arXiv subjects

Changgu Lee

Publications and source records attributed to Changgu Lee.

At least 19 recordsLinked to original sources

arXiv2026

Contrasting anisotropic electron-phonon-spin coupling in Fe$_{3}$GeTe$_{2}$ and Fe$_{5}$GeTe$_{2}$: A helicity-resolved Raman study

Two-dimensional van der Waals ferromagnets Fe$_3$GeTe$ _2$ (F3GT) and Fe$_5$GeTe$_2$ (F5GT) exhibit pronounced magneto-optical responses, which open promising platforms for investigating the interplay among lattice, electronic, and magnetic degrees of freedom. Here, we present a comparative study of optical resonance-induced anisotropic electron-phonon coupling and its association with magnetic ordering in these systems using wavelength- and temperature-dependent helicity-resolved Raman spectroscopy. By resolving the doubly degenerate E modes under left- and right-circularly polarized excitations, we demonstrate that the temperature evolution of the chiral mode splitting ($\Delta f$) does not track the magnetization behavior, indicating that the helicity-dependent Raman response arises not solely from time-reversal symmetry breaking due to magnetic order, but also from spin-orbit-coupled electronic interactions. Notably, in F3GT, the out-of-plane magnetization indirectly governs the in-plane anisotropic electron-phonon coupling under optical resonance, whereas F5GT exhibits static anisotropic interactions. The Fano asymmetry parameter $1/q$ reveals mode- and temperature-dependent coupling strengths between phonons and the electronic continuum, with pronounced angular anisotropy in F3GT but isotropic behavior in F5GT--- a consequence of its multiple Fe sites and enhanced interlayer hybridization in the latter. Our results demonstrate the role of crystal structure and magnetic anisotropy in shaping the anisotropically coupled electron-phonon-spin dynamics in these layered metallic ferromagnets, and highlight Fe$_x$GeTe$_2$ as a versatile platform for microscopic insight into chiral light-matter interactions in layered metallic ferromagnets.

cond-mat.str-el↗

arXiv2026

Activated Migration of Localized Ligand-Field Excitons in Atomically Thin CrCl3

Two-dimensional crystals with densely packed atoms exhibit a range of emerging properties, particularly a wide variety of excitonic behaviors. Thickness-variable layered chromium trihalides with finite surface recombination sites provide an ideal system for understanding how excitons confined in octahedral ligand fields migrate on nanometer length scales, a regime that defies conventional transport probes. In this work, we demonstrate that Cr3+-derived photoluminescence in CrCl3 is spectrally thickness-independent, but its relaxation dynamics are strongly sensitive to thickness and temperature, thereby indicating significant activated migration. A diffusion-coupled surface recombination model reveals an effective out-of-plane diffusivity of 4.5 x 10-6 cm2/s for the ligand-field excitons and a diffusion activation energy of 130 meV. The latter is comparable to the reorganization energy independently estimated from optical Stokes shifts, suggesting that exciton transport is coupled to local lattice relaxation. Furthermore, we show that the relaxation dynamics can be systematically tuned by either enhancing or suppressing surface recombination through controlled surface reactions or encapsulation. This work not only reveals the nanoscopic transport of localized ligand-field excitons but also establishes a spectroscopic transport probe applicable to various 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic electron-phonon coupling and chiral phonons in van der Waals room temperature ferromagnet Fe$_{5}$GeTe$_{2}$

The layered van der Waals Fe$_5$GeTe$_2$ (F5GT) compound exhibits room-temperature ferromagnetism, making it a promising candidate for technological applications. In our study, combined temperature, wavelength, and polarization-dependent Raman measurements, along with {\it ab initio} calculations reveal important aspects of lattice dynamics and electron-phonon interactions. The angle-resolved Raman intensity under linear polarization configurations exhibits a strong tilt in the laboratory coordinate system, indicating the existence of anisotropic electron-phonon coupling. The temperature evolution of this anisotropy is discussed by extracting the phase factor of the Raman tensor elements from the angle-resolved intensity measured at different temperatures, also uncovering a spin-orbit coupling-mediated electron-phonon response in F5GT. The thermal evolution of electron-phonon coupling is also examined by measuring the temperature dependence of the Fano parameter of the asymmetric peak in the Raman spectra, while wavelength-dependent measurements establish the role of optical resonance in enhancing the anisotropic interaction. Finally, the threefold rotational symmetry guarantees the existence of chiral phonons. We present direct spectroscopic evidence for these chiral vibrational modes through cross-circularly polarized Raman measurements, complemented by theoretical calculations of phonon circular polarization. Together, these results identify F5GT as an ideal platform for investigating emergent couplings among lattice, electronic, and magnetic degrees of freedom and for advancing the understanding of chiral phonons in magnetic van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2023

Pulsed-mode metalorganic vapor-phase epitaxy of GaN on graphene-coated c-sapphire for freestanding GaN thin films

