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Harold Y. Hwang

Publications and source records attributed to Harold Y. Hwang.

At least 19 recordsLinked to original sources
arXiv2026

Bend Contour Electron Tomography (BCET): Quantitative strain and topography mapping

Freestanding thin films of quantum materials naturally develop sub-micrometer, nonuniform strain fields that strongly affect their electronic, magnetic, and structural properties in both equilibrium and nonequilibrium conditions. However, current methods to quantitatively resolve these mesoscopic features are primarily restricted to scanning probes, making it challenging for dynamical measurements such as single-shot imaging and femtosecond microscopy. Here, we present a new computational framework which we denote as bend contour electron tomography (BCET), which efficiently converts bend contours in transmission electron microscope images into quantitative two-dimensional maps of strain and topography. By iteratively minimizing a designed loss function between experimental and simulated bend contour images, BCET retrieves both surface morphology and in-plane strain tensor fields without requiring scanning or diffraction mapping. We applied BCET to freestanding SrTiO$_3$ thin films, demonstrating the successful reconstruction of the local strain distribution and curvature field with high fidelity. Our approach provides a quantitative framework for characterizing mesoscale structures in freestanding films using wide-field imaging, opening new avenues to investigate how spatial inhomogeneity governs phase transitions and nonequilibrium dynamics in two-dimensional quantum materials.

cond-mat.mes-hall↗
arXiv2026

Freestanding Antiferromagnetic Oxide Membranes: Synthesis and Characterization of Cr$_2$O$_3$

Antiferromagnetic (AFM) insulators stand out as a promising class of materials for fast switching, energy-efficient magnon-based technology, but our understanding of how to tune these properties with the lattice remains limited. As an AFM insulator with strong magnetoelectric response, Cr$_2$O$_3$ membranes stand out as a promising platform to explore multi-modal tunability via strain. However, fabricating these membranes is challenging due to the scarcity of etchable sacrificial layers with compatible lattice constants and sufficient surface quality. We address this issue by utilizing La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) as a sacrificial layer, enabling successful fabrication of millimeter-scale Cr$_2$O$_3$ membranes. Substrate-free characterization reveals that strain due to the lattice mismatch during growth is released by the formation of small polycrystalline domains, preserving single-crystalline order over ~90% of the membrane area. Bulk-like structural properties are confirmed by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and second-harmonic generation spectroscopy. Platinum Hall measurements reveal an above-room-temperature N$é$el transition. These results establish Cr$_2$O$_3$ membranes as a viable platform for strain-tuning antiferromagnetism and magnetoelectricity.

cond-mat.mtrl-sci↗
arXiv2026

Designing heterostructures to control oxygen stoichiometry in helimagnetic perovskite strontium ferrite

A large challenge in determining the physics of helimagnetic SrFeO3 is in stabilizing the stoichiometric chemical phase over long enough time scales to conduct extensive measurements. Degradation in SrFeO3 manifests mainly as a crossover from metallic to insulating behavior. Using a combination of electronic transport and density functional theory, we show that this degradation is dominated by oxygen loss, possibly on the order of one percent. We further demonstrate that high quality SrFeO3 thin films can be stabilized long-term by combining a nanoscale band insulator capping layer with an ex situ ozone anneal. We show that this produces a nearly-pristine cation sublattice and preserves metallicity for at least several weeks. These results establish a reliable pathway for producing chemically stable SrFeO3 thin films, enabling reproducible studies of its unusual helimagnetism.

cond-mat.mtrl-sci↗
arXiv2026

Doping dependence of local moments in infinite layer nickelates

The infinite layer nickelates are notable for their lack of long-range antiferromagnetic ordering, in contrast to the parent compounds of the superconducting cuprates. Instead, the nickelates show evidence of short-range glassy behavior in both the undoped and optimally-doped regimes, implying that local electronic moments exist independent of superconductivity. However, the systematic doping-dependent magnetic behavior is not yet fully resolved, and characterizing it could uncover the relationship between local moments and the superconducting dome. In this work, we use muon spin rotation ($μ$SR) on a (La,Sr)NiO$_2$ doping series from the undoped parent compound, through the superconducting dome, to the over-doped normal state (Sr substitution 0% $\leq$ x $\leq$ 25%) to probe the magnetic ground state and the temperature-dependent static and dynamic behavior. We find that local moments experience spin freezing into a glassy state at temperatures on the order of a few tens of kelvin regardless of the doping level. We also observe a subtle destabilization of the glassy state with increased hole doping. These observations suggest that magnetism and superconductivity are largely decoupled phenomena with indirect interactions described in a multi-orbital framework.

