Research

Research Overview

My research bridges fundamental condensed matter physics with practical applications in quantum technologies and energy storage. I combine theoretical insights with computational tools to discover and understand novel quantum phenomena.

Weyl Fermions, causal structure, correlations, correlated systems, condensed matter, Interacting system, Wei-Chi Chiu, Nature Communications 14, 2228 (2023) The spacetime light cone is central to the definition of causality in the theory of relativity. Recently, links between relativistic and condensed matter physics have been uncovered, where relativistic particles can emerge as quasiparticles in the energy-momentum space of matter. Here, we unveil an energy-momentum analogue of the spacetime light cone by mapping time to energy, space to momentum, and the light cone to the Weyl cone. We show that two Weyl quasiparticles can only interact to open a global energy gap if they lie in each other’s energy-momentum dispersion cones–analogous to two events that can only have a causal connection if they lie in each other’s light cones. Moreover, we demonstrate that the causality of surface chiral modes in quantum matter is entangled with the causality of bulk Weyl fermions. Furthermore, we identify a unique quantum horizon region and an associated ‘thick horizon’ in the emergent causal structure.

Causal Structure in
Energy-Momentum space

Wei-Chi Chiu; Strain-Induced Charge Density Waves with Emergent Topological States in Monolayer NbSe2; Emergence of topological states in strongly correlated systems, particularly two- dimensional (2D) transition-metal dichalcogenides, presents unique opportunities for manipulating electronic properties in quantum materials. However, a comprehensive understanding of the intricate interplay between correlations and topology remains elusive. Here we employ first-principles modeling to reveal two distinct 2×2 charge density wave (CDW) phases in monolayer 1H-NbSe2, which become energetically favorable over the conventional 3×3 CDWs under modest biaxial tensile strain of about 1%. These strain-induced CDW phases coexist with numerous topological states characterized by Z2 topology, high mirror Chern numbers, topological nodal lines, and higher-order topo- logical states, which we have verified rigorously by computing the topological indices and the presence of robust edge states and localized corner states. Remarkably, these topological properties emerge because of the CDW rather than a pre-existing topology in the pristine phase. Our findings advance the fundamental understanding of the interplay between correlations, topology, and geometry in 2D materials, opening new opportunities to engineer topological states through strain-induced correlation effects in materials with initially trivial topology. These results hold promise for applications in electronics, spintronics, and other advanced quantum devices, where robust and tunable topological states are crucial.

Correlated Systems

Wei-Chi Chiu, Nature 617, 493–498 (2023) Photocathodes—materials that convert photons into electrons through a phenomenon known as the photoelectric effect—are important for many modern technologies that rely on light detection or electron-beam generation. However, current photocathodes are based on conventional metals and semiconductors that were mostly discovered six decades ago with sound theoretical underpinnings. Progress in this field has been limited to refinements in photocathode performance based on sophisticated materials engineering. Here we report unusual photoemission properties of the reconstructed surface of single crystals of the perovskite oxide SrTiO3(100), which were prepared by simple vacuum annealing. These properties are different from the existing theoretical descriptions. In contrast to other photocathodes with a positive electron affinity, our SrTiO3 surface produces, at room temperature, discrete secondary photoemission spectra, which are characteristic of efficient photocathode materials with a negative electron affinity. At low temperatures, the photoemission peak intensity is enhanced substantially and the electron beam obtained from non-threshold excitations shows longitudinal and transverse coherence that differs from previous results by at least an order of magnitude. The observed emergence of coherence in secondary photoemission points to the development of a previously undescribed underlying process in addition to those of the current theoretical photoemission framework. SrTiO3 is an example of a fundamentally new class of photocathode quantum materials that could be used for applications that require intense coherent electron beams, without the need for monochromatic excitations.

Novel Photoemission

Wei-Chi Chiu, Condens. Matter 2020, 5(2), 39 With the rising demand for sustainable energy storage, sodium-ion batteries have gained immense attention as a cost-effective and abundant alternative to lithium-ion batteries. However, a key challenge remains: developing high-performance anode materials that offer both exceptional energy capacity and superior conductivity. Unlocking next-generation sodium-ion battery materials requires innovative approaches to bridge this gap. Our research addresses this by investigating Na₃Bi, a topological Dirac semimetal with excellent conductivity, and Na₃Sb, a high-capacity anode material (660 mAh/g) that suffers from poor conductivity. By engineering Na₃BixSb₁₋ₓ alloys, we reveal a topological phase transition, unlocking the potential for next-generation high-capacity, highly conductive sodium-ion battery anodes. This breakthrough paves the way for faster-charging, longer-lasting, and more efficient energy storage solutions, crucial for powering the future of renewable energy and electric mobility.

Topological Batteries

Wei-Chi Chiu, Phys. Rev. B 107, 174524 (2023) The advancement of quantum computing and superconducting electronics hinges on a deep understanding of Josephson junctions (JJs), particularly those integrating transition metal dichalcogenides (TMDs). These hybrid systems leverage strong spin-orbit coupling, tunable band topology, and unconventional superconducting correlations, making them promising candidates for next-generation quantum technologies. However, accurate modeling of their behavior demands sophisticated theoretical approaches. This work presents a Non-Equilibrium Green’s Function (NEGF) framework for computing supercurrents in Josephson junctions, enabling precise modeling of quantum transport, superconducting coherence, and current-phase relationships, while incorporating disorder, interfacial interactions, and spin-polarized Andreev bound states. By leveraging this method, we apply atomistic modeling to investigate key mechanisms governing proximity effects, Andreev bound states, and quantum coherence in superconductor–TMD–superconductor JJ. Our analysis of interface-induced band modifications, spin-dependent transport, and local density of states variations reveals fundamental interactions shaping superconducting behavior in TMD-based junctions. Our findings provide a solid foundation for the rational design of high-coherence Josephson junctions, essential for quantum circuits, robust qubit architectures, and energy-efficient superconducting logic.

Josephson Junctions Modeling

Topological Materials

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