PCSI2024 Session PCSI-MoA2: Topological Materials & Interfaces I

It is conventionally thought is that a single material will exhibit a single kind of conduction polarity, either n-type or p-type, uniformly along all directions of the crystal. Then, in all modern electronic devices, functionality is achieved by integrating together these p-type or n-type materials together. Here we will describe our recent work in the synthesis, properties, and applications of metals and semiconducting materials that exhibit either n-type or p-type conduction behavior depending on the crystallographic direction, a phenomenon we refer to as “goniopolarity”. We will establish the origin of this exotic behavior and the band structure design principles for identifying new goniopolar materials.^[1] This has led to a large expansion in the number of compounds that we have experimentally demonstrated to exhibit this effect, such as NaSn₂As₂, NaSnAs, WSi₂ and PdSe₂.^[2-5] Finally, we will show that the unique charge separation in goniopolar materials can overcome limitations of energy-harvesting technologies including thermoelectrics and photocatalysis.

Topological semimetals hold promise for their use in low-powered electronics and spintronic devices [1-4] but these applications await targeted growth on conventional semiconducting substrates and the exploration of their properties in the ultrathin limit. Weak antilocalization (WAL) has been used extensively in the study of surface states in topological insulators and shows promise for the study of surface states in Weyl semimetals (WSMs). WAL is a quantum interference effect that results in an increase in a system’s conductivity owing to the suppression of back-scattering from self-intersecting carrier paths. This quantum interference requires carriers maintain phase coherence over multiple scattering events. The length over which carriers maintain coherence is defined as the decoherence length. In an applied field, the WAL is destroyed when the magnetic length approaches the decoherence length, offering a natural insight into the localizing disorder length scales. Here we report on insights from WAL into the surface state and interface properties of the recently synthesized, single-crystal-like ultrathin films of Weyl semimetal TaAs(001) grown on GaAs(001) substrates [5-7].

Figure 1 shows atomic force microscopy and magnetoconductance for representative TaAs ultrathin films. AFM on a 10 nm thick TaAs film on GaAs reveals oriented, rod-like growth along – the (1\bar10) direction. At low temperatures, the magnetoconductance exhibits clear signatures of WAL. Intriguingly, we find that the number of apparent WAL conduction channels depends on the orientation of the applied current relative to the film topography as well as the number of GaAs/TaAs interfaces. We hypothesize that this unique anisotropic WAL stems from a topological and trivial state with different decoherence lengths localized at each interface.

References:

[1] J. Hu, S. Xu, Z. Mao, Annual Review of Materials Research 49:1, 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] J. N. Nelson et. al., Matter 6, 2886 (2023).

[6] J. Sadowski et. al., Crystal Growth and Design 22, 6039 (2022).

[7] I. A. Leahy et. al., In Revision at ACS Nano (2023).

⁺Author for correspondence: Ian.Leahy@nrel.gov

Magnetic skyrmions are chiral spin textures whose non-trivial real space topology is often created by an interfacial anisotropic Dzyaloshinkii-Moriya exchange interaction (DMI) that originates from spin-orbit coupling and broken inversion symmetry [1]. They have been observed in a wide variety of bulk single crystals such as MnSi [2] and thin films such as Fe_1-xCo_xSi [3]. More recently, magnetic skyrmions have been probed at ferromagnet/topological insulator interfaces [4] and in magnetic Weyl semimetals [5]. This motivates similar explorations of skyrmion formation in Dirac semimetals (DSMs). We investigate the formation of skyrmions at the interface of a canonical DSM (Cd₃As₂) and a ferromagnetic semiconductor (In_1-xMn_xAs) with perpendicular magnetic anisotropy. Our calculations indicate nonzero spin susceptibility in such heterostructures due to Rashba spin-orbit coupling from broken inversion symmetry, implying the DM interaction necessary for skyrmions. To experimentally test this idea, we grew Cd₃As₂/In_1-xMn_xAs bilayers (Fig. 1a) and mapped out the behavior of the Hall effect as a function of temperature, magnetic field, and gate voltage in electrostatically top gated devices. Below T = 6 K, we observe an emergent gate-tunable topological Hall effect (THE) indicated by an excess Hall resistance (Fig. 1b). This signature is most pronounced at the charge neutrality point, suggesting the formation of a Dirac-electron mediated chiral spin texture at the DSM/ferromagnet interface. Our study provides a new platform to study the interplay between the topological states in DSMs and the chiral spin textures associated with the THE. Supported by the NSF Graduate Research Fellowship Program (Grant No. DGE1255832).

