The search for exotic superconductors


image: The crystal structure of two materials that John Harter will examine in his CAREER project which aims to discover odd parity superconductors with close polar phases
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Credit: UC Santa Barbara

John Harter, assistant professor of materials at UC Santa Barbara, received a National Science Foundation (NSF) Early CAREER Award, the foundation’s highest honor for young faculty. He will receive more than $715,000 over five years to support his cutting-edge quantum materials science research and educational activities.

“Professor John Harter is pursuing cutting-edge research that could accelerate the development of quantum technologies,” said Tresa Pollock, acting dean of UCSB’s College of Engineering (COE) and Alcoa Professor Emeritus of Materials. “We extend our heartfelt congratulations to him and believe that his work reflects the intellectual diversity and impact of the research conducted by our faculty.”

Harter is the eleventh junior WCC faculty member since April 2020 to receive the prestigious NSF award, and the third Assistant Professor in the Department of Materials to earn the recognition in the past nine months.

“Winning a CAREER award is a great honor, and it means the NSF believes the research conducted by my group has value,” said Harter, who joined UCSB in 2017 after earning a Ph.D. in physics from Cornell University. “The award is substantial and will support a dedicated Ph.D. student to work on the project, as well as necessary supplies and refill fees for shared facilities, over a five-year period. Such guaranteed long-term support will help my group push the field of unconventional superconductivity in exciting new directions.

Superconductivity is a phase of matter that emerges when a metal, such as mercury or lead, is cooled to sufficiently low temperatures. At this point, the material loses all electrical resistance and does not allow magnetic fields to penetrate, which means that an electrical current can persist in the material indefinitely. Microscopically, superconductivity results from the pairing of electrons orbiting each other, and materials scientists classify different types of superconductivity according to the shape of these orbits. Typically, their mutual orbital motion is symmetric, meaning that if space flipped or the pair flipped, the orbit would look exactly the same – a feature of inversion symmetry called even parity.

Harter will conduct experimental research on odd parity superconductivity, which results from pairs of electrons whose orbits reverse their direction when space is reversed. Spontaneous breaking of inversion symmetry by electron pairs leads to properties not found in more conventional types of superconductivity.

“Odd-parity superconductors are rare and interesting because they are expected to exhibit many exotic properties,” Harter explained. “Perhaps the most exciting prospect of odd-parity superconductivity is the possibility of finding a so-called ‘topological’ superconducting phase, which is an even rarer distinction related to how electron pairs can become entangled. in the superconductor.”

In quantum entanglement, the particles remain connected so that actions performed on one affect the other regardless of the distance between them. Due to this phenomenon, topological superconductors should theoretically be stable against environmental fluctuations and have applications in future quantum information technologies, namely the construction of a topological quantum computer naturally immune to decoherence. Overcoming decoherence, or the loss of quantum properties caused by “noise” or environmental conditions, is the main obstacle to building a functioning and scalable quantum computer. According to Harter, topological superconductors could be the “secret ingredient” scientists have been looking for to solve the difficult problem of decoherence.

“Just as the advent of the computing age is strongly linked to the exploitation of silicon to build solid-state transistor circuits, the development of quantum computers will also require a robust hardware platform capable of overcoming quantum decoherence. “, said Harter. “Odd-parity superconductors may be the key to achieving this goal.”

To discover new odd-parity superconductors, Harter’s research group will draw on recent theoretical developments that have revealed a link between odd-parity superconductivity and materials that exhibit both strong spin-orbit coupling and fluctuations. of an auxiliary polar order. Spin-orbit coupling is a relativistic effect that occurs when an electron approaches the speed of light as it orbits an atomic nucleus. When this happens, the spin of the electron becomes correlated with its orbital motion. Polar order is any distortion of a material, such as a displacement of the atoms in the crystal unit away from their equilibrium positions, which violates spatial inversion symmetry, so that one direction in the crystal becomes distinct from the opposite direction.

“What theoretical models have shown is that if we can find a material that combines these two key ingredients – spin-orbit coupling and polar order – electrons are able to orbit in exactly the right way. to form superconducting pairs of odd parity,” he said.

Harter’s group will first identify superconductors with strong spin-orbit coupling and polar order using a specialized laser, where all the energy from the laser beam is packed into extremely short pulses of light. They will use the laser to measure a material’s second harmonic generation, which occurs when light shines on a material and the frequency of a small component of the reflected light is doubled. Typically, light reflected from a material has exactly the same frequency. The frequency can double, however, if a material breaks inversion symmetry and possesses polar order.

“Using this technique, we can map the magnitude of the polar order in a candidate material,” Harter said. “If this polar order exists near the superconducting phase, we have a good candidate to pursue.”

Once a polar-order superconductor has been found, the researchers will use two techniques to remove the polar order. First, they will alter the overall concentration of electrons in the material by replacing a fraction of the atoms with other elements that donate or accept electrons, a process known as chemical doping. They will also apply pressure to the material in one or more directions to change its chemical bond lengths and crystal lattice constants.

“These steps will act as tuning knobs, allowing us to adjust the strength of the polar order in the material under study. By adjusting down to zero, we can accentuate the fluctuations,” Harter explained.

Once they have improved the polar fluctuations in the material, the researchers will study the parity symmetry of the resulting superconducting phase with a combination of conventional techniques, including Raman spectroscopy and non-reciprocal charge transport, as well as developing new techniques such as nonlinear terahertz spectroscopy.

Harter will initially focus on three promising families of materials – doped SrTiO3, the Cd superconducting pyrochlore2D2Oseven, and the quasi- and filled-skutterudites.

“The positive results of this CAREER project will confirm long-held theoretical insights and could have far-reaching implications for the development of quantum technology,” Harter said. “The ultimate goal of my research is to develop new experimental methods to reliably characterize the ground states of unconventional superconductors and other quantum materials. This is an extremely difficult assignment that I expect to work on for most of my college career, but this award will serve as a springboard to help my group quickly begin to progress in this area.


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