Quantum Materials

Staff Professor Kenji Ishida
Associate Professor Shingo Yonezawa
Assistant Professor Shunsaku Kitagawa

We are studying a variety of physical phenomena emerging in a strongly correlated system, where electrons in a solid strongly interact with each other. At temperatures much lower than room temperature, material properties are dominated by quantum-statistical-mechanics effects, and phenomena far away from common sense, such as superconductivity, occur. Aiming to discover and understand such novel phenomena, we perform various types of experiments.

Topological quantum phenomena

Prof. Maeno and Assistant Prof. Yonezawa's Group is mainly studying "topological quantum phenomena", novel phenomena caused by the "shape (topology)" of the electron wave function of the quantum condensation system. Looking at things from the perspective of the wave function topology, you can notice similarity among phenomena whose relationships are not seen at all at the first glance.

Time-reversal-symmetry broken superconductor Sr_2RuO_4

There are two possible spin states of Cooper pair responsible for the superconducting state, the spin-singlet and spin-triplet states, but most superconductors have singlet states with no spin degrees of freedom. There has been accumulating evidence that Sr_2RuO_4 is one of the most promising candidates for spin-triplet superconductors. Moreover, a chiral superconducting state, in which the time reversal symmetry is spontaneously broken, is believed to be realized in this oxide. In recent years, it has been uncovered that such chiral spin-triplet superconductivity has an aspect as a "topological superconductor", showing superconducting topological quantum phenomena. Furthermore, the Mott insulator Ca_2RuO_4, with Sr replaced by Ca, also shows superconductivity under pressure. We aim to discover and understand topological quantum phenomena peculiar to superconductors broken in time reversal symmetry, through experiments on bulk superconducting properties of Sr_2RuO_4, superconducting devices using Sr_2RuO_4, and external field effects such as pressure and electric field to Ca_2RuO_4.

Search for new material

In order to discover unknown physical phenomena, we must find materials that may exhibit interesting properties. We thus search for and develop materials that may exhibit interesting properties (especially superconductivity), mainly among transition-metal oxides. We have found various interesting materials, such as the first anti-perovskite oxide superconductor Sr_{3-x}SnO as well as several new superconductors and the oxide conductors PdCoO_2 showing huge magnetoresistance.

Microscopic properties of materials

Prof. Ishida and Assistant Prof. Kitagawa's group is studying various superconductors and magnetic materials from the microscopic point of view using nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin resonance (muSR) techniques. Recent topics are introduced below.

Superconductivity on uranium-based ferromagnets

Superconductivity with Meissner effect was thought to be incompatible with ferromagnetism (like magnets). In recent years, however, U-based materials that are ferromagnetic but show superconductivity have been discovered and are attracting much interest. In our laboratory, we have studied the ferromagnetic (FM) superconductor UCoGe. We have showed that UCoGe possesses unique FM fluctuations with strong uniaxial (Ising-type) anisotropy, which are working as a pairing glue for UCoGe. We also study metamagnetic material UCoAl which can induce strong Ising ferromagnetism by applying a magnetic field, and we are studying magnetic properties of this compound using NMR.

Iron-based superconductors

Recently, iron-based high temperature superconductors which contain iron was discovered by Professor Hosono's group at Tokyo Institute of Technology. A parent compound of iron-based superconductors are antiferromagnets, but they show superconductivity by substituting elements or applying pressure. Our laboratory revealed that superconductivity occurs near the magnetic phase in the LaFeAs(O,F) system and that the superconductivity of iron-based superconductor is unconventional superconductivity. We also report on the relation between antiferromagnetic fluctuations, magnetic order and superconductivity in BaFe_2(As,P)_2 and the FM quantum critical behavior on related compound Ce(Fe,Ru)PO. By the way, the figure on the right shows the antiferromagnetic ordering temperature of the parent material in various unconventional superconductors and the highest superconducting transition temperature of each system. Very interestingly, it seems that there is a linear relation between the antiferromagnetism of the parent material and the strength of superconductivity. We would like to continue to understand the superconducting mechanism from a broad perspective.