Tc Conductors Research Articles

Angular magnetoresistance provides texture information on high- Tc conductors

Angular magnetoresistance measurements are performed by rotating a superconductor to an angle θ in a fixed magnetic field, while monitoring the resistance R. It is argued theoretically that for fields well above the lower critical field, H⪢ H cl, the bulk resistivity of the conductor is independent of θ if the crystallites of which it is composed are randomly oriented. Non-random orientation (a key aspect of texture) is revealed, therefore, by variations of R with θ. Dips in R indicate that the field is parallel to the copper oxide planes in a significant fraction of the current-carrying crystallites. C-axis, a-axis, and other textured film conductors are used to illustrate the technique. The angular magnetoresistance is found to be an important supplement to conventional texture determinations by microscopy and X-ray diffraction.

ANALYSIS AND IMPROVEMENT OF THE PRIMARY FIELD WAVEFORM OF THE GROUND TEM METHOD*

ABSTRACTMany ground TEM systems use an on‐off type of primary field waveform and measure the secondary field only in the transmitter off‐time, so as to realize the pure secondary field measurement. The width of the trailing edge of the primary pulses and the duration of the transmitter on‐time form a time‐constant (Tc) window. Most effective excitation is achieved if the target conductor has a Tc falling into this window.To be able to measure in the transmitter on‐time, two primary field waveforms are proposed. The first is a series of unipolar pulses with identical steep ramps at both leading and trailing edges. Measurements made in the on‐time after the leading edges simply reduce the stacking time needed and power consumption by half. Also, long‐ Tc conductors are better excited. The second is a series of bipolar pulses with long ramps as the leading edges and short ramps as the trailing edges. This waveform presents two different Tc‐windows at the same time and helps to improve the detection of long‐Tc conductors in the presence of short‐ Tc ones.