Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q15: Focus Session: Advances in Scanned Probe Microscopy III: Spectroscopic Techniques at Low Temperatures |
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Sponsoring Units: GIMS Chair: Andreas Heinrich, IBM Almaden Research Center Room: B114 |
Wednesday, March 17, 2010 11:15AM - 11:51AM |
Q15.00001: Scanning Probe Microscopy at mK Temperatures * Invited Speaker: Scanning probe microscopy has made significant advances with a wealth of new physics emerging as cryogenic instruments have been developed in the last decade allowing high resolution spectroscopic studies with spatial atomic resolution [1]. Most low temperature SPM instruments today operate at 4 K using liquid $^{4}$He, with a few exceptions [2]. In this talk, we describe the next generation of ultra low temperature scanning probe microscope (SPM) with high magnetic field (15 T) capability operating at 10 mK using the circulation of a $^{3}$He-$^{4}$He mixture in a dilution refrigerator (DR). With this system operating at 10 mK, we can extend the capability of scanning tunneling spectroscopy to higher energy resolution ($\approx $3 $\mu $eV) for a range of applications in nanoscale systems. To achieve the design goal of mK operation for SPM applications we designed and constructed an ultra-high vacuum (UHV) SPM-compatible DR, an ultra-low temperature compatible SPM module, and extensive vibration isolation and RF shielding components. The DR was designed and constructed with features specific for UHV SPM applications, such as a Joule-Thomson (JT) condenser for lower noise operation. Noise measurements of the tunneling current show virtually no circulation-induced noise using the JT condenser, in contrast to noisy operation with a 1K pot. The custom-designed SPM module, with a three-axis position stage, is made from coin silver and ceramics for rigidity and thermal conduction in the mK regime. We also developed and constructed a low temperature current pre-amplifier, operating on the still at 650 mK, to circumvent problems due to long cable capacitances. Extensive noise measurements and first scanning measurements on graphene samples will be described. *In collaboration with Alexander F. Otte, Young Kuk, Phillip N. First, Walt A. de Heer, and Joseph A. Stroscio [1] D. L. Miller, \textit{et al.}, Science \textbf{324}, 924 (2009) [2] A. J. Heinrich, \textit{et al.}, Science \textbf{306}, 466 (2004) [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q15.00002: Realization of Artificial Graphene via Atomic Manipulation W. Mar, K. K. Gomes, W. Ko, H. C. Manoharan Graphene has emerged as a prototype 2D material for realizing Dirac electrons in condensed matter physics. The physics of these Dirac fermions stems from the symmetries of the honeycomb lattice, naturally present in planar carbon. Here we demonstrate an artificial variant of graphene not found in nature, constructed by atomic manipulation of two-dimensional electron gases into a honeycomb lattice. We present scanning tunneling microscopy (STM) studies on artificial graphene that confirm the key electronic properties of graphene, and extend these investigations to regimes difficult to achieve in natural graphene, including atomically tunable hopping, precise defects, edges, and strain. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q15.00003: Determining the Magnetism of Single Atoms on a Semiconductor Surface Alexander Khajetoorians, Bruno Chilian, Jens Wiebe, Roland Wiesendanger We demonstrate a method in which we combine spin-resolved Landau level spectroscopy and inelastic tunneling spectroscopy (IETS) to determine the magnetization and anisotropy of single Fe atoms coupled to a 2D electron gas on a III-V (110) semiconductor surface. We show here, using ultra-low temperature (300mK) scanning tunneling spectroscopy in high magnetic fields (12T) that the states of the Fe atom couple to the spin-split Landau levels thereby producing an overall asymmetry in the local density of states (LDOS) for a given Landau level. By probing the LDOS with changing magnetic field, we determine the magnetization of the atom. Furthermore, we observe spin excitations of the Fe atom by IETS. From these excitations, we observe a zero-field splitting of the Fe spin which we attribute to magnetic anisotropy. We relate these two measurements using a simple quantum magnetic Hamiltonian which suitably describes both experimental observations. