arXiv subjects

Se Young Park

Publications and source records attributed to Se Young Park.

At least 19 recordsLinked to original sources

arXiv2026

Systematic display of the spin splitting in band structures of representative altermagnetic crystals

In this work we demonstrate a novel approach to exhibit the unique spin splitting that is typical of collinear altermagnets. This approach is to plot band structures on Brillouin zone paths that sample general k-points to show a representative picture that corresponds to Brillouin zone averages. This is in contrast to conventional band structure plotting which plots band structures on the highest-symmetry points and lines, and thus in many cases can fail to show any altermagnetic spin splitting at all. Our investigation compares the new approach with the band structures of collinear altermagnets using conventional band structure plotting. We report the band structure and symmetry analysis for MnTe, CrSb, SmFeO3, ScCrO3, LaMnO3, TlCrO3, HoFeO3, InCrO3 and DyFeO3. This result clearly demonstrates the advantage of this novel method for displaying the spin splitting of collinear altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

AlterSeeK-Path: Systematic construction of generalized band-structure paths for displaying altermagnetic spin splitting

Altermagnetic materials exhibit spin splitting in their electronic band structures while maintaining zero net magnetization. However, conventional high-symmetry k-paths generally hide this splitting because they follow symmetry lines that in most cases enforce spin degeneracy. We present AlterSeeK-Path, an open-source Python tool that systematically constructs generalized band-structure paths for collinear altermagnets. The method selects the centroid of the conventional irreducible wedge used in routine band-structure calculations as the representative general k-point, maps it to a spin-flip-related partner, and inserts paired segments through these points into the standard high-symmetry path; these segments systematically sample the interior of the irreducible wedge. We demonstrate the construction for all 54 three-dimensional combinations of extended Bravais lattice type and spin Laue group across the six crystal systems that support collinear altermagnetism, and for the 12 two-dimensional cases spanning the four compatible two-dimensional Bravais lattices. Representative band structures are shown for the distinct lattice/path cases. With AlterSeeK-Path, these band structures can be constructed with essentially the same effort as conventional band structures.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelectric amplitude switching and continuous memory

Although ferroelectric systems inherently exhibit binary switching behavior, recent advances in analog memory device have spurred growing interest in achieving continuous memory states. In this work, we demonstrate ferroelectric amplitude switching at the mesoscopic scale in compositionally graded Ba1-xSrxTiO3 heterostructures, enabling continuous modulation of polarization magnitude without altering its direction, which we defined as amplitude switching. Using switching current measurement, piezoresponse force microscopy and Landau-Ginzburg-Devonshire simulations, we reveal that compositionally graded ferroelectric heterostructure can possess amplitude switching behavior through a double well potential with flattened minima. This behavior supports stable, continuous polarization states and establishes a new platform for analog memory applications. These findings introduce amplitude switching as a new dynamic of the order parameter, paving the way for energy-efficient and reliable analog memory systems.

cond-mat.mtrl-sci↗

arXiv2025

G-type Antiferromagnetic BiFeO$_3$ is a Multiferroic $g$-wave Altermagnet

G-type antiferromagnetic BiFeO$_3$ is shown to be an altermagnet. We present the band structure using an unconventional scheme designed to highlight the distinctive spin splitting which is characteristic of altermagnets. We define and show plots of the spin-splitting function in reciprocal space. We show that the nodal surfaces of the spin-splitting function that follow from symmetry can be classified into two types, which we call symmetry-enforced and continuity-enforced. We describe the spin-splitting function with a simple parametrization in a basis of symmetry-adapted plane waves. Using group-theory analysis based on irreducible representations of the crystallographic Laue group, we confirm that the altermagnetism of G-type BiFeO$_3$ is $g$-wave and present a complete classification table for the general three-dimensional case. Finally, we discuss the effect of ferroelectric switching on the altermagnetic order, and identify three classes of ferroelectric altermagnets.

cond-mat.mtrl-sci↗

arXiv2025

Precision Analysis of $\mathrm{^{12}C / ^{13}C}$ Ratios in Orion IRc2 Acetylene Isotopologues via $χ^2$ Fitting

