arXiv subjects

Eun Kyo Ko

Publications and source records attributed to Eun Kyo Ko.

11 recordsLinked to original sources

arXiv2026

Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

The discovery of high-$T_c$ superconductivity in Ruddlesden-Popper nickelates has sparked substantial effort towards understanding unconventional electronic states beyond a traditional cuprate-like $d^9$ configurational ground state. An understanding of the interplay between magnetic ground states and multi-orbital physics is key for establishing a microscopic mechanism for superconductivity. In the bilayer nickelates, spin density wave (SDW) order is a prominent feature in the non-superconducting regime, yet its relation to superconducting pairing remains an open question. Here, we use resonant x-ray scattering to examine the existence of SDW order in superconducting bilayer nickelate thin films La$_2$PrNi$_2$O$_7$ (LPNO). Comparing superconducting and oxygen-deficient LPNO thin films, we find that superconductivity occurs in SDW-free, oxygen-stoichiometric regions, whereas oxygen-deficiency promotes SDW order, indicating phase segregation of SDW and superconductivity. Furthermore, Ni-$L_3$ and O-$K$ edge spectroscopy reveals distinct electronic structures - particularly along the $c$-axis - between the two regions. Our results identify oxygen stoichiometry as a key parameter controlling interlayer coupling and thus the electronic structure of bilayer nickelates. In concert with theory, we propose that a ligand hole primarily resides at the inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state, which serves as the ground state for superconducting bilayer nickelates.

cond-mat.str-el↗

arXiv2026

Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

Since the discovery of superconductivity in infinite-layer nickelates, there have been extensive efforts to unravel their electronic structure and pairing mechanism. In particular, understanding how the electronic structure evolves with doping is essential for clarifying theoretical models of superconductivity in nickelates. Here we present studies of the optical conductivity of Nd1-xSrxNiO2 thin films spanning the full phase diagram 0.025 < x < 0.30 using spectroscopic ellipsometry. The data are consistent with a two-band Drude model, which allows the decomposition of the intraband response into distinct contributions. One is from a "narrow" Drude term which we associate with electron bands, and the other a "broad" Drude term linked to the hole band with strong correlations. Increasing Sr doping leads to an expansion of the hole band spectral weight, and a corresponding reduction in the electron band, indicative of the multiband electronic structure and a doping-dependent reconstruction of the Fermi surface. Both doping and temperature-dependent optical spectra display significant spectral weight transfer from high to low energy, a hallmark of strong electronic correlations. In the superconducting state at optimal doping (x = 0.15), both electron and hole bands contribute to the superconducting condensate, signifying multiband superconductivity.

cond-mat.supr-con↗

arXiv2025

Superconductivity and normal-state transport in compressively strained La$_2$PrNi$_2$O$_7$ thin films

The discovery of superconductivity under high pressure in Ruddlesden-Popper phases of bulk nickelates has sparked great interest in stabilizing ambient pressure superconductivity in thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, albeit with broad and two-step-like transitions. Here, we report intrinsic superconductivity and normal-state transport properties in compressively strained La$_2$PrNi$_2$O$_7$ thin films, achieved through a combination of isovalent Pr substitution, growth optimization, and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties.

cond-mat.supr-con↗

arXiv2025

Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$

The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the naïve expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum.

cond-mat.supr-con↗

arXiv2025

Resolving Structural Origins for Superconductivity in Strain-Engineered La$_3$Ni$_2$O$_7$ Thin Films

The discovery of high-temperature superconductivity in bulk La$_3$Ni$_2$O$_7$ under high hydrostatic pressure and, more recently, biaxial compression in epitaxial thin films has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La$_3$Ni$_2$O$_7$ films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography to directly measure the atomic-scale structural evolution of La$_3$Ni$_2$O$_7$ thin films across a wide range of biaxial strains tuned via substrate. By resolving both the cation and oxygen sublattices, we study strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity. Rather than previously supposed $c$-axis compression, our results highlight the importance of in-plane biaxial compression in superconducting thin films, which suggests an alternative -- possibly cuprate-like -- understanding of the electronic structure. Identifying local regions of inhomogeneous oxygen stoichiometry and high internal strain near crystalline defects, we suggest potential pathways for improving the sharpness and temperature of the superconducting transition.

cond-mat.supr-con↗

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↗

arXiv2022

Tuning orbital-selective phase transitions in a two-dimensional Hund's correlated system

Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in $\textit{J}$ related phenomena without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$ orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the $\textit{d$_x$$_y$}$ band and a Mott gap in the $\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials.

cond-mat.str-el↗

arXiv2022

Heteroepitaxial control of Fermi liquid, Hund metal, and Mott insulator phases in the single-atomic-layer limit

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenomena appear within a few atomic layers from the interface, thus limiting the application of available experimental probes. Here, we utilize atomic-scale epitaxy and photoemission spectroscopy to demonstrate the interface control of correlated electronic phases in atomic-scale ruthenate--titanate heterostructures. While bulk SrRuO$_3$ is a ferromagnetic metal, the heterointerfaces exclusively realize three distinct correlated phases in the single-atomic-layer limit. Our theory reveals that atomic-scale structural proximity effects lead to the emergence of Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO$_3$. These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure.

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↗

arXiv2020

Two-channel anomalous Hall effect in SrRuO3

The Hall effect in SrRuO$_3$ thin-films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in SrRuO$_3$ can change as a function of saturation magnetization. Here we report Hall peaks in SrRuO$_3$ in which volumetric magnetometry measurements and magnetic force microscopy indicate that the peaks result from the superposition of two anomalous Hall channels with opposite sign. These channels likely form due to thickness variations in SrRuO$_3$, creating two spatially separated magnetic regions with different saturation magnetizations and coercive fields. The results are central to the development of strongly correlated materials for spintronics.

cond-mat.str-el↗

arXiv2019

Controllable thickness inhomogeneity and Berry-curvature-engineering of anomalous Hall effect in SrRuO3 ultrathin films

In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for determining and understanding topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect (THE) or inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we systematically investigated the AHE of SrRuO3 ultrathin films with controllable inhomogeneities in film thickness (tSRO). By harnessing the step-flow growth of SrRuO3 films, we induced microscopically-ordered stripes with one-unit-cell differences in tSRO. The resultant spatial distribution of momentum-space Berry curvatures enables a two-channel AHE, which shows hump-like anomalies similar to the THE and can be continuously engineered via sub-unit-cell control of tSRO. In these inhomogeneous SRO films, we microscopically identified a two-step magnetic switching and stripe-like ferromagnetic domains. These features are fingerprints for distinguishing the two-channel AHE from the skyrmion-induced THE.

cond-mat.mtrl-sci↗