Kazuto AKIBA

Research Overview

The properties of materials that we encounter in our daily lives represent only a small part of their full potential. Materials exhibit a wide variety of physical properties depending on their environment—such as temperature, pressure, and magnetic field. In particular, under so-called "extreme conditions" that are far removed from everyday environments, even well-known materials can reveal unexpected and unknown aspects. This is because exotic states that cannot be stabilized under normal conditions may become the most stable under extreme conditions. Thus, the realm of extreme environments, where our everyday assumptions no longer apply, represents a vast unexplored territory for discovering novel physical phenomena.

My research aims to discover new physics and functionalities under extreme conditions by developing precision measurement techniques under ultra-low temperatures (~0.1 K), high pressures (up to 10 GPa), and strong magnetic fields (over 10 T), and using these to study the electronic states of materials. Accurately and comprehensively measuring physical properties under such conditions is far more challenging than in typical settings, which limits the amount of information that can be obtained. I strive to overcome these experimental challenges and develop techniques that enable previously impossible measurements. In my research, I emphasize the importance of interpreting experimental results based on established theoretical frameworks and explaining them quantitatively and thoroughly. Most experiments reveal results where many complex phenomena are intertwined. I believe that by carefully disentangling these phenomena one by one, the unexplained aspects that remain are where truly novel physics resides.

• Realization of an Ideal Nodal Ring Semimetal State in Black Phosphorus under Pressure

  1. K. Akiba, Y. Akahama, M. Tokunaga, and T. C. Kobayashi, Phys. Rev. B 109, L201103 (2024). "Editors' Suggestion"
  

  Three-dimensional topological materials with linear band crossings near the Fermi level are of great interest, as their properties are governed by Dirac fermions—particles that obey the laws of relativistic quantum mechanics. Well-known topological materials like Dirac and Weyl semimetals feature linear band crossings at discrete points in momentum space. In contrast, nodal line or nodal ring semimetals, which have recently attracted attention, exhibit band crossings that form continuous lines or rings. Candidate materials with such band structures, including MP₃ (where M is an alkaline earth or rare-earth element) based on the black phosphorus structure, have been proposed, and a variety of novel physical properties arising from these features have been theorized. However, it remains challenging to experimentally realize a situation where the physical properties are governed solely by the nodal lines or rings and to reveal their universal behavior.

  We addressed this challenge by focusing on black phosphorus, a prototypical elemental semiconductor that can be regarded as the parent compound of the aforementioned MP₃ materials. It was known that applying about 1.5 GPa pressure closes the energy gap of black phosphorus, making it semimetallic, but its detailed electronic structure had not been clarified. By using high-quality single crystals synthesized in Japan and a rotatable indenter-type pressure cell in magnetic fields, we performed detailed quantum oscillation measurements under pressure and elucidated the full electronic structure (i.e., the Fermi surface) of semimetallic black phosphorus. The experimentally obtained Fermi surface matched the nodal ring structure predicted by first-principles calculations in remarkable detail, demonstrating that black phosphorus under pressure is indeed a nodal ring semimetal. In this system, only carriers originating from the nodal ring determine the material's properties, and the shape and carrier density of the nodal ring can be easily controlled by pressure, making it unique among known materials. We also found that the large and anisotropic magnetoresistance observed in semimetallic black phosphorus is significantly influenced by a direction-dependent scattering relaxation time along the crystal axes. These results suggest that black phosphorus under pressure can serve as a promising platform for systematic studies of nodal ring semimetal physics. This work was selected as an Editors’ Suggestion in the international journal Physical Review B, published by the American Physical Society.

• Correlation Between Superconductivity and Charge Density Waves in LaAgSb₂

  1. K. Akiba, N. Umeshita, and T. C. Kobayashi, Phys. Rev. B 106, L161113 (2022).
  2. K. Akiba and T. C. Kobayashi, Phys. Rev. B 107, 245117 (2023).
  

