Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D1: Induced Superconductivity in Carbon Nanotubes and Graphene |
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Sponsoring Units: DCMP Chair: Chun Ning (Jeanie) Lau, University of California, Riverside Room: Colorado Convention Center Four Seasons 2-3 |
Monday, March 5, 2007 2:30PM - 3:06PM |
D1.00001: Induced Superconductivity in Nanowires and Nanotubes Invited Speaker: We study experimentally electron transport in 1 dimensional semiconductor nanowires (consisting of InAs and InP combinations) and carbon nanotubes. The wires are connected to superconducting source-drain contacts with gate electrodes in the substrate or on the surface. In the regime of weak coupling to the contacts we observe Coulomb blockade effects. We present level spectroscopy including a determination of the spin states. In the regime of strong coupling to the contacts interference effects are observed. In this regime and using superconducting contacts, we find supercurrents flowing through InAs-nanowires over micrometer length scales. The critical current is tunable by gate voltage, thus realizing so-called JOFETs (Josephson FETs) [1]. When we define quantum dots in between superconducting contacts the direction of the supercurrent is determined by the single electron spin state in the quantum dot [2,3]. \newline \newline \textbf{1.} Yong-Joo Doh, Jorden A. van Dam, Aarnoud L. Roest, Erik P. A. M. Bakkers, Leo P. Kouwenhoven, and Silvano De Franceschi, \textit{Tunable supercurrent through semiconductor nanowires, }Science \textbf{309}, 272-275 (2005) \newline \textbf{2.} P. Jarillo-Herrero, J.A. van Dam and L.P. Kouwenhoven, \textit{Quantum supercurrent transistors in carbon nanotubes, }Nature 439, 953-956 (2006) \newline \textbf{3.} Jorden A. Van Dam, Yuli V. Nazarov, Erik P.A.M. Bakkers, Silvano De Franceschi and Leo P. Kouwenhoven, \textit{Supercurrent reversal in quantum dots, }Nature \textbf{442}, 667-670 (2006) [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D1.00002: Andreev reflection in graphene Invited Speaker: Relativity and superconductivity have no common ground in ordinary matter, because the velocity of electrons is only a small fraction of the velocity of light. The unusual band structure of a single layer of carbon atoms (graphene) contains negatively and positively charged particles that move as relativistic electrons and positrons. The electron-like particles in the conduction band can be converted into positron-like particles in the valence band when they are reflected by a superconductor. (The missing charge of $2e$ enters the superconductor as a Cooper pair.) This {\em inter}band reflection process can be distinguished from the usual {\em intra}band Andreev reflection, because the reflection angle has the opposite sign. A new phenomenology of graphene--superconductor junctions is predicted, including an anomalous scaling of the supercurrent with the length of the junction and the existence of charge-neutral modes propagating along the interface. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D1.00003: Superconducting Nanotube Dots Invited Speaker: In this talk, I will focus on charge transport in carbon nanotube devices with superconducting source and drain contacts in the finite-bias non-equilibrium transport regime. As contact material, bi-layers of Au and Al were used and transport has been studied at temperatures in the 0.1 K range. Because carbon nanotubes are quantum dots (qdots), we in fact explore the physics of qdots with superconducting contacts, something which only recently became possible thanks to carbon nanotubes and most recently to semiconducting nanowires. In my talk, I will first summarize our pioneering work on multiwalled carbon nanotubes in which we could demonstrate proximity induced effects both in the weak and the strong coupling regime. In the latter an intriguing interplay between superconductivity and Kondo physics appears. Then, I will discuss the physics of multiple Andreev reflection in a situation when only one resonant state is present and compare this with experimental results. Finally, I will compare our early results with our recent measurements on single-wall carbon nanotubes. This work has been supported by the Swiss Institute on Nanoscience, the Swiss National Science Foundation, EU projects DIENOW and HYSWITCH. I gratefully acknowledge contribution of the following persons to this work (in alphabetic order): B. Babic, W. Belzig, C. Bruder, M. R. Buitelaar, J.-C. Cuevas, A. Eichler, L. Forro, J. Gobrecht, M. Gr\"{a}ber, M. Iqbal, T. Kontos, A. Levy Yeyati, A. Martin-Rodero, T. Nussbaumer, S. Oberholzer, C. Strunk, H. Scharf, J. Trbovic, E. Vecino, M. Weiss [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D1.00004: Josephson junctions with tuneable single wall carbon nanotubes as barriers Invited Speaker: Weak superconductivity induced in a barrier between two stronger superconductors constitutes the essence of a Josephson junction. The original prediction of Josephson assumed a thin potential barrier much higher than the energy gap of the superconductors on either side. The Josephson junction does, however, not require a potential barrier, a metallic barrier may be a sufficient to reduce the penetration of Cooper pairs. The barrier can also be of geometric origin as for superconducting microbridges smaller than the coherence length. In the last two situations the Cooper pair Josephson transmission is most conveniently pictured as a multiple Andreev reflection process at the barrier/superconductor interfaces. We have studied Josephson junctions, where the barrier region is a single wall carbon nanotube (CNT) with a long mean free path. The junction between the 1-dimensional CNT and the superconductor often constitutes an additional thin potential barrier, I. The Josephson junction has therefore the structure S/I/CNT/I/S. The CNT has a capacitance, C, to the external electrodes, which typically yields a Coulomb blockade energy larger than the energy gap of the superconductors and the finite length of the CNT (smaller than it's mean free path) gives rise to Fabry-Perot eigenenergies. The Coulomb blockade is only strongly developed if the resistance R of the barrier, I, is large (R$>$h/e$^{2})$. If R$\sim $h/4e$^{2}$ we have an almost adiabatic transmission, where the Josephson critical current may approach the quantized value 2$\Delta$e/$\hbar$. In the region where R$\sim $h/2e$^{2}$ Josephson tunnelling can be observed simultaneously with charge quantization and Kondo resonance tunneling. In this regime we study gate imposed even-odd electron number effect leading to characteristic variation of the supercurrent exhibiting an alternation between 0- and $\pi \quad -$Josephson junctions. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D1.00005: Gate-controlled superconductivity in diffusive multiwalled carbon nanotube Invited Speaker: We have investigated electrical transport in a diffusive, PECVD-grown multiwalled carbon nanotube contacted using superconducting leads made of Al/Ti sandwich structure. We find proximity-induced superconductivity with measured critical currents up to $I_{cm} = 1.3$ nA, tunable by gate voltage. The supercurrent branch displays a finite zero bias resistance which varies as $R_0 \propto I_{cm}^{-\alpha}$ with $\alpha =0.74$. We discuss the interpretation of these findings in terms of the RCSJ-model as well as the diffusive junction model for long SNS structures. In addition, we will compare the results with our recent data on proximity-induced supercurrents in singlewalled carbon nanotubes. [Preview Abstract] |
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