Phys. Rev. B 99, 085124 (2019) - Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal ${\mathrm{Bi}}_{0.97}{\mathrm{Sb}}

Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal ${Bi}_{0.97} {Sb}_{0.03}$

Jorrit C. de Boer, Daan H. Wielens, Joris A. Voerman, Bob de Ronde, Yingkai Huang, Mark S. Golden, Chuan Li, and Alexander Brinkman

Phys. Rev. B 99, 085124 – Published 13 February 2019

Abstract

The field of topological materials science has recently been focusing on three-dimensional Dirac semimetals, which exhibit robust Dirac phases in the bulk. However, the absence of characteristic surface states in accidental Dirac semimetals (DSMs) makes it difficult to experimentally verify claims about the topological nature using commonly used surface-sensitive techniques. The chiral magnetic effect (CME), which originates from the Weyl nodes, causes an $E \cdot B$ -dependent chiral charge polarization, which manifests itself as negative magnetoresistance. We exploit the extended lifetime of the chirally polarized charge and study the CME through both local and nonlocal measurements in Hall bar structures fabricated from single crystalline flakes of the DSM ${Bi}_{0.97} {Sb}_{0.03}$ . From the nonlocal measurement results we find a chiral charge relaxation time, which is over one order of magnitude larger than the Drude transport lifetime, underlining the topological nature of ${Bi}_{0.97} {Sb}_{0.03}$ .

Received 19 November 2018
Revised 9 January 2019

DOI:https://doi.org/10.1103/PhysRevB.99.085124

Physics Subject Headings (PhySH)

Magnetoresistance

Nanostructures Topological materials

Atomic force microscopy Drude model Shubnikov-de Haas effect

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jorrit C. de Boer^1,*, Daan H. Wielens^1,*, Joris A. Voerman¹, Bob de Ronde¹, Yingkai Huang², Mark S. Golden², Chuan Li¹, and Alexander Brinkman¹

¹MESA+ Institute for Nanotechnology, University of Twente, The Netherlands
²Van der Waals - Zeeman Institute, IoP, University of Amsterdam, The Netherlands

^*These two authors contributed equally to this work.

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Vol. 99, Iss. 8 — 15 February 2019

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Figure 1
Dirac semimetals and the chiral magnetic effect. (a) Two superposed Weyl cones in a Dirac semimetal. In an external electric field, the electrons on a shifted Fermi surface relax to their equilibrium state with characteristic time $τ_{c}$ . (b) In parallel external electric and magnetic fields, the two Weyl cones are separated in momentum space and exhibit different chemical potentials. Intercone relaxation of chirally polarized electrons occurs with characteristic time $τ_{CME}$ .
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Figure 2
Local magnetoresistance. (a) Local longitudinal magnetoresistance for perpendicular and parallel electric and magnetic fields. For parallel fields, the MR is strongly negative due to the chiral magnetic effect. Inset: schematic illustration of the device used for local transport. Current is sourced through two of the four outer leads, while the inner four are used as probes for the longitudinal and Hall resistances, as is typical for Hall measurements. (b) Results of the Hall measurement for perpendicular electric and magnetic fields. (c) Drude multiband fit on the conductance. The conductances have been obtained from the measured resistances $R_{xx}$ and $R_{xy}$ through tensor inversion. The low mobility, high carrier density surface state contributions are based on literature [3].
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Figure 3
Nonlocal setup. A schematic illustration of the device used for nonlocal transport measurements (gray represents the ${Bi}_{0.97} {Sb}_{0.03}$ flake and yellow the metallic contacts). Current is sourced parallel to the external magnetic field, through two contacts on the side, and the chiral polarization that diffuses into the central channel, is measured at the contacts further along this channel.
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Figure 4
Nonlocal signals. (a) Measured voltages at distances 1.5, 3 and 4.5 $μ m$ . The inset shows the local voltage, measured at the source contacts. (b) Same as in (a), but for a varying source current $I_{s} (B)$ such that $V_{L} = cst.$ (inset). (c) Symmetrized nonlocal voltages for fixed local electric field, $V_{NL} (E = cst., B)$ , from (b).
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Figure 5
Diffusion lengths. Top: Atomic force microscopy image of a ${Bi}_{0.97} {Sb}_{0.03}$ single-crystal flake, structured into a device for nonlocal measurements and contacted with Nb leads. Bottom: Normalized strength of the Ohmic (zero field) and CME (from fit) contributions to the signals measured at the voltage probes. The CME strength persists over longer distances than the Ohmic signal.
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Figure 6
Temperature dependence of the CME. (a) Atomic force microscopy image of the device used for nonlocal measurements of the CME on a ${Bi}_{0.97} {Sb}_{0.03}$ flake at different temperatures. (b) Temperature dependence of the voltage at the nonlocal contact closest to the source. (c) Temperature dependence of the chiral charge diffusion length $L_{CME}$ and the chiral polarization lifetime $τ_{CME}$ as extracted through Eq. (3). For more information, see Sec. F of the Supplemental Material [9].
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Physical Review B

covering condensed matter and materials physics

Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal ${Bi}_{0.97} {Sb}_{0.03}$

Jorrit C. de Boer, Daan H. Wielens, Joris A. Voerman, Bob de Ronde, Yingkai Huang, Mark S. Golden, Chuan Li, and Alexander Brinkman

Phys. Rev. B 99, 085124 – Published 13 February 2019

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Physical Review B

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Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal Bi0.97Sb0.03

Jorrit C. de Boer, Daan H. Wielens, Joris A. Voerman, Bob de Ronde, Yingkai Huang, Mark S. Golden, Chuan Li, and Alexander Brinkman

Phys. Rev. B 99, 085124 – Published 13 February 2019

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Nonlocal signatures of the chiral magnetic effect in the Dirac semimetal ${Bi}_{0.97} {Sb}_{0.03}$