This content will become publicly available on December 21, 2023
 Award ID(s):
 1702594
 Publication Date:
 NSFPAR ID:
 10410290
 Journal Name:
 Journal of Applied Physics
 Volume:
 132
 Issue:
 23
 Page Range or eLocationID:
 234302
 ISSN:
 00218979
 Sponsoring Org:
 National Science Foundation
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Observation of intrinsic quantum transport properties of twodimensional (2D) topological semimetals can be challenging due to suppression of high mobility caused by extrinsic factors introduced during fabrication. We demonstrate current annealing as a method to substantially improve electronic transport properties of 2D topological semimetal flakes. Contact resistance and resistivity were improved by factors up to [Formula: see text] and [Formula: see text], respectively, in devices based on exfoliated flakes of two topological semimetals, ZrSiSe and BaMnSb 2 . Using this method, carrier mobility in ZrSiSe was improved by a factor of 3800, resulting in observation of recordhigh mobility for exfoliated ZrSiSe. Quantum oscillations in annealed ZrSiSe appeared at magnetic fields as low as 5 T, and magnetoresistance increased by a factor of 10 4 . We argue that a thermal process underlies this improvement. Finally, Raman spectroscopy and analysis of quantum oscillations in ZrSiSe indicate that the phonon modes and Fermi surface area are unchanged by current annealing.

Abstract Twodimensional electron systems subjected to high transverse magnetic fields can exhibit Fractional Quantum Hall Effects (FQHE). In the GaAs/AlGaAs 2D electron system, a double degeneracy of Landau levels due to electronspin, is removed by a small Zeeman spin splitting,
, comparable to the correlation energy. Then, a change of the Zeeman splitting relative to the correlation energy can lead to a reordering between spin polarized, partially polarized, and unpolarized many body ground states at a constant filling factor. We show here that tuning the spin energy can produce fractionally quantized Hall effect transitions that include both a change in$$g \mu _B B$$ $g{\mu}_{B}B$ for the$$\nu$$ $\nu $ minimum, e.g., from$$R_{xx}$$ ${R}_{\mathrm{xx}}$ to$$\nu = 11/7$$ $\nu =11/7$ , and a corresponding change in the$$\nu = 8/5$$ $\nu =8/5$ , e.g., from$$R_{xy}$$ ${R}_{\mathrm{xy}}$ to$$R_{xy}/R_{K} = (11/7)^{1}$$ ${R}_{\mathrm{xy}}/{R}_{K}={(11/7)}^{1}$ , with increasing tilt angle. Further, we exhibit a striking size dependence in the tilt angle interval for the vanishing of the$$R_{xy}/R_{K} = (8/5)^{1}$$ ${R}_{\mathrm{xy}}/{R}_{K}={(8/5)}^{1}$ and$$\nu = 4/3$$ $\nu =4/3$ resistance minima, including “avoided crossing” type lineshape characteristics, and observable shifts of$$\nu = 7/5$$ $\nu =7/5$ at the$$R_{xy}$$ ${R}_{\mathrm{xy}}$ minima the latter occurring for$$R_{xx}$$ ${R}_{\mathrm{xx}}$ and the 10/7. The results demonstrate both size dependence and the possibility, not just of competition between different spin polarized states at the same$$\nu = 4/3, 7/5$$ $\nu =4/3,7/5$ and$$\nu$$ $\nu $ , but also the tilt or Zeemanenergydependent crossover between distinct FQHE associated withmore »$$R_{xy}$$ ${R}_{\mathrm{xy}}$ 
The moleculebased ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE] x , x [Formula: see text] 2) has garnered interest from the quantum information community due to its excellent coherent magnonic properties and ease of onchip integration. Despite these attractive properties, a detailed understanding of the electronic structure and mechanism for longrange magnetic ordering have remained elusive due to a lack of detailed atomic and electronic structural information. Previous studies via xray absorption near edge spectroscopy and the extended xray absorption fine structure have led to various proposed structures, and in general, V[TCNE] x is believed to be a threedimensional network of octahedrally coordinated V 2+ , each bonded to six TCNE molecules. Here, we elucidate the electronic structure, structural ordering, and degradation pathways of V[TCNE] x films by correlating calculations of density functional theory (DFT) with scanning transmission electron microscopy and electron energyloss spectroscopy (EELS) of V[TCNE] x films. Lowloss EELS measurements reveal a bandgap and an excited state structure that agree quantitatively with DFT modeling, including an energy splitting between apical and equatorial TCNE ligands within the structure, providing experimental results directly backed by theoretical descriptions of the electronic structure driving the robust magnetic ordering in these films. Coreloss EELS confirmsmore »

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