On the basis of pioneering works related to superconductivity research, the purpose of our unit is to improve the functionality of the high-temperature superconductors, elucidate the fundamental mechanism of superconductivity, search and design strategic novel superconducting materials, develop the technology for physical property evaluation, and promote research for design and synthesis of new high-temperature superconductors so that the challenges related to the enhancement of superconducting transition temperature can be addressed.
■Synthesis and characterization of bulk and thin films of high-Tc superconductors: (Materials Science and Engineering: Matsumoto, Horide, Jha)
■Synthesis of novel superconducting materials: (Electrical and Electronics Engineering Research Division: Matsuhira)
■Synthesis of host materials for novel high-Tc superconductor: (Basic Science Research: Tanaka)
■Evaluation of physical properties at high pressure for high-Tc superconductors: (Basic Science Research: Mito)
■Precise electrical transport and magnetic measurements of high-Tc superconductors: (Basic Science Research: Deguchi)
■First-principles calculations for high-Tc superconductors (Basic Science Research: Nakamura)
■Fundamental theory of the mechanism of high-Tc superconductors (Basic Science Research: Watanabe)
Synthesis of bulk and thin film samples of high-Tc superconductors, and their characterization (in-charge: Matsumoto, Horide, Jha)
This group is involved in the synthesis of epitaxial superconducting thin films deposited on a substrate, and evaluation of crystal structure and microstructure together with the material properties of these thin films. Usually, the synthesis and characterizations, in this group, are carried out on cuprate-superconductors. In future, however, the deposition of epitaxial thin film of new superconducting material studied by other groups is also planned in order to collaborate with other groups and provide suitable samples for Physics Research. In addition, taking advantage of the thin-film deposition technique, the application of strain to the superconducting layer, formation of non-equilibrium phases, formation and control of surface and interface of two-dimensional material, and experiments using an electric field effect and intercalation will also be considered. Together with pulsed laser deposition technique, sputtering and chemical solution deposition methods are also used for the deposition of superconducting thin films. Thin film XRD and high-resolution TEM systems are successfully employed to evaluate the crystal structure and microstructures of the thin film samples. The as-deposited thin film sample are subjected to patterning using photolithography with wet chemical etching or laser etching for low temperature electrical transport measurements using a physical property measurement system.
Collaborative research with other teams
The main contribution of this team is to provide bulk and thin film samples as required in the high-pressure experiments and magnetic and electrical measurements conducted by Deguchi and Mito groups of the property evaluation team. Also, the deposition of epitaxial thin films will be carried out for novel bulk superconductors discovered by Matsuhira group of the synthesis team. In addition, this team will also provide the facility for evaluation of the physical properties of such samples. The theoretical team of Nakamura and Watanabe groups is expected to propose new superconductors and their crystal structures. Realization of such theoretical propositions will also be attempted experimentally by this team.
Synthesis of novel superconducting materials (in-charge: Matsuhira)
Our group has explored novel superconductors emerged by carrier doping for materials including valence skipper (Bi, Pb, Sn, etc.), and 5d electron Mott insulator (especially strongly correlated Ir oxide). In addition, thermal expansion measurement is carried out in order to study the effect of the pressure on the phase transition temperature. From the thermal expansion and specific heat measurements, by applying Ehrenfest’s second-order phase transition equation:
we can get the pressure dependence of superconducting transition temperature because the superconducting transition at zero magnetic field in type-II superconductor is almost second order phase transition; when pressure is applied, as is always positive, if is positive, the superconducting transition temperature is an increasing function of pressure. Thermal expansion measurements are carried out by standard strain gauge method and by capacitance method that allows more accuracy in the measurement. Feedback on physical properties evaluation of synthesized materials and theoretical calculations lead to the enhancement of phase transition temperature. Moreover, by analyzing the results obtained in high pressure experiments, it is possible to get important findings leading to elucidation of the mechanism of superconductivity.
With physical properties evaluation team (Mito group): magnetic measurements, compression effect evaluation through the thermal measurement, and comparison with the results of the high-pressure experiments of the physical properties evaluation team. Moreover, attempts will be made to synthesize new materials proposed by the property evaluation team and the theoretical group.
Synthesis of Host Materials for Novel High-Tc Superconductor (in-charge: Tanaka)
Evaluation of physical properties of high-Tc superconductors at high-pressure (in-charge: Mito)
Superconductivity is a multi-body cooperative phenomenon of charge and spin. This multi-body phenomenon changes if the lattice is distorted, because all atoms in the lattice have some artificial perturbation. High-pressure experiment is a practical method for distorting the crystal lattice which can be used to manipulate the physical properties improving the functionality of the material. In recent years, the first-principles calculations are developed remarkably, and the synergetic effect of these calculations, together with the high-pressure experiments has led to the development of the new frontiers in superconductivity research.
High-pressure experiments are conducted not only to apply strain in a sample but also to apply giant strain in a material. Thus, subjecting superconducting materials under extreme environmental conditions to utilize the potential properties is expected to be very useful. In addition to obtaining structural information, such high-pressure experiments are very useful in making the design guidelines for high- Tc superconductors with the help of theoretical calculations.
There are four specific research themes of this group which are as follows:
(1) Study of the effect of hydrostatic pressure on cuprate-superconductors.
(2) Study of the uniaxial strain effect on cuprate-superconductors.
(3) Study of the uniaxial strain efficiency in thin films of cuprate-superconductors.
