Evidence for Weyl fermions in metallic and in semi-metallic systems
| Date/Time: | Thursday, 04 May 2017 from 4:30 pm to 5:20 pm |
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| Location: | Note new location: Science 102 |
| Phone: | 515-294-7377 |
| Channel: | College of Liberal Arts and Sciences |
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Here, we discuss physical systems claimed to display Weyl-fermion-like quasi-particles. First we will discuss a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one[1]. This effect is correlated with the interlayer coupling disappearing for fields applied along the so-called Yamaji angles where the interlayer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral (for example, Weyl) fermions. Its observation in PdCoO2, PtCoO2 and Sr2RuO4 suggests that the anomaly affects the transport of clean conductors, in particular near the quantum limit. The second system to be discussed is the orthorhombic MoTe2 which together with its isostructural compound WTe2, were recently claimed to belong to a new class (type II) of Weyl semi-metals, i.e. characterized by a linear touching between hole and electron Fermi surfaces. To validate this prediction, we synthesized high quality single-crystals[2,3,4] of ?-MoTe2, finding that its superconducting transition temperature seems to depend on disorder suggesting an unconventional superconducting state [2]. Similarly to WTe2, its magnetoresistivity does not saturate at high magnetic fields and can easily surpass 106 %. An analysis of the quantum oscillatory signal superimposed onto the magnetic susceptibility indicates that geometry of the Fermi surfaces evolves with magnetic field preventing the accurate extraction of its Berry phase which is predicted to acquire a value of ? for Weyl type-II systems. Surprisingly, and in sharp contrast to recent ARPES results, the geometry of the Fermi surface, as extracted from the quantum oscillations, is found to be markedly distinct from the calculated one. Synchrotron X-ray analysis indicates that its crystallographic structure evolves considerably upon cooling which might explain the discrepancy between our measurements, the DFT calculations and the ARPES results.
1. N. Kikugawa et al., Nature Comm. 7, 10903 (2016).
2. D. Rhodes et al., arXiv:1605.09065 (2016);
3. Q. Zhou et al., Phys. Rev. B 94, 121101(R) (2016).
4. D. Rhodes et al., Nano Letters 17, 1616 (2017)