arXiv subjects

Joongwon Lee

Publications and source records attributed to Joongwon Lee.

7 recordsLinked to original sources

arXiv2026

Second-Order Optical Nonlinearity of AlScN Films Grown By Molecular Beam Epitaxy

Alloys of AlN have rapidly emerged as a material platform for nonlinear optics. In this paper, we measure the second-order optical nonlinearity of AlScN films grown directly on nitrided c-plane sapphire by molecular beam epitaxy. This direct growth approach, which bypasses a thick AlN buffer layer, allows us to isolate the true nonlinear response of the AlScN film. Our results show a large enhancement of d31, but a suppression of d33 in AlScN films compared to AlN. We observe that d31 can be as high as 4.92 pm/V , which is 60 times larger than that of AlN. The development of AlScN-based photonic devices can enable energy-efficient nonlinear optical operations that can be epitaxially integrated with electronic and photonic devices based on Si, GaN and AlN.

cond-mat.mtrl-sci↗

arXiv2026

Magneto-Optical Detection of Anisotropic Spin Currents in Altermagnetic RuO2

Altermagnets are a recently identified class of collinear antiferromagnets that host large spin-split electronic bands, offering a promising platform for efficient spin-current generation. Among proposed candidates, the metallic oxide RuO2 is predicted to exhibit strong altermagnetic spin splitting; however, whether it sustains robust magnetic order beyond the ultrathin thickness limit remains unresolved. Here, we employ optical probes to investigate charge-to-spin conversion in a 12-nm-thick (101)-oriented RuO2 film grown on sapphire. Polarization-resolved second-harmonic generation reveals nonlinear optical responses consistent with the surface symmetry and N\'eel order of RuO2. Under an applied current, both second-harmonic generation and polar magneto-optical Kerr effect measurements detect a pronounced, directionally anisotropic spin polarization, exhibiting enhanced signals for current along [010] and strongly suppressed responses for current along [-101], in agreement with the symmetry of the altermagnetic spin-splitter effect. Non-magnetic or Rashba-type mechanisms cannot explain this symmetry-selective response. Scanning transmission electron microscopy further reveals that substantial strain persists even in relatively thick films, providing a possible explanation for the observed behavior. Therefore, these results establish RuO2 as an efficient spin source and demonstrate the potential of altermagnets for field-free spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Unusual dependence on the angle of magnetic field for the spin Hall magnetoresistance of monodomain epitaxial BiFeO3 thin films

Spin Hall magnetoresistance (SMR) measurements provide a way to probe the surface spin structure of insulating magnetic materials. Such measurements produce resistance signals of the form {$Δ$}R {$\propto$} cos[2($α$-$α$$_0$)], where $α$ is the angle between the current and the external in-plane magnetic field. Previous experiments on a wide range of materials have found $α$$_0$ = 0$°$ for ferromagnets and $α$$_0$ = 90$°$ for antiferromagnets. Here we investigate SMR in bilayers of Pt with monodomain BiFeO$_3$ multiferroic epitaxial thin films. We observe signals of the form {$Δ$}R {$\propto$} cos[2($α$-$α$$_0$)] but surprisingly the angle $α$$_0$ can take values very different from 90$°$ or 0$°$, with large variations from sample to sample. The aim of the paper is to report this striking departure from the expected magnetic field dependence of SMR and to encourage consideration of possible microscopic mechanisms.

cond-mat.mtrl-sci↗

arXiv2025

FragFM: Hierarchical Framework for Efficient Molecule Generation via Fragment-Level Discrete Flow Matching

We introduce FragFM, a novel hierarchical framework via fragment-level discrete flow matching for efficient molecular graph generation. FragFM generates molecules at the fragment level, leveraging a coarse-to-fine autoencoder to reconstruct details at the atom level. Together with a stochastic fragment bag strategy to effectively handle a large fragment space, our framework enables more efficient, scalable molecular generation. We demonstrate that our fragment-based approach achieves better property control than the atom-based method and additional flexibility through conditioning the fragment bag. We also propose a Natural Product Generation benchmark (NPGen) to evaluate the ability of modern molecular graph generative models to generate natural product-like molecules. Since natural products are biologically prevalidated and differ from typical drug-like molecules, our benchmark provides a more challenging yet meaningful evaluation relevant to drug discovery. We conduct a comparative study of FragFM against various models on diverse molecular generation benchmarks, including NPGen, demonstrating superior performance. The results highlight the potential of fragment-based generative modeling for large-scale, property-aware molecular design, paving the way for more efficient exploration of chemical space.

cs.LG↗

arXiv2024

Ferroelectric AlBN Films by Molecular Beam Epitaxy

We report the properties of molecular beam epitaxy deposited AlBN thin films on a recently developed epitaxial nitride metal electrode Nb2N. While a control AlN thin film exhibits standard capacitive behavior, distinct ferroelectric switching is observed in the AlBN films with increasing Boron mole fraction. The measured remnant polarization Pr of 15 uC/cm2 and coercive field Ec of 1.45 MV/cm in these films are smaller than those recently reported on films deposited by sputtering, due to incomplete wake-up, limited by current leakage. Because AlBN preserves the ultrawide energy bandgap of AlN compared to other nitride hi-K dielectrics and ferroelectrics, and it can be epitaxially integrated with GaN and AlN semiconductors, its development will enable several opportunities for unique electronic, photonic, and memory devices.

cond-mat.mtrl-sci↗

arXiv2024

BInD: Bond and Interaction-generating Diffusion Model for Multi-objective Structure-based Drug Design

Recent remarkable advancements in geometric deep generative models, coupled with accumulated structural data, enable structure-based drug design (SBDD) using only target protein information. However, existing models often struggle to balance multiple objectives, excelling only in specific tasks. BInD, a diffusion model with knowledge-based guidance, is introduced to address this limitation by co-generating molecules and their interactions with a target protein. This approach ensures balanced consideration of key objectives, including target-specific interactions, molecular properties, and local geometry. Comprehensive evaluations demonstrate that BInD achieves robust performance across all objectives, matching or surpassing state-of-the-art methods. Additionally, an NCI-driven molecule design and optimization method is proposed, enabling the enhancement of target binding and specificity by elaborating the adequate interaction patterns.

q-bio.BM↗

arXiv2022

Antiferromagnetic Spin Orientation and Magnetic Domain Structure in Epitaxially Grown MnN Studied using Optical Second Harmonic Generation

MnN is a centrosymmetric collinear antiferromagnet belonging to the transition metal nitride family with a high Neel temperature, a low anisotropy field, and a large magnetic moment per Mn atom. Despite several recent experimental and theoretical studies, the spin symmetry (magnetic point group) and magnetic domain structure of the material remain unknown. In this work, we use optical second harmonic generation (SHG) to study the magnetic structure of thin epitaxially-grown single-crystal (001) MnN films. Our work shows that spin moments in MnN are tilted away from the [001] direction and the components of the spin moments in the (001) plane are aligned along one of the two possible in-plane symmetry axes ([100] or [110]) resulting in a magnetic point group symmetry of 2/m1'. Our work rules out magnetic point group symmetries 4/mmm1' and mmm1' that have been previously discussed in the literature. Four different spin domains consistent with the 2/m1' magnetic point group symmetry are possible in MnN. A statistical model based on the observed variations in the polarization-dependent intensity of the second harmonic signal collected over large sample areas puts an upper bound of 0.65 microns on the mean domain size. Our results show that SHG can be used to probe the magnetic order in metallic antiferromagnets. This work is expected to contribute to the recent efforts in using antiferromagnets for spintronic applications.

cond-mat.mtrl-sci↗