Session F60: Topological Materials: Magnetism and Hybrid Structures

Topological phenomena in magnetic topological insulator thin films

Quantum anomalous Hall effect (QAHE), which is related to the momentum-space Berry phase, was experimentally realized in Cr-doped (Bi,Sb)₂Te₃ (BST) and V-doped BST thin films. The full quantization of anomalous Hall effect (AHE) in these two systems requires extremely low temperature (<50mK)[1][2]. Magnetic inhomogeneity has been proposed as one of the main reasons that limit the temperature for the realization of QAHE. In the first part, we will present direct visualization of long-range ferromagnetism in the Cr,V co-doped BST films, by using the MFM and in-situ transport measurement[3]. The magnetization reversal process reveals typical ferromagnetic domain behavior, i.e. domain nucleation and domain wall propagation. In the second part, we will discuss topological Hall effect(THE), a real-space Berry phase effect that originates from the non-coplanar spin textures[4]. Intrinsic THE was revealed at different temperatures in the magnetic topological insulator heterostructure Sb₂Te₃/V-doped Sb₂Te₃, after a back-gate voltage was applied to zero the AHE. The high-temperature THE persists above the Curie temperature, which originates from the spin chirality fluctuations[5]. At temperatures well below the Curie temperature, THE emerges around the coercive fields, when the ferromagnetic bubble domains form. The low-temperature THE likely results from the static topological charges at the chiral domain walls.

[1] C.-Z. Chang et al., Science 340, 167 (2013).
[2] C.-Z. Chang et al., Nature Materials 14, 473–477 (2015).
[3] W. Wang et al., Nature Physics 14, 791 (2018).
[4] Y. Taguchi et al., Science 291, 2573 (2001).
[5] W. Wang et al., Nature Materials 18, 1054 (2019).   

Magneto-transport in PbxSn1-xSe Quantum Well

Pb_xSn1_-xSe (0.16<x<0.4) is a narrow gap semiconductor that exhibits a topological phase transition to a topological crystalline insulator (TCI) when the concentration of Sn increases. It is a valley degenerate topological system that can exhibit exotic quantum Hall effects and strong coupling between the crystal and electronic structures. Here we investigated the transport properties of PbEuSe/Pb_xSn_1-xSe/PbEuSe quantum wells, of which the thickness is 30 and 50nm and x=0.3. By doping Bismuth during the growth, we can efficiently tune the carrier type from p type to n type with carrier densities as low as 1E18cm^-3 and high mobility (>10000cm²/ (Vs)). We found out that the sample showed linear magnetoresistance starting at relatively low field (<1T). The slope of ΔR/R₀ is proportional to the mobility. We applied several model to explain this phenomenon. Further process like gating is also used to investigate the mechanism. With high mobility and low carrier density in the quantum well, it will be a great platform to study 2D quantum phenomenon in the landau quantized regime.

  

Magnetotransport properties in antimony single crystals

The magnetoresistance of antimony single crystals has been measured with the measuring current perpendicular and parallel to the magnetic field, respectively. Extremely large positive transverse magnetoresistance (TMR) up to 1,000,000% was obtained in a magnetic field of 7 T. Field dependence of TMR followed the power law MR ∝ H^2a in all measured temperatures from 2 K to 300 K. The extremely large TMR is attributed to the well compensated electron and hole carriers with high mobility. A very pronounced negative longitudinal magnetoresistance (LMR) at high field and a valley at zero field was observed. The general behavior appears to be consistent with the chiral magneto effect predicted in quantum solids. Moreover, a very unusual negative potential from voltage leads was detected by IV curve, dc, and ac resistance measurements at high magnetic field. This observation cannot be understood in terms of available theories and may be related to some exotic topological characteristics predicted for antimony.   

Thickness Dependence of Electronic Properties of Topological Antiferromagnet MnBi2Te4 Films

The prerequisite for the realization of the quantum anomalous Hall (QAH) state is an interplay between topology and magnetism. More recently, MnBi₂Te₄ was experimentally demonstrated to be a material in which the topological and magnetic states intrinsically coexist. The QAH and axion insulator states were predicted in thin films of MnBi₂Te₄ when the number of layers is odd and even, respectively. Here, we synthesized MnBi₂Te₄ thin films with thicknesses down to one layer using molecular beam epitaxy (MBE) and systematically studied the thickness dependence of electronic properties. Both Hall and Magneto-Optical Kerr Effect measurements show a clear ferromagnetic hysteresis in the one-layer sample. For films thicker than one layer, similar to the exfoliated MnBi₂Te₄ devices, the samples show a small ferromagnetic hysteresis loop at the low magnetic field and spin flop transitions at the high magnetic field. We also found the Hall resistance under zero magnetic field displays an odd/even oscillation, i.e., the samples with odd (even) layers exhibit an anomalous Hall (AH) hysteresis loop with a negative (positive) sign.   

