Hyunsoo Yang
Degree: PhD
Position: Professor
Affiliation: NUS – Department of Electrical and Computer Engineering
Research Type: Experiment
Office: E5-03-14
Email: eleyang@nus.edu.sg
Contact: (65) 6516 7217
Website: https://www.yang-group.com/
Research Interests:
Spintronics
Carbon electronics
Topological Insulators
THz devices
Spin waves
CA2DM Publications:
2022 |
Wang, Yi; Yang, Hyunsoo Spin-Orbit Torques Based on Topological Materials Journal Article ACCOUNTS OF MATERIALS RESEARCH , 2022. @article{ISI:000854752300001, title = {Spin-Orbit Torques Based on Topological Materials }, author = {Yi Wang and Hyunsoo Yang}, doi = {10.1021/accountsmr.2c00123}, times_cited = {0}, year = {2022}, date = {2022-09-05}, journal = {ACCOUNTS OF MATERIALS RESEARCH }, publisher = {AMER CHEMICAL SOC }, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA }, abstract = {CONSPECTUS: Widespread applications of magnetic devices, such as magnetic random-access memory (MRAM), logic-in memory, and neuromorphic computing devices, require an efficient method to manipulate the local magnetization. One superior mechanism is the spin-orbit torque (SOT) associated with spin currents generated by charge currents in a material with strong spin-orbit coupling. A higher SOT efficiency (i.e., a higher charge-to-spin conversion efficiency) is crucial for low-power SOT device operation. Therefore, materials possessing the merit of a high SOT are urgently desired for the innovation of SOT devices. Among all of the SOT materials, topological materials such as topological insulators and topological semimetals have attracted considerable attention recently because of their nontrivial band structures with strong spin-orbit coupling and expected giant SOT. Therefore, topological materials are regarded as promising candidates for future energy-efficient device applications. However, research in the area of topological material-based spintronics is still in an early stage, and the related physical, material and device issues still need to be addressed. In this Account, we review our recent progress regarding the charge-to-spin conversion and SOT-driven magnetization switching using the emerging topological materials, with an emphasis on topological insulators and Weyl semimetals. First, we introduce the extraordinary physical features of single-layer topological insulators associated with electron transport, charge-spin interconversion, and unique spin textures and subsequently present our SOT results for topological materials mainly for Bi2Se3. This material shows pronounced SOT with the in-plane and out-of-plane component due to the spin momentum locking and hexagonal warping effect, respectively. The SOT can be efficiently modulated by varying the film crystal directions or by interface engineering between Bi2Se3 and a ferromagnet. Thereafter, room-temperature magnetization switching by the electron-mediated or magnon-mediated spin torque is demonstrated in topological insulator-based devices, and the switching current density J(C) is on the order of similar to 10(5) A/cm(2), which is approximately 1 to 2 orders of magnitude smaller than that in heavy metals. Second, we summarize our SOT results in Weyl semimetals. Due to the broken crystal symmetry, Weyl semimetals (e.g., WTe2) can show an additional sizable out-of-plane spin polarization, which can be detected by electrical and optical techniques. Therefore, the Weyl semimetals can possess unconventional out-of-plane damping-like SOT, which will facilitate the field-free magnetization switching. The interface Dzyaloshinskii-Moriya interaction (DMI) is observed in Weyl semimetal/ferromagnet heterostructures, which can affect the domain wall motion. We also demonstrate the energy-efficient SOT-induced magnetization switching in Weyl semimetal-based devices. Third, we discuss the advances in SOT devices with wafer-scale topological materials prepared by industry-compatible techniques such as magnetron sputtering and chemical vapor deposition. These films may have stoichiometry similar to that of single-crystalline topological materials and SOTs as large as in single-crystalline topological materials. Finally, we present our perspectives for practical applications using this emerging family of quantum