Charge distribution in the STO/LAO/STO heterostructure. (up) The charge-density map of the STO/LAO/STO heterostructure obtained by the in-line electron holography technique. The top and bottom interfaces clearly show the positive and negative charges, respectively. (down) The STEM–ADF image of the entire STO/LAO/STO heterostructure. Scale bar, 5 nm.

The discovery of a two-dimensional electron gas (2DEG) at the LaAlO₃/SrTiO₃ interface has resulted in the observation of many properties not present in conventional semiconductor heterostructures, and so become a focal point for device applications. Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed, the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO₃/LaAlO₃/SrTiO₃ heterostructures. Using electrical transport measurements and in-line electron holography, we provide direct evidence of a 2DHG that coexists with a 2DEG at complementary heterointerfaces in the same structure. First-principles calculations, coherent Bragg rod analysis and depth-resolved cathodoluminescence spectroscopy consistently support our finding that to eliminate ionic point defects is key to realizing a 2DHG. The coexistence of a 2DEG and a 2DHG in a single oxide heterostructure provides a platform for the exciting physics of confined electron– hole systems and for developing applications.

Störmer, H. L., and W-T. Tsang. "Two-dimensional hole gas at a semiconductor heterojunction interface." Applied Physics Letters 36.8 (1980): 685-687.

Bark, C. W., et al. "Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain." Proceedings of the National Academy of Sciences 108.12 (2011): 4720-4724.

Podkaminer, J. P., et al. "Creation of a two-dimensional electron gas and conductivity switching of nanowires at the LaAlO3/SrTiO3 interface grown by 90o off-axis sputtering." Applied Physics Letters 103.7 (2013): 071604.

RHEED diffraction pattern of a Sr₃SnO thin film along the [100] and [110] crystallographic directions. The band structure of Sr₃SnO calculated by DFT and measured by ARPES. The agreement between the bulk theoretical calculations and the experiments suggest the TCI nature of Sr₃SnO.

Topological materials are derived from the interplay between symmetry and topology. Recent advance in topological band theories has identified antiperovskite oxide Sr₃SnO as a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystal point group symmetries. The realization of such material, however, is challenging. Guided by thermodynamic calculations, we designed a deposition approach to achieve the adsorption-controlled growth of epitaxial Sr₃SnO single crystal films. In-situ transport and Angular Resolved Photoemission Spectroscopy analysis revealed the metallic and non-trivial topological nature of the as-grown samples. Compared with conventional deposition techniques, the synthesis route demonstrated in this work shows remarkable superiority in sample quality and is readily to be adapted for producing other topological systems with antiperovskite structures. The successful realization of thin films of topological crystalline insulators opens opportunities to manipulate topological states by tuning symmetries via epitaxial strain and heterostructuring.

Ma, Yanjun, et al. "Realization of epitaxial thin films of the topological crystalline insulator Sr3SnO." Advanced Materials 32.34 (2020): 2000809.

Superconducting radio-frequency (SRF) resonator cavities provide extremely high quality factors > 10¹⁰ at 1-2 GHz and 2K in large linear accelerators of high-energy particles. The maximum accelerating field of SRF cavities is limited by penetration of vortices into the superconductor. Present state-of-the-art Nb cavities can withstand up to 50 MV/m accelerating gradients and magnetic fields of 200-240 mT which destroy the low-dissipative Meissner state. Achieving higher accelerating gradients requires superconductors with higher thermodynamic critical fields, of which Nb₃Sn has emerged as a leading material for the next generation accelerators. To overcome the problem of low vortex penetration field in Nb₃Sn, it has been proposed to coat Nb cavities with thin film Nb3Sn multilayers with dielectric interlayers. Here, we report the growth and multi-technique characterization of stoichiometric Nb₃Sn/Al₂O₃ multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al₂O₃ and Nb₃Sn. Low-field RF measurements suggest that our multilayers have quality factor comparable with cavity-grade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers which could overcome the intrinsic limits of the Nb cavity technology.

Vaswani, C., et al. "Terahertz second-harmonic generation from lightwave acceleration of symmetry-breaking nonlinear supercurrents." Physical Review Letters 124.20 (2020): 207003.

