About

Harold Y. Hwang is the Shadi and Omid Farokhzad Professor of Applied Physics at Stanford University. His research interests encompass condensed matter physics, with a focus on probing correlated electrons at artificial interfaces and in confined systems. He specializes in atomic scale synthesis and control of complex oxide heterostructures, low-dimensional superconductivity, and developing novel devices based on interface states in oxides. His work involves exploring materials physics through advanced experimental techniques to understand and manipulate electronic properties at the nanoscale, contributing to the fields of nanoscience and quantum engineering.

Research topics

• Materials science
• Physics
• Chemistry
• Crystallography
• Condensed matter physics
• Biochemistry
• Composite material
• Biology
• Nanotechnology
• Metallurgy
• Chemical physics

Selected publications

• Atomic-Scale Moiré and Electronic Structure Analysis of Twisted Epitaxial MoS ₂ –Au–MoS ₂ Heterostructures

  Nano Letters · 2026-02-18

  article

  Twisted epitaxy enables precise orientation control of nanostructures confined within van der Waals (vdW) gaps. Here, we investigate the moiré and electronic structure of a representative twisted epitaxial system, where Au nanodiscs are grown inside twisted bilayer MoS2 with a 6° interlayer twist, inducing a 3° symmetrical misalignment of Au relative to each MoS2 layer (MoS2–Au–MoS2). Using multislice electron ptychography (MEP), we resolve the three-dimensional “moiré-of-moirés” structure of MoS2–Au–MoS2 with atomic resolution. Electron energy loss spectroscopy (EELS) shows that MoS2 encapsulation significantly reduces the plasmon energy of Au nanodiscs compared with their unencapsulated counterparts. Furthermore, first-principles calculations reveal that Au insertion alters the electronic band alignment near the Fermi level of bilayer MoS2. Our results introduce a twisted MoS2–Au–MoS2 heterostructure as a structurally and electronically rich material system and establish twisted epitaxy as a new strategy for moiré engineering and the synthesis of 2D-confined materials with tunable optoelectronic properties.

  PublisherDOI

• Fermi-liquid transport beyond the upper critical field in superconducting La2PrNi2O7 thin films

  Nature Communications · 2026-03-10

  articleOpen accessSenior author

  Unconventional superconductivity typically emerges out of a strongly correlated normal state, manifesting as a highly renormalised Fermi liquid or a strange metal with T-linear resistivity. In Ruddlesden-Popper bilayer nickelates, superconductivity with a critical temperature T_c exceeding 80 and 40 K has been respectively realised in pressurised 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_c superconductivity. Here we report detailed magnetotransport experiments on superconducting La₂PrNi₂O₇ (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 T² temperature dependence of resistivity and Hall angle, and H² magnetoresistance obeying Kohler scaling. Using the empirical Kadowaki-Woods ratio, we estimate a quasiparticle effective mass m^*/m_e ≃ 10, thereby revealing the highly renormalised Fermi liquid state therein. Our results demonstrate that thin-film LPNO follows the same T_c/T_F scaling observed across a myriad of strongly correlated superconductors and establish key normal-state characteristics of strained bilayer superconducting nickelates.

  PublisherDOI

• Freestanding SrNbO3 membranes as flexible transparent conductors

  APL Materials · 2026-01-01

  articleOpen accessSenior author

  Synthesizing flexible transparent conducting materials is important for applications in flexible optoelectronics. Metallic oxide membranes offer great promise due to their large elasticity and high transparency. Among them, SrNbO3, a correlated metal, stands out as a promise transparent conducting oxide. However, synthesizing freestanding SrNbO3 membranes poses a challenge because it easily undergoes oxidation, resulting in degraded conductivity. To address this, we utilize epitaxial BaTiO3 capping layers, which effectively prevents SrNbO3 oxidation. We successfully fabricate millimeter-sized transparent conducting BaTiO3/SrNbO3/BaTiO3 membranes using water-soluble sacrificial layers. The obtained SrNbO3 (10 nm thick) membrane exhibits a room temperature sheet resistance (RS) of 174 Ω/sq and a transmittance (T) of 92% at a wavelength of 550 nm. The figure of merit (T10/RS) of this transparent conductor reaches 2.2 × 10−3 Ω−1 in the visible regime, comparable to SrNbO3 films on rigid substrates. These results open possibilities for applications as flexible transparent conductors.