materials. }, keywords = {}, pubstate = {published}, tppubtype = {article} } CONSPECTUS: Widespread applications of magnetic devices, such as magnetic random-access memory (MRAM), logic-in memory, and neuromorphic computing devices, require an efficient method to manipulate the local magnetization. One superior mechanism is the spin-orbit torque (SOT) associated with spin currents generated by charge currents in a material with strong spin-orbit coupling. A higher SOT efficiency (i.e., a higher charge-to-spin conversion efficiency) is crucial for low-power SOT device operation. Therefore, materials possessing the merit of a high SOT are urgently desired for the innovation of SOT devices. Among all of the SOT materials, topological materials such as topological insulators and topological semimetals have attracted considerable attention recently because of their nontrivial band structures with strong spin-orbit coupling and expected giant SOT. Therefore, topological materials are regarded as promising candidates for future energy-efficient device applications. However, research in the area of topological material-based spintronics is still in an early stage, and the related physical, material and device issues still need to be addressed. In this Account, we review our recent progress regarding the charge-to-spin conversion and SOT-driven magnetization switching using the emerging topological materials, with an emphasis on topological insulators and Weyl semimetals. First, we introduce the extraordinary physical features of single-layer topological insulators associated with electron transport, charge-spin interconversion, and unique spin textures and subsequently present our SOT results for topological materials mainly for Bi2Se3. This material shows pronounced SOT with the in-plane and out-of-plane component due to the spin momentum locking and hexagonal warping effect, respectively. The SOT can be efficiently modulated by varying the film crystal directions or by interface engineering between Bi2Se3 and a ferromagnet. Thereafter, room-temperature magnetization switching by the electron-mediated or magnon-mediated spin torque is demonstrated in topological insulator-based devices, and the switching current density J(C) is on the order of similar to 10(5) A/cm(2), which is approximately 1 to 2 orders of magnitude smaller than that in heavy metals. Second, we summarize our SOT results in Weyl semimetals. Due to the broken crystal symmetry, Weyl semimetals (e.g., WTe2) can show an additional sizable out-of-plane spin polarization, which can be detected by electrical and optical techniques. Therefore, the Weyl semimetals can possess unconventional out-of-plane damping-like SOT, which will facilitate the field-free magnetization switching. The interface Dzyaloshinskii-Moriya interaction (DMI) is observed in Weyl semimetal/ferromagnet heterostructures, which can affect the domain wall motion. We also demonstrate the energy-efficient SOT-induced magnetization switching in Weyl semimetal-based devices. Third, we discuss the advances in SOT devices with wafer-scale topological materials prepared by industry-compatible techniques such as magnetron sputtering and chemical vapor deposition. These films may have stoichiometry similar to that of single-crystalline topological materials and SOTs as large as in single-crystalline topological materials. Finally, we present our perspectives for practical applications using this emerging family of quantum materials. |
Yang, Hyunsoo; Valenzuela, Sergio O; Chshiev, Mairbek; Couet, Sébastien; Dieny, Bernard; Dlubak, Bruno; Fert, Albert; Garello, Kevin; Jamet, Matthieu; Jeong, Dae-Eun; Lee, Kangho; Lee, Taeyoung; Martin, Marie-Blandine; Kar, Gouri Sankar; Sénéor, Pierre; Shin, Hyeon-Jin; Roche, Stephan Two-dimensional materials prospects for non-volatile spintronic memories Journal Article NATURE, 606 , pp. 663–673, 2022, ISSN: 1476-4687. @article{ISI:000739656004687, title = {Two-dimensional materials prospects for non-volatile spintronic memories}, author = {Hyunsoo Yang and Sergio O. Valenzuela and Mairbek Chshiev and Sébastien Couet and Bernard Dieny and Bruno Dlubak and Albert Fert and Kevin Garello and Matthieu Jamet and Dae-Eun Jeong and Kangho Lee and Taeyoung Lee and Marie-Blandine Martin and Gouri Sankar Kar and Pierre Sénéor and Hyeon-Jin Shin and Stephan Roche}, doi = {10.1038/s41586-022-04768-0}, times_cited = {0}, issn = {1476-4687}, year = {2022}, date = {2022-06-22}, journal = {NATURE}, volume = {606}, pages = {663–673}, abstract = {Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes. |
