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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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Role of Fe intercalation on the electronic correlation in resistively switchable antiferromagnet Fe$_{x}$NbS$_2$
Authors:
Wenxin Li,
Jonathan T. Reichanadter,
Shan Wu,
Ji Seop Oh,
Rourav Basak,
Shannon C. Haley,
Siqi Wang,
Joshua E. Chaparro Mata,
Elio Vescovo,
Donghui Lu,
Makoto Hashimoto,
Christoph Klewe,
Suchismita Sarker,
Jessica L. McChesney,
Alex Frañó,
James G. Analytis,
Robert J. Birgeneau,
Jeffrey B. Neaton,
Yu He
Abstract:
Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe$_{x}$NbS$_2$ is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of $x_c$ = 1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle…
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Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe$_{x}$NbS$_2$ is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of $x_c$ = 1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the $x_c$. Moment-carrying Fe 3$d_{z^2}$ electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in TMD layer, rapidly lose coherence above $x_c$, and drive a transformation of the magnetic ground state via modification of the effective Fe-Fe exchange interaction. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems upon charge doping, shedding light on the tunability of magnetic exchange interactions beyond nearest-neighbor and electronic correlation in magnetic TMDs.
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Submitted 13 March, 2026; v1 submitted 3 September, 2025;
originally announced September 2025.
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Kramers nodal lines in intercalated TaS$_2$ superconductors
Authors:
Yichen Zhang,
Yuxiang Gao,
Aki Pulkkinen,
Xingyao Guo,
Jianwei Huang,
Yucheng Guo,
Ziqin Yue,
Ji Seop Oh,
Alex Moon,
Mohamed Oudah,
Xue-Jian Gao,
Alberto Marmodoro,
Alexei Fedorov,
Sung-Kwan Mo,
Makoto Hashimoto,
Donghui Lu,
Anil Rajapitamahuni,
Elio Vescovo,
Junichiro Kono,
Alannah M. Hallas,
Robert J. Birgeneau,
Luis Balicas,
Ján Minár,
Pavan Hosur,
Kam Tuen Law
, et al. (2 additional authors not shown)
Abstract:
Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. Howev…
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Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. However, an experimental demonstration for ideal KNLs well isolated at the Fermi level is lacking. Here, we establish a class of noncentrosymmetric achiral intercalated transition metal dichalcogenide superconductors with large Ising-type spin-orbit coupling, represented by In$_x$TaS$_2$, to host an ideal KNL phase. We provide evidence from angle-resolved photoemission spectroscopy with spin resolution, angle-dependent quantum oscillation measurements, and ab-initio calculations. Our work not only provides a realistic platform for realizing and tuning KNLs in layered materials, but also paves the way for exploring the interplay between KNLs and superconductivity, as well as applications pertaining to spintronics, valleytronics, and nonlinear transport.
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Submitted 29 May, 2025; v1 submitted 11 March, 2025;
originally announced March 2025.
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Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor
Authors:
Zehao Wang,
Yucheng Guo,
Hsiao Yu Huang,
Fang Xie,
Yuefei Huang,
Bin Gao,
Ji Seop Oh,
Han Wu,
Jun Okamoto,
Ganesha Channagowdra,
Chien Te Chen,
Feng Ye,
Xingye Lu,
Zhaoyu Liu,
Zheng Ren,
Yuan Fang,
Yiming Wang,
Ananya Biswas,
Yichen Zhang,
Ziqin Yue,
Cheng Hu,
Chris Jozwiak,
Aaron Bostwick,
Eli Rotenberg,
Makoto Hashimoto
, et al. (11 additional authors not shown)
Abstract:
In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FB…
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In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.
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Submitted 24 November, 2025; v1 submitted 7 June, 2024;
originally announced June 2024.
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Tunability of charge density wave in a magnetic kagome metal
Authors:
Ji Seop Oh,
Ananya Biswas,
Mason Klemm,
Hengxin Tan,
Makoto Hashimoto,
Donghui Lu,
Binghai Yan,
Pengcheng Dai,
Robert J. Birgeneau,
Ming Yi
Abstract:
The discovery of the charge density wave order (CDW) within a magnetically ordered phase in the kagome lattice FeGe has provided a promising platform to investigate intertwined degrees of freedom in kagome lattices. Recently, a method based on post-annealing has been suggested to manipulate the CDW order in kagome FeGe towards either long-range or suppressed orders. Here, we provide a comprehensiv…
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The discovery of the charge density wave order (CDW) within a magnetically ordered phase in the kagome lattice FeGe has provided a promising platform to investigate intertwined degrees of freedom in kagome lattices. Recently, a method based on post-annealing has been suggested to manipulate the CDW order in kagome FeGe towards either long-range or suppressed orders. Here, we provide a comprehensive comparison of the experimentally measured electronic structures of FeGe crystals that have undergone different post-annealing procedures and demonstrate the remarkable effectiveness on tuning the CDW gap without strong perturbation on the underlying electronic structure. Moreover, we observe an additional low temperature transition that only appears in crystals with a long-range CDW order, which we associate with a lattice-spin coupled order. Our work indicates a likely strong sensitivity of the CDW order to disorder in FeGe, and provides evidence for strong coupling between the electronic, lattice, and spin degrees of freedom in this kagome magnet.
