We investigate the magnetotransport and electronic properties of unconventional superconductors. Current interests include tunneling spectroscopy of electron-doped SrTiO₃, where we are trying to understand the nature of pairing in the dilute limit, where the Fermi energy is far below the Debye energy in the system. We are studying the density and disorder tuned superconductor-to-metal transition in two dimensions using dual-gate devices. Recently we have discovered superconductivity in infinite-layer nickelates, which leads to many questions on the origin of pairing in this system, and its relation to cuprates.

• D. Li, B. Y. Wang, K. Lee, S. P. Harvey, M. Osada, B. H. Goodge, L. F. Kourkoutis, and H. Y. Hwang, "Superconducting Dome in Nd_1−xSr_xNiO₂ Infinite Layer Films." Phys. Rev. Lett. 125, 027001 (2020).
• D. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita, and H. Y. Hwang, "Superconductivity in an Infinite-Layer Nickelate." Nature 572, 624 (2019).
• A. G. Swartz, A. K. C. Cheung, H. Yoon, Z. Y. Chen, Y. Hikita, S. Raghu, and H. Y. Hwang, "Superconducting Tunneling Spectroscopy of Spin-Orbit Coupling and Orbital Depairing in Nb:SrTiO₃." Phys. Rev. Lett. 121, 167003 (2018).
• Z. Y. Chen, A. G. Swartz, H. Yoon, H. Inoue, T. A. Merz, D. Lu, Y. W. Xie, H. T. Yuan, Y. Hikita, S. Raghu, and H. Y. Hwang, "Carrier Density and Disorder Tuned Superconductor-Metal Transition in a Two-Dimensional Electron System." Nat. Commun. 9, 4008 (2018).
• A. G. Swartz, H. Inoue, T. A. Merz, Y. Hikita, S. Raghu, T. P. Devereaux, S. Johnston, and H. Y. Hwang, "Polaronic Behavior in a Weak Coupling Superconductor." Proc. Natl. Acad. Sci. U. S. A. 115, 1475 (2018).

Since the discovery of graphene as a stable, 2D crystal, research in 2D materials has greatly expanded and evolved. Our lab bridges the worlds of 2D materials and those synthesized through thin-film growth. We have developed techniques for the release of freestanding complex oxide films and heterostructures using a water-soluble buffer layer. The resulting complex oxide membranes can be transferred onto any substrate or platform and then stretched, bent, and poked to study nanomechanical and electromechanical response. We study a variety of systems in our lab, ranging from magnets, superconductors, and ferroelectrics.

• R. Xu, J. Huang, E. S. Barnard, S. S. Hong, P. Singh, E. K. Wong, T. Jansen, V. Harbola, J. Xiao, B. Y. Wang, S. Crossley, D. Lu, S. Liu, and H. Y. Hwang, "Strain-Induced Room-Temperature Ferroelectricity in SrTiO₃ Membranes." Nat. Commun. 11, 3141 (2020).
• S. S. Hong, M. Gu, M. Verma, V. Harbola, B. Y. Wang, D. Lu, A. Vailionis, Y. Hikita, R. Pentcheva, J. M. Rondinelli, and H. Y. Hwang, "Extreme Tensile Strain States in La_0.7Ca_0.3MnO₃ Membranes." Science 368, 71 (2020).
• P. Singh, A. G. Swartz, D. Lu, S. S. Hong, K. Lee, K. Nishio, Y. Hikita, and H. Y. Hwang, "Large-Area Crystalline BaSnO₃ Membranes with High Electron Mobilities." ACS Appl. Electron. Mater. 1, 1269 (2019).
• Z. Y. Chen, B. Y. Wang, B. H. Goodge, D. Lu, S. S. Hong, D. F. Li, L. F. Kourkoutis, Y. Hikita, and H. Y. Hwang, "Freestanding Crystalline YBa₂Cu₃O_7-x Heterostructure Membranes." Phys. Rev. Mater. (Rapid Communications) 3, 060801 (2019).
• S. S. Hong, J. H. Yu, D. Lu, A. F. Marshall, Y. Hikita, Y. Cui, and H. Y. Hwang, "Two-Dimensional Limit of Crystalline Order in Perovskite Membrane Films." Sci. Adv. 3, eaao5173 (2017).
• D. Lu, D. J. Baek, S. S. Hong, L. F. Kourkoutis, Y. Hikita, and H. Y. Hwang, "A New Synthetic Route to Freestanding Single Crystal Perovskite Films and Heterostructures." Nat. Mater. 15, 1255 (2016).

Many of the materials we study have very interesting photoelectrochemical properties, in addition to their relevance for condensed matter physics. In particular, we are applying our expertise in the design and growth of atomic scale heterostructures to engineer and investigate their surface chemistry, with a focus on solar water spltting, of central interest for developing new renewable energy technologies.

• K. Lee, M. Osada, H. Y. Hwang, and Y. Hikita, "Engineering the Oxygen Evolution Reaction in IrO_x/SrIrO₃ Catalysts: Correlations between Structural Parameters and Catalytic Activity." J. Phys. Chem. Lett. 10, 1516 (2019).
• L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Norskov, and T. F. Jaramillo, "A Highly Active and Stable IrO_x/SrIrO₃ Catalyst for the Oxygen Evolution Reaction." Science 353, 1011 (2016).
• Y. Hikita, K. Nishio, L. C. Seitz, P. Chakthranont, T. Tachikawa, T. F. Jaramillo, and H. Y. Hwang, "Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic-Scale Control." Adv. Energy Mater. 6, 1502154 (2016).

We investigate coherent electronic systems using new techniques we are developing that enable deep-sub-micron nanolithography and epitaxial overgrowth. A current effort is in probing the 2D-1D superconducting transition, and electron interferometry in the viscinity of superconductivity. We also develop electrical devices based on the fundamental developments in our group.

• B. S. Y. Kim, Y. Hikita, T. Yajima, and H. Y. Hwang, "Heteroepitaxial Vertical Perovskite Hot-Electron Transistors Down to the Monolayer Limit." Nat. Commun. 10, 5312 (2019).
• H. Inoue, H. Yoon, T. A. Merz, A. G. Swartz, S. S. Hong, Y. Hikita, and H. Y. Hwang, "Delta-Doped SrTiO₃ Top-Gated Field Effect Transistor." Appl. Phys. Lett. 114, 231605 (2019).
• D. Lu, S. Crossley, R. Xu, Y. Hikita, and H. Y. Hwang, "Freestanding Oxide Ferroelectric Tunnel Junction Memories Transferred onto Silicon." Nano Lett. 19, 3999 (2019).
• B. S. Y. Kim, M. Minohara, Y. Hikita, C. Bell, and H. Y. Hwang, "Atomically-Engineered Epitaxial Anatase TiO₂ Metal-Semiconductor Field-Effect Transistors." Appl. Phys. Lett. 112, 133506 (2018).