PCSI2023 Session PCSI-WeM1: Topological and Magnetic Materials

Cd₃As₂ is a prototypical Dirac semi-metal, a class of materials with gapless topologically protected electronic states. These materials could play a role in a large number of applications, including transistors, spintronics, photodetectors, and thermoelectrics. To be used in these technologies, however, significant progress needs to be made in developing routes to tune their properties as well as combining them with materials that are already technologically relevant. Here, a II-VI/III-V platform is first developed which allows for growth of Cd₃As₂ on GaAs(111) with high electron mobility. This platform is extended for both (110) and (001) film orientations, ultimately allowing for growth of heterostructures relevant for photodetectors. Analogous approaches also allow for integration of Cd₃As₂ with Si(001). Ways to alter the electronic properties of Cd₃As₂ will also be discussed. This work demonstrates routes toward developing quantum materials for a variety of applications and may be extended to a variety of other materials system.

Author for Correspondence: anthony.rice@nrel.gov

Topological semimetals (TSMs) are emerging as materials with potential use in low powered electronics and spintronic devices [1-4]. In order to translate the useful properties of TSMs to device applications, studies focusing on reliable epitaxial growth, disorder, and the control of electronic states in TSM films are needed. Here, wefocus on the use of alloying with Zn to modify the electronic structure and electrical transport of (Cd_1-xZn_x)₃As₂ with x = 0-0.23.

Zn doping of Cd₃As₂ has been used to lower the carrier concentration and move the Fermi energy closer to the Dirac point. However, the addition of Zn is also expected to modify the band structure, causing a change in the electronic structure from Dirac semimetal to semiconductor [5]. By tuning the growth conditions to suppress native defects in our films [6] we are able to produce (Cd_1-xZn_x)₃As₂ films with carrier concentrations a full order of magnitude smaller (as shown in Fig. 1b, ~10¹⁷ cm^-3) than other literature reports (>10¹⁸ cm^-3) [7]. Lowering the starting carrier concentration enables us to tune the Fermi energy with smaller amounts of Zn doping.

Figure 1 shows the Zn doping dependence of the low-field mobility and the carrier density for our films. For x < 0.1, we observe a slight reduction in mobility with increasing x paired with an order of magnitude reduction in the carrier density. By x = 0.23, the dominant carrier switches from n-type to p-type accompanied by a 100x reduction of the carrier mobility, consistent with the transition from TSM Cd₃As₂ to semiconducting Zn₃As₂ behavior. We will present a careful analysis of the electrical transport properties to explore the low Zn doping regime where the n-type carrier densities reach their lowest values before the electronic structure is significantly altered.

[1] J. Hu, S. Xu, Z. Mao,Annual Review of Materials Research49:1, 207-252 (2019).

[2] I. Leahy, et. al., Proceedings of the National Academy of Sciences, 1808747115 (2018).

[3] H. Chorsi, et. al., Advanced Functional Materials, 32:19, 2110655 (2022).

[4] B. Zhao, et. al., Advanced Materials, 32:38, 200818 (2020).

[5] H. Li, et. al., Scientific Reports 7, 3148 (2017).

[6] A. D. Rice, et. al., Physical Review Materials 3, 121201 (R) (2019).

[7] S. Nishihaya, et. al., Physical Review B 87, 245103 (2018).

⁺Author for correspondence: Ian.Leahy@nrel.gov [mailto:Ian.Leahy@nrel.gov]

⁺Author for correspondence: jocienne.nelson@nrel.gov

Magnetic topological insulator could achieve quantum anomalous Hall (QAH) effect and spin-orbit torque (SOT) switching in the same structure. This is promising for its future applications in memory or switching applications with its robust surface properties by topological protection. In this work we have grown Cr:(Bi_xSb_1-x)₂Te₃ and MnBi₂Te₄ on GaAs (111) substrate through modulation doping by MBE (Molecular Beam Epitaxy). The doping level and the thickness of each layer is examined to tune the bandgap and the Fermi level of the whole sample. In this way, we can tune the Fermi level into the bandgap and optimize the total resistivity to achieve quantization.

Considering the van der Waals nature of the epitaxial layers, it has very weak van der Waals bonding with the substrate. This gives rise to a quasi Van der Waals epitaxial growth mode at the interface of GaAs (111) and epitaxial layers. In this growth mode strain relaxes quickly within the 1^st epitaxial layer. Growth mechanism and the influence on its transport properties are also discussed.

