29 March 1991
Vol 251, Issue 5001

About The Cover

Scanning tunneling microscope image of a high-transition temperature film of sputtered YBa2Cu3O7 on MgO showing spiral grains with step heights of about one unit cell (12 Å). The perspective-shaded plot was generated with a computer program called SCOPE; colors represent increasing altitude from blue to red-violet. See page 1587. [Image by Melvin L. Prueitt, computer graphics group, and data by Marilyn Hawley, MST-7, Los Alamos National Laboratory]