Correlated Quantum Matter Mini-Workshop @ NTHU

Friday 6 Nov 2026, 09:00 → 17:00 Asia/Taipei

R620 (NTHU)

On the occasion of Prof. Gabriel Kotliar’s (Rutgers University) visit to National Tsing Hua University, we are organizing a one-day mini-workshop on strongly correlated physics.

Strongly correlated systems, in which electron–electron interactions cannot be treated as small perturbations, host some of the most striking phenomena in condensed matter physics, including Mott transitions, unconventional superconductivity, heavy-fermion behaviour, strange metals, and exotic magnetic and topological phases. Understanding these systems requires a combination of new theoretical concepts, advanced computational methods, and close dialogue with experiment.

The workshop aims to bring together researchers in Taiwan working on correlated quantum matter from complementary perspectives, including dynamical mean-field theory and its extensions, tensor-network and quantum Monte Carlo methods, field-theoretical approaches, quantum information perspectives on many-body systems, and first-principles studies of quantum materials. Through talks and open discussions, we hope to foster exchange of ideas, highlight recent progress, and stimulate new collaborations between local groups and our visitor.

Graduate students and postdocs are especially encouraged to attend. Participation is free, but registration is requested for planning purposes.

Confirmed speakers:
1. Ying-Jer Kao (NTU) 

2. Yu-Te Hsu (NTHU)

3. Chung-Yu Mou (NTHU)

4. Atsushi Fujimori (UTokyo/NTHU)

5. Ian McCulloch (NTHU)

6. Tsung-Han Lee (NCCU)

09:30 → 10:15 Ghost Rotationally-Invariant Slave-Boson Approach to Correlated Quantum Materials, Prof. Tsung-Han Lee

Accurately describing correlated quantum materials from first principles remains a major challenge in condensed matter physics. Conventional density functional theory (DFT) often fails to capture strong electronic correlations, while more sophisticated many-body approaches, such as dynamical mean-field theory (DMFT) and quantum Monte Carlo (QMC), can be computationally demanding. In this talk, I will introduce the ghost rotationally-invariant slave-boson (gRISB) method, also known as the ghost-Gutzwiller approximation [1], as a computationally efficient framework for describing strongly correlated electron systems. I will first discuss recent extensions of the gRISB framework to finite temperatures [2] and dynamical response functions [3], which broaden its applicability to thermodynamic properties and dynamical correlations. I will then present recent developments in combining gRISB with DFT (DFT+gRISB), highlighting its systematic improvement over conventional RISB in describing the electronic properties of correlated materials [4-6]. Finally, I will discuss our recent application of DFT+gRISB to structural distortions in SrMoO₃, where its integration with machine-learning techniques enables efficient exploration of a complex energy landscape involving competing structural phases. Together, these developments establish gRISB as a promising framework for investigating the interplay between electronic correlations, dynamical properties, and structural degrees of freedom in quantum materials.

[1] Nicola Lanatà, Tsung-Han Lee, Yong-Xin Yao, and Vladimir Dobrosavljević, Phys. Rev. B 96, 195126 (2017).
[2] Samuele Giuli, Tsung-Han Lee, Yong-Xin Yao, Gabriel Kotliar, Andrei E. Ruckenstein, Olivier Gingras, and Nicola Lanatà, arXiv:2603.20559 (2026).
[3] Tsung-Han Lee, Corey Melnick, Nicola Lanatà, and Gabriel Kotliar, arXiv:2610.08575 (2026).
[4] Tsung-Han Lee, Nicola Lanatà, and Gabriel Kotliar, Phys. Rev. B 107, L121104 (2023).
[5] Tsung-Han Lee, Corey Melnick, Ran Adler, Nicola Lanatà, and Gabriel Kotliar, Phys. Rev. B 108, 245147 (2023).
[6] Tsung-Han Lee, Corey Melnick, Ran Adler, Xue Sun, Yongxin Yao, Nicola Lanatà, and Gabriel Kotliar, Phys. Rev. B 110, 115126 (2024).

