arXiv subjects

Alex Taekyung Lee

Publications and source records attributed to Alex Taekyung Lee.

17 recordsLinked to original sources

arXiv2026

Site-Specific Orbital Symmetry Breaking in Epitaxial Graphene on Silicon Carbide

The epitaxial graphene grown on SiC interface has been the subject of tremendous research efforts to control its electronic transport properties in the quest for graphene electronic applications. The electronic structure of the epitaxial graphene buffer layer on SiC(0001) is investigated at the atomic scale using monochromated electron energy loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope (STEM). The C $K$-edge reveals spectral fine structures distinctly different from free-standing graphene layer, providing direct experimental evidence of a modified bonding environment. Ab initio calculations providing a site-resolved probe of orbital character to elucidate the origin of these spectral features: covalent bonding between one-quarter of the buffer-layer carbon atoms and the substrate silicon atoms induces partial $\mathrm{sp}^3$ hybridization, quenching the out-of-plane $π$ states and disrupting $π$-conjugation across the interfacial layer. Substrate-imposed symmetry breaking further lifts the in-plane orbital degeneracy. This orbital-engineering framework, rooted in $π$-state quenching and site-specific symmetry breaking, establishes a quantitative basis for tailoring graphene-substrate interfaces toward next-generation low-energy transistor applications.

cond-mat.mtrl-sci↗

arXiv2026

Revealing Short- and Long-range Li-ion diffusion in Li$_2$MnO$_3$ from finite-temperature dynamical mean field theory

Li$_2$MnO$_3$ is a key component of Li-excess layered cathodes of the form $(1-x),\mathrm{LiMO_2} + x,\mathrm{Li_2MnO_3}$ ($M$ = Mn, Ni, Co, \dots), yet its role in setting Li-ion transport limitations remains under debate. Here we combine DFT+$U$, finite-temperature DFT+DMFT with a continuous-time quantum Monte Carlo impurity solver, and nudged-elastic-band (NEB) calculations to study Li$^{+}$ migration in paramagnetic Li$_2$MnO$_3$ in the presence of a single Li vacancy. Evaluating DMFT total energies along the DFT+$U$ NEB geometries reveals that dynamical correlations strongly renormalize the lowest-barrier processes, reducing the activation energies to $E_a = 0.18$ eV for the shortest-range hop and $E_a = 0.50$ eV for the next-lowest (transport-controlling) step. The 0.18 eV barrier quantitatively reproduces the short-range activation energy from $μ^{+}$SR, while the 0.50 eV barrier is consistent with the long-range transport scale extracted from ac-impedance measurements. This single-vacancy, paramagnetic DMFT description thus provides a unified interpretation of local and macroscopic probes without invoking clustered vacancy configurations or strong extrinsic disorder, consistent with nearly stoichiometric Li$_2$MnO$_3$ powders. More broadly, our results highlight finite-temperature dynamical correlations as an essential ingredient for predicting ionic migration energetics in correlated oxide electrodes.

cond-mat.mtrl-sci↗

arXiv2025

Impact of structural distortions on the correlated electronic structure of orbital-selective Mott insulating Na$_3$Co$_2$SbO$_6$ under strains

Na$_{3}$Co$_{2}$SbO$_6$ is a promising candidate to realize the Kitaev spin liquid phase since the large Kitaev spin exchange interaction is tunable via the change in electronic structure, such as the trigonal crystal field splitting ($Δ_{TCF}$). Here, we show that the uncorrelated electronic structure of Na$_{3}$Co$_{2}$SbO$_6$ is rather insensitive to the strain effect due to the low crystal symmetry accompanied by oxygen displacements and the presence of Sb $s$ orbitals. This suggests that the Kitaev spin-exchange interaction obtained from perturbation theory also does not depend much on the strain effect. Using density functional theory plus dynamical mean field theory, we find that the correlated electronic structure of Na$_{3}$Co$_{2}$SbO$_6$ is an orbital selective Mott insulating state where the trigonal $a_{1g}$ orbital is insulating due to correlation-assisted hybridization, while other $d$ orbitals behave as typical Mott insulators, resulting in tunability of $Δ_{TCF}$ under the strain effect effectively. Our results show that the local Co-site symmetry and dynamical correlation effects will play an important role in engineering the novel magnetic phase in this and related materials.

