Physics students learn that the Hall effect—the creation of a voltage difference when an electric current is deflected by a magnetic field—can be used to tell whether a material’s mobile charge carriers are positively or negatively charged. In p-type semiconductors, whose currents are carried by positively charged holes, the Hall voltage gradient is in the direction of the cross product of the current and the field. In n-type semiconductors, whose charge carriers are electrons, the gradient is in the opposite direction.
But that simple relationship does not always hold, as Martin Wegener and colleagues of the Karlsruhe Institute of Technology in Germany have now experimentally shown. The researchers crafted a metamaterial out of an n-type semiconductor—specifically zinc oxide, although any other n-type material would also have worked. When the researchers measured the structure’s Hall voltage, they found that it behaved like a p-type material. The metamaterial, shown here, is a periodic arrangement of interlocking rings. Mathematicians Marc Briane and Graeme Milton, who predicted the effect in 2009, were partly inspired by medieval chain-mail armor.
The literature is full of examples of metamaterials with electromagnetic, acoustic, or mechanical properties outside the range of their constituent materials. (See, for example, Physics Today, February 2007, page 19.) But in most of those cases, the curious behavior arises from an internal resonance, so it’s dependent on incident waves of just the right resonant frequency. The Hall-effect reversal is different: It works when the current and the field are both stationary, with no need for resonant waves. (C. Kern, M. Kadic, M. Wegener, Phys. Rev. Lett., in press.)
