Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal C d 3 A s 2

RAPID COMMUNICATIONS PHYSICAL REVIEW B 95, 241113(R) (2017) Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd 3 As 2 Timo Schumann, 1,* Manik Goyal, 1 David A. Kealhofer, 2 and Susanne Stemmer 1,† Materials Department, University of California, Santa Barbara, California 93106-5050, USA Department of Physics, University of California, Santa Barbara, California 93106-9530, USA (Received 9 April 2017; revised manuscript received 5 June 2017; published 26 June 2017) Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, nonsaturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions that have a defined chirality. Classical mechanisms, due to disorder or nonuniform current injection, can, however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd 3 As 2 , a three-dimensional Dirac semimetal. Quasilinear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasilinear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features. DOI: 10.1103/PhysRevB.95.241113 Recent experimental realizations of three-dimensional topological semimetals have opened up numerous exciting opportunities to test quasiparticle behavior that mimics mass- less, relativistic Dirac or Weyl fermions. A signature of Weyl fermions is the “chiral anomaly”, when charge flows from one Weyl node to one of opposite chirality under parallel electric and magnetic fields, thereby giving rise to an unusual negative longitudinal magnetoresistance (MR) [1,2]. In Dirac semimet- als the Dirac nodes split into Weyl nodes in the magnetic field and similar physics is expected. Recent observations [3–9] of longitudinal negative MR in Dirac and Weyl semimetals have therefore generated significant attention. While most studies consider the negative longitudinal MR to be evidence of the chiral anomaly, current jetting has been proposed as an alternative explanation [10,11]. Current jetting arises under inhomogeneous current injection into high-mobility materials that have a large conductivity anisotropy [12]. It leads to a strong preference of the current to flow in the direction of the magnetic field, resulting in a negative longitudinal MR [12]. Other studies claim to have ruled out current jetting caused by nonuniform current injection [7]. Negative longitudinal MR can also be due to conductivity fluctuations and has been reported for disordered semiconductors [13,14]. A recent study [15] showed that mobility fluctuations are a likely origin of linear, positive transverse MR in Cd 3 As 2 , a three-dimensional Dirac semimetal [16–20]. Most studies that attribute the negative longitudinal MR to chiral anomaly have been carried out on bulk materials. Thin films offer several important advantages towards resolving the origin of the negative longitudinal MR in Dirac and Weyl semimetals. In particular, thin-film-device geometries are much less susceptible to current jetting. Furthermore, the effects of nonuniformities can be studied by varying the film thickness and growth conditions. In this work, we study the magnetotransport properties of epitaxial Cd 3 As 2 thin films. schumann.timo@gmx.net stemmer@mrl.ucsb.edu We show that both the transverse positive MR and the negative longitudinal MR are due to conductivity fluctuations. Cd 3 As 2 films were grown by molecular beam epitaxy on relaxed, 180-nm-thick GaSb buffer layers on GaAs (111)B ¯ direction), as de- substrates (1 ◦ miscut towards the [ 1 ¯ 12] scribed elsewhere [21]. The beam equivalent pressure was 2 × 10 −6 Torr, the substrate temperatures ranged between 150 ◦ C and 170 ◦ C, and the growth time was between 5 min and 60 min (see Table I). Surface morphologies were investigated by optical, atomic force, and scanning electron microscopies. The film’s structure and alignment with the substrate were probed by x-ray diffraction (XRD) using Cu Kα radiation. The films were patterned into Hall bar structures with widths and lengths of 100 μm using standard optical lithography and Ar + ion milling. The thicknesses of the Cd 3 As 2 layers were deter- mined by scanning electron microscopy of the patterned Hall bar structures. Magnetoresistance measurements were carried out in a Quantum Design Dynacool PPMS, at temperatures between 300 K and 2 K and magnetic fields up to 9 T. DC excitation currents of 10 μA were used in all measurements. Shubnikov–de Haas oscillations were detected in the thinnest films (See Fig. S1 in the Supplemental Material [22]). Representative optical and atomic force micrographs of a Cd 3 As 2 film are shown in Figs. 1(a) and 1(b). In most areas, the film is smooth with atomically stepped surfaces ˚ matches the interplanar [Fig. 1(b)]. The step height (∼4 A) spacing of the Cd 3 As 2 (112) planes, which form the surface [21]. The defects in the optical micrograph [Fig. 1(a)] are likely areas that grew in a three-dimensional (island) growth mode. Their density was about 10 3 −10 4 cm −2 . Out-of-plane XRD confirmed single-phase, epitaxial Cd 3 As 2 [Fig. 1(c)]. High-resolution XRD showed Laue fringes from the GaSb buffer layer and for the thinnest Cd 3 As 2 layers [arrows in Fig. 1(d)]. The three-dimensional carrier densities determined from low-field Hall measurements were similar for all films grown at 150 ◦ C (see Table I) and ranged from ∼(0.8−1.8) × 10 17 cm −3 at 2 K to (5.0−8.0) × 10 17 cm −3 at 300 K. These carrier densities are lower than those of single crystals reported in the literature (typically > 10 18 cm −3 [15,17,23]), indicating ©2017 American Physical Society