About

Tai-Chang Chiang is a Research Professor at the University of Illinois Urbana-Champaign, affiliated with the Grainger College of Engineering. He received his B.S. in physics from National Taiwan University in 1971 and his Ph.D. in physics from the University of California, Berkeley in 1978. After working as a postdoctoral fellow at the IBM T.J. Watson Research Center, he joined the Department of Physics at the University of Illinois in 1980. Professor Chiang has conducted seminal research on the electronic properties, lattice structure, and dynamic behavior of surfaces, interfaces, and ultrathin films, employing molecular beam epitaxy techniques to create thin films and composite systems made of metals, semiconductors, topological insulators, superconductors, and charge-density-wave compounds. He is credited with being the first to create atomically uniform films of varying thicknesses, which function as miniature electron interferometers, enabling precise measurements of electronic wavelengths and electron kinetics. An outstanding theorist, he developed models for his experimental results and pioneered applications of angle-resolved and core-level photoemission to surface and thin film research, leading to advances in surface structure analysis and band structure mapping. His research also includes x-ray thermal diffuse scattering for phonon mapping, conducted at synchrotron radiation facilities worldwide. Currently, his research focuses on the physics of surfaces, interfaces, and tailored thin film structures relevant to quantum and nanoscale science, exploring electronic, spintronic, and atomistic behaviors in systems prepared by deposition, self-assembly, and artificial layering. His work emphasizes the interplay of quantum confinement, reduced dimensions, spin texture, and topological order, with particular attention to phenomena such as quantum well states, spin polarization effects, and emergent properties in complex, engineered materials.

Research topics

• Physics
• Condensed matter physics
• Quantum mechanics
• Computer Science
• Geometry
• Materials science
• Artificial Intelligence
• Combinatorics
• Algorithm
• Mathematics

Selected publications

• Topological Surface Superconductivity via Josephson Coupling in Bi ₂ Te ₃ /Nb

  Nano Letters · 2025-11-08

  articleOpen accessSenior authorCorresponding

  Since discoveries of protected conducting surface states, topological superconducting qubits have enchanted quantum science as prime elements in future fault-tolerant devices, particularly those based on Josephson junctions containing topological insulators. Still, Josephson coupling is often eclipsed by other proximity effects that can dilute topological superconducting pairing at the nontrivial insulator’s boundaries. Here, however, using an ultra-low-temperature scanning tunneling microscope, we detect Josephson physics in topological Bi2Te3 films on superconducting Nb. At low temperatures, a previously undetected proximity gap varies little with Bi2Te3 thickness and the density of states exhibits normal and superconducting components. Such observations are rationalized via Josephson pair tunneling through the (nearly) insulating Bi2Te3 bulk, creating a rare, pure topological superconducting sheet. Our findings establish routes toward accessible topological superconducting states in qubits.

  PublisherDOI

• In-Plane Anisotropy of Charge Density Wave Fluctuations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mi>T</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mi>TiSe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

  Physical Review Letters · 2025-09-25 · 1 citations

  articleOpen access

  We report measurements of anisotropic triple-q charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1T-TiSe_{2} over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, T_{CDW}, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.

  PublisherOA PDFDOI

• Measurement of the dynamic charge susceptibility near the charge density wave transition in ErTe ₃

  Proceedings of the National Academy of Sciences · 2025-06-20

  articleOpen access

  A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of valence electron density coupled with lattice distortion. Its formation is closely tied to the dynamical charge susceptibility, [Formula: see text], which reflects the collective electron dynamics of the material. Despite decades of study, [Formula: see text] near a CDW transition has never been measured at nonzero momentum, [Formula: see text], with meV energy resolution. Here, we investigate the canonical CDW transition in ErTe[Formula: see text] using momentum-resolved electron energy loss spectroscopy, a technique uniquely sensitive to valence band charge excitations. Unlike phonons, which soften via the Kohn anomaly, we find the electronic excitations exhibit purely relaxational dynamics well described by a diffusive model, with the diffusivity peaking just below the critical temperature, [Formula: see text]. Additionally, we report for the first time a divergence in the real part of [Formula: see text] in the static limit ([Formula: see text]), a long-predicted hallmark of CDWs. Unexpectedly, this divergence occurs as [Formula: see text], with only a weak thermodynamic signature at [Formula: see text]. Our study necessitates a reexamination of the traditional description of CDW formation in quantum materials.

