BK21 Seminar: Dr. Dongwook Kim (TU Wien, Austria)2026. 4. 20. (Mon), 15:00
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BK21 SEMINAR
♧ Speaker: Dr. Dongwook Kim (TU Wien, Austria)
♧ Date: 2026. 4. 20. (Mon), 15:00
♧ Venue: Chemistry Bldg. Room #310
A tale of two gap structures in mid-infrared regime of the optical conductivity in the high-temperature superconductors
Mid-infrared spectra in the optical conductivity of the strongly correlated systems can be associated with various physical origins. Since it corresponds to the energy scale between 0.1 - 1.0 eV, in the single-orbital Hubbard model with U ≫ 1 eV, the mid-infrared peak is expected not to exist.
However, the high-temperature superconductors (Cuprates [1] and infinite-layer Nickelates [2]) both showed the mid-infrared peak below 0.5 eV, while the local Coulomb repulsion U ≈ 4.
Although the O−2p orbital below the Fermi level hybridized to the Cu-3dx2−y2 in Cuprates can lead additional optical spectra, it is expected to generate the optical spectra above 1 eV, which is relevant to the experimentally observed mid-infrared peak. Furthermore, infinite-layer Nickelates also show similar mid-infrared peak, although Ni-3dx2−y2 doesn’t hybridize to the O−2p orbital due to its Mott-Hubbardness [3].
In this study we show the optical conductivity in the hole-doped single-orbital square-lattice Hubbard model calculated by the Ladder Dynamical Vertex Approximation (LDΓA), which is a nonlocal extension of Dynamical Mean Field Theory (DMFT) posing the locality of the 2-particle irreducible vertex. Considering only the ladder diagrams for the spin-Fermion coupling driven self-energy in Schwinger-Dyson equation, all the gap structures in the self-energy with the nonlocal origin is relevant to the antiferromagnetic (AFM) like magnons.
In the underdoped regime, the mid-infrared peak red-shifts as the increase of the doping, consistent to experiment, which we associate to the strong AFM fluctuation getting weakened as doping. In the overdoped regime, the flat mid-infrared spectra is made at some point and vanishes frequency-independently as doping. This corresponds to the waterfall regime where the seperated lower Hubbard band (LHB) no more exists and start to get merged into the conduction band [4]. We show that the corresponding spectral function also frequency-independently vanishes from the waterfall regime, i.e. overall vanishing of the optical spectra resembles the behavior of the death of LHB. For both characteristic mid-infrared spectra in different doping regimes, we perform a semianalytic analysis with modelized self-energy with single pole structure with 1-D rectangular bare density of state (DOS). The agreement with LDΓA results demonstrates that the mid-infrared spectra originate from gap-induced pole structures in the self-energy, rather than artifacts of analytic continuation or the underlying bare density of states.
References
[1] C. Kumar, A. Akrap, C. C. Homes, E. Martino, B. Klebel-Knobloch, W. Tabis, O. S. Barišić, D. K. Sunko, and N. Barišić, Physical Review B 107, 144515 (2023).
[2] W. J. Kim, K. Lee, E. K. Ko, J. Son, Y. Lee, Y. Yu, S. J. Moon, T. W. Noh, and H. Y. Hwang, arXiv preprint arXiv:2602.09567 (2026).
[3] M. Kitatani, L. Si, O. Janson, R. Arita, Z. Zhong, and K. Held, npj Quantum Materials 5, 59 (2020).
[4] J. Krsnik and K. Held, Nature communications 16, 255 (2025).
Host: Prof. Jihoon Shim (T.279-2344)
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