Superconductors - keep it in the family?

29 October 2018 by Kathryn Boast

Author: Franziska Kirschner

Fran recently published a paper on an insightful family of superconductors, available here:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.094505.
In this post, she explores what this work really means for superconductivity.

Since the discovery of high-temperature superconductors over 30 years ago, there has been a huge effort to try and find a superconductor that works at room temperature. While there have been many experimental successes in getting superconductors to work at higher and higher temperatures, what is really lacking is a complete theory of high-temperature superconductivity. If scientists were able to find such a theory, it is hoped that the search for room temperature superconductivity might be a little easier!

In order to construct such a theory, it is important to look at the similarities and differences between different families of superconductors to try and understand which microscopic details are important in determining the superconducting properties of a material. The members of a family of superconductors often have very similar crystal structures (the arrangements of the atoms inside the crystals) and electronic structures (the energy levels of their electrons).

When a superconductor superconducts, its electrons pair up into Cooper pairs. When you heat up a superconductor, the Cooper pairs gain extra thermal energy and can break up via something we call the pair-breaking mechanism. Two types of pair-breaking mechanism are important for this work, and they correspond to fully-gapped and nodal superconductors. A fully-gapped superconductor can have some of its Cooper pairs broken up at any temperature — even just above absolute zero! A nodal superconductor, on the other hand, can only have its Cooper pairs broken up once the thermal energy is larger than some amount: this is the energy gap. Whether a superconductor is nodal or fully-gapped is intricately linked to its electronic structure. So if the members of a family of superconductors are meant to have similar electronic structures, we’d expect them to have the same pair-breaking mechanism.

LuxZr1-xB12 is an example of a family of superconductors, where Lu is lutetium, Zr is zirconium and B is boron. We can change the value of x using chemical doping, meaning that we can study LuB12, ZrB12, Lu0.8Zr0.2B12 (meaning 20% of the lutetium atoms are substituted for zirconium atoms) and (hypothetically!) anything in-between. All of the members of this family have a very similar crystal structure: the boron atoms are in the shape of a cage, with the zirconium or lutetium atoms nestled between them. Not much is known about their electronic structure, but we think it ought not to change too much between different members of the family.franFig1.png

In order to study the pair-breaking mechanism of superconductors, we can use a technique called µSR. µSR fires a fundamental particle (an antimuon) into a piece of superconductor, and then studies its interaction with the magnetic field of the superconductor. From this interaction, we can determine a whole range of properties of a superconductor, including its critical temperature (the temperature below which it superconducts) and its pair-breaking mechanism.

In our recently published paper, we present a study of several members of the LuxZr1-xB12 family of superconductors using µSR. Remarkably, we find that not all members of this family have the same pair-breaking mechanism. If x is low (and so most of the rare earth atoms in the material are zirconium), we have a nodal superconductor. If x is high (meaning most of the rare earth atoms are lutetium), we have a fully-gapped superconductor. This is fascinating because it shows that very small changes in the microscopic crystal and electronic structures can lead to dramatically different superconducting behaviour. As the compounds in this family are all so similar, it provides us with a very good system in which we can precisely isolate the cause of this change. And if we can isolate the cause, we have further information with which we can construct that elusive theory of high-temperature superconductivity!

So now we’re left with a challenge — what is precisely the cause of this crossover from nodal to fully-gapped superconductivity? We need to do more experiments, including further studies on more members of the LuxZr1-xB12 family so that we can pinpoint precisely where the crossover occurs. We already have a good idea of the crystal structures, but more experiments need to be done to calculate the electronic structures of these superconductors. It’s likely that the way the electrons in the lutetium, zirconium and boron interact with one another is the cause for this crossover, but we need to determine exactly how and why the electrons behave this way.

I haven’t mentioned the temperatures at which these superconductors work yet. And that’s because they’re not particularly exciting: the critical temperature of ZrB12 is 6K, while for LuB12 it's 0.6K (and the compounds in between have critical temperatures somewhere in between too). But what is exciting is that the nodal-to-fully-gapped crossover has only been seen in a handful of compounds before: namely in the iron-based superconductors. The iron-based superconductors work at much higher temperatures, and so what we can learn from this crossover may be useful right at the forefront of high-temperature superconductivity.

One final observation concerns not the superconductivity, but the magnetism of these materials. As with many superconductors, the LuxZr1-xB12 family are difficult to synthesise. While you might expect the Lu3+ and Zr4+ in Lu0.07Zr0.96B12 to be randomly mixed up, it’s sometimes possible that the Lu atoms can cluster together in one part of the crystal. We find, also using µSR, that these clusters of Lu3+ may be magnetic, which is strange because Lu3+ itself is non-magnetic! We don’t quite understand how non-magnets can clump up to become magnets, but we do know that this has happened before: in some families of high-temperature cuprate (copper-based) superconductors. So if we study the magnetic properties of the LuxZr1-xB12, we may be able to learn more about even more high-temperature superconductors.

So what at first appeared a set of vanilla, low-temperature superconductors has turned out to be a family rich in interesting physics, combining properties of two major groups of high-temperature superconductors: iron-based and cuprate. Many of the struggles involved in finding a theory of high-temperature superconductivity have revolved around difficulties in reconciling the properties of the iron-based superconductors with those of the cuprates. Our work may be a first step in solving some of these difficulties.

Categories: research | paper | superconductors | quantum materials