PCSI2027 Session PCSI-WeA1: Topological Thin Films & Heterostructures

Interfacing topological materials with other functional materials offers promising platforms for realizing and manipulating novel quantum phenomena, yet their properties are often limited by defects [1], which are compounded by complex materials processing at epitaxial interfaces. Molecular beam epitaxy (MBE) gives the highest level of control over thin-film synthesis, including composition, thickness, substrate termination, strain, and interfacial structure. I will discuss how this high level of control can enable the integration of topological materials with magnetism and superconductivity. Examples include epitaxial MnBi₂Te₄ [2], where thickness and growth conditions reveal the relationship between intrinsic magnetism, defects, and anomalous Hall response, and Fe(Te,Se)/Bi-Te heterostructures [3-4], where engineered interfaces enhance superconductivity in low-dimensional films. I will also highlight how MBE can suppress structural defects in Bi₂Se₃ [5] and selectively stabilize distinct polymorphs of the novel magnetic material MnTe [6]. Together, these results demonstrate how MBE enables both deeper understanding and deliberate design of emergent properties in topological quantum materials.

⁺Author for correspondence: brahlekm@ornl.gov [mailto:brahlekm@ornl.gov]

[1] Brahlek, M. “Criteria for Realizing Room-Temperature Electrical Transport Applications of Topological Materials.” Advanced Materials 32, 2005698 (2020) https://doi.org/10.1002/adma.202005698

[2] Mallick, D., et al. “Intrinsic Even-Odd Thickness-Driven Anomalous Hall Effect in Epitaxial MnBi2Te4 Thin Films.” Physical Review Materials 10, 044203 (2026) https://doi.org/10.1103/pqj7-prc1

[3] Moore, R. G., et al. “Monolayer Superconductivity and Tunable Topological Electronic Structure at the Fe(Te,Se)/Bi2Te3 Interface.” Advanced Materials 35, 2210940 (2023). https://doi.org/10.1002/adma.202210940

[4] Chen, A.-H., et al. “Interfacially Enhanced Superconductivity in Fe(Te,Se)/Bi4Te3 Heterostructures.” Advanced Materials 36, 2401809 (2024). https://doi.org/10.1002/adma.202401809

[5] Mazza, A. R., et al. “Nucleation and Antiphase Twin Control in Bi2Se3 via Step-Terminated Al2O3 Substrates.” Advanced Functional Materials 36, e13054 (2026) https://doi.org/10.1002/adfm.202513054

[6] Chen, A.-H., et al. “Programmable Phase Selection between Altermagnetic and Noncentrosymmetric Polymorphs of MnTe on InP via Molecular Beam Epitaxy.” ACS Applied Materials & Interfaces 18, 15654–15664 (2026) https://doi.org/10.1021/acsami.5c14582

The field angle dependence of magnetoresistance (MR) in topological semimetals can contain information about mobility anisotropy and topological transport [1-4]. Here, we report on angle dependent magnetotransport in (001)-oriented Cd₃As₂ thin films with ultralow carrier densities (~2-5 ⋅ 10¹⁶ cm^-3) near their quantum limit. With increasing field magnitude, we identify a striking crossover in the H∥I transport from negative longitudinal MR to a huge positive longitudinal MR with magnitude comparable to the transverse MR (Figure 1). Using systematic transport measurements and theoretical modeling of the transport with an 8 x 8 k⋅p model, we identify a unique quantum-limit contribution governed by Fermi surface anisotropy. We discuss these results within the context of previously undescribed field-angle dependent quantum limit transport in other semimetals.

References:

¹ I. A. Leahy, A. D. Rice, J. N. Nelson, H. Ness, M. van Schilfgaarde, D. Graf, A. Suslov, W. Pan, and K. Alberi, Interplay of Quasi-Quantum Hall Effect and Coulomb Disorder in Semimetals, Nature Communications Materials [https://www.nature.com/articles/s43246-026-01282-y] (2026).

²A. Collaudin, B. Fauque, Y. Fuscya, W. Kang, and K. Behnia, Angle Dependence of the Orbital MR in Bismuth, Physical Review X, 5, 021022 [https://journals.aps.org/prx/pdf/10.1103/PhysRevX.5.021022] (2015).

³ J. Wang, H. Yang, L. Ding, W. You, C. Xi, J. Cheng, Z. Shi, C. Cao, Y. Luo, Z. Zhu, J. Dai, M. Tian, and Y. Li, Angle-dependent MR and its implications for Lifshitz transition in W₂As₃, npj quantum materials, 4, 58 [https://www.nature.com/articles/s41535-019-0197-5] (2019).

⁴ S. Ghosh, A. Low, N. Devaraj, S. Changdar, A. Narayan, S. Thirupathaiah, Extremely large and angle dependent MR in Kagome Dirac Semimetal RFe₆Sn₆ (R = Ho, Dy), Journal of Alloys and Compounds, 1040 183506 [https://www.sciencedirect.com/science/article/pii/S0925838825050674] (2025).

Bi_1-xSb_x was the first experimentally identified three-dimensional topological insulator, hosts spin-momentum-locked surface states protected by time-reversal symmetry [1,2]. Its high mobility and large spin Hall angle make it attractive for spin–orbit-torque devices [3]. In ultrathin films, surface-state hybridization opens a gap [4], quantum confinement forms subbands, and the interfacial asymmetry induces Rashba splitting [5]. As a result, the conducting channels and their Fermi surfaces are expected to depend strongly on thickness in the few-bilayer (BL) range.