Yang, X., et al. "Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity." Nature materials 17.7 (2018): 586-591.

Patterned Nb₃Sn thin films on Al₂O₃ to make a coplanar waveguide to determine figures of merit of Nb₃Sn for use in RF superconducting resonators.

Multiferroic BiFeO₃ has attracted great interest due to its promising application to magnetoelectric devices. Due to this low symmetry, eight possible polarization (ferroelectric) variants, which correspond to four structural (ferroelastic) domains, may form in the films, leading to complex domain patterns with both {100} and {101} twin boundaries. Such a complex domain structure can deteriorate the ferroelectric response of the system by external electric field, and complicates the examination of the coupling between magnetic and ferroelectric order parameters in BiFeO₃. We have demonstrated the selection of domain structure variants in epitaxial BiFeO₃ films and consequently significant improvement in ferroelectric switching behavior and leakage current by employing miscut in cubic (001) SrTiO₃ substrates.

Price, N. Waterfield, et al. "Strain Engineering a Multiferroic Monodomain in Thin-Film Bi Fe O 3." Physical Review Applied 11.2 (2019): 024035.

Saenrang, Wittawat, et al. "Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO 3 heterostructures." Nature communications 8.1 (2017): 1-8.

Price, N. Waterfield, et al. "Electrical Switching of Magnetic Polarity in a Multiferroic BiFeO 3 Device at Room Temperature." Physical Review Applied 8.1 (2017): 014033.

Zhao, T., et al. "Electrically controllable antiferromagnets: Nanoscale observation of coupling between antiferromagnetism and ferroelectricity in multiferroic BiFeO3." Nature Mater 5 (2006): 823-829.

In-plane PFM images, P-E loops and leakage currents of of 4, 2 and 1 variant (001) BiFeO₃ thin films.

Schematic of BaBiO₃, BaPbO₃ superlattice structures. Full temperature range resistivity vs temperature. The inset of overlays the transition probed by resistive measurements (blue) with out-of-plane (black) and in-plane (gray) onset of diamagnetism (red dashed line) for m = 2. Transition region of superconducting samples.

The isotropic, nonmagnetic doped BaBiO₃ superconductors maintain some similarities to high-Tc cuprates, while also providing a cleaner system for isolating charge density wave (CDW) physics that commonly competes with superconductivity. Artificial layered superlattices offer the possibility of engineering the interaction between superconductivity and CDW. Here we stabilize a low-temperature, fluctuating short-range CDW order by using artificially layered epitaxial (BaPbO₃)3m/(BaBiO₃)m (m = 1–10 unit cells) superlattices that are not present in the optimally doped BaPb_0.75Bi_0.25O₃ alloy with the same overall chemical formula. Charge transfer from BaBiO₃ to BaPbO₃ effectively dopes the former and suppresses the long-range CDW; however, as the short-range CDW fluctuations strengthen at low temperatures charge appears to localize and superconductivity is weakened. The monolayer structural control demonstrated here provides compelling implications to access controllable, local density wave orders absent in bulk alloys and manipulate phase competition in unconventional superconductors.

Harris, D. T., et al. "Charge density wave modulation in superconducting BaPb O 3/BaBi O 3 superlattices." Physical Review B 101.6 (2020): 064509.

Harris, David T., et al. "Superconductivity-localization interplay and fluctuation magnetoresistance in epitaxial BaPb 1− x Bi x O 3 thin films." Physical Review Materials 2.4 (2018): 041801.

Electrical resistivity versus temperature of VO_2−δ (8 nm)/VO₂ (8 nm) bilayer (solid line) and 8-nm-thick VO_2−δ single layer (black dashed line), measured upon cooling. Metallic and insulating phases are represented by red and blue colors, respectively. (B) Monoclinic portion as a function of temperature. (C) Temperature dependence of monoclinic Raman shift. Error bars denote SD of the fitted Raman peak position. We fit the measured Raman peak with a Gaussian curve.

