Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U20: Focus Session: Mesoscopics - Preparation, Superconductivity and Magnetism |
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Sponsoring Units: DMP Chair: Jacobo Santamaria, Universidad Complutense Room: 322 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U20.00001: A Novel Nano-Assembly Technique for the Creation of Ultra-Low Disorder, Locally-Tunable One-Dimensional Systems with Carbon Nanotubes Jonah Waissman, Maayan Honig, Sharon Pecker, Avishai Benyamini, Assaf Hamo, Shahal Ilani Carbon nanotubes offer exciting prospects for studies of fundamental physics in one dimension due to their propensity for clean, defect-free growth, and long lengths. Recent technological advances have allowed for the creation of zero-dimensional ultra-clean nanotube devices, leading to new physics. But to date, the full potential of these molecules for full-fledged experiments in extended one-dimensional geometries is still unrealized, owing to fundamental limitations in making complex and clean devices. In this talk, we will describe a new nano-assembly technique to create suspended carbon nanotube devices of large complexity and with extremely low levels of electronic disorder. We demonstrate the creation of devices with multiple electrostatic gates and devices that combine several nanotubes positioned at chosen distances from each other. These capabilities open the door to a wide array of new experiments on the physics of electrons, spins and mechanics in one dimension. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U20.00002: Towards FIB patterning of commercial SiN membranes for sensitive magneto-calorimetry Kurtis Wickey, Thomas Kent, Roberto Myers, Joseph Heremans, Ezekiel Johnston-Halperin Investigating magnetocaloric effects in thin films, spin-thermal coupling, and the heat capacity of 2D materials such as graphene, germanene, and MoS$_{\mathrm{2}}$ requires small (hundreds of microns and less) thermally isolated platforms with sensitivity to comparably small heat capacities. Previously, calorimeters fabricated on amorphous SiNx membranes have been used due to their low thermal conductivity and compatibility with standard fabrication techniques. Here, we use a focused ion beam (FIB) to remove large portions of commercial SiNx membranes, leaving a platform that is thermally isolated from the Si frame by narrow supporting legs. This approach allows the fabrication of the calorimeter around existing samples such as flakes of MoS$_{\mathrm{2}}$, pre-patterned mesas of magnetic thin films, etc. The thermal isolation of the platform ensures uniform temperature without the use of the thermally conducting layer present in unpatterned membrane calorimeters, further improving the sensitivity of our calorimeters. We will discuss our progress towards realizing these calorimeters. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U20.00003: Atomic Calligraphy Matthias Imboden, Flavio Pardo, Cristian Bolle, Han Han, Ammar Tareen, Jackson Chang, Jason Christopher, Benjamin Corman, David Bishop Here we present a MEMS based method to fabricate devices with a small number of atoms. In standard semiconductor fabrication, a large amount of material is deposited, after which etching removes what is not wanted. This technique breaks down for structures that approach the single atom limit, as it is inconceivable to etch away all but one atom. What is needed is a bottom up method with single or near single atom precision. We demonstrate a MEMS device that enables nanometer position controlled deposition of gold atoms. A digitally driven plate is swept as a flux of gold atoms passes through an aperture. Appling voltages on four comb capacitors connected to the central plate by tethers enable nanometer lateral precision in the xy plane over 15x15 sq. microns. Typical MEMS structures have manufacturing resolutions on the order of a micron. Using a FIB it is possible to mill apertures as small as 10 nm in diameter. Assuming a low incident atomic flux, as well as an integrated MEMS based shutter with microsecond response time, it becomes possible to deposit single atoms. Due to their small size and low power consumption, such nano-printers can be mounted directly in a cryogenic system at ultrahigh vacuum to deposit clean quench condensed metallic structures. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U20.00004: Oxidation of atomic scale patterns prepared by scanning probe techniques Kai Li, Namboodiri Pradeep, Joseph Fu, Lei Chen, Richard Silver Scanning probes offer a potential alternative technology pathway in practical atomic scale devices and developing atom-based dimensional standards. However, the process steps, such as atomic scale lithography and subsequent pattern transfer need considerable optimization before the technology can be utilized for manufacturing applications. Nanoscale patterns are prepared in UHV on a hydrogen passivated silicon surface using STM by selectively removing H atoms. These patterns can then be used for further chemical processing such as oxidation and RIE. Conventional Si oxidation processes that require a high temperature and moisture-rich environment are known to damage the hydrogen-protected area. The challenge is to produce a strong SiO$_{\mathrm{2}}$ hard etch mask on the patterned area without affecting the hydrogen passivation layer. Currently we are developing a new low temperature oxidation process that starts with exposing the patterned areas to oxide/moisture at temperatures below H desorption. The presentation will focus on the details of near atomic scale oxide chemistry relevant to processing nanoscale patterns.