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Submitted 2 April, 2024;
originally announced April 2024.
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Tailoring Physical Properties of Crystals through Synthetic Temperature Control: A Case Study for new Polymorphic NbFeTe2 phases
Authors:
Hanlin Wu,
Sheng Li,
Yan Lyu,
Yucheng Guo,
Wenhao Liu,
Ji Seop Oh,
Yichen Zhang,
Sung-Kwan Mo,
Clarina dela Cruz,
Robert J. Birgeneau,
Keith M. Taddei,
Ming Yi,
Li Yang,
Bing Lv
Abstract:
Growth parameters play a significant role in the crystal quality and physical properties of layered materials. Here we present a case study on a van der Waals magnetic NbFeTe2 material. Two different types of polymorphic NbFeTe2 phases, synthesized at different temperatures, display significantly different behaviors in crystal symmetry, electronic structure, electrical transport, and magnetism. Wh…
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Growth parameters play a significant role in the crystal quality and physical properties of layered materials. Here we present a case study on a van der Waals magnetic NbFeTe2 material. Two different types of polymorphic NbFeTe2 phases, synthesized at different temperatures, display significantly different behaviors in crystal symmetry, electronic structure, electrical transport, and magnetism. While the phase synthesized at low temperature showing behavior consistent with previous reports, the new phase synthesized at high temperature, has completely different physical properties, such as metallic resistivity, long-range ferromagnetic order, anomalous Hall effect, negative magnetoresistance, and distinct electronic structures. Neutron diffraction reveals out-of-plane ferromagnetism below 70K, consistent with the electrical transport and magnetic susceptibility studies. Our work suggests that simply tuning synthetic parameters in a controlled manner could be an effective route to alter the physical properties of existing materials potentially unlocking new states of matter, or even discovering new materials.
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Submitted 20 March, 2024;
originally announced March 2024.
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Two-Step Electronic Response to Magnetic Ordering in a van der Waals Ferromagnet
Authors:
Han Wu,
Jian-Xin Zhu,
Lebing Chen,
Matthew W Butcher,
Ziqin Yue,
Dongsheng Yuan,
Yu He,
Ji Seop Oh,
Jianwei Huang,
Shan Wu,
Cheng Gong,
Yucheng Guo,
Sung-Kwan Mo,
Jonathan D. Denlinger,
Donghui Lu,
Makoto Hashimoto,
Matthew B. Stone,
Alexander I. Kolesnikov,
Songxue Chi,
Junichiro Kono,
Andriy H. Nevidomskyy,
Robert J. Birgeneau,
Pengcheng Dai,
Ming Yi
Abstract:
The two-dimensional (2D) material Cr$_2$Ge$_2$Te$_6$ is a member of the class of insulating van der Waals magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the e…
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The two-dimensional (2D) material Cr$_2$Ge$_2$Te$_6$ is a member of the class of insulating van der Waals magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the electronic structure. Specifically, we observe Te $p$-orbital-dominated bands to undergo changes at the Curie transition temperature T$_C$ while the Cr $d$-orbital-dominated bands begin evolving at a higher temperature scale. Combined with neutron scattering, density functional theory calculations, and Monte Carlo simulations, we find that the electronic system can be consistently understood to respond sequentially to the distinct temperatures at which in-plane and out-of-plane spin correlations exceed a characteristic length scale. Our findings reveal the sensitivity of the orbital-selective electronic structure for probing the dynamical evolution of local moment correlations in vdW insulating magnets.
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Submitted 20 December, 2023; v1 submitted 18 December, 2023;
originally announced December 2023.
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Reversible Non-Volatile Electronic Switching in a Near Room Temperature van der Waals Ferromagnet
Authors:
Han Wu,
Lei Chen,
Paul Malinowski,
Jianwei Huang,
Qinwen Deng,
Kirsty Scott,
Bo Gyu Jang,
Jacob P. C. Ruff,
Yu He,
Xiang Chen,
Chaowei Hu,
Ziqin Yue,
Ji Seop Oh,
Xiaokun Teng,
Yucheng Guo,
Mason Klemm,
Chuqiao Shi,
Yue Shi,
Chandan Setty,
Tyler Werner,
Makoto Hashimoto,
Donghui Lu,
T. Yilmaz,
Elio Vescovo,
Sung-Kwan Mo
, et al. (15 additional authors not shown)
Abstract:
The ability to reversibly toggle between two distinct states in a non-volatile method is important for information storage applications. Such devices have been realized for phase-change materials, which utilizes local heating methods to toggle between a crystalline and an amorphous state with distinct electrical properties. To expand such kind of switching between two topologically distinct phases…
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The ability to reversibly toggle between two distinct states in a non-volatile method is important for information storage applications. Such devices have been realized for phase-change materials, which utilizes local heating methods to toggle between a crystalline and an amorphous state with distinct electrical properties. To expand such kind of switching between two topologically distinct phases requires non-volatile switching between two crystalline phases with distinct symmetries. Here we report the observation of reversible and non-volatile switching between two stable and closely-related crystal structures with remarkably distinct electronic structures in the near room temperature van der Waals ferromagnet Fe$_{5-δ}$GeTe$_2$. From a combination of characterization techniques we show that the switching is enabled by the ordering and disordering of an Fe site vacancy that results in distinct crystalline symmetries of the two phases that can be controlled by a thermal annealing and quenching method. Furthermore, from symmetry analysis as well as first principle calculations, we provide understanding of the key distinction in the observed electronic structures of the two phases: topological nodal lines compatible with the preserved global inversion symmetry in the site-disordered phase, and flat bands resulting from quantum destructive interference on a bipartite crystaline lattice formed by the presence of the site order as well as the lifting of the topological degeneracy due to the broken inversion symmetry in the site-ordered phase. Our work not only reveals a rich variety of quantum phases emergent in the metallic van der Waals ferromagnets due to the presence of site ordering, but also demonstrates the potential of these highly tunable two-dimensional magnets for memory and spintronics applications.