10:30 → 11:00 TOWARD COMPLETING THE PHASE DIAGRAM OF HIGH-Tc CUPRATE SUPERCONDUCTORS, Prof Chung-Yu Mou

Despite four decades of research on high-Tc cuprate superconductors, a complete understanding of their physics remains elusive. In particular, the establishment of the global phase diagram has been hindered by the observation of various orders. In this talk, I am going to show you that a quantum critical point (QCP) that is buried under the superconducting dome is potentially responsible for the observed various orders. The existence of this QCP is confirmed by the scaling analysis on the dynamical charge-charge correlation obtained by using the high-resolution resonant inelastic X-ray scattering (RIXS) [1]. In particular, it shows that the QCP appears to be in the universality class characterized by the O(4) symmetry. Furthermore, by using the renormalized mean-field theory, we solved the phase diagram of density waves for the t-J model. It indicates that the O(4) critical point results from the vanishing of the order parameter composed by the charge density wave (CDW) and pair density wave (PDW). Furthermore, the CDW order n(Q) and PDW order ∆(Q) generally prevail in the phase diagram with Q being either (±π/2,0), (0,±π/2), (±π/4,0), or (0,±π/4). Finally, combining the obtained phase diagram and the elastic scattering part of RIXS at Q, we determine the pseudo-gap T line as the cross-over line for vanishing of the PDW correlation functions and found that it is in consistent with T line found by other experimental methods. Our results thus provide a complete phase diagram for high-Tc cuprate superconductors.

REFERENCE
[1] Quantum critical behavior of cuprate superconductors observed by inelastic X-ray scattering, Nature Communications 17, 7040 (2026), H. Y. Huang, C. Y. Mou, A. Singh, J. S. Su, J. Okamoto, S. Komiya, C. T. Chen, T. K. Lee, A. Fujimori, and D. J. Huang.

11:15 → 12:00 Emergent frustrated magnetism in strain-patterned graphene, Prof. Ying-Jer Kao

Geometrically frustrated magnetism conventionally arises from pre-existing magnetic moments on lattices whose geometry prevents their interactions from being simultaneously satisfied, giving rise to highly degenerate states and rich collective behavior. Creating such frustration in an intrinsically non-magnetic material presents a fundamentally different challenge, requiring both the magnetism and the competing interactions to emerge from correlated electrons. Here we show that this can be realized in graphene through lithographically programmable strain engineering. Patterning strain and the associated pseudo-magnetic field (PMF) into superlattices creates a correlated electronic system with flat bands and strong interactions. Transport measurements reveal interaction-driven insulating behavior, anisotropic magnetic hysteresis, and slow relaxation dynamics reminiscent of spin freezing, qualitatively captured by Monte Carlo simulations of competing magnetic moments on the PMF-defined ruby superlattice. Cryogenic magnetic force microscopy further reveals magnetic textures associated with the PMF landscape. These observations demonstrate frustrated magnetism emerging from correlated electrons in otherwise non-magnetic graphene. Our results establish lithographic strain engineering as a general and scalable route to flat bands and correlated states in van der Waals materials, providing a versatile platform for programmable quantum matter.

Ref. arXiv:2609.31581

13:30 → 14:15 Experimental signatures of strong correlations in 3d- and 5d-electron insulators, Prof. Yu-Te Hsu