cond-mat.str-el↗

arXiv2025

First-principle Study of Multiple Metastable Charge Ordering States in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$

La doped SrFeO$_{3}$, La$_{1/3}$Sr$_{2/3}$FeO$_{3}$, exhibits a metal-to-insulator transition accompanied by both antiferromagnetic and charge ordering states along with the Fe-O bond disproportionation below a critical temperature near 200K. Unconventionally slow charge dynamics measured in this material near the critical temperature shows that its excited charge ordering states can exhibit novel electronic structures with nontrivial energy profiles. Here, we reveal possible metastable states of charge ordering structures in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ using the first-principle and climbing image nudged elastic band methods. In the strong correlation regime, La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ is an antiferromagnetic insulator with a charge ordering state of the big-small-big pattern, consistent with the experimental measurement of this material at the low temperature. As the correlation effect becomes weak, we find at least two possible metastable charge ordering states with the distinct Fe-O bond disproportionation. Remarkably, a ferroelectric metallic state emerges with the small energy barrier of $\sim$7 meV, driven by a metastable CO state of the small-medium-big pattern. The electronic structures of these metastable charge ordering states are noticeably different from those of the ground-state. Our results can provide an insightful explanation to multiple metastable charge ordering states and the slow charge dynamics of this and related oxide materials.

cond-mat.str-el↗

arXiv2024

Exploring the Coexistence of Spin States in [Fe-(tpy-ph)$_2$]$^{2+}$ Complexes on Au(111) using ab initio calculations

In this work, we systematically study the electronic structure and stability of spin states of the [Fe-(tpy-ph)$_2$]$^{2+}$ molecule in both gas phase and on a Au(111) substrate using density functional theory +U (DFT+U) calculations. We find that the stability of the Fe$^{2+}$ ion's spin states is significantly influenced by the Hubbard U parameter. In the gas phase, the low-spin (LS, S=0) state is found to be energetically favorable for U(Fe) $\leq$ 3 eV, whereas the high-spin (HS, S=2) state is stabilized for U(Fe) > 3 eV. Interaction with the Au(111) substrate is found to elevate the critical U for the spin-state transition to 3.5 eV. Additionally, we perform L-edge X-ray absorption spectroscopy (XAS) calculations based on time-dependent DFT (TD-DFT) for both HS and LS states. The calculated XAS suggests that the HS state more closely aligns with the experimental observations, indicating the potential coexistence of the HS state as the initial state during the X-ray excitation process. These findings enrich our understanding of spin-state dynamics in [Fe-(tpy-ph)$_2$]$^{2+}$.

cond-mat.mtrl-sci↗

arXiv2023

Delocalized polaron and Burstein-Moss shift induced by Li in $α$-$\textrm{V}_{2}\textrm{O}_{5}$: DFT+DMFT study

We performed density functional theory (DFT)+$U$ and dynamical mean field theory (DMFT) calculations with continuous time quantum Monte Carlo impurity solver to investigate the electronic properties of V$_2$O$_5$ and Li$_x$V$_2$O$_5$ ($x$ = 0.125 and 0.25). Pristine V$_2$O$_5$ is a charge-transfer insulator with strong O $p$-V $d$ hybridization, and exhibits a large band gap ($E_{\textrm{gap}}$) as well as non-zero conduction band (CB) gap. We show that the band gap, the number of $d$ electrons of vanadium, $N_d$, and conduction band (CB) gap for V$_2$O$_5$ obtained from our DMFT calculations are in excellent agreement with the experimental values. While the DFT+$U$ approach replicates the experimental band gap, it overestimates the value of $N_d$ and underestimates the CB gap. In the presence of low Li doping, the electronic properties of V$_2$O$_5$ are mainly driven by a polaronic mechanism, the electron spin resonance and electron nuclear double resonance spectroscopies observed the coexistence of free and bound polarons. Notably, our DMFT results identify both polaron types, with the bound polaron being energetically preferred, while DFT+$U$ method predicts only the free polaron. Our DMFT analysis also reveals that increased Li doping leads to electron filling in the conduction band, shifting the Fermi level, this result consistent with the observed Burstein-Moss shift upon enhanced Li doping and we thus demonstrate that the DFT+DMFT approach can be used for accurate and realistic description of strongly correlated materials.