  PublisherDOI

• Signatures of Kramers-Weyl fermions in the charge density wave material (TaSe4)2I

  Communications Materials · 2025-10-20 · 1 citations

  articleOpen access

  The quasi-one-dimensional charge density wave (CDW) material (TaSe4)2I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle-resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe4)2I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. Our findings highlight the unique topological nature of (TaSe4)2I and provide a pathway for identifying KW fermions in other chiral materials. It has been predicted that the quasi-one-dimensional charge density wave material (TaSe4)2I hosts Kramers-Weyl fermions, but direct spectroscopic evidence of this is limited. Here, ARPES and theoretical calculations reveal signatures that may indicate the presence of Kramers-Weyl fermions.

  PublisherOA PDFDOI

• Unconventional Spectral Gaps Induced by Charge Density Waves in the Weyl Semimetal (TaSe₄)₂I

  Nano Letters · 2024-07-08 · 6 citations

  articleOpen accessSenior authorCorresponding

  Coupling Weyl quasiparticles and charge density waves (CDWs) can lead to fascinating band renormalization and many-body effects beyond band folding and Peierls gaps. For the quasi-one-dimensional chiral compound (TaSe4)2I with an incommensurate CDW transition at TC = 263 K, photoemission mappings thus far are intriguing due to suppressed emission near the Fermi level. Models for this unconventional behavior include axion insulator phases, correlation pseudogaps, polaron subbands, bipolaron bound states, etc. Our photoemission measurements show sharp quasiparticle bands crossing the Fermi level at T > TC, but for T < TC, these bands retain their dispersions with no Peierls or axion gaps at the Weyl points. Instead, occupied band edges recede from the Fermi level, opening a spectral gap. Our results confirm localization of quasiparticles (holes created by photoemission) is the key physics, which suppresses spectral weights over an energy window governed by incommensurate modulation and inherent phase defects of CDW.

  PublisherOA PDFDOI

• Evidence of directional structural superlubricity and Lévy flights in a van der Waals heterostructure

  arXiv (Cornell University) · 2024-06-24 · 1 citations

  preprintOpen access

  Structural superlubricity is a special frictionless contact in which two crystals are in incommensurate arrangement such that relative in-plane translation is associated with vanishing energy barrier crossing. So far, it has been realized in multilayer graphene and other van der Waals two-dimensional crystals with hexagonal or triangular crystalline symmetries, leading to isotropic frictionless contacts. Directional structural superlubricity, to date unrealized in two-dimensional systems, is possible when the reciprocal lattices of the two crystals coincide in one direction only. Here, we evidence directional structural superlubricity a $α$-bismuthene/graphite van der Waals system, manifested by spontaneous hopping of the islands over hundreds of nanometres at room temperature, resolved by low-energy electron microscopy and supported by registry simulations. Statistical analysis of individual and collective $α$-bismuthene islands populations reveal a heavy-tailed distribution of the hopping lengths and sticking times indicative of L{é}vy flight dynamics, largely unobserved in condensed-matter systems.