This work uses Shubnikov–de Haas (SdH) oscillations to track the Fermi surface of Bi₀.₈₅Sb₀.₁₅ films from 2 BL to 10 BL grown on InSb(111)B. The native oxide on the InSb(111)B substrate was removed by annealing with an impinging atomic H-flux at 300 °C in ultrahigh vacuum (UHV), giving a (3x3) surface reconstruction. The surface was subsequently exposed to an Sb₄ flux to mitigate In droplet formation, followed by annealing to obtain the well-ordered (3×3) surface reconstruction. BiSb was grown on this surface by MBE with the substrate held at room temperature, and then post-growth it was annealed at 80–120 °C for 1h, forming a (1×1) surface reconstruction. Growth was monitored by RHEED, and surface morphology was characterized by STM [Fig. 1(a)]. Hall bars with a channel length of 20 µm and width of 5 µm were fabricated to minimize geometric effects and enable accurate magnetotransport measurements. SdH frequencies, effective masses, and tilt-angle dependence together assign each oscillation to a specific channel (hybridized surface state, confined subband, or interface state). At 2 BL, an increase in resistance to the MΩ range at 1.8 K, together with STM evidence of island formation, indicates incomplete film coalescence and substrate-dominated transport. The challenges of achieving continuous films in this ultrathin limit, and the growth optimizations addressing them, will be discussed. The 3 BL film lies between the hybridization-gapped 2 BL and effectively gapless 4 BL regimes [4]. In this film, R_xx shows multiple distinct SdH frequencies [Fig. 1(b,c)] with effective masses of ≈ 0.30, 0.35, and 0.34 mₑ. The nearly equal masses of the latter two disfavor independent Dirac pockets and point to spin-split contours of a single band. The full thickness series will show how the Fermi surface pockets evolve from the hybridized to the topological regime, with implications for engineering transport channels in BiSb-based spintronic devices.

Topological insulators present an obstruction to the description of connected bands by Wannier functions in real space. Starting with a set of Wannier functions, the electronic dispersion can be obtained using tight binding (TB) methods. In the case of gapped systems under such description, there is no obvious ways of distinguishing if the connected valence/conduction bands have trivial or non-trivial topology.

Symmetry techniques are developed to identify connected bands with trivial topology, leaving others as symmetry indicated as having non-trivial topology. A key aspect of the symmetry analysis requires the recognition of the orientability of the Brillouin zone (BZ) (torus). The BZ of the honeycomb lattice is illustrated in Fig.1. Such orientation changes under some space group operations. The presence of such operations in the space group forbids topological invariants, such as Chern numbers, in connected bands with an invariant band description induced from Wannier functions. This gives rise to the basic selection rule on the topological invariants in connected bands with invariant band description. Connected bands without an invariant band description under TB are symmetry-indicated as having non-trivial topology. The hierarchy of connected band in terms of Wannier function description is shown in Fig.2

Under the TB model, the obstruction to real-space Wannier function description manifests as a lack of invariant band description of the connected bands. The configuration of band connectivity of connected bands with trivial/non-trivial topology is established from Wannier functions using the projection operator technique and provides the corresponding classification of band topology.

The TQC criteria of classifying topological phases are modified from “having atomic limit” to “having an invariant band description induced from Wannier functions”. In turn, Wannier functions, single or multi-centred, can only be defined from Fourier transform of connected bands with trivial topology.

The analysis is illustrated with the honeycomb lattice (graphene). Earlier work was reported in PCSI49.

Two-dimensional correlated topological semimetals provide a unique platform in which strong electron correlations, nontrivial band topology, and broken symmetries converge to generate exceptionally strong nonlinear electrodynamics in THz regime. In the first part of the talk, I will introduce our recent experimental effort to study the unique interplay among the quantum geometrical properties, gate-tunable electron correlation and THz electrodynamics in 2D correlated topological semimetals TaIrTe₄. Building upon the nonlinear Hall effect as a new mechanism for THz rectification, we have observed a large zero-bias responsivity (~ 0.3 A/W), ultralow NEP (~ pW/Hz^1/2), broadband THz response (0.1 to 10 THz) and ultrafast intrinsic speed (~ ps) at room temperature. The THz sensing performance can be further enhanced by introducing gate-tunable electron correlations [1].

Building on this strong THz light-matter interaction, we explore whether light can move beyond probing electronic order to actively program it. We will discuss how helical optical and THz fields provide a route to reshape this delicate energy landscape, stabilize chiral charge order, and on demand control its handedness [2]. The observed light-induced chiral charge order intertwined with electron correlation and topology establishes the necessary symmetry-breaking conditions to access more exotic phases, such as fractional quantum anomalous Hall states and chiral superconductivity. Together, these results establish THz electrodynamics in 2D correlated topological semimetals as a pathway from high-performance sensing to the programmable creation and control of chiral quantum order.

[1] T. Xi et al., Nature Electronics 8, 578 (2025).

[2] T. Xi et al., submitted (2026).

⁺Author for correspondence: jun.xiao@wisc.edu