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Normal Dirac Semimetal Phase and Zeeman-Induced Topological Fermi Arc in PtSr5
Authors:
Inkyou Lee,
Churlhi Lyi,
Youngkuk Kim
Abstract:
Pt-Sr binary intermetallics encompass a broad range of stoichiometries and crystal structures, stabilized by complex bonding and multivalent chemistry. The Sr-rich end member, PtSr5, is recently identified via artificial-intelligence-guided materials design as a body-centered tetragonal compound (I4/m). Using first-principles calculations, we show that PtSr5 hosts a Dirac semimetal phase with triv…
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Pt-Sr binary intermetallics encompass a broad range of stoichiometries and crystal structures, stabilized by complex bonding and multivalent chemistry. The Sr-rich end member, PtSr5, is recently identified via artificial-intelligence-guided materials design as a body-centered tetragonal compound (I4/m). Using first-principles calculations, we show that PtSr5 hosts a Dirac semimetal phase with trivial Z2 topology, classified as a normal Dirac semimetal. A symmetry-indicator analysis based on parity eigenvalues at the eight time-reversal-invariant momenta confirms that all Z2 invariants-evaluated on time-reversal-invariant two-dimensional subspaces of momentum space with a direct band gap-are trivial, thereby establishing the topologically trivial nature of the Dirac semimetal phase. Nonetheless, our calculations reveal that applying an external Zeeman magnetic field along the z-axis drives the system into a Weyl semimetal phase, as corroborated by characteristic changes in the computed surface states. This work demonstrates the tunability of topological phases in PtSr5 via external perturbations and highlights the effectiveness of AI-based materials exploration in discovering new quantum materials.
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Submitted 26 October, 2025;
originally announced October 2025.
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Coexisting Kagome and Heavy Fermion Flat Bands in YbCr$_6$Ge$_6$
Authors:
Hanoh Lee,
Churlhi Lyi,
Taehee Lee,
Hyeonhui Na,
Jinyoung Kim,
Sangjae Lee,
Younsik Kim,
Anil Rajapitamahuni,
Asish K. Kundu,
Elio Vescovo,
Byeong-Gyu Park,
Changyoung Kim,
Charles H. Ahn,
Frederick J. Walker,
Ji Seop Oh,
Bo Gyu Jang,
Youngkuk Kim,
Byungmin Sohn,
Tuson Park
Abstract:
Flat bands, emergent in strongly correlated electron systems, stand at the frontier of condensed matter physics, providing fertile ground for unconventional quantum phases. Recent observations of dispersionless bands at the Fermi level in kagome lattice open the possibility of unifying the disjoint paradigms of topology and correlation-driven heavy fermion liquids. Here, we report the unprecedente…
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Flat bands, emergent in strongly correlated electron systems, stand at the frontier of condensed matter physics, providing fertile ground for unconventional quantum phases. Recent observations of dispersionless bands at the Fermi level in kagome lattice open the possibility of unifying the disjoint paradigms of topology and correlation-driven heavy fermion liquids. Here, we report the unprecedented coexistence of these mechanisms in the layered kagome metal YbCr6Ge6. At high temperatures, an intrinsic kagome flat band-arising from the frustrated hopping on the kagome lattice-dominates the Fermi level. Upon cooling, localized Yb 4f-states hybridize with the topological kagome flat bands, transforming this state into the Kondo resonance states that are nearly dispersionless across the entire Brillouin zone. Crystalline symmetry forbids hybridization along specific high-symmetry lines, which stabilizes Dirac crossings of heavy-fermion character. Topological analysis of the resulting gaps reveals both trivial and nontrivial Z2 invariants, establishing the emergence of a Dirac-Kondo semimetal phase. Taken together, these results identify YbCr6Ge6 as a prototype of a topological heavy-fermion system and a platform where geometric frustration, strong correlations, and topology converge, with broad implications for correlated quantum matter.
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Submitted 19 March, 2026; v1 submitted 5 September, 2025;
originally announced September 2025.
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Tunable bulk photovoltaic effect in strained $γ$-GeSe
Authors:
Hong-Guk Min,
Churlhi Lyi,
Youngkuk Kim
Abstract:
Recently, Lee \textit{et. al.} [Nano Lett. \textbf{21}, 4305 (2021)] newly synthesized monochalcogenide GeSe in a polar phase, referred to as $γ$-phase. Motivated by this work, we study shift current of $γ$-GeSe and its tunability via an in-plane uniaxial strain. Using first-principles calculations, we uncover the electronic structure of the strained $γ$-GeSe systems. We then calculate frequency-d…
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Recently, Lee \textit{et. al.} [Nano Lett. \textbf{21}, 4305 (2021)] newly synthesized monochalcogenide GeSe in a polar phase, referred to as $γ$-phase. Motivated by this work, we study shift current of $γ$-GeSe and its tunability via an in-plane uniaxial strain. Using first-principles calculations, we uncover the electronic structure of the strained $γ$-GeSe systems. We then calculate frequency-dependent shift current conductivities at various strains. The tunability is demonstrated to enhance the shift current up to $\sim$ 20 $μ$A/V$^2$. Moreover, the direction of shift current can be inverted by a light strain. Markedly, an anomalous behavior is found in the zero-frequency limit, which can be an indicative of band inversion and a potential topological phase transition driven by the strain. Our results suggest that shift current can be a tangible prove of bulk electronic states of $γ$-GeSe.
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Submitted 15 November, 2022; v1 submitted 13 March, 2022;
originally announced March 2022.
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Magnetic properties of S = 5/2 anisotropic triangular chain Bi3FeMo2O12
Authors:
K. Boya,
K. Nam,
A. K. Manna,
J. Kang,
C. Lyi,
A. Jain,
S. M. Yusuf,
P. Khuntia,
B. Sana,
V. Kumar,
A. V. Mahajan,
Deepak. R. Patil,
Kee Hoon Kim,
S. K. Panda,
B. Koteswararao
Abstract:
Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptib…
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Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility suggests the presence of short-range spin correlations. Magnetic susceptibility data do not fit to S=5/2 uniform spin chain model due to the presence of 2nd nearest-neighbor coupling (J2) along with the 1st nearest-neighbor coupling J1 of the zig-zag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1~1.1) with a negligible inter-chain interaction (J'/J ~ 0.01), implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature of -40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S = 5/2 anisotropic triangular chain exhibits the gapless excitations.
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Submitted 23 October, 2021;
originally announced October 2021.