APS March Meeting 2017
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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.

Michelle Tomczyk is a graduate student in the Department of Physics and Astronomy at Pitt.

She works in the Levy lab where she studies quantum transport phenomena at the lanthanum aluminate and strontium titanate interface. She creates nanostructures at the interface using an innovative AFM lithography technique in order to study emergent phenomena like superconductivity and magnetism. This information can benefit classical as well as quantum computing.

Michelle won a travel award at the Science 2014 poster session for her poster on “Electron Pairing Without Superconductivity”.

Sofia Sondoval describes her experience as Artist-in-Residence in Levylab.

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 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.

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.

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.

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.

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.

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.

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.

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.

"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.

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.

Megan Kirkendall is a graduate student in the Department of Physics and Astronomy at Pitt.

She works in the Levy lab where she researches quantum simulation at the lanthanum aluminate strontium titanate interface. Her research involves engineering a lattice interface on the nanometer scale, and then using that information to simulate a quantum system that can be studied. This process provides insight into quantum systems that cannot be simulated with a normal computer.

Megan won the grand prize at the Science 2014 poster session for her poster on “Experimental Quantum Simulation Using 1D LaAlO3/SrTiO3”.

Alexandre is an undergraduate senior who majors in physics at the University of Pittsburgh. He began his research with Professor Jeremy Levy when he was a freshman. Gauthier’s research focuses on the production of an advanced canvas analyzer, used to measure the electrical properties of multiterminal devices, and a low temperature scanning probe microscope, used to study electromechanical properties of single-electron transistors. He was awarded the Goldwater Scholarship for his innovations which was described by Chancellor Mark A. Nordenberg as “the highest national honor that can be won by undergraduate students studying science, math, or engineering, which makes the entire Pitt community particularly proud of Alexandre's selection.” 

A moment of inspiration that evolved into multi-million dollar quantum computing concept. Jeremy is Professor in the Department of Physics and Astronomy at the University of Pittsburgh. He received his B.S. in Physics from Harvard University and his Ph.D., also in Physics, from the University of California at Santa Barbara. His work focuses on exploring novel phenomena in solid state systems to provide the foundation for future technologies in the areas of nonlinear dynamics, semiconductor spintronics, quantum computing, and oxide nanoelectronics. In 2000, he became Director of the Center for Oxide-Semiconductor Materials for Quantum Computation which is funded by the Department of Defense and is dedicated to creating a viable approach to quantum computation using electron spin as a quantum bit. He is recipient of the Chancellor's Distinguished Research and Distinguished Teaching Awards, the Nanotech Briefs Nano 50 Award, and was named a Fellow of the American Physical Society.