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1. Electronic Transport and Interaction of Lattice Dynamics in Topological Nodalline Semimetal HfAs ₂ Single Crystals

   https://doi.org/10.1002/adfm.202316775  

   Muhammad, Zahir; Hussain, Ghulam; Islam, Rajbul; Zawadzka, Natalia; Hossain, Md_Shafayat; Iqbal, Obaid; Babiński, Adam; Molas, Maciej_R; Xue, Fei; Zhang, Yue; et al (June 2024, Advanced Functional Materials)

   Abstract Topological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs₂. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs₂are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals. 

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2. Semi-Dirac Fermions in a Topological Metal

   https://doi.org/10.1103/PhysRevX.14.041057  

   Shao, Yinming; Moon, Seongphill; Rudenko, A N; Wang, Jie; Herzog-Arbeitman, Jonah; Ozerov, Mykhaylo; Graf, David; Sun, Zhiyuan; Queiroz, Raquel; Lee, Seng Huat; et al (December 2024, Physical Review X)

   Topological semimetals with massless Dirac and Weyl fermions represent the forefront of quantum materials research. In two dimensions, a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction was predicted 16 years ago. These highly exotic quasiparticles—the semi-Dirac fermions—ignited intense theoretical and experimental interest but remain undetected. Using magneto-optical spectroscopy, we demonstrate the defining feature of semi-Dirac fermions— $B^{2/3}$scaling of Landau levels—in a prototypical nodal-line metal ZrSiS. In topological metals, including ZrSiS, nodal lines extend the band degeneracies from isolated points to lines, loops, or even chains in the momentum space. With calculations and theoretical modeling, we pinpoint the observed semi-Dirac spectrum to the crossing points of nodal lines in ZrSiS. Crossing nodal lines exhibit a continuum absorption spectrum but with singularities that scale as $B^{2/3}$at the crossing. Our work sheds light on the hidden quasiparticles emerging from the intricate topology of crossing nodal lines and highlights the potential to explore quantum geometry with linear optical responses. Published by the American Physical Society2024 

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3. Large anomalous Hall effect and negative magnetoresistance in half-topological semimetals

   https://doi.org/10.1038/s42005-023-01469-6  

   Zhu, Yanglin; Huang, Cheng-Yi; Wang, Yu; Graf, David; Lin, Hsin; Lee, Seng Huat; Singleton, John; Min, Lujin; Palmstrom, Johanna C.; Bansil, Arun; et al (November 2023, Communications Physics)

   Abstract Proposed mechanisms for large intrinsic anomalous Hall effect (AHE) in magnetic topological semimetals include diverging Berry curvatures of Weyl nodes, anticrossing nodal rings or points of non-trivial bands. Here we demonstrate that a half-topological semimetal (HTS) state near a topological critical point can provide an alternative mechanism for a large AHE via systematic studies on an antiferromagnetic (AFM) half-Heusler compound TbPdBi. We not only observe a large AHE with tanΘ^H≈ 2 in its field-driven ferromagnetic (FM) phase, but also find a distinct Hall resistivity peak in its canted AFM phase. Moreover, we observe a large negative magnetoresistance with a value of ~98%. Our in-depth theoretical modelling indicates that these exotic transport properties originate from the HTS state which exhibits Berry curvature cancellation between the trivial spin-up and nontrivial spin-down bands. Our study offers alternative strategies for improved materials design for spintronics and other applications. 

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4. Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa4

   https://doi.org/10.1038/s41467-023-40767-z  

   Lei, Shiming; Allen, Kevin; Huang, Jianwei; Moya, Jaime M.; Wu, Tsz Chun; Casas, Brian; Zhang, Yichen; Oh, Ji Seop; Hashimoto, Makoto; Lu, Donghui; et al (December 2023, Nature Communications)

   Magnetic topological semimetals allow for an effective control of the topological electronic states by tuning the spin configuration. Among them, Weyl nodal line semimetals are thought to have the greatest tunability, yet they are the least studied experimentally 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 EuGa₄as a magnetic Weyl nodal ring semimetal, in which the line nodes form closed rings near the Fermi level. The Weyl nodal ring 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 EuGa₄exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic topological semimetals. Our theoretical model suggests that the non-saturating magnetoresistance up to 40 T arises as a consequence of the nodal ring state. 

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5. Topological quadratic-node semimetal in a photonic microring lattice

   https://doi.org/10.1038/s41467-023-38861-3  

   Gao, Zihe; Zhao, Haoqi; Wu, Tianwei; Feng, Xilin; Zhang, Zhifeng; Qiao, Xingdu; Chiu, Ching-Kai; Feng, Liang (December 2023, Nature Communications)

   Abstract Graphene, with its two linearly dispersing Dirac points with opposite windings, is the minimal topological nodal configuration in the hexagonal Brillouin zone. Topological semimetals with higher-order nodes beyond the Dirac points have recently attracted considerable interest due to their rich chiral physics and their potential for the design of next-generation integrated devices. Here we report the experimental realization of the topological semimetal with quadratic nodes in a photonic microring lattice. Our structure hosts a robust second-order node at the center of the Brillouin zone and two Dirac points at the Brillouin zone boundary—the second minimal configuration, next to graphene, that satisfies the Nielsen–Ninomiya theorem. The symmetry-protected quadratic nodal point, together with the Dirac points, leads to the coexistence of massive and massless components in a hybrid chiral particle. This gives rise to unique transport properties, which we demonstrate by directly imaging simultaneous Klein and anti-Klein tunnelling in the microring lattice. 

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