arXiv subjects

Hyunseok Kim

Publications and source records attributed to Hyunseok Kim.

12 recordsLinked to original sources

arXiv2025

Optimal regularity results in Sobolev-Lorentz spaces for linear elliptic equations with $L^1$- or measure data

It has been well known that if $Ω$ is a bounded $C^1$-domain in $\R^n,\ n \ge 2$, then for every Radon measure $f$ on $Ω$ with finite total variation, there exists a unique weak solution $u\in W_0^{1,1}(Ω)$ of the Poisson equation $-Δu=f$ in $Ω$ satisfying $\nabla u \in L^{n/(n-1),\infty}(Ω;\R^n )$. In this paper, optimal regularity properties of the solution $u$ are established in Sobolev-Lorentz spaces $L_α^{p,q}(Ω)$ of order $ α$ less than but arbitrarily close to $2$. More precisely, for any $0 \le α<1$, we show that $u\in L_{α+1}^{p(α),\infty}(Ω)$, where $p(α)= n/(n-1+α)$. Moreover, using an embedding result for Sobolev-Lorentz spaces $L_α^{p,q}(Ω)$ into classical Besov spaces $B_α^{p,q}(Ω)$, we deduce that $u\in B_{α+1}^{p(α),\infty}(Ω)$. Indeed, these regularity results are proved for solutions of the Dirichlet problems for more general linear elliptic equations with nonhomogeneous boundary data. On the other hand, it is known that if $Ω$ is of class $C^{1,1}$, then for each $G\in L^1 (Ω;\R^n )$ there exists a unique very weak solution $v\in L^{n/(n-1),\infty} (Ω)$ of $-Δv= {\rm div}\, G$ in $Ω$ satisfying the boundary condition $v=0$ in some sense. We prove that $v$ has the optimal regularity property, that is, $v\in L_α^{p(α),\infty}(Ω)\cap B_α^{p(α),\infty}(Ω)$ for every $0 \le α< 1$. This regularity result is also proved for more general equations with nonhomogeneous boundary data.

math.AP↗

arXiv2023

On linear elliptic equations with drift terms in critical weak spaces

We study the Dirichlet problem for a second order linear elliptic equation in a bounded smooth domain $Ω$ in $\mathbb{R}^n$, $n \ge 3$, with the drift $\mathbf{b} $ belonging to the critical weak space $L^{n,\infty}(Ω)$. We decompose the drift $\mathbf{b} = \mathbf{b}_1 + \mathbf{b}_2$ in which $\text{div} \mathbf{b}_1 \geq 0$ and $\mathbf{b}_2$ is small only in a small scale quasi-norm of $L^{n,\infty}(Ω)$. Under this new smallness condition, we prove existence, uniqueness, and regularity estimates of weak solutions to the problem and its dual. Hölder regularity and derivative estimates of weak solutions to the dual problem are also established. As a result, we prove uniqueness of very weak solutions slightly below the threshold. When $\mathbf{b}_2 =0$, our results recover those by Kim and Tsai in [SIAM J. Math. Anal. 52 (2020)]. Due to the new small scale quasi-norm, our results are new even when $\mathbf{b}_1=0$.

math.AP↗

arXiv2023

Global existence and uniqueness of weak solutions of a Stokes-Magneto system with fractional diffusions

We consider a Stokes-Magneto system in $\mathbb{R}^d$ ($d\geq 2$) with fractional diffusions $Λ^{2α}\boldsymbol{u}$ and $Λ^{2β}\boldsymbol{b}$ for the velocity $\boldsymbol{u}$ and the magnetic field $\boldsymbol{b}$, respectively. Here $α,β$ are positive constants and $Λ^s = (-Δ)^{s/2}$ is the fractional Laplacian of order $s$. We establish global existence of weak solutions of the Stokes-Magneto system for any initial data in $L_{2}$ when $α$, $β$ satisfy $1/2<α<(d+1)/2$, $β>0$, and $\min\{α+β,2α+β-1\}>d/2$. It is also shown that weak solutions are unique if $β\geq 1$ and $\min \{α+β,2α+β-1\}\geq d/2+1$, in addition.

math.AP↗

arXiv2022

Dirichlet problems for second order linear elliptic equations with $L^{1}$-data

We consider the Dirichlet problems for second order linear elliptic equations in non-divergence and divergence forms on a bounded domain $Ω$ in $\mathbb{R}^n$, $n \ge 2$: $$ -\sum_{i,j=1}^n a^{ij}D_{ij} u + b \cdot D u + cu = f \;\;\text{ in $Ω$} \quad \text{and} \quad u=0 \;\;\text{ on $\partial Ω$} $$ and $$ - {\rm div} \left( A D u \right) + {\rm div}(ub) + cu = {\rm div} F \;\;\text{ in $Ω$} \quad \text{and} \quad u=0 \;\;\text{ on $\partial Ω$} , $$ where $A=[a^{ij}]$ is symmetric, uniformly elliptic, and of vanishing mean oscillation (VMO). The main purposes of this paper is to study unique solvability for both problems with $L^1$-data. We prove that if $Ω$ is of class $C^{1}$, $ {\rm div} A + b\in L^{n,1}(Ω;\mathbb{R}^n)$, $c\in L^{\frac{n}{2},1}(Ω) \cap L^s(Ω)$ for some $1<s<\frac{3}{2}$, and $c\ge0$ in $Ω$, then for each $f\in L^1 (Ω)$, there exists a unique weak solution in $W^{1,\frac{n}{n-1},\infty}_0 (Ω)$ of the first problem. Moreover, under the additional condition that $Ω$ is of class $C^{1,1}$ and $c\in L^{n,1}(Ω)$, we show that for each $F \in L^1 (Ω; \mathbb{R}^n)$, the second problem has a unique very weak solution in $L^{\frac{n}{n-1},\infty}(Ω)$.

math.AP↗

arXiv2022

Multiplication of freestanding semiconductor membranes from a single wafer by advanced remote epitaxy

Freestanding single-crystalline membranes are an important building block for functional electronics. Especially, compounds semiconductor membranes such as III-N and III-V offer great opportunities for optoelectronics, high-power electronics, and high-speed computing. Despite huge efforts to produce such membranes by detaching epitaxial layers from donor wafers, however, it is still challenging to harvest epitaxial layers using practical processes. Here, we demonstrate a method to grow and harvest multiple epitaxial membranes with extremely high throughput at the wafer scale. For this, 2D materials are directly formed on III-N and III-V substrates in epitaxy systems, which enables an advanced remote epitaxy scheme comprised of multiple alternating layers of 2D materials and epitaxial layers that can be formed by a single epitaxy run. Each epilayer in the multi-stack structure is then harvested by layer-by-layer peeling, producing multiple freestanding membranes with unprecedented throughput from a single wafer. Because 2D materials allow peeling at the interface without damaging the epilayer or the substrate, wafers can be reused for subsequent membrane production. Therefore, this work represents a meaningful step toward high-throughput and low-cost production of single-crystal membranes that can be heterointegrated.

physics.app-ph↗

arXiv2021

Dirichlet and Neumann problems for elliptic equations with singular drifts on Lipschitz domains

We consider the Dirichlet and Neumann problems for second-order linear elliptic equations: \[ -\triangle u +\mathrm{div}(u\mathbf{b}) =f \quad\text{ and }\quad -\triangle v -\mathbf{b} \cdot \nabla v =g \] in a bounded Lipschitz domain $Ω$ in $\mathbb{R}^n$ $(n\geq 3)$, where $\mathbf{b}:Ω\rightarrow \mathbb{R}^n$ is a given vector field. Under the assumption that $\mathbf{b} \in L^{n}(Ω)^n$, we first establish existence and uniqueness of solutions in $L_α^{p}(Ω)$ for the Dirichlet and Neumann problems. Here $L_α^{p}(Ω)$ denotes the Sobolev space (or Bessel potential space) with the pair $(α,p)$ satisfying certain conditions. These results extend the classical works of Jerison-Kenig [17] and Fabes-Mendez-Mitrea [12] for the Poisson equation. We also prove existence and uniqueness of solutions of the Dirichlet problem with boundary data in $L^{2}(\partialΩ)$. Our results for the Dirichlet problems hold even for the case $n=2$.

math.AP↗

arXiv2021

Interpolation inequalities in function spaces of Sobolev-Lorentz type

Interpolation inequalities in Triebel-Lizorkin-Lorentz spaces and Besov-Lorentz spaces are studied for both inhomogeneous and homogeneous cases. First we establish interpolation inequalities under quite general assumptions on the parameters of the function spaces. Several results on necessary conditions are also provided. Next, utilizing the interpolation inequalities together with some embedding results, we prove Gagliardo-Nirenberg inequalities for fractional derivatives in Lorentz spaces, which do hold even for the limiting case when one of the parameters is equal to 1 or $\infty$.

math.FA↗

arXiv2021

Structured World Belief for Reinforcement Learning in POMDP

Object-centric world models provide structured representation of the scene and can be an important backbone in reinforcement learning and planning. However, existing approaches suffer in partially-observable environments due to the lack of belief states. In this paper, we propose Structured World Belief, a model for learning and inference of object-centric belief states. Inferred by Sequential Monte Carlo (SMC), our belief states provide multiple object-centric scene hypotheses. To synergize the benefits of SMC particles with object representations, we also propose a new object-centric dynamics model that considers the inductive bias of object permanence. This enables tracking of object states even when they are invisible for a long time. To further facilitate object tracking in this regime, we allow our model to attend flexibly to any spatial location in the image which was restricted in previous models. In experiments, we show that object-centric belief provides a more accurate and robust performance for filtering and generation. Furthermore, we show the efficacy of structured world belief in improving the performance of reinforcement learning, planning and supervised reasoning.

cs.LG↗

arXiv2020

Mapping Charge Recombination and the Effect of Point Defect Insertion in Gallium Arsenide Nanowire Heterojunctions

Electronic devices are extremely sensitive to defects in their constituent semiconductors, but locating electronic point defects in bulk semiconductors has previously been impossible. Here we apply scanning transmission electron microscopy (STEM) electron beam-induced current (EBIC) imaging to map electronic defects in a GaAs nanowire Schottky diode. Imaging with a non-damaging 80 or 200 kV STEM acceleration potential reveals a minority-carrier diffusion length that decreases near the surface of the hexagonal nanowire, thereby demonstrating that the device's charge collection efficiency (CCE) is limited by surface defects. Imaging with a 300 keV STEM beam introduces vacancy-interstitial (VI, or Frenkel) defects in the GaAs that increase carrier recombination and reduce the CCE of the diode. We create, locate, and characterize a single insertion event, determining that a defect inserted 7 nm from the Schottky interface broadly reduces the CCE by 10% across the entire nanowire device. Variable-energy STEM EBIC imaging thus allows both benign mapping and pinpoint modification of a device's e-h recombination landscape, enabling controlled experiments that illuminate the impact of both extended (1D and 2D) and point (0D) defects on semiconductor device performance.

cond-mat.mes-hall↗

arXiv2018

Existence, uniqueness, and regularity results for elliptic equations with drift terms in critical weak spaces

We consider Dirichlet problems for linear elliptic equations of second order in divergence form on a bounded or exterior smooth domain $Ω$ in $\mathbb{R}^n$, $n \ge 3$, with drifts $\mathbf{b}$ in the critical weak $L^n$-space $L^{n,\infty}(Ω; \mathbb{R}^n )$. First, assuming that the drift $\mathbf{b}$ has nonnegative weak divergence in $L^{n/2, \infty }(Ω)$, we establish existence and uniqueness of weak solutions in $W^{1,p}(Ω)$ or $D^{1,p}(Ω)$ for any $p$ with $n' = n/(n-1)< p < n$. By duality, a similar result also holds for the dual problem. Next, we prove $W^{1,n+\varepsilon}$ or $W^{2, n/2+δ}$-regularity of weak solutions of the dual problem for some $\varepsilon, δ>0$ when the domain $Ω$ is bounded. By duality, these results enable us to obtain a quite general uniqueness result as well as an existence result for weak solutions belonging to $\bigcap_{p< n' }W^{1,p}(Ω)$. Finally, we prove a uniqueness result for exterior problems, which implies in particular that (very weak) solutions are unique in both $L^{n/(n-2),\infty}(Ω)$ and $L^{n,\infty}(Ω)$.

math.AP↗

arXiv2018

Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures

Vertically stacked van der Waals heterostructures constitute a promising platform for providing tailored band alignment with enhanced excitonic systems. Here we report observations of neutral and charged interlayer excitons in trilayer WSe2-MoSe2-WSe2 van der Waals heterostructures and their dynamics. The addition of a WSe2 layer in the trilayer leads to significantly higher photoluminescence quantum yields and tunable spectral resonance compared to its bilayer heterostructures at cryogenic temperatures. The observed enhancement in the photoluminescence quantum yield is due to significantly larger electron-hole overlap and higher light absorbance in the trilayer heterostructure, supported via first-principle pseudopotential calculations based on spin-polarized density functional theory. We further uncover the temperature- and power-dependence, as well as time-resolved photoluminescence of the trilayer heterostructure interlayer neutral excitons and trions. Our study elucidates the prospects of manipulating light emission from interlayer excitons and designing atomic heterostructures from first-principles for optoelectronics.

cond-mat.mes-hall↗

arXiv2017

Monolithic InGaAs nanowire array lasers on silicon-on-insulator operating at room temperature

Chip-scale integrated light sources are a crucial component in a broad range of photonics applications. III-V semiconductor nanowire emitters have gained attention as a fascinating approach due to their superior material properties, extremely compact size, and the capability to grow directly on lattice-mismatched silicon substrates. Although there have been remarkable advances in nanowire-based emitters, their practical applications are still in the early stages due to the difficulties in integrating nanowire emitters with photonic integrated circuits (PICs). Here, we demonstrate for the first time optically pumped III-V nanowire array lasers monolithically integrated on silicon-on-insulator (SOI) platform. Selective-area growth of purely single-crystalline InGaAs/InGaP core/shell nanowires on an SOI substrate enables the nanowire array to form a photonic crystal nanobeam cavity with superior optical and structural properties, resulting in the laser to operate at room temperature. We also show that the nanowire array lasers are effectively coupled with SOI waveguides by employing nanoepitaxy on a pre-patterned SOI platform. These results represent a new platform for ultra-compact and energy-efficient optical links, and unambiguously point the way toward practical and functional nanowire lasers.

cond-mat.mes-hall↗