Research

What have we been striving for all along?

Long ago, condensed matter physics was concerned with very concrete questions: Why do metals conduct electricity? Why are some materials insulating? Why do crystals form specific structures? Why do objects made of iron become magnetized? In the early 20th century, with the birth of quantum mechanics, physicists began to use quantum mechanics to understand materials. Quantum and semiclassical theories of solids, crystals, and metals were developed, followed by rigorous quantum mechanical concepts such as the Pauli exclusion principle, Fermi–Dirac statistics, and Bloch’s theorem. The band theory of solids, the Fermi surface, and other frameworks that are now fundamental to condensed matter physics gradually became established. After World War II, methods from quantum field theory began to be applied to many-body problems, and the BCS theory provided an explanation for superconductivity. The idea that a large number of particles can give rise to entirely new collective behavior became increasingly clear.

In the decades that followed, many new states of matter and phenomena were discovered: strongly correlated systems, the quantum Hall effect, low-dimensional electron systems, high-temperature superconductivity, quantum spin liquids, heavy-fermion systems, topological phases, and, more recently, two-dimensional and layered materials. Yet all of these developments ultimately return to one fundamental question: When a large number of particles come together, why can the whole system exhibit properties that none of the individual particles possess? This is one of the central ideas of condensed matter physics—emergence.

A single electron does not possess properties such as zero electrical resistance, magnetism, superconductivity, or topology. But when large numbers of electrons and atoms organize themselves in particular ways and interact with one another, entirely new macroscopic properties can emerge. For example, a single electron cannot form a magnet, but interactions between the spins of many electrons can give rise to ferromagnetic or antiferromagnetic order. Interactions between electrons can also drive a material into a superconducting state, characterized by zero electrical resistance and the Meissner effect. These properties cannot simply be understood as the sum of the properties of individual particles; rather, they emerge from the collective behavior of the system as a whole. This is what we mean by emergent phenomena. From this perspective, condensed matter physics is not simply the study of “solids.” It is the study of how microscopic interactions in complex many-body systems give rise to new collective behavior.

Within such systems, researchers typically focus on several fundamental layers. At the microscopic level are the atoms, electrons, spins, and lattice degrees of freedom in a material. These degrees of freedom interact through mechanisms such as electron–electron interactions, electron–lattice coupling, and magnetic exchange interactions. When many such degrees of freedom interact, they can form different quantum states and ordered phases, including magnetic order, superconductivity, charge density waves (CDWs), liquid-crystalline phases, and various topological states.

A central goal of condensed matter physics is therefore to understand the connection between these microscopic interactions and macroscopic properties: Why does a particular material develop a particular ordered state? How do different orders compete with or couple to one another? And what are the characteristic energy scales of the excitations above these quantum states?

The term quantum materials generally refers to materials whose macroscopic properties depend strongly on quantum mechanics and quantum many-body interactions. While many conventional materials can be understood using relatively classical pictures, in quantum materials, quantum properties of electrons—such as spin, orbital degrees of freedom, electronic band structure, quantum entanglement, topology, and electronic correlations—often directly determine their macroscopic behavior. Materials such as graphene, topological insulators, unconventional superconductors, quantum magnets, and kagome materials can exhibit quantum phenomena that are absent from, or difficult to explain using, conventional pictures of matter. Quantum materials therefore provide important experimental platforms for studying emergent phenomena in modern condensed matter physics.

As an experimental condensed matter physics group, we grow materials and use experimental techniques to probe the behavior of electrons, spins, and lattices. Ultimately, we seek to address one of the most fundamental questions in condensed matter physics:

How do microscopic quantum laws, through the interactions of a vast number of particles, ultimately give rise to the macroscopic properties of matter that we observe in experiments?

What do we do?

As an experimental condensed matter physics group, we focus on the growth, characterization, and neutron scattering studies of bulk quantum materials. We grow high-quality bulk single crystals using various crystal growth techniques, as neutron scattering generally requires samples with sufficient volume and mass to produce a measurable scattering signal.

We characterize their macroscopic properties through thermoelectric transport and magnetic measurements to identify phase transitions and anomalies. We then use neutron scattering to probe magnetic order, spin excitations, and lattice vibrations at the microscopic level, and analyze the data to connect these microscopic behaviors with macroscopic properties.

In short:

Bulk material growth → Macroscopic property measurements → Neutron scattering → Data analysis → Understanding quantum phenomena.