Observation of the orbital Hall effect in a light metal Ti.

The orbital Hall effect1 refers to the generation of electron orbital angular momentum flow transverse to an external electric field. Contrary to the common belief that the orbital angular momentum is quenched in solids, theoretical studies2,3 predict that the orbital Hall effect can be strong and is a fundamental origin of the spin Hall effect4-7 in many transition metals. Despite the growing circumstantial evidence8-11, its direct detection remains elusive. Here we report the magneto-optical observation of the orbital Hall effect in the light metal titanium (Ti). The Kerr rotation by the orbital magnetic moment accumulated at Ti surfaces owing to the orbital Hall current is measured, and the result agrees with theoretical calculations semi-quantitatively and is supported by the orbital torque12 measurement in Ti-based magnetic heterostructures. This result confirms the orbital Hall effect and indicates that the orbital angular momentum is an important dynamic degree of freedom in solids. Moreover, this calls for renewed studies of the orbital effect on other degrees of freedom such as spin2,3,13,14, valley15,16, phonon17-19 and magnon20,21 dynamics.

Pseudo-Hydrodynamic Flow of Quasiparticles in Semimetal WTe2 at Room Temperature.

Recently, much interest has emerged in fluid-like electric charge transport in various solid-state systems. The hydrodynamic behavior of the electronic fluid reveals itself as a decrease of the electrical resistance with increasing temperature (the Gurzhi effect) in narrow channels, polynomial scaling of the resistance as a function of the channel width, violation of the Wiedemann-Franz law supported by the emergence of the Poiseuille flow. Similar to whirlpools in flowing water, the viscous electronic flow generates vortices, resulting in abnormal sign-changing electrical response driven by backflow. However, the question of whether the long-ranged sign-changing electrical response can be produced by a mechanism other than hydrodynamics has not been addressed so far. Here polarization-sensitive laser microscopy is used to demonstrate the emergence of visually similar abnormal sign-alternating patterns in semi-metallic tungsten ditelluride at room temperature where this material does not exhibit true hydrodynamics. It is found that the neutral quasiparticle current consisting of electrons and holes obeys an equation remarkably similar to the Navier-Stokes equation. In particular, the momentum relaxation is replaced by the much slower process of quasiparticle recombination. This pseudo-hydrodynamic flow of quasiparticles leads to a sign-changing charge accumulation pattern via different diffusivities of electrons and holes.

Covalent-bonding-induced strong phonon scattering in the atomically thin WSe2 layer.

In nano-device applications using two-dimensional (2D) van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered (n = 1) and bi-layered (n = 2) WSe2 flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al2O3 substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe2 layers increases. In particular, as the metal is changed from Al, to Au, and to Ti, we find an interesting trend of TBR depending on the WSe2 thickness; when referenced to TBR for a system without WSe2, TBR for n = 1 decreases, but that for n = 2 increases. This result clearly demonstrates that the stronger bonding for Ti leads to a better thermal conduction between the metal and the WSe2 layer, but in return gives rise to a large mismatch in the phonon density of states between the first and second WSe2 layers so that the WSe2-WSe2 interface becomes a major thermal resistance for n = 2. By using photoemission spectroscopy and optical second harmonic generation technique, we confirm that the metallization induces a change in the valence state of W-ions, and also recovers a non-centrosymmetry for the bi-layered WSe2.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films.

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO3 thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO3 substrate, an epitaxial polycrystalline SrRuO3 thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.