He, Pan; Koon, Gavin Kok Wai; Isobe, Hiroki; Tan, Jun You; Hu, Junxiong; Neto, Antonio Castro H; Fu, Liang; Yang, Hyunsoo Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons Journal Article NATURE NANOTECHNOLOGY , 2022, ISSN: 1748-3387. @article{ISI:000751676800001, title = {Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons }, author = {Pan He and Gavin Kok Wai Koon and Hiroki Isobe and Jun You Tan and Junxiong Hu and Antonio Castro H Neto and Liang Fu and Hyunsoo Yang}, doi = {10.1038/s41565-021-01060-6}, times_cited = {0}, issn = {1748-3387}, year = {2022}, date = {2022-02-03}, journal = {NATURE NANOTECHNOLOGY }, publisher = {NATURE PORTFOLIO }, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY }, abstract = {Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification. |
He, Pan; Koon, Gavin Kok Wai; Isobe, Hiroki; Tan, Jun You; Hu, Junxiong; Neto, Antonio Castro H; Fu, Liang; Yang, Hyunsoo Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons Journal Article NATURE NANOTECHNOLOGY, 17 (4), pp. 378-+, 2022, ISSN: 1748-3387. @article{ISI:000751676800001, title = {Graphene moire superlattices with giant quantum nonlinearity of chiral Bloch electrons}, author = {Pan He and Gavin Kok Wai Koon and Hiroki Isobe and Jun You Tan and Junxiong Hu and Antonio Castro H Neto and Liang Fu and Hyunsoo Yang}, doi = {10.1038/s41565-021-01060-6}, times_cited = {0}, issn = {1748-3387}, year = {2022}, date = {2022-02-03}, journal = {NATURE NANOTECHNOLOGY}, volume = {17}, number = {4}, pages = {378-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moire superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moire bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification. |
2021 |
Shi, Shuyuan; Li, Jie; Hsu, Chuang-Han; Lee, Kyusup; Wang, Yi; Yang, Li; Wang, Junyong; Wang, Qisheng; Wu, Hao; Zhang, Wenfeng; Eda, Goki; Liang, Gengchiau; Chang, Haixin; Yang, Hyunsoo Observation of the Out-of-Plane Polarized Spin Current from CVD Grown WTe2 Journal Article ADVANCED QUANTUM TECHNOLOGIES, 4 (8), 2021. @article{ISI:000663272500001, title = {Observation of the Out-of-Plane Polarized Spin Current from CVD Grown WTe_{2}}, author = {Shuyuan Shi and Jie Li and Chuang-Han Hsu and Kyusup Lee and Yi Wang and Li Yang and Junyong Wang and Qisheng Wang and Hao Wu and Wenfeng Zhang and Goki Eda and Gengchiau Liang and Haixin Chang and Hyunsoo Yang}, doi = {10.1002/qute.202100038}, times_cited = {0}, year = {2021}, date = {2021-06-19}, journal = {ADVANCED QUANTUM TECHNOLOGIES}, volume = {4}, number = {8}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Weyl semimetal Td-phase WTe2 possesses the spin-resolved band structure with strong spin-orbit coupling, holding promises as a useful spin source material. The noncentrosymmetric crystalline structure of Td-WTe2 endows the generation of the out-of-plane polarized spin, which is of great interest in magnetic memory applications. Previously, WTe2 was explored in spin devices based on mechanically exfoliated single crystal flakes with a size of micrometers. For practical spintronics applications, it is highly desirable to implement wafer-scale thin films. In this work, centimeter-scale chemical vapor deposition (CVD) grown Td-WTe2 thin films are used and the spin current generation is studied by the spin torque ferromagnetic resonance (ST-FMR) technique. The in-plane and out-of-plane spin conductivities of 7.36 x 10(3) (PLANCK CONSTANT OVER TWO PI/2e) (ohm m)(-1) and 1.76 x 10(3) (PLANCK CONSTANT OVER TWO PI/2e) (ohm m)(-1), respectively, are found in CVD-growth 5 nm-WTe2. The current-induced magnetization switching in WTe2/NiFe is demonstrated at room temperature in the domain wall motion regime, which may invigorate potential spintronic device innovations based on Weyl semimetals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Weyl semimetal Td-phase WTe2 possesses the spin-resolved band structure with strong spin-orbit coupling, holding