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    AuthorZhang, Xixiang (5)De Wolf, Stefaan (2)Li, Peng (2)Manchon, Aurelien (2)Wang, Mao (2)View MoreDepartment
    Materials Science and Engineering Program (9)
    Physical Sciences and Engineering (PSE) Division (9)KAUST Solar Center (KSC) (3)Chemical Science Program (1)Imaging and Characterization Core Lab (1)View MoreJournalPhysical Review Materials (3)Advanced Functional Materials (1)Nature Energy (1)Physical Review Applied (1)Physical Review B (1)View MoreKAUST Acknowledged Support Unit
    Office of Sponsored Research (OSR) (9)
    CCF (2)KAUST Solar Center (1)KAUST Supercomputing Centre (1)scientific illustrator (1)View MoreKAUST Grant NumberOSR-2017-CRG6-3427 (2)CRF-2017-3427-CRG6 (1)CRF-2018-3717-CRG7 (1)Grant No. OSR-2015-CRG4-2626 (1)OSR-2015-CRG4-2626 (1)View MorePublisherAmerican Physical Society (APS) (5)American Association for the Advancement of Science (AAAS) (1)American Chemical Society (ACS) (1)Springer Science and Business Media LLC (1)Wiley (1)TypeArticle (9)Year (Issue Date)2019 (9)Item Availability
    Open Access (9)

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    Chirality switching of an antiferromagnetic spiral wall and its effect on magnetic anisotropy

    Li, Q.; Yang, M.; N'Diaye, A. T.; Klewe, C.; Shafer, P.; Gao, N.; Wang, T. Y.; Arenholz, E.; Zhang, Xixiang; Hwang, C.; Li, J.; Qiu, Z. Q. (Physical Review Materials, American Physical Society (APS), 2019-11-26) [Article]
    An antiferromagnetic NiO spiral wall in Fe/NiO/Co0.5Ni0.5O/vicinal Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180° spiral wall, we find that the NiO spiral wall always switches its chirality at ~ 90° rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90° to 180°. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on Py/NiO/Co0.5Ni0.5O/vicinal Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180° spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.
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    Carrier Extraction from Perovskite to Polymeric Charge Transport Layers Probed by Ultrafast Transient Absorption Spectroscopy.

    Ugur, Esma; Khan, Jafar Iqbal; Aydin, Erkan; Wang, Mingcong; Kirkus, Mindaugas; Neophytou, Marios; McCulloch, Iain; De Wolf, Stefaan; Laquai, Frédéric (The journal of physical chemistry letters, American Chemical Society (ACS), 2019-10-22) [Article]
    The efficiency of state-of-the-art perovskite solar cells is limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward toward the Shockley-Queisser limit. Here, we demonstrate that ultrafast transient absorption spectroscopy can directly probe hole extraction and recombination dynamics at perovskite/hole transport layer (HTL) interfaces. To illustrate this, we employed PDPP-3T as HTL because its ground-state absorption is at lower energy than the perovskite's photobleach, enabling direct monitoring of interfacial hole extraction and recombination. Moreover, by fitting the carrier dynamics using a diffusion model, we determined the carrier mobility. Afterwards, by varying the perovskite thickness, we distinguished between carrier diffusion and carrier extraction at the interface. Lastly, we prepared device-like structures, TiO2/perovskite/PDPP-3T stacks, and observed reduced carrier recombination in the perovskite. From PDPP-3T carrier dynamics, we deduced that hole extraction is one order faster than recombination of holes at the interface.
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    Self-Healing and Stretchable 3D-Printed Organic Thermoelectrics

    Kee, Seyoung; Haque, Mohammed; Corzo Diaz, Daniel Alejandro; Alshareef, Husam N.; Baran, Derya (Advanced Functional Materials, Wiley, 2019-01-01) [Article]
    With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy-harvesting solutions that are compatible with such wearables. However, many of the proposed energy-harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self-healability is essential to realize self-powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self-healing and stretchable properties produced via 3D-printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self-healable and solution-processable composite, 3D-printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self-healing. This approach represents a significant step in achieving damage-free and truly wearable 3D-printed organic thermoelectrics.
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    Competition between Electronic and Magnonic Spin Currents in Metallic Antiferromagnets

    Wen, Yan; Zhuo, Fengjun; Zhao, Yuelei; Li, Peng; Zhang, Qiang; Manchon, Aurelien; Zhang, Xixiang (Physical Review Applied, American Physical Society (APS), 2019-11-13) [Article]
    We investigate the spin-orbit torque in a Ta/Ir−Mn/Cu/Ni−Fe multilayer heterostructure and relate it to spin current transmission through the Ir−Mn layer. We identify several spin current transport regimes as a function of the temperature and the thickness of the Ir−Mn layer. To interpret this experiment, we develope a drift-diffusion model accounting for both electron and magnon transport in the heterostructures. This model allows us to discriminate between the contributions of electrons and magnons to the total spin current in Ir−Mn. We find that the electron-magnon spin convertance is one order of magnitude larger than the interfacial electronic spin conductance, while the magnon diffusion length is about ten times longer than the electronic spin relaxation length. This study demonstrates that magnonic spin transport dominates over electronic spin transport even in disorder metallic antiferromagnets.
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    Critical behavior of intercalated quasi-van der Waals ferromagnet Fe0.26TaS2

