Browsing by Author "Zhao, WY"

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    Antiferromagnetic topological insulating state in Tb0.02Bi1.08Sb0.9Te2S single crystals
    (American Physical Society (APS), 2023-03-13) Guo, L; Zhao, WY; Li, Q; Xu, M; Chen, L; Bake, A; Vu, THY; He, YH; Fang, Y; Cortie, DL; Mo, SK; Edmonds, MT; Wang, XL; Dong, S; Karel, J; Zheng, RK
    Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantized anomalous Hall insulator, which has been observed in magnetic-topological-insulator-based devices. In this work, we report a successful doping of rare earth element Tb into Bi1.08Sb0.9Te2S topological insulator single crystals, in which the Tb moments are antiferromagnetically ordered below ∼10 K. Benefiting from the in-bulk-gap Fermi level, transport behavior dominant by the topological surface states is observed below ∼150 K. At low temperatures, strong Shubnikov-de Haas oscillations are observed, which exhibit 2D-like behavior. The topological insulator with long range magnetic ordering in rare earth doped Bi1.08Sb0.9Te2S single crystal provides an ideal platform for quantum transport studies and potential applications. ©2023 American Physical Society.
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    Creating thin magnetic layers at the surface of Sb2Te3 topological insulators using a low-energy chromium ion beam
    (AIP Publishing, 2020-05-11) Cortie, DL; Zhao, WY; Yue, Z; Li, Z; Bake, A; Marenych, O; Pastuovic, Z; Nancarrow, M; Zhang, ZM; Qi, DC; Evans, PJ; Mitchell, DRG; Wang, XL
    The surfaces of Sb2Te3 topological insulator crystals were implanted using a 40 keV chromium ion beam. To facilitate uniform doping, the Sb2Te3 was passivated with a thin TiO2 film before the implantation step. The resulting chemical structure was studied using atomic-resolution transmission electron microscopy. A fluence of 7 × 1015 ions/cm2 at 40 keV lead to amorphization of the Sb2Te3 surface, with chromium predominantly incorporated in the amorphous layer. Heating to 200 °C caused the amorphous region to recrystallize and led to the formation of a thin chromium-rich interfacial layer. Near-edge x-ray absorption spectroscopy indicates a uniform valence state of Cr3+ throughout, with no evidence of metallic clustering. High-temperature superparamagnetic behavior was detected up to 300 K, with an increased magnetic moment below 50 K. © 2020 Author(s).
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    Giant linear magnetoresistance in half-metallic Sr2CrMoO6 thin films
    (Springer Nature, 2021-12-01) Wang, ZC; Chen, L; Li, SS; Ying, JS; Tang, F; Gao, GY; Fang, Y; Zhao, WY; Cortie, DL; Wang, XL; Zheng, RK
    Linear magnetoresistance (LMR) is a special case of a magnetic-field induced resistivity response, which has been reported in highly disordered semiconductor systems and in topological materials. In this work, we observe LMR effect in half-metallic perovskite Sr2CrMoO6 thin films, of which the maximum MR value exceeds +1600% at 2 K and 14 T. It is an unusual behavior in ferrimagnetic double perovskite material like Sr2CrMoO6, which are known for intrinsic tunneling-type negative magnetoresistance. In the thin films, the high carriers’ density (~1022 cm−3) and ultrahigh mobility (~104 cm2 V−1 s−1) provide a low-resistivity (~10 nΩ·cm) platform for spin-polarized current. Our DFT calculations and magnetic measurements further support the half-metal band structure. The LMR effect in Sr2CrMoO6 could possibly originate from transport behavior that is governed by the guiding center motion of cyclotron orbitals, where the magnetic domain structure possibly provides disordered potential. The ultrahigh mobility and LMR in this system could broaden the applications of perovskites, and introduce more research on metallic oxide ferri-/ferro-magnetic materials. © The Author(s) 2021 - Open Access CC BY licence.
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    Lamellae preparation for atomic-resolution STEM imaging from ion-beam-sensitive topological insulator crystals
    (AIP Publishing, 2022-04-06) Bake, A; Zhao, WY; Mitchell, DRG; Wang, XL; Nancarrow, M; Cortie, DL
    Good specimen quality is a key factor in achieving successful scanning transmission electron microscope analysis. Thin and damage-free specimens are prerequisites for obtaining atomic-resolution imaging. Topological insulator single crystals and thin films in the chalcogenide family such as Sb2Te3 are sensitive to electron and ion beams. It is, therefore, challenging to prepare a lamella suitable for high-resolution imaging from these topological insulator materials using standard focused ion-beam instruments. We have developed a modified method to fabricate thin focused ion-beam (FIB) lamellae with minimal ion-beam damage and artefacts. The technique described in the current study enables the reliable preparation of high-quality transmission electron microscope (TEM) specimens necessary for studying ultra-thin surface regions. We have successfully demonstrated that the careful selection of FIB milling parameters at each stage minimizes the damage layer without the need for post-treatment. © 2022 Author(s). Published under an exclusive license by the AVS.