(4) Study of the effect of giant strain on conventional metal superconductors.
In association with Nakamura and Watanabe groups of the theory team and Matsumoto group of the synthesis team, we will conduct a systematic research on the physical properties of the superconducting materials subjected to lattice distortion by applying stress.
Precise measurement of electrical resistance and magnetic moment of high-Tc superconductors (in-charge: Deguchi)
Highly anisotropic (d-wave) wave function of the Cooper pair and low-dimensionality (two-dimensional) in the CuO2 plane in cuprate superconductors need to be modulated to achieve a unique superconducting vortex glass phase. From the application point of view, studies on the magnetic vortex glass phase under high magnetic field have been extensively carried out in the superconducting thin films. Flux pinning centers have been incorporated into the ceramics and thin films by using non-superconducting nanoparticles. We have carried out research on novel superconducting vortex glass phase under low magnetic field. Normally, in type II superconductors (such as cuprate superconductors), the magnetic flux do not penetrate into the superconductor below the first critical field in order to remain in the Meissner state. However, in the sintered samples such as ceramics, the magnetic flux penetrates into the voids between the particles, and in the thin films, magnetic flux penetrate into pinning centers. The magnetic flux is pinned by the non-superconducting portions that become normal and the emergence of specific flux glass phase is expected. Chiral glass phase due to the freezing of chirality by the superconducting loop current as shown in figure below have been proposed even in the superconducting ceramics in theoretical studies. Further, in thin films, paramagnetic Meissner magnetization opposite to magnetization appears as is observed from the diamagnetic Meissner effect.
More specifically, the investigation of the novel magnetic flux glass phase at low magnetic fields in the thin film and ceramic samples of the high-Tc superconductors incorporating magnetic flux pinning centers is carried out by precise electrical resistance and magnetic measurements. Usually, the linear and non-linear resistance, and linear and non-linear magnetic susceptibility is examined in detail by the AC method and with the study of the phase transition of the vortex glass phase by measuring the DC resistance, and magnetization, it is aimed to discover new superconducting phase.
Cooperative relationship with other teams: For ceramic and thin film sample preparation, we will work with Matsumoto group of the synthesis team. In coordination with Watanabe and Nakamura groups of the theory team, a new physical property derived from the low magnetic field flux glass phase will be developed for the superconducting phase.
First-principle calculations for the high-Tc superconductors (in-charge: Nakamura)
We develop first-principle calculation techniques and program codes to evaluate the superconducting transition temperature (Tc) of the real materials for theoretical material design and the superconducting mechanism elucidation. We analyze the correlation between crystal structure and calculated Tc data for various materials, which provides a theoretical guidance for the high-Tc superconductor search. In cooperation with the experimental groups, we aim at the discovery of the new high- Tc superconductors from Kyushu Institute of Technology.
The transition temperature Tc is estimated by the Macmillan formula
where is an electronic-phonon interaction parameter, is an effective electronic interaction parameter, and is the characteristic phonon frequency. From inputs of the crystal structure and the atomic number of the constituent elements of the material, the non-empirical evaluation of Tc is now possible with the help of the ab initio calculations using the above parameters. Recently discovered high-pressure hydrogen sulfide (Tc = 190 K) and monolayer iron-based superconductor (Tc = 50-60 K) are good targets in the initial stage of our study.
Cooperative relationship with other teams: We are intensively discussing with the members of the high-pressure physics team (Mito group) and the superconducting material synthesis team (Matsumoto and Matsuhira groups). We also collaborate with Deguchi and Watanabe groups for microscopic understanding of the superconducting transition mechanism.
Fundamental theory of mechanism of high-Tc superconductivity (in-charge: Watanabe)
By applying pressure and magnetic field to materials, continuous transition temperature to the ordered phase such as magnetic orders can be suppressed to zero temperature, which is referred to the quantum critical point (QCP). It is known that near the QCP, quantum critical phenomena emerge, where physical quantities such as electrical resistivity and specific heat exhibit anomalous behaviors distinct from those in normal metals. Since critical fluctuations enhanced near the QCP can induce the other ordered phase such as superconductivity, quantum critical phenomena in correlated electron systems have attracted much attention from the viewpoint of realization of high-temperature superconductivity. In fact, the mechanism of magnetic-fluctuation mediated superconductivity near the magnetic QCP has been extensively studied in relation to high-temperature superconductivity observed in cuprates.
Recently, a new type of quantum critical phenomena, not following the conventional magnetic criticality well understood so far, has been discovered in Yb-based and Ce-based compounds, which has been a critical issue in strongly-correlated electron systems. In these materials, superconductivity has also been observed, which suggests an interesting possibility that a new mechanism different from the spin-fluctuation mediated Cooper pairing, which has been discussed near the magnetic QCP so far, is realized.
Hence, in this project, we will try to clarify the fundamental properties of the new quantum critical phenomena in strongly-correlated electron systems. We will construct the theoretical framework and clarify the nature of the electronic states near the QCP. Through the clarification of the mechanism of instability of electronic states caused by the enhanced critical fluctuations near the QCP, we will try to get insight into the mechanism of the superconductivity to draw up the guiding principle of material design for realization of superconductors with higher transition temperatures.
In cooperation with synthesis team (Matsumoto and Matsuhira groups), physical property evaluation team (Mito and Deguchi groups), and theory team (Nakamura group), the fundamental theory of the new quantum critical phenomena will be developed.