“Seek and you shall find”: pushing strange metals to the extreme.

Researchers at the HFML used Europe’s strongest continuous magnetic field to study the behaviour of an exotic superconducting metal at freezing temperatures, down to almost -273 oC. They discovered evidence of strange metallic behaviour - something they didn’t expect to find. New questions arise.

Room temperature superconductivity – the perpetual flow of electrical current – remains a holy grail for physicists. The temperature at which a metal goes superconducting depends primarily on the strength of the interaction that causes the electrons to form superconducting pairs. However, it also depends on the fraction of electrons that actually ‘feel’ that interaction. Thus, if you want to maximize superconductivity, ideally you want an interaction that is strong and affects all of the electrons. A nematic interaction provides the best of both worlds, but first, you need to identify materials that harbour such an interaction, understand how the electrons feel this interaction and then learn how to maximise it. This work represents an important step along that road.

phase transitions nematic phase

Phase transitions
If you place a glass of water in a freezer and wait a while, it turns to ice. This liquid-to-solid transition is what’s known as a phase transition. Cool a liquid crystal and the molecules enter an intermediate phase between liquid and solid – all individual molecules are still mobile but all of a sudden, they point in the same direction like matches in a matchbox. This is the nematic phase in liquid crystals. In a nematic metal, it is not elongated molecules that align, but the electrons themselves that align their direction of travel.

Cool an elemental metal and the resistivity falls. As the temperature approaches 0 Kelvin or -273o C, which scientists call absolute zero, the resistivity levels off at a finite value. Certain elements, however, undergo a remarkable transition a few degrees above 0 Kelvin. The resistivity suddenly drops to zero, and the metal becomes a superconductor. This means no heat or any other form of energy is dissipated as current flows inside the material. Scientists are always trying to figure out how to achieve superconductivity at ever higher temperatures in order to avoid the economic benefits of dissipation-less energy transmission, levitated transport etc.

Strange metals and the quantum critical point
It is not only elemental or ordinary metals that superconduct however. Today, scientists are also fascinated by ‘strange metals’, that undergo phase transitions (e.g. to a magnetic state) before superconductivity sets in. If they find the right tuning knob, scientists can suppress the temperature at which this transition occurs, driving it to 0 K.  When doing so, the electrons start fluctuating quantum mechanically between the magnetic and non-magnetic phases without any thermal agitation. This is called: the quantum critical point. Imagine water turning into ice turning into gas and back. All at the same temperature. Pretty amazing. Now scientists know that this point exists but it is hard to access experimentally. This is because in many cases, superconductivity gets in the way and it is not that easy to get rid of. But accessing this quantum critical point is important for two reasons. Firstly, when the temperature is just above the critical point, the resistivity of the strange metal acts in a way that goes against every conventional theory. Secondly, it is highly likely that these critical fluctuations that cause the strange resistance are also responsible for inducing superconductivity, so if scientists can access this strange metallic state and study its behaviour, they might be able to identify the interaction what causes superconductivity in these exotic systems.

So it ends here?
Then you don’t know the real scientists. They like a tough challenge. PhD student Salvatore Licciardello and his team, led by Prof. Nigel Hussey, thought of a way to make superconductivity disappear in a very special material and explore the metallic state down to the lowest temperatures possible. How did they do it?

  1. First of all, they searched for the right compound and found it: iron chalcogenide FeSe, where the electrons appear to undergo a transition to a purely nematic phase at a temperature close to 100 K.
    But we just learnt that the quantum critical point occurs at 0 K. So how to reach it?
  2. The Japanese collaborators within the team found a neat trick: they substituted exactly 1/6 of the Se atoms with S atoms. By doing this, the transition temperature to the nematic phase is suppressed to 0 K.
  3. The quantum critical point itself is still ‘protected’ by a veil of superconductivity that must first be removed. Here is where the high field magnets come in handy. Superconductivity is a pretty robust state but subject it to a strong enough magnetic field and it will eventually succumb.

Then, the strange metallic state revealed itself, extending all the way down to temperatures just one degree above absolute zero. Licciardello also discovered that as the quantum critical point is approached, the electrons become progressively heavier, ‘weighed down’ by the intensifying fluctuations of the nematic order.

Wow, this must give a lot of insight
It is quite amazing, that’s why it is published in Nature. But actually, the outcome was different from what the team expected. First of all, strange metal behaviour had only been seen previously in systems close to a magnetic quantum critical point, never close to a purely nematic one. Indeed, there are currently no theoretical model that predicts this behaviour in such a nematic state. Secondly, while the critical fluctuations become stronger the nearer the system is to the nematic quantum critical point, the superconductivity itself does not become stronger. It raises new questions and begs for more research. Hussey: “We need to understand how electrons interact with nematic fluctuations to cause strange metallic behaviour in the first place. Then, we need to understand why superconductivity in this particular system is not enhanced near the quantum critical point. Nematic fluctuations are one possible route to high temperature superconductivity, but not, it seems, in the iron chalcogenides. Nevertheless, understanding what first inhibits the growth of superconductivity can help us understand what makes it grow. Salvatore’s work is an important first step in this endeavour.”

So the puzzle continues….

Related publication
Electrical resistivity across a nematic quantum critical point,S. Licciardello, J. Buhot, J. Lu, J. Ayres, S. Kasahara, Y. Matsuda, T. Shibauchi and N.E. Hussey, Nature (2019)

Contact: Nigel Hussey