Research

Writing and erasing nanowires at the interface of LAO/STO


Writing and erasing nanowires at the interface of LAO/STO

In our lab we use conductive-AFM (C-AFM) lithography to locally control the metal-insulator transition at the interface of LaAlO3/SrTiO3.  The technique is reversible and allows for creation of a wide range of nanostructures at oxide interfaces simply by applying positive or negative voltages to the AFM tip and scanning across the LaAlO3 surface.   As illustrated above, we work with TiO2-terminated SrTiO3 with approximately 3 unit cells LaAlO3, a layer that is approximately 1.2nm.  The LaAlO3/SrTiO3 interface is initially insulating. However, by applying a positive voltage with a c-AFM tip, we protonate the top surface and these ions attract electrons at the interface, thus locally inducing the metal-insulator transition at the interface[1,2]. Nanowires and other structures can be created at the interface by moving tip on the surface. The width of nanowires can be as thin as 2 nm by properly controlling the tip voltage, although typical widths are about 10 nm. This process is reversible. By applying a negative voltage to the AFM tip, the interface can restore insulating state and nanowires can be erased. With c-AFM lithography, we are able to create various nanostructures such as single-electron transistors (SETs)[3], broadband THz emitters[4] and ballistic electron waveguides[5].  
  1. C. Cen, S. Thiel, J. Mannhart, and J. Levy, "Oxide Nanoelectronics on Demand," Science 323, 1026 (2009). 
    http://dx.doi.org/10.1126/science.1168294
  2. C. Cen, S. Thiel, G. Hammerl, C. W. Schneider, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nature Materials 7, 298 (2008). 
    http://dx.doi.org/10.1038/nmat2136 
  3. G. Cheng, P. F. Siles, F. Bi, C. Cen, D. F. Bogorin, C. W. Bark, C. M. Folkman, J. W. Park, C. B. Eom, G. Medeiros-Ribeiro, and J. Levy, "Sketched Oxide Single-Electron Transistor," Nature Nanotechnology 6, 343 (2011). 
    http://dx.doi.org/10.1038/nnano.2011.56
  4. Y. Ma, M. Huang, S. Ryu, C. W. Bark, C.-B. Eom, P. Irvin, and J. Levy, "Broadband Terahertz Generation and Detection at 10 nm scale," Nano Letters 13, 2884 (2013). 
    http://dx.doi.org/10.1021/nl401219v
  5. M. Tomczyk, G. Cheng, H. Lee, S. Lu, A. Annadi, J. P. Veazey, M. Huang, P. Irvin, S. Ryu, C.-B. Eom, and J. Levy, "Micrometer-Scale Ballistic Transport of Electron Pairs in LaAlO3/SrTiO3 nanowires," Physical Review Letters 117, 096801 (2016). 
    http://dx.doi.org/10.1103/PhysRevLett.117.096801

1D Quantum Simulation Using a Solid State Platform


1D Quantum Simulation Using a Solid State Platform

The motivation for this work is to develop a solid state quantum simulation platform.  One potential candidate is the oxide heterostructure LaAlO3/SrTiO3, which exhibits nearly every phenomenon in the solid state and has key ingredients for quantum simulation including: strong, controllable, sign changing electron-electron interactions [1], and the ability to be controlled on the nanoscale [2].  The level of control that we have over the interface will allow us to create a lattice of sites for putting electrons that doesn’t have to bear any resemblance to the underlying lattice of the system, and create any arbitrary 1D or 2D lattice to simulate a wide range of interesting Hamiltonians.

The ultimate goal of this project is to create 2D lattices that can be used to simulate quantum systems that are difficult or impossible to model classically.  An extensive amount of work has been done towards this goal in the field of cold atomic gases which can be very precisely controlled and are well described by the Hubbard model Hamiltonian, which describes the interactions of particles in a lattice. But it is difficult for these systems to reach temperatures where correlated ground states occur.  A solid state quantum simulation platform would have to advantage of naturally supporting the solid state physics that cold atoms are attempting to simulate and study.

We began the project by trying to simulate 1D systems because they are more theoretically tractable.  We will take advantage of the extreme nanoscale control that we have over the LaAlO3/SrTiO3 interface using conductive atomic force microscope lithography to create artificial 1D systems.  Additionally, similar nanowire devices at the LaAlO3/SrTiO3 interface show a gate-tunable transition from attractive to repulsive electron-electron interactions which should allow us to simulate interesting quantum systems.  Experimental simulations of 1D superlattices are currently underway and results from these experiments should help to create and constrain a model in order to better understand the system and how to develop the tools needed to preform solid state quantum simulation.

