Dirac electronic materials beyond graphene and topological insulators have recently attracted considerable attention. Cd3As2 is a Dirac semimetal with linear dispersion along all three momentum directions and can be viewed as a three-dimensional analogue of graphene. By breaking of either time-reversal symmetry or spatial inversion symmetry, the Dirac semimetal is believed to transform into a Weyl semimetal with an exotic chiral anomaly effect, however the experimental evidence of the chiral anomaly is still missing in Cd3As2. Here we show a large negative magnetoresistance with magnitude of −63% at 60 K and −11% at 300 K in individual Cd3As2 nanowires. The negative magnetoresistance can be modulated by gate voltage and temperature through tuning the density of chiral states at the Fermi level and the inter-valley scatterings between Weyl nodes. The results give evidence of the chiral anomaly effect and are valuable for understanding the Weyl fermions in Dirac semimetals.

Three-dimensional Dirac semimetals, three-dimensional analogues of graphene, are unusual quantum materials with massless Dirac fermions, which can be further converted to Weyl fermions by breaking time reversal or inversion symmetry. Topological surface states with Fermi arcs are predicted on the surface and have been observed by angle-resolved photoemission spectroscopy experiments. Although the exotic transport properties of the bulk Dirac cones have been demonstrated, it is still a challenge to reveal the surface states via transport measurements due to the highly conductive bulk states. Here, we show Aharonov–Bohm oscillations in individual single-crystal Cd3As2 nanowires with low carrier concentration and large surface-to-volume ratio, providing transport evidence of the surface state in three-dimensional Dirac semimetals. Moreover, the quantum transport can be modulated by tuning the Fermi level using a gate voltage, enabling a deeper understanding of the rich physics residing in Dirac semimetals.

We report the magnetotransport properties of individual Bi2Se3 nanoplates. The carrier Hall mobility is up to 104 cm2/Vs. A large positive linear magnetoresistance (MR) approaching to 400% without sign of saturation was observed at 14 T. By angular dependence measurements, we demonstrate that the linear MR originates from a two-dimensional transport. Furthermore, by comparing the Hall mobility and longitudinal resistance under different temperatures, we give very clear evidence that reveals the close relationship between magnetoresistance and mobility.

Cd 3 As 2 is a model material of Dirac semimetal with a linear dispersion relation along all three directions in the momentum space. The unique band structure of Cd 3 As 2 makes it with both Dirac and topological properties. It can be driven into a Weyl semimetal by the symmetry breaking or a topological insulator by enhancing the spin-orbit coupling. Here we report the temperature and gate voltage dependent magnetotransport properties of Cd 3 As 2 nanoplates with Fermi level near the Dirac point. The Hall anomaly demonstrates the two-carrier transport accompanied by a transition from n-type to p-type conduction with decreasing temperature. The carrier-type transition is explained by considering the temperature dependent spin-orbit coupling. The magnetoresistance exhibits a large non-saturating value up to 2000% at high temperatures, which is ascribed to the electron-hole compensation in the system. Our results are valuable for understanding the experimental observations related to the two-carrier transport in Dirac/Weyl semimetals, such as Na 3 Bi, ZrTe 5 , TaAs, NbAs, and HfTe 5 .KEYWORDS: Dirac semimetal, magnetoresistance, two-band transport, temperature dependence, Hall resistance Three-dimensional (3D) Dirac semimetal 1-3 is a quantum material, where the conduction bands and valence bands touch at discrete points, known as Dirac points.In momentum space, it has linear dispersion along all three directions near the Dirac points, which are protected by the rotational crystalline symmetry. 3,4 Upon breaking of time-reversal symmetry or spatial inversion symmetry, the Dirac point splits into a pair of Weyl nodes with opposite chiralities, thus the Dirac semimetal changes into Weyl semimetal. 5,6 The distinct electronic structures of Dirac semimetal give rise to many other topological phases, for examples, topological insulator 7,8 and topological superconductor. 9 Recently, the negative magnetoresistance (MR) has been observed in Dirac semimetal 10-13 and Weyl semimetal, 14 which is attributed to the chiral anomaly in the presence of parallel electric field and magnetic field. 15,16 In the family of 3D Dirac semimetal, Cd 3 As 2 is a model material that has a pair of Dirac points near the Γ point in the Brillouin zone. 3,[17][18][19] Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy both reveal the linear dispersion near the Dirac ResultsNanoplate Characterizations. The as-grown Cd 3 As 2 nanoplates are single crystalline. 31 As shown in Figure 1a, the scanning electron microscopy (SEM) image indicates that the lateral dimension of the nanoplates ranges from several micrometers to tens of micrometers. A corner of a typical nanoplate is shown by the transmission electron microscopy (TEM) image in Figure 1b. The high-resolution TEM image in Si substrate with 285 nm SiO 2 thin layer serves as the back gate. Figure 2b shows the temperature dependence of the longitudinal resistivity, which exhibits a semiconducting behavior at high temperatures and a metallic behavi...