Magnetic proximity interactions (MPIs) between atomically-thin semiconductors and two-dimensional magnets provide a means to manipulate spin and valley degrees of freedom in nonmagnetic monolayers, without the use of applied magnetic fields.In such van der Waals (vdW) heterostructures, MPIs originate in the nanometer-scale coupling between the spin-dependent electronic wavefunctions in the two materials, and typically their overall effect is regarded as an effective magnetic field acting on the semiconductor monolayer. Here we demonstrate that this picture, while appealing, is incomplete: The effects of MPIs in vdW heterostructures can be markedly asymmetric, in contrast to that from an applied magnetic field [1].Valley-resolved optical reflection spectroscopy of MoSe₂/CrBr₃ vdW structures reveals strikingly different energy shifts in the K and K' valleys of the MoSe₂, due to ferromagnetism in the CrBr₃ layer. Strong asymmetry is observed at both the A- and B-exciton resonances. Density-functional calculations indicate that valley-asymmetric MPIs depend sensitively on the spin-dependent hybridization of overlapping bands, and as such are likely a general feature of such hybrid vdW structures. These studies suggest routes to selectively control specific spin and valley states in monolayer semiconductors.

Kagome magnets are attractive family of materials due to complex spin textures and topological band structures [1]. As a typical example of kagome magnet, Fe₃Sn₂ has been shown to exhibit spin frustration [2] and magnetic skyrmions [3] in real space, and massive Dirac fermions [4] in momentum space. However, most of these studies were done on bulk crystals. The development of epitaxially grown Fe₃Sn₂ thin films will be an exciting future direction, as the thin films of kagome magnets enable potential applications in devices as well as the discovery of new phenomena.

In this presentation, we report our progress in atomic layer molecular beam epitaxy (AL-MBE) growth of kagome magnet Fe₃Sn₂ thin films on Pt(111) buffer layer on Al₂O₃(0001) substrates. During the growth, the RHEED intensity shows oscillatory behavior, indicating layer-by-layer growth mode (Fig. 1(a)). AL-MBE allows us to grow Fe₃Sn₂ at much lower temperatures and therefore produces a sharp interface. The high quality of the sample is confirmed by various methods (Fig. 1(b)). The magnetic properties of Fe₃Sn₂ thin films are also presented here (Fig. 1(c)) [5]. We further show that the anomalous Hall effect (AHE) only has intrinsic contribution, suggesting the magnetic Weyl semimetal nature of Fe₃Sn₂ (Fig 1(d)).

[1] Kuroda, K., et al. Nature Materials16.11 (2017): 1090-1095.

[2]Fenner, L. A., et al. Journal of Physics: Condensed Matter21.45 (2009): 452202.

[3]Hou, Zhipeng, et al. Advanced Materials29.29 (2017): 1701144.

[4] Ye, Linda, et al. Nature555.7698 (2018): 638-642.

[5] Cheng, Shuyu, et al. APL Materials10, 061112 (2022)

Kagome antiferromagnet Mn₃Sn has garnered attention due to the presence of interesting properties such as anomalous Hall effect below 420K¹, Nernst effects and presence of exchange bias². Until now most thin film growths have been conducted by using sputter deposition or by cleaving. Recently, Higo et al reported the finding of the 100% perpendicular full switching of the (0110) oriented Mn₃Sn which was grown using molecular beam epitaxy on a MgO (110) substrate having a thin W buffer layer ³. This result indicated that the orientation and possible strain of the film can be key for film transport properties and therefore its of great interest to explore how one can achieve different crystalline film orientation of Mn₃Sn.

In this talk, we demonstrate the synthesis of crystalline Mn₃Sn on Al₂O₃(0001) without a buffer layer using molecular beam epitaxy. The samples were deposited at two different temperatures T_h (500 ± 9 °C) and T_l (416 ± 9 °C) with Mn: Sn flux ratio of 3.2:1 for 90 minutes. Our analysis indicates that for the two temperatures, the resulting orientations of the films are different, with the T_h sample being predominantly c-oriented and T_l sample being 43% a-oriented but in our recent template growth method at room temperature we achieved 82% a-oriented film. In both cases the reflection high energy electron diffraction (RHEED) patterns were streaky indicating a crystalline film and the cross-sectional scanning tunneling electron microscopy (STEM) gave an insight into the morphology of the samples. In both cases the sample are discontiguous with presence of 3D morphology for sample grown at T_h and quasi-2D morphology for the T_l grown sample. Template grown sample are contiguous and show streaky RHEED patterns throughout the growth. Orientation relationships between the Mn₃Sn films and the sapphire substrate are determined from in-plane and out-of-plane measurements. The composition of the samples, the strain effects as well as the pseudomorphic overlay will be discussed in detail. Furthermore, we are in the process of doing STM measurements of the samples and performing theoretical calculations.

¹Z. Zhao, Q. Guo, F. Chen, K. Zhang, and Y. Jiang. Physica B: Condensed Matter, 604, 412692 (2021).

²X.F. Zhou, X.Z. Chen, Y.F. You, L.Y. Liao, H. Bai,R.Q. Zhang, Y.J. Zhou, H.Q. Wu, C. Song, and F. Pan,Phys. Rev. Applied 14, 054037 (2020).

³T. Higo, K. Kondou, T. Nomoto, M. Shiga, S. Sakamoto, X. Chen, D.N.-Hamane, R. Arita, Y. Otani, S.Miwa and S. Nakatsuji, Nature 607, 474-479 (2022).