promises as a useful spin source material. The noncentrosymmetric crystalline structure of Td-WTe2 endows the generation of the out-of-plane polarized spin, which is of great interest in magnetic memory applications. Previously, WTe2 was explored in spin devices based on mechanically exfoliated single crystal flakes with a size of micrometers. For practical spintronics applications, it is highly desirable to implement wafer-scale thin films. In this work, centimeter-scale chemical vapor deposition (CVD) grown Td-WTe2 thin films are used and the spin current generation is studied by the spin torque ferromagnetic resonance (ST-FMR) technique. The in-plane and out-of-plane spin conductivities of 7.36 x 10(3) (PLANCK CONSTANT OVER TWO PI/2e) (ohm m)(-1) and 1.76 x 10(3) (PLANCK CONSTANT OVER TWO PI/2e) (ohm m)(-1), respectively, are found in CVD-growth 5 nm-WTe2. The current-induced magnetization switching in WTe2/NiFe is demonstrated at room temperature in the domain wall motion regime, which may invigorate potential spintronic device innovations based on Weyl semimetals. |
Kumar, Dushyant; Hsu, Chuang-Han; Sharma, Raghav; Chang, Tay-Rong; Yu, Peng; Wang, Junyong; Eda, Goki; Liang, Gengchiau; Yang, Hyunsoo Room-temperature nonlinear Hall effect and wireless radiofrequency rectification in Weyl semimetal TaIrTe4 Journal Article NATURE NANOTECHNOLOGY, 16 (4), pp. 421-+, 2021, ISSN: 1748-3387. @article{ISI:000611456700002, title = {Room-temperature nonlinear Hall effect and wireless radiofrequency rectification in Weyl semimetal TaIrTe_{4}}, author = {Dushyant Kumar and Chuang-Han Hsu and Raghav Sharma and Tay-Rong Chang and Peng Yu and Junyong Wang and Goki Eda and Gengchiau Liang and Hyunsoo Yang}, doi = {10.1038/s41565-020-00839-3}, times_cited = {0}, issn = {1748-3387}, year = {2021}, date = {2021-01-25}, journal = {NATURE NANOTECHNOLOGY}, volume = {16}, number = {4}, pages = {421-+}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling(1,2). However, the low-temperature detection of the NLHE limits its applications(3,4). Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling(1,2). However, the low-temperature detection of the NLHE limits its applications(3,4). Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals. |
2020 |
Liang, Shiheng; Shi, Shuyuan; Hsu, Chuang-Han; Cai, Kaiming; Wang, Yi; He, Pan; Wu, Yang; Pereira, Vitor M; Yang, Hyunsoo Spin-Orbit Torque Magnetization Switching in MoTe2/Permalloy Heterostructures Journal Article ADVANCED MATERIALS, 32 (37), 2020, ISSN: 0935-9648. @article{ISI:000554472200001, title = {Spin-Orbit Torque Magnetization Switching in MoTe_{2}/Permalloy Heterostructures}, author = {Shiheng Liang and Shuyuan Shi and Chuang-Han Hsu and Kaiming Cai and Yi Wang and Pan He and Yang Wu and Vitor M Pereira and Hyunsoo Yang}, doi = {10.1002/adma.202002799}, times_cited = {8}, issn = {0935-9648}, year = {2020}, date = {2020-08-02}, journal = {ADVANCED MATERIALS}, volume = {32}, number = {37}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path toward high-performance, nonvolatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires the harnessing of both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T '-MoTe(2)adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness-dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe(2)for low-power semimetal-material-based spin devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path toward high-performance, nonvolatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires the harnessing of both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T '-MoTe(2)adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness-dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe(2)for low-power semimetal-material-based spin devices. |
2019 |
