Discovery of an unconventional charge density wave (CDW) in purple bronze, a molybdenum oxide, points to a possible new pathway to high temperature superconductivity. A CDW is a state of matter where electrons bunch together periodically, like a standing wave of light or water. CDWs and superconductivity are frenemies, since they share a common origin and often coexist, yet compete for dominance. Conventional CDWs and superconductivity arise from interactions of electrons with lattice vibrations (phonons), while electron-electron interactions are the likely origin of unconventional, high temperature superconductivity such as found in copper and iron based compounds. Missing was an example of an unconventional, electron-electron driven CDW. In purple bronze we show the existence of a CDW in the apparent absence of a strong electron-phonon interaction and an extraordinary increase in the CDW transition temperature from 130 K in bulk to 220 K at the surface. This enhancement of critical temperature is accompanied by a huge energy gap increase—a key parameter essential for both CDW and high temperature superconductivity.
Image of the Fermi surface (left) and band dispersion (right) along a red line cut. At 130 K a gapped branch appears that is due to the surface CDW
Discovery of an Unconventional Charge Density Wave at the Surface of K0.9Mo6O17