Surprising electronic disorder in a copper oxide-based ceramic
Cuprates, a class of copper-oxide ceramics that share a common building block of copper and oxygen atoms in a flat square lattice, have been studied for their ability to be superconducting at relatively high temperatures. In their pristine state, however, they are a special kind of insulator (a material that does not readily conduct electricity) known as a Mott insulator.
When electrical charge carriers—either electrons or the lack of electrons, known as “holes"—are added to an insulator in a process called doping, the insulator may become a metal, which readily conducts electricity, or a semiconductor, which can conduct electricity depending on the environment. Cuprates, however, behave neither like a normal insulator nor like a normal metal because of strong interactions between their electrons. To avoid the large energy cost arising from these interactions, the electrons spontaneously organize in a collective state where the motion of each particle is tied to all the other ones.
One example is the superconducting state, where electrons move in unison and drift with zero net friction when a potential is applied, a zero-resistance state which is a defining characteristic of a superconductor. Another collective electronic state is a “charge density wave,” a term coined from the wave-like modulation in the density of electrons, in which electrons “freeze” into periodic and static patterns, at the same time hindering electron flow. This state is antagonistic to the superconducting state, and, therefore, important to study and understand. In cuprates, charge-density-waves prefer to align to the atomic rows of copper and oxygen atoms that make up the underlying crystal structure, with wave “‘crests” occurring every three to five unit cells, depending on the material and doping level.