cond-mat.str-el↗

arXiv2023

Effect of Off-Diagonal Elements in Wannier Hamiltonian on DFT+DMFT for low-symmetry material: Study of Li$_2$MnO$_3$

We study the effect of the off-diagonal elements of the Wannier Hamiltonian on the electronic structure of low-symmetry material Li$_2$MnO$_3$ ($C2/m$), using dynamical mean field theory calculations with continuous-time Quantum Monte Carlo impurity solver. Presence of significant off-diagonal elements leads to a pronounced suppression of the energy gap. The off-diagonal elements are largest when the Wannier projection is used based on the global coordinate, and they remain substantial even with the projection using the local coordinate close to the direction of Mn-O bonds. We show that the energy gap is enhanced by the diagonalization of the Mn $d$ block in the full $p$-$d$ Hamiltonian, with applying unitary rotation matrix. Additionally, the inclusion of a small double counting energy is crucial for achieving the experimental gap by reducing $p$-$d$ hybridization. Furthermore, we establish the efficiency of a low-energy ($d$-only basis) model for studying the electronic structure of Li$_2$MnO$3$, as the Wannier basis represents a hybridized state of Mn $d$ and O $p$ orbitals. These findings suggest an appropriate new approach for investigating low-symmetry materials using the DFT+DMFT method. To the best of our knowledge, no systematic study of the effect of off-diagonal terms has been conducted thus far. We also find that the antiferromagnetic ground state $Γ_{2u}$ is stable with $U \leq 2$ eV within density functional theory+$U$ calculations, which is much smaller than widely used $U$=5 eV.

cond-mat.str-el↗

arXiv2023

Low-energy electronic interactions in ferrimagnetic Sr2CrReO6 thin films

We reveal in this study the fundamental low-energy landscape in the ferrimagnetic Sr2CrReO6 double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both Sr2CrReO6 powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund's coupling, and the strain-induced tetragonal crystal field. We also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV ($\pm$25 meV) through X-L-W-U high-symmetry points and is dispersive toward $Γ$

cond-mat.str-el↗

arXiv2023

Spin and electronic excitations in $4f$ atomic chains on Au(111) substrates

High spin systems, like those that incorporate rare-earth $4f$ elements (REEs), are increasingly relevant in many fields. Although research in such systems is sparse, the large Hilbert spaces they occupy are promising for many applications. In this work, we examine a one-dimensional linear array of europium (Eu) atoms on a Au(111) surface and study their electronic and magnetic excitations. Ab initio calculations using VASP with PBE+U are employed to study the structure. We find Eu atoms to have a net charge when on gold, consistent with a net magnetic momemt of $\simeq 3.5 μ_B$. Examining various spin-projection configurations, we can evaluate first and second neighbor exchange energies in an isotropic Heisenberg model between spin-$\frac{7}{2}$ moments to obtain $J_1 \approx -1.2 \, \mathrm{K}$ and $J_2 \approx 0.2 \, \mathrm{K}$ for the relaxed-chain atomic separation of $a \approx 5$ $\mathrm{\dot{A}}$. These parameters are used to obtain the full spin excitation spectrum of a physically realizable four-atom chain. The large $|J_1|/J_2$ ratio results in a highly degenerate ferromagnetic ground state that is split by a significant easy plane single ion anisotropy of $0.6$ K. Spin-flip excitations are calculated to extract differential conductance profiles as those obtained by scanning tunneling microscopy techniques. We uncover interesting behavior of local spin excitations, especially as we track their dispersion with applied magnetic fields.

