Research

The field of condensed matter physics in 21st century pays much attentions to the quantum materials, which refers to materials with collective properties governed by quantum effects, such as special magnetic and electronic order phenomena that could lead to revolutionary, next-generation technologies, like quantum computing, atomic clocks, telecommunication, spintronics and spin batteries. Quantum materials include, but are not limited to: superconducting materials, correlated electron materials, topological materials, and atomic layer materials. Apparently, materials are at the basis of all our breakthrough in physics, as well as sophisticated modern technologies. Although they are largely taken for granted, without the continuing discovery and development of new materials, technology would not continue to grow. So as in the field of quantum materials, which moves toward greater understanding of physical phenomena, new materials very often drive fields of research and development in new directions. 

The overall goal of my research group is to synthesis and characterize high-quality materials that are appealing to the field of condensed matter physics. However, we are never just a support group. The rapid and continuous development of quantum materials must depend on novel materials, in spite of the ones already being predicted or proposed. Our research experience in condensed matter physics, solid state chemistry and materials science will direct our discovery activities in new superconductors, topological materials, and quantum spin liquids, and we keep our eyes open for other interesting properties as well.

In all cases, we employ the principles and analytical methods to search, design and synthesize new quantum materials of interest. Such process yields fruitful results, along with publications on unreported crystal structure, crystal growth method, and novel electronic or magnetic properties. When these preliminary results are intriguing, further neutron/synchrotron/muon experiments can be conducted at public facilities, and other collaborations can be extended to the whole world.

The research interests of our group lie in development of novel quantum materials, particularly in the following three topics: frustrated magnets, new superconductors, and topological matters. 

Frustrated Magnets

A hot topic in condensed matter physics in recent years is related to the quantum spin liquid (QSL) state, in which strong quantum fluctuations prevent spins from ordering or freezing and they remain in a disordered liquid-like states even at absolute zero. Previous studies have shown that QSL state tends to emerge in the geometrically frustrated systems with two-dimensional low-spin magnetic lattices, in which the interactions among the limited magnetic degrees of freedom is restricted by crystal geometry. When Ising spins are placed on a lattice with triangular motifs (a), AFM interactions cannot be simultaneously satisfied for all positions, therefore leading to the “frustration” of the system. This is called geometric frustration. Unlike QSLs, Kitaev QSL arises from strong anisotropy and bond-dependent interactions that frustrate the spin configuration on a single site of a honeycombb lattice (b). This is called exchange frustration. The Kitaev model, which is an exactly solvable model of honeycombb lattice magnetism, has attracted considerable attention, as it gives rise to quantum and topological spin liquids and emergent Majorana quasiparticles. 

Novel Superconductors

Superconductivity is a set of physical properties observed in materials that show zero electrical resistance and Meissner effect below a critical temperature. Superconductor material under ambient pressure can be a chemical element (e.g. mercury, lead, niobium, etc.), intermetallic alloy (e.g. lead-bismuth, niobium-tin, indium-telluride, etc.), or compound (such as cuprate and iron pnictide). The discovery of high temperature superconductivity in the late 1980s in copper oxides stimulates a “superconductivity fever” in the condensed matter physics community, followed by the much more recent discovery of superconductivity with high critical temperatures in iron-based pnictides and chalcogenides. However, the mechanism of those unconventional superconductors remains ambiguous after being studied for more than three decades. Other than directly unveiling the mechanism in the two famous high-Tc Fe- and Cu-based superconductors, which attracts intensive research activities but still remains challenge, exploring other related systems may provide new understanding.

Topological Matters

A topological insulator is a material that behaves as an insulator in the bulk but shows exotic conducting states on its surface which are protected by the time-reversal symmetry. The interplay of magnetism and topology is a key research subject in condensed matter physics, which offers great opportunities to explore emerging new physics, such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. As a new extension of the current study of topological insulators, antiferromagnetic topological insulators (AFMTI), in which the antiferromagnetic long-range order in a topological insulator spontaneously breaks time-reversal symmetry. Proposed by most cutting-edge research, MnBi2Te4, GdBiPt are two promising examples of this topological class. The as-grown MnBi2Te4 single crystal exhibits a van der Waals layered structure, which is composed of septuple Te-Bi-Te-Mn-Te-Bi-Te sequences. However, this compound is not ideal from the perspective of inorganic chemistry, since cation intermixing is ubiquitous in such structure type, and thus non-stoichiometry has been noticed in several spectroscopy experiments. Forthcoming endeavors in our group can be focused on optimizing this material by atomic substitution with rare-earth elements, which migh be able to solve the intermixing problem and meanwhile exhibt a gapped Dirac-cone-like dispersion, suggested by first-principles calculations.