Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDC) have attracted intense research interest in the past decade. Their unique electronic and optical properties offer the promise of novel optoelectronic applications in the terahertz regime. Recently, generation and detection of broadband terahertz (~10 THz bandwidth) emission from 10-nm-scale LaAlO3/SrTiO3 nanostructures created by conductive atomic force microscope (c-AFM) lithography has been demonstrated. This unprecedented control of THz emission at 10 nm length scales creates a pathway toward hybrid THz functionality in 2D-material/LaAlO3/SrTiO3 heterostructures. Here we report initial efforts in THz spectroscopy of 2D nanoscale materials with resolution comparable to the dimensions of the nanowire (10 nm). Systems under investigation include graphene, single-layer molybdenum disulfide (MoS2), and tungsten diselenide (WSe2) nanoflakes.
The interface between perovskite oxide semiconductors LaAlO3 and SrTiO3 exhibits remarkable conducting, superconducting, magnetic, and spintronic properties that are strongly influenced by electron density. Scanning probe methods have the ability to probe local properties of interest. For example, magnetic force microscopy (MFM) has be used to measure magnetism at the LaAlO3/SrTiO3 interface, while piezoelectric force microscopy has been used to measure the local electron density. Here we directly compare these two methods to provide further insight into the relationship between electron density and magnetic properties.
Aharanov-Bohm (AB) interference can arise in transport experiments when magnetic flux threads through two or more transport channels. The existence of this behavior requires long-range ballistic transport and is typically observed only in exceptionally clean materials. We observe AB interference in wide (w∼100 nm) channels created at the LaAlO3/SrTiO3 interface using conductive AFM lithography. Interference occurs above a critical field B∼4 T and increases in magnitude with increasing magnetic field. The period of oscillation implies a ballistic length that greatly exceeds the micron-scale length of the channel, consistent with Fabry-Perot interference in 1D channels. The conditions under which AB oscillations are observed will be discussed in the context of the electron pairing mechanism in LaAlO3/SrTiO3.
Interfacial ferromagnetism in LaAlO3/SrTiO3 (LAO/STO) heterostructures has been probed by a variety of techniques. Recently, magnetic force microscopy (MFM) was used to image ferromagnetic domains that are electrically tunable at room temperature when the samples were grown in certain conditions. Optical techniques provide powerful tools for probing magnetic phenomena, and recently magnetic circular dichroism has been observed in reduced bulk STO crystals. Here we describe a scanning magneto-optical Kerr imaging system that could achieve sub-micrometer precision and 10−4 rad/√ Hz sensitivity with a 150 fs pulsed-laser centered at 425 nm. Such capability would make pump and probe measurement on the gate-tunable LAO/STO ferromagnetism and ultrafast imaging of domain dynamics possible.
We investigate the interaction between high-temperature superconductor Bi2Sr2CaCu2O8+δ (BSCCO) flakes deposited on the oxide heterostructure LaAlO3/SrTiO3 (LAO/STO). Conductive-atomic force microscope (c-AFM) lithography will be used to create nanowires at the LAO/STO interface that couple to the BSCCO. Through coupling of these materials, we will be able to study phenomena such as the proximity effect and coulomb drag.
The interface of LaAlO3/SrTiO3 presents a locally tunable metal-insulator transition that can be utilized to create complex nanostructures. Using conducting AFM lithography techniques, we can create a variety of nanoscale devices such as sketched single-electron transistors (SketchSETs). Due to the piezoelectric properties of LaAlO3/SrTiO3, there exists the possibility of locally modulating the local electron density using the pressure applied by an AFM tip. Some of the most interesting properties are only observed at cryogenic temperature. For this purpose we utilize a cryogenic AFM system. I will describe our efforts to perform nanomechanical imaging of conductive structures, which can be helpful in mapping the electronic properties of oxide nanostructures.
Understanding the properties of large quantum systems can be challenging both theoretically and numerically. One experimental approach– quantum simulation–involves mapping a quantum system of interest onto a physical system that is programmable and experimentally accessible. A tremendous amount of work has been performed with quantum simulators formed from optical lattices; by contrast, solid-state platforms have had only limited success. Our experimental approach to quantum simulation takes advantage of nanoscale control of a metalinsulator transition at the interface between two insulating complex oxide materials. This system naturally exhibits a wide variety of ground states (e.g., ferromagnetic, superconducting) and can be configured into a variety of complex geometries. We will describe initial experiments that explore the magnetotransport properties of one-dimensional superlattices with spatial periods as small as 4 nm, comparable to the Fermi wavelength. The results demonstrate the potential of this solid-state quantum simulation approach, and also provide empirical constraints for physical models that describe the underlying oxide material properties.
The electron system at the interface of two complex oxides, LaAlO3 and SrTiO3, exhibits a number of interesting strongly-correlated electronic properties, such as superconductivity and spin-orbit coupling. Reduced dimensionality is made accessible through nanowire devices created with conducting AFM lithography. Here, we describe an electrostatically-controlled dimensionality crossover in weak antilocalization behavior of LaAlO3/SrTiO3 nanowires at low temperature. These measurements give insight to the interplay of spin-orbit coupling and dimensionality. Characterizing the behavior of the strongly-correlated electronic properties in these reduced dimensions is necessary in order to develop this system as a multifunctional nanoelectronics platform.
Quasi-1D nanowires are created using conductive AFM (c-AFM) lithography at the LaAlO3/SrTiO3 (110) interface along the (001) and (110) crystallographic directions. The superconducting properties of nanowires were investigated under transport measurements with respect to the crystallography and orbital hierarchy. We observe anisotropic superconductivity where the upper critical magnetic field along the (001) and (110) directions are markedly different with a superconducting dome that is shifted for the two orientations as a function of gate voltages. The superconducting dome shift can be explained by anisotropic band structures along the two different directions combined with the Lifshitz transition.
Graphene and LaAlO3/SrTiO3 (LAO/STO) are both two-dimensional electronic systems with a fascinating range of properties. The coupling between these two 2DEG’s has the potential to produce various novel phenomena and create new functionalities. Successful integration of these two systems must overcome a number of technical challenges. Graphene-complex-oxide (GCO) heterostructures are created using Hyflon AD (2,2,4-trifluoro-5 trifluoromethoxy-1,3 dioxole) as a support layer for transferring and patterning CVD graphene on LAO/STO. This approach has advantages over more traditional methods that use Poly(Methyl Methacrylate) (PPMA) to transfer CVD graphene in that the Hyflon is easier to remove from the oxide surface after processing. To test the quality of GCO heterostructures, a graphene Hall bar structure is created. The quantum Hall regime can routinely be reached in the graphene layer, while preserving the ability of the LAO/STO to be patterned using AFM lithography. This approach opens up the possibility for the exploration of a wide range of GCO devices.