arXiv subjects

Young-Kwon Han

Publications and source records attributed to Young-Kwon Han.

7 recordsLinked to original sources

arXiv2026

Enhancement of exciton radius near a band-gap closing through quantum geometry

Exciton engineering traditionally focuses on modifying semiclassical material properties, such as the effective mass and dielectric screening, while largely overlooking the quantum geometry of the underlying electron and hole Bloch states. This approximation is adequate for many materials but breaks down near a topological band-gap closing, where the quantum metric around the band extrema becomes strongly enhanced. In this regime, Bloch states at different momenta become less similar, reducing the projected electron--hole Coulomb matrix elements and consequently weakening exciton binding. We demonstrate this mechanism in a spin--orbit-coupled Lieb-lattice model tuned toward a topological phase transition. The suppressed Coulomb matrix elements narrow the exciton wavefunction in momentum space, leading to an enlarged exciton radius in real space. This increase in exciton size produces an experimentally accessible enhancement of the weak-field diamagnetic response. Our results show that quantum geometry can fundamentally reshape exciton properties near a topological phase transition, revealing a previously underexplored route for engineering excitonic states.

cond-mat.mes-hall↗

arXiv2025

Topology in Holographic Mean-Field Theory at Zero and Finite Temperature

We investigate topological invariants in strongly interacting many-body systems within holographic mean-field theory (H-MFT) framework. Analytic expressions for retarded Green's functions are obtained for all possible fermionic bilinear interactions in the limit of probe background limit $\mathrm{AdS}_4$, from which we construct topological Hamiltonians. Integrating Berry curvature over the momentum domain for the gapped spectra yields well-defined and quantized Chern numbers, enabling a systematic classification of them across interaction types. These topological invariants remain robust under deformation parameters like interaction and temperature, indicating that H-MFT encodes effective single-particle-state topology near a quantum critical point in strongly correlated systems. We point out why topological number is defined in the holographic theories while it is not in the perturbative field theory.

hep-th↗

arXiv2025

Hall Angle of a Spatially Random Vector Model

Strange metals exhibit linear resistivity and anomalous Hall transport, yet a comprehensive theory that accounts for both phenomena is still lacking. Recent studies have shown SYK-like spatially random couplings between a Fermi surface and a bosonic field, either scalar or vector type, can yield linear-$T$ resistivity. In this paper, we continue the investigation on a vector coupling in the presence of a magnetic field. We compute the fermion and boson propagators, along with the self-energy and polarization functions, and determine their dependence on the magnetic field. Although the Hall angle does not exhibit the signature of strange-metal, the linear-in-temperature resistivity remains at low temperatures. Results indicate that random interactions can robustly support linear transport, though additional ingredients may be required to capture the full phenomenology of strange metals.

hep-th↗

arXiv2024

Holographic mean field theory and Kondo lattice

We first study a non-relativistic field theory model for the Kondo lattice by introducing the Kondo condensation, whose main effect is the hybridization of the flat band of the localized electron with dispersive one of the itinerant electron. The problem here is that the resulting Kondo condensation arises only in strong coupling where the validity of the mean field theory is questionable. Therefore, we build a holographic mean field theory of the Kondo lattice with strong coupling by identifying the effect of the lattice with the fermion's spectral shape due to the coupling with the order parameter representing the symmetry breaking. For the flat band spectrum we use the mixed quantization, and for the dispersive spectrum we intoduce the second fermion in standard quantization. The coupling of the two fermions with the scalar order representing the Kondo condensation provides the hybrization of the two spectrum, reproducing the main feature of the Kondo lattice together with the fuzzy character of the spectrum of the strongly coupled system.

hep-th↗

arXiv2024

Classes of Holographic Mott Gaps

The fermion gaps are classified into order gap or Mott gap depending on the presence/absence of the order parameter. We construct the holographic model of the Mott gap using the field that is supported by the density only without introducing any order parameter. We then classify the Mott gap, depending on the shape of the gap in the density of states and whether the Fermi surface is touching the valence bond or not, into three classes: i) Symmetric gap, ii) Asymmetric gap with isolated Fermi sea. iii) Asymmetric gap with Fermi sea touching the valence band. Finally, we identify possible non-minimal gauge interactions that produce a flatband without symmetry breaking.

hep-th↗

arXiv2022

ABC-stacked multilayer graphene in holography

A flat band can be studied an infinitely strong coupling, realized in a simple system. Therefore, its holographic realization should be interesting. Laia and Tong gave a realization of the flat band over the entire momentum region by introducing a particular boundary term. Here, we give a model with a flat band over a finite region of momentum space using a bulk interaction term instead of the boundary term. We find that the spectrum of our model is precisely analogous to that of the ABC stacked multilayer graphene. In the presence of the chemical potential, the flat band is bent in our holographic model, which is very close to the band deformation due to the spin-orbit

hep-th↗

arXiv2022

Holographic Lieb lattice and gapping its Dirac band

We first point out that the Laia-Tong model realizes the Lieb lattice in the holographic setup. It generates a flat band of sharp particle spectrum together with a Dirac band of unparticle spectrum. We then construct a model which opens a gap to the Dirac band so that one can realize a well-separated flat band, which can play the role of the hydrogen atom of strongly correlated systems. We then study the phase transition between the gapped and gapless phases analytically. We also made methodological progress to find a few other quantizations and we express the Green functions in any quantization in terms of that in the standard quantization.

hep-th↗