^{Photo: NIMS}

Growth of the semiconductor industry increases computer power exponentially. Our daily life is supported by accurate computer simulations as represented by weather forecasts. Computer simulations are realized by running software on computers. Many years ago, I started software development of a phonon calculation code, phonopy (A.Togo et al., Journal of Physics: Condensed Matter 35, 353001 (2023); A.Togo, Journal of the Physical Society of Japan 92, 012001 (2023)), for our own scientific interest. By distributing it as open source software, many people started to use it. I also contribute to the software development of a crystallographic symmetry finder, spglib (A.Togo, I.Tanaka). Since crystal symmetry is ubiquitous in solid state science, many other software projects rely on it like a comic shown below. Popular scientific software is infrastructure. I would like to present how we manage these scientific software projects in the modern era of the scientific community. Presently, I am interested in electron-phonon interaction calculation (L. Chaput et al., Physical Review B 100, 174304 (2019); M. Engel et al., Physical Review B 106, 094316 (2022)) and magnetic symmetry (K. Shinohara et al.). I would like to talk about our recent research activities.

Dr. Togo was awarded the Sir Martin Wood Prize at the Millennium Science Forum which took place in November 2021. The Millennium Science Forum was established in 1998 to promote scientific exchange between Britain and Japan and recognize the work of outstanding young Japanese researchers. The prize is named after Sir Martin Wood, founder of Oxford Instruments. more

^{Photo: Takeshi Kondo}

High-temperature superconductivity, which occurs by carrier doping to a cupper oxide (cuprate) Mott insulator, is one of the most significant discoveries in 20^th-century physics. Immediately after its discovery, attracted attention was given to the anomalous behaviours of various physical properties observed above the superconducting transition temperature T_c. The electronic state relevant to these is a pseudogap, which opens above T_c in the density of states close to the Fermi level. The pseudogap was first discovered in the measurement of nuclear magnetic relaxation (NMR) rate 1/T₁ : 1/T₁T at high temperatures follows the Curie-Weiss law, whereas, on cooling, it becomes peaked at a certain temperature much higher than T_c, and then begins to decrease. This phenomenon was called "spin gap" because a gap seems to appear in the spin excitation spectrum. An energy gap was later found to open at temperatures higher than T_c in the density of states near the Fermi level by photoelectron spectroscopy and tunnelling spectroscopy, and since then, this
gap has been called “pseudogap”.

In the underdoped regime close to the Mott insulating phase, theory predicted the formation of "small Fermi pocket". However, an arc-shaped Fermi surface, that can be defined neither as a "large Fermi surface" nor as a "small Fermi pocket", was observed, due to the opening of the pseudogap at partial segments of the Fermi surface. The Fermi-arc is a strange electronic state, and understanding the relation of the pseudogap with the superconductivity has been thought of as crucial to elucidate the mechanism of high-T_c superconductivity.On this issue, two controversial views have been given: the first is that the pseudogap arises due to the pair formation of electrons as a precursor phenomenon of superconductivity. The second is that the pseudogap is, in contrast, generated by an electronic state distinct from preformed pairs, such as charge order or charge density wave, which compete with superconductivity.

In my talk, I will introduce my study of the pseudogap state in cuprates by high-resolution angle-resolved photoelectron spectroscopy (ARPES), clearly demonstrating that the pseudogap state competes with superconductivity. I will also present the successful observation of a "small Fermi pocket" near the Mott phase. The investigation of the lightly-doped Mott state may lead to the elucidation of the relationship between the Fermi arc and small Fermi pocket in cuprates, which has been a theoretical difficulty for many years. Importantly, the substitution of elements for carrier doping causes significant inhomogeneity in electronic states, making it hard to investigate the intrinsic electronic properties of the lightly-doped Mott state. The multi-layered cuprates focused on in my study has inner CuO₂ planes avoiding direct contact with charge reservoir layers, which realize a clean electronic system without the disorder. These compounds became the key to our successful observation of "small Fermi pockets", which could allow further development of the study of cuprates, as will be discussed in my lecture.

Associate Professor Kondo was awarded the Sir Martin Wood Prize at the Millennium Science Forum which took place in November 2020. The Millennium Science Forum was established in 1998 to promote scientific exchange between Britain and Japan and recognize the work of outstanding young Japanese researchers. The prize is named after Sir MartinWood, founder of Oxford Instruments. more

^{Photo: Keiichi Inoue}

Microbial rhodopsins form a family of photoreceptive membrane proteins in unicellular microbes. In the 20th century, microbial rhodopsins were considered to have a limited range in species such as hyperhalophilic archaea. However, recent genomic and environmental metagenomic analyses identified many thousands of microbial rhodopsins from diverse microorganisms, such as bacteria, archaea, algae, fungi, and even giant viruses, which use sunlight energy for various biological events.