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Photoluminescence Path Bifurcations by Spin Flip in Two-Dimensional CrPS4

Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters owing to long-range spin correlation and efficient transition between high and low-spin excited states of Cr3+ ions. In this study, we report on dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing novel magneto-optic functions and devices.

cond-mat.mtrl-sci↗

arXiv2021

Spin dynamics slowdown near the antiferromagnetic critical point in atomically thin FePS3

Two-dimensional (2D) magnetic materials have attracted much recent interest with unique properties emerging at the few-layer limit. Beyond the reported impacts on the static magnetic properties, the effects of reducing the dimensionality on the magnetization dynamics are also of fundamental interest and importance for 2D device development. In this report, we investigate the spin dynamics in atomically-thin antiferromagnetic FePS3 of varying layer numbers using ultrafast pump-probe spectroscopy. Following the absorption of an optical pump pulse, the time evolution of the antiferromagnetic order parameter is probed by magnetic linear birefringence. We observe a strong divergence in the demagnetization time near the Néel temperature. The divergence can be characterized by a power-law dependence on the reduced temperature, with an exponent decreasing with sample thickness. We compare our results to expectations from critical slowing down and a two-temperature model involving spins and phonons, and discuss the possible relevance of spin-substrate phonon interactions.

cond-mat.mes-hall↗

arXiv2020

Polarized Raman Spectra and Complex Raman Tensors of Antiferromagnetic Semiconductor CrPS4

Chromium thiophosphate (CrPS4), a monoclinic crystal of C_2^3 space group, is a ternary layered semiconductor with an optical bandgap of 1.4 eV and exhibits antiferromagnetism below 36 K. Despite its potential in optoelectronic and magnetic applications, the symmetry of its lattice vibrations has not been systematically studied. In this work, we performed a polarized Raman spectroscopy of bulk CrPS4 using three different excitation wavelengths of 457, 514, and 633 nm. High-quality crystals grown by the chemical vapor transport method were mechanically exfoliated or polished to expose three orthogonal crystallographic facets. Polarized Raman spectra were obtained in parallel and cross configurations by rotating samples about the surface normal to each of the facets. Among 33 Raman active modes (16 A and 17 B) at the Brillouin zone center, 19 (8) peaks observed in the parallel (cross) configuration were assigned as A (B) modes. Complex-valued Raman tensors were determined for 7 major A modes using the angle-dependent Raman spectra of the three facets. The results can also be used in determining the crystallographic orientations of CrPS4 unequivocally.

physics.chem-ph↗

arXiv2020

Interlayer magnetism in Fe3-xGeTe2

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

cond-mat.mtrl-sci↗

arXiv2020

On-Stack Two-Dimensional Conversion of MoS2 into MoO3

Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D 'on-stack' chemical conversion of single-layer crystalline MoS2 into MoO3 with a precise layer control that enables truly 2D MoO3 and MoO3/MoS2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS2. The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.

cond-mat.mtrl-sci↗

arXiv2020

Crossover between Photochemical and Photothermal Oxidations of Atomically Thin Magnetic Semiconductor CrPS4

Many two-dimensional (2D) semiconductors represented by transition metal dichalcogenides have tunable optical bandgaps in the visible or near IR-range standing as a promising candidate for optoelectronic devices. Despite this potential, however, their photoreactions are not well understood or controversial in the mechanistic details. In this work, we report a unique thickness-dependent photoreaction sensitivity and a switchover between two competing reaction mechanisms in atomically thin chromium thiophosphate (CrPS4), a two-dimensional antiferromagnetic semiconductor. CrPS4 showed a threshold power density 2 orders of magnitude smaller than that for MoS2 obeying a photothermal reaction route. In addition, reaction cross section quantified with Raman spectroscopy revealed distinctive power dependences in the low and high power regimes. On the basis of optical in situ thermometric measurements and control experiments against O2, water, and photon energy, we proposed a photochemical oxidation mechanism involving singlet O2 in the low power regime with a photothermal route for the other. We also demonstrated a highly effective encapsulation with Al2O3 as a protection against the destructive photoinduced and ambient oxidations.

cond-mat.mtrl-sci↗

arXiv2020

Structural and optical properties of single- and few-layer magnetic semiconductor CrPS4

Atomically thin binary 2-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure and thickness. By adding another element in those materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS4), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffractions. Transmission electron microscopy showed that preferentially-cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy for the first time and exhibited significant dependence on thickness of crystals exfoliated down to single layer. Optical absorption determined by reflectance contrast was dominated by d-d type transitions localized at Cr3+ ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness-dependence and hinted a high photochemical activity for single layer CrPS4. The current structural and optical investigation will provide a firm basis for future study and application of this novel magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2019

Antisymmetric magnetoresistance in van der Waals Fe3GeTe2/graphite/Fe3GeTe2 tri-layer heterostructures

