Showing papers by "Saburo Takahashi published in 2019"

Spin transport in insulators without exchange stiffness.

Abstract: The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator Gd3Ga5O12 (GGG), a standard substrate for growing magnetic films, can be a spin current generator, but has never been considered as a superior conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, spin transport persists up to temperatures of 100 K $$\gg$$ Tg = 180 mK, the magnetic glass-like transition temperature of GGG. At 5 K and 3.5 T, we find a spin diffusion length λGGG = 1.8 ± 0.2 μm and a spin conductivity σGGG = (7.3 ± 0.3) × 104 Sm−1 that is larger than that of the record quality magnet Y3Fe5O12 (YIG). We conclude that exchange stiffness is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices. Long-range spin transport is essential for spintronics applications, but so far has only been achieved in magnets below their Curie temperature. Here, the authors report on efficient spin transport in paramagnetic insulator Gd3Ga5O12 exposed to a moderate magnetic field exhibiting a spin diffusion length of 1.8 μm.

Spin pumping from nuclear spin waves

Abstract: Various spintronic phenomena originate from the exchange of angular momentum between the spin of electrons and other degrees of freedom in crystalline materials. Many degrees of freedom, such as magnetization1 and mechanical motion2, have already been united into this exchange framework. However, the nuclear spin—a key angular momentum—has yet to be incorporated. Here we observe spin pumping from nuclear magnetic resonance (NMR), in which nuclear spin dynamics emits a spin current, a flow of spin angular momentum of electrons. By using the canted antiferromagnet MnCO3, in which typical nuclear spin-wave formation is established due to the reinforced hyperfine coupling, we find that a spin current is generated from an NMR. Nuclear spins are indispensable for quantum information technology3 and are also frequently used in various sensors, such as in magnetic resonance imaging4. The observed NMR spin pumping allows spin-current generation from nuclei and will enable spintronic detection of nuclear spin states. Spin current is generated by pumping from nuclear spin waves. The nuclear magnetic resonance is used to transfer angular momentum from the nuclei of an antiferromagnet to a propagating spin current that is subsequently collected in a distant electrode.

Vortex rectenna powered by environmental fluctuations

Abstract: A rectenna, standing for a rectifying antenna, is an apparatus which generates d.c. electricity from electric fluctuations. It is expected to realize wireless power transmission as well as energy harvesting from environmental radio waves. To realize such rectification, devices that are made up of internal atomic asymmetry such as an asymmetric junction have been necessary so far. Here we report a material that spontaneously generates electricity by rectifying environmental fluctuations without using atomic asymmetry. The sample is a common superconductor without lowered crystalline symmetry, but, just by putting it in an asymmetric magnetic environment, it turns into a rectifier and starts generating electricity. Superconducting vortex strings only annihilate and nucleate at surfaces, and this allows the bulk electrons to feel surface fluctuations in an asymmetric environment: a vortex rectenna. The rectification and generation can be switched on and off with only a slight change in temperature or external magnetic fields.

Transport properties on an ionically disordered surface of topological insulators: Toward high-performance thermoelectrics

Abstract: Motivated by recent experiments that report a strong energy dependence of the Seebeck coefficient S, we study theoretically thermoelectric (TE) transport on a surface of a three-dimensional, gap-opened, and ionically disordered topological insulator (TI). We calculate the TE coefficients and the dimensionless figure of merit Z T by using a semiclassical transport theory, taking into account the electron scattering due to ionic disorders. The ionic disorder on the TI surface is modeled by charge impurities with randomly distributed long-range potential (Coulomb impurities) and short-range potential (normal impurity). As a result, S differs significantly between two impurities in the gapless limit. Conversely, in the presence of a surface subgap, Z T for both impurity models is as high as ∼ 2 below room temperature within a permissible range of impurity parameters. In addition, by comparing the results of the theoretical model with the results of recently reported experiments, we find that the contribution of Coulomb impurities to TE transport is comparable to or dominates that of normal impurities.

Ambipolar Seebeck power generator based on topological insulator surfaces

Abstract: We propose a thermoelectric (TE) energy conversion device based on the surface of a three-dimensional topological insulator (TI) that is magnetically gap-opened and ionically disordered. A pair of top and bottom surfaces of a single TI film takes the role of a vertical p–n junction with ambipolar conduction, which can be considered as a TE module consisting of two dissimilar TE materials. By tuning the surface carrier screening by means of electric gating, we find that the figure of merit ZT of the device exceeds 1 in the low-temperature-regime below 300 K. Our model may represent one direction for the implementation of TE energy conversion and heat management in nanodevices.We propose a thermoelectric (TE) energy conversion device based on the surface of a three-dimensional topological insulator (TI) that is magnetically gap-opened and ionically disordered. A pair of top and bottom surfaces of a single TI film takes the role of a vertical p–n junction with ambipolar conduction, which can be considered as a TE module consisting of two dissimilar TE materials. By tuning the surface carrier screening by means of electric gating, we find that the figure of merit ZT of the device exceeds 1 in the low-temperature-regime below 300 K. Our model may represent one direction for the implementation of TE energy conversion and heat management in nanodevices.