Theoretical Basis for Future Bismuthate Research

It would seem that the carrier identification would be readily resolved by experiment. In fact, numerous experiments have found the carriers to be electrons in BaPb B Og (78)(79), the carrier concentration and Tc readily scaling with Bi content until a CDW decreases the carrier density. This interpretation is well supported by a magnetic susceptibility study (80). Even though the parent compound BaBiOg is a hole type semiconductor (81), the carrier type has been shown to be n or electronic in character (75) in Ba K BiOg. 

Perhaps the best guideline to use in designing high Tc materials is to look for electron localization mechanisms, as suggested by Sleight 

From the above discussions, it is clear that the major contribution of theory in guiding future experimental work is the demand it places for high quality empirically determined data. Theory has rarely, if ever, contributed to a rise in Tc, and, without a clearly defined mechanism, the contributions of theory to the pursuit of high Tc remains low. 

The rather low critical current densities expected in bulk samples of these granular superconductor materials (103)(104), and their relatively low magnetic critical fields (50)(78)(105)(106), allied with the relatively low Tc s observed would appear to hinder development of superconducting applications for these materials. Even the critical current of 5 X 105 A/cm2 observed for single crystalline thin films (39) is now considered low for a superconductor at 4.2K. However, when considering the applicability of a material to a task, 

For example, a low critical current density has been shown useful in using BaPb B Og as a microwave switch. The transmission of 2.8 GHz microwaves through a polycrystalline thin film was switched on in less than 30 ns via a high current pulse (107), resulting in short microwave pulses. 

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