~We will also present our approach to fabricating stable, atomically defined calibration standards based on the crystal lattice. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U20.00005: Fabrication of Flat Freestanding Silicon Nanomembranes Kyle McElhinny, David Czaplewski, Gokul Gopalakrishnan, Martin Holt, Paul Evans Silicon nanomembranes are suspended single crystal sheets of silicon, tens of nanometers thick, with areas in the thousands of square micrometers. Freestanding nanomembranes provide an ideal system for studying the physics of nanoscale crystalline materials and find application in novel electronic and photonic materials and devices. Challenges in fabrication arise due to buckling in response to stresses in the silicon-on-insulator starting material. In equilibrium, the elastic energy of the membrane is minimized by distributing the buckling distortion across the entire membrane. We demonstrate that flat nanomembranes can be created by utilizing a modification of traditional membrane fabrication procedures. This new scheme produces an elastically metastable structure, in which the buckling is redistributed to a small area near the edges of the membrane. An energetically favorable mechanism for this redistribution will be discussed. Membranes with thicknesses from 315 nm down to 6 nm have been fabricated, showing vertical deviations of less than 10 nm across an area covering 100 $\mu$m $\times$ 100 $\mu$m. X-ray scattering experiments performed on these structures demonstrate the importance of the ability to fabricate crystallographically uniform and flat nanomembranes. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U20.00006: Mesoscopic relaxations in homoepitaxial systems and their effect on oxygen adsorption Oleg O. Brovko, Wuwei Feng, Holger L. Meyerheim, Valeri S. Stepanyuk, J\"urgen Kirschner The importance of mesoscopic relaxations in heteroepitaxial systems has been recognized quite a while ago. Both theoretical predictions and subsequent experimental observations have clearly shown the importance of mesoscopic relaxations for electronic, magnetic and geometric properties of heteroepitaxial nanostructures. The implications of mesoscopic relaxations in \emph{homoepitaxial} systems, however, despite theoretical predictions of their importance, are still not fully understood. In the present joint experimental and theoretical paper, by the example of Fe nanoislands grown homoepitaxially on a p(1x1)O/Fe(001) surface we demonstrate that relaxations at the edges of nanoislands do not only determine the electronic and geometric structures of nanoislands' rims but also govern the oxygen adsorption thereon. Contraction of metallic bonds at the edge of Fe nanoislands leads to a corrugation of the edges and the substrate around, which inevitably leads to a change in adsorption height and electronic structure of oxygen atoms residing on the island. Our results outline the importance of mesoscopic relaxations in homoepitaxial nanostructures for the system's electronic and structural properties and the adsorption of light elements and molecules thereon. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U20.00007: Synthesis of Low Density Metallic Nanowire Network Edward Burks, Chad Flores, Dustin Gilbert, Kai Liu, Thomas Felter, Supakit Charnvanichborikarn, Sergei Kucheyev, Jeffery Colvin Highly porous metallic nanostructures have been shown to possess interesting thermal, electrical and mechanical properties due in part to their high surface areas and low densities. In this work, ion track-etched membranes were used as a template for electrodeposition to realize a low density interconnected copper nanowire network. Polycarbonate membranes (3-6 microns thick) were first irradiated with energetic Xe$^{6+}$ ions at normal incidence and multiple 45 degree azimuthal angles. The total irradiation density was 2x10$^{9}$ tracks/cm$^{2}$. Following a UV/ozone treatment, NaOH was used to preferentially etch the latent tracks of ion damage, creating intersecting nanopores in the polycarbonate matrix. A thin metal layer was then sputtered onto one side of the now-porous membrane to be used as a working electrode. Selected metals such as Cu and Co were then electrodeposited from a sulfate electrolyte into the pores, filling the membrane with an interconnected wire network. The polycarbonate membrane was then folded onto itself several times, and dichloromethane was used to dissolve away the polycarbonate. So far densities as low as 40mg/cm$^{3}$ have been achieved. Structural and magnetic properties of such networks have been investigated. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U20.00008: Superconducting vortex dynamics in nanostructured hybrids based on Fe single-crystal nanotriangles Jose Vicent, Alicia Gomez, Elvira Gonzalez, Miguel Iglesias, Javier Palomares, Nadia Sanchez, Federico Cebollada, Jesus Gonzalez Arrays of Fe single-crystal nanotriangles have been fabricated by Electron Beam Lithography. These arrays are embedded in superconducting Nb thin films. We have studied the superconducting vortex lattice motion on the periodic pinning potentials induced by the magnetic arrays. The vortex dynamics can be controlled through tailoring the magnetic stray field configurations. Which are due to different magnetic remanent states of the Fe single-crystal nanostructures. These configurations have been modified by changing the direction of the saturating applied field and also by using different orientations of the Fe magneto-crystalline easy axes within the triangles. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U20.00009: Suppression of Superconductivity in Small Clusters of Proximity-Coupled Superconducting Islands Malcolm Durkin, Serena Eley, Sarang Gopalakrishnan, Nadya Mason We report transport measurements of proximity-coupled arrays of mesoscopic niobium islands patterned on gold films. We show that superconductivity in the individual islands depends on the number of nearest neighbors, even for island diameters much larger than the superconducting coherence length. We also investigate the length scale where superconductivity in single islands approaches the bilayer approximation. This work is relevant to the understanding of metallic states and quasi-superconductivity in 2D systems [1]. [1] S. Eley, S. Gopalakrishnan, P. Goldbart, and N. Mason, Nature Phys. 8, 59-62 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U20.00010: Electric-field induced superconducting ball formation: new physics of superconductors or a flawed experiment? R.S.B. Ghosh, J.E. Hirsch In 1999, Rongjia Tao, P.W. Anderson and coworkers reported the discovery of a surprising new effect in high temperature superconductors (Phys. Rev. Lett. 83, 5575 (1999)): in the presence of a large electric field, millions of superconducting microparticles spontaneously aggregated into balls of macroscopic dimensions. Subsequently, Tao and coworkers reported that the same effect takes place in low temperature conventional superconductors (Physica C 377, 357 (2002)). If true, this effect would be evidence for novel physics of superconductors, not described by BCS theory. However our experimental studies with high temperature superconductors show that (i) ball formation also occurs in the absence of an applied electric field, and (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature. Possible origins of the phenomenon and implications for theories of superconductivity are discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U20.00011: Electric-Field Induced Formation of Superconducting Balls R. Tao, X. Xu, E. Amr, H. Tang Ghosh and Hirsch recently claimed that many micrometer-size particles in liquid nitrogen, as large as between 25 $\mu m$ and 32 $\mu m$, can be aggregated into balls by shaking. It turns out that they performed their experiments with liquid nitrogen in open air, the moisture condensed on their particle surface leading to ball aggregation by shaking. We repeated their shaking experiment and found that dry BSCCO, YBCO and Pb powders in liquid nitrogen do not form any balls by shaking in a glove bag filled with dry nitrogen gas. No matter how we shake the samples, these powders do not aggregate together. However, when we open the glove bag and let the air come to the samples, BSCCO, YBCO and Pb all form some balls quickly by shaking. Also inside the dry glove bag, when we apply an electric field and slowly increase it, superconducting particles form balls within two critical electric fields, $E_{c1}$ and $E_{c2}$ ($E_{c1} < E_{c2}$), while non-superconducting particles do not form balls at all. The electric field induced superconducting ball formation reveals that the area of interaction between electric field and superconductors requires more investigation. However, the phenomenon can be explained within the BCS theory. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U20.00012: Characterization of spin induced subgap states in superconductor/quantum dot/superconductor junctions Gediminas Kirsanskas, Brian Andersen, Karsten Flensberg, Jens Paaske We examine the emergence of subgap states in a junction consisting of two superconducting leads coupled to spinful Colomb blockaded quantum dot. The system is modeled by an effective Kondo model, which gives rise to so-called Yu-Shiba-Rusinov states inside the gap. We determine the dispersion of these states with an applied phase difference across the junction and study their dependence on an applied magnetic field. Also the effects of coupling asymmetry to the leads and deviation from the particle-hole symmetric point are addressed. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U20.00013: High critical-current superconductor-InAs nanowire-superconductor junctions Simon Abay, Henrik Nilsson, Fan Wu, C.M. Wilson, H.Q. Xu, Per Delsing We report on InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift-off nanofabrication method to get very short nanowire devices with Ohmic contacts. We observe very high critical currents of up to 800\,nA in a wire with a diameter of 80\,nm. The current-voltage characteristics of longer and suspended nanowires display either Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device. In addition, both the conductance and the critical current of the suspended devices increased step-wise as a function of the local gate voltage. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U20.00014: In-Plane Magnetic Field Tolerance of a Nanobridge SQUID Magnetometer Natania Antler, Eli M. Levenson-Falk, Ravi Naik, Shay Hacohen-Gourgy, R. Vijay, I. Siddiqi We describe the operation of a nanobridge SQUID magnetometer subject to an in-plane magnetic field of up to 60 mT. The magnetometer is comprised of a nanobridge SQUID with two aluminum weak links embedded in a 4-8 GHz microwave tank circuit for dispersive readout. We obtain a flux sensitivity of 17 n$\Phi_0/$Hz$^{1/2}$ with 50 MHz of instantaneous bandwidth in zero magnetic field. This effectively corresponds to single spin resolution, within a 1 Hz bandwidth, for nanomagnets placed within 100-200 nm from the nanobridge edge. We find that the effective flux sensitivity only degrades by a factor of $\sim$3 up to 60 mT of applied field. Finally, we describe progress towards magnetization dynamics measurements in different spin species such as Cobalt nanoclusters and Bismuth implanted in Silicon-28. [Preview Abstract] |
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