Browsing by Author "Teng, Xiaokun"

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    Interplay of Spin, Charge, and Lattice in Kagome Antiferromagnet FeGe
    (2024-08-08) Teng, Xiaokun; Dai, Pengcheng; Yi, Ming
    Strongly correlated quantum materials feature complex phase diagrams with intertwined phases that have nearly degenerate ground-state energies. A notable example is copper oxides, where charge density waves (CDWs) coexist with magnetic order and compete with superconductivity. Recently, similar rich phase diagrams have been observed in correlated topological materials such as 2D kagome lattice metals. These materials are composed of corner-sharing triangles exhibiting flat bands, magnetic order, superconductivity, and CDW order. In this thesis, we present the discovery of CDW in the antiferromagnetic (AFM) ordered phase of kagome lattice FeGe (Chapter 3). This marks the first observation of CDW in a correlated magnetic-ordered kagome metal. The CDW in FeGe occurs at wavevectors identical to those in the non-magnetic $\rm AV_3Sb_5$ (A = K, Rb, Cs) and enhances the AFM ordered moment. Our findings suggest that the CDW in FeGe arises from electron correlations-driven AFM order and Van Hove singularities-driven instability, contrasting with copper oxides and nickelates where CDW typically precedes or accompanies magnetic order. Using angle-resolved photoemission spectroscopy (ARPES), we identified all three electronic signatures of the kagome lattice in FeGe (Chapter 4). This includes flat bands induced by destructive interference of electronic wavefunctions, topological Dirac crossings, and Van Hove singularities. Below antiferromagnetic transition temperature, driven by magnetic exchange splitting, Van Hove singularities move near the Fermi level, and gaps open in the vicinity of the CDW transition. This behavior highlights the interplay between charge order and magnetism in FeGe. These observations suggest that magnetic interactions drive band modifications, resulting in the formation of the charge density wave, indicating that emergent magnetism and charge order are intertwined in this moderately correlated kagome metal. We then further investigated the spin and lattice excitations in FeGe using inelastic neutron scattering (Chapter 5). Our results show that spin excitations below around 100 meV can be modeled by a spin-1 Heisenberg Hamiltonian. However, higher energy excitations are centered around the Brillouin zone boundary, appearing rod-like, and extend to around 180 meV, consistent with quasiparticle excitations across spin-polarized electron-hole Fermi surfaces. This supports that FeGe is a Hund’s metal in the intermediate correlated regime, with magnetism arising from both itinerant and localized electrons. Moreover, The $c$-axis spin wave dispersion and Fe-Ge optical phonon modes harden below the CDW transition temperature $T_{\rm CDW}$ due to spin-charge-lattice coupling. In addition to these findings, this thesis includes an introduction to the fundamental concepts of magnetism, charge density waves, and the unique properties of kagome materials (Chapter 1). It also details experimental techniques, such as elastic, inelastic neutron scattering, and ARPES (Chapter 2). Overall, this research advances our understanding of the interplay between magnetic, electronic, and structural properties in correlated kagome materials and motivates future studies to further examine the competing phases in these systems.
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    Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet
    (Springer Nature, 2024) Wu, Han; Chen, Lei; Malinowski, Paul; Jang, Bo Gyu; Deng, Qinwen; Scott, Kirsty; Huang, Jianwei; Ruff, Jacob P. C.; He, Yu; Chen, Xiang; Hu, Chaowei; Yue, Ziqin; Oh, Ji Seop; Teng, Xiaokun; Guo, Yucheng; Klemm, Mason; Shi, Chuqiao; Shi, Yue; Setty, Chandan; Werner, Tyler; Hashimoto, Makoto; Lu, Donghui; Yilmaz, Turgut; Vescovo, Elio; Mo, Sung-Kwan; Fedorov, Alexei; Denlinger, Jonathan D.; Xie, Yaofeng; Gao, Bin; Kono, Junichiro; Dai, Pengcheng; Han, Yimo; Xu, Xiaodong; Birgeneau, Robert J.; Zhu, Jian-Xin; da Silva Neto, Eduardo H.; Wu, Liang; Chu, Jiun-Haw; Si, Qimiao; Yi, Ming; Rice Center for Quantum Materials
    Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled 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 Fe5−δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.
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    Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe
    (American Physical Society, 2024) Wu, Shangfei; Klemm, Mason L.; Shah, Jay; Ritz, Ethan T.; Duan, Chunruo; Teng, Xiaokun; Gao, Bin; Ye, Feng; Matsuda, Masaaki; Li, Fankang; Xu, Xianghan; Yi, Ming; Birol, Turan; Dai, Pengcheng; Blumberg, Girsh
    Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature TN≈400 K and a charge density wave (CDW) transition at TCDW≈110 K, followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10−4 and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.