J. D. Jorgensen

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

We have determined the crystal structure of the single‐phase stoichiometric high‐temperature superconductor in the Y‐Ba‐Cu‐O system using high‐resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a=3.8231 A, b=3.8864 A, and c=11.6807 A. The structure consists of ‘‘dimpled’’ CuO2 layers in the a‐b planes loosely bonded by one‐dimensional fencelike square‐planar CuO3 chains in the b‐c planes.

Rietveld Refinement with Spallation Neutron Powder Diffraction Data

The mathematical functions necessary for Rietveld refinement of time-of-flight neutron powder diffraction patterns from spallation sources are developed and a computer program for least-squares analysis is described. The results of Rietveld refinements of nickel and a low-carbon steel are described and discussed. The method fully exploits the high resolution (Δd/d ~ 0.3 ~ 0.5%) available with powder diffractometers currently in operation on these sources and examples are given of precise determination of atom coordinates, thermal parameters, lattice parameters and the detection of small strains.

Time-dependent structural phenomena at room temperature in quenched YBa2Cu3O6.41 : local oxygen ordering and superconductivity

Abstract The superconducting transition temperature, T c , of YBa 2 Cu 3 O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a -and c -axes contract by 0.04%, while the percentage shortening of the b -axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b -axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.

Structure and superconductivity of HgBa2CuO4+δ

Abstract We have used neutron powder diffraction to investigate the defect structure of HgBa 2 CuO 4+δ . An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T c onset , of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.

Specific heat of Mg11B2: evidence for a second energy gap.

We report specific-heat measurements on two samples of Mg 11 B2, one powder and one sintered, that give essentially identical results. Both samples are of exceptionally high quality: At the critical temperature the discontinuity in specific heat is higher than that of other samples, the transition is sharper than for most samples, and the signature feature of the small, non-BCS energy gap is particularly pronounced. The results are compared with a phenomenological model for a multi-gap superconductor, with band-structure calculations, and with spectroscopic determinations of the energy gaps.

The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect.

Magnesium diboride, MgB2, was recently observed to become superconducting at 39 K, which is the highest known transition temperature for a non-copper-oxide bulk material. Isotope-effect measurements, in which atoms are substituted by isotopes of different mass to systematically change the phonon frequencies, are one of the fundamental tests of the nature of the superconducting mechanism in a material. In a conventional Bardeen–Cooper–Schrieffer (BCS) superconductor, where the mechanism is mediated by electron–phonon coupling, the total isotope-effect coefficient (in this case, the sum of both the Mg and B coefficients) should be about 0.5. The boron isotope effect was previously shown to be large and that was sufficient to establish that MgB2 is a conventional superconductor, but the Mg effect has not hitherto been measured. Here we report the determination of the Mg isotope effect, which is small but measurable. The total reduced isotope-effect coefficient is 0.32, which is much lower than the value expected for a typical BCS superconductor. The low value could be due to complex materials properties, and would seem to require both a large electron–phonon coupling constant and a value of μ* (the repulsive electron–electron interaction) larger than found for most simple metals.The recent discovery of superconductivity at ~39 K in MgB2, by Nagamatsu et al.1 establishes this simple binary compound as having the highest bulk superconducting transition temperature, Tc, of any non-copper-oxide material. Much of the initial research has focused on whether MgB2 is a conventional BCS, electron-phonon mediated superconductor, and, if it is, why Tc is so high. Isotope effect measurements, in which the atom masses are manipulated to systematically change the phonon frequencies, are one of the fundamental experimental tests of electron-phonon mediated superconductivity. One would expect the total isotope effect coefficient, i.e. the sum of both the Mg and B coefficients, to be 1/2 for a highTc, phonon mediated, simple sp orbital superconductor like MgB2. We find a value of 0.32(1), much reduced from the canonical BCS value of 0.5. This requires large values of λ (electron-phonon coupling constant) and μ* (repulsive electron-electron pseudopotential) to account for the high Tc and, thus, constrains any theoretical model for superconductivity in MgB2.

Electronically Focused Time-of-Flight Powder Diffractometers at the Intense Pulsed Neutron Source

Two time-of-flight powder diffractometers have operated at the Intense Pulsed Neutron Source (IPNS) since August 1981. These instruments use dedicated microcomputers to focus time-of-flight events so that data from different detectors can be summed into a single histogram. Thus, large multidetector arrays can be employed at any scattering angle from 12 to 157 °. This design permits data to be collected over a uniquely wide range of d spacings while maintaining high resolution and count rates. The performance of the two instruments is evaluated by analyzing data from a standard A1203 sample by the Rietveld method. These instruments provide the capability for moderate- to high-resolution measurements with the duration of a typical run being a few hours.

Phenomenological two-gap model for the specific heat of MgB2

The authors show that the specific heat of the superconductor MgB{sub 2} in zero field, for which significant non-BCS features have been reported, can be fitted, essentially within experimental error, over the entire range of temperature to T{sub c} by a phenomenological two-gap model. The resulting gap parameters agree with previous determinations from band-structure calculations, and from various spectroscopic experiments. The determination from specific heat, a bulk property, shows that the presence of two superconducting gaps in MgB{sub 2} is a volume effect.

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Pressure-induced charge transfer and dTc/dP in YBa2Cu3O7−x

Abstract Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.