We studied the molecular functional mechanisms of various microbial rhodopsins by time-resolved laser spectroscopy and vibrational spectroscopy to reveal their structure-function correlation. All microbial rhodopsins have the common chromophore, all-trans retinal, which isomerizes to 13-cis upon illumination. Our spectroscopic studies revealed that the protein moiety of each rhodopsin shows different structural changes in response to retinal isomerization, which leads to different biological function.

We also found many rhodopsin genes having widely different amino acid sequences compared with previously reported ones. As a result, light-driven outward Na+ pump and inward H+ pump microbial rhodopsins were newly identified, and their transport mechanism was also revealed.
In 2018, a new distinct class of rhodopsin was discovered by functional metagenomics, and it was named heliorhodopsin (HeR) meaning “the rhodopsin of the sun” in Greek. HeR has an orientation in membrane in which N- and C-termini face cytoplasmic and extracellular side, respectively. This is the opposite to all classical microbial rhodopsins. Furthermore, bestrhodopsin, which is a natural fusion protein between 1 or 2 microbial rhodopsins and bestrophin, forms a gigantic 700 kDa pentameric complex, also found in marine algae in 2022. Bestrhodopsin transports ions through the central pore of the bestrophin domain and it is regulated by the rhodopsin domains in a light dependent manner. The variety of rhodopsins is expected to continue to expand, and many new insights about the essence of protein functionality will be obtained by studying them.

Associate Professor Inoue was awarded the Sir Martin Wood Prize at the Millennium Science Forum which took place in November 2019. The Millennium Science Forum was established in 1998 to promote scientific exchange between Britain and Japan and recognize the work of outstanding young Japanese researchers. The prize is named after Sir Martin Wood, founder of Oxford Instruments. more

^{Photo: Takeshi Kondo}

Superconductors characterized by zero resistivity have numerous technological applications, such as magnetic resonance imaging (MRI), and experiments have been conducted for the future 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. Perhaps, the most exciting application may be Superconducting Magnetic Levitation Railway, planned to become operational in 2027 in Japan; the technology utilizing superconductors will finally become visible in our daily lives.

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 realized 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 certainly to find the mechanism of high-Tc superconductivity accepted by all, that is not everything. These compounds have rich physical properties which 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, that is the insulator established in the extreme condition where electrons are localized 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. In the webinar, I will share the fascination of cuprates by introducing the anomalous electronic properties standing out under the strong electron correlation effect. In particular, I use angle-resolved photoemission spectroscopy, a very powerful technique to directly observe the electronic structure of matters, and reveal the properties of enigmatic states in cuprates, called "pseudogap" and "small Fermi pocket". more

^{Photo: Keiichi Inoue}

The sun is the source of most living things on earth, and living organisms use light energy as a source of energy to drive their physiological activities and as a source of information to perceive the surrounding environment, which is useful for their own survival. The two most familiar strategies for the use of light are the vision of animals, including humans, and the photosynthesis of plants.

In photosynthesis, complexes of chlorophyll pigments and large proteins, called the photo­system exists in chloroplasts and other parts of plants, absorb the energy of sunlight and undergoes a highly efficient charge-separation reaction to produce the chemical energy necessary for the synthesis of adenosine triphosphate (ATP) and carbohydrates. On the other hand, in animal vision, the shape and color of objects seen by the eye are recognized by rhodopsin, a photoreceptor membrane protein located in the retina, which captures light entering through a lens in eye and transmits this information to the brain through the optic nerve.

In recent years, it was revealed that many microbes such as bacteria and unicellular eu­karyotic microbes, has their own photoreceptive proteins called microbial rhodopsin, similar to animal rhodopsin. Both of these are membrane proteins consisting of seven ɑ-helices, with a retinal pigment, a derivative of vitamin A, bound to the protein to absorb visible light. How­ever, unlike animal rhodopsin, microbial rhodopsin uses the light energy to transport various ions, such as protons, sodium and chloride ions, into and out of the cells, and to control gene expression with light, to regulate enzymatic activity in a light-dependent manner, etc.

In recent years, these microbial rhodopsins have been used as a major molecular tool in "optogenetics," a new methodology to manipulate the neural activity in animals by light. In this webinar, I will present the photobiology of microbial rhodopsins, the chemical and molecular mechanisms, as well as the applications in optogenetics.

Associate Professor Inoue was awarded the Sir Martin Wood Prize at the Millennium Science Forum which took place in November 2019. The Millennium Science Forum was established in 1998 to promote scientific exchange between Britain and Japan and recognize the work of outstanding young Japanease researchers. The prize is named after Sir Martin Wood, founder of Oxford Instruments. more