cond-mat.mes-hall↗

arXiv2021

Origin of the orbital polarization of Co$^{2+}$ in La$_2$CoTiO$_6$ and (LaCoO$_3$)$_1$+(LaTiO$_3$)$_1$ : a DFT+$U$ and DMFT study

The unequal electronic occupation of localized orbitals (orbital polarization), and associated lowering of symmetry and degeneracy, play an important role in the properties of transition metal oxides. Here, we examine systematically the underlying origin of orbital polarization, taking as exemplar the 3$d$ manifold of Co$^{2+}$ in a variety of spin, orbital and structural phases in the double perovskite La$_2$CoTiO$_6$ and the (001) superlattice (LaCoO$_3$)$_1$+(LaTiO$_3$)$_1$ systems. Superlattices are of specific interest due to the large experimentally observed orbital polarization of their Co cations. Based on first principles calculations, we find that robust and observable orbital polarization requires symmetry reduction through the lattice structure; the role of local electronic interactions is to greatly enhance the orbital polarization.

cond-mat.mtrl-sci↗

arXiv2019

Magnetism of (LaCoO$_3$)$_n$+(LaTiO$_3$)$_n$ superlattices with $n=1,2$

LaCoO$_3$ provides a poignant example of a transition metal oxide where the cobalt cations display multiple spin states and spin transitions and which continues to garner substantial attention. In this work, we describe first principles studies, based on DFT+$U$ theory, of superlattices containing LaCoO$_3$, specifically (LaCoO$_3$)$_n$+(LaTiO$_3$)$_n$ for $n=1,2$. The superlattices show strong electron transfer from Ti to Co resulting in Co$^{2+}$, significant structural distortions and a robust orbital polarization of Co$^{2+}$. We predict high-spin Co$^{2+}$ and a checkerboard or G-type antiferromagnetic (AFM) ground state. We provide a detailed analysis of the magnetic interactions and phases in the superlattices. We predict that ferromagnetic order on the Co${2+}$ can be stabilized by hole doping (e.g., replacing La by Sr) which is rather unusual for Co$^{2+}$ cations.

cond-mat.mtrl-sci↗

arXiv2017

Structural and metal-insulator transitions in rhenium based double perovskites via orbital ordering

Re-based double perovskites (DPs) exhibit a complex interplay of structural and metal-insulator transitions. Here we systematically study the ground state electronic and structural properties for a family of Re-based DPs $A_2B$ReO$_6$ ($A$=Sr, Ca and $B$=Cr, Fe), which are related by a common low energy Hamiltonian, using density functional theory + $U$ calculations. We show that the on-site interaction $U$ of Re induces orbital ordering (denoted COO), with each Re site having an occupied $d_{xy}$ orbital and a C-type alternation among $d_{xz}/d_{yz}$, resulting in an insulating state consistent with experimentally determined insulators Sr$_2$CrReO$_6$, Ca$_2$CrReO$_6$, and Ca$_2$FeReO$_6$. The threshold value of $U_{Re}$ for orbital ordering is reduced by inducing $E_g$ octahedral distortions of the same C-type wavelength (denoted COD), which serves as a structural signature of the orbital ordering; octahedral tilting also reduces the threshold. The COO, and the concomitant COD, are a spontaneously broken symmetry for the Sr based materials (i.e. $a^0a^0c^-$ tilt pattern), while not for the Ca based systems (i.e. $a^-a^-b^+$ tilt pattern). Spin-orbit coupling does not qualitatively change the physics of the COO/COD, but can induce relevant quantitative changes. We prove that a single set of $U_{Cr}$,$U_{Fe}$,$U_{Re}$ capture the experimentally observed metallic state in Sr$_2$FeReO$_6$ and insulating states in the other three systems. We predict that the COO is the origin of the insulating state in Sr$_2$CrReO$_6$, and that the concomitant COD may be experimentally observed at sufficiently low temperatures (ie. space group P$4_2/m$). Additionally, given our prescribed values of $U$, we show that the COO induced insulating state in Ca$_2$CrReO$_6$ will survive even if the COD amplitude is suppressed (e.g. due to thermal fluctuations).

