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Research Spotlight

Professor Hae-Young Kee - Quantum Condensed Matter Physics
Fall 2022 Newsletter Research Spotlight Image 2

"Evolution of interorbital superconductor to intraorbital spin-density wave in layered ruthenates” published in May 2022 in Physical Review Research

Authors: Austin W. Lindquist, Jonathan Clepkens and Hae-Young Kee

We spoke to graduate students Austin W. Lindquist and Jonathan Clepkens about this work.

What are the layered ruthenate materials that you’ve studied and why are they a topic of interest?

The strontium ruthenate family that we study refers to a class of materials in which a variable number of layers of ruthenium atoms surrounded by oxygen atoms are stacked on top of each other. The material where the individual layers are well separated, which we refer to as the single layer material, has long been thought to host an exotic form of superconductivity. However, even after 25 years of intense studies, the superconducting state is still not well understood. Furthermore, when two layers are stacked directly on top of each other, referred to as the bilayer material, it exhibits a magnetic ordering at low temperatures in a magnetic field. This adds another puzzle piece to the longstanding mystery: why does a simple stacking of two layers change the physical state from an exotic superconductor to a magnet?

What was the purpose of this work?

The purpose of this work is to help us better understand strontium ruthenate, both the single layer and the bilayer. While many recent publications have focused on the single layer only, here we consider both systems and explore a theme that can unify the two. We wanted to show that our model could simultaneously explain the experimental observations of superconductivity in the single layer, as well as the magnetic ordering in the bilayer.

Can you describe the Kanamori Hamiltonian used for this work?

The Kanamori Hamiltonian is an equation that describes how electrons interact within orbitals around an atom, in the materials we consider these are ruthenium atoms. We use it to describe the Coulomb repulsion and fermionic exchange among electrons. These interactions between electrons can give rise to phases of matter such as superconductivity or magnetism. What’s interesting about the superconductivity from this model is that it actually occurs between electrons in different orbitals, in contrast to more conventional routes to superconductivity which occur between electrons in the same orbital.

What were the results of the work?

We found that we can unify the superconductivity and magnetic order in the single layer and the bilayer within a single framework. While both systems have very similar models overall, the addition of the second layer modifies the pairing between different orbitals and destroys the superconductivity, while also placing the system very near a magnetic instability, allowing order to form once a magnetic field is applied. We think that being able to explain both of these systems simultaneously means that we are closer to understanding the origin of superconductivity.

What is next for Hae-Young Kee’s group?

The puzzle of superconductivity in the single layer strontium ruthenate is still incomplete. We’ve proposed a new type of spin-triplet superconductor, and its elementary excitations are yet to be explored. We would like to understand the nature of the excitations, as the presence of Majorana fermions inside of vortices are critical for the application of this material to topological quantum computing proposals. We’ve also proposed an experimental route to increase the transition temperature of superconducting strontium ruthenate. We hope that this can be confirmed by experimentalists, both to provide validation of our theories, but also because increasing superconducting critical temperatures is always desirable.

Read the full paper here:

More on Professor Hae-Young Kee’s Group here: