Physics and devices of two-dimensional electron gas at complex oxide interfaces

Complex oxides are rich with properties, owing to the strong interaction among their charge, spin, orbital, and structural degrees of freedom. Artificial interfacial modifications in complex oxide heterostructures, including defects, formal polarization, structural symmetry breaking, and interlayer interaction, can lead to even richer and novel properties not found in the bulk form. These emergent properties not only can serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality. Our research in this field focusses on:
• Emergent properties at the interface [Nat. Commun. 2011]
• New-types of oxide interfaces [Nat. Commun. 2013, Science 2015, Nano Letter 2016]
• The mechanism of the charge transfer [PRX 2013, PRL 2018]
• Electric-field control of the interfacial properties [PRL 2016, PRL 2018]

Quantum materials by design

Two-dimensional materials exhibit exceptional properties with unique band structures, while complex oxides with the strong correlations among degrees of freedom can lead to multifunctionality. This enables us to exploit the enhanced possibilities arising from a hybrid combination of the two-dimensional materials with complex oxides, making the hybrid heterostructures and interfaces a potential platform for quantum materials by design with tunable and new or improved functionalities. Our current research direction is on the physics and devices of hybrid two-dimensional-material/complex-oxide heterostructures, such as disorder-induced magnetoresistance, pseudomagnetism, and tunable Moire pattern in 2d-material/ferroelectric-oxide (Graphene/hBN/BaTiO3) heterostructures [Advanced Materials 2020].

Novel (topological) magnetic spin textures, spin-orbit-coupled based devices, and low-energy switching devices

Spin-based logic has been identified as a strong contender for non-charge-based computing. In contrast to charge-based devices, bits in spin-based devices are generally switched by flipping spins without moving any charge in space, thus preventing heat dissipation. Although some energy is still dissipated in flipping spins, it is significantly less than energy associated with current flow. In addition, the exchange interaction between the spins makes all of them rotate together in unison. This gives spin an advantage over charge as a state variable to encode information in (computing) devices. The traditionally proposed spin logic devices unfortunately still suffer from the inefficiency, scaling, low speed or noise. My research focuses on the interfacial magnetoelectric effect and new topological spin textures (such as Skyrmion) that have been identified to overcome the power-hungry issue of the traditional spin devices. As such devices require material systems with a large spin-orbit coupling, complex oxides and interfaces are strong contenders, as they have been shown to exhibit large spin-orbit coupling [Small 2020].

Oxide Electronics beyond Moore

The rapidly approaching “end” of Moore’s law is full of opportunities for the incorporation of new technologies which have been in the background so far. The primary mover behind this paradigm shift has been the march towards 3D circuits where wafers of circuits are bonded with intermediate thru wafer vias to enhance the circuit density without compromising speed. One of the interesting fall offs of this approach is the fact that all the wafers do not have to go through the same device process and further the wafers need no longer all be silicon! This provides a new opportunity for hetero-integration of different material systems and for novel device technologies based on 2D materials and oxides and heterostructures that have manifested new/ novel physical phenomena, of relevance to the information processing/ communication/ storage industry.[more]

While oxides in their bulk form have shown a tremendous spectrum of physical characteristics, oxide heterostructures provide a versatile pathway to combine these various physical phenomena and create novel electronic phases and eventually devices. We are now at the stage where we can produce customized oxide heterostructures with atomic precision. Well-controlled thin films, heterostructures, interfaces and quantum wells based on oxide materials have been fabricated and are being explored for a possible new generation of oxide electronic devices. A highly sophisticated epitaxial deposition technique, namely PLD-MBE is available in our lab, will be a key enabler for the preparation of our atomically precise controlled systems.

Oxide Interfaces

In many ionic materials, including the oxides, surfaces created along specific directions become electrically charged. Equivalently, such electronic charging, or ‘polarization’, can also occur at the interface connecting two materials. Theoretically, in certain systems this could lead to the build up of an ever increasing voltage in the materials, a situation known as a ‘polarity catastrophe’. Of course, this cannot occur for energetic reasons in practical systems, and nature deals with this situation by reconstructing the electronic configuration of the interface. This can occur by the shifting of charge across the interface or by structural reconstructions, meaning the displacement of atoms. For the oxide materials especially, a tantalizing consequence of these reconstructions is that it provides a means to create novel electronic phases, stabilized by the interface and which cannot exist in the bulk. Studies have already shown 2-dimensional conducting planes to emerge between otherwise insulating materials. Using the state of the art thin-film deposition techniques, we will study interfaces involving various other interesting systems, including other titanates, as well as cuprates, manganates and vanadates. In the cuprate materials, the multivalency and charge transfer properties of the Cu-O system are at the heart of high–Tc superconductivity. The vanadates are of interest for their wide range of possible valence states and the possibility to also realize p-type doping, and the manganates for their magnetic and Colossal Magneto-Resistance properties.

High-temperature superconductivity: Novel materials and quantum phase transition

The discovery that a copper oxide could superconduct at a higher temperature had a profound influence on physics. Even higher temperature superconductivity and its superconducting mechanism remain unresolved problems and continue to be a challenging and exciting field of research. We are excited about the possible new high-temperature materials, which can perhaps shed light on theories of high-temperature superconductivity.
Further, high-T c superconductivity in cuprates results from doping charge carriers into Mott insulators. The magnetic and superconducting properties of Mott insulators doped by holes (p-type carriers) are different from those doped by electrons (n-type carriers), showing asymmetry in the phase diagrams. Moreover, their transport properties depend on the type of carriers, for example, in-plane normal-state resistivity exhibits quadratic temperature (T ) dependence for n-type cuprates while linear T dependence is seen for p-type cuprates with optimal doping. Such comparisons of electron- and holedoping asymmetry (symmetry) in cuprates should help to further our understanding of the cuprate superconductors. The typical electron-hole asymmetry (symmetry) investigation is usually based on cuprates with different crystallographic structures such as NCCO (T-structure) and La2−xSrxCuO4 (T-structure). These materials have different parent Mott insulators and thus exhibit different properties even without doping. Therefore, it is desirable to dope electrons and holes into a Mott insulator without changing the crystallographic structure and address the physics in both sides of such “ambipolar” cuprate.

Doping charge carriers will causes the change of cuprates from antiferromagnetic Mott insulators to high-Tc superconductors, and then normal metal. Among these transitions, superconductor-insulator transition (SIT) in the two-dimensional limit is of particular interest. Continuous changing of carrier density is necessary to understand the nature of such phase transition, and thus, further our understanding of cuprate superconductors. Electric field-effect doping is a potential avenue for this investigation and it is reversible and exempt from chemical disorders, enabling the study of intrinsic properties of cuprates.
These fuels our research in this field, and we focus on these aspects:

  • Superconductivity in infinite layer nickelates [PRL 2020]
  • Electric-field tuning and quantum phase transition in high-Tc [PRB 2015]
  • Ambipolar superconductors [PRB2012, PRL 2016]

Resistive Switching

One of the great unresolved issues in resistive switching is its driving mechanism. A deep understanding of this mechanism is required in order to optimize Resistive Random Access Memory (ReRAM) device characteristics and develop guidelines for scaling, reliability, and reproducibility. Based on our experiments, we have proposed a new model based on the so-called “quasi conduction band” which can give a more stable characteristic, thereby enabling device scaling with enhanced reproducibility.