cond-mat.str-el↗

arXiv2017

Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor

We have explored a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single crystal Sr$_2$VO$_3$FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a non-trivial $C_4$ (2$\times$2) order, not achievable by thermal excitation with unpolarized current. Our tunneling spectroscopy study shows that the induced $C_4$ (2$\times$2) order has characteristics of plaquette antiferromagnetic order in Fe layer and strongly suppressed superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the $C_4$ state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.

cond-mat.supr-con↗

arXiv2017

Correlation of Fe-based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c \approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

cond-mat.supr-con↗

arXiv2014

Magnetic proximity effect at Bi$_2$Se$_3$/EuS interface with broken inversion symmetry

We investigate a magnetic proximity effect on Dirac surface states of topological insulator (TI) induced by a Bi$_2$Se$_3$/EuS interface, using density-functional theory (DFT) and a low-energy effective model, motivated by a recent experimental realization of the interface. We consider a thin ferromagnetic insulator EuS film stacked on top of Bi$_2$Se$_3$(111) slabs of three or five quintuple layers (QLs) with the magnetization of EuS normal to the interface ($z$ axis), which breaks both time-reversal and inversion symmetry. It is found that a charge transfer and surface relaxation makes the Dirac cones electron-doped. For both 3 and 5 QLs, the top-surface Dirac cone has an energy gap of 9 meV, while the bottom surface Dirac cone remains gapless. This feature is due to the short-ranged induced magnetic moment of the EuS film. For the 5 QLs, an additional Dirac cone with an energy gap of 2 meV is formed right below the bottom-surface Dirac point, while for 3 QLs, there is no additional Dirac cone. We also examine the spin-orbital texture of the Dirac surface states with broken inversion symmetry, using DFT and the effective model. We find that the $p_z$ orbital is coupled to the $z$ component of the spin moment in the opposite sign to the $p_x$ and $p_y$ orbitals. The $p_z$ and radial $p$ orbitals are coupled to the in-plane spin texture in the opposite handedness to the tangential $p$ orbital. The result obtained from the effective model agrees with our DFT calculations. The calculated spin-orbital texture may be tested from spin-polarized angle-resolved photoemission spectroscopy.

cond-mat.mes-hall↗

arXiv2013

Structure, strain, and control of ground state property in LaTiO$_3$/LaAlO$_3$ superlattice

Using first-principles density functional theory calculations, we examined the ground state property of LaTiO$_3$/LaAlO$_3$ superlattice. Total energy calculations, taking account of the structural distortions, $U$ dependence, and exchange correlation functional dependence, show that the spin and orbital ground state can be controlled systematically by the epitaxial strain. In the wide range of strains, ferromagnetic spin and antiferro orbital ordering are stabilized, which is notably different from the previously reported ground state in titanate systems. By applying large tensile strains, the system can be transformed into an antiferromagnetic spin and ferro-orbital-ordered phase.

cond-mat.mtrl-sci↗

arXiv2013

Charge transfer, confinement, and ferromagnetism in LaMnO$_3$/LaNiO$_3$ (001)-superlattice

Using first-principles density functional theory calculations, we investigated the electronic structure and magnetic properties of (LaMnO$_3$)$_m$/(LaNiO$_3$)$_n$ superlattices stacked along (001)-direction. The electrons are transferred from Mn to Ni, and the magnetic moments are induced at Ni sites that are paramagnetic in bulk and other types of superlattices. The size of induced moment is linearly proportional to the amount of transferred electrons, but it is larger than the net charge transfer because the spin and orbital directions play important roles and complicate the transfer process. The charge transfer and magnetic properties of the ($m$,$n$) superlattice can be controlled by changing the $m/n$ ratio. Considering the ferromagnetic couplings between Mn and Ni spins and the charge transfer characteristic, we propose the (2,1) superlattice as the largest moment superlattice carrying $\sim8μ_B$ per formula unit.

cond-mat.mtrl-sci↗