Eric T. Ahrens

Phase separation and superconductivity in La2CuO4+δ: Effects of oxygen diffusion

Abstract In oxygen-annealed La 2 CuO 4+δ (δ ∼0.03), bulk magnetization measurements show an increase of ∼4K in the superconducting T c when the sample is cooled slowly through a narrow temperature range near 195K. At this temperature, 139 La NQR spectra exhibit a concomitant appearance of an anomalous feature associated with the metallic phase whose spectral weight increases with cooling rate. The data suggests a distribution of local structures and annealing-dependent volume fraction in this part of the sample. Interpretation of these results in terms of oxygen diffusion in a phase separated material is considered and a picture consistent with the observed changes in T c is obtained. The activation energy of oxygen diffusion derived from the model, Q ∼ 0.25 eV, is in the same order of magnitude as the results obtained in La 2−x Sr x CuO 4− y .

209Bi NMR in the heavy-electron system YbBiPt

Abstract Bismuth NMR Knight shift and spin lattice relaxation rate 1/T1 data are reported between 35 and 325 K in the low-carrier heavy fermion system YbBiPt. The Knight shift is strongly temperature dependent and negative. Its temperature dependence tracks the bulk susceptibility with a hyperfine coupling constant Ahf = −7.88 kOe/μB. At low temperatures 1/T1 exhibits a dramatic increase, such that the average 4f spin correlation time τf shows a crossover behavior at about 75 K. The rate 1 τ f is proportional to temperature above 75 K, consistent with non-interacting 4f local moments which are relaxed via Korringa-type scattering with the conduction electrons. We discuss the behavior below 75 K in terms of crystal-field effects or a strongly temperature dependent contribution from non-zero q regime of the dynamical susceptibility.

NQR study of local structure and cooling rate‐dependent superconductivity in La2CuO4+δ

Structural properties of oxygen-annealed polycrystals of La{sub 2}CuO{sub 4 + {delta}} ({delta}{approximately}0.03) have been studied using {sup 139}La NQR spectroscopy. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range at about 195K. Preliminary analysis of the {sup 139}La NQR spectra suggest that the oxygen-rich phase-separated region is composed of two structurally distinct phases, both of which are metallic and super-conducting. One phase has a structure closely related to the stoichiometric oxygen-poor compound. The second shows a considerable amount of apical oxygen disorder, a large shift in NQR frequency {nu}{sub Q}, and a volume fraction which increases with cooling rate. The formation of the second phase below {minus}200K is indicative of the freezing Of CuO{sub 6} octahedral tilting. Abrupt shifts in {nu}{sub Q} above {Tc} were also observed for both phases, suggestive of a local structural anomaly or charge transfer to the Cu-O plane.

9Be and 11B NMR study of superconductivity in boron doped UBe13

Abstract We present 9 Be and 11 B NMR spin-lattice relaxation rate (1/ T 1 ) measurements in UBe 13− x B x for x = 0.03 and 0.07 over the temperature range 0.096 K to 2 K. The temperature dependence of 1/ T 1 shows a strong B concentration dependence, especially at low temperatures. We interpret this behavior as consistent with gapless superconductivity induced by the addition of B impurities. The ratio of the 9 Be to 11 B 1/ T 1 increases with decreasing temperature below T c indicating additional contributions to the 9 Be relaxation rate, possibly from nuclear spin diffusion to normal-state vortex cores or paramagnetic impurities.

Microscopic Study of Local Structure and Charge Distribution in Metallic La2CuO4+δ

We employ NMR and NQR spectroscopy as probes of local structure and charge environments in metallic La2CuO4+δ (Tc = 38 K). We discuss the effect of annealing the sample at various temperatures T a (T c < T a < 300 K) on the superconducting T c. The dependence of T c on T a indicates that annealing allows the development of structural order which is important for T c. The 139La quadrupole frequency VQ in the in metallic phase is smaller than in the undoped material. This is unexpected and may indicate a smaller charge on the apex oxygen in the doped material and thus a different distribution of charge between the La-O layer and the planes. We find that the presence of doped holes induces a distribution of displacements of the apex oxygen off of the vertical La-Cu bond axis. The distribution in displacements extends from zero to the value observed in lightly doped (antiferromagnetic) La2CuO4+δ . This directly demonstrates considerable inhomogeneity in the crystal structure of the La-O layer. Copper NQR spectroscopy shows that there are two distinct copper sites in the CuO2 planes and thus that either the structure or the charge distribution in the planes is inhqmogeneous as well. These inhomogeneities are the intrinsic response of the crystal to doped holes; they are not the result of distortions of the lattice due to the presence of interstitial oxygen atoms.

Resonant ultrasound measurements in single crystals of intrinsic and superconducting copper‐oxygen plane compounds

Resonant ultrasound measurements in very pure single crystals of La2CuO4 have been made. The material was prepared such that its resistivity at 300 K is extremely high (170 Ω cm), and untwinned crystals of about 0.1×0.1×0.1 cm were used. Oxygen was then introduced, eventually leading to superconductivity. Here, data on the three longitudinal sound velocities and attenuations as a function of temperature for different oxygen dopings are presented and the effects of magnetic and superconducting interactions are discussed. All data were taken with a resonant ultrasound method developed by us in which the transducers are flexible and are less than 1% of the sample mass, minimizing transducer loading effects.