Physicists studying high-temperature superconductors are seeing quantum waves, and if they've interpreted the undulations correctly, a much-debated theory of how these materials work might be washed up.
According to the "stripes" theory, electric charges within copper-and-oxygen-based superconductors collect in long lines, and pairs of charges then glide along these stripes unhindered. But now a team of physicists led by J. C. Séamus Davis of the University of California (UC), Berkeley, suggests that stripes might be an illusion. By examining bismuth strontium calcium copper oxide (BSCCO), they show how stripes might be a subtle effect of overlapping quantum waves of electric charge.
As reported online 25 July by Science, Davis and colleagues studied the surface of a single crystal of BSCCO with a tiny, fingerlike electrical probe called a scanning tunneling microscope (STM). As they slowly moved the probe across the surface, the current between the tip and the material rose and fell in ripples--a pattern that other researchers had ascribed to stripes of charge lying just beneath the surface of the material. But going beyond the earlier studies, the UC Berkeley researchers found that as they varied the voltage between tip and surface, the spacing of the ripples changed. That shouldn't happen if stripes were causing the ripples, they argue. Moreover, Davis and colleagues found that the spacing varied just as it would if the ripples sprang from a quantum wave of electrical charge, reflecting off an imperfection in the crystal and then interfering with itself to produce peaks and valleys. The new data don't rule out stripes entirely, Davis says, but stripes aren't needed to explain the observed undulations.
Yet some researchers argue that the UC Berkeley group overlooked the stripes in its own data. The STM readings also show small signs of ripples whose spacing does not change with voltage, says Aharon Kapitulnik, a physicist at Stanford University in Palo Alto, California, who previously studied BSCCO with an STM and says he saw stripes. In plots where Davis and colleagues see only one peak that moves as the voltage changes, he sees a second that doesn't (see figure). "It's in their own data," Kapitulnik says.