University of Minnesota
School of Physics & Astronomy

Research Spotlight

Finding the Glue in Unconventional Superconductors

Rafael Fernandes
Rafael Fernandes
Alex Schumann

Most metals, when cooled below a certain temperature, become superconductors. A superconductor not only carries electric current without dissipation but it also expels the magnetic field from its interior. "Microscopically we understand that the system is in a new quantum state where pairs of electrons form bound states, called Cooper pairs, in a coherent way, " says Rafael Fernandes, a theoretical physicist who studies superconductivity.

In conventional superconductors, the bound state occurs because of a small attraction between the electrons promoted by fluctuations of the underlying crystalline lattice. Physicists have understood this phenomenon since the late 1950s – despite the fact that it had been discovered in the early 1910s.

In the late 1980s and again in the late 2000s, other classes of superconductors were discovered that do not fit this explanation. These materials display rather high superconducting transition temperatures – at least compared to those of conventional superconductors. If they could be elevated to room temperature, these superconductors would have a huge potential for technological applications. One of the main challenges of Fernandes’ research is to understand what is going on with these unconventional superconductors, in particular, whether the electrons still form bound states and, if so, what is the underlying microscopic "glue." Unlike in conventional superconductors, where the lattice provides this glue, in unconventional superconductors it has been proposed that the electrons themselves provide the glue via fluctuations of the normal state. "We want to understand if there is anything unusual in the normal state of these materials, and whether this may affect superconductivity," Fernandes says. The normal states of some of these unconventional superconductors display a rather unusual ordering called electronic nematic. Fernandes and his group are trying to understand whether this ordering is connected to unconventional superconductivity.

The name "electronic nematic" comes from an analogy with liquid crystals, which are systems commonly found in our daily lives, for example in LCD displays. In the nematic phase of a liquid crystal, rotational symmetry is broken but translational symmetry is not. This is an intermediate situation between a liquid and a crystalline solid, where both symmetries are either preserved or broken, respectively. When electrons interact with each other, they may behave analogously to these liquid crystals. Specifically, in an electronic nematic phase, the rotational symmetry of the system is lowered, whereas the translational symmetry remains unaffected. “This is a very unusual state. It’s an odd thing that the electrons, instead of the lattice, would drive this rotational symmetry breaking.” Lately, he has focused on not only understanding microscopically this nematic state, but also proposing specific experimental signatures of this somewhat elusive electronic phase. So far, Fernandes says, experiments have been in good agreement with their models.

Now that Fernandes and his collaborators have studied the nematic phase extensively, the next step is to find whether – and how – it is connected to superconductivity. Does nematicity help or harm the superconducting state? How important is it to form the Cooper pairs? "I don’t believe that the nematic phase is irrelevant for superconductivity," Fernandes says, "but we are trying to understand exactly what its impact is, because there are also other ordered phases near the onset of the superconducting state, such as magnetism." Whether or not the nematic phase turns out to be important for the microscopic mechanism behind unconventional superconductivity, Fernandes and his group will have brought about a better understanding of this elusive phase of matter.