cond-mat.supr-con↗
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

Reconfigurable Strain Gradient Polarity in Crystalline Oxide Nanomembranes for Controlled Bending of Functional Materials

We report the fabrication and mechanical characterization of a "bubble" geometry for accessing local strain gradients using freestanding, single-crystalline manganite nanomembranes: a switchable bistable nanodrum, with opposite strain gradient polarities. By leveraging epitaxial strain as a source of pre-strain and with control of geometrical and mechanical boundary conditions, the fabricated device can support strain gradients with strain variation ranging from 0.01% to 1%. Switching energetics can be designed to configure the bubble morphology. By providing a mechanical framework for sustained strain gradients, this platform supports scalable oxide membrane applications such as the mechanical manipulation of magnetism, coupled to local probes.

cond-mat.mtrl-sci↗
arXiv2026

Defect-engineered scaling of lead-free ferroelectrics with ultralow-voltage switching

Scaling ferroelectrics to nanometer thicknesses remains a central challenge for low-power, nonvolatile electronics, as leakage currents increasingly dominate with reduced dimensions. Alkali-based, lead-free ferroelectrics offer an environmentally sustainable alternative to lead-based systems, yet their scaling is severely limited by leakage arising from volatile alkali constituents. Here, we show that this intrinsic limitation can be transformed into an advantageous degree of freedom through defect engineering. By precisely modulating alkali deficiency during thin-film synthesis, we engineer clustered defect complexes that function as deep trap states, strongly suppressing leakage and enabling robust ferroelectric operation in ultrathin films down to the sub-10 nm regime at voltages below 100 mV. Our results establish defect-enabled scaling as a viable pathway for advancing environmentally benign ferroelectrics toward ultra-low-power, non-volatile electronic technologies.

cond-mat.mtrl-sci↗
arXiv2026

Origin of Spin Stripes in Bilayer Nickelate La$_3$Ni$_2$O$_7$

The bilayer nickelate La$_3$Ni$_2$O$_7$ has recently emerged as a high-temperature superconductor with unusual spin stripe order in its ambient pressure phase. We propose a microscopic Hamiltonian that faithfully reflects the crystalline symmetry of this system, with the primary aim of addressing its unconventional magnetism at ambient pressure. Using state-of-the-art density matrix renormalization group calculations, we show that $(π/2,π/2)$ spin stripe order arises in our model at sizable Hund's coupling $J_H$ from a hidden quasi-one-dimensionality and persists over a range of electron concentrations. In the more symmetric high-pressure regime, our model exhibits enhanced interlayer pairing tendencies when the interlayer antiferromagnetic coupling $J_{\bot}$ becomes sufficiently large. Our results provide a microscopic origin of the diagonal spin stripes and identify Hund's coupling $J_H$ and interlayer coupling $J_{\bot}$ as key ingredients governing magnetic order and pairing tendencies in La$_3$Ni$_2$O$_7$.

cond-mat.supr-con↗
arXiv2026

Nearly Isotropic Magnon Transport in Epitaxial Lithium Aluminum Ferrite Thin Films

Low-loss magnetic insulating thin films are promising for information transport via magnons, where isotropic in-plane magnon propagation is desirable. We report nonlocal measurements of electrically and thermally generated magnons in epitaxial (001) lithium aluminum ferrite Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$ thin films with pronounced fourfold in-plane magnetic anisotropy. By measuring the inverse spin Hall signal as a function of the magnon diffusion distance, we deduce magnon diffusion lengths that are nearly identical along the [100] and [110] directions at 250~K. This isotropy is consistent with a nearly isotropic exchange stiffness. These results highlight spinel ferrites as viable platforms for isotropic magnon transport.

cond-mat.mtrl-sci↗
arXiv2026

High-temperature superconductivity in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ membranes under pressure

