arXiv subjects

Junho Park

Publications and source records attributed to Junho Park.

21 records · Page 2Linked to original sources

arXiv2021

Interlayer coupling and ultrafast hot electron transfer dynamics in metallic VSe2/graphene van der Waals heterostructures

Atomically thin vanadium diselenide (VSe2 ) is a two-dimensional transition metal dichalcogenide exhibiting attractive properties due to its metallic 1T-phase. With the recent development of methods to manufacture high-quality monolayer VSe 2 on van der Waals materials, the outstanding properties of VSe2 -based heterostructures have been widely studied for diverse applications. Dimensional reduction and interlayer coupling with a van der Waals substrate lead to its distinguishable characteristics from its bulk counterparts. However, only a few fundamental studies have investigated the interlayer coupling effects and hot electron transfer dynamics in VSe2 heterostructures. In this work, we reveal ultrafast and efficient interlayer hot electron transfer and interlayer coupling effects in VSe2 /graphene heterostructures. Femtosecond time-resolved reflectivity measurements showed that hot electrons in VSe 2 were transferred to graphene within a 100-fs timescale with high efficiency. Besides, coherent acoustic phonon dynamics indicated interlayer coupling in VSe2 /graphene heterostructures and efficient thermal energy transfer to three-dimensional substrates. Our results provide valuable insights into the intriguing properties of metallic transition metal dichalcogenide heterostructures and motivate designing optoelectronic and photonic devices with tailored properties.

cond-mat.str-el↗

arXiv2020

Horizontal shear instabilities in rotating stellar radiation zones: II. Effects of the full Coriolis acceleration

Stellar interiors are the seat of efficient transport of angular momentum all along their evolution. Understanding the dependence of the turbulent transport triggered by the shear instabilities due to the differential rotation in stellar radiation zones is mandatory. Indeed, it constitutes one of the cornerstones of the rotational transport and mixing theory which is implemented in stellar evolution codes to predict the rotational and chemical evolutions of stars. We investigate horizontal shear instabilities in stellar radiation zones by considering the full Coriolis acceleration with both the dimensionless horizontal component $\tilde{f}$ and the vertical component $f$. We performed a linear stability analysis for a horizontal shear flow with a hyperbolic tangent profile, both numerically and asymptotically using the WKBJ approximation. As in the traditional approximation, we identified the inflectional and inertial instabilities. The inflectional instability is destabilized as $\tilde{f}$ increases and its maximum growth rate increases significantly, while the thermal diffusivity stabilizes the inflectional instability similarly to the traditional case. The inertial instability is also strongly affected; for instance, the inertially unstable regime is extended in the non-diffusive limit as $0<f<1+\tilde{f}^{2}/N^{2}$, where $N$ is the dimensionless Brunt-Väisälä frequency. More strikingly, in the high-thermal-diffusivity limit, it is always inertially unstable at any colatitude $θ$ except at the poles (i.e., $0^{\circ}<θ<180^{\circ}$). Using the asymptotic and numerical results, we propose a prescription for the effective turbulent viscosities induced by the instabilities to be possibly used in stellar evolution models. The characteristic time of this turbulence is short enough to redistribute efficiently angular momentum and mix chemicals in the radiation zones.

astro-ph.SR↗

arXiv2020

Horizontal shear instabilities in rotating stellar radiation zones: I. Inflectional and inertial instabilities and the effects of thermal diffusion

Rotational mixing, the key process in stellar evolution, transports angular momentum and chemical elements in stellar radiative zones. In the past two decades, an emphasis has been placed on the turbulent transport induced by the vertical shear instability. However, instabilities arising from horizontal shear and the strength of the anisotropic turbulent transport that they may trigger remain relatively unexplored. This paper investigates the combined effects of stable stratification, rotation, and thermal diffusion on the horizontal shear instabilities in the context of stellar radiative zones. The eigenvalue problem describing linear instabilities of a flow with a hyperbolic-tangent horizontal shear profile was solved numerically for a wide range of parameters. As a first step, we consider a polar $f$-plane where the gravity and rotation vector are aligned. Two types of instabilities are identified: the inflectional and inertial instabilities. The inflectional instability that arises from the inflection point is the most unstable when at a zero vertical wavenumber and a finite wavenumber in the streamwise direction along the imposed-flow direction. The three-dimensional inflectional instability is destabilized by stratification, while it is stabilized by thermal diffusion. The inertial instability is rotationally driven, and a WKBJ analysis reveals that its growth rate reaches the maximum $\sqrt{f(1-f)}$ in the inviscid limit as the vertical wavenumber goes to infinity, where $f$ is the dimensionless Coriolis parameter. The inertial instability for a finite vertical wavenumber is stabilized as the stratification increases, whereas it is destabilized by the thermal diffusion. Furthermore, we found a self-similarity in both the inflectional and inertial instabilities based on the rescaled parameter $PeN^2$ with the Péclet number $Pe$ and the Brunt-Väisälä frequency $N$.

astro-ph.SR↗