Van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next generation spintronic devices. When layered with non-magnetic vdW materials, such as graphene and/or topological insulators, vdW heterostructures can be assembled (with no requirement for lattice matching) to provide otherwise unattainable device structures and functionalities. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in van der Waals heterostructured Fe3GeTe2/graphite/Fe3GeTe2 devices. Unlike conventional giant magnetoresistance (GMR) which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high, intermediate and low resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three resistance behavior was attributed to a spin momentum locking induced spin polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures.

cond-mat.mes-hall↗

arXiv2018

Topological Insulator-Based van der Waals Heterostructures for Effective Control of Massless and Massive Dirac Fermions

Three dimensional (3D) topological insulators (TIs) are an important class of materials with applications in electronics, spintronics and quantum computing. With the recent development of truly bulk insulating 3D TIs, it has become possible to realize surface dominated phenomena in electrical transport measurements e.g. the quantum Hall (QH) effect of massless Dirac fermions in topological surface states (TSS). However, to realize more advanced devices and phenomena, there is a need for a platform to tune the TSS or modify them e.g. gap them by proximity with magnetic insulators, in a clean manner. Here we introduce van der Waals (vdW) heterostructures in the form of topological insulator/insulator/graphite to effectively control chemical potential of the TSS. Two types of gate dielectrics, normal insulator hexagonal boron nitride (hBN) and ferromagnetic insulator Cr2Ge2Te6 (CGT) are utilized to tune charge density of TSS in the quaternary TI BiSbTeSe2. hBN/graphite gating in the QH regime shows improved quantization of TSS by suppression of magnetoconductivity of massless Dirac fermions. CGT/graphite gating of massive Dirac fermions in the QH regime yields half-quantized Hall conductance steps and a measure of the Dirac gap. Our work shows the promise of the vdW platform in creating advanced high-quality TI-based devices.

cond-mat.mes-hall↗

arXiv2018

Hard magnetic properties in nanoflake van der Waals Fe3GeTe2

Two dimensional (2D) van der Waals (vdW) materials have demonstrated fascinating optical, electrical and thickness-dependent characteristics. These have been explored by numerous authors but reports on magnetic properties and spintronic applications of 2D vdW materials are scarce by comparison. By performing anomalous Hall effect transport measurements, we have characterised the thickness dependent magnetic properties of single crystalline vdW Fe3GeTe2. The nanoflakes of this vdW metallic material exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, we confirmed the existence of magnetic coupling between vdW atomic layers and obtained an estimated coupling length of ~ 5 vdW layers in Fe3GeTe2. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into vdW magnetic heterostructures, paving the way for spintronic research and applications based on these devices.

cond-mat.mtrl-sci↗

arXiv2015

Anomalous polarization dependence of Raman scattering and crystallographic orientation of black phosphorus

We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscope analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag1 and Ag2 modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy.

cond-mat.mes-hall↗

arXiv2014

Designed Three-Dimensional Freestanding Single-Crystal Carbon Architectures

Single-crystal carbon nanomaterials have led to great advances in nanotechnology. The first single-crystal carbon nanomaterial, fullerene, was fabricated in a zero-dimensional form. One-dimensional carbon nanotubes and two-dimensional graphene have since followed and continue to provide further impetus to this field. In this study, we fabricated designed three-dimensional (3D) single-crystal carbon architectures by using silicon carbide templates. For this method, a designed 3D SiC structure was transformed into a 3D freestanding single-crystal carbon structure that retained the original SiC structure by performing a simple single-step thermal process. The SiC structure inside the 3D carbon structure is self-etched, which results in a 3D freestanding carbon structure. The 3D carbon structure is a single crystal with the same hexagonal close-packed structure as graphene. The size of the carbon structures can be controlled from the nanoscale to the microscale, and arrays of these structures can be scaled up to the wafer scale. The 3D freestanding carbon structures were found to be mechanically stable even after repeated loading. The relationship between the reversible mechanical deformation of a carbon structure and its electrical conductance was also investigated. Our method of fabricating designed 3D freestanding single-crystal graphene architectures opens up prospects in the field of single-crystal carbon nanomaterials, and paves the way for the development of 3D single-crystal carbon devices.

cond-mat.mes-hall↗

arXiv2012

Observation of tightly bound trions in monolayer MoS2

Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS2 is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on-off ratio exceeding 10^8 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasi-particles, which can be created with valley and spin polarized holes, have no analogue in other semiconducting materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues both for fundamental studies of many-body interactions and for opto-electronic and valleytronic applications in 2D atomic crystals.

cond-mat.mtrl-sci↗

arXiv2010

Anomalous Lattice Vibrations of Single and Few-Layer MoS2

Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the 3-dimensional to the 2-dimensional regime.

cond-mat.mtrl-sci↗