High-resolution electron microscopy with superconducting lenses at liquid helium temperatures.

Following our first successful imaging experiments'-3 with high-field superconducting solenoid lenses, numerous methodological and conceptual problems remained to be solved before the specific advantages of this approach could be adequately established for high-resolution electron microscopy. These problems included: (a) incorporation of superconducting solenoid lenses and associated cryogenic components into high-performance electron microscope systems; (b) precise control and reproducible current setability for focusing superconducting solenoid lenses; (c) satisfactory specimen mounting to prevent temperature drift and achieve high degree of stability during irradiation; (d) stabilization of lens excitation currents and accelerating voltage, and improved electron source characteristics for low-temperature microscopy; (e) reduction of magnetic, electrical, and mechanical perturbations under carefully controlled conditions in the requisite cryogenic environment; (f) adequate continuous recording of images without breaking high vacuum under cryogenic conditions. Meeting these requirements has involved a major development and research effort in this laboratory during the past few years. Pursuing a comprehensive research program with different types of cryo-electron microscopes, our earlier workl-6 has been extended, confirming the exceptional stability of high-quality images (50-100-A resolution) recorded exclusively with superconducting solenoid lenses at 4-32 kilogauss and 4-50 kv accelerating potential. Using a superconducting niobium-zirconium objective lens, operating in a specially designed liquid helium cryostat with superconducting stigmators and regulating circuitry, it has now been possible to record, for the first time, electron micrographs of biological specimens at 4.20K, reproducibly attaining resolutions of 10-20 A with minimized specimen modifications. In addition, the unique combination of high magnetic fields, liquid helium temperature, and high electron optical magnifications has enabled us to make preliminary observations on characteristic electron optical phenomena associated with trapped fluxes in thin superconducting films. Salient aspects of this work are described in the present report. Experimental.-All of the equipment used was specially designed, developed, and tested in our laboratories, making use, wherever possible, of commercially available electronic and cryogenic components. In view of the stringent requirements, even special superconducting equipment which was built to our specifications by commercial firms had to be extensively modified and adapted in our workshops. In general, our work has centered on two approaches: (1) Cryo-electron microscope optical bench system (Fig. 1) comprises a modified air-core liquid helium Dewar with different types of niobium-zirconium solenoid lenses' operating at 4-32 kilogauss without pole pieces, and with modified objective, and objective-projector pole pieces. The objective pole pieces were essentially of conventional design with focal lengths of 1.6-2.6 mm. Available 25 amp regulated power supplies with storage batteries had to be considerably improved and used in conjunction with additional current vernier control circuits. Under suitable conditions this system permits adjustable current changes of 10-9 for achieving reproducible "super-