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Submitted 6 July, 2023;
originally announced July 2023.
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Observation of Flat Bands and Dirac Cones in a Pyrochlore Lattice Superconductor
Authors:
Jianwei Huang,
Chandan Setty,
Liangzi Deng,
Jing-Yang You,
Hongxiong Liu,
Sen Shao,
Ji Seop Oh,
Yucheng Guo,
Yichen Zhang,
Ziqin Yue,
Jia-Xin Yin,
Makoto Hashimoto,
Donghui Lu,
Sergey Gorovikov,
Pengcheng Dai,
Jonathan D. Denlinger,
M. Zahid Hasan,
Yuan-Ping Feng,
Robert J. Birgeneau,
Youguo Shi,
Ching-Wu Chu,
Guoqing Chang,
Qimiao Si,
Ming Yi
Abstract:
Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases…
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Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network-a pyrochlore lattice-can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu$_2$. We observe flat bands originating from both the Ce 4$f$ orbitals as well as from the 3D destructive interference of the Ru 4$d$ orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cones at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions.
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Submitted 14 December, 2023; v1 submitted 18 April, 2023;
originally announced April 2023.
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Charge order induced Dirac pockets in the nonsymmorphic crystal TaTe$_4$
Authors:
Yichen Zhang,
Ruixiang Zhou,
Hanlin Wu,
Ji Seop Oh,
Sheng Li,
Jianwei Huang,
Jonathan D. Denlinger,
Makoto Hashimoto,
Donghui Lu,
Sung-Kwan Mo,
Kevin F. Kelly,
Gregory T. McCandless,
Julia Y. Chan,
Robert J. Birgeneau,
Bing Lv,
Gang Li,
Ming Yi
Abstract:
The interplay between charge order (CO) and nontrivial band topology has spurred tremendous interest in understanding topological excitations beyond the single-particle description. In a quasi-one-dimensional nonsymmorphic crystal TaTe$_4$, the (2a$\times$2b$\times$3c) charge ordered ground state drives the system into a space group where the symmetry indicator features the emergence of Dirac ferm…
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The interplay between charge order (CO) and nontrivial band topology has spurred tremendous interest in understanding topological excitations beyond the single-particle description. In a quasi-one-dimensional nonsymmorphic crystal TaTe$_4$, the (2a$\times$2b$\times$3c) charge ordered ground state drives the system into a space group where the symmetry indicator features the emergence of Dirac fermions and unconventional double Dirac fermions. Using angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of the CO induced Dirac fermion-related bands near the Fermi level. Furthermore, the band folding at the Fermi level is compatible with the new periodicity dictated by the CO, indicating that the electrons near the Fermi level follow the crystalline symmetries needed to host double Dirac fermions in this system.
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Submitted 25 March, 2024; v1 submitted 1 April, 2023;
originally announced April 2023.
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Ideal Weak Topological Insulator and Protected Helical Saddle Points
Authors:
Ji Seop Oh,
Tianyi Xu,
Nikhil Dhale,
Sheng Li,
Chao Lei,
Chiho Yoon,
Wenhao Liu,
Jianwei Huang,
Hanlin Wu,
Makoto Hashimoto,
Donghui Lu,
Chris Jozwiak,
Aaron Bostwick,
Eli Rotenberg,
Chun Ning Lau,
Bing Lv,
Fan Zhang,
Robert Birgeneau,
Ming Yi
Abstract:
The paradigm of classifying three-dimensional (3D) topological insulators into strong and weak ones (STI and WTI) opens the door for the discovery of various topological phases of matter protected by different symmetries and defined in different dimensions. However, in contrast to the vast realization of STIs, very few materials have been experimentally identified as being close to WTI. Even among…
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The paradigm of classifying three-dimensional (3D) topological insulators into strong and weak ones (STI and WTI) opens the door for the discovery of various topological phases of matter protected by different symmetries and defined in different dimensions. However, in contrast to the vast realization of STIs, very few materials have been experimentally identified as being close to WTI. Even amongst those identified, none exists with topological surface states (TSS) exposed in a global bulk band gap that is stable at all temperatures. Here we report the design and observation of an ideal WTI in a quasi-one-dimensional (quasi-1D) bismuth halide, Bi$_{4}$I$_{1.2}$Br$_{2.8}$ (BIB). Via angle-resolved photoemission spectroscopy (ARPES), we identify that BIB hosts TSS on the (100)$\prime$ side surface in the form of two anisotropic $π$-offset Dirac cones (DCs) separated in momentum while topologically dark on the (001) top surface. The ARPES data fully determine a unique side-surface Hamiltonian and thereby identify two pairs of non-degenerate helical saddle points and a series of four Lifshitz transitions. The fact that both the surface Dirac and saddle points are in the global bulk band gap of 195 meV, combined with the small Dirac velocities, nontrivial spin texture, and the near-gap chemical potential, qualifies BIB to be not only an ideal WTI but also a fertile ground for topological many-body physics.