The metal-insulator transition in correlated materials is usually coupled to a symmetry lowering structural phase transition. This coupling not only complicates the understanding of the basic mechanism of this phenomenon but also limits the speed and endurance of prospective electronic devices. We demonstrate an isostructural, purely electronically driven metal-insulator transition in epitaxial heterostructures of an archetypal correlated material, vanadium dioxide. A combination of thin-film synthesis, structural and electrical characterization, and theoretical modeling reveals that an interface interaction suppresses the electronic correlations without changing the crystal structure in this otherwise correlated insulator. This interaction stabilizes a nonequilibrium metallic phase and leads to an isostructural metal-insulator transition. This discovery will provide insights into phase transitions of correlated materials and may aid the design of device functionalities.

Lee, D. A. E. S. U., et al. "Isostructural metal-insulator transition in VO2." Science 362.6418 (2018): 1037-1040.

Briggs, Ryan M., Imogen M. Pryce, and Harry A. Atwater. "Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition." Optics express 18.11 (2010): 11192-11201.

Lee, Jaeseong, et al. "Epitaxial VO2 thin film-based radio-frequency switches with thermal activation." Applied Physics Letters 111.6 (2017): 063110.

Point defects have played a major role in tuning the properties of materials over the last few decades. In quantum heterostructures based on complex oxides, however, the control of individual point defects continues to be challenging arising mostly from inherent non-stoichiometry. Herein, the ability to tune point defects in oxide-based quantum heterostructure LaAlO₃/SrTiO₃ (LAO/STO) is demonstrated using a newly-developed metal-organic pulsed laser deposition (MOPLD) technique. Density functional theory (DFT) calculations predict that increasing the ratio of [antisite Ti (Ti_Sr)]/[strontium vacancies (V_Sr)] reduces the equilibrium amount of oxygen vacancies (V_O) which is the most dominant scattering center at low temperatures. X-ray diffraction and Raman spectroscopy show that the MOPLD technique, with titanium tetraisopropoxide (TTIP) used as Ti source, provides a wide process window of controlling the stoichiometry of STO without inducing structural alterations. Depth-resolved cathodoluminescence spectroscopy reveals that STO films grown at larger TTIP flux have a higher ratio of [Ti_Sr]/[V_Sr], and lower [V_O], leading to high electron mobility of ~6,260 cm²/V*s at the interface of LAO/STO at low temperature and clear Shubnikov–de Haas oscillations. These results provide new insights into point defect engineering of the next generation of complex oxide thin films and heterostructures to investigate novel quantum phenomena.

Top-down schematic view of the metal-organic pulsed laser deposition system. Red line is 808 nm diode laser for substrate heating. Yellow line is 248 nm excimer laser for laser ablation of the target material (SrO single crystal). Green line is 30 KeV electron beam for RHEED diffraction.

The metal organic precursor selectively adsorbs onto the Sr-terminated surface and not the Ti-terminated surface meaning a process window with 1:1 Ti:Sr stoichiometry is possible with this growth technique. In this growth window the point defect populations can be controlled.

Exfoliation of epitaxial PMN-PT film from an SrTiO₃ substrate (top left) frees it from the effect of substrate clamping. By coupling with a similarly-fabricated CoFe₂O₄ (CFO) (top right), the process of crystal stacking allows to create novel heterostructures that have been unachievable by conventional heteroepitaxy growth methods. Freed from the substrate clamping, the CFO/PMN-PT heterostructure shows an increase in strain-mediated magnetoelectric coupling. The bottom left shows the converse ME coupling between PMN-PT and CFO, while the bottom right demonstrates an increase in the direct ME coupling.

Relaxor-ferroelectrics like PMN-PT are well known for their giant piezoelectricity, i.e. the ability to generate large strains under application of an electric field (converse piezoelectric effect). Bulk single crystals show some of the largest piezoelectric coefficients of any known materials and find wide spread use as piezoelectric transducers in medical imaging, micro-positioners, and energy storage. However, by coupling with a magnetic material exhibiting large magnetostriction, the ferroelectric (FE) and ferromagnetic (FM) heterostructures can achieve enhanced functionality through the strain-mediated magnetoelectric (ME) coupling effect, where electric (magnetic) field control of magnetism (polarization) can lead to novel devices with applications in memory storage, magnetic/electric field sensing, and energy harvesting.

