Stanford Advisors

All Publications

  • Isotropic Pauli-limited superconductivity in the infinite-layer nickelate Nd0.775Sr0.225NiO2 NATURE PHYSICS Wang, B., Li, D., Goodge, B. H., Lee, K., Osada, M., Harvey, S. P., Kourkoutis, L. F., Beasley, M. R., Hwang, H. Y. 2021
  • Superconducting Dome in Nd1-xSrxNiO2 Infinite Layer Films PHYSICAL REVIEW LETTERS Li, D., Wang, B., Lee, K., Harvey, S. P., Osada, M., Goodge, B. H., Kourkoutis, L. F., Hwang, H. Y. 2020; 125 (2)
  • Superconductivity in an infinite-layer nickelate. Nature Li, D., Lee, K., Wang, B. Y., Osada, M., Crossley, S., Lee, H. R., Cui, Y., Hikita, Y., Hwang, H. Y. 2019; 572 (7771): 624–27


    The discovery of unconventional superconductivity in (La,Ba)2CuO4 (ref. 1) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting Sr2RuO4 (ref. 2) and surface-electron-doped Sr2IrO4, which exhibits spectroscopic signatures consistent with a superconducting gap3,4, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates5,6, as well as thin-film heterostructures designed to host superconductivity. One such structure is the LaAlO3/LaNiO3 superlattice7-9, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied [Formula: see text] band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the eg orbitals10. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides11-13. Using soft-chemistry topotactic reduction14-20, NdNiO2 and Nd0.8Sr0.2NiO2 single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas NdNiO2 exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of Nd0.8Sr0.2NiO2 indicate a superconducting transition temperature of about 9 to 15 kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures21-23, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides24 and pnictides25.

    View details for DOI 10.1038/s41586-019-1496-5

    View details for PubMedID 31462797

  • Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes. Nano letters Li, D., Adamo, C., Wang, B. Y., Yoon, H., Chen, Z., Hong, S. S., Lu, D., Cui, Y., Hikita, Y., Hwang, H. Y. 2021


    A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO3, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO3 membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.

    View details for DOI 10.1021/acs.nanolett.1c01194

    View details for PubMedID 33989008

  • Epitaxial Stabilization and Oxygen Evolution Reaction Activity of Metastable Columbite Iridium Oxide ACS APPLIED ENERGY MATERIALS Lee, K., Flores, R. A., Liu, Y., Wang, B., Hikita, Y., Sinclair, R., Bajdich, M., Hwang, H. Y. 2021; 4 (4): 3074-3082
  • Universal behavior of the bosonic metallic ground state in a two-dimensional superconductor NPJ QUANTUM MATERIALS Chen, Z., Wang, B., Swartz, A. G., Yoon, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2021; 6 (1)
  • Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates. Proceedings of the National Academy of Sciences of the United States of America Goodge, B. H., Li, D., Lee, K., Osada, M., Wang, B. Y., Sawatzky, G. A., Hwang, H. Y., Kourkoutis, L. F. 2021; 118 (2)


    The recent observation of superconductivity in [Formula: see text] has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott-Hubbard character of [Formula: see text] Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

    View details for DOI 10.1073/pnas.2007683118

    View details for PubMedID 33397720

  • Phase diagram of infinite layer praseodymium nickelate Pr1-xSrxNiO2 thin films PHYSICAL REVIEW MATERIALS Osada, M., Wang, B., Lee, K., Li, D., Hwang, H. Y. 2020; 4 (12)
  • Superconducting Dome in Nd_{1-x}Sr_{x}NiO_{2} Infinite Layer Films. Physical review letters Li, D., Wang, B. Y., Lee, K., Harvey, S. P., Osada, M., Goodge, B. H., Kourkoutis, L. F., Hwang, H. Y. 2020; 125 (2): 027001


    We report the phase diagram of Nd_{1-x}Sr_{x}NiO_{2} infinite layer thin films grown on SrTiO_{3}. A superconducting dome spanning 0.125

    View details for DOI 10.1103/PhysRevLett.125.027001

    View details for PubMedID 32701320

  • A Superconducting Praseodymium Nickelate with Infinite Layer Structure. Nano letters Osada, M., Wang, B. Y., Goodge, B. H., Lee, K., Yoon, H., Sakuma, K., Li, D., Miura, M., Kourkoutis, L. F., Hwang, H. Y. 2020


    A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd0.8Sr0.2NiO2, epitaxially stabilized on SrTiO3 substrates via topotactic reduction from the perovskite precursor phase. Here, we present the synthesis and properties of PrNiO2 thin films on SrTiO3. Upon doping in Pr0.8Sr0.2NiO2, we observe superconductivity with a transition temperature of 7-12 K and robust critical current density at 2 K of 334 kA/cm2. These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4f configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO2 planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

    View details for DOI 10.1021/acs.nanolett.0c01392

    View details for PubMedID 32574061

  • Aspects of the synthesis of thin film superconducting infinite-layer nickelates APL MATERIALS Lee, K., Goodge, B. H., Li, D., Osada, M., Wang, B., Cui, Y., Kourkoutis, L. F., Hwang, H. Y. 2020; 8 (4)

    View details for DOI 10.1063/5.0005103

    View details for Web of Science ID 000526748400002

  • Extreme tensile strain states in La0.7Ca0.3MnO3 membranes. Science (New York, N.Y.) Hong, S. S., Gu, M. n., Verma, M. n., Harbola, V. n., Wang, B. Y., Lu, D. n., Vailionis, A. n., Hikita, Y. n., Pentcheva, R. n., Rondinelli, J. M., Hwang, H. Y. 2020; 368 (6486): 71–76


    A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale La0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.

    View details for DOI 10.1126/science.aax9753

    View details for PubMedID 32241944

  • Strain-induced room-temperature ferroelectricity in SrTiO3 membranes. Nature communications Xu, R. n., Huang, J. n., Barnard, E. S., Hong, S. S., Singh, P. n., Wong, E. K., Jansen, T. n., Harbola, V. n., Xiao, J. n., Wang, B. Y., Crossley, S. n., Lu, D. n., Liu, S. n., Hwang, H. Y. 2020; 11 (1): 3141


    Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

    View details for DOI 10.1038/s41467-020-16912-3

    View details for PubMedID 32561835

  • Freestanding crystalline YBa2Cu3O7-x heterostructure membranes PHYSICAL REVIEW MATERIALS Chen, Z., Wang, B., Goodge, B. H., Lu, D., Hong, S., Li, D., Kourkoutis, L. F., Hikita, Y., Hwang, H. Y. 2019; 3 (6)