Observation of the Chiral-Anomaly-Induced Negative Magnetoresistance in 3D Weyl Semimetal TaAs

Weyl semimetal is the three-dimensional analog of graphene. According to quantum field theory, the appearance of Weyl points near the Fermi level will cause novel transport phenomena related to chiral anomaly. In the present paper, we report the experimental evidence for the long-anticipated negative magnetoresistance generated by the chiral anomaly in a newly predicted time-reversal-invariant Weyl semimetal material TaAs. Clear Shubnikov de Haas (SdH) oscillations have been detected starting from a very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits as $\pi$, indicating the existence of Weyl points.

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When two nondegenerate bands cross in three-dimensional momentum space, the crossing points are called Weyl points, which can be viewed as topological defects in band structure, like “knots” on a rope. Weyl points always appear in pairs characterized by opposite chiralities. For materials with Weyl points located near the Fermi level—Weyl semimetals—we expect exotic low-energy physics that includes Fermi arcs on the surfaces and chiral-anomaly-induced quantum transport. The chiral anomaly refers to the violation of number conservation of electrons for a given chirality; it manifests itself via a negative magnetoresistance in the parallel electric and magnetic fields. However, because of the lack of suitable candidates, this quantum anomaly phenomenon has been examined only occasionally by theorists. Here, we report experimental evidence for the long-anticipated chiral anomaly in the newly identified time-reversal-invariant Weyl semimetal material TaAs single crystals.

Weyl semimetals can be thought of as three-dimensional analogs of graphene. We grow single polyhedral crystals of TaAs with dimensions in millimeters, and we conduct experiments in temperatures as low as 1.8 K and magnetic fields as strong as 9 T. We discover extremely large positive magnetoresistance with obvious Shubnikov de Haas oscillations for magnetic fields perpendicular to the current. Our analysis of the Shubnikov de Haas oscillation peaks shows that the Berry phase accumulated along the cyclotron orbits is $\pi$. When the magnetic field is rotated parallel to the current, notable negative magnetoresistance is observed, implying the existence of the chiral anomaly and the concomitant Weyl points in TaAs. We furthermore find an ultrahigh carrier mobility in this material.

Our work provides the earliest transport data in a Weyl semimetal that hint at the presence of the chiral anomaly. Our results pave the way for utilizing Weyl fermions in valleytronic applications.

Figure 1

Structure and symmetry of a TaAs single crystal. (a) The crystal structure of TaAs with a nonsymmorphic space group of $I{4}_{1}md$. Blue and violet balls represent a Ta atom and an As atom, respectively. (b) Schematic diagram of a dozen pairs of Weyl points projected on the (001) facet. “$+$” and “$-$” denote Weyl points with positive and negative chiralities, respectively. The circles show that there are two Weyl points with the same chirality projected on the same point in the (001) facet. $\mathrm{\Gamma }$, X, and M are the high symmetry points in the Brillouin zone. (c) X-ray diffraction pattern of a TaAs single crystal. The inset shows an optical image of a typical sample at the millimeter scale. (d) Schematic diagram of bulk Landau levels of a pair of Weyl nodes. The dotted lines represent the zeroth quantum Landau Level with “$+$” (blue) and “$-$” (red) chiralities in a magnetic field parallel to the electric current.

Figure 2

Angular and field dependence of MR in a TaAs single crystal at 1.8 K. (a) Magnetoresistance with respect to the magnetic field ($B$) at different angles between $B$ and the electric current ($I$) ($\theta =0°–90°$). The inset zooms in on the lower MR part, showing negative MR at $\theta =90°$ (longitudinal negative MR), and it depicts the corresponding measurement configurations. (b) Magnetoresistance measured in different rotating angles around $\theta =90°$ with the interval of every 0.2 °. The negative MR appeared at a narrow region around $\theta =90°$, and most obviously when $B//I$. Either positive or negative deviations from 90 ° would degenerate and ultimately kill the negative MR in the whole range of the magnetic field. Inset: The minima of MR curves at different angles (88 °–92.2 °) in a magnetic field from 1 to 6 T. (c) The negative MR at $\theta =90°$ (open circles) and fitting curves (red dashed lines) at various temperatures. $T=1.8$, 10, 25, 50, 75, and 100 K. (d) Magnetoresistance in the perpendicular magnetic field component, $B\cos \theta$. The misalignment indicates the 3D nature of the electronic states.

Figure 3

Temperature dependence of Hall resistivity and resistivity for TaAs. (a) Hall resistivity measured at various temperatures from 2 to 300 K. The lower-left and the upper-right insets show the Hall resistivity at 2 and 300 K, respectively. The obvious SdH oscillation demonstrates the high quality of the sample. (b) Hall resistivity at $\mathrm{T}=90$, 100, 120, and 150 K. (c) Temperature dependence of carrier mobility ${\mu }_e$ and ${\mu }_h$ of electrons and holes deduced by the two-carrier model. Main panel: Fitting with ${\sigma }_{xy}$; inset: fitting with ${\sigma }_{xx}$. (d) Magnetic field dependence of resistivity with $\theta =0°$ at representative temperatures. (e) Main panel: Oscillatory components of ${\rho }_{\mathrm{xx}}$ at 1.8 K, obtained by subtracting the ${\rho }_{\mathrm{xx}}$ at 20 K. The open circles are the experimental data, and the red line is the best fit by two oscillatory frequency components. Upper inset: Two frequency components $\alpha =16\mathrm{T}$ and $\beta =7\mathrm{T}$ extracted from raw oscillation patterns in the main panel. Lower inset: Landau-level index plots $1/B$ versus $n$ for different oscillation frequencies $\alpha =16\mathrm{T}$ and $\beta =7\mathrm{T}$. We assigned integer indices to the $\mathrm{\Delta }{\rho }_{\mathrm{xx}}$ peak positions in $1/B$ and indicated them with blue and pink dashed lines in the main panel. (f) The temperature dependence of resistivity in the magnetic field perpendicular to the electric current. The red arrow indicates that the resistivity peak moves to high temperatures under much higher magnetic fields. The inset gives the measurement configuration and zooms in on the case of 0 T.

Figure 4

Theoretical analyzing of Fermi surfaces in TaAs. (a,b) First-principles-calculated Fermi surfaces in the top view and side view, respectively. (c) The band structure with SOC included. (d–f) and (g–i) Band structure around Weyl nodes in the $k_z=0.592\pi$ (W1) and $k_z=0$ (W2) planes along the $k_x$, $k_y$, and $k_z$ directions, respectively.