Shi, Shuyuan; Liang, Shiheng; Zhu, Zhifeng; Cai, Kaiming; Pollard, Shawn D; Wang, Yi; Wang, Junyong; Wang, Qisheng; He, Pan; Yu, Jiawei; Eda, Goki; Liang, Gengchiau; Yang, Hyunsoo All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures Journal Article NATURE NANOTECHNOLOGY, 14 (10), pp. 945-+, 2019, ISSN: 1748-3387. @article{ISI:000488977100013, title = {All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe_{2}/ferromagnet heterostructures}, author = {Shuyuan Shi and Shiheng Liang and Zhifeng Zhu and Kaiming Cai and Shawn D Pollard and Yi Wang and Junyong Wang and Qisheng Wang and Pan He and Jiawei Yu and Goki Eda and Gengchiau Liang and Hyunsoo Yang}, doi = {10.1038/s41565-019-0525-8}, times_cited = {0}, issn = {1748-3387}, year = {2019}, date = {2019-10-01}, journal = {NATURE NANOTECHNOLOGY}, volume = {14}, number = {10}, pages = {945-+}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals(1-3) or topological insulators(4,5) provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure(6,7); the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation(8). The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling(6,9) and non-trivial band structures(10) with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states(9-11). Atomically flat surfaces, which can be produced with high quality(12), facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of similar to 2.96 x 10(5) A cm(-2) suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 +/- 0.06 mJ m(-2) induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.}, keywords = {}, pubstate = {published}, tppubtype = {article} } All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals(1-3) or topological insulators(4,5) provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure(6,7); the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation(8). The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling(6,9) and non-trivial band structures(10) with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states(9-11). Atomically flat surfaces, which can be produced with high quality(12), facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of similar to 2.96 x 10(5) A cm(-2) suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 +/- 0.06 mJ m(-2) induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures. |
He, Pan; Hsu, Chuang-Han; Shi, Shuyuan; Cai, Kaiming; Wang, Junyong; Wang, Qisheng; Eda, Goki; Lin, Hsin; Pereira, Vitor M; Yang, Hyunsoo Nonlinear magnetotransport shaped by Fermi surface topology and convexity Journal Article NATURE COMMUNICATIONS, 10 , 2019, ISSN: 2041-1723. @article{ISI:000461757900007, title = {Nonlinear magnetotransport shaped by Fermi surface topology and convexity}, author = {Pan He and Chuang-Han Hsu and Shuyuan Shi and Kaiming Cai and Junyong Wang and Qisheng Wang and Goki Eda and Hsin Lin and Vitor M Pereira and Hyunsoo Yang}, doi = {10.1038/s41467-019-09208-8}, times_cited = {0}, issn = {2041-1723}, year = {2019}, date = {2019-03-20}, journal = {NATURE COMMUNICATIONS}, volume = {10}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The nature of Fermi surface defines the physical properties of conductors and many physical phenomena can be traced to its shape. Although the recent discovery of a current-dependent nonlinear magnetoresistance in spin-polarized non-magnetic materials has attracted considerable attention in spintronics, correlations between this phenomenon and the underlying fermiology remain unexplored. Here, we report the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe2, with an interesting temperature-driven inversion. Theoretical calculations reproduce the nonlinear transport measurements and allow us to attribute the inversion to temperature-induced changes in Fermi surface convexity. We also report a large anisotropy of nonlinear magnetoresistance in WTe2, due to its low symmetry of Fermi surfaces. The good agreement between experiments and theoretical modeling reveals the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. These results lay a new path to explore ramifications of distinct fermiology for nonlinear transport in condensed-matter.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The nature of Fermi surface defines the physical properties of conductors and many physical phenomena can be traced to its shape. Although the recent discovery of a current-dependent nonlinear magnetoresistance in spin-polarized non-magnetic materials has attracted considerable attention in spintronics, correlations between this phenomenon and the underlying fermiology remain unexplored. Here, we report the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe2, with an interesting temperature-driven inversion. Theoretical calculations reproduce the nonlinear transport measurements and allow us to attribute the inversion to temperature-induced changes in Fermi surface convexity. We also report a large anisotropy of nonlinear magnetoresistance in WTe2, due to its low symmetry of Fermi surfaces. The good agreement between experiments and theoretical modeling reveals the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. These results lay a new path to explore ramifications of distinct fermiology for nonlinear transport in condensed-matter. |