  1. Cheng, G., Tomczyk, M., Tacla, A. B., Lee, H., Lu, S., Veazey, J. P., Huang, M., Irvin, P., Ryu, S., Eom, C.-B., Daley, A., Pekker, D. & Levy, J. "Tunable electron-electron Interactions in LaAlO3/SrTiO3 Nanostructures,"  arXiv:1602.06029 (2016). 
    http://arxiv.org/abs/1602.06029
  2. Cen, C., Thiel, S., Hammerl, G., Schneider, C. W., Anderson, K. E., Hellberg, C. S., Mannhart, J. & Levy, J. "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nature Materials 7, 298 (2008). 
    http://dx.doi.org/doi:10.1038/nmat2136

 


Broadband THz spectroscopy for a single nanoscale object

Terahertz frequency (0.1−30 THz) is known as the characteristic frequency for intramolecular and intermolecular motions. Spectroscopy with terahertz radiation has played an important role in fields like chemical sensing, characterization of biological material and drug analysis. Recently, broadband terahertz (10 THz bandwidth) generation and detection at 10 nm scales has been demonstrated using LaAlO3/SrTiO3 (LAO/STO) nanostructures [1]. This unprecedented control of terahertz radiation, on a scale of four orders of magnitude smaller than the diffraction limit, provides a useful technique to investigate a variety of nanoscale objects, such as Au nanorods, PbS quantum dots, graphene and single-layer molybdenum disulfide (MoS2) nanoflakes.

Nanoscale conducting region can be created and erased at LAO/STO interface with the method of conductive atomic force microscope (c-AFM) lithography [2]. These created nanostructures can be used as photodetectors [3] as well as THz generators and detectors. After creating a nanostructure composed of 10nm wide nanowires and a junction with comparable width, ultrafast pulses coming from a Ti: Sapphire laser go through a Michelson interferometer and then focused onto the junction. Photo-induced voltage change across the junction is measured as a function of time delay between the two pulses. When time delay is around 0 fs, there is a sharp decrease in the 4-terminal voltage, indicating the generation of THz radiation. The mechanism behind this phenomenon is the  process, where the nonlinear polarization is a product of the electric field across the junction, the third-order susceptibility at the junction, and two optical fields.

This powerful technique has been proved to be able to probe the response of a single plasmonic Au nanorod [4]. Commercial gold nanorods with a well-defined plasmon resonance peak around 810 nm is deposited onto the surface of LAO/STO sample. After an AFM image is taken to locate an ideal single nanorod, a four terminal structure with a junction located right on top of the nanorod will be designed and created. When the light polarization is parallel to the long axis of nanorod, the THz signal will be largely enhanced. This is due to the plasmonic coupling with induced THz radiation at the nanojunction. With this powerful THz spectrometer as a platform, various other nanoscale materials can be studied, such as PbS quantum dots and single-layer MoS2 nanoflakes. Graphene plasmonics is also under investigation.

  1. Ma, Y., Huang, M., Ryu, S., Bark, C. W., Eom, C.-B., Irvin, P., & Levy, J. "Broadband Terahertz Generation and Detection at 10 nm scale," Nano Letters 13, 2884−2888 (2013). 
    http://dx.doi.org/10.1021/nl401219v
  2. Cen, C., Thiel, S., Hammerl, G., Schneider, C. W., Anderson, K. E., Hellberg, C. S., Mannhart, J. & Levy, J. "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nature Materials 7, 298 (2008). 
    http://dx.doi.org/doi:10.1038/nmat2136 
  3. Irvin, P., Ma, Y., Bogorin, D. F., Cen, C., Bark, C. W., Folkman, C. M., Eom, C.-B. & Levy, J. "Rewritable Nanoscale Oxide Photodetector," Nature Photonics 4, 849-852 (2010). 
    http://dx.doi.org/10.1038/nphoton.2010.238
  4. Jnawali, G., Chen, L., Huang, M., Lee, H., Ryu, S., Podkaminer, J. P., Eom, C.-B., Irvin, P. & Levy, J. "Photoconductive Response of a Single Au Nanorod Coupled to LaAlO3/SrTiO3 Nanowires," APL 106, 211101 (2015). 
    http://dx.doi.org/10.1063/1.4921750

 

Superlattice patterning on Graphene/LAO/STO


Superlattice patterning on Graphene/LAO/STO

Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. The two-dimensional electron gas (2DEG) appearing at the Lanthanum aluminate-strontium titanate (LaAlO3/SrTiO3) interface makes it an ideal substrate to pattern superlattice on graphene through Conductive atomic force microscopy (c-AFM) lithography­ [1]­. Owing to the high dielectric constant of SrTiO3 in low temperature, it is possible to heavily dope graphene and tune the carrier density dramatically. Thus, LAO3/STO3 makes it possible to tune the fermi level and created nanostructures reversibly on graphene.