We present a detailed analysis of acetylene (C$_2$H$_2$) and its isotopologues in the Orion IRc2 region, focusing on the determination of $^{12}$C/$^{13}$C isotopic ratios using high-resolution infrared spectra from SOFIA. By employing a robust $χ^2$ fitting method, we simultaneously determined temperature and column density, achieving a $^{12}$C/$^{13}$C ratio of $18.72^{+1.54}_{-1.46}$ for the blue clump and $15.07^{+1.61}_{-1.60}$ for the red clump. These results revealed significant discrepancies with the traditional rotational diagram method, which overestimated the ratios by 12.1% and 23.9%, respectively. Our $χ^2$ approach also reduced uncertainties by up to 75%, providing more precise and reliable isotopic ratios. Additionally, we extended the analysis to isotopologues not covered in HITRAN, calculating vibrational and rotational constants through quantum chemical calculations. This allowed us to model subtle isotopic shifts induced by $^{13}$C and deuterium substitution, enabling accurate isotopologue detection in astrophysical environments. The Python package (TOPSEGI) developed in this study facilitates efficient $χ^2$ fitting and isotopic ratio analysis, making it a valuable tool for future high-resolution observations. This work highlights the critical role of advanced spectral models and fitting techniques in understanding isotopic fractionation and the chemical evolution of interstellar matter.

astro-ph.GA↗

arXiv2025

First-principles study of dielectric properties of ferroelectric perovskite oxides with on-site and inter-site Hubbard interactions

We study the atomic and electronic structures of ferroelectric perovskite oxides, BaTiO$_3$, LiNbO$_3$, and PbTiO$_3$ using ab initio extended Hubbard functionals in which the on-site and inter-site Hubbard interactions are determined self-consistently, adapted from the pseudohybrid density functional proposed by Agapito-Curtarolo-Buongiorno Nardelli. Band structures, ferroelectric distortions, polarization, Born effective charges, and switching barriers are calculated with extended Hubbard functionals, that are compared with those using local density approximation (LDA), generalized gradient approximation (GGA), and Hybrid (HSE06) functionals. The properties of all three compounds calculated by extended Hubbard functionals are in good agreement with experimental data. We find a substantial increase in band gaps due to the inter-site Coulomb interactions, which show better agreement with $GW$ results compared to those from LDA and GGA functionals. The crucial role of the inter-site Coulomb interactions in restoring the suppressed polar instability, which is computed when only the on-site Hubbard interactions are considered, is also highlighted. Overall, we find that the properties calculated using our extended Hubbard functionals exhibit trends similar to those obtained with the HSE06 functional, while reducing computational costs by over an order of magnitude. Thus, we propose that the current method is well-suited for high-throughput calculations for perovskite oxides, offering significantly improved accuracy in computing band gap and other related physical properties such as the shift current photovoltaic effect and band alignments in ferroelectric heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Spin-orbit-splitting-driven nonlinear Hall effect in NbIrTe4

The Berry curvature dipole (BCD) serves as a one of the fundamental contributors to emergence of the nonlinear Hall effect (NLHE). Despite intense interest due to its potential for new technologies reaching beyond the quantum efficiency limit, the interplay between BCD and NLHE has been barely understood yet in the absence of a systematic study on the electronic band structure. Here, we report NLHE realized in NbIrTe4 that persists above room temperature coupled with a sign change in the Hall conductivity at 150 K. First-principles calculations combined with angle-resolved photoemission spectroscopy (ARPES) measurements show that BCD tuned by the partial occupancy of spin-orbit split bands via temperature is responsible for the temperature-dependent NLHE. Our findings highlight the correlation between BCD and the electronic band structure, providing a viable route to create and engineer the non-trivial Hall effect by tuning the geometric properties of quasiparticles in transition-metal chalcogen compounds.

cond-mat.mtrl-sci↗

arXiv2024

Electronic structure of above-room-temperature van der Waals ferromagnet Fe$_3$GaTe$_2$

Fe$_3$GaTe$_2$, a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature ($\textit{T}$$_C$). In this study, we reveal the electronic structure of Fe$_3$GaTe$_2$ in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction ($\textit{J}$$_{ex}$) and magnetic anisotropy energy to the development of the high-$\textit{T}$$_C$ ferromagnetic ordering in Fe$_3$GaTe$_2$. Intriguingly, we observe substantial modifications to these crucial driving factors through doping, which we attribute to alterations in multiple spin-splitting bands near the Fermi level. These findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high-$\textit{T}$$_C$ ferromagnetic ordering in Fe$_3$GaTe$_2$.

