arXiv subjects

Sehwan Song

Publications and source records attributed to Sehwan Song.

14 recordsLinked to original sources

arXiv2025

Metallicity and Anomalous Hall Effect in Epitaxially-Strained, Atomically-thin RuO2 Films

The anomalous Hall effect (AHE), a hallmark of time-reversal symmetry breaking, has been reported in rutile RuO2, a debated metallic altermagnetic candidate. Previously, AHE in RuO2 was observed only in strain-relaxed thick films under extremely high magnetic fields (~50 T). Yet, in ultrathin strained films with distinctive anisotropic electronic structures, there are no reports, likely due to disorder and defects suppressing metallicity thus hindering its detection. Here, we demonstrate that ultrathin, fully-strained 2 nm TiO2/t nm RuO2/TiO2 (110) heterostructures, grown by hybrid molecular beam epitaxy, retain metallicity and exhibit a sizeable AHE at a significantly lower magnetic field (< 9 T). Density functional theory calculations reveal that epitaxial strain stabilizes a non-compensated magnetic ground state and reconfigures magnetic ordering in RuO2 (110) thin films. These findings establish ultrathin RuO2 as a platform for strain-engineered magnetism and underscore the transformative potential of epitaxial design in advancing spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2024

Dimensionality Engineering of Magnetic Anisotropy from Anomalous Hall Effect in Synthetic SrRuO3 Crystals

Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here, we investigate the impact of dimensionality of epitaxially-grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designing oxide superlattices with a correlated ferromagnetic SrRuO3 and nonmagnetic SrTiO3 layers, we observed modulated ferromagnetic behavior with the change of the SrRuO3 thickness. Especially, for three-unit-cell-thick layers, we observe a significant 1,500% improvement of coercive field in the anomalous Hall effect, which cannot be solely attributed to the dimensional crossover in ferromagnetism. The atomic-scale heterostructures further reveal the systematic modulation of anisotropy for the lattice structure and orbital hybridization, explaining the enhanced magnetic anisotropy. Our findings provide valuable insights into engineering the anisotropic hybridization of synthetic magnetic crystals, offering a tunable spin order for various applications.

cond-mat.mtrl-sci↗

arXiv2024

Electrically Tunable Spin Exchange Splitting in Graphene Hybrid Heterostructure

Graphene, with spin and valley degrees of freedom, fosters unexpected physical and chemical properties for the realization of next-generation quantum devices. However, the spin symmetry of graphene is rather robustly protected, hampering manipulation of the spin degrees of freedom for the application of spintronic devices such as electric gate tunable spin filters. We demonstrate that a hybrid heterostructure composed of graphene and LaCoO3 epitaxial thin film exhibits an electrically tunable spin exchange splitting. The large and adjustable spin exchange splitting of 155.9 - 306.5 meV was obtained by the characteristic shifts in both the spin symmetry broken quantum Hall states and the Shubnikov-de-Haas oscillations. Strong hybridization induced charge transfer across the hybrid heterointerface has been identified for the observed spin exchange splitting. The substantial and facile controllability of the spin exchange splitting provides an opportunity for spintronics applications with the electrically-tunable spin polarization in hybrid heterostructures.

cond-mat.str-el↗

arXiv2024

Tunable electron scattering mechanism in plasmonic SrMoO$_3$ thin films

4d transition metal perovskite oxides serve as suitable testbeds for the study of strongly correlated metallic properties. Among these, $SrMoO_{3}$ (SMO) exhibits remarkable electrical conductivity at room temperature. The temperature-dependent resistivity $(\rho(T))$ exhibits a Fermi-liquid behavior below the transition temperature $T^{*}$, reflecting the dominant electron-electron interaction. Above $T^{*}$, electron-phonon interaction becomes more appreciable. In this study, we employed the power-law scaling of $\rho(T)$ to rigorously determine the $T^{*}$. We further demonstrate that the $T^{*}$ can be modified substantially by ~40 K in epitaxial thin films. It turns out that the structural quality determines $T^{*}$. Whereas the plasma frequency could be tuned by the change in the electron-electron interaction via the effective mass enhancement, we show that the plasmonic properties are more directly governed by the electron-impurity scattering. The facile control of the electron scattering mechanism through structural quality modulation can be useful for plasmonic sensing applications in the visible region.

cond-mat.str-el↗

arXiv2023

Exotic magnetic anisotropy near digitized dimensional Mott boundary

The magnetic anisotropy of low-dimensional Mott systems exhibits unexpected magnetotransport behavior useful for spin-based quantum electronics. Yet, the anisotropy of natural materials is inherently determined by the crystal structure, highly limiting its engineering. We demonstrate the magnetic anisotropy modulation near a digitized dimensional Mott boundary in artificial superlattices composed of a correlated magnetic monolayer SrRuO3 and nonmagnetic SrTiO3. The magnetic anisotropy is initially engineered by modulating the interlayer coupling strength between the magnetic monolayers. Interestingly, when the interlayer coupling strength is maximized, a nearly degenerate state is realized, in which the anisotropic magnetotransport is strongly influenced by both the thermal and magnetic energy scales. Our results offer a new digitized control for magnetic anisotropy in low-dimensional Mott systems, inspiring promising integration of Mottronics and spintronics.

