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Magnetotransport properties of Cd3As2 nanostructures

Three-dimensional (3D) topological Dirac semimetal is a new kind of material that has a linear energy dispersion in 3D momentum space and can be viewed as an analog of graphene. Extensive efforts have been devoted to the understanding of bulk materials, but yet it remains a challenge to explore the intriguing physics in low-dimensional Dirac semimetals. Here, we report on the synthesis of Cd3As2 nanowires and nanobelts and a systematic investigation of their magnetotransport properties. Temperature-dependent ambipolar behavior is evidently demonstrated, suggesting the presence of finite-size of bandgap in nanowires. Cd3As2 nanobelts, however, exhibit metallic characteristics with a high carrier mobility exceeding 32,000 cm2V-1s-1 and pronounced anomalous double-period Shubnikov-de Haas (SdH) oscillations. Unlike the bulk counterpart, the Cd3As2 nanobelts reveal the possibility of unusual change of the Fermi sphere owing to the suppression of the dimensionality. More importantly, their SdH oscillations can be effectively tuned by the gate voltage. The successful synthesis of Cd3As2 nanostructures and their rich physics open up exciting nanoelectronic applications of 3D Dirac semimetals.

and spintronics. [10][11][12] Recently, theory predicts the existence of three-dimensional (3D) Dirac fermions where the Dirac nodes are developed via the point contact of conduction-valence bands. The potential candidates involve β-BiO2, Na3Bi and Cd3As2. [13][14][15] Interestingly, these 3D topological Dirac semimetals can be driven into topological insulators 14 or Weyl semimetals 14,16,17 by breaking symmetries that may lead to the discovery of novel physical phenomena such as quantum spin Hall effect and topological superconductivity. 14 Soon after the theoretical predictions, experiments like angle-resolved photoemission spectroscopy (ARPES) 3, 18-20 and scanning tunneling microscopy 21 were carried out on Na3Bi and Cd3As2 to investigate the 3D Dirac fermions.
Cd3As2 has been paid special attentions due to its chemical stability in air and extremely high mobility at both low and room temperatures. 20,22 Studies based on Cd3As2 bulk materials have shown ultrahigh mobility of 9×10 6 cm 2 V -1 s -1 at 5 K 23 and up to 1.5×10 4 cm 2 V -1 s -1 at room temperature. 22 Giant and linear magnetoresistance 23,24 and nontrivial  Berry's phase 25 of Dirac fermions were demonstrated in transport experiments. Recently, a superconductivity phase was also identified in a Cd3As2 crystal making it an interesting candidate of the topological superconductors. [26][27][28][29] However, despite the extensive studies on Cd3As2 bulk materials, few efforts have been devoted to the low-dimensional nanostructures, such as nanowires 30,31 or nanobelts.
In this letter, we present the magnetotransport properties of Cd3As2 nanostructures.
The superb crystallinity of as-grown Cd3As2 nanowires allows us to observe the semiconductor-like ambipolar effect, indicating the band gap opening. In contrast, Cd3As2 nanobelts exhibit high carrier mobility and pronounced double-period Shubnikov-de Haas (SdH) oscillations that can be effectively modulated by applied back-gate voltage (VBG). The reduced dimensionality in these nanostructures, compared with the bulk crystals, offers the artificial suppression of Fermi sphere thus allowing for the observation of unique magnetotransport properties.
Transmission electron microscopy (TEM) was carried out to determine the structural characteristics of the synthesized Cd3As2 nanostructures. Figure 1a is a TEM image of a typical Cd3As2 nanowire with a diameter of ~80 nm. Figure 1b Figure 2c shows the shift of the threshold voltage VTH towards the negative direction as temperature increases. This is originated from the increasing electron charge carriers in the channel at elevated temperatures. 35 Also, temperature-dependent electron and hole mobility can be extracted from the linear region of the transfer characteristics using the equation, 36- where d /d is the slope of the transfer curve in the linear regime, L is the channel length, and ⁄ is the capacitance between the channel and the back gate through the SiO2 gate dielectric (εr = 3.9; tox= 6 285 nm; r is the radius of the Cd3As2 nanowire). As shown in Figure 2d, the mobility reaches the maximum value at ~20 K and decreases as temperate is increased which is attributed to the electron-phonon scattering. 39 The temperature dependence of mobility typically follows the relation ∝ − , where the temperature damping factor γ depends on electron-phonon coupling in the nanowire channel. By performing the best fit to the linear part of the curves (dash lines in Figure 2d), γ can be obtained to be ~0.5 and 1.7 for electrons and holes, respectively. Further suppression of phonon scattering can be realized by encapsulation of the nanowire in high-κ dielectric environment. 40 Nanobelt is another form of nanostructures to study the physics in Cd3As2. To investigate the magnetotransport properties of the Cd3As2 nanobelts, back-gate devices with a standard Hall-bar geometry were fabricated, as schematically illustrated in Figure 3a (also see the SEM image, inset of Figure 3b). The longitudinal resistance (Rxx) as a function of temperature (Rxx-T) at zero magnetic field is acquired from the device with a 120 nm-thick Cd3As2 nanobelt (device 01, Figure 3b). Clearly, it shows a metallic behavior that is different from amorphous semiconducting Cd3As2 thin films. 41 In contrast with the quantum confinement effect in nanowires (gap opening), the Cd3As2 nanobelts are thick enough to incur the widely-observed band inversion (gapless), 14 leading to the high mobility and metallic characteristics. Figure 3c depicts the electron Hall slopes at different temperatures, from which the temperaturedependent Hall mobility and carrier density can be extracted (Figure 3d). The mobility reaches up to 32,000 cm 2 V -1 s -1 and drops as temperature rises because of the enhanced electron-phonon scattering. 40 We also observed a clear Hall anomaly and attribute it to 7 the crystal anisotropy at low temperatures which is consistent with former transport experiments of Cd3As2 bulk materials. 23 Owing to the high mobility of electrons in Cd3As2 nanobelts, we were able to resolve the ShH oscillations in the longitudinal magnetoresistance (Rxx) of the samples. The slopes yield two distinct frequencies F1=66. 8  The observation of SdH oscillations is vital to analyze important parameters of the carrier transport. 47 In order to calculate the cyclotron mass (mcyc), the temperature-  Table 1.
To further probe the nature of SdH oscillations, the angular-dependent magnetotransport measurements were performed by tilting the sample from =0° to 90°.
2 is Berry's phase, 2 is the additional phase shift that changes from 0 for a quasi-