cond-mat.mtrl-sci↗

arXiv2022

Bulk-interface correspondence from quantum distance in flat band systems

The bulk-boundary correspondence is an integral feature of topological analysis and the existence of boundary or interface modes offers direct insight into the topological structure of the Bloch wave function. While only the topology of the wave function has been considered relevant to boundary modes, we demonstrate that another geometric quantity, the so-called quantum distance, can also host a bulk-interface correspondence. We consider a generic class of two-dimensional flat band systems, where the flat band has a parabolic band-crossing with another dispersive band. While such flat bands are known to be topologically trivial, we show that the nonzero maximum quantum distance between the eigenstates of the flat band around the touching point guarantees the existence of boundary modes at the interfaces between two domains with different chemical potentials or different maximum quantum distance. Moreover, the maximum quantum distance can predict even the explicit form of the dispersion relation and decay length of the interface modes.

cond-mat.str-el↗

arXiv2022

Flexoelectric control of a ferromagnetic metal

Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experimentally unexplored in metals. Here, using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields can polarize a metallic oxide SrRuO3 with unexpectedly large Ru off-center displacements. We also observe that the flexoelectrically induced polar state of SrRuO3 leads to sizable lattice expansion, similar to the electrostrictive expansion caused by ionic displacements in dielectrics under an external electric field. We further suggest that flexoelectrically driven Ru off-centering promotes strong coupling between lattice and electronic degrees of freedom, possibly enhancing the ferromagnetism of SrRuO3. Beyond conventional electric fields, flexoelectric fields may universally engender novel electronic states and their control via pure atomic displacements in a nondestructive and fast manner.

cond-mat.mtrl-sci↗

arXiv2021

Stable hump-like Hall effect and non-coplanar spin textures in SrRuO$_3$ ultrathin film

We observed a hump-like feature in Hall effects of SrRuO$_3$ ultrathin films, and systematically investigated it with controlling thicknesses, temperatures and magnetic fields. The hump-like feature is extremely stable, even surviving as a magnetic field is tilted by as much as 85$^\circ$. Based on the atomic-level structural analysis of a SrRuO$_3$ ultrathin film with a theoretical calculation, we reveal that atomic rumplings at the thin-film surface enhance Dzyaloshinskii-Moriya interaction, which can generate stable chiral spin textures and a hump-like Hall effect. Moreover, temperature dependent resonant X-ray measurements at Ru L-edge under a magnetic field showed that the intensity modulation of unexpected peaks was correlated with the hump region in the Hall effect. We verify that the two-dimensional property of ultrathin films generates stable non-coplanar spin textures having a magnetic order in a ferromagnetic oxide material.

cond-mat.str-el↗

arXiv2021

Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite oxide thin films

Magnetism and spin-orbit coupling (SOC) are two quintessential ingredients underlying novel topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by SOC, the nodal structures become a source of Berry curvature; this leads to a large anomalous Hall effect (AHE). Contrary to three-dimensional systems that naturally host nodal points/lines, two-dimensional (2D) systems can possess stable nodal structures only when proper crystalline symmetry exists. Here we show that 2D spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the AHE. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO$_3$, a representative metallic ferromagnet with SOC. We show that the sign-changing AHE upon variation in the film thickness, magnetization, and chemical potential can be well explained by theoretical models. Our study is the first to directly characterize the topological band structure of 2D spin-polarized bands and the corresponding AHE, which could facilitate new switchable devices based on ferromagnetic ultrathin films.

cond-mat.str-el↗

arXiv2019

Magnetic and electrical anisotropy with correlation and orbital effects in dimerized honeycomb ruthenate Li$_2$RuO$_3$

Li2RuO3 undergoes a structural transition at a relatively high temperature of 550 K with a distinct dimerization of Ru-Ru bonds on the otherwise isotropic honeycomb lattice. It exhibits a unique herringbone dimerization pattern with a largest ever reported value of the bond shrinkage of about ~ 0.5 Å. Despite extensive studies, both theoretical and experimental, however, its origin and its effect on physical properties still remain to be understood. In this work, using high quality single crystals we investigated the anisotropy of resistivity ($ρ$) and magnetic susceptibility ($χ$) to find a very clear anisotropy: $ρ$$_c*$ > $ρ$$_b$ > $ρ$$_a$ and $χ$$_b$ > $χ$$_a$ > $χ$$_c*$. For possible theoretical interpretations, we carried out density functional calculations to conclude that these anisotropic behavior is due to the correlation effects combined with the unique orbital structure and the dimerization of Ru 4d bands.

