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Extremely large magnetoresistance in the "ordinary" metal ReO3

Huancheng Chen, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas-van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance (MR) of ReO3 with a simple cubic structure, an "ordinary" nonmagnetic metal considered previously. We found that ReO3 exhibits almost all the characteristics of XMR semimetals: the nearly quadratic field dependence of MR, a field-induced upturn in resistivity followed by a plateau at low temperatures, and high mobilities of charge carriers. It was found that for magnetic field H applied along the c axis, the MR exhibits an unsaturated H-1.75 dependence, which was argued to arise from the complete carrier compensation supported by the Hall resistivity measurements. For H applied along the direction of 15 degrees relative to the c axis, an unsaturated H-1.90 dependence of MR up to (9.43 x 10(3))% at 10 K and 9 T was observed, which was explained by the existence of electron open orbits extending along the k(x) direction. Two mechanisms responsible for XMR observed usually in the semimetals occur also in the simple metal ReO3 due to its peculiar FS (two closed electron pockets and one open electron pocket), once again indicating that the details of FS geometrical configuration are a key factor for the observed XMR in materials.

AMER PHYSICAL SOC2021

Observation of a singular Weyl point surrounded by charged nodal walls in PtGa

Jan Hugo Dil, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

Constrained by the Nielsen-Ninomiya no-go theorem, in all so-far experimentally determined Weyl semimetals (WSMs) the Weyl points (WPs) always appear in pairs in the momentum space with no exception. As a consequence, Fermi arcs occur on surfaces which connect the projections of the WPs with opposite chiral charges. However, this situation can be circumvented in the case of unpaired WP, without relevant surface Fermi arc connecting its surface projection, appearing singularly, while its Berry curvature field is absorbed by nontrivial charged nodal walls. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, we show experimentally that a singular Weyl point emerges in PtGa at the center of the Brillouin zone (BZ), which is surrounded by closed Weyl nodal walls located at the BZ boundaries and there is no Fermi arc connecting its surface projection. Our results reveal that nontrivial band crossings of different dimensionalities can emerge concomitantly in condensed matter, while their coexistence ensures the net topological charge of different dimensional topological objects to be zero. Our observation extends the applicable range of the original Nielsen-Ninomiya no-go theorem which was derived from zero dimensional paired WPs with opposite chirality. In all experimentally observed Weyl semimetals so far, the Weyl points always appear in pairs in the momentum space. Here, the authors report one unpaired Weyl point without surface Fermi arc emerging at the center of the Brillouin zone, which is surrounded by charged Weyl nodal walls in PtGa.

NATURE RESEARCH2021

Large magnetoresistance and nonzero Berry phase in the nodal-line semimetal MoO2

Huancheng Chen, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity rho(xx)(T,H), Hall resistivity rho(xy)(T,H), and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with a monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching 5.03 x 10 4 % at 2 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of rho(xx)(T), the characteristics common for many topologically nontrivial as well as trivial semimetals, emerge also in MoO2. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for magnetic field along the current direction and the nonzero Berry phase in de Haas-van Alphen measurements indicate that pairs of Weyl points appear in MoO2, which may be due to the crystal symmetry breaking. These results highlight MoO2 as a platform for studying the topological properties of oxides.

2020