Magnetotransport measurements of topological insulators are very important to reveal the exotic topological surface states for spintronic applications. However, the novel properties related to the surface Dirac fermions are usually accompanied by a large linear magnetoresistance under perpendicular magnetic field, which makes the identification of the surface states obscure. Here, we report prominent Shubnikov-de Haas (SdH) oscillations under an in-plane magnetic field, which are identified to originate from the surface states in the sidewalls of topological insulator Bi2Se3 nanoplates. Importantly, the SdH oscillations appear with a dramatically weakened magnetoresistance background, offering an easy path to probe the surface states directly when the coexistence of surface states and bulk conduction is inevitable. Moreover, under a perpendicular magnetic field, the oscillations in Hall conductivity have peak-to-valley amplitudes of 2 e2/h, giving confidence to achieve a quantum Hall effect in this system. A cross-section view of the nanoplate shows that the sidewall is (015) facet dominant and therefore forms a 58° angle with regard to the top/bottom surface instead of being perpendicular; this gives credit to the surface states' behavior as two-dimensional transport.

Dirac semimetals [1], such as Cd 3 As 2 or Na 3 Bi [2][3][4][5][6][7][8][9][10], show a linear electronic dispersion in three dimensions described by two copies of the Weyl equation. Applying a magnetic field can break the time reversal symmetry, and the Dirac semimetal is transformed into a Weyl semimetal with the two Weyl nodes separated in the momentum space [10,11]. Chiral charge pumping between the Weyl nodes with different chirality is predicted, which brings the Weyl fermions into the experimental realm. Recently, anomalous transport properties signaled by a pronounced negative magnetoresistance are observed as the evidence for the chiral anomaly effect [10,12].Besides this, the surface dispersion-relation of a Weyl semimetal is topologically equivalent to a non-compact Riemann surface without equal-energy contour that encloses the projection of the Weyl point [13], leading to the emergence of surface Fermi arcs [14]. Lots of angle-resolved photoemission spectroscopy (ARPES) experiments [7,[15][16][17][18] [16,19,[32][33][34][35][36][37][38]. Although the one-dimensional helical transport has been demonstrated in topological insulator nanowires through measuring the AB oscillations is the flux quantum and , where is the measured magnetic field periodicity ( in this case) and S is the cross-sectional area.From the magnetic field periodicity, we can deduce the cross-sectional area to be , which is consistent with the nanowire diameter ~58 nm. In To further clearly present the conductance oscillations, we plot the mapping of ∆G versus gate voltage and magnetic field in Fig. 1d. Clearly there are two kinds of phase modulations on the interference. One is tuned by gate voltage, and the other is influenced by the magnetic field. At a fixed gate voltage, if the conductance reaches the minimum at zero magnetic field, the conductance will be the maximum at half integer multiple of ; if the conductance is maximum at zero magnetic field, the conductance will be the maximum at integer multiple of . The phase of the AB interference is strongly dependent on gate voltage. while when the chirality is -1, the energy dispersion has a similar form with a sign 6 change. This physics picture is depicted in Fig. 2. At zero magnetic field, that is =0, the original linear energy dispersion becomes gapped with a series of sub-bands, as shown in Fig. 2a. The red and blue lines represent the chirality to be +1 and -1, respectively. According to the surface band splitting, there should emerge a periodic oscillation when the Fermi level crosses the sub-bands continuously. This is what happens in our Cd 3 As 2 nanowires, as shown in Fig. 1b.When a magnetic field is applied, the corresponding AB oscillation term should be considered. The surface energy band diagrams at the magnetic flux and are depicted in Fig. 2b, where the letter L and R denote the chirality of Weyl nodes to be +1 or −1. Apparently, when the magnetic flux is half integer of Ф 0 , the linear energy band with specified chirality emerges. The quantum transport can be...

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Topological insulators have exotic surface states that are massless Dirac fermions, manifesting special magnetotransport properties, such as the Aharonov-Bohm effect, Shubnikov-de Haas oscillations, and weak antilocalization effects. In the surface Dirac cone, the band structures are typically closely related to the p-orbitals and possess helical orbital texture. Here we report on the tunability of the transport properties via the interaction between the magnetic field and the spin-orbital angular momentum of the surface states in individual Bi2Se3 nanoribbons. Because the surface states have a large Landé factor and helical spin-orbital texture, the in-plane magnetic field induced Zeeman energy will result in the deformation of the Dirac cone, which gives rise to spin polarization of the surface states. The spin-dependent scattering of the conducting electrons on the existing local magnetic moments produces a giant negative magnetoresistance. The negative magnetoresistance is robust with a ratio of -20% at 2 K and -0.5% at 300 K under 14 T. The results are valuable for possible orbital-electronics based on topological insulators.