d-electron systems feature strong correlations and offer a fertile ground for novel quantum phases. In this talk, I will discuss experimental evidence for novel electronic ground states realized in transition-metal-based materials with a proximate metal-insulator transition.
Sr2IrO4 is a model material that shares several defining characteristics with the high-Tc cuprates, including a half-filled, effective spin-1/2 state hosted in a quasi-2D lattice. Doping the spin-orbit coupled Mott insulating state does not lead to superconductivity, however, instead it leads to an asymmetric suppression of the antiferromagnetic phase boundary and the emergence of anisotropic energy gaps. These observations hint at a disconnect between superconductivity and pseudogap. Here we present new evidence for the existence of a thermodynamic pseudogap in electron-doped Sr2IrO4 [1] and an unconventional antiferromagnetic quantum phases transition in hole-doped Sr2IrO4 [2], and discuss the implication for understanding the cuprate phase diagram.
FeSi is long-known material which exhibits several characteristics of Kondo insulators despite the absence of f-electrons [3]. Proposed to be a topological insulator, there has been mounting evidence for a conducting surface state in FeSi though its nature remains to be clarified. Here we present transport and thermodynamic evidence for time-reversal-symmetry breaking in the low-T regime where surface conduction becomes dominant. The observation of an intrinsic anomalous Hall effect in a bulk non-magnetic insulating state provides a new insight for understanding the unconventional ground state of this 3d correlated narrow-gap insulator.

Reference
[1] YT Hsu et al., Nat. Phys. 20, 1596 (2024)
[2] TN Yen et al., in preparation
[3] JM Tomczak, J. Phys.: Condens. Matter 30, 183001 (2018)
[4] YC Pan et al., arXiv:2609.28418

14:30 → 15:15 Nodal metal versus hole pockets and interlayer coupling in multilayer cuprates, Prof. Atsushi Fujimori

In multilayer cuprates, the maximum Tc,max is the highest at n = 3 (triple-layer) in all cuprate families. For the inner CuO2 planes of n > 5, antiferromagnetic (AFM) order has been confirmed by NMR and ARPES. ARPES studies have shown small hole Fermi pockets are formed in the inner CuO2 planes as a result of AFM band folding [1].

Here, we report ARPES data of the triple-layer cuprates Bi2223 over a wide doping range covering from underdoped to overdoped regions. In the inner CuO2 plane of underdoped samples, the doping level became extremely low but the d-wave superconducting gap is enhanced to an unprecedentedly large value of nodal gap Δ0 ~ 100 meV, and the gap persisted up to ~2Tc without the appearance of a Fermi arc, indicating a robust nodal metal. We attribute this nodal-metallic behavior to the unique local environment of the inner CuO2 plane in the triple-layer cuprates, sandwiched by optimally-doped two outer CuO2 planes and hence subject to strong proximity effect from both sides. Such a situation is realized only in triple-layer cuprates, and explains why the Tc reaches the maximum at n = 3 in every cuprate family [2].

While it has been estalished that underdoped to optimally doped Bi2223 show distinct Fermi surfaces for the inner and outer CuO2 planes [3], a resent ARPES study on overdoped Bi2223 has shown further splitting of the outer-plane Fermi surface, resulting in three Fermi surfaces [4]. Our additional ARPES data have shown that the transition from the two to three Fermi surfaces is accompanied by enhanced interlayer single-particle hopping and interlayer pairing as the system enters the overdoped region [5].

References
1. S. Kunisada et al., Science 369, 833–838 (2020)
2. S. Ideta et al., Nat. Commun. 16, 9470 (2025).
3. S. Ideta et al., Phys. Rev. Lett. 127, 217004 (2021).
4. Luo, X. et al., Nat. Phys. 19, 1841 (2023).
5. S. Ideta et al., in preparation.

15:30 → 16:15 From Bethe ansatz to precision many-body spectra, Prof Ian McCulloch

Exactly solvable models provide essential benchmarks for strongly correlated quantum systems, but obtaining quantitative dynamical observables from their exact solutions remains challenging. I will introduce the Bethe library, a precision-generic framework for Bethe-ansatz calculations motivated by comparisons with tensor-network methods. The main focus will be the dynamical spin structure factor of the spin-1/2 Heisenberg antiferromagnet. I will present a corrected, high-accuracy calculation of the four-spinon contribution and new six-spinon results, explaining how analytical reductions, soft-region coordinates and controlled numerical integration make these calculations practical. Together, the two-, four- and six-spinon contributions account for approximately 99% of the integrated intensity and first frequency moment. Spectral thresholds, sum rules and time-domain correlations provide complementary tests, while unusually slow finite-size convergence illustrates the difficulty of extracting thermodynamic dynamics from finite chains.