Superconductors may be grouped into two major classes. The first is conventional metallic, whose pairing mechanism is explained by the BCS theory and electron-phonon coupling. The second we call unconventional, and the precise pairing mechanism has still to be worked out. All of the unconventional superconductors have electronic properties that are highly tunable, either by doping or pressure, from a non-superconducting parent compound, to a superconductor, to a non-superconducting Fermi liquid, thus defining a superconducting ‘dome’ in the phase diagram. More than 40 families of such materials, including the high-temperature superconductors, exhibit this ubiquitous phase diagram. In the underdoped phases, all of these materials show intriguing correlated electron states above the dome, and researches agree that the understanding of this “electron matter” holds the key to the pairing mechanism. Finally, I will show how we have found that point contact spectroscopy is exquisitely sensitive detecting electron matter.
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