To realize such devices that meet the demands of high sensitivity and low power, thin films of both the FE and FM materials must be used. While bulk single crystals of PMN-PT show giant piezoelectricity (under application of ~100s V), thin films suffer from mechanical clamping by the passive substrate which reduces their piezoelectricity to almost zero. Therefore, the PMN-PT must be removed from its substrate, creating free-standing PMN-PT membranes, to achieve the giant piezoelectric effect once again (under application of ~1s V). Through a process of crystal stacking, we can create novel heterostructures that couple the PMN-PT with a variety of materials. By coupling with CoFe₂O₄ (CFO), we achieved large magnetoelectric coupling effects, including manipulation of the CFO’s magnetic anisotropy under application of only 5V. By applying this crystal stacking process to any materials, including complex oxides, 2-D materials, III-V’s, and many more, an unprecedented number of new combinations can lead to the discovery of novel physical phenomena, as well as the creation of innovative devices.

Kum, Hyun S., et al. "Heterogeneous integration of single-crystalline complex-oxide membranes." Nature 578.7793 (2020): 75-81.

(a) Unit cell of Pr₂Ir₂O₇ with only the cation sublattices shown. Oxygen atoms are not shown for clarity. (b), (c) “All-in–all-out” and “2-in–2-out” spin configurations on the Pr corner-sharing tetrahedra. (d), (e) Corner-sharing Ir tetrahedra surrounded by a Pr hexagonal ring in the (111) plane of the bulk and thin film, respectively. The red arrows in the Pr atoms indicate the Pr 4f moments. Due to the cubic symmetry breaking in the thin film, Ir local moments can be established indicated here in (e) by the blue arrows in the Ir atoms.

Rare-earth pyrochlore iridates (RE₂Ir₂O₇) consist of two interpenetrating cation sublattices, the RE with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. The Pr₂Ir₂O₇ thin film was grown by solid phase epitaxy, a novel growth technique, where amorphous Pr₂Ir₂O₇ was deposited on substrate via magnetron sputtering at room temperature with subsequent post annealing at high temperature. This novel growth technique broadens the growth condition of the composition including volatile species.

Guo, Lu, et al. "Spontaneous Hall effect enhanced by local Ir moments in epitaxial P r 2 I r 2 O 7 thin films." Physical Review B 101.10 (2020): 104405.

Emergent superconductivity in parent Ba-122 by superlattice design. (A) Fe sublattice and tetrahedral geometry in Ba-122 bulk single crystal below magnetostructural phase transition temperature. (B) Superlattice design for the control of Fe sublattice structure and FeAs4 tetrahedron in the presence of tetragonal-to-orthorhombic transition. (C) Emergent superconductivity by proximity to the regular tetrahedron via reduction of the Ba-122 layer thickness t in a Ba-122/SrTiO₃ superlattice.

Fe-based superconductors exhibit a diverse interplay between charge, orbital, and magnetic ordering. Variations in atomic geometry affect electron hopping between Fe atoms and the Fermi surface topology, influencing magnetic frustration and the pairing strength through changes of orbital overlap and occupancies. Here, we experimentally demonstrate a systematic approach to realize superconductivity without chemical doping in BaFe₂As₂, employing geometric design within an epitaxial heterostructure. We control both tetragonality and orthorhombicity in BaFe₂As₂ through superlattice engineering, which we experimentally find to induce superconductivity when the As−Fe−As bond angle approaches that in a regular tetrahedron. This approach to superlattice design could lead to insights into lowdimensional superconductivity in Fe-based superconductors.

Kang, Jong-Hoon, et al. "Superconductivity in undoped BaFe2As2 by tetrahedral geometry design." Proceedings of the National Academy of Sciences 117.35 (2020): 21170-21174.

Kang, Jong-Hoon, et al. "Control of epitaxial BaFe2As2 atomic configurations with substrate surface terminations." Nano letters 18.10 (2018): 6347-6352.