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    Modulation of crystal and electronic structures in topological insulators by rare-earth doping
    (American Chemical Society, 2019-08-26) Yue, ZJ; Zhao, WY; Cortie, DL; Yang, G; Li, Z; Wang, XL
    We study magnetotransport in a rare-earth-doped topological insulator, Sm0.1Sb1.9Te3 single crystals, under magnetic fields up to 14 T. It is found that that the crystals exhibit Shubnikov-de Haas (SdH) oscillations in their magnetotransport behavior at low temperatures and high magnetic fields. The SdH oscillations result from the mixed contributions of bulk and surface states. We also investigate the SdH oscillations in different orientations of the magnetic field, which reveal a three-dimensional Fermi surface topology. By fitting the oscillatory resistance with the Lifshitz-Kosevich theory, we draw a Landau-level fan diagram that displays the expected nontrivial phase. In addition, the density functional theory calculations show that Sm doping changes the crystal structure and electronic structure compared with those of pure Sb2Te3. This work demonstrates that rare-earth doping is an effective way to manipulate the Fermi surface of topological insulators. Our results hold potential for the realization of exotic topological effects in magnetic topological insulators. Copyright © 2024 American Chemical Society.
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    Spin reorientation transition and negative magnetoresistance in ferromagnetic NdCrSb3 single crystals
    (MDPI, 2023-02-20) Chen, L; Zhao, WY; Wang, ZC; Tang, F; Fang, Y; Zeng, Z; Xia, ZC; Cheng, ZX; Cortie, DL; Rule, KC; Wang, XL; Zheng, RK
    High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii–Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties. © 2023 The Authors, Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.
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    Top-down patterning of topological surface and edge states using a focused ion beam
    (Springer Nature, 2023-03-27) Bake, A; Zhang, Q; Ho, CS; Causer, GL; Zhao, WY; Yue, ZJ; Nguyen, A; Akhgar, G; Karel, J; Mitchell, DRG; Pastuovic, Z; Lewis, RA; Cole, JH; Nancarrow, M; Wang, XL; Cortie, DL
    The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ2 to pattern conducting channels. It is shown that modifying Sb2Te3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics. Open Access This article is licensed under a Creative Commons Attribution 4.0 © Crown Copyright 2023
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    Topological insulator VxBi1.08-x Sn0.02Sb0.9Te2S as a promising n-type thermoelectric material
    (Elsevier, 2022-10) Chen, L; Zhao, WY; Li, M; Yang, G; Guo, L; Bake, A; Liu, P; Cortie, DL; Zheng, RK; Cheng, ZX; Wang, XL
    As one of the most important n-type thermoelectric (TE) materials, Bi2Te3 has been studied for decades, with efforts to enhance the thermoelectric performance based on element doping, band engineering, etc. In this study, we report a novel bulk-insulating topological material system as a replacement for n-type Bi2Te3 materials: V doped Bi1.08Sn0.02Sb0.9Te2S (V:BSSTS). The V:BSSTS is a bulk insulator with robust metallic topological surface states. Furthermore, the bulk band gap can be tuned by the doping level of V, which is verified by magnetotransport measurements. Large linear magnetoresistance is observed in all samples. Excellent thermoelectric performance is obtained in the V:BSSTS samples, e.g., the highest figure of merit ZT of ~ 0.8 is achieved in the 2% V doped sample (denoted as V0.02) at 530 K. The high thermoelectric performance of V:BSSTS can be attributed to two synergistic effects: (1) the low conductive secondary phases Sb2S3, and V2S3 are believed to be important scattering centers for phonons, leading to lower lattice thermal conductivity; and (2) the electrical conductivity is increased due to the high-mobility topological surface states at the boundaries. In addition, by replacing one third of costly tellurium with abundant, low-cost, and less-toxic sulfur element, the newly produced BSSTS material is inexpensive but still has comparable TE performance to the traditional Bi2Te3-based materials, which offers a cheaper plan for the electronics and thermoelectric industries. Our results demonstrate that topological materials with unique band structures can provide a new platform in the search for new high performance TE materials. © 2022 Elsevier B.V.
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    Transport measurements in porous Bi2Te3 thin films
    (American Physical Society, 2022-03-16) Akhgar, G; Nguyen, A; Cortie, DL; Bake, A; Zhao, WY; Liu, C; Fuhrer, MS; Culcer, D; Hamilton, AR; Edmonds, MT; Karel, J
    Recent theoretical work has predicted the existence of disordered topological insulators , however, minimal experimental work has been conducted on disordered TIs. Here we used molecular-beam epitaxy (MBE) to grow Bi2Te3 thin films that were comprised of nanocrystals embedded in an amorphous matrix. Further disorder was introduced through Ne ion irradiation which produced porosity in the films. In this talk we will present magnetoresistance measurements on porous Bi2Te3, where weak anti-localisation (WAL) was observed. The magnetoresistance curves were fitted using a Dirac Fermion model specifically derived to model weak antilocalization in TIs. Our results also show that the temperature dependence of the phase coherence length in porous Bi2Te3, with an increased surface to volume ratio, exhibits 2D-like transport.