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q15.00004: STM/STS at milliKelvin temperatures Mark Gubrud, Anita Roychowdhury, Michael Dreyer Scanning tunneling microscopy and spectroscopy systems operating at milliKelvin temperatures in dilution refrigerators have been built in a modest number of labs around the world. Certain issues and problems have been consistently encountered despite the variety of approaches that have been taken. Results that have been obtained indicate the most promising approaches for future improvement. I will review this accumulated experience from the literature and relate it to the design of a new system, located at the University of Maryland, College Park, that will be used initially to test the feasibility of a novel scheme for Josephson phase microscopy. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q15.00005: A dual tip STM for superconducting phase-difference detection Anita Roychowhdury, M.A. Gubrud, Dan Sullivan, Michael Dreyer, J.R. Anderson, C.J. Lobb, F.C. Wellstood We have built a dual tipped Pan-style STM, with each tip capable of independently scanning a sample. We have tested it at room temperature on graphite samples, and calibrated both scanners using atomic steps seen on graphite. We intend to use the STM at ultra-low (mK) temperatures to measure the spatial variation of the gauge-invariant phase difference in superconducting samples at the atomic scale. The two tips will function as the junctions of an asymmetric SQUID, with one tip acting as a reference while the other scans the sample. We will reduce fluctuations in the Josephson phase of the scanning tip by adding capacitance across the junctions and minimizing the geometric inductance. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q15.00006: Towards a Dual-Tip STM Applications in Mesoscopic Surface Transport Rami Dana, Yishay Manassen A DTSTM based on the mechanically controllable break junction (MCBJ) with two fabricated electron beam induced deposition (EBID) nanotips was developed. Unlike the traditional bending which applies lateral force on the junction, in the new design, the breaking mechanism applies torque on a virtual axle running through the junction. The junction is curved in Si wafer by double-sided anisotropic etching to form 30 micron wide bridge as a base for EBID tips. Nanotips with controlled architecture and from variety of materials are then fabricated on each side of the junction to establish a dual-tip system. Integration of the special characteristics of MCBJ and EBID, leads to a DTSTM capable of $\sim $50 nm probe separation, as presented in this work. On these scales more local and less averaged information can be collected; thus, new insight on electron transport phenomena on the nanoscale will hopefully be gained. The nature of current flow on these scales can be interesting from both fundamental physics and device application points of views. More on this work can be found at http://physweb.bgu.ac.il/$\sim$ramid/ [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q15.00007: Nanosecond scanning tunneling microscopy Andreas Heinrich, Sebastian Loth, Markus Etzkorn, Donald Eigler, Christopher Lutz Scanning tunneling microscopy is a powerful tool primarily because of the very high spatial resolution. In addition, the spectroscopic imaging modes have significantly enhanced the capabilities of the technique. However the time resolution of this technique has to date been governed by the slow response of the current to voltage converter used to measure the tunnelling current. Here we present a novel approach for achieving high time resolution with a DC current measurement. We show that this technique can be applied to a large number of different physical systems. We will focus on its use for the real-time measurement of spin relaxation of single atoms on surfaces. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q15.00008: Competition between spin coupling and a magnetic field in the Kondo effect A.F. Otte, M. Ternes, C.F. Hirjibehedin, S. Loth, C.P. Lutz, A.J. Heinrich Experiments in various systems have shown that the resonance of the Kondo effect, an intriguing many-body phenomenon, can be split by an external magnetic field. Here we show that coupling a single magnetic atom to a Kondo system can have the same result. We use a low temperature scanning tunneling microscope to study an Fe atom coupled to a Kondo-screened Co atom, bound on a thin insulating Cu$_2$N layer. Scanning tunneling spectroscopy measurements at zero magnetic field show the Kondo peak to be split due to the presence of the Fe atom, which couples antiferromagnetically to the Co atom. An externally applied magnetic field of appropriate magnitude can compensate the effect of the spin coupling and reconstitute the peak. These experiments provide a unique way to understand the interplay between Kondo physics, exchange coupling and magneto- crystalline anisotropy. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q15.00009: Spin-transfer torque on a single magnetic adatom Fernando Delgado, Juan Jos\'e Palacios, Joaqu\'in Fern\'andez-Rossier We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunnelling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunnelling. Experiments with Mn adatoms deposited on a Cu2N surface have been reported for non-polarized currents [1-2]. We show that by changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunnelling conductance. Therefore, this effect opens the possibility of writing/reading a single spin without the need of a local magnetic field.\\[4pt] [1] C.F. Hirjibehedin, C. P. Lutz, A. J. Heinrich, Science 312, 1021 (2006).\\[0pt] [2] C. Hirjibehedin et al., Science 317, 1199 (2007). [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q15.00010: Low Temperature Scanning Force Microscopy for Probing the Edge of Quantum Hall Systems James Hedberg, Ashwin Lal, Yoichi Miyahara, Guillaume Gervais, Peter Grutter, Michael Hilke, L.N. Pfeiffer, K.W. West Using our recently implemented ultra-low temperature, high magnetic field scanning force microscope, we have further developed methods to probe charge transport in semiconductor materials, specifically edge states of a 2-dimensional electron gas (2DEG) in the quantum hall regime. Among other techniques, we apply an AC excitation signal to a buried two-dimensional electron gas and monitor the electrostatic potential distribution via the oscillation dynamics of a piezoelectric quartz tuning fork with an etched metallic probe attached to one tine. The quartz tuning fork allows for ultrasensitive detection of electric forces as a consequence of its small oscillation amplitude. Additionally, the 2DEG sample is prepared with a cleaved edge overgrowth structure offering an extra electrode separated from the 2DEG by a atomically defined tunneling barrier, resulting in an addressable edge. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q15.00011: Development of Low Temperature Scanning Gate Microscope Halvar Trodahl, Jesse Berezovsky, R.M. Westervelt Two-dimensional electron gas (2DEG) and semiconductor nanowire systems are of interest as they are attractive candidates for nanoelectronics, spintronics and quantum information processing. A capacitively coupled scanning probe microscope (SPM) tip can be used to study the motion of electrons in these systems by imaging electron flow through a 2DEG [1] and tuning the charge of a nanowire quantum dot [2]. In this talk we will outline the key design aspects of the third generation SPM built in the Westervelt lab to measure conductance of nanometer scale electronic devices. This microscope will have the ability to measure devices at approximately 500 mK in a 7 T magnetic field. An innovative approach to low temperature coarse positioning has been incorporated into the system. \\[4pt] [1] M. A. Topinka, et al., \textit{Nature} 410, 183-186 (2001). \\[0pt] [2] A. C. Bleszynski, et al., \textit{Nano Letters} 7, 2559-2562 (2007). [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q15.00012: Modeling single-electron resonances of electric-field-sensitive scanning probes Stuart Tessmer, Irma Kuljanishvili, Morewell Gasseller Electric-field sensitive scanning probe methods have proven to be valuable tools to study nanoelectronics systems. We have developed a modeling method suitable to analyze single-electron resonances detected by these techniques. The method is based on basic electrostatics and a numerical boundary-element approach. The results compare well with experimental single-electron capacitance-voltage curves and capacitance images. [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q15.00013: Scanning Tunneling Microscope with Two-Dimensional Coarse Approach John Nichols, Kwok-Wai Ng Since the invention of the Scanning Tunneling Microscope (STM), it has been a powerful tool for probing the electronic properties of materials. Typically STM designs capable of obtaining resolution on the atomic scale are limited to a small area which can be probed. We have built a STM with a coarse approach in two dimensions, the z- and x- directions which are respectively parallel and perpendicular to the tip. This allows us to image samples with very high sensitivity at sites separated by macroscopic distances. This device is a single unit with a compact design making it very stable with the potential of use at cryogenic temperatures. This STM is capable of obtaining atomic resolution on HOPG. I will discuss the design of this STM and share images illustrating its capabilities. [Preview Abstract] |
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