arXiv subjects

Jieun Lee

Publications and source records attributed to Jieun Lee.

39 records · Page 3Linked to original sources

arXiv2018

Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas

Using a high-speed and broadband radio frequency (RF) (0.1-1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2-D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a non-modal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (< 500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1-3 μs) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.

physics.plasm-ph↗

arXiv2017

Valley Magnetoelectricity in Single-Layer MoS2

Magnetoelectric (ME) effect, the phenomenon of inducing magnetization by application of an electric field or vice versa, holds great promise for magnetic sensing and switching applications. Studies of the ME effect have so far focused on the control of the electron spin degree of freedom (DOF) in materials such as multiferroics and conventional semiconductors. Here, we report a new form of the ME effect based on the valley DOF in two-dimensional (2D) Dirac materials. By breaking the three-fold rotational symmetry in single-layer MoS2 via a uniaxial stress, we have demonstrated the pure electrical generation of valley magnetization in this material, and its direct imaging by Kerr rotation microscopy. The observed out-of-plane magnetization is independent of in-plane magnetic field, linearly proportional to the in-plane current density, and optimized when the current is orthogonal to the strain-induced piezoelectric field. These results are fully consistent with a theoretical model of valley magnetoelectricity driven by Berry curvature effects. Furthermore, the effect persists at room temperature, opening possibilities for practical valleytronic devices.

cond-mat.mes-hall↗

arXiv2015

Electrical control of the valley Hall effect in bilayer MoS2 transistors

The valley degree of freedom of electrons in solids has been proposed as a new type of information carriers beyond the electronic charge and spin. Recent experimental demonstrations of the optical orientation of the valley polarization and generation of the valley current through the valley Hall effect in monolayer MoS2 have shown the potential of two-dimensional semiconductor transition metal dichalcogenides for valley based electronic and optoelectronic applications. The valley Hall conductivity in monolayer MoS2, a non-centrosymmetric crystal, however, cannot be easily tuned, presenting a challenge for valley-based applications. Here we report the control of the valley Hall effect in bilayer MoS2 transistors through a gate. The inversion symmetry present in bilayer MoS2 was broken by the gate applied electric field perpendicular to the plane. The valley polarization near the edges of the device channels induced by the longitudinal electrical current was imaged by use of Kerr rotation microscopy. The polarization is out-of-plane, has opposite sign for the two edges, and is strongly dependent on the gate voltage. The observation is consistent with the symmetry dependent Berry curvature and valley Hall conductivity in bilayer MoS2. Our results are another step towards information processing based on the valley degree of freedom.

cond-mat.mes-hall↗