2018 |
Wang, Qisheng; Li, Jie; Besbas, Jean; Hsu, Chuang-Han; Cai, Kaiming; Yang, Li; Cheng, Shuai; Wu, Yang; Zhang, Wenfeng; Wang, Kaiyou; Chang, Tay-Rong; Lin, Hsin; Chang, Haixin; Yang, Hyunsoo Room-Temperature Nanoseconds Spin Relaxation in WTe2 and MoTe2 Thin Films Journal Article ADVANCED SCIENCE, 5 (6), 2018, ISSN: 2198-3844. @article{ISI:000435765900023, title = {Room-Temperature Nanoseconds Spin Relaxation in WTe_{2} and MoTe_{2} Thin Films}, author = {Qisheng Wang and Jie Li and Jean Besbas and Chuang-Han Hsu and Kaiming Cai and Li Yang and Shuai Cheng and Yang Wu and Wenfeng Zhang and Kaiyou Wang and Tay-Rong Chang and Hsin Lin and Haixin Chang and Hyunsoo Yang}, doi = {10.1002/advs.201700912}, times_cited = {0}, issn = {2198-3844}, year = {2018}, date = {2018-06-01}, journal = {ADVANCED SCIENCE}, volume = {5}, number = {6}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin-polarized states and can carry a very large current density. In addition, the intrinsic non-centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room-temperature spin relaxation time of 1.2ns (0.4ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate, and suppression of backscattering required by time-reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin-polarized states and can carry a very large current density. In addition, the intrinsic non-centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room-temperature spin relaxation time of 1.2ns (0.4ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate, and suppression of backscattering required by time-reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature. |
2017 |
Wang, Yi; Zhu, Dapeng; Wu, Yang; Yang, Yumeng; Yu, Jiawei; Ramaswamy, Rajagopalan; Mishra, Rahul; Shi, Shuyuan; Elyasi, Mehrdad; Teo, Kie-Leong; Wu, Yihong; Yang, Hyunsoo Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques Journal Article NATURE COMMUNICATIONS, 8 , 2017, ISSN: 2041-1723. @article{ISI:000414662500005, title = {Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques}, author = {Yi Wang and Dapeng Zhu and Yang Wu and Yumeng Yang and Jiawei Yu and Rajagopalan Ramaswamy and Rahul Mishra and Shuyuan Shi and Mehrdad Elyasi and Kie-Leong Teo and Yihong Wu and Hyunsoo Yang}, doi = {10.1038/s41467-017-01583-4}, times_cited = {0}, issn = {2041-1723}, year = {2017}, date = {2017-11-08}, journal = {NATURE COMMUNICATIONS}, volume = {8}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Topological insulators with spin-momentum-locked topological surface states are expected to exhibit a giant spin-orbit torque in the topological insulator/ferromagnet systems. To date, the topological insulator spin-orbit torque-driven magnetization switching is solely reported in a Cr-doped topological insulator at 1.9 K. Here we directly show giant spin-orbit torque-driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge-to-spin conversion efficiency of similar to 1-1.75 in the thin Bi2Se3 films, where the topological surface states are dominant. In addition, we find the current density required for the magnetization switching is extremely low, similar to 6 x 10(5) A cm(-2), which is one to two orders of magnitude smaller than that with heavy metals. Our demonstration of room temperature magnetization switching of a conventional 3d ferromagnet using Bi2Se3 may lead to potential innovations in topological insulator-based spintronic applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Topological insulators with spin-momentum-locked topological surface states are expected to exhibit a giant spin-orbit torque in the topological insulator/ferromagnet systems. To date, the topological insulator spin-orbit torque-driven magnetization switching is solely reported in a Cr-doped topological insulator at 1.9 K. Here we directly show giant spin-orbit torque-driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge-to-spin conversion efficiency of similar to 1-1.75 in the thin Bi2Se3 films, where the topological surface states are dominant. In addition, we find the current density required for the magnetization switching is extremely low, similar to 6 x 10(5) A cm(-2), which is one to two orders of magnitude smaller than that with heavy metals. Our demonstration of room temperature magnetization switching of a conventional 3d ferromagnet using Bi2Se3 may lead to potential innovations in topological insulator-based spintronic applications. |