Discovery of a quantum topological kagome magnet by vector magnetic field-based STM

Kagome magnets have been known to exhibit rich topologically based vector magnetic field responses, including giant and anisotropic spin-orbit tunability (J.-X. Yin et al. Nature 562, 91-95 (2018)) and negative flatband magnetism (J.-X. Yin et al. Nature Physics 15, 443-448 (2019)). Here we use the same vector magnetic field-based STM technique combined with Berry curvature sensitive transport to discover a new quantum topological kagome magnet. We observe striking Landau quantization of the kagome lattice, dissipationless chiral edge state, and quantum-limit Hall response. These combined results point to the realization of a high-temperature quantum topological kagome magnet.   

Quantum oscillations in thin films of antiferromagnetic kagome metal FeSn

FeSn is an antiferromagnetic metal, consisting of two-dimensional layers of corner-sharing triangles of Fe, separated by honeycomb Sn spacer layers. This geometrical arrangement of Fe atoms, known as the kagome lattice, is predicted to give rise to Dirac fermions and topological flat bands. Recent photoemission and quantum oscillation studies on FeSn bulk single crystals have experimentally confirmed the existence of these features in their electronic structures [1]. Here we report the quantum oscillation studies on thin films of FeSn. The observed oscillation frequency reproduces the previous measurements on FeSn bulk single crystals. Temperature and field-angle dependent oscillation frequencies respectively reveal the scattering timescale and morphology of the observed Fermi pocket. The realization of high quality FeSn thin films offers a promising platform to explore the interplay of magnetism and electronic topology via quantum confinement or electrostatic gating [2].
References:
[1] M. Kang, L. Ye, et al., arXiv:1906.02167 (2019)
[2] H. Inoue, M. Han, et al., Appl. Phys. Lett. 115, 072403 (2019)   

Dirac-Surface-State-Modulated Bilinear Magnetoresistance in Topological Insulators

Topological insulators (TIs) are a class of quantum materials in which the interior is insulating but electrons can travel through the two-dimensional (2D) conducting surfaces known as Dirac surface states. The Dirac surface states harbor a helical spin texture and thus provide a good platform for potential spintronic applications. Prior experiments have shown that the helical spin texture of the Dirac surface states can be resolved using bilinear magnetoresistance (BMR) measurements. The observation of BMR in previous studies was attributed to the presence of the hexagonally warped Fermi surface at the Dirac surface state in samples with heavy doping. Here, we fabricated a series of (Bi_1-xSb_x)₂Te₃ samples with different x and systematically studied their BMR. When the chemical potential is near the Dirac point and the warping effect of the Dirac surface state disappears, the BMR reaches a maximum rather than vanishes. This observation is further confirmed in a dual-gate-tuned device. Our work indicates that BMR has more complex origins than the warping effect of the Dirac surface state alone.   

Dirac Band Engineering of Topological Insulator/Graphene Hybrid Structures

Dirac materials such as graphene and topological insulators (TI) have attracted much attention because of their unique electrical properties. Stacking these two Dirac materials to form a heterostructure provides a platform for the exploration of the interesting physics of coupled Dirac fermions. We recently used molecular beam epitaxy to fabricate the (Bi_1-xSb_x)₂Te₃/graphene hybrid structures and systematically studied the electronic band structures as a function of x and thickness. Because the ratio between the lattice constants of Sb₂Te₃ and graphene is √3, the Dirac cone of graphene can be folded to the Γ point of the Brillouin zone of Sb₂Te₃. The coexistence of two different Dirac low-energy states is indeed observed in angle-resolved photoemission spectroscopy measurements. This phenomenon is further demonstrated by scanning tunneling microscopy results and first-principles calculations. Our work is of great importance for understanding the interplay of Dirac fermions across the TI/graphene interface and paves a new way to study band engineering in Dirac materials.   