  Many studies, including those on high-temperature superconductors and heavy-fermion systems, have shown that unconventional superconductivity can emerge near quantum critical points associated with magnetic order. However, systematic studies on the relationship between superconductivity and charge order are limited, and many aspects remain unclear. We previously demonstrated that LaAgSb₂ serves as a promising platform for exploring novel phenomena near the critical pressure of a charge density wave (CDW). In this study, we further discovered that LaAgSb₂ is a superconductor at ambient pressure with a transition temperature T_c = 0.3 K. Electrical resistivity measurements under pressure in the dilution refrigerator temperature range revealed a sharp peak in T_c precisely at the CDW critical pressure, indicating a strong correlation between superconductivity and CDW. The suppression of T_c away from the critical pressure suggests that some superconductivity-enhancing mechanism is active only near the CDW critical point. To understand this behavior, we investigated the superconducting transition temperature T_c expected from the conventional phonon-mediated mechanism, based on first-principles electron-phonon coupling calculations. The results showed that the calculated electron-phonon coupling strength was small, and the theoretical T_c was on the order of a few millikelvin. Therefore, the experimentally observed T_c of around 1 K cannot be explained by this mechanism alone and suggests the involvement of a nontrivial mechanism related to the collapse of CDW order. We also found that carriers with relatively strong electron-phonon coupling are localized on two-dimensional Fermi surfaces formed by p_x and p_y orbitals of Sb atoms in the square lattice. This implies that the Sb square lattice plays a crucial role in the formation of CDWs, the emergence of highly mobile carriers, and also in superconductivity.

  In addition, we investigated LaCuSb₂, which has the same crystal structure as LaAgSb₂. Unlike LaAgSb₂, LaCuSb₂ shows no anomalies indicative of a CDW transition at ambient pressure. Although previous studies were inconclusive, LaCuSb₂ has been reported to be a superconductor with T_c ≈ 1 K at ambient pressure. Noting that the unit cell volume of LaCuSb₂ is significantly smaller than that of LaAgSb₂, we hypothesized that it might already lie near the CDW critical point at ambient pressure and closely examined its electronic state and superconducting properties. From resistivity, magnetization, and specific heat measurements, we established that LaCuSb₂ is a bulk superconductor. We also used theoretical calculations to analyze transport phenomena under magnetic fields and determined its Fermi surface. Based on the experimentally determined Fermi surface, we conducted electron-phonon interaction calculations, revealing that: (1) Low-frequency phonon bands exhibit relatively strong electron-phonon coupling, and (2) Carriers interacting strongly with phonons are distributed across the entire Fermi surface. These findings differ from LaAgSb₂, where such carriers were localized to specific Fermi surfaces. The theoretical T_c closely matched experimental values, leading us to conclude that LaCuSb₂ is a typical phonon-mediated superconductor. The higher T_c compared to LaAgSb₂ can be attributed to factors (1) and (2), and no CDW-related critical phenomena appear to be involved in LaCuSb₂.

• Electronic States of LaAgSb₂ with a Square Sb Lattice Studied via Quantum Oscillation and Transport Measurements under Pressure

  1. K. Akiba, H. Nishimori, N. Umeshita, and T. C. Kobayashi, Phys. Rev. B 103, 085134 (2021).
  2. K. Akiba, N. Umeshita, and T. C. Kobayashi, Phys. Rev. B 105, 035108 (2022).
  

  LaAgSb₂ is known to exhibit successive charge density wave (CDW) transitions at ambient pressure at T_CDW1 = 210 K and T_CDW2 = 190 K. Through experiments, we revealed the temperature-pressure phase diagram of CDWs in LaAgSb₂ and identified the critical pressures as P_CDW1 = 3.2 GPa and P_CDW2 = 1.7 GPa for CDW1 and CDW2, respectively. We also found that the Shubnikov–de Haas oscillation patterns changed distinctly at these critical pressures, providing direct evidence of Fermi surface reconstruction associated with the disappearance of the CDWs. In the high-pressure regime above P_CDW1, where CDWs are absent, we observed a metallic state with high mobility and conductivity.

  Furthermore, we developed a rotatable mechanism for an indenter-type pressure cell, enabling measurement of transport properties with respect to magnetic field direction at pressures up to 4 GPa and temperatures down to 1.6 K. Using this setup, we clarified the Fermi surface anisotropy and magnetoresistance behavior under pressure. By comparing these experimental results with numerical simulations of electrical conductivity based on predicted pressure-induced Fermi surface evolution, we demonstrated that an anisotropic Dirac band originating from the Sb square lattice—hidden under the CDW gap at ambient pressure—emerges above P_CDW1 and dominates transport properties in that regime.