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Insulators, metals, and superconductors differ in their response to an applied electric field. No electrical charge flows in an insulator. In a superconductor, a supercurrent flows without resistance and cancels the applied field. Only metals tolerate the simultaneous presence of an electric field and a dissipative current flow. These three distinct behaviors are hallmarks of the electrons’ organization in materials—they are attached to atoms in an insulator, they form a condensate of coherent Cooper pairs in a superconductor, and they are free and mobile in metals. There is a long-held belief, however, that confining electrons to 2D would deprive them of the metallic option at zero temperature. In a Colloquium article for Reviews of Modern Physics, a trio of physicists now challenge this view. They identify a fundamental flaw in our assumptions about electron organization in 2D materials [1]. Based on a body of published data and on theoretical arguments, the researchers show that in these materials an unexpected and anomalous form of metallicity can exist at low temperatures at the transition between an insulator and a superconductor.

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