  PublisherOA PDFDOI

• Reducing the Strain Required for Ambient‐Pressure Superconductivity in Ruddlesden‐Popper Bilayer Nickelates

  Advanced Materials · 2026-02-12 · 6 citations

  articleOpen accessSenior authorCorresponding

  ABSTRACT The discovery of high‐temperature superconductivity in pressurized bulk Ruddlesden‐Popper (RP) bilayer nickelates has prompted the conjecture that epitaxial compressive strain might mimic essential aspects of hydrostatic pressure. The realization of superconductivity in films on SrLaAlO 4 (001) (SLAO) supports this correspondence, yet it remains unclear whether the pressure–temperature phase diagram of RP bilayer nickelates can be systematically mapped (and studied at ambient pressure) as a function of epitaxial strain. To this end, experimental access near the elusive edge of the superconducting phase boundary would provide invaluable insight into the nature of the superconducting state and the ground state from which it emerges. Here we report superconducting RP bilayer nickelates grown on LaAlO 3 (001) (LAO), where the compressive strain required for ambient‐pressure superconductivity is nearly halved to −1.2%. These films exhibit a superconducting onset above 10 K and reach zero resistance at 3 K, with normal‐state transport properties differing from those of films grown on SLAO. Our comparative study shows that strain–rather than interfacial structure is the primary factor governing the superconductivity and normal‐state properties. This work offers a new opportunity to probe emergent phenomena near the superconducting phase boundary in the strain–temperature phase diagram of RP bilayer nickelates.

  PublisherOA PDFDOI

• Surface preparation method for investigating the three-dimensional electronic structure of perovskite nickelates

  Physical review. B./Physical review. B · 2025-06-27

  articleOpen access

  The investigation of the electronic structures on perovskite oxides using surface-sensitive spectroscopy techniques is often hindered by their “uncleavable” nature, typically requiring expensive and complex experimental setups that integrate the capabilities of sample synthesis and spectroscopy measurement under ultrahigh vacuum condition. Here, we address this challenge by developing an ozone-annealing process that yields atomically flat surfaces on perovskite oxide thin films, making them suitable for high-resolution angle-resolved photoemission spectroscopy measurements. Using this method, we present a three-dimensional electronic structure study of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>Nd</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi>Sr</a:mi><a:mi>x</a:mi></a:msub><a:msub><a:mi>NiO</a:mi><a:mn>3</a:mn></a:msub></a:mrow></a:math> (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>x</b:mi><b:mo>=</b:mo><b:mn>0</b:mn></b:mrow></b:math> and 0.175) thin films with unprecedented accuracy. The experimentally determined low-energy fermiology exhibits quantitative agreements with two-band tight-binding simulations, which is further validated by first-principles calculations considering the material's actual crystal structure. This work provides an accessible approach for ARPES measurements on perovskite oxides and other strongly correlated oxides, including the recently discovered high-<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>T</c:mi><c:mtext>c</c:mtext></c:msub></c:math> nickelates.

