Fermi Surface Transforms at the Onset of the Pseudogap State in a Cuprate Superconductor

Published October 18, 2022

Fermi surfaces calculated from angle dependent magnetoresistance (ADMR) data (left) inside the pseudogap phase at p=0.21, showing four small pockets (resembling those that would be created by antiferromagnetism) and (right) outside the pseudogap phase at p=0.24, showing the single large pocket of a simple metal.

In high-temperature superconductors, a region exists between the superconducting and normal states known as the pseudogap state. Using the 45T hybrid magnet, scientists have determined that magnetism plays a previously unknown role in the development of the pseudogap phase.

What did scientists discover?

A 'Fermi surface' provides a map that shows where in energy/momentum space all of a metal's electrons are to be found. How electrons arrange themselves and interact with other electrons and the array of atoms in a crystalline lattice determines the physical properties of a material. This is especially true in the quantum realm. In high-temperature superconductors, MagLab users found that the Fermi surface transforms at a certain "critical point," the carrier concentration that corresponds to the onset of the pseudogap state.

Why is this important?

There have long been hints that magnetism and superconductivity are intertwined in high-temperature superconductors. This experiment shows that this magnetism appears at the onset of the pseudogap state. The magnetism transforms the Fermi surface into four small pockets, dramatically reducing the number of charge carriers when the pseudogap state forms. This experiment provides direct evidence that the critical point associated with the onset of the pseudogap is also associated with the onset of magnetism, a clue that might help solve the mechanism of high temperature superconductivity in the cuprate superconductors.

Who did the research?

Y. Fang¹, G. Grissonnanche^1,2, A. Legros^2,3, S. Verret², F. Laliberté², C. Collignon², A. Ataei², M. Dion², J. Zhou⁴, D. Graf⁵, M.J. Lawler¹, P.A. Goddard⁶, L. Taillefer^2,7 & B.J. Ramshaw^1,7

¹Cornell University; ²Université de Sherbrooke; ³Université Paris-Saclay; ⁴University of Texas at Austin; ⁵National MagLab; ⁶University of Warwick; ⁷Canadian Institute for Advanced Research

Why did they need the MagLab?

The MagLab's 45T hybrid magnet was essential to suppress superconductivity and to bend the electron trajectories sufficiently to reveal the structure of the Fermi surfaces in the samples studied. MagLab techniques enable magnetoresistance measurements with noise levels less than 0.1% of the signal measured as the sample is rotated around two different axes.