| 1. |
- Bing, Zhao, 1990, et al.
(författare)
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Unconventional Charge–Spin Conversion in Weyl-Semimetal WTe2
- 2020
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 32:38
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Tidskriftsartikel (refereegranskat)abstract
- An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba–Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe2, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin–orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.
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| 2. |
- Dankert, André, 1986, et al.
(författare)
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Hall sensors batch-fabricated on all-CVD h-BN/graphene/h-BN heterostructures
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The two-dimensional (2D) material graphene is highly promising for Hall sensors due to its potential of having high charge carrier mobility and low carrier concentration at room temperature. Here, we report the scalable batch-fabrication of magnetic Hall sensors on graphene encapsulated in hexagonal boron nitride (h-BN) using commercially available large area CVD grown materials. The all-CVD grown h-BN/graphene/h-BN van der Waals heterostructures were prepared by layer transfer technique and Hall sensors were batch-fabricated with 1D edge metal contacts. The current-related Hall sensitivities up to 97 V/AT are measured at room temperature. The Hall sensors showed robust performance over the wafer scale with stable characteristics over six months in ambient environment. This work opens avenues for further development of growth and fabrication technologies of all-CVD 2D material heterostructures and allows further improvements in Hall sensor performance for practical applications.
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| 3. |
- Dankert, André, 1986, et al.
(författare)
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Origin and evolution of surface spin current in topological insulators
- 2018
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 97:12
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Tidskriftsartikel (refereegranskat)abstract
- The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezin g out the bulk conductivity in semiconducting Bi1.5Sb0.5Te1.7Se1.3 below 100K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2and100K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.
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| 4. |
- Hoque, Anamul Md, 1988, et al.
(författare)
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All-electrical creation and control of spin-galvanic signal in graphene and molybdenum ditelluride heterostructures at room temperature
- 2021
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- The ability to engineer new states of matter and control their spintronic properties by electric fields is at the heart of future information technology. Here, we report a gate-tunable spin-galvanic effect in van der Waals heterostructures of graphene with a semimetal of molybdenum ditelluride at room temperature due to an efficient spin-charge conversion process. Measurements in different device geometries with control over the spin orientations exhibit spin-switch and Hanle spin precession behavior, confirming the spin origin of the signal. The control experiments with the pristine graphene channels do not show any such signals. We explain the experimental spin-galvanic signals by theoretical calculations considering the spin-orbit induced spin-splitting in the bands of the graphene in the heterostructure. The calculations also reveal an unusual spin texture in graphene heterostructure with an anisotropic out-of-plane and in-plane spin polarization. These findings open opportunities to utilize graphene-based heterostructures for gate-controlled spintronic devices.
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| 5. |
- Hoque, Anamul Md, 1988, et al.
(författare)
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Charge-spin conversion in layered semimetal TaTe2 and spin injection in van der Waals heterostructures
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- A spin-polarized current source using nonmagnetic layered materials is promising for next-generation all-electrical spintronic science and technology. Here we electrically created spin polarization in a layered semimetal TaTe2 via the charge-spin conversion process. Using a hybrid device of TaTe2 in a van der Waals heterostructure with graphene, the spin polarization in TaTe2 is efficiently injected and detected by nonlocal spin-switch, Hanle spin precession, and inverse spin Hall effect measurements. Systematic experiments at different bias currents and gate voltages in a vertical geometry prove the TaTe2 as a nonmagnetic spin source at room temperature. These findings demonstrate the possibility of making an all-electrical spintronic device in a two-dimensional van der Waals heterostructure, which can be essential building blocks in energy-efficient spin-orbit technology.
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| 6. |
- Karpiak, Bogdan, 1992, et al.
(författare)
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1D ferromagnetic edge contacts to 2D graphene/h-BN heterostructures
- 2018
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:1, s. 014001-
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Tidskriftsartikel (refereegranskat)abstract
- We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a change in sign of the signal for the electron-and hole-doped regimes, which is in contrast to the expected behavior of the spin signal. Calculations show that the contact-induced fringe fields are typically on the order of hundreds of mT, but can be reduced below 100 mT with careful optimization of the contact geometry. There may be an additional contribution from magnetoresistance effects due to tunneling anisotropy in the contacts, which needs further investigation. These studies are useful for optimization of spin injection and detection in 2D material heterostructures through 1D edge contacts.
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| 7. |
- Karpiak, Bogdan, 1992, et al.
(författare)
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Gate-tunable Hall sensors on large area CVD graphene protected by h-BN with 1D edge contacts
- 2017
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 122:5
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Tidskriftsartikel (refereegranskat)abstract
- Graphene is an excellent material for Hall sensors due to its atomically thin structure, high carrier mobility, and low carrier density. However, graphene devices need to be protected from the environment for reliable and durable performance in different environmental conditions. Here we present magnetic Hall sensors fabricated on large area commercially available chemical vapor deposited (CVD) graphene protected by exfoliated hexagonal boron nitride (h-BN). To connect the graphene active regions of Hall samples to the outputs, 1D edge contacts were utilized which show reliable and stable electrical properties. The operation of the Hall sensors shows the current-related sensitivity up to 345 V/(AT). By changing the carrier concentration and type in graphene by the application of gate voltage, we are able to tune the Hall sensitivity.
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| 8. |
- Karpiak, Bogdan, 1992, et al.
(författare)
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Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene
- 2020
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices.
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| 9. |
- Karpiak, Bogdan, 1992
(författare)
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Spin and magneto transport in van der Waals heterostructures of graphene with ferromagnets
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increasing demand for information and communication technologies has augmented the requirements of electronic devices with improved speed, sensitivity, and reduced power consumption. The utilization of novel electronic materials and the use of the spin degree of freedom as a state variable for information processing and storage are expected to fulfill these demands. In this direction, two-dimensional (2D) materials have attracted a significant research effort with the long-term goal of creating electronic devices with novel functionalities. Graphene has shown excellent potential for future device applications due to its outstanding electronic carrier mobility and spin coherence time at room temperature. Followed by the successful advent of graphene, a vast plethora of 2D materials with complementary electronic properties have been discovered, such as insulating hexagonal boron nitride (hBN), magnets and topological semimetals. We observed that engineering 2D material heterostructures by combining the best of different materials in one ultimate unit offers the possibility of the creation of new phases of matter and novel opportunities in device design. For example, graphene is shown to acquire magnetic properties because of proximity-induced interactions with a magnetic insulator in van der Waals heterostructure. On the other hand, topological semimetal candidates such as WTe2 and ZeTe5 allowed us to observe unconventional charge-to-spin conversion and anomalous Hall effects due to their enormous spin-orbit coupling, lower crystal symmetry, and larger fictitious magnetic field in the crystals. Furthermore, the performance of heterostructures comprised of graphene and hBN with one-dimensional ferromagnetic edge contacts and a path for optimizing such device geometry is outlined. These experimental findings on 2D materials and heterostructure device architectures can contribute to developing a new platform for spintronic as well as quantum science and technology.
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| 10. |
- Khokhriakov, Dmitrii, 1991, et al.
(författare)
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Gate-tunable spin-galvanic effect in graphene-topological insulator van der Waals heterostructures at room temperature
- 2020
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate-tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a non-equilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin-galvanic signal. These findings provide an efficient route for realizing all-electrical and gate-tunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits.
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