Novel superconductor creates waves with magnetic field

In 2019, the University of California (UC) Santa Barbara received a $25 million grant to establish the first US National Science Foundation (NSF) Quantum Foundry. Since then, researchers affiliated with the foundry have been working to develop materials that can enable quantum information–based technologies for such applications as quantum computing, communications, sensing and simulation.

Now, in a paper in Nature Materials, a team led by foundry co-director and UC Santa Barbara materials professor Stephen Wilson reports that a material developed in the Quantum Foundry as a candidate superconductor — a material in which electrical resistance disappears and magnetic fields are expelled— could prove useful for future quantum computation.

In a previous paper in Physical Review Letters, Wilson and his group reported a new material, cesium vanadium antimonide (CsV₃Sb₅), that exhibited a surprising mixture of characteristics involving a self-organized patterning of charge intertwined with a superconducting state. As it turns out, these characteristics are shared by a number of related vanadium-containing materials, including RbV₃Sb₅ and KV₃Sb₅. The latter (a mixture of potassium, vanadium and antimony) is the subject of the Nature Materials paper.

Materials in this group of compounds, Wilson noted, “are predicted to host interesting charge density wave physics [that is, their electrons self-organize into a non-uniform pattern across the metal sites in the compound]. The peculiar nature of this self-organized patterning of electrons is the focus of the current work.”

This predicted charge density wave state and other exotic physics stem from the network of vanadium ions inside these materials, which form a corner-sharing network of triangles known as a kagome lattice. KV₃Sb₅ was discovered to be a rare superconducting metal built from kagome lattice planes. Some of the material’s other characteristics led researchers to speculate that charges in it may form tiny loops of current that create local magnetic fields.

Materials scientists and physicists have long predicted that a material could be made that would exhibit a type of charge density wave order that breaks what is called time reversal symmetry. “That means that it has a magnetic moment, or a field, associated with it,” Wilson explained. “You can imagine that there are certain patterns on the kagome lattice where the charge is moving around in a little loop. That loop is like a current loop, and it will give you a magnetic field. Such a state would be a new electronic state of matter and would have important consequences for the underlying unconventional superconductivity.”

The role of Wilson’s group was to make KV₃Sb₅ and characterize its bulk properties. Their collaborators at Princeton University then used high-resolution scanning tunnelling microscopy (STM) to look for what they believe are the signatures of such a state. According to Wilson, these signatures “are also hypothesized to exist in other anomalous superconductors, such as those that superconduct at high temperature, though it has not been definitively shown”.

STM works by scanning a very sharp metal wire tip over a surface. By bringing the tip extremely close to the surface and applying an electrical voltage to the tip or to the sample, the surface can be imaged down to the scale of individual atoms. In the paper, the researchers report seeing and analyzing a pattern of order in the electronic charge, which changes as a magnetic field is applied. This coupling to an external magnetic field suggests a charge density wave state that creates its own magnetic field.

This is exactly the kind of work for which the Quantum Foundry was established. “The foundry’s contribution is important,” Wilson said. “It has played a leading role in developing these materials, and foundry researchers discovered superconductivity in them and then found signatures indicating that they may possess a charge density wave. Now, the materials are being studied worldwide, because they have various aspects that are of interest to many different communities.

“They are of interest, for instance, to people in quantum information as potential topological superconductors. They are of interest to people who study new physics in topological metals, because they potentially host interesting correlation effects, defined as the electrons’ interacting with one another, and that is potentially what provides the genesis of this charge density wave state. And they’re of interest to people who are pursuing high-temperature superconductivity, because they have elements that seem to link them to some of the features seen in those materials, even though KV₃Sb₅ superconducts at a fairly low temperature.”

If KV₃Sb₅ turns out to be what it is suspected of being, it could be used to make a topological qubit for quantum information applications. “In making a topological computer, one wants to make qubits whose performance is enhanced by the symmetries in the material, meaning that they don’t tend to decohere [decoherence of fleeting entangled quantum states being a major obstacle in quantum computing] and therefore have a diminished need for conventional error correction," Wilson said.

“There are only certain kinds of states you can find that can serve as a topological qubit, and a topological superconductor is expected to host one. Such materials are rare. This system may be of interest for that, but it’s far from confirmed, and it’s hard to confirm whether it is or not. There is a lot left to be done in understanding this new class of superconductors.”

This story is adapted from material from the University of California, Santa Barbara, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.