Since receiving a $25 million grant in 2019 to become the first National
Science Foundation (NSF) Quantum Foundry, UC Santa Barbara 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.
They may have done it.
In a new paper, published in the journal Nature Materials, foundry
co-director and UCSB materials professor Stephen Wilson, and multiple
co-authors, including key collaborators at Princeton University, study a new
material developed in the Quantum Foundry as a candidate superconductor — a
material in which electrical resistance disappears and magnetic fields are
expelled— that could be useful in future quantum computation.
A previous paper published by Wilson’s group in the journal Physical Review
Letters and featured in Physics magazine described a new material, cesium
vanadium antimonide (CsV3Sb5), that exhibits a surprising mixture of
characteristics involving a self-organized patterning of charge intertwined
with a superconducting state. The discovery was made by Elings Postdoctoral
Fellow Brenden R. Ortiz. As it turns out, Wilson said, those characteristics
are shared by a number of related materials, including RbV3Sb5 and KV3Sb5,
the latter (a mixture of potassium, vanadium and antimony) being the subject
of this most recent paper, titled “Discovery of unconventional chiral charge
order in kagome superconductor KV3Sb5.”
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 (V) ions inside these materials, which form a
corner-sharing network of triangles known as a kagome lattice. KV3Sb5 was
discovered to be a rare metal built from kagome lattice planes, one that
also superconducts. 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 said. “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 the material and characterize its
bulk properties. The Princeton team then used high-resolution scanning
tunnelling microscopy (STM) to identify what they believe are the signatures
of such a state, which, Wilson said “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 resolving individual atoms and where the electrons group. In the
paper the researchers describe 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,” he continued. “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 useful in quantum information applications. For
instance, Wilson said, “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.
“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,” he added. “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.”
Reference:
Unconventional chiral charge order in kagome superconductor KV3Sb5 by
Yu-Xiao Jiang, Jia-Xin Yin, M. Michael Denner, Nana Shumiya, Brenden R.
Ortiz, Gang Xu, Zurab Guguchia, Junyi He, Md Shafayat Hossain, Xiaoxiong
Liu, Jacob Ruff, Linus Kautzsch, Songtian S. Zhang, Guoqing Chang, Ilya
Belopolski, Qi Zhang, Tyler A. Cochran, Daniel Multer, Maksim Litskevich,
Zi-Jia Cheng, Xian P. Yang, Ziqiang Wang, Ronny Thomale, Titus Neupert,
Stephen D. Wilson and M. Zahid Hasan, 10 June 2021, Nature Materials.
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