Although the best-known transition is undeniably the liquid-solid phase transition, other transitions (paramagnetic-ferromagnetic, metal-insulator, etc.) are textbook examples for physicists. These changes in state are controlled by thermal or quantum fluctuations that destabilize one phase at the benefit of another phase (organized differently). This helps explain the enormous surprise of researchers at the discovery of a transition between two identical magnetic phases in the CeCoIn5 superconductor, in the presence of a magnetic field.
CeCoIn5 is a model system for studying the interactions between magnetism and superconductivity. When 5% of the cerium atoms in CeCoIn5 are substituted for neodymium atoms, a magnetic order appears within the superconducting phase. This order is analogous to antiferromagnetism, and is made up of a wave of ordered magnetic moments called the spin wave. The researchers studied the behavior of this phase under a magnetic field using neutron diffraction experiments at the Institut Laue-Langevin (Grenoble) and the Institut Paul Scherrer (Switzerland).
They observed that the application of an 8-tesla field erases the spin-density wave, according to a well-understood process. Quite unexpectedly, the magnetic order then reappeared again (still with the same 8-tesla magnetic field value) and reformed exactly the same spin-density wave. The existence of this instability suggests to physicists that there is a change in superconducting properties in the presence of a magnetic field, specifically a change in symmetry of the wave function of Cooper pairs (related to superconductivity). The magnetic order and superconductivity eventually disappear when the magnetic field reaches 11 teslas.
This result questions the understanding of states of matter in which magnetism and superconductivity intertwine. It will certainly inspire other works, both theoretical and experimental.
Figure: Neutron diffraction intensity as a function of magnetic field and temperature for 5 %-Nd substituted CeCoIn5: a phase transition at 8 T separates two antiferromagnetic phases with identical magnetic symmetry within a superconducting phase.