D. H. Liebenberg

Magnetic Susceptibility of Superconductors and Other Spin Systems

Basic Experimental Techniques and Concepts: Multipurpose Cryostat for Low Temperature Magnetic and Electric Measurements of Solids (C. Rillo et al.). ac Inductive Measurements (Q.Y. Chen). Theoretical Considerations and Phenomenological Models: ac Losses in Type II Superconductors (J.R. Clem). Bulk and Single Crystal Samples: NonLinear ac Susceptibility Response Near the Irreversibility Line (L. Civale et al.). Thin Films: ac Susceptibility Techniques Applied to Thin Film Superconductors (J.H. Classen). Magnetically Ordered and SpinGlass Systems: ac Susceptibility of Dilute Magnetic Systems (G. Williams). Specialized Measuring Techniques: Magnetically Modulated Resistance Techniques (B. Kim et al.). AppendixIrreversibility Line Debate. 26 additional articles. Index.

Mixed‐state behavior in large magnetic fields of high‐temperature superconducting films prepared by metalorganic deposition

Films of Ba2YCu3O7−x prepared by metalorganic deposition (MOD) are demonstrated to have large critical current densities in high magnetic fields. The current‐voltage (I‐V) characteristics are reported for an MOD film in applied fields from 0 to 6 T at 77 K. The zero field critical current density was 600 000 A/cm2 and fell to 60 000 A/cm2 at 1 T. The critical current is approximately proportional to 1/H1/2. The variation in I‐V curves above the critical current indicates the voltage has a power law behavior with values for the current exponent varying from 5.2 to 1.7 for magnetic fields from 0 to 6 T.

Angular dependence of critical currents of high temperature superconducting films in high magnetic fields prepared by metalorganic deposition

Abstract Critical current measurements have been obtained as a function of angle of the magnetic field to the plane of the thin film metelorganic deposited YBa 2 Cu 3 O 7- x (YBCO). In the present measurements the current flowed in the a , b plane and was always perpendicular to the applied field. In contrast to the usual observations that show a maximum critical current for H | a , b and a minimum with H | c these new measurements of films with a high critical current density show a secondary maximum with this latter field orientation. Adaptation of a model for angular torque dependence gives a qualitative understanding of the results.

Effect of magnetic field on d.c. transport critical current density in grain orientated YBaCuO

Abstract The d.c. transport critical current density in samples of grain orientated bulk Y-Ba-Cu-O has been measured at 77 K, in steady state magnetic fields up to 12 T. The critical current density of these orientated materials decreases rapidly at low fields but more slowly with increasing magnetic field. The measured critical current densities ranged from 1500 to 2500 A cm−2 in magnetic fields of 5–12 T. These values are orders of magnitude larger than those of sintered materials.

Magnetic field dependence of d.c. critical current for large samples of textured bulk YBaCuO

Abstract Large samples of textured YBaCuO were prepared at the Texas Center for Superconductivity using a continuous process. These were cut into a rectangular shape with typical dimensions 4.0 cm × 0.5 cm × 0.25 cm. The large physical size of these samples and their ability to carry high currents particularly in zero magnetic field required low resistance contacts to avoid Joule heating at the sample current - electrode interface. Accordingly, low resistance metal contact pads for current and voltage electrodes were applied at NIST Boulder, using a sputter etch, sputter deposition, thermal evaporation and oxygen anneal procedure. Transport critical current density measurements at 77 K, and in magnetic fields up to 6 T were performed at NRL. A holder was designed to hold the samples securely in such a way that would allow for contraction during cool-down and to support them against the anticipated large Lorenz forces. The highest critical current measured with no external field was 393 A. This corresponds to a critical current density of 1871 A cm −2 . At 1 T, these values decreased to 229 A or 1090 A cm −2 . These critical current values, found in material that was fabricated using a technique capable of being scaled up for larger sizes, demonstrate a potential usefulness in large scale applications.

Effects of pressure on high Tc superconductors

Pressure is an important research tool in understanding the nature of superconductivity and in the development of new superconducting materials. Experiments have been performed at NRL at pressures to 7 GPa on single crystals of Tl2Ba2CaCu2O8+δ, Tl2Ba2Ca2Cu3O10−γ, and NdBa2Cu3O7−δ. All three materials exhibit a positive dTc/dP which decreases with increasing pressure; in several instances, an extremium in Tc(P) is found. For Tl2Ba2Ca2Cu3O10−x, Tc was observed to increase from 116 K at atmospheric pressure, to a value as high as 131.8±0.5 K at 7 GPa. Measurements of Tc(P) on NdBa2Cu3O7−δ were found to be in conflict with an empirical prediction for this material.

Pressure dependence of Tc in Tl0.2Hg0.8Ba2Ca2Cu3Ox

We report results of our study of the pressure dependence of Tc in Tl0.2Hg0.8Ba2Ca2Cu3Ox to 6 GPa in a diamond anvil cell. The sample studied has an ambient Tc of 136.4 K and application of quasihydrostatic pressure raised Tc to 144.4 K at 6 GPa. We will also describe recent modifications of our ac susceptibility apparatus that have improved our sensitivity while simplifying the coil design and the balancing.

Critical current angle-dependent measurements of thin (15nm) chemically-derived YBa2Cu3O7−d films in fields to 5.5 T

Abstract Critical current measurements were made using films of YBa 2 Cu 3 O 7−d prepared by a metalorganic process. The film thickness was purposely reduced to about 15 nm so that measurements of the critical current could be made at temperatures of 4.2 K with reasonable width. Magnetic fields to 5.53 T were applied parallel to the film plane and as a function of angle from that plane. The current and applied field were always orthogonal. Comparison with measurements at 77 K show that at low fields, 0.5 T, the peak in critical near H//a,b is broadened at the lower temperature. Increasing fields tend to sharpen the peak more rapidly at 4 K. Measurements with H//c show a decrease by a factor of 4 at 5.5 T and did not change during a second thermal cycle between room temperature and 4.2 K. The power law exponents were determined from the I–V data ranging from 1–150μV.

Angular dependence of transport current near critical at fields to 4 T in metalorganic thin films

Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa2Cu3O7−d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andH∥c a secondary maximum is observed whenJc> 1 MA/cm2. NearH∥a, b a large increase inIc is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveIc is determined and the fit to a power lawV∼In shows a strong correlation betweenIc andn.