cond-mat.str-el↗

arXiv2023

Optical detection of bond-dependent and frustrated spin in the two-dimensional cobalt-based honeycomb antiferromagnet Cu3Co2SbO6

Two-dimensional honeycomb antiferromagnet becomes an important class of materials as it can provide a route to Kitaev quantum spin liquid, characterized by massive quantum entanglement and fractional excitations. The signatures of its proximity to Kitaev quantum spin liquid in the honeycomb antiferromagnet includes anisotropic bond-dependent magnetic responses and persistent fluctuation by frustration in paramagnetic regime. Here, we propose Cu3Co2SbO6 heterostructures as an intriguing honeycomb antiferromagnet for quantum spin liquid, wherein bond-dependent and frustrated spins interact with optical excitons. This system exhibits antiferromagnetism at 16 K with different spin-flip magnetic fields between a bond-parallel and bond-perpendicular directions, aligning more closely with the generalized Heisenberg-Kitaev than the XXZ model. Optical spectroscopy reveals a strong excitonic transition coupled to the antiferromagnetism, enabling optical detection of its spin states. Particularly, such spin-exciton coupling presents anisotropic responses between bond-parallel and bond-perpendicular magnetic field as well as a finite spin-spin correlation function around 40 K, higher than twice its N\'eel temperature. The characteristic temperature that remains barely changed even under strong magnetic fields highlights the robustness of the spin-fluctuation region. Our results demonstrate Cu3Co2SbO6 as a unique candidate for the quantum spin liquid phase, where the spin Hamiltonian and quasiparticle excitations can be probed and potentially controlled by light.

cond-mat.str-el↗

arXiv2023

Honeycomb oxide heterostructure: a new platform for Kitaev quantum spin liquid

Kitaev quantum spin liquid, massively quantum entangled states, is so scarce in nature that searching for new candidate systems remains a great challenge. Honeycomb heterostructure could be a promising route to realize and utilize such an exotic quantum phase by providing additional controllability of Hamiltonian and device compatibility, respectively. Here, we provide epitaxial honeycomb oxide thin film Na3Co2SbO6, a candidate of Kitaev quantum spin liquid proposed recently. We found a spin glass and antiferromagnetic ground states depending on Na stoichiometry, signifying not only the importance of Na vacancy control but also strong frustration in Na3Co2SbO6. Despite its classical ground state, the field-dependent magnetic susceptibility shows remarkable scaling collapse with a single critical exponent, which can be interpreted as evidence of quantum criticality. Its electronic ground state and derived spin Hamiltonian from spectroscopies are consistent with the predicted Kitaev model. Our work provides a unique route to the realization and utilization of Kitaev quantum spin liquid.

cond-mat.str-el↗

arXiv2023

Atomic-scale Modulation of Synthetic Magnetic Order in Oxide Superlattices

Atomic-scale precision control of magnetic interactions facilitates a synthetic spin order useful for spintronics, including advanced memory and quantum logic devices. Conventional modulation of synthetic spin order has been limited to metallic heterostructures that exploit RKKY interaction through a nonmagnetic metallic spacer; however, they face problems arising from Joule heating and/or electric breakdown. The practical realization and observation of a synthetic spin order across a nonmagnetic insulating spacer would lead to the development of spin-related devices with a completely different concept. Herein, we report the atomic-scale modulation of the synthetic spiral spin order in oxide superlattices composed of ferromagnetic metal and nonmagnetic insulator layers. The atomically controlled superlattice exhibit an oscillatory magnetic behavior, representing the existence of a spiral spin structure. Depth-sensitive polarized neutron reflectometry evidences modulated spiral spin structures as a function of the nonmagnetic insulator layer thickness. Atomic-scale customization of the spin state could lead the field one step further to actual spintronic applications.

cond-mat.str-el↗

arXiv2022

Tunable Ferromagnetism in LaCoO3 Epitaxial Thin Films

Ferromagnetic insulators play a crucial role in the development of low-dissipation quantum magnetic devices for spintronics. Epitaxial LaCoO3 thin film is a prominent ferromagnetic insulator, in which the robust ferromagnetic ordering emerges owing to epitaxial strain. Whereas it is evident that strong spin-lattice coupling induces ferromagnetism, the reported ferromagnetic properties of epitaxially strained LaCoO3 thin films were highly consistent. For example, even under largely modulated degree of strain, the reported Curie temperatures of epitaxially strained LaCoO3 thin films lie within 80-85 K, without much deviation. In this study, substantial enhancement (~18%) in the Curie temperature of epitaxial LaCoO3 thin films is demonstrated via crystallographic orientation dependence. By changing the crystallographic orientation of the films from (111) to (110), the crystal-field energy was reduced and the charge transfer between the Co and O orbitals was enhanced. These modifications led to a considerable enhancement of the ferromagnetic properties (including the Curie temperature and magnetization), despite the identical nominal degree of epitaxial strain. The findings of this study provide insights into facile tunability of ferromagnetic properties via structural symmetry control in LaCoO3.

