Cuprate Superconductors
A high temperature cuprate superconductor is obtained by doping a Mott insulator with holes or electrons. The corresponding phase diagrams are shown below. An explanation for the phenomenon of high-Tc superconductivity has to deal with the underlying pairing mechanism and why it happens at such high temperatures. For this one needs to understand the original state, which was destabilized to give rise to superconductivity. We will call this non-superconducting state, the “normal state”. Hence, it is essential to understand the normal state of HTSC in order to explain the mechanism behind the high critical temperatures.
There is significant evidence that the normal state of cuprates is not a Fermi liquid. In fact, a striking feature in the normal state is the pseudogap (PG), which signifies the suppression of the density of states near the Fermi level and there is evidence that the PG disappears at higher dopings. Hence, it is crucial in particular to understand the origin of this PG and its behavior with doping.
The suggested origin of the pseudogap is either due to a phase, which is a precursor to the superconducting phase in which electron pairs exist without the phase stiffness required to carry a supercurrent, or it is due to the proximity to a quantum critical point separating the superconducting state from another nearby competing ground state. Figuring out which model is the correct description of the actual phenomenon, will be a very important clue towards the solution of the high-Tc problem. One possible way to distinguish between the two models is to observe the effect of a magnetic field on the PG. The precursor superconductivity scenario suggests that the superconducting gap and the PG have the same origin and hence a magnetic field, which is a strong depairing field, should have a noticeable effect on the PG. On the other hand if the PG is due to a competing order parameter of the nearby ground state, it will be very robust and the effect of a magnetic field will be negligible. We use point contact spectroscopy in magnetic fields of ~30 T to study of the formation of the pseudogap in n-doped cuprates and determining its origin using. Click here to learn more about our technique and results.