T c , with the critical concentration, x cr = 0.56, for complete suppression of superconductivity. Evidence for magnetic correlations in the superconducting state is presented for x near x cr . Heat capacity and magnetic susceptibility anomalies indicate that it is the Pr ions that order, in agreement with neutron elastic scattering studies. The results suggest that the Pr ions may be almost tetravalent.

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y1−xPrx)Ba2Cu3O7 [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu−O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (Tc=75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>Tc, and by the vortex state forT<Tc. Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.

Superconducting and Magnetic Phase Boundaries in Doped Cu-Based Superconducting Oxides: Contrasting Charging and Pair Breaking Mechanisms

After several years of intensive study of the high transition temperature superconducting oxides (HTS), the nature and origin of the pairing mechanism has eluded investigators. Theories with local, real space pairing and more conventional models with k-space pairing have evolved and a variety of mechanisms have been proposed.1–5 A useful and now well known method of probing the relevant interactions in both low and high temperature superconductivity is through the use of impurities selected to investigate specific aspects of the response of the system to controlled perturbations, with particular emphasis given to changes of the superconducting properties.