G. H. Myer

SIMPLIFIED PREPARATION OF REBA2CU3O7-X VIA THE ACETATE METHOD

High-quality bulk REB a 2 Cu 3 O 7− x (RE = Y,Eu,Gd,Nd,La) was synthesized by a solution method. Stoichiometric amounts of yttrium, barium, and copper acetates were dissolved in glacial acetic acid. The acid was boiled away leaving a glassy acetate precursor. This precursor has shown amorphous scattering in XRD, a phenomenon consistent with atomic level mixing of reactants. The glassy precursor was subsequently heat-treated with a 5 °C/min ramp to 900 °C and a 2 h soak at 900 °C in air. The final product was obtained after heat treatment under oxygen at 550 °C with slow cooling to room temperature. Final products were analyzed by XRD, SEM, and four probe de-resistivity measurements. The mechanism of both precursor and product formation was examined through substitution studies and XRD. It was found that a combination of a rare earth acetate, barium acetate, and acetic acid was necessary for the formation of an amorphous precursor.

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.

Specific heat and magnetic susceptibility of the high-Tc superconductor (Bi, Pb, Sb)2Sr2Ca2Cu3O10

Abstract The temperature dependence of the specific heat and magnetic susceptibility of single phase (Bi, Pb, Sb) 2 Sr 2 Ca 2 Cu 3 O 10 has been measured. A distinct jump in the specific heat C ( T ) at the superconducting transition temperature T c is obtained with a δ C at T c =6.2±0.2 J/mole K and T c =109 K. This result combined with an estimate of the Sommerfeld constant γ obtained from scaling the temperature independent contribution of the magnetic susceptibility χ( T ) is close to the value of δ C expected assuming a weak coupling BCS theory. The γ-value obtained from δC =1.43 γT c and the temperature independent contribution to χ( T ) is ∼40 mJ/mole K 2 which indicates that the electrons are highly correlated with a band mass ≈10 m 0 where m 0 is the free electron mass. These γ-values and the effective electron mass are very similar to those previously reported for other high- T c superconductors, e.g., YBa 2 Cu 3 O 7 . At low temperatures C ( T ) can be fitted to A / T 2 + γT + βT 3 + C E ( T ) with A =40 mJK/mole, γ⋍0.0 and β =1.76 mJ/mole K 4 and C E ( T ) representing a small Einstein-like contribution to C ( T ) with an Einstein temperature of ∼4 3 K.

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

Abstract The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature ( T N =2.3 K) of GdBa 2 Cu 3 O 7- x . Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Neel temperatures, extrapolated to zero measuring field, are identical: T N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, d T N /d P =+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T T N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T N approaching T =0 K at about 2.5 T.

Magnetic properties of a high Tc superconductivity related system Y1−xPrxBa2Fe3O8

We studied Pr substitution for Y in a 123 high T c superconductor related system, Y1−x Pr x Ba2Fe3O8 using of x‐ray diffraction, magnetization, and 57Fe transmission Mossbauer spectroscopy. The Fe sublattices are magnetically ordered through the whole system. The Pr substitution, however, suppressed the Fe ordering temperature. The low temperature susceptibility across the system increases with increased Pr concentration x, and an additional antiferromagnetic ordering at 24 K is observed for x=1, indicating a possible Pr ordering. The averaged magnetic hyperfine field at Fe sites measured at 15 K is also reduced as x increases. The dramatic effect of Pr substitution for Y in this system, which is not observed in studies of Y substitution by other rare‐earth elements, is interpreted as the effect of the hybridization between the Pr 4‐f electrons and the electrons in the Fe(2)O2 layers, similar to the Pr hybridization in Y1−x Pr x Ba2Cu3O7−y system.

Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors

Abstract The temperature dependence of the specific heat C(T) has been measured for YBa2(Cu1−xMx)3O7 (YBCO) with M = Zn and Ni and 1.2 λ LT = C(T) T at T = 1.5 K reaching values of 100–150 mJ⧸mol K2 for modest Zn and Ni doping. This γLT dependency for Zn doped YBCO has been associated with a localization of d-holes within the CuO2 planes caused by a disorder induced disruption of hybridization within the planes.

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.

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.

Epr Spectroscopy on Zn-Substituted YBa 2 Cu 3 O 7

The temperature and concentration dependence of the electron paramagnetic resonance spectra of YBa 2 (Cu 1−x Zn x ) 3 O 7 has been measured for 0≤x≤0.16. Zn substituted on the Cu-site cause a rapid nearly linear depression of the superconducting transition temperature T c with T c going to zero in the vicinity of x=0.10. Only weak EPR spectra due to ≤1% of the Cu-ions are seen in the superconducting phase of YBa 2 Cu 3 O 7 . These spectra are most likely attributed to Cu-ions in the vicinity of grain boundaries and do not represent a bulk response. However, a strong asymmetric resonance is observed in the Zn-substituted samples with x>0.1. The EPR results obtained for YBa 2 (Cu 1−x Zn x ) 3 O 7 are discussed in terms of a possible localization of d-electrons.