  PublisherDOI

• Ground optical communication array terminal (GOCArT)

  2025-09-18

  article

  The Ground Optical Communication Array Terminal (GOCArT) is designed as a scalable-aperture, deployable ground terminal with the following innovations under investigation 1) telescope array transmitter with Lateral Transfer Retroreflectors for boresight auto-alignment, 2) low-excess-noise high-sensitivity avalanche photodiode (APD) 2.5 Gbpscapable detectors 3) large-area 2.5 Gbps-capable APD detectors 4) “fly’s-eye” array receiver using single-detector. We present our analysis and design of a ground terminal that is compatible with the Space Development Agency (SDA) standard for 2.5 Gbps space-ground free-space optical communication links. Building on this SDA Phase II SBIR effort, Relative Dynamics’ end goal is to provide operational GOCArT capability for the SDA's Layered Network of Military Satellites, known as the Proliferated Warfighter Space Architecture (PWSA). Two important free-space optical communication ground terminal design drivers are receiver sensitivity and atmospheric turbulence mitigation. We compare approaches to achieve high receiver sensitivity for 2.5 Gbps OOK. We report results for GaAsSb/AlGaAsSb APDs for use at telecom C-band (1550 nm) with extremely low-excess noise factor. We report results for InGaAs/InAlAs APDs for use at telecom C-band (1550 nm) with large detection area (&gt; 100 micron diameter) to improve GOCArT performance in atmospheric turbulence. We present analysis showing that a telescope array provides only minimal improvement for direct-detection receiver systems that are thermal-noise limited and use forward error correction codes. We introduce a fly’s eye receiver with a multimode fiber-optic combiner as an option. We show analysis, design, and tests of a fly’s eye receiver prototype system to assess the performance. GOCArT uses a small array of small aperture transmitter telescopes to mitigate the effects of atmospheric turbulence on the uplink.

  PublisherDOI

• Superconductivity in compressed quasi−one-dimensional face-sharing hexagonal perovskite chalcogenides

  Science Advances · 2025-09-12 · 2 citations

  articleOpen access

  Oxide perovskite superconductors typically feature stacks of metal-oxygen octahedra or planar blocks connected through corners, forming three-dimensional (3D) or 2D layered structures. Here, we find a group of quasi-1D superconducting materials among hexagonal perovskite chalcogenides with face-sharing connectivity. Resistance and magnetization measurements demonstrate anisotropic superconductivity in compressed barium titanium trisulfide (BaTiS 3 ) at a low hole carrier concentration of (1.6 ± 0.1) × 10 21 per cubic centimeter, with the highest superconducting temperature ( T c ) reaching ~9.3 kelvin. Synchrotron x-ray diffraction indicates that the superconducting phase retains a hexagonal perovskite structure consisting of quasi-1D infinite titanium hexasulfide chains. Density functional theory calculations, combined with the observed decrease in the maximum T c from ~9.3 to ~6.2 kelvin upon substituting sulfur with selenium, suggest that electron-phonon interactions play a key role in the pairing mechanism of superconducting BaTiX 3 (X = sulfur and selenium). Our study offers a quasi-1D platform with face-sharing metal-chalcogen octahedra for understanding the mechanism of emerging electronic states in perovskite materials.

  PublisherDOI

• Dynamic Doping of Nickelates with Lithium Reveals a Widely Tunable Insulator–Metal Transition with Charge Filling and Band Renormalization Regimes

  ACS Nano · 2025-08-01 · 1 citations

  article

  The insertion of electron-donating ions has emerged as a powerful technique to manipulate the electronic structure of correlated oxides. However, the resulting electronic structure remains poorly understood, with challenges in quantifying dopant concentration, unexplained differences with substitutionally doped films, and a poor understanding of how dopant atoms interact with insulator–metal transitions (IMTs). Here, these issues are addressed in the context of the rare earth nickelates, a prototypical correlated oxide family with widely tunable electronic behavior under the insertion of protons and alkali metals as interstitial dopants. RNiO3 (R = Pr, Nd) epitaxial thin films are synthesized, lithium dopants are introduced and quantified using electrochemical and synchrotron-based techniques, and the resulting electronic structure is studied. From electronic transport measurements of LixLiRNiO3, lithium is found to affect the metal–insulator transition, causing more than an order of magnitude reduction in ground-state resistivity at fractions xLi < 0.18, a systematic lowering of transition temperature, and successively smaller ON/OFF ratios over 0.00 < xLi < 0.25. At larger fractions xLi > 0.25, the transition is destroyed, and insulating behavior is observed over T = 5–300 K. Angle-resolved photoemission (ARPES) confirms transport results and reveals band renormalization occurring over 0.10 < xLi ≤ 0.71. ARPES and X-ray absorption spectroscopy (XAS) combined with density functional theory indicate that rigid band filling models are generally insufficient to explain doping from lithium, especially at low temperatures, but could approximate room temperature effects in the low doping regime (xLi < 0.10). Broadly, the results indicate that interstitial dopants lead to complex interactions with metal–insulator transitions and the emergence of an exciting family of correlated electronic phases.

