There and back again: from magnets to superconductors
A theory of novel phase formation near quantum critical points suggests that large fluctuations lead to magnetic analogs of inhomogeneous superconductivity.
Phase diagram of the itinerant ferromagnet with a spin spiral phase as a function of pressure p, magnetic field H, and temperature T. QCP denotes a quantum critical point, TCP a tricritical point, and FM a uniform ferromagnetic phase.
Interactions often modify the behavior of quantum particles and reorganize them into new phases of matter. One dramatic example is the superconducting state, where electrons with attractive interactions form Cooper pairs. The most pertinent interactions in metals are often effective ones—mediated by lattice vibrations and magnetism—and they can be altered by changing the electron environment. Changes to chemical composition, applying magnetic fields and or pressure have the most dramatic effect on the interactions near a so-called quantum critical point. This is the point in the phase diagram where, at zero temperature, a metal is just on the verge of developing magnetic order. In this vicinity electrons interact via intense quantum and thermal fluctuations of that incipient order. Those long-range interactions both modify the metallic properties and also lead to new low-temperature phases such as superconductivity. In a paper in Physical Review Letters, Gareth Conduit and Ben Simons at the Cavendish Laboratory in Cambridge and Andrew Green at the School of Physics and Astronomy at St. Andrews, both in the UK, undertake a theoretical study of metals near a ferromagnetic quantum critical point. They show that critical fluctuations there can favor a low-temperature phase that is the magnetic analog of a superconducting state.
This textured magnetic phase has never been cleanly observed in the solid state, however the study underpinned the first experimental exploration of its properties in an ultracold atomic gas. Moreover, quantum fluctuations mean that a variety of other inhomogeneous magnetic states are energetically favorable, with our suggestion of a spin spiral state being first observed in CeFePO in 2012.
For further details see Inhomogeneous phase formation on the border of itinerant ferromagnetism, Editors' Suggestion in Phys. Rev. Lett. 103, 207201 (2009).