Lattice compression has emerged as a fundamental tuning parameter for nickelate superconductivity. Pressure acts as a trigger to induce superconductivity in bulk Ruddlesden-Popper nickelates. For infinite-layer nickelate thin films, compressive epitaxial strain and rare-earth ion chemical pressure have been used to substantially enhance the superconducting transition temperature ($T_c$). Efforts to go further have been constrained by the limits of epitaxial stability or the challenges of measuring thin films in high-pressure environments. Here, we overcome this limitation by developing a technique to incorporate freestanding infinite-layer $\mathrm{Nd_{0.85}Sr_{0.15}NiO_2}$ membranes into a diamond anvil cell. Using this platform, we observe a strong increase in $T_c$ up to our highest measurement pressure of $\sim$90 GPa, where a superconducting downturn can be observed near liquid nitrogen temperatures. Strikingly, we find a simple linear enhancement of $T_c$ at a rate of 0.65 K GPa$^{-1}$, with no signs of saturation. This suggests that the pairing strength in infinite-layer nickelates can be raised to a surprisingly high scale, using an approach that can be broadly applied to many two-dimensional materials.

cond-mat.supr-con↗
arXiv2026

Evolution of the Superfluid Density in Infinite-Layer Nickelates

Nickelate superconductors provide a valuable new platform for the study of unconventional superconductivity that is complementary to the cuprates. One of the central puzzles about high-temperature superconductors is what factors determine the scale of their superconducting transition temperature ($T_\mathrm{c}$). To address this question for infinite-layer nickelates, we present a systematic mutual inductance study of the superfluid density across the doping-dependent superconducting dome of $\mathrm{Nd}_{1-x}\mathrm{Sr}_x\mathrm{NiO}_2$. We observe a weak superfluid stiffness that exhibits an approximately square-root correlation with $T_\mathrm{c}$. We also find a strong interplay between Nd magnetism and the superconducting phase, manifested as a substantial low-temperature suppression of superfluid density. These observations highlight the importance of superconducting phase fluctuations in limiting $T_\mathrm{c}$ and unexpectedly strong coupling between the Nd 4$f$ moments and the superfluid.

cond-mat.supr-con↗
arXiv2026

Machine Learning Reconstruction of High-Dimensional Electronic Structure from Angle-Resolved Photoemission Spectroscopy

The emergent behavior of quantum materials is governed by their electronic structure, which can be experimentally probed by photoemission spectroscopy techniques that generate a four-dimensional dataset of energy and momentum. However, the quantitative extraction of Hamiltonian parameters from these high-dimensional spectra remains a significant challenge, currently relying on labor-intensive, expert-dependent analysis rather than standardized workflows. Here, we introduce a deep learning framework based on implicit neural representations to accelerate the retrieval of Hamiltonian parameters in two types of transition-metal oxides: perovskite nickelates and manganites. Our approach outperforms traditional analytical fitting procedures, yielding superior agreement with experimental Fermi surface topologies and energy-momentum dispersions. This work highlights the potential of deep learning tools to bridge the gap between theory and experiment, paving the way for high-throughput, autonomous discovery pipelines in quantum materials.

cond-mat.str-el↗
arXiv2026

A superconducting half-dome in bilayer nickelates

Understanding how superconductivity emerges and collapses in correlated electron systems remains a central challenge in condensed matter physics. As a recently discovered member of the high temperature superconductor family, bilayer nickelates provide a new opportunity for examining this problem. Their pronounced sensitivity to oxygen stoichiometry, while posing challenges for stabilizing superconductivity, simultaneously offers an effective control parameter for tuning electronic phases. Here we report a superconducting half-dome in compressively strained bilayer nickelate thin films as a function of continuous tuning of oxygen stoichiometry. Starting from an optimally superconducting state, increasing oxygen stoichiometry gradually suppresses superconductivity toward a metallic phase, whereas decreasing oxygen stoichiometry drives a granular superconductor-to-insulator transition while leaving the superconducting onset intact. This half-dome structure can be understood to arise from the contrasting roles played by interstitial oxygen versus oxygen vacancies - namely the dominance of doping versus scattering. Notably, the half-dome emerges consistently across samples with different rare-earth combinations, with or without alkaline-earth doping, revealing a general feature of the bilayer nickelate phase diagram.

