Research

Reseach overview

Our research field can be broadly described as solid-state physics. However, the specific topics we are interested in—such as unconventional superconductivity (including high-temperature superconductivity), heavy electron systems, quantum critical phenomena, quantum spin liquids, and topological phenomena—are rich in the essential elements of modern physics, including strong correlations, symmetry breaking, and topology.

Depending on the research theme, our work is closely connected to a wide range of fields, including statistical physics, quantum information and quantum computation, particle and nuclear physics, and even technological applications. This interdisciplinary nature, with the potential to impact various other research areas, is one of the most exciting aspects of our field.

The quantum phenomena we are interested in often emerge at low temperatures. Therefore, we utilize ultra-low temperature environments—below a few kelvin—and control material states using high magnetic and electric fields. Our core experimental approach involves precise measurements of physical properties under multiple extreme conditions. Depending on the topic, we employ a variety of measurement techniques, including electrical and thermal transport measurements, thermodynamic property measurements, and magnetic measurements.

We are also engaged in the development of novel experimental and measurement techniques, as well as in material synthesis. In particular, our materials research goes beyond conventional chemical synthesis: we are also exploring the creation of artificial superlattices using state-of-the-art thin-film fabrication techniques, enabling us to engineer material systems that do not exist in nature. 

Unconventional superconductivity

Superconductivity is a state in which electrical resistance drops to zero—an idea many are familiar with through its applications in technologies such as magnetic levitation (maglev) trains and MRI machines. It is one of the most dramatic manifestations of quantum phenomena on a macroscopic scale, arising from the formation of electron pairs (Cooper pairs) that undergo Bose-Einstein condensation.

More than a century has passed since the discovery of superconductivity, yet it remains a central topic in modern condensed matter physics. While the basic understanding of conventional superconductors was established over 50 years ago through the BCS theory, a growing number of unconventional superconductors have since been discovered—systems that cannot be explained within the BCS framework. These include high-temperature superconductors found in cuprates and iron-based compounds, and understanding the nature and mechanisms of superconductivity in these materials is one of the major challenges in the field.

Two important keywords in the study of unconventional superconductivity are symmetry breaking and exotic superconducting states. The former is a fundamental and universal concept in physics: superconductivity is generally characterized by the breaking of gauge symmetry. However, in recent years, superconductors that break symmetries other than gauge symmetry have been discovered, sparking significant interest.

The latter includes superconducting states where Cooper pairs carry a finite center-of-mass momentum or where the system exhibits topological superconductivity. The realization and verification of these states in real materials remain at the forefront of research. Notably, finite-momentum Cooper pairing is also discussed in the context of neutron stars, linking this topic to nuclear physics. In our lab, we aim to identify broken symmetries and search for novel superconducting states through high-precision measurements of physical properties.

Quantum spin liquid

In conventional magnetic materials, spins typically “freeze” as the temperature is lowered, much like how matter itself solidifies. However, in systems dominated by quantum fluctuations, spins can remain in a disordered, liquid-like state all the way down to absolute zero. Such a state is known as a quantum spin liquid. While magnons—the quantized spin waves—are well-known quasiparticles that describe spin excitations in ordered magnets, quantum spin liquids are predicted to host even more exotic quasiparticles. Our research aims to explore and understand these emergent excitations.

One notable example from our recent work involves a special type of quantum spin liquid known as a Kitaev quantum spin liquid. In this state, an exotic neutral fermion called a Majorana particle emerges as a quasiparticle. Majorana fermions, which are their own antiparticles, have attracted significant attention due to their unique properties. They have been proposed as the basis for non-Abelian anyons, a key ingredient in fault-tolerant quantum computing. While Majorana particles have also been discussed as a possible identity for neutrinos, their existence remained unconfirmed for over 80 years since their theoretical prediction.

Our research group has provided experimental evidence for Majorana quasiparticles in a magnetic insulator—a candidate material for the Kitaev quantum spin liquid. Specifically, we observed the half-integer thermal quantum Hall effect, a signature that indicates the presence of Majorana fermions in the material (＊). The quantum Hall effect is a hallmark of topological physics, and our observation was the first to demonstrate a topologically protected quantum state in a quantum spin liquid.

The understanding of Majorana particles is still in its early stages. We continue to investigate the mechanisms behind their emergence and their universality across different materials. Furthermore, to move toward practical applications—including quantum computation—we are also developing new techniques for detecting and manipulating Majorana particles.

(＊) Y. Kasahara et al., Nature 559, 227–231 (2018); T. Yokoi, Y. Kasahara et al., Science 373, 568–572 (2021).

Development of new materials using artificial superlattices

A crystal lattice with a periodic structure longer than the fundamental unit cell, formed by overlaying multiple types of crystal lattices, is known as a superlattice. When such a structure is artificially created by alternately stacking different materials, it is referred to as an artificial superlattice.

Artificial superlattices enable the fabrication of unprecedented layered structures—materials that do not exist in nature. This opens the door to controlling the dimensionality of quantum states, artificially introducing inversion symmetry, and modulating electronic states at interfaces. As a result, novel quantum phases that are not observed in the individual constituent materials can emerge.

Our laboratory takes on the challenge of developing new materials by utilizing advanced atomic-layer thin film fabrication techniques such as pulsed laser deposition (PLD) and molecular beam epitaxy (MBE).

Quantum critical phenomena

Quantum phase transitions are phase transitions driven by quantum fluctuations originating from the uncertainty principle. These transitions occur at absolute zero temperature when a control parameter such as pressure is varied. The boundary between distinct ground states under such conditions is known as a quantum critical point.

Near the quantum critical point, not only does an anomalous metallic state emerge—deviating from the standard Fermi liquid theory of conventional metals—but unconventional superconductivity often appears as well.

Our laboratory investigates a wide range of materials to explore the universality of quantum critical phenomena and their potential connection to superconductivity.