  PublisherDOI

• Author Correction: Superconductivity and normal-state transport in compressively strained La2PrNi2O7 thin films

  Nature Materials · 2025-10-10 · 2 citations

  erratumOpen accessSenior author
  PublisherOA PDFDOI

• Controllable dynamic magnetism in reconfigurable ferroelastic domain-wall networks engineered via nanocavities

  Physical review. B./Physical review. B · 2025-11-04

  article

  Ferroelastic domain walls (DWs) offer a unique platform for engineering dynamic magnetism through controlled symmetry breaking. Here, we demonstrate deterministic magnetic field generation in reconfigurable DW networks using nanocavity-patterned ferroelastic matrices. Atomistic simulations reveal that propagating kinks along polar twin walls induce strain-gradient polarization and displacement current vortices, producing localized magnetic fields $(\ensuremath{\sim}{10}^{\ensuremath{-}7}\text{--}{10}^{\ensuremath{-}6} \mathrm{T})$ via flexoelectric coupling. These fields exhibit scale-free avalanche dynamics with universal power-law exponents, mirroring mechanical energy release events during kink nucleation and depinning transitions. To overcome intrinsic disorder limitations, we mimic lithographically defined nanocavities that guide orthogonal DW propagation with cycle-to-cycle reproducibility, achieving local field enhancement compared to stochastic networks. Scanning superconducting quantum interference device (SQUID) microscopy on ${\mathrm{SrTiO}}_{3}$ detects out-of-plane magnetic signatures $(\ensuremath{\sim}{10}^{\ensuremath{-}7} \mathrm{T})$ persisting for milliseconds, quantitatively matching simulations and indicating extended lifetimes. Crucially, kink velocity-dependent dynamic magnetism is established through synchronized magnetic and energy jerk profiles, resolving the atomic-scale mechanism linking DW motion to emergent magnetism. This work establishes a multiscale framework for defect-engineered ferroelastic materials, bridging atomic-scale polarization dynamics, mesoscale DW circuit design, and macroscale nonvolatile memory functionality. By decoupling magnetic responses from intrinsic disorder, our approach advances ferroelastic DW networks toward practical applications in strain-programmable spintronics and ultrahigh-density racetrack memories.

  PublisherDOI

Frequent coauthors

• Yasuyuki Hikita

  Nitto (Japan)

  348 shared

• Christopher Bell

  University of Bristol

  203 shared

• Kyuho Lee

  130 shared

• Bai Yang Wang

  SLAC National Accelerator Laboratory

  124 shared

• Motoki Osada

  SLAC National Accelerator Laboratory

  116 shared

• Danfeng Li

  City University of Hong Kong

  112 shared

• Lena F. Kourkoutis

  Cornell University

  106 shared

• Yi Cui

  Stanford University

  90 shared

Labs

• Hwang LabPI

  Research on oxide thin films, electrochemistry, and energy-related applications

Education

• Ph.D., Applied Physics

  Stanford University

  1990

• M.S., Applied Physics

  Stanford University

  1986

• B.S., Physics

  California Institute of Technology

  1982