  PublisherOA PDFDOI

• Evidence of Directional Structural Superlubricity and Lévy Flights in a van der Waals Heterostructure

  Small · 2024-11-26 · 6 citations

  articleOpen access

  Structural superlubricity is a special frictionless contact in which two crystals are in incommensurate arrangement such that relative in-plane translation is associated with vanishing energy barrier crossing. So far, it has been realized in multilayer graphene and other van der Waals (2D crystals with hexagonal or triangular crystalline symmetries, leading to isotropic frictionless contacts. Directional structural superlubricity, to date unrealized in 2D systems, is possible when the reciprocal lattices of the two crystals coincide in one direction only. Here, directional structural superlubricity a α-bismuthene/graphite van der Waals system is evidenced, manifested by spontaneous hopping of the islands over hundreds of nanometers at room temperature, resolved by low-energy electron microscopy and supported by registry simulations. Statistical analysis of individual and collective α-bismuthene islands populations reveal a heavy-tailed distribution of the hopping lengths and sticking times indicative of Lévy flight dynamics, largely unobserved in condensed-matter systems.

  PublisherOA PDFDOI

• Topological Quantum Well States in Pb/Sb Thin-Film Heterostructures

  ACS Nano · 2024-03-26 · 2 citations

  articleOpen accessSenior authorCorresponding

  Composite topological heterostructures, wherein topologically protected states are electronically tuned due to their proximity to other matter, are key avenues for exploring emergent physical phenomena. Particularly, pairing a topological material with a superconductor such as Pb is a promising means for generating a topological superconducting phase with exotic Majorana quasiparticles, but oft-neglected is the emergence of bulklike spin-polarized states that are quite relevant to applications. Using high-resolution photoemission spectroscopy and first-principles calculations, we report the emergence of bulk-like spin-polarized topological quantum well states with long coherence lengths in Pb films grown on the topological semimetal Sb. The results establish Pb/Sb heterostructures as topological superconductor candidates and advance the current understanding of topological coupling effects required for realizing emergent physics and for designing advanced spintronic device architectures.

  PublisherOA PDFDOI

• Conformally invariant charge fluctuations in a strange metal

  arXiv (Cornell University) · 2024-11-17 · 2 citations

  preprintOpen access

  The strange metal is a peculiar phase of matter in which the electron scattering rate, $τ^{-1} \sim k_B T/\hbar$, which determines the electrical resistance, is universal across a wide family of materials and determined only by fundamental constants. In 1989, theorists hypothesized that this universality would manifest as scale-invariant behavior in the dynamic charge susceptibility, $χ''(q,ω)$. Here, we present momentum-resolved inelastic electron scattering measurements of the strange metal Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ showing that the susceptibility has the scale-invariant form $χ''(q,ω) = T^{-ν} f(ω/T)$, with exponent $ν= 0.93$. We find the response is consistent with conformal invariance, meaning the dynamics may be thought of as occurring on a circle of radius $1/T$ in imaginary time, characterized by conformal dimension $Δ= 0.05$. Our study indicates that the strange metal is a universal phenomenon whose properties are not determined by microscopic properties of a particular material.

  PublisherOA PDFDOI

• Realization of a two-dimensional Weyl semimetal and topological Fermi strings

  Nature Communications · 2024-07-17 · 29 citations

  articleOpen access

  A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.

  PublisherOA PDFDOI

Recent grants

• Coupled Electronic and Lattice Structures in Thin Crystalline Films

  NSF · $646k · 2017–2023

• Electronic and Atomistic Effects in Quantum Structures

  NSF · $480k · 2005–2009

• Electronic and Atomistic Effects in Nanostructures

  NSF · $615k · 2013–2017

• Electronic and Atomistic Effects in Nanostructures

  NSF · $515k · 2009–2014

Frequent coauthors

• T. Miller

  Harvard University

  114 shared

• T. Miller

  80 shared

• Guang Bian

  University of Missouri

  68 shared

• M. Y. Chou

  Institute of Atomic and Molecular Sciences, Academia Sinica

  61 shared

• Xiaoxiong Wang

  58 shared

• Hawoong Hong

  Argonne National Laboratory

  55 shared

• Deng-Sung Lin

  National Tsing Hua University

  48 shared

• Ro-Ya Liu

  National Synchrotron Radiation Research Center

  40 shared

Education

• PhD , Physics

  University of California Berkeley

  1978