Wu, Yang; Qiu, Xuepeng; Liu, Hongwei; Liu, Jingbo; Chen, Yuanfu; Ke, Lin; Yang, Hyunsoo Tunable terahertz reflection of graphene via ionic liquid gating Journal Article NANOTECHNOLOGY, 28 (9), 2017, ISSN: 0957-4484. @article{ISI:000395071700001, title = {Tunable terahertz reflection of graphene via ionic liquid gating}, author = {Yang Wu and Xuepeng Qiu and Hongwei Liu and Jingbo Liu and Yuanfu Chen and Lin Ke and Hyunsoo Yang}, doi = {10.1088/1361-6528/aa57ad}, times_cited = {0}, issn = {0957-4484}, year = {2017}, date = {2017-03-03}, journal = {NANOTECHNOLOGY}, volume = {28}, number = {9}, publisher = {IOP PUBLISHING LTD}, address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND}, abstract = {We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to +/- 3V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1-1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to +/- 3V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1-1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components. |
2016 |
Wu, Yang; Niu, Jing; Danesh, Mohammad; Liu, Jingbo; Chen, Yuanfu; Ke, Lin; Qiu, Chengwei; Yang, Hyunsoo Localized surface plasmon resonance in graphene nanomesh with Au nanostructures Journal Article APPLIED PHYSICS LETTERS, 109 (4), 2016, ISSN: 0003-6951. @article{ISI:000381688900006, title = {Localized surface plasmon resonance in graphene nanomesh with Au nanostructures}, author = {Yang Wu and Jing Niu and Mohammad Danesh and Jingbo Liu and Yuanfu Chen and Lin Ke and Chengwei Qiu and Hyunsoo Yang}, doi = {10.1063/1.4959833}, times_cited = {0}, issn = {0003-6951}, year = {2016}, date = {2016-07-25}, journal = {APPLIED PHYSICS LETTERS}, volume = {109}, number = {4}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {A hybrid structure of a graphene nanomesh with the gold nanodisks is studied to enhance the light absorption by the localized surface plasmon resonance. From the reflection spectra of the visible range for graphene nanomesh samples without and with nanodisks, it is found that the absorption of graphene nanomesh structures is greatly enhanced in the presence of gold nanodisks around the resonance wavelength. Simulation results based on the finite-difference time-domain method support the experimental observations. This study demonstrates the potential of constructing graphene based photodetectors with a high light absorption efficiency and wavelength selectivity. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A hybrid structure of a graphene nanomesh with the gold nanodisks is studied to enhance the light absorption by the localized surface plasmon resonance. From the reflection spectra of the visible range for graphene nanomesh samples without and with nanodisks, it is found that the absorption of graphene nanomesh structures is greatly enhanced in the presence of gold nanodisks around the resonance wavelength. Simulation results based on the finite-difference time-domain method support the experimental observations. This study demonstrates the potential of constructing graphene based photodetectors with a high light absorption efficiency and wavelength selectivity. Published by AIP Publishing. |
2015 |