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Submitted 30 January, 2023; v1 submitted 29 January, 2023;
originally announced January 2023.
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Kramers nodal lines and Weyl fermions in SmAlSi
Authors:
Yichen Zhang,
Yuxiang Gao,
Xue-Jian Gao,
Shiming Lei,
Zhuoliang Ni,
Ji Seop Oh,
Jianwei Huang,
Ziqin Yue,
Marta Zonno,
Sergey Gorovikov,
Makoto Hashimoto,
Donghui Lu,
Jonathan D. Denlinger,
Robert J. Birgeneau,
Junichiro Kono,
Liang Wu,
Kam Tuen Law,
Emilia Morosan,
Ming Yi
Abstract:
Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been…
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Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi.
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Submitted 9 June, 2023; v1 submitted 24 October, 2022;
originally announced October 2022.
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Intertwined magnetism and charge density wave order in kagome FeGe
Authors:
Xiaokun Teng,
Ji Seop Oh,
Hengxin Tan,
Lebing Chen,
Jianwei Huang,
Bin Gao,
Jia-Xin Yin,
Jiun-Haw Chu,
Makoto Hashimoto,
Donghui Lu,
Chris Jozwiak,
Aaron Bostwick,
Eli Rotenberg,
Garrett E. Granroth,
Binghai Yan,
Robert J. Birgeneau,
Pengcheng Dai,
Ming Yi
Abstract:
Electron correlations often lead to emergent orders in quantum materials. Kagome lattice materials are emerging as an exciting platform for realizing quantum topology in the presence of electron correlations. This proposal stems from the key signatures of electronic structures associated with its lattice geometry: flat band induced by destructive interference of the electronic wavefunctions, topol…
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Electron correlations often lead to emergent orders in quantum materials. Kagome lattice materials are emerging as an exciting platform for realizing quantum topology in the presence of electron correlations. This proposal stems from the key signatures of electronic structures associated with its lattice geometry: flat band induced by destructive interference of the electronic wavefunctions, topological Dirac crossing, and a pair of van Hove singularities (vHSs). A plethora of correlated electronic phases have been discovered amongst kagome lattice materials, including magnetism, charge density wave (CDW), nematicity, and superconductivity. These materials can be largely organized into two types: those that host magnetism and those that host CDW order. Recently, a CDW order has been discovered in the magnetic kagome FeGe, providing a new platform for understanding the interplay between CDW and magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we observe all three types of electronic signatures of the kagome lattice: flat bands, Dirac crossings, and vHSs. From both the observation of a temperature-dependent shift of the vHSs towards the Fermi level as well as guidance via first-principle calculations, we identify the presence of the vHSs near the Fermi level (EF) to be driven by the development of underlying magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW order as gaps that open on the near-EF vHS bands, as well as evidence of electron-phonon coupling from a kink on the vHS band together with phonon hardening observed by inelastic neutron scattering. Our observation points to the magnetic interaction-driven band modification resulting in the formation of the CDW order, indicating an intertwined connection between the emergent magnetism and vHS charge order in this moderately-correlated kagome metal.
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Submitted 12 October, 2022;
originally announced October 2022.
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Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa$_4$
Authors:
Shiming Lei,
Kevin Allen,
Jianwei Huang,
Jaime M. Moya,
Tsz Chun Wu,
Brian Casas,
Yichen Zhang,
Ji Seop Oh,
Makoto Hashimoto,
Donghui Lu,
Jonathan Denlinger,
Chris Jozwiak,
Aaron Bostwick,
Eli Rotenberg,
Luis Balicas,
Robert Birgeneau,
Matthew S. Foster,
Ming Yi,
Yan Sun,
Emilia Morosan
Abstract:
Magnetic topological semimetals (TSMs) allow for an effective control of the topological electronic states by tuning the spin configuration, and therefore are promising materials for next-generation electronic and spintronic applications. Of magnetic TSMs, Weyl nodal-line (NL) semimetals likely have the most tunability, and yet they are the least experimentally studied so far due to the scarcity o…
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Magnetic topological semimetals (TSMs) allow for an effective control of the topological electronic states by tuning the spin configuration, and therefore are promising materials for next-generation electronic and spintronic applications. Of magnetic TSMs, Weyl nodal-line (NL) semimetals likely have the most tunability, and yet they are the least experimentally studied so far due to the scarcity of material candidates. Here, using a combination of angle-resolved photoemission spectroscopy and quantum oscillation measurements, together with density functional theory calculations, we identify the square-net compound EuGa4 as a new magnetic Weyl nodal ring (NR) semimetal, in which the line nodes form closed rings in the vicinity of the Fermi level. Remarkably, the Weyl NR states show distinct Landau quantization with clear spin splitting upon application of a magnetic field. At 2 K in a field of 14 T, the transverse magnetoresistance of EuGa4 exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic TSMs. High field magnetoresistance measurements indicate no saturation up to 40 T. Our theoretical model indicates that the nonsaturating MR naturally arises as a consequence of the Weyl NR state. Our work thus point to the realization of Weyl NR states in square-net magnetic materials, and opens new avenues for the design of magnetic TSMs with very large magnetoresistance.