+Author for correspondence: ski5160@psu.edu, nxs16@psu.edu

Layered two-dimensional materials host a variety of crystalline defects, including dislocations either in-plane or out-of-plane with respect to the 2D layered structure. Recently, twisted multilayer 2D material structures have been of interest due to the presence of flat bands and other emergent properties associated with moiré superlattices.[1] Periodic regions of crystalline commensurability making up these superlattices are now understood to be separated by interlayer dislocations, with Burgers vectors and line directions in the plane of the 2D material, and having either edge or screw character.[2] Using density functional theory and quantum Monte Carlo-fitted total energy tight-binding calculations, we show that out-of-plane relaxation of the structures makes possible unique helical dislocations in bilayer graphene, and that the presence of these helical dislocation lines coincides precisely with the so-called magic-angle condition at which unconventional superconductivity is observed.[3] We then illustrate a different dislocation structure, with line direction oriented out-of-plane, but which also has a helical structure. Such a screw dislocation, which adopts a double-helix dislocation core configuration in bilayer structures, is expected to create conditions for exotic transport properties in certain classes of layered topological insulator materials. We present initial results demonstrating this possibility in BiTe and BiSe compounds. In these examples, we present relaxed dislocation core structures computed using first-principles methods, and show that the observed configurations match both experimental observations and the theoretical conditions that are expected to lead to quantum conduction in these otherwise topologically insulating materials.

[1] Cao, Y. et al., Unconventional superconductivity in magic-angle graphene superlattices, Nature 556, 43-50 (2018).

[2] Pochet, P., B. C. McGuigan, J. Coraux, and H. T. Johnson, Toward moiré engineering in 2D materials via dislocation theory, Applied Materials Today 9, 240-250 (2017).

[3] Rakib, T., P. Pochet, E. Ertekin, and H. T. Johnson, Helical dislocation in twisted bilayer graphene, Extreme Mechanics Letters, 63, 102053 (2023).

⁺ Author for correspondence: htj@illinois.edu

Besides providing an interesting platform to interrogate fundamental physics questions, two-dimensional transition metal dichalcogenides (TMDCs) are well suited to advance the development of optoelectronic technologies. In this work, we investigated the growth and the optical properties of ZrTe₂, a candidate topological Dirac semimetal, grown using molecular beam epitaxy. During the growth of 12 unit cells (u.c.) of ZrTe₂ on a sapphire substrate, we obtained in-situ spectroscopic ellipsometry after the deposition of each u.c. Additionally, we obtained temperature dependent ellipsometry data on the sample between 20 ℃ and 350 ℃. After the deposition of the ZrTe₂ layers, a Te capping layer was deposited in order to protect the TMDC film. Post-growth X-ray reflectivity measurements indicated that the total thickness of ZrTe₂ and the thickness of Te to be 5.95 nm and 19 nm, respectively.

A standard inversion technique was used to model the ellipsometry spectra by specifying a three layer model (i.e., sapphire substrate, ZrTe₂ layer and the Te capping layer) to fit the final ellipsometry spectra. The thicknesses obtained from X-ray reflectivity allowed us to obtain the precise dielectric function of the final ZrTe₂ layer (i.e., 12 u.c.), which was converted to the optical conductivity. Subsequently, we fit the remaining ellipsometry spectra obtained for 11 u.c. through 1 u.c. ZrTe₂ layers. Clearly, the optical conductivity shows a noticeable change with the thickness of the ZrTe₂ layers, where the real part increases with the thickness of ZrTe₂, as shown in Fig. 1. The layer-dependent conductivity was further analyzed by incorporating a Drude oscillator to account for free electrons, and two Kramers-Kronig-consistent oscillators to represent the band-to-band transitions. Interestingly, we find that the Drude contribution reduces as the thickness of ZrTe₂ gets smaller, suggesting that its metallic character diminishes as the thickness reduces.