    Zhang, Chenhui; Yuan, Ye; Wang, Mao; Li, Peng; Zhang, Junwei; Wen, Yan; Zhou, Shengqiang; Zhang, Xixiang (Physical Review Materials, American Physical Society (APS), 2019-11-05) [Article]
    In the present work, single-crystalline quasi-van der Waals ferromagnet Fe0.26TaS2 was successfully synthesized with Fe atoms intercalated at ordered positions between TaS2 layers. Its critical behavior was systematically studied by measuring the magnetization around ferromagnetic to paramagnetic phase transition temperature, TC ∼ 100.7 K, under different magnetic fields. The critical exponent β for the spontaneous magnetization below TC, γ for the inverse initial susceptibility above TC, and δ for the magnetic isotherm at TC were determined with modified Arrott plots, the Kouvel-Fisher method, the Widom scaling law, and critical isotherm analysis, and found to be the following values: β = 0.459(6), γ = 1.205(11), and δ = 3.69(1). The obtained critical exponents are self-consistent and follow the scaling equation, indicating the reliability and intrinsicality of these parameters. A close analysis within the framework of renormalization group theory reveals that the spin coupling inside Fe0.26TaS2 crystal is of the three-dimensional Heisenberg ({d : n}={3:3}) type with long-range magnetic interaction and that the exchange interaction decays with distance as J(r) ∼ r−4.71.
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    Quantum anomalous Hall effect and Anderson-Chern insulating regime in the noncollinear antiferromagnetic 3Q state

    Ndiaye, Papa Birame; Abbout, Adel; Goli, V. M. L. D. P.; Manchon, Aurelien (Physical Review B, American Physical Society (APS), 2019-10-28) [Article]
    We investigate the emergence of both quantum anomalous Hall and disorder-induced Anderson-Chern insulating phases in two-dimensional hexagonal lattices, with an antiferromagnetically ordered 3Q state and in the absence of spin-orbit coupling. Using tight-binding modeling, we show that such systems display not only a spin-polarized edge-localized current, the chirality of which is energy dependent, but also an impurity-induced transition from trivial metallic to topological insulating regimes, through one edge mode plateau. We compute the gaps' phase diagrams and demonstrate the robustness of the edge channel against deformation and disorder. Our study hints at the 3Q state as a promising building block for dissipationless spintronics based on antiferromagnets.
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    p-type codoping effect in (Ga,Mn)As: Mn lattice location versus magnetic properties

    Xu, Chi; Zhang, Chenhui; Wang, Mao; Xie, Yufang; Hübner, René; Heller, René; Yuan, Ye; Helm, Manfred; Zhang, Xixiang; Zhou, Shengqiang (Physical Review Materials, American Physical Society (APS), 2019-08-22) [Article]
    In the present work, we perform a systematic investigation on p-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature (TC) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both TC and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.
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    Passivating contacts for crystalline silicon solar cells

    Allen, Thomas; Bullock, James; Yang, Xinbo; Javey, Ali; De Wolf, Stefaan (Nature Energy, Springer Science and Business Media LLC, 2019-09-16) [Article]
    The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed.
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    Current-driven magnetization switching in a van der Waals ferromagnet Fe3GeTe2.

    Wang, Xiao; Tang, Jian; Xia, Xiuxin; He, Congli; Zhang, Junwei; Liu, Yizhou; Wan, Caihua; Fang, Chi; Guo, Chenyang; Yang, Wenlong; Guang, Yao; Zhang, Xiaomin; Xu, Hongjun; Wei, Jinwu; Liao, Mengzhou; Lu, Xiaobo; Feng, Jiafeng; Li, Xiaoxi; Peng, Yong; Wei, Hongxiang; Yang, Rong; Shi, Dongxia; Zhang, Xixiang; Han, Zheng; Zhang, Zhidong; Zhang, Guangyu; Yu, Guoqiang; Han, Xiufeng (Science advances, American Association for the Advancement of Science (AAAS), 2019-08-23) [Article]
    The recent discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials holds promises for spintronic devices with exceptional properties. However, to use 2D vdW magnets for building spintronic nanodevices such as magnetic memories, key challenges remain in terms of effectively switching the magnetization from one state to the other electrically. Here, we devise a bilayer structure of Fe3GeTe2/Pt, in which the magnetization of few-layered Fe3GeTe2 can be effectively switched by the spin-orbit torques (SOTs) originated from the current flowing in the Pt layer. The effective magnetic fields corresponding to the SOTs are further quantitatively characterized using harmonic measurements. Our demonstration of the SOT-driven magnetization switching in a 2D vdW magnet could pave the way for implementing low-dimensional materials in the next-generation spintronic applications.
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