We here introduce research of the Macroscopic Quantum Physics Group. The Macroscopic Quantum Physics Group focus on an exploratory synthesis and characterization of strongly correlated materials such as oxides, chalcogenides, and intermetallics. Particularly, we are interested in magnetic, transport, and optical properties of correlated electronic systems including quantum magnets, superconductors, and quantum Hall liquids. The ultimate goal is to discover novel macroscopic quantum properties that cannot be described in the existing frameworks.

A system with a large Coulomb interaction in comparison with the electron bandwidth is called a strongly correlated electron system. In the strong coupling limit of the Coulomb interaction, the electrons are localized at the atomic position, so that the system does not exhibit electrical conductivity. Such insulators are called Mott insulators, which are distinguished from band insulators that are simply understood by the band theory. Strongly correlated electron systems near the Mott insulator are intriguing, since they show novel electronic properties including high-temperature superconductivity, magnetically induced ferroelectricity, giant magnetoresistance, and quantum anomalous Hall effect, as a consequence of the mutual coupling among the internal degrees of freedom such as spin, orbital, and quantum phases.

We will introduce the research on superconductivity and ferroelectricity in the iron-based compound BaFe₂X₃(X ＝S and Se) with a quasi-one-dimensional ladder structure. As shown in Fig. 1(a), this material has a ladder structure of iron atoms. Reflecting strong one-dimensionality, the electron correlation effect becomes strong, and BaFe₂X₃ becomes a Mott insulator at ambient pressure. This is in contrast to the itinerant nature of iron-based superconductors with a two-dimensional square lattice, which has been intensively studied since the discovery in 2005. Therefore, BaFe₂X₃ can be expected to be an excellent platform for studying electron correlation effects in iron-based compounds. These materials undergo an antiferromagnetic transition at low temperature; the magnetic structure are the striped –type and block-type ones in BaFe₂S₃ and BaFe₂Se₃, respectively (Fig. 1(b, c)).

(a) The crystal structure of BaFe₂X₃ (X = S and Se), in which Fe atoms form a two-leg ladder structure. Magnetic structures in the local ladder of (b) BaFe₂S₃ and (c) BaFe₂Se₃.

Applying pressure has the effect of reducing the interatomic distance, which corresponds to the increase in the bandwidth. As the bandwidth increases with respect to the Coulomb repulsion, the Mott insulating state melts into a metallic state. Figure 2(a) shows the pressure dependence of resistivity of BaFe₂S₃ as a function of temperature. The pressure is generated up to 15 GPa with a use of a pressure device called a diamond anvil cell or a cubic anvil cell (1 GPa = 10,000 atm). It can be seen that the electrical resistivity of BaFe₂S₃ decreases with the pressure application and the system shows a metal-insulator transition at around11 GPa. Upon entering to a metallic state, the material exhibits the superconducting transition at low temperatures. Since it is revealed that the magnetic order disappears near the metal-insulator transition, it is considered that spin fluctuations contribute to the formation of Cooper pairs. However, iron-based compounds have orbital degrees of freedom and charge fluctuations develop near the Mott transition; these fluctuations are likely relevant to the emergence of the superconductivity. The further study is expected to reveal the microscopic mechanism of the superconductivity.

An isostructural compound BaFe₂Se₃ also shows interesting properties. In contrast to a strip-type antiferromagnetic order in BaFe₂S₃, BaFe₂Se₃ exhibits a block-type antiferromagnetic order (Fig. 1(c)). Since this block-type magnetic structure has lower symmetry with respect to the crystal structure of the paramagnetic phase, the crystal structure must be low-symmetric near the magnetic order. Such magnetically induced structural phase transitions are frequently observed in frustrated magnets. In particular, when spontaneous polarization appears owing to the structural phase transition associated with an magnetic order, the phenomena is called multiferroics. To check a possibility that BaFe₂Se₃ shows multiferroic properties, we measured the optical second harmonic generation (SHG), which is sensitive to the crystal symmetry. As a result, SHG signal was observed below around 400 K, and it was found that BaFe₂Se₃ actually has polarization (Fig. 2(b)). It was also found that the polar transition temperature and the magnetic transition temperature are separated with each other. This is an interesting result, suggesting that the spin and the lattice systems are separated owing to the low dimensionality of the ladder structure. The relationship between superconductivity and multiferroic properties under pressure should be clarified in a future study.

In this way, we are conducting researches on macroscopic quantum phenomena in strongly correlated electron systems from various viewpoints and techniques.