arXiv subjects

K. Hwang

Publications and source records attributed to K. Hwang.

3 recordsLinked to original sources

arXiv2020

Thermodynamic evidence for field-angle dependent Majorana gap in a Kitaev spin liquid

The exactly-solvable Kitaev model of two-dimensional honeycomb magnet leads to a quantum spin liquid (QSL) characterized by Majorana fermions, relevant for fault-tolerant topological quantum computations. In the high-field paramagnetic state of $\alpha$-RuCl3, half-integer quantization of thermal Hall conductivity has been reported as a signature of edge current, but the bulk nature of this state remains elusive. Here, from high-resolution heat capacity measurements under in-plane field rotation, we find strongly angle-dependent low-energy excitations in bulk $\alpha$-RuCl3. The excitation gap has a sextuple node structure, and the gap amplitude increases with field, exactly as expected for itinerant Majorana fermions in the Kitaev model. Our thermodynamic results are fully linked with the transport quantization properties, providing the first demonstration of the bulk-edge correspondence in a Kitaev QSL. Moreover, at higher fields where the quantum thermal Hall effect vanishes, we find the possible emergence of a novel nematic QSL state with two-fold rotational symmetry.

cond-mat.str-el↗

arXiv2020

A free-floating or wide-orbit planet in the microlensing event OGLE-2019-BLG-0551

High-cadence observations of the Galactic bulge by the microlensing surveys led to the discovery of a handful of extremely short-timescale microlensing events that can be attributed to free-floating or wide-orbit planets. Here, we report the discovery of another strong free-floating planet candidate, which was found from the analysis of the gravitational microlensing event OGLE-2019-BLG-0551. The light curve of the event is characterized by a very short duration (<3 d) and a very small amplitude (< 0.1 mag). From modeling of the light curve, we find that the Einstein timescale, tE = 0.381 +/- 0.017 d, is much shorter, and the angular Einstein radius, thetaE = 4.35 +/- 0.34 uas, is much smaller than those of typical lensing events produced by stellar-mass lenses (tE ~ 20 d, thetaE ~ 0.3 mas), indicating that the lens is very likely to be a planetary-mass object. We conduct an extensive search for possible signatures of a companion star in the light curve of the event, finding no significant evidence for the putative host star. For the first time, we also demonstrate that the angular Einstein radius of the lens does not depend on blending in the low-magnification events with strong finite source effects.

astro-ph.EP↗

arXiv2018

Anisotropic spin-orbit torque generation in epitaxial SrIrO3 by symmetry design

Spin-orbit coupling (SOC), the interaction between the electron spin and the orbital angular momentum, can unlock rich phenomena at interfaces, in particular interconverting spin and charge currents. Conventional heavy metals have been extensively explored due to their strong SOC of conduction electrons. However, spin-orbit effects in classes of materials such as epitaxial 5d-electron transition metal complex oxides, which also host strong SOC, remain largely unreported. In addition to strong SOC, these complex oxides can also provide the additional tuning knob of epitaxy to control the electronic structure and the engineering of spin-to-charge conversion by crystalline symmetry. Here, we demonstrate room-temperature generation of spin-orbit torque on a ferromagnet with extremely high efficiency via the spin-Hall effect in epitaxial metastable perovskite SrIrO3. We first predict a large intrinsic spin-Hall conductivity in orthorhombic bulk SrIrO3 arising from the Berry curvature in the electronic band structure. By manipulating the intricate interplay between SOC and crystalline symmetry, we control the spin-Hall torque ratio by engineering the tilt of the corner-sharing oxygen octahedra in perovskite SrIrO3 through epitaxial strain. This allows the presence of an anisotropic spin-Hall effect due to a characteristic structural anisotropy in SrIrO3 with orthorhombic symmetry. Our experimental findings demonstrate the heteroepitaxial symmetry design approach to engineer spin-orbit effects. We therefore anticipate that these epitaxial 5d transition-metal oxide thin films can be an ideal building block for low-power spintronics.

cond-mat.mtrl-sci↗