Research

Unconventional Superconductivity

Superconductivity is one of the most dramatic phase transition phenomena in nature. Since the discovery of unconventional superconductivity that falls outside the framework of the conventional BCS theory, elucidating its underlying mechanisms has become a central issue in condensed matter physics. In our laboratory, we investigate various exotic superconducting states, including the following topics:

Spin-triplet Superconductivity
In the superconducting state, electrons are known to form pairs. While these electrons typically form spin-singlet states with their spins pointing in opposite directions, superconductivity in which electrons form pairs in spin-triplet states is called spin-triplet superconductivity. This state is expected to provide a promising platform for topological superconductivity. By focusing on candidate materials such as uranium-based superconductors, we aim to clarify superconducting symmetry and detect topological quasiparticle excitations through precision measurements under ultra-low temperatures and magnetic fields [1].
[1] S. Suetsugu et al., Sci. Adv. 10, eadk3772 (2024). 

Chiral superconductivity
Chiral superconductivity is a superconducting state in which time-reversal symmetry is spontaneously broken. Such states are expected to host topologically nontrivial edge states and vortex states. By performing experiments under extreme conditions, including ultralow temperatures and high magnetic fields, our laboratory seeks to achieve experimental verification of chiral superconductivity.

Quantum spin liquid

In quantum spin liquids, strong quantum fluctuations originating from the Heisenberg uncertainty principle prevent localized spins from ordering or freezing even at absolute zero temperature. As a result, entirely new quantum-mechanical spin states emerge, characterized by long-range quantum entanglement. Quantum spin liquid states have attracted considerable attention due to their potential applications in fault-tolerant quantum computation and have rapidly developed into a major research field in recent years. Our laboratory investigates various novel spin states, including the following:

Exotic quasiparticle excitations in quantum spin liquids
Quantum spin liquid states are known to host fractionalized exotic quasiparticles, such as spinons and Majorana fermions, which are described by degrees of freedom entirely different from those of the original electron spin. We explore these fractionalized excitations and clarify their topological properties through ultra-high-precision thermal Hall effect measurements, as well as magnetic and thermal measurements at ultra-low temperatures [1, 2].
[1] K. Imamura, S. Suetsugu, et al., Sci. Adv. 10, eadk3539 (2024).
[2] S. Suetsugu, et al., arXiv:2407.16208 (2024).

Emergent quantum phases near the quantum spin liquid phase
In the vicinity of quantum spin liquid states, various peculiar spin states such as quantum magnetization plateau states and spin nematic phases are expected to emerge. For example, in kagome lattice antiferromagnets, where the effects of geometric frustration arising from competing spin interactions are pronounced, magnetization plateau states of purely quantum-mechanical origin can appear. Our laboratory works on the discovery and characterization of these novel quantum phases that manifest near the quantum spin liquid phase [1].
[1] S. Suetsugu, et al., Phys. Rev. Lett. 132, 226701 (2024). 

Emergent quantum phases in strongly correlated systems

In strongly correlated electron systems, strong interactions between electrons lead to the formation of collective quantum states, also known as quantum many-body states. While superconductivity and quantum spin liquids are representative examples, these systems are also expected to host various other quantum ordered phases that spontaneously break specific symmetries. In our laboratory, we aim to elucidate such novel quantum ordered phases, including the following:

Imaginary charge density wave in kagome metals
In strongly correlated electron systems with geometric frustration, conventional charge and spin orderings are often suppressed, giving rise to nontrivial quantum ordered phases. For example, in strongly correlated metals on a two-dimensional kagome lattice, an imaginary charge density wave state, characterized by a modulation of the imaginary component of the electron hopping integral, is expected to emerge. This imaginary charge density wave corresponds to a loop-current ordered phase in which nanoscale circulating currents flow dissipationlessly within the crystal lattice. Such a state is also anticipated to exhibit topological electronic responses originating from a nontrivial Berry curvature. Our laboratory investigates these novel quantum ordered phases in kagome metals [1].
[1] T. Asaba, et al., Nat. Phys. 20, 40-46 (2024).