Topological Properties of SnSe/EuS and SnTe/CaTe Heterostructures

We use density functional theory (DFT) to study epitaxial interfaces of the topological crystalline insulators SnSe and SnTe with the ferromagnetic semiconductor EuS and the nonmagnetic semiconductor CaTe, respectively. We consider both surface slab models and periodic "sandwich" heterostructures. We find that a gapped surface state arises in the SnSe/EuS interface due to the magnetic proximity effect. By comparison, the gapless topological surface state at the interface of SnTe/CaTe is unperturbed.   

Synthesis of hybrid oxide/chalcogenide interfaces to explore topological crystalline insulator properties

The class of topological crystalline insulators (TCIs), including SnTe, provides new opportunities to explore topological behaviors. Surface carriers in TCIs experience the same spin-momentum locking as conventional topological insulators, but are protected by crystalline symmetry, which makes possible controlling topological states in SnTe through electric field gating. This work presents growth of SnTe thin enough to achieve topological state control on a substrate, SrTiO₃, easily integrated into field-effect devices. We grow SnTe films by co-sublimation-deposition, where a high-quality thick film is controllably thinned by elevating the substrate temperature in constant flux. X-ray diffraction and atomic force microscopy confirm SnTe films grown by this process are uniform down to 7 nm. Carrier density over a range of film thickness, extracted from Hall measurements, reveals surface-dominated transport in SnTe films less than 16 nm. Weak antilocalization (WAL) behavior observed in magnetotransport is consistent with topological conduction through these surface states, and we fit WAL to theory to extract details of the topological transport. The growth method presented here opens up future opportunities to design heterostructures to further explore topological behavior.   

Modular Arithmetic with Nodal Lines: Drumhead Surface States in ZrSiTe

We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically.
We find two different surface states in ZrSiTe - topological drumhead surface states and trivial floating band surface states, which can be easily distinguished in ARPES experiments. Using the spectra of Wilson loops, we show that a non-trivial Berry phase that exist in a confined region within the Brillouin Zone gives rise to the topological drumhead-type surface states. The Z₂ structure of the Berry phase induces a Z₂ 'modular arithmetic' of the surface states, allowing surface states deriving from different nodal lines to hybridize and gap out, which can be probed by a set of Wilson loops. Our findings are confirmed by ab-initio calculations and angle-resolved photoemission experiments, which are in excellent agreement with each other and the topological analysis.
Ref: arXiv:1909.02154   

NMR investigation of topological quantum material ZrTe5

In this work, we performed ¹²⁵Te NMR measurements on the topological quantum material ZrTe₅. The measured spin-lattice relaxation rates (1/T₁) can be well-explained by a theoretical model for Dirac electron systems, which reveals that the topological characteristic of ZrTe₅ is T-dependent. Combined with DFT calculations, the topological transition ordering can be confirmed from weak topological state to strong topological state with temperature increasing, the reverse of what has been proposed. The T-dependence of Dirac band gap can be derived from NMR results, demonstrating a gapless Dirac semimetal state occurring at a Lifshitz transition temperature, T_c = 85 K in our crystals. We also show that the changes in NMR shift at the Lifshitz transition provide direct evidence of band inversion when the topological phase transition occurs.   

Heteroepitaxial growth of single layer ferromagnetic VSe2 on graphene and Bi2Se3

Recent work has shown evidence of ferromagnetic order at room temperature in epitaxial single layer VSe₂, providing promising opportunities for emergent properties particularly when integrated with other 2D materials. In this work, we report the results of the heteroepitaxial growth of VSe₂ on epitaxial graphene/6H-SiC(0001) and topological insulator Bi₂Se₃. Using in situ scanning tunneling microscopy, we find markedly different growth behaviors on these two substrates. On top of graphene, hexagonal VSe₂ islands first nucleate randomly within the terraces, and then coalesce to form single layer VSe₂ upon further growth. In contrast, the epitaxial Bi₂Se₃(111) substrate surface exhibits spirals with atomically smooth terraces up to 50 nm in width. Here, the VSe₂ epilayer preferentially nucleates at the Bi₂Se₃ spiral step edges forming a distinct lateral Bi₂Se₃ -VSe₂ heterojunctions. This growth mode of VSe₂ on Bi₂Se₃ provides unique opportunities to explore one-dimensional topological-magnetic heterostructures whose structural and electronic properties will be discussed at the meeting.