R. H. Heffner

Transport‐magnetism correlations in the ferromagnetic oxide La0.7Ca0.3MnO3

We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post‐annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed‐laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.

Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

Lattice dynamics and correlated atomic motion from the atomic pair distribution function

The mean-square relative displacements ~MSRD! of atomic pair motions in crystals are studied as a function of pair distance and temperature using the atomic pair distribution function ~PDF!. The effects of the lattice vibrations on the PDF peak widths are modelled using both a multi-parameter Born‐von Karman ~BvK! force model and a single-parameter Debye model. These results are compared to experimentally determined PDFs. We find that the near-neighbor atomic motions are strongly correlated, and that the extent of this correlation depends both on the interatomic interactions and crystal structure. These results suggest that proper account of the lattice vibrational effects on the PDF peak width is important in extracting information on static disorder in a disordered system such as an alloy. Good agreement is obtained between the BvK model calculations of PDF peak widths and the experimentally determined peak widths. The Debye model successfully explains the average, though not detailed, natures of the MSRDs of atomic pair motion with just one parameter. Also the temperature dependence of the Debye model largely agrees with the BvK model predictions. Therefore, the Debye model provides a simple description of the effects of lattice vibrations on the PDF peak widths.

Field Induced Reduction of the Low-Temperature Superfluid Density in YBa{sub 2}Cu{sub 3}O{sub 6.95}

A novel high magnetic field (8 T) spectrometer for muon spin rotation has been used to measure the temperature dependence of the in-plane magnetic penetration depth {lambda}{sub ab} in YBa{sub 2}Cu {sub 3}O{sub 6.95} . At low H and low T , {lambda}{sub ab} exhibits the characteristic linear T dependence associated with the energy gap of a d{sub x{sup 2}-y{sup 2}}-wave superconductor. However, at higher fields {lambda}{sub ab} is essentially temperature independent at low T . We discuss possible interpretations of this surprising new feature in the low-energy excitation spectrum. (c) 1999 The American Physical Society.

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.

63Cu NMR and hole depletion in the normal state of yttrium-rich Y1-xPrxBa2Cu3O7

The {sup 63}Cu Knight shift {ital K} and spin-lattice relaxation rate 1/{ital T}{sub 1} have been measured in the superconducting cuprate system Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}, 0.05{le}{ital x}{le}0.20. With Pr doping {ital K} decreases and develops a temperature dependence at both plane and chain sites. This resembles the behavior of the Cu and Y Knight shifts as well as the bulk susceptibility in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}. The orbital contribution to {ital K} and the anisotropy of the Cu hyperfine coupling remain essentially unchanged over the entire Pr concentration range. No appreciable direct effect of Pr magnetism on the conduction-band susceptibility was found. Instead, analysis of the bulk susceptibility and NMR data indicate that pair breaking and hole depletion both take part in the suppression of the superconducting transition temperature {ital T}{sub {ital c}}. The temperature dependence of 1/{ital T}{sub 1} for magnetic field parallel to the {ital c} axis is also similar to that for the oxygen-deficient compound. This agreement leads to a consistent picture of the role of antiferromagnetic fluctuations in these materials. An analysis of the data in the framework of the phenomenological theory of Millis, Monien, and Pines is given.

Local lattice disorder in the geometrically frustrated spin-glass pyrochlore Y 2 Mo 2 O 7

The geometrically frustrated spin-glass Y{sub 2}Mo{sub 2}O{sub 7} has been widely considered to be crystallographically ordered with a unique nearest-neighbor magnetic exchange interaction, J. To test this assertion, we present x-ray-absorption fine-structure results for the Mo and Y K edges as a function of temperature and compare them to results from a well-ordered pyrochlore, Tl{sub 2}Mn{sub 2}O{sub 7}. We find that the Mo-Mo pair distances are significantly disordered at approximately right angles to the Y-Mo pairs. These results strongly suggest that lattice disorder nucleates the spin-glass phase in this material. (c) 2000 The American Physical Society.

Cuprous oxide manometer for high-pressure magnetic resonance experiments

We present a manometer designed to measure pressures of 1–20 kbar in temperatures between 4–300 K in cylinder‐piston type chambers, with an accuracy of ∼100 bar. The manometer is based on pressure‐dependent zero‐field 63Cu nuclear quadrupole resonance frequency corresponding to ‖±3/2〉↔‖±1/2〉 transition in Cu2O. The nuclear quadrupole resonance frequency νQ varies linearly with pressure and its temperature dependence is adequately explained by a model of lattice vibrational modes in O—Cu—O bonds. This manometer is particularly convenient for zero or high‐field magnetic resonance experiments.

Magnetic and superconducting properties of the heavy-fermion superconductor UPd2Al3

Zero- and transverse-field muon-spin rotation (μ+ SR) spectroscopy, as well as critical-field investigations were performed on the newly discovered antiferromagnetic superconductor UPd2Al3. In contrast to the situation in UNi2Al3, no spontaneous frequency could be observed in the zero-field spectroscopy, suggesting a symmetric muon stopping site. The coexistence between magnetism and superconductivity could unambiguously be confirmed by the μ+ SR data. The magnetic-penetration depth λ(0) has been obtained from the increase of the μ+ SR line width below Tc and from the slopes of the critical field at Tc. The results are compatible with a value of λ(0) = (6250 ± 1250) and suggest that UPd2Al3 is a BCS-type weak-coupling superconductor.

sup 139 La NMR study of phase separation in single-crystal La sub 2 CuO sub 4+. delta

We report a study of single-crystal La{sub 2}CuO{sub 4+{delta}}, a 38-K superconductor produced by high-pressure oxygenation. We employ {sup 139}La NMR to probe the behavior of copper spins in the CuO{sub 2} planes. At low temperatures we observe two signals, one originating from regions of the crystal rich in oxygen and a second having no excess oxygen ({delta}{approximately}0). Upon warming through 265{plus minus}5 K, the volume fraction of the crystal poor in oxygen goes to zero. The magnetic shift of the peak intensity of the line originating in the oxygen-rich portion of the crystal does not change in the vicinity of 265 K. These observations provide direct microscopic evidence for phase separation in the crystal.