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Submitted 14 December, 2022; v1 submitted 12 August, 2022;
originally announced August 2022.
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Uniaxial ferromagnetism in the kagome metal TbV${_6}$Sn${_6}$
Authors:
Elliott Rosenberg,
Jonathan M. DeStefano,
Yucheng Guo,
Ji Seop Oh,
Makoto Hashimoto,
Donghui Lu,
Robert J. Birgeneau,
Yongbin Lee,
Liqin Ke,
Ming Yi,
Jiun-Haw Chu
Abstract:
The synthesis and characterization of the vanadium-based kagome metal TbV${_6}$Sn${_6}$ is presented. X-ray measurements confirm this material forms with the same crystal structure type as the recently investigated kagome metals GdV$_6$Sn$_6$ and YV$_6$Sn$_6$, with space group symmetry P6/mmm. A signature of a phase transition at 4.1K is observed in heat capacity, resistivity, and magnetic suscept…
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The synthesis and characterization of the vanadium-based kagome metal TbV${_6}$Sn${_6}$ is presented. X-ray measurements confirm this material forms with the same crystal structure type as the recently investigated kagome metals GdV$_6$Sn$_6$ and YV$_6$Sn$_6$, with space group symmetry P6/mmm. A signature of a phase transition at 4.1K is observed in heat capacity, resistivity, and magnetic susceptibility measurements, and both resistivity and magnetization measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly large anisotropy in the magnetic susceptibility was observed, with the c-axis susceptibility nearly 100 times the ab plane susceptibility at 2K. This is highly suggestive of uniaxial ferromagnetism, and the large size of 9.4$μ_b$/f.u. indicates the Tb$^{3+}$ $4f$ electronic moments cooperatively align perpendicular to the V kagome lattice plane. The entropy at the phase transition is nearly Rln(2), indicating that the CEF ground state of the Tb$^{3+}$ ion is a doublet, and therefore the sublattice of $4f$ electrons in this material can be shown to map at low temperatures to the Ising model in a D$_{6h}$ symmetry environment. Hall measurements at temperatures from 300K to 1.7K can be described by two-band carrier transport at temperatures below around 150K, with a large increase in both hole and electron mobilities, similar to YV$_6$Sn$_6$, and an anomalous Hall effect is seen below the ordering temperature. Angle-resolved photoemission measurements above the magnetic ordering temperature reveal typical kagome dispersions. Our study presents TbV${_6}$Sn${_6}$ as an ideal system to study the interplay between Ising ferromagnetism and non-trivial electronic states emerging from a kagome lattice.
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Submitted 29 May, 2022;
originally announced May 2022.
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Spectral Evidence for Unidirectional Charge Density Wave in Detwinned BaNi$_2$As$_2$
Authors:
Yucheng Guo,
Mason Klemm,
Ji Seop Oh,
Yaofeng Xie,
Bing-Hua Lei,
Sergey Gorovikov,
Tor Pedersen,
Matteo Michiardi,
Sergey Zhdanovich,
Andrea Damascelli,
Jonathan Denlinger,
Makoto Hashimoto,
Donghui Lu,
Sung-Kwan Mo,
Rob G. Moore,
Robert J. Birgeneau,
David J. Singh,
Pengcheng Dai,
Ming Yi
Abstract:
The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi$_2$As$_2$, like its well-known iron-based analog BaFe$_2$As$_2$, also hosts a symmetry-breaking structural transition that is coupled to a u…
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The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi$_2$As$_2$, like its well-known iron-based analog BaFe$_2$As$_2$, also hosts a symmetry-breaking structural transition that is coupled to a unidirectional charge density wave (CDW), providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning $B_1g$ uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in BaFe$_2$As$_2$, the structural and CDW order parameters in BaNi$_2$As$_2$ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Our measurements point to a likely lattice origin of the CDW in BaNi$_2$As$_2$.
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Submitted 28 May, 2022;
originally announced May 2022.
