Dr. Takeshi Kondo is the winner of Oxford Instruments NanoScience’s 2020 Sir Martin Wood Prize - established in 1998 to promote scientific exchange between Britain and Japan and recognise the work of outstanding young Japanese researchers.

In this blog post, Dr. Kondo shares his award-winning research into high critical temperature superconductors, explaining the physical properties of cuprate high-Tc superconductors and how to use the experimental technique - angle-resolved photoemission spectroscopy (ARPES) - to observe the electronic structure of matter.

"Although the goal of fundamental physicists studying cuprates is to find the mechanism of high-Tc superconductivity accepted by all, it’s not everything. These compounds have rich physical properties that have been fascinating researchers since their discovery more than 30 years ago"

What are real-world applications of high critical temperature superconductors?

Superconductors characterised by zero resistivity have numerous technological applications and have now become visible in our daily lives. They are key materials used for life-saving applications, magnetic resonance imaging (MRI) in the healthcare industry. In the utilities sector, experiments have been conducted to improve the performance of wind power generators, or on a larger scale, to build the infrastructure to transmit electricity around the world using power lines with no energy loss in the future. A particularly exciting application of superconductors is underway in the transport industry, with the engineering of The Superconducting Magnetic Levitation Railway (Maglev) in Japan, due in 2027. It’s set to be the world’s fastest bullet train, utilising the force of superconductors to enable the train to levitate off the tracks with minimal friction.

What are the physical properties of copper oxide high-Tc superconductors?

The conventional superconductors need to be cooled to cryogenic temperatures close to absolute zero. The copper oxide superconductors with a high critical temperature (high-Tc cuprates) can function at temperatures even above 100K, higher than the liquid nitrogen temperature. Last year, room-temperature superconductivity was reported to be realised in a simple compound containing hydrogen, sulfur, and carbon. However, it is achieved only under extremely high pressure, like obtained at the core of the earth. Cuprates, therefore, still hold the record of the highest Tc achievable at the ambient pressure crucial for an actual application.

Although the goal of fundamental physicists studying cuprates is to find the mechanism of high-Tc superconductivity accepted by all, it’s not everything. These compounds have rich physical properties that have been fascinating researchers since their discovery more than 30 years ago. High-Tc superconductivity in cuprates occurs by a carrier doping to a Mott insulator - the insulator established in the extreme condition where electrons are localised to stop their conduction due to the Coulomb repulsion among them. The related electron correlation effect has been thought of as the main source leading to complicated yet fascinating properties of cuprates.

How do you use ARPES to observe the electronic structure of matters and reveal the pseudogap?

ARPES uses the photoelectric effect established by Albert Einstein: if you illuminate light into a solid, the electron absorbs the energy of a photon and it is excited out of the solid as a photoelectron. The photoelectrons conserve the energy and momentum possessed in solids. Therefore, the kinetic energies and angles of photoelectrons measured by ARPES can be directly converted to energy and momentum in solids, determining the electronic structure illustrated as a function of these two parameters. The pseudogap in cuprates develops in the momentum space called the antinodal region, which corresponds to certain angles in ARPES measurements. The spectrum exhibits an energy gap, characterized as starting to open at temperatures even higher than Tc. In addition, it is very broad, in contrast to the spectra with a superconducting gap, indicating electrons have a very short lifetime due to a strong correlation effect. In my study, I revealed that the pseudogap and superconducting states are competing in the antinodal region.

Can you explain more about exotic states emerging by doping to a strongly correlated Mott insulator?

The Mott insulator is a special kind of insulator, in which carriers are localized on each ion site. This electronic system gets unstable if electrons come to the same site due to a strong correlation effect, thus it naturally selects a localized state losing metallicity. By doping carriers to it, the system gains metallicity since those carriers can easily hop to other sites of ions. When highly doped, the carriers screen each other and can behave as if nearly free electrons. In the underdoped region, the correlation effect and metallicity get both important, making the situation more complicated. The pseudogap and small Fermi pockets emerge in such an electronic system where neither localization nor itinerary is established.

Anything else to add?

Superconductivity is still one of the most intriguing phenomena in physics. One will be thrilled when witnessing the resistivity of samples suddenly drop to zero on cooling. Similarly, it is also very exciting to see that the coherent sharp peaks quickly grow together with the opening of a superconducting gap in ARPES spectra once the sample is cooled below the transition temperature. On the other hand, the pseudogap state emerging between localization and itinerary of electrons is still mysterious and fascinating. One can enjoy the fun and depth of solid-state physics through the research of high-Tc cuprate superconductors full of these charms.

Dr. Takeshi Kondo