APS March Meeting Videos

In an electronic system with two closely spaced but isolated conductors, current that is sourced in one conductor can induce a current or voltage in the second conductor. This phenomenon, known as “Coulomb drag”, represents a powerful approach to probe Coulomb interactions and electron correlations. Here we examine Coulomb drag in a pair of nanowires created with conductive-AFM lithography at the LaAlO3/SrTiO3 interface. Coulomb drag measurements are performed by sourcing current in one wire and measuring the induced voltage or current in the other wire. Experimental features depend strongly on magnetic field. At low magnetic fields, the wires can be superconducting, leading to large drag resistance when the wire is driven past the critical current. At high magnetic field, distinct oscillations are observed that are associated with the electron subband structure in the wires. 

Nano-engineered graphene devices can exhibit novel and useful electronic and optical properties, many of which depend critically on controlling the chemical potential relative to the charge-neutrality point. Complex-oxide heterostructures enable reconfigurable control of conductive nanostructures, making them an interesting platform for controlling the electronic properties of graphene at nanoscale dimensions. Here we report the fabrication of graphene/LaAlO3/SrTiO3 heterostructures with nanoscale programmable control of the charge-neutrality point. Magnetotransport measurements of superlattice structures show characteristic interference features that can be associated with the electronically patterned interface. We discuss possible new directions based on this highly versatile hybrid platform.

Graphene is a promising tunable plasmonic material in the terahertz regime. Plasmons can be induced in graphene by femtosecond laser excitation, and their resonance frequency can be gate-tuned over a broad terahertz range. Another 2D electron system, the complex-oxide heterostructure LaAlO3/SrTiO3, has been shown to exhibit great promise for control and detection of broadband THz emission at extreme nanoscale dimensions. Recently, we have successfully integrated these two platforms: we have created graphene/LaAlO3/SrTiO3 structures with high mobility in the graphene channel and oxide nanostructures directly underneath the graphene layer. Here we describe new experiments that probe graphene plasmonic behavior using this nanoscale THz spectrometer using ultrafast optical techniques. This unprecedented control of THz radiation at 10 nm length scales creates a pathway toward hybrid THz functionality in graphene/LaAlO3/SrTiO3 heterostructures.

  Clean one-dimensional electron transport has been observed in very few material systems. The development of exceptionally clean electron waveguides formed at the interface between complex oxides LaAlO$_3$ and SrTiO$_3$ enables low-dimensional transport to be explored with newfound flexibility. This material system not only supports ballistic one-dimensional transport, but possesses a rich phase diagram and strong attractive electron-electron interactions which are not present in other solid-state systems. Here we report an unusual phenomenon in which quantized conductance increases by steps that themselves increase sequentially in multiples of e2/h. The overall conductance exhibits a Pascal-like sequence: 1, 3, 6, 10 … e2/h, which we ascribe to ballistic transport of 1, 2, 3, 4 ... “bunches” of electrons. We will discuss how subband degeneracies can occur in non-interacting models that have carefully tuned parameters. Strong attractive interactions are required, however, for these subbands to “lock” together. This Pascal liquid phase provides a striking example of the consequences of strong attractive interactions in low-dimensional environments.

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.

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.

"Coulomb drag'' is a transport phenomenon where Coulomb interaction between two close but electrically isolated conductors induces voltage in one conductor when an electric current is injected in the other conductor. It is a powerful approach to probe electronic correlations. Here we examine 1D electronic correlations in a proximally coupled nanowire system where two parallel nanowires are created with conductive atomic force microscopy at the LaAlO3/SrTiO3 interface. Coulomb drag measurements are made by injecting current into one wire (drive wire) and measuring the induced voltage in the other wire (drag wire). This geometry offers experimental insights into the interplay of electron pairing and superconductivity in reduced dimensions.

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