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Discovery of charge density wave in a correlated kagome lattice antiferromagnet
Authors:
Xiaokun Teng,
Lebing Chen,
Feng Ye,
Elliott Rosenberg,
Zhaoyu Liu,
Jia-Xin Yin,
Yu-Xiao Jiang,
Ji Seop Oh,
M. Zahid Hasan,
Kelly J. Neubauer,
Bin Gao,
Yaofeng Xie,
Makoto Hashimoto,
Donghui Lu,
Chris Jozwiak,
Aaron Bostwick,
Eli Rotenberg,
Robert J. Birgeneau,
Jiun-Haw Chu,
Ming Yi,
Pengcheng Dai
Abstract:
A hallmark of strongly correlated quantum materials is the rich phase diagram resulting from competing and intertwined phases with nearly degenerate ground state energies. A well-known example is the copper oxides, where a charge density wave (CDW) is ordered well above and strongly coupled to the magnetic order to form spin-charge separated stripes that compete with superconductivity. Recently, s…
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A hallmark of strongly correlated quantum materials is the rich phase diagram resulting from competing and intertwined phases with nearly degenerate ground state energies. A well-known example is the copper oxides, where a charge density wave (CDW) is ordered well above and strongly coupled to the magnetic order to form spin-charge separated stripes that compete with superconductivity. Recently, such rich phase diagrams have also been revealed in correlated topological materials. In two-dimensional kagome lattice metals consisting of corner-sharing triangles, the geometry of the lattice can produce flat bands with localized electrons, non-trivial topology, chiral magnetic order, superconductivity and CDW order. While CDW has been found in weakly electron correlated nonmagnetic AV3Sb5 (A = K, Rb, Cs), it has not yet been observed in correlated magnetic ordered kagome lattice metals. Here we report the discovery of CDW within the antiferromagnetic (AFM) ordered phase of kagome lattice FeGe. The CDW in FeGe occurs at wavevectors identical to that of AV3Sb5, enhances the AFM ordered moment, and induces an emergent anomalous Hall effect. Our findings suggest that CDW in FeGe arises from the combination of electron correlations-driven AFM order and van Hove singularities-driven instability possibly associated with a chiral flux phase, in stark contrast to strongly correlated copper oxides and nickelates, where the CDW precedes or accompanies the magnetic order.
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Submitted 22 March, 2022;
originally announced March 2022.
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Nonsymmorphic Symmetry-Protected Band Crossings in a Square-Net Metal PtPb$_4$
Authors:
Han Wu,
Alannah M. Hallas,
Xiaochan Cai,
Jianwei Huang,
Ji Seop Oh,
Vaideesh Loganathan,
Ashley Weiland,
Gregory T. McCandless,
Julia Y. Chan,
Sung-Kwan Mo,
Donghui Lu,
Makoto Hashimoto,
Jonathan Denlinger,
Robert J. Birgeneau,
Andriy H. Nevidomskyy,
Gang Li,
Emilia Morosan,
Ming Yi
Abstract:
Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Bril…
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Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb$_4$, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin-orbit coupling (SOC), the SOC in this case plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb$_4$ to be a material system with narrow crossings approximately protected by non-symmorhpic crystalline symmetries.
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Submitted 25 March, 2022; v1 submitted 14 February, 2022;
originally announced February 2022.
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Correlation-Driven Electronic Reconstruction in FeTe$_{1-x}$Se$_x$
Authors:
Jianwei Huang,
Rong Yu,
Zhijun Xu,
Jian-Xin Zhu,
Ji Seop Oh,
Qianni Jiang,
Meng Wang,
Han Wu,
Tong Chen,
Jonathan D. Denlinger,
Sung-Kwan Mo,
Makoto Hashimoto,
Matteo Michiardi,
Tor M. Pedersen,
Sergey Gorovikov,
Sergey Zhdanovich,
Andrea Damascelli,
Genda Gu,
Pengcheng Dai,
Jiun-Haw Chu,
Donghui Lu,
Qimiao Si,
Robert J. Birgeneau,
Ming Yi
Abstract:
Electronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstructio…
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Electronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe$_{1-x}$Se$_x$. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized $d_{xy}$ orbital with the remaining itinerant iron 3$d$ orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in $d_{xy}$ as Se concentration is reduced.
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Submitted 7 January, 2022;
originally announced January 2022.
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Transport anomalies in the layered compound BaPt4Se6
Authors:
Sheng Li,
Yichen Zhang,
Hanlin Wu,
Huifei Zhai,
Wenhao Liu,
Daniel Peirano Petit,
Ji Seop Oh,
Jonathan Denlinger,
Gregory T. McCandless,
Julia Y. Chan,
Robert J. Birgeneau,
Gang Li,
Ming Yi,
Bing Lv
Abstract:
We report a layered ternary selenide BaPt4Se6 featuring sesqui-selenide Pt2Se3 layers sandwiched by Ba atoms. The Pt2Se3 layers in this compound can be derived from the Dirac-semimetal PtSe2 phase with Se vacancies that form a honeycomb structure. This structure results in a Pt (VI) and Pt (II) mixed-valence compound with both PtSe6 octahedra and PtSe4 square net coordination configurations. Tempe…
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We report a layered ternary selenide BaPt4Se6 featuring sesqui-selenide Pt2Se3 layers sandwiched by Ba atoms. The Pt2Se3 layers in this compound can be derived from the Dirac-semimetal PtSe2 phase with Se vacancies that form a honeycomb structure. This structure results in a Pt (VI) and Pt (II) mixed-valence compound with both PtSe6 octahedra and PtSe4 square net coordination configurations. Temperature dependent electrical transport measurements suggest two distinct anomalies: a resistivity crossover, mimic to the metal-insulator (M-I) transition at ~150K, and a resistivity plateau at temperatures below 10K. The resistivity crossover is not associated with any structural, magnetic or charge order modulated phase transitions. Magnetoresistivity, Hall and heat capacity measurements concurrently suggest an existing hidden state below 5K in this system. Angle-resolved photoemission spectroscopy measurements reveal a metallic state and no dramatic reconstruction of the electronic structure up to 200K.
