Fermi-level-dependent charge-to-spin current conversion by Dirac surface states of topological insulators

Kondou, Kouta; Yoshimi, Ryutaro; Tsukazaki, Atsushi; Fukuma, Yasuhiro; Matsuno, Jobu; Takahashi, Kei; Kawasaki, Masashi; Tokura, Y.; Otani, Y.

doi:10.1038/nphys3833

Cited by 369 publications

(368 citation statements)

References 34 publications

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“…The charge-to-spin conversion efficiency (q ICS ) in Fig. 6d is defined as the ratio of the spin-current density across the interface to the charge current density along the interface 101 . A large q ICS is observed as long as E F is located within the bulk bandgap, as seen in Fig.…”

Section: Topological Electronicsmentioning

confidence: 99%

Emergent functions of quantum materials

2017

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Materials can harbour quantum many-body systems, most typically in the form of strongly correlated electrons in solids, that lead to novel and remarkable functions thanks to emergence-collective behaviours that arise from strong interactions among the elements. These include the Mott transition, high-temperature superconductivity, topological superconductivity, colossal magnetoresistance, giant magnetoelectric e ect, and topological insulators. These phenomena will probably be crucial for developing the next-generation quantum technologies that will meet the urgent technological demands for achieving a sustainable and safe society. Dissipationless electronics using topological currents and quantum spins, energy harvesting such as photovoltaics and thermoelectrics, and secure quantum computing and communication are the three major fields of applications working towards this goal. Here, we review the basic principles and the current status of the emergent phenomena and functions in materials from the viewpoint of strong correlation and topology.E mergence is a concept developed for many-body systems, indicating the properties, phenomena, and functions that never appear in the individual elements but are realized only when a huge number of elements get together 1 . Inside materials, emergent phenomena are often seen due to the interaction of electronic states; with recent developments on electronic states in solids schematically summarized in Fig. 1a. Electrons in solids have several degrees of freedom: charge, spin and orbital, and are characterized by the topological nature determined by the atomic potential on the crystal lattice structure, as schematically shown in Fig. 1b. These five attributes are coupled together and determine the overall responses to stimuli, which appear as materials' electrical, magnetic, optical, thermal, and mechanical properties.These collective electrons demonstrate various macroscopic quantum phenomena, the representative example of which is superconductivity. One of the big challenges in condensed matter physics is to increase the temperature (T c ) at which superconductivity can be observed to above room temperature. However, there are many other macroscopic quantum phenomena that are already observable above room temperature. One is magnetism (by the Bohr-van Leeuwen theorem), and the other is ferroelectricity 2,3 . Remarkably, there are common features of these three macroscopic quantum phenomena: the order parameter, which is of quantum origin, behaves as a classical quantity, and the quantum topology plays a crucial role.The topological nature of the electronic states is the key concept in the most recent developments in understanding quantum materials. The quantum Hall effect, topological insulators, and topological superconductors are all characterized by nontrivial topologies in Hilbert space. The Berry phase, which describes the connection and curvature of the subspace of Hilbert space, plays the central role in the unified principle to describe this topological nature 4 . ...

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Section: Topological Electronicsmentioning

confidence: 99%

Emergent functions of quantum materials

2017

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“…Since the two branches of Rashba splitting bands are wrapped by TSS linear bands, it is generally assumed that the Rashba bands play a less role in spin-current generation than TSS [24]. In addition, the effective spin polarization of the Rashba splitting bands was found to be opposite to that of TSS, which may cause a partial cancellation of the spin polarizations from TSS and thus a reduction of SOT from TI surfaces [25,26]. However, recent ARPES measurements reveal that the strength of Rashba splitting on the three-dimensional (3D) TI, Bi2Se3, surface can be tuned by involving adsorbates, such as Pd and Cs [19,21].…”

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confidence: 99%

“…Recently, Rashba induced charge-spin interconversions, with a higher efficiency than that from SHE, have been detected in Bi/Ag, α-Sn/Ag [7][8][9][10], LaAlO3/SrTiO3 [11]. On the other hand, the topological surface states (TSS) of topological insulators (TIs) are another interfacial mechanism for achieving efficient charge-to-spin conversion due to the strong spin orbit coupling (SOC) and inherent spinmomentum locking, despite of the inevitable bulk transport [3,4,[12][13][14][15][16][17]. Therefore, understanding and optimization of the interfacial effects is of significance and great interest for efficient spincurrent generation in SOT based spintronics devices.…”

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confidence: 99%

“…For the samples with tAg > 0 nm, the interfacial Rashba effect comes into play and, in literature, the spin polarization of the Rashba effect on TI was found to be generally opposite to that of the TSS [25,26]. However, from our measurements, it is clear that accumulated spins due to the Bi2Se3/Ag interface effect and Bi2Se3 TSS are in the same direction, as the spin accumulation is greatly enhanced in the Bi2Se3/Ag/CoFeB samples compared with the Bi2Se3/CoFeB sample.…”

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Efficient charge-spin conversion and magnetization switching through the Rashba effect at topological-insulator/Ag interfaces

Shi

Wang

et al. 2018

Phys. Rev. B

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We report the observation of efficient charge-to-spin conversion in the three-dimensional topological insulator (TI) Bi2Se3 and Ag bilayer by the spin-torque ferromagnetic resonance technique. The spin orbit torque ratio in the Bi2Se3/Ag/CoFeB heterostructure shows a significant enhancement as the Ag thickness increases to ~2 nm and reaches a value of 0.5 for 5 nm Ag, which is ~3 times higher than that of Bi2Se3/CoFeB at room temperature. The observation reveals the interfacial effect of Bi2Se3/Ag exceeds that of the topological surface states (TSS) in the Bi2Se3 layer and plays a dominant role in the charge-to-spin conversion in the Bi2Se3/Ag/CoFeB system. Based on the first-principles calculations, we attribute our observation to the large Rashba-splitting bands which wrap the TSS band and has the same net spin polarization direction as TSS of Bi2Se3.Subsequently, we demonstrate for the first time the Rashba induced magnetization switching in Bi2Se3/Ag/Py with a low current density of 5 5.8 10  A/cm 2 .

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“…[7][8][9][10][11][12][13][14][15] In Bi 2 Se 3 systems, bulk-insulating samples are obtained by chemical substitution, e.g. Sb substitution in Bi sites and Te substitution in Se sites.…”

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Linear magnetoresistance in a topological insulator Ru2Sn3

Shiomi

Saitoh

2017

AIP Advances

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We have studied magnetotransport properties of a topological insulator material Ru2Sn3. Bulk single crystals of Ru2Sn3 were grown by a Bi flux method. The resistivity is semiconducting at high temperatures above 160 K, while it becomes metallic below 160 K. Nonlinear field dependence of Hall resistivity in the metallic region shows conduction of multiple carriers at low temperatures. In the high-temperature semiconducting region, magnetoresistance exhibits a conventional quadratic magnetic-field dependence. In the low-temperature metallic region, however, high-field magnetoresistance is clearly linear with magnetic fields, signaling a linear dispersion in the low-temperature electronic structure. Small changes in the magnetoresistance magnitude with respect to the magnetic field angle indicate that bulk electron carriers are responsible mainly for the observed linear magnetoresistance.

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Fermi-level-dependent charge-to-spin current conversion by Dirac surface states of topological insulators

Cited by 369 publications

References 34 publications

Emergent functions of quantum materials

Emergent functions of quantum materials

Efficient charge-spin conversion and magnetization switching through the Rashba effect at topological-insulator/Ag interfaces

Linear magnetoresistance in a topological insulator Ru2Sn3

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