cond-mat.mtrl-sci↗

arXiv2022

Preparation of large Cu3Sn single crystal by Czochralski method

Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport properties of Cu3Sn single crystals were analyzed to verify the crystal quality. Notably, compared to the mm-sized crystals from a molten Sn-flux, the cm-sized crystals obtained by the CZ method are free from contamination from flux materials, paving the way for the follow-up works.

cond-mat.mtrl-sci↗

arXiv2021

Defect engineering of magnetic ground state in EuTiO$_3$ epitaxial thin films

Atomistic defect engineering through the pulsed laser epitaxy of perovskite transition metal oxides offers facile control of their emergent opto-electromagnetic and energy properties. Among the various perovskite oxides, EuTiO3 exhibits a strong coupling between the lattice, electronic, and magnetic degrees of freedom. This coupling is highly susceptible to atomistic defects. In this study, we investigated the magnetic phase of EuTiO$_3$ epitaxial thin films via systematic defect engineering. A magnetic phase transition from an antiferromagnet to a ferromagnet was observed when the unit cell volume of EuTiO3 expanded due to the introduction of Eu-O vacancies. Optical spectroscopy and density functional theory calculations show that the change in the electronic structure as the ferromagnetic phase emerges can be attributed to the weakened Eu-Ti-Eu super-exchange interaction and the developed ferromagnetic Eu-O-Eu interaction. Facile defect engineering in EuTiO$_3$ thin films facilitates understanding and tailoring of their magnetic ground state.

cond-mat.mtrl-sci↗

arXiv2021

Epitaxial stabilization of metastable 3C BaRuO3 thin film with ferromagnetic non-Fermi liquid phase

Thin films of perovskite Ruthenates of the general formula ARuO3 (A = Ca and Sr) are versatile electrical conductors for viable oxide electronics. They are also scientifically intriguing, as they exhibit non-trivial electromagnetic ground states depending on the A-site element. Among them, realization of the cubic perovskite (3C) BaRuO3 thin film has been a challenge so far, because the 3C phase is metastable with the largest formation energy among the various polymorph phases of BaRuO3. In this study, we successfully prepared 3C BaRuO3 thin films employing epitaxial stabilization. The 3C BaRuO3 thin films show itinerant ferromagnetism with a transition temperature of ~48 K and a non-Fermi liquid phase. The epitaxial stabilization of the 3C BaRuO3 further enabled us to make a standard comparison of perovskite Ruthenates thin films, thereby establishing the importance of the Ru-O orbital hybridization in understanding the itinerant magnetic system.

cond-mat.mtrl-sci↗

arXiv2021

Tailoring topological Hall effect in SrRuO3/SrTiO3 superlattices

Investigating the effects of the complex magnetic interactions on the formation of nontrivial magnetic phases enables a better understanding of magnetic materials. Moreover, an effective method to systematically control those interactions and phases could be extensively utilized in spintronic devices. SrRuO3 heterostructures function as a suitable material system to investigate the complex magnetic interactions and the resultant formation of topological magnetic phases, as the heterostructuring approach provides an accessible controllability to modulate the magnetic interactions. In this study, we have observed that the Hall effect of SrRuO3/SrTiO3 superlattices varies nonmonotonically with the repetition number (z). Using Monte Carlo simulations, we identify a possible origin of this experimental observation: the interplay between the Dzyaloshinskii-Moriya interaction and dipole-dipole interaction, which have differing z-dependence, might result in a z-dependent modulation of topological magnetic phases. This approach provides not only a collective understanding of the magnetic interactions in artificial heterostructures but also a facile control over the skyrmion phases.

cond-mat.str-el↗

arXiv2020

Propagation control of octahedral tilt in SrRuO3 via artificial heterostructuring

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilting of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, we propose a method to selectively engineer the octahedral bonding geometries, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering has been developed using atomically designed SrRuO3/SrTiO3 superlattices. In particular, the propagation of RuO6 octahedral tilting within the SrRuO3 layers having identical thicknesses was systematically controlled by varying the thickness of adjacent SrTiO3 layers. This led to a substantial modification in the electromagnetic properties of the SrRuO3 layer, significantly enhancing the magnetic moment of Ru. Our approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

cond-mat.str-el↗