Cd₃As₂ is a prototypical Dirac semi-metal, a class of materials with gapless topologically protected electronic states. In this system, these topological electronic states are close to the intrinsic Fermi level and are well isolated from non-trivial bands. Additionally, this system is air stable and compatible with molecular beam epitaxy, including lattice matching to III-Sb and II-Te layers, and similar elements to conventional semiconductors. These materials could play a role in a large number of applications, including transistors, spintronics, photodetectors, and thermoelectrics. To do this, however, significant progress must be made on achieving tunability of these materials. In particular, routes to altering its typical n-type carrier concentration must be developed.

Previous attempts to dope Cd₃As₂(112) with group VI elements, including Te and Se, were successful, allowing for increases of n_3d from 5e17 cm^-3 up to slightly over 3e18 cm^-3 [1]. Concentration vs mobility relationships appeared similar to doping in convention semiconductors, with mobility decreasing with increased ionized impurities. Attempts to increase doping beyond this level by using increased group VI fluxes resulted in lower measured Hall concentrations and even larger decreases in mobility, a sign that compensating defects are forming. When similar doping is attempted on the (001) surface, however, doping beyond 1e19 cm^-3 is possible. Furthermore, an order of magnitude smaller Te fluxes are required to achieve similar doping levels. Finally, smaller unintentionally doped carrier concentrations are achievable on this surface. This work highlights the role of surface kinetics in defect incorporation in topological semi-metals.

[1] A.D. Rice et al. Appl. Phys. Lett. 122, 061901 (2023)

⁺Author for correspondence: Anthony.rice@nrel.gov

With low carrier concentrations and high mobilities, the pentatelluride material family (HfTe₅ and ZrTe₅) typically have a quantum limit of a few Tesla, making them an ideal platform to study ultra-quantum limit phenomena and magnetic field-induced effects in a three dimensional (3D) Dirac fermion system. In the quantum limit, the magnetic field is strong enough to confine all the electrons to their lowest Landau levels, resulting in a quantized in-plane dispersion, lower effective dimensionality, and a system that is more unstable to electronic correlations. Previous experiments in the pentatellurides have revealed many exotic high-field phenomena including an interaction driven instability [1] and a field induced Lifshitz transition [2]. These materials sit right at the cusp of a strong-to-weak 3D topological phase transition, resulting in a band structure and electronic properties that are extremely sensitive to external tuning parameters such as magnetic field and strain [3]. Consequently, while these systems are promising for the extrinsic control of topological properties, there are many controversies surrounding their intrinsic behavior as the electronic properties depend on the sample growth and preparation conditions [4].

In this work we report on the ultra-quantum limit electronic properties and magnetic field-temperature phase diagram of flux grown, bulk HfTe₅ as revealed by magnetotransport measurements in pulsed magnetic fields up to 65 T (Fig. 1). These samples show a purely insulating resistance vs temperature behavior in zero field. We find a strong and non-monotonic angle dependence of the magnetoresistance and several high field features in the ultra-quantum limit. The interpretation of these features will be discussed during the presentation.

[1] L. Yanwen et al. Nat. Commun. 7, 12516 (2016)

[2] S. Galeski et al. Nat. Commun. 13, 7418 (2022)

[3] J. Mutch et al. Sci. Adv. 5, eaav9771 (2019)

[4] J. Liu et al. arXiv:2304.09072 (2023)

⁺Author for correspondence: jpalmstrom@lanl.gov [mailto:jpalmstrom@lanl.gov]