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Submitted 28 September, 2021;
originally announced September 2021.
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Room-Temperature Topological Phase Transition in Quasi-One-Dimensional Material Bi$_4$I$_4$
Authors:
Jianwei Huang,
Sheng Li,
Chiho Yoon,
Ji Seop Oh,
Han Wu,
Xiaoyuan Liu,
Nikhil Dhale,
Yan-Feng Zhou,
Yucheng Guo,
Yichen Zhang,
Makoto Hashimoto,
Donghui Lu,
Jonathan Denlinger,
Xiqu Wang,
Chun Ning Lau,
Robert J. Birgeneau,
Fan Zhang,
Bing Lv,
Ming Yi
Abstract:
Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In thi…
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Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi$_4$I$_4$, mediated via a first order structural transition between two distinct stacking orders of the weakly-coupled chains. Using high resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high temperature $β$ phase to be the first weak topological insulator with gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low temperature $α$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.
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Submitted 27 May, 2021;
originally announced May 2021.
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Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe$_{2}$
Authors:
B. K. Choi,
S. Ulstrup,
S. M. Gunasekera,
J. Kim,
S. Y. Lim,
L. Moreschini,
J. S. Oh,
S. -H. Chun,
C. Jozwiak,
A. Bostwick,
E. Rotenberg,
H. Cheong,
I. -W. Lyo,
M. Mucha-Kruczynski,
Y. J. Chang
Abstract:
Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemissio…
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Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials, the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications on the gap following surface-doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe$_{2}$. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals.
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Submitted 29 May, 2020;
originally announced May 2020.
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Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite oxide thin films
Authors:
Byungmin Sohn,
Eunwoo Lee,
Se Young Park,
Wonshik Kyung,
Jinwoong Hwang,
Jonathan D. Denlinger,
Minsoo Kim,
Donghan Kim,
Bongju Kim,
Hanyoung Ryu,
Soonsang Huh,
Ji Seop Oh,
Jong Keun Jung,
Dongjin Oh,
Younsik Kim,
Moonsup Han,
Tae Won Noh,
Bohm-Jung Yang,
Changyoung Kim
Abstract:
Magnetism and spin-orbit coupling (SOC) are two quintessential ingredients underlying novel topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by SOC, the nodal structures become a source of Berry curvature; this leads to a large anomalous Hall effect (AHE). Contrary to three-dimensional systems th…
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Magnetism and spin-orbit coupling (SOC) are two quintessential ingredients underlying novel topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by SOC, the nodal structures become a source of Berry curvature; this leads to a large anomalous Hall effect (AHE). Contrary to three-dimensional systems that naturally host nodal points/lines, two-dimensional (2D) systems can possess stable nodal structures only when proper crystalline symmetry exists. Here we show that 2D spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the AHE. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO$_3$, a representative metallic ferromagnet with SOC. We show that the sign-changing AHE upon variation in the film thickness, magnetization, and chemical potential can be well explained by theoretical models. Our study is the first to directly characterize the topological band structure of 2D spin-polarized bands and the corresponding AHE, which could facilitate new switchable devices based on ferromagnetic ultrathin films.
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Submitted 4 July, 2021; v1 submitted 10 December, 2019;
originally announced December 2019.
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Element-Specific Orbital Character in a Nearly-Free-Electron Superconductor Ag$_5$Pb$_2$O$_6$ Revealed by Core-Level Photoemission
Authors:
Soobin Sinn,
Kyung Dong Lee,
Choong Jae Won,
Ji Seop Oh,
Moonsup Han,
Young Jun Chang,
Namjung Hur,
Byeong-Gyu Park,
Changyoung Kim,
Hyeong-Do Kim,
Tae Won Noh
Abstract:
Ag$_5$Pb$_2$O$_6$ has attracted considerable attention due to its novel nearly-free-electron superconductivity, but its electronic structure and orbital character of the Cooper-pair electrons remain controversial. Here, we present a method utilizing core-level photoemission to show that Pb 6$s$ electrons dominate near the Fermi level. We observe a strongly asymmetric Pb 4$f_{7/2}$ core-level spect…
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Ag$_5$Pb$_2$O$_6$ has attracted considerable attention due to its novel nearly-free-electron superconductivity, but its electronic structure and orbital character of the Cooper-pair electrons remain controversial. Here, we present a method utilizing core-level photoemission to show that Pb 6$s$ electrons dominate near the Fermi level. We observe a strongly asymmetric Pb 4$f_{7/2}$ core-level spectrum, while a Ag 3$d_{5/2}$ spectrum is well explained by two symmetric peaks. The asymmetry in the Pb 4$f_{7/2}$ spectrum originates from the local attractive interaction between conducting Pb 6$s$ electrons and a Pb 4$f_{7/2}$ core hole, which implies a dominant Pb 6$s$ contribution to the metallic conduction. In addition, the observed Pb 4$f_{7/2}$ spectrum is not explained by the well-known Doniach-Šunjić lineshape for a simple metal. The spectrum is successfully generated by employing a Pb 6$s$ partial density of states from local density approximation calculations, thus confirming the Pb 6$s$ dominant character and free-electron-like density of states of Ag$_5$Pb$_2$O$_6$.