cond-mat.supr-con↗
arXiv2026

Superconductivity and magnetism in bilayer nickelates: itinerant perspective

We study superconductivity and magnetism in bilayer nickelates from an itinerant perspective. Starting from a tight binding fit to recent ARPES measurements on compressively strained thin films, we incorporate the standard set of onsite repulsive interactions among partially filled $e_g$ orbitals: intra-orbital $U$, inter-orbital $U'$, Hund's coupling $J_H$ and a pair hopping $J_P$. We obtain the effective pairing interaction by dressing these bare interactions with particle-hole fluctuations via the RPA. In the strong Hund's coupling regime, we find that $s$-wave superconductivity and $(π/2, π/2)$ SDW order are the favored ground states. With weaker Hund's coupling, we find that $d$-wave pairing and $(π, π)$ SDW are the leading ground states. Our results are qualitatively consistent with earlier DMRG studies, and point to the key role played by Hund's coupling in determining the nature of superconductivity and magnetism in this system.

cond-mat.supr-con↗
arXiv2026

Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3

We demonstrate control of helimagnetic order in biaxially strained SrFeO3 thin films using neutron diffraction and resonant soft x-ray scattering. SrFeO3, a negative charge-transfer oxide, exhibits a complex magnetic phase diagram that includes multi-q spin structures. Tensile epitaxial strain produces a pronounced shortening of the helimagnetic ordering length and a tilting of the magnetic ordering vector. We interpret this behavior in terms of chemical expansion: lattice dilation under tensile strain lowers the energetic cost of oxygen vacancies, leading to an expanded unit cell that modifies Fe-O hybridization and enhances superexchange relative to double exchange. These results reveal how epitaxial strain can indirectly tune helimagnetism through defect-driven chemical expansion, highlighting the strong coupling between lattice, chemistry, and magnetic order in transition-metal oxides. Our findings establish chemical expansion as an effective mechanism for engineering complex magnetic textures in oxide thin films, with implications for spintronic, magnonic, and quantum information applications.

cond-mat.mtrl-sci↗
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↗
arXiv2026

Fermi-liquid transport beyond the upper critical field in superconducting La$_2$PrNi$_2$O$_7$ thin films

Unconventional superconductivity typically emerges out of a strongly correlated normal state, manifesting as a highly renormalized Fermi liquid or a strange metal with $T$-linear resistivity. In Ruddlesden-Popper bilayer nickelates, superconductivity with a critical temperature $T_{\rm c}$ exceeding 80 and 40~K has been respectively realised in pressurized bulk crystals and epitaxially strained thin films. These advancements call for the characterisation of fundamental normal-state and superconducting parameters in these new materials platforms of high-$T_{\rm c}$ superconductivity. Here we report detailed magnetotransport experiments on superconducting La$_2$PrNi$_2$O$_7$ (LPNO) thin films under pulsed magnetic fields up to 64~T and access the normal-state behaviour over a wide temperature range between 1.5 and 300~K. We find that the normal state of thin-film LPNO exhibits the hallmarks of Fermi-liquid transport, including $T^2$ temperature dependence of resistivity and Hall angle, and $H^2$ magnetoresistance obeying Kohler scaling. Using the empirical Kadowaki-Woods ratio, we estimate a quasiparticle effective mass $m^*/m_e \simeq 10$, thereby revealing the highly renormalized Fermi liquid state therein. Our results demonstrate that thin-film LPNO follows the same $T_{\rm c}/T_{\rm F}$ scaling observed across a myriad of strongly correlated superconductors and establish key normal-state characteristics of strained bilayer superconducting nickelates.

cond-mat.supr-con↗
arXiv2026

High spin, low spin or gapped spins: magnetism in the bilayer nickelates

Inspired by the recent discovery of high-temperature superconductivity in bilayer nickelates, we investigate the role of magnetism emerging from a hypothetical insulating $d^8$ parent state. We demonstrate that due to the interplay of superexchange and Hund's coupling, the system can be in a high-spin, low-spin or spin-gapped state. The low-spin state has singlets across the bilayer in the $d_{z^2}$ orbital, with charge carriers in the $d_{x^2-y^2}$ orbital. Thus, at low energy scales, it behaves as an effective one band system when hole doped. By contrast, the high-spin state is a more robust, spin-1 antiferromagnet. Using Hartree-Fock methods, we find that for fixed interaction strength and doping, high-spin magnetism remains more robust than the low-spin counterpart. Whether this implies that the high spin state provides a stronger pairing glue, or more strongly competes with superconductivity remains an open question. Our analysis therefore underscores the importance of identifying the spin state for understanding superconductivity in nickelates.

cond-mat.str-el↗