The growth of the graphene is achieved by atmospheric pressure chemical vapor deposition (APCVD) on ultra-flat diamond turned copper substrates [2]. Then the Graphene is transferred to LAO/STO substrated and patterned to a hall bar structure with an amorphous perfluoropolymer Hyflon AD60, which will leave less residue compared with PMMA. Superlattice was written by Conductive atomic force microscopy (c-AFM) lithography with alternating positive and negative five volts. As a control group, Rxx2 region was written uniformly with 5V.

The propagation of nearly free electrons through a weak periodic potential results in the opening of bandgaps near points of the reciprocal lattice. In the graphene superlattices, there is no gap opening at the Dirac point. Instead, a new set of Dirac points will emergent at an energy determined by the superlattice size [3]. Longitudinal resistance was measured as a function of back-gate under 5 Tesla at 2K. In the Landau fan diagram, a replica of Dirac point shows up next to the charge neutrality point only in the superlattice region, which is a sign of superlattice potential.

  1. Cen, C. et al. "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nat Mater 7, 298-302(2008). 
    http://dx.doi.org/10.1038/nmat2136
  2. S. Dhingra, J.-F. H., I. Vlassiouk, and B. D’Urso. "Chemical Vapor Deposition of Graphene on Large-Domain Ultra-Flat Copper," Carbon 69 (2014). 
    http://dx.doi.org/10.1016/j.carbon.2013.12.014
  3. Ando, T. & Nakanishi, T. "Impurity Scattering in Carbon Nanotubes – Absence of Back Scattering –," J. Phys. Soc. Jpn. 67, 1704-1713. 
    http://dx.doi.org/10.1143/JPSJ.67.1704

 

Tunneling experiments reveal a new electronic phase


Tunneling experiments reveal a new electronic phase

The nature of superconductivity, a phase in which electron form pairs and condense into a special zero-resistance state, is not understood in SrTiO3. PQI researcher Guanglei Cheng’s work, which appeared in Nature [1], provides a surprising new insight into this unconventional superconducting system, thereby revealing a novel phase in which electrons remain paired far outside of the superconducting regime.

Superconductivity can persist to very low carrier densities in STO [2], placing it outside conventional BCS theory and prompting early speculation about the possibility of real-space electron pairs existing outside of the superconducting state [3]. However, no evidence for such pairs was observed until Cheng et al’s experiments utilizing a quantum dot geometry (left panel). In this geometry, individual electrons tunnel onto and off of a conducting island, permitting single-electron spectroscopy of the electronic states on the dot and their evolution as a function of external parameters such as magnetic field and temperature.

Surprisingly, at a magnetic field around 2 Tesla (but sometimes as large as 7 Tesla!), the tunneling conductance peaks (the bright green and yellow features, right panel) start to bifurcate. As the magnetic field increases further, the split peaks shift with typical Zeeman energy. This curious behavior suggests that below ~2 Tesla, each conductance peak actually corresponds to the tunneling of not one, but two electrons. These electron pairs are extraordinarily strong, persisting to magnetic fields an order of magnitude larger than the critical field for superconductivity, 0.2 Tesla. Therefore, this constitutes clear evidence for electron pairs persisting in a non-condensed, non-superconducting phase. 

  1. Cheng, G. et al. "Electron Pairing without Superconductivity," Nature 521, 196-199 (2015). 
    http://dx.doi.org/10.1038/nature14398
  2. Schooley, J. F., Hosler, W. R. & Cohen, M. L. "Superconductivity in Semiconducting SrTiO3," Phys. Rev. Lett. 12, 474–475 (1964). 
    http://dx.doi.org/10.1103/PhysRevLett.12.474
  3. Eagles, D. M. "Possible Pairing without Superconductivity at Low Carrier Concentrations in Bulk and Thin-Film Superconducting Semiconductors," Phys. Rev. 186, 456–463 (1969). 
    http://dx.doi.org/10.1103/PhysRev.186.456

 

Ferromagnetism is controlled with a gate voltage and detected from atomic force microscope


Ferromagnetism is controlled with a gate voltage and detected from atomic force microscope

A new type of ferromagnetism that can be turned on and off electrically has been found on the interface of two non-magnetic oxide materials. The results have been accepted by Nature Communication, and will open a gateway to new types of spintronic and quantum devices.

Previous PQI graduate student Feng has found that on the interface of LaAlO3/SrTiO3, when LaAlO3 is 12 unit cells thick, will demonstrate tunable ferromagnetism by external electric field in room temperature. The interface was known to be highly conductive with the presence of 2D electron gas. Feng found that when the 2D electron gas was driven away by electric field, ferromagnetism would appear on the interface. The magnetism was measured with a highly sensitive atomic force microscope with a magnetized tip, and magnetic dipoles appeared to be in the same plane as the oxide interface. 

  1. Bi, F. et al. "Room-Temperature Electronically-Controlled Ferromagnetism at the LaAlO3/SrTiO3 Interface," Nature Communication 5, 5019 (2014). 
    http://dx.doi.org/10.1038/ncomms6019