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Submitted 29 March, 2017;
originally announced March 2017.
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Electronic Structure of the Kitaev Material $α$-$\textrm{RuCl}_3$ Probed by Photoemission and Inverse Photoemission Spectroscopies
Authors:
Soobin Sinn,
Choong Hyun Kim,
Beom Hyun Kim,
Kyung Dong Lee,
Choong Jae Won,
Ji Seop Oh,
Moonsup Han,
Young Jun Chang,
Namjung Hur,
Hitoshi Sato,
Byeong-Gyu Park,
Changyoung Kim,
Hyeong-Do Kim,
Tae Won Noh
Abstract:
Recently, $α$-$\textrm{RuCl}_3$ has attracted much attention as a possible material realization of the honeycomb Kitaev model, which may stabilize a quantum-spin-liquid state. Compared to extensive studies on its magnetic properties, there is still a lack of understanding on its electronic structure, which is strongly related with its Kitaev physics. Here, the electronic structure of $α$-…
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Recently, $α$-$\textrm{RuCl}_3$ has attracted much attention as a possible material realization of the honeycomb Kitaev model, which may stabilize a quantum-spin-liquid state. Compared to extensive studies on its magnetic properties, there is still a lack of understanding on its electronic structure, which is strongly related with its Kitaev physics. Here, the electronic structure of $α$-$\textrm{RuCl}_3$ is investigated by photoemission (PE) and inverse photoemission (IPE) spectroscopies. The band gap, directly measured from PE/IPE spectra, is found to be 1.9 eV, much larger than previous estimations. The LDA calculations show that the on-site Coulomb interaction $\textit{U}$ can open the band gap without spin-orbit coupling (SOC). However, the SOC should also be incorporated to reproduce the proper gap size, indicating that the interplay between $\textit{U}$ and SOC plays an essential role in the physics of $α$-$\textrm{RuCl}_3$. There exist some spectral features in PE/IPE spectra which cannot be explained by the LDA calculations. To explain such discrepancies, we perform the configuration-interaction calculations for a ${\textrm{RuCl}}_6^{3-}$ cluster. The experimental data and calculations demonstrate that the 4$\textit{d}$ compound $α$-$\textrm{RuCl}_3$ is a $J_{\textrm{eff}}$ = 1/2 Mott insulator rather than a quasimolecular-orbital insulator. Our study also provides important physical parameters, required in verifying the proposed Kitaev physics in $α$-$\textrm{RuCl}_3$.
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Submitted 22 August, 2016;
originally announced August 2016.
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Monte Carlo simulations of the structure of Pt-based bimetallic nanoparticles
Authors:
Kayoung Yun,
Yong-Hun Cho,
Pil-Ryung Cha,
Jaegab Lee,
Jung Soo Oh,
Jung-Hae Choi,
Seung-Cheol Lee,
Ho-Seok Nam
Abstract:
Pt-based bimetallic nanoparticles have attracted significant attention as a promising replacement for expensive Pt nanoparticles. In the systematic design of bimetallic nanoparticles, it is important to understand their preferred atomic structures. However, compared with unary systems, alloy nanoparticles present more structural complexity with various compositional configurations, such as mixed-a…
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Pt-based bimetallic nanoparticles have attracted significant attention as a promising replacement for expensive Pt nanoparticles. In the systematic design of bimetallic nanoparticles, it is important to understand their preferred atomic structures. However, compared with unary systems, alloy nanoparticles present more structural complexity with various compositional configurations, such as mixed-alloy, core-shell, and multishell structures. In this paper, we developed a unified empirical potential model for various Pt-based binary alloys, such as Pd-Pt, Cu-Pt, Au-Pt, and Ag-Pt. Within this framework, we performed a series of Monte Carlo (MC) simulations that quantify the energetically favorable atomic arrangements of Pt-based alloy nanoparticles: an intermetallic compound structure for the Pd-Pt alloy, an onion-like multi-shell structure for the Cu-Pt alloy, and core-shell structures (Au@Pt and Ag@Pt) for the Au-Pt and Ag-Pt alloys. The equilibrium nanoparticle structures for the four alloy types were compared with each other, and the structural features can be interpreted by the interplay of their material properties, such as the surface energy and heat of formation.
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Submitted 18 February, 2012; v1 submitted 15 February, 2012;
originally announced February 2012.