Strong electron correlations are one of central issues of condensed matter physics. In systems, where charge, spin and orbital degrees of freedom are coupled with the lattice dynamics those interactions may result in rich variety of physical phenomena. In my talk, I will present my investigations of two systems where such couplings result in different ground states: a Mott insulator and heavy fermion rare-earth compound.
Layered perovskite Ca2RuO4 has been attracting a great interest as much as the spin triplet superconductor Sr2RuO4 because of its rich properties. Isovalent Ca substitution for Sr changes the system from superconductor to Mott insulator. Ca2RuO4 is an antiferromagnetic insulator below TN=110 K and exhibits a metal-insulator transition (MIT) at ~357 K accompanied by a crystal structure distortion [1, 2]. Despite a large number of experimental and theoretical studies, the mechanism of the MIT and magnetic transition in Ca2RuO4 is still hotly debated.
The 4f electrons are localized inside the atom due to strong Coulomb interaction, yet hybridize with the itinerant conduction electrons. As a result, various anomalous phenomena can emerge, such as valence fluctuations, unconventional superconductivity, heavy-fermion behavior, and spin/charge ordering [3, 4]. A ground-state features of the system are characterized by the competition between Kondo interactions which quench the magnetic moments and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions that leads to the magnetic ordering. The magnetic properties of such systems are directly related to the valence states of rare-earth ions and can be tuned by chemical substitution, external pressure and magnetic field. I will present the chemical substitution controlled physical property on Yb- and Eu-compounds. The focus will be on the results of photoelectron spectroscopy and my investigations of the electronic structural change across the phase transitions of these systems and the mechanism of phase transitions.
In the last part of my lecture, I will also mention the future perspective of my research taking advantage of KaCIF project. Exotic phenomena, e.g. superconductivity, magnetic ordering, and non-Fermi liquid behavior, in heavy fermion systems tend to appear at low temperature, and/or under pressure, that are partially inaccessible by photoelectron spectroscopy. To study such physics, in addition to spectroscopic methods, it is necessary to tackle the problem with complemental experimental techniques. In particular, transport measurements under extreme condition (low temperature, high pressure, and magnetic field) enable us to study the heavy fermion physics.
References
[1] S. Nakatsuji, et al., J. Phys. Soc. Jpn. 66, 1868 (1997).
[2] H. Fukazawa, et al., Physica B 281-282, 613 (2000).
[3] C. M. Varma, Rev. Mod. Phys. 48, 219 (1976).
[4] V. N. Antonov, et al., Adv. Condens. Matter Phys. 2011, 107 (2011).