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Shift charge and spin photocurrents in Dirac surface states of topological insulator
Kim, Kun Woo,Morimoto, Takahiro,Nagaosa, Naoto American Physical Society 2017 Physical Review B Vol.95 No.3
<P>The generation of photocurrent in condensed matter is of main interest for photovoltaic and optoelectronic applications. Shift current, a nonlinear photoresponse, has attracted recent intensive attention as a dominant player of bulk photovoltaic effect in ferroelectric materials. In three-dimensional topological insulators Bi2X3 (X = Te, Se), we find that Dirac surface states with a hexagonal warping term support shift current by linearly polarized light. Moreover, we study 'shift spin current' that arises in Dirac surface states by introducing time-reversal symmetry breaking perturbation. The estimate for the magnitudes of the shift charge and spin current densities are 0.13I(0) and 0.40I(0) (nA/m) for Bi2Te3 with the intensity of light I-0 measured in (W/m(2)), respectively, which can offer a useful method to generate these currents efficiently.</P>
Nonlinear spin current generation in noncentrosymmetric spin-orbit coupled systems
Hamamoto, Keita,Ezawa, Motohiko,Kim, Kun Woo,Morimoto, Takahiro,Nagaosa, Naoto American Physical Society 2017 Physical Review B Vol.95 No.22
<P>Spin current plays a central role in spintronics. In particular, finding more efficientways to generate spin current has been an important issue and has been studied actively. For example, representative methods of spin-current generation include spin-polarized current injections from ferromagnetic metals, the spin Hall effect, and the spin battery. Here, we theoretically propose a mechanism of spin-current generation based on nonlinear phenomena. By using Boltzmann transport theory, we showthat a simple application of the electric field E induces spin current proportional to E-2 in noncentrosymmetric spin-orbit coupled systems. We demonstrate that the nonlinear spin current of the proposed mechanism is supported in the surface state of three-dimensional topological insulators and two-dimensional semiconductors with the Rashba and/or Dresselhaus interaction. In the latter case, the angular dependence of the nonlinear spin current can be manipulated by the direction of the electric field and by the ratio of the Rashba and Dresselhaus interactions. We find that the magnitude of the spin current largely exceeds those in the previous methods for a reasonable magnitude of the electric field. Furthermore, we show that application of ac electric fields (e.g., terahertz light) leads to the rectifying effect of the spin current, where dc spin current is generated. These findings will pave a route to manipulate the spin current in noncentrosymmetric crystals.</P>
Real-space observation of a two-dimensional skyrmion crystal
Yu, X. Z.,Onose, Y.,Kanazawa, N.,Park, J. H.,Han, J. H.,Matsui, Y.,Nagaosa, N.,Tokura, Y. Macmillan Publishers Limited. All rights reserved 2010 Nature Vol.465 No.7300
Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin??electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T??B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe<SUB>1−x</SUB>Co<SUB>x</SUB>Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe<SUB>0.5</SUB>Co<SUB>0.5</SUB>Si using Lorentz transmission electron microscopy. With a magnetic field of 50??70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T??B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.
Quantum Dots Formed in Three-dimensional Dirac Semimetal Cd<sub>3</sub>As<sub>2</sub> Nanowires
Jung, Minkyung,Yoshida, Kenji,Park, Kidong,Zhang, Xiao-Xiao,Yesilyurt, Can,Siu, Zhuo Bin,Jalil, Mansoor B. A.,Park, Jinwan,Park, Jeunghee,Nagaosa, Naoto,Seo, Jungpil,Hirakawa, Kazuhiko American Chemical Society 2018 NANO LETTERS Vol.18 No.3
<P>We demonstrate quantum dot (QD) formation in three-dimensional Dirac semimetal Cd<SUB>3</SUB>As<SUB>2</SUB> nanowires using two electrostatically tuned p-n junctions with a gate and magnetic fields. The linear conductance measured as a function of gate voltage under high magnetic fields is strongly suppressed at the Dirac point close to zero conductance, showing strong conductance oscillations. Remarkably, in this regime, the Cd<SUB>3</SUB>As<SUB>2</SUB> nanowire device exhibits Coulomb diamond features, indicating that a clean single QD forms in the Dirac semimetal nanowire. Our results show that a p-type QD can be formed between two n-type leads underneath metal contacts in the nanowire by applying gate voltages under strong magnetic fields. Analysis of the quantum confinement in the gapless band structure confirms that p-n junctions formed between the p-type QD and two neighboring n-type leads under high magnetic fields behave as resistive tunnel barriers due to cyclotron motion, resulting in the suppression of Klein tunneling. The p-type QD with magnetic field-induced confinement shows a single hole filling. Our results will open up a route to quantum devices such as QDs or quantum point contacts based on Dirac and Weyl semimetals.</P> [FIG OMISSION]</BR>