cond-mat.str-el↗

arXiv2019

Unconventional crystal field splitting in non-centrosymmetric BaTiO$_3$ thin films

Understanding the crystal field splitting and orbital polarization in non-centrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking BaTiO$_3$ (BTO) as a representative material we investigate titanium crystal field splitting and orbital polarization in non-centrosymmetric TiO$_6$ octahedra with resonant X-ray linear dichroism at Ti $L_{2,3}$-edge. The high-quality BaTiO$_3$ thin films were deposited on DyScO$_3$ (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction (RHEED) and element specific X-ray absorption spectroscopy (XAS) were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal field splitting and orbital configuration ($d_{xz}$/$d_{yz}$ $<$ $d_{xy}$ $<$ $d_{3z^2-r^2}$ $<$ $d_{x^2-y^2}$ or $d_{xy}$ $<$ $d_{xz}$/$d_{yz}$ $<$ $d_{x^2-y^2}$ $<$ $d_{3z^2-r^2}$) according to Jahn-Teller effect, it is revealed that $d_{xz}$, $d_{yz}$, and $d_{xy}$ orbitals are nearly degenerate, whereas $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbitals are split with an energy gap $\sim$ 100 meV in the epitaxial BTO films. The unexpected degenerate states $d_{xz}$/$d_{yz}$/$d_{xy}$ are coupled to Ti-O displacements resulting from competition between polar and Jahn-Teller distortions in non-centrosymmetric TiO$_6$ octhedra of BTO films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.

cond-mat.str-el↗

arXiv2019

Superlattice-induced ferroelectricity in charge-ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3}$

Charge-order-driven ferroelectrics are an emerging class of functional materials, distinct from conventional ferroelectrics, where electron-dominated switching can occur at high frequency. Despite their promise, only a few systems exhibiting this behavior have been experimentally realized thus far, motivating the need for new materials. Here, we use density functional theory to study the effect of artificial structuring on mixed-valence solid-solution La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (LSFO), a system well-studied experimentally. Our calculations show that A-site cation (111)-layered LSFO exhibits a ferroelectric charge-ordered phase in which inversion symmetry is broken by changing the registry of the charge order with respect to the superlattice layering. The phase is energetically degenerate with a ground-state centrosymmetric phase, and the computed switching polarization is 39 $μ$C/cm$^{2}$, a significant value arising from electron transfer between Fe ions. Our calculations reveal that artificial structuring of LSFO and other mixed valence oxides with robust charge ordering in the solid solution phase can lead to charge-order-induced ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2018

Artificial two-dimensional polar metal at room temperature

Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO$_3$/SrTiO$_3$/LaTiO$_3$. A combination of atomic resolution scanning transmission electron microscopy with electron energy loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases.

cond-mat.mtrl-sci↗

arXiv2016

Charge-order-induced ferroelectricity in LaVO$_{3}$/SrVO$_{3}$ superlattices

The structure and properties of the 1:1 superlattice of LaVO$_{3}$ and SrVO$_{3}$ are investigated with a first-principles density-functional-theory-plus-$U$ (DFT+$U$) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered cation ordering to produce a polar structure with nonzero spontaneous polarization normal to the interfaces. This polarization is produced by electron transfer between the V$^{3+}$ and V$^{4+}$ layers, and is comparable to that of conventional ferroelectrics. The energy of this polar state relative to the nonpolar ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the polar phase can be induced by applied electric field, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the nonpolar state on removal of the field, a ferroelectric-type hysteresis loop could be observed.

cond-mat.mtrl-sci↗

arXiv2016

Flux states and topological phases from spontaneous time-reversal symmetry breaking in CrSi(Ge)Te$_3$-based systems

We study adatom-covered single layers of CrSiTe$_3$ and CrGeTe$_3$ using first-principles calculations based on hybrid functionals. We find that the insulating ground state of a monolayer of La (Lu) deposited on single-layer CrSiTe$_3$ (CrGeTe$_3$) carries spontaneously generated current loops around the Cr sites, These "flux states" induce antiferromagnetically ordered orbital moments on the Cr sites and are also associated with nontrivial topological properties. The calculated Chern numbers for these systems are predicted to be $\pm 1$ even in the absence of spin-orbit coupling, with sizable gaps on the order of 100 meV. The flux states and the associated topological phases result from spontaneous time-reversal symmetry breaking due to the presence of nonlocal Coulomb interactions.

cond-mat.mtrl-sci↗