Gopinadhan, Kalon; Shin, Young Jun; Jalil, Rashid; Venkatesan, Thirumalai; Geim, Andre K; Neto, Antonio Castro H; Yang, Hyunsoo Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures Journal Article NATURE COMMUNICATIONS, 6 , 2015, ISSN: 2041-1723. @article{ISI:000363021700001, title = {Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures}, author = {Kalon Gopinadhan and Young Jun Shin and Rashid Jalil and Thirumalai Venkatesan and Andre K Geim and Antonio Castro H Neto and Hyunsoo Yang}, doi = {10.1038/ncomms9337}, times_cited = {0}, issn = {2041-1723}, year = {2015}, date = {2015-09-01}, journal = {NATURE COMMUNICATIONS}, volume = {6}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Understanding magnetoresistance, the change in electrical resistance under an external magnetic field, at the atomic level is of great interest both fundamentally and technologically. Graphene and other two-dimensional layered materials provide an unprecedented opportunity to explore magnetoresistance at its nascent stage of structural formation. Here we report an extremely large local magnetoresistance of similar to 2,000% at 400 K and a non-local magnetoresistance of 490,000% in an applied magnetic field of 9 T at 300 K in few-layer graphene/boron-nitride heterostructures. The local magnetoresistance is understood to arise from large differential transport parameters, such as the carrier mobility, across various layers of few-layer graphene upon a normal magnetic field, whereas the non-local magnetoresistance is due to the magnetic field induced Ettingshausen-Nernst effect. Non-local magnetoresistance suggests the possibility of a graphene-based gate tunable thermal switch. In addition, our results demonstrate that graphene heterostructures may be promising for magnetic field sensing applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Understanding magnetoresistance, the change in electrical resistance under an external magnetic field, at the atomic level is of great interest both fundamentally and technologically. Graphene and other two-dimensional layered materials provide an unprecedented opportunity to explore magnetoresistance at its nascent stage of structural formation. Here we report an extremely large local magnetoresistance of similar to 2,000% at 400 K and a non-local magnetoresistance of 490,000% in an applied magnetic field of 9 T at 300 K in few-layer graphene/boron-nitride heterostructures. The local magnetoresistance is understood to arise from large differential transport parameters, such as the carrier mobility, across various layers of few-layer graphene upon a normal magnetic field, whereas the non-local magnetoresistance is due to the magnetic field induced Ettingshausen-Nernst effect. Non-local magnetoresistance suggests the possibility of a graphene-based gate tunable thermal switch. In addition, our results demonstrate that graphene heterostructures may be promising for magnetic field sensing applications. |
Wu, Yang; La-o-Vorakiat, Chan; Qiu, Xuepeng; Liu, Jingbo; Deorani, Praveen; Banerjee, Karan; Son, Jaesung; Chen, Yuanfu; Chia, Elbert E M; Yang, Hyunsoo Graphene Terahertz Modulators by Ionic Liquid Gating Journal Article ADVANCED MATERIALS, 27 (11), pp. 1874-+, 2015, ISSN: 0935-9648. @article{ISI:000351216500008, title = {Graphene Terahertz Modulators by Ionic Liquid Gating}, author = {Yang Wu and Chan La-o-Vorakiat and Xuepeng Qiu and Jingbo Liu and Praveen Deorani and Karan Banerjee and Jaesung Son and Yuanfu Chen and Elbert E M Chia and Hyunsoo Yang}, doi = {10.1002/adma.201405251}, times_cited = {0}, issn = {0935-9648}, year = {2015}, date = {2015-03-18}, journal = {ADVANCED MATERIALS}, volume = {27}, number = {11}, pages = {1874-+}, publisher = {WILEY-V C H VERLAG GMBH}, address = {BOSCHSTRASSE 12, D-69469 WEINHEIM, GERMANY}, abstract = {Excellent-performance terahertz (THz) modulators based on graphene/ionic liquid/graphene sandwich structures are demonstrated. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with a modulation depth of up to 99% by applying a small gate voltage of 3 V. The outstanding performance of the proposed devices is due to the conical band structure of the graphene and the powerful gating effect of the ionic liquid in proximity to the graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Excellent-performance terahertz (THz) modulators based on graphene/ionic liquid/graphene sandwich structures are demonstrated. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with a modulation depth of up to 99% by applying a small gate voltage of 3 V. The outstanding performance of the proposed devices is due to the conical band structure of the graphene and the powerful gating effect of the ionic liquid in proximity to the graphene. |