Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B36: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures |
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Sponsoring Units: GIMS Chair: Joseph A. Stroscio, National Institute of Standards and Technology Room: Morial Convention Center 228 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B36.00001: Subkelvin spin polarized STM: measuring magnetization curves of individual adatoms Invited Speaker: Magnetic nanostructures consisting of a few atoms on non-magnetic substrates are explored as model systems for miniaturized data storage devices and for the implementation of novel spin-based computation techniques. Since these nanostructures are well defined and controllable on the atomic scale, they are ideally suited to study the fundamentals of magnetic interactions. We used spin polarized scanning tunneling spectroscopy at subkelvin temperatures to image the magnetization of individual adatoms as a function of an external magnetic field. This allows to directly measure their magnetic interactions at very low energy scale. We will present the design of the 300mK STM [1] and then focus on the results. Interestingly, Co atoms on Pt(111) behave paramagnetic even at very low temperatures, 300 times smaller than the previously reported giant barrier between up and down spin [2]. A peculiar variation in the saturation flux density, which is measured for each atom, is found. This is attributed to their mutual indirect exchange via the substrate electrons. Indeed, we observe an interaction between the adatom and a Co monolayer stripe oscillating with distance between ferromagnetic and antiferromagnetic coupling on the scale of the Fermi wavelength. \newline [1] J. Wiebe et al., Rev. Sci. Instrum. 75, 4871 (2004). \newline [2] P. Gambardella et al., Science 300, 1130 (2003). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B36.00002: Investigation of Acceptor States and Landau Levels in (In,Mn)As by Scanning Tunneling Spectroscopy Young Jae Song, Nikolai Zhitenev, Joseph Stroscio, Gregory Rutter, Phillip First Increased interest in spin-based electronics as a replacement for charge-based electronics has led to significant scientific attention on dilute magnetic semiconductors (DMS). Magnetically doped III-V semiconductors are a strong research focus, with the aim of achieving higher Curie temperatures by understanding the microscopic nature of ferromagnetism in these DMS materials. In this presentation, we discuss our recent study of single Mn acceptor states in InAs(110). Mn impurities deposited at low temperature are substituted by using STM atom manipulation techniques to exchange a Mn atom with a surface In atom [1]. Voltage-dependent imaging and scanning tunneling spectroscopy (STS) reveal a number of electronic states associated with the Mn acceptor state and the Landau levels in the 2D subbands of an accumulation layer as a function of applied magnetic field. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement, NSF grant ECS-0404084, and Dept. of Commerce/NIST grant 60NANB7D6166. [1] Dale Kitchen \textit{et al}, Nature 442, 436 (2006) [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B36.00003: Atomic manipulation and tunneling spectroscopy on vacancy of Ag(111) surface studied with LT-STM Danda P. Acharya, Kai F. Braun, Saw W. Hla The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investigated at an atomic scale by combining scanning tunneling microscopy and spectroscopy and atom manipulation at 5 K. Various vacancy sizes, from one atom to hundreds of atoms, on Ag(111) are first created by tip-sample contact in a controlled manner. Using lateral manipulation, a vacancy is filled one at a time and the corresponding differential conductance spectra are recorded. Small energy shifts in the onset of surface state are observed. The shift is more pronounced for small size vacancy and becomes less and less pronounced for larger size vacancies. The observed dI/dV intensities at different size of vacancies clearly reveals that the surface state onset on Ag(111) disappears after reaching towards the 6$^{th}$ layers. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B36.00004: Direct detection of force gradient using atomic force microscopy with very small oscillation amplitude Toshu An, Atsushi Nomura, Takahiro Nishio, Toyoaki Eguchi, Kotone Akiyama, Yukio Hasegawa Using a quartz, which has a self-sensing capability, simple configuration of atomic force microscopy (AFM) is realized, and because of its high stiffness frequency-modulation (FM) operations with a small oscillation amplitude below 100 pm is possible. The small amplitude AFM operation enhances sensitivity of short range forces. Moreover, a force gradient can be directly detected from force vs. distance measurements. We carried out FM-AFM at low temperature (LT) in ultra-high vacuum using a quartz length-extension resonator with a tungsten tip (Spring constant and resonant frequency of the resonator are 540 000 N/m and 1 MHz, respectively) (An et al., APL \textbf{76}, 133114 '05). The system was simply made by attaching an extra electrode to the tip-holder of our LT-scanning tunneling microscope cooled by $^{3}$He (APL \textbf{88}, 113115 '06). Direct detection of the force gradient was performed on the Si(111) 7x7 surface using very small oscillation amplitude of 70 pm at 2.4 K. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B36.00005: Low-Temperature Nanotribology of Au and Pb using a Quartz Tunning-Fork Carlos Untiedt, Giovanni Saenz-Arce Quartz Tunning-Fork (TF) has been in recent years successfully implemented in force detection schemes for scanning probe microscopy (SPM) applications. Here we report its use as a nanotribometer for measuring friction in atomic size areas. The idea behind such a friction detector, is to take advantage of the large Q-factor of a TF ($Q_{air}\sim6000$, $Q_{vac}\sim20000$ at room temperature) which in our set-up depends of the SPM tip-sample dissipative forces. We have measured the Q-factor and the resonance frequency for various TFs. As a first step in the use of a TF as a nanotribometer, we studied the reactive forces that are associated with the combined local elastic properties of the sample and tip and calculated the damping rate associate with changes of the tip-sample distance. Finally, we show the variation of the measured damping rate and local spring constants with tip-sample distances for Au and Pb. For the measurement we have used our TF-nanotribometer at different temperatures ranging from 1.5K to room temperature in high vacuum. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B36.00006: A Nano-Scale Scanning SQUID Susceptometer for the Measurement of Isolated Magnetic Moments Nicholas C. Koshnick, Martin E. Huber, Julie Bert, Hendrik Bluhm, Jeffrey Large, Hal Edwards, Kathryn A. Moler Superconducting Quantum Interference Devices (SQUIDs) are well known as excellent magnetic field sensors. We present a scanning DC SQUID susceptometer that is designed to couple well to nanometer-sized objects. Its gradiometric design and local field coils allow for cancellation of the applied field so that dynamic range issues do not limit the SQUID's sensitivity. Integrated modulation coils linearize the signal and allow for optimal performance at all applied fields. Planar coaxial shielding, enabled by a multi-layer niobium process, results in a low inductance (100 pH) millimeter scale design where the pickup loops can be optimized independently from the junction and shunt resistor parameters. The sensor loop is on a terraced structure so that it can be scanned approximately 100 nm from the sample surface. Focused Ion Beam milling is used to fabricate pickup loops with inner diameters between 250 nm and 2 microns with line widths of approximately 200 nm. A white noise sensitivity of 0.8 $\mu\Phi_0/\sqrt{Hz}$ gives an estimated spin sensitivity of 80 $\mu_B/\sqrt{Hz}$ at 4 Kelvin. We will also report on on-going scanning susceptometry measurements, and on the spin sensitivity at low temperatures. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B36.00007: Absolute in-situ calibration piezoelectric quartz tuning fork force Sensors. Sanjay Bidasaria, Alexei Marchenkov A method has been developed for absolute calibration of piezoelectric quartz tuning forks for use as force sensors with nano-newton resolution. The performance of the forks in a cryogenic environment is investigated. The mechanical properties of the forks are extracted from the frequency dependent admittance and compared to the exact model of a vibrating cantilever in a helium atmosphere. The method is verified by simultaneous application of calibrated point loads to the cantilever in vacuum. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B36.00008: A UHV-LT-STM System for Optical Experiments David R. Daughton, Donghun Lee, Jay A. Gupta The combination of optical techniques and scanning tunneling microscopy (STM) provides insight into a diverse set of physical processes including surface chemistry, surface-photon interactions, and spin scattering in semiconductors. We present a novel cryogenic temperature, ultrahigh vacuum STM which incorporates a maneuverable, high numeric aperture lens, with sub-5 micron spot size, in proximity to the tunnel junction. Modifications to our microscope have been made to improve upon the 12.5 K base temperature and 10 pm tip stability. Our initial efforts are focused on studies of photo-chemical reactions and chemical identification by tip-enhanced Raman spectroscopy (TERS). UHV deposition techniques have been developed for a variety of molecules well suited for TERS studies including azulene, azobenzene, methylene blue, and C60. Electrochemically and chemically-etched Ag and Au tips are optimized for field enhancement with characterization by scanning electron microscopy and collection of the plasmon emission from the tip. Raman spectra have been collected from molecule-coated tips in vacuum to test the optical setup for TERS. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B36.00009: Photon-induced Molecular Motion Probed by STM Jun Zhang, Kevin Kelly To understand the mechanics of nanoscale motion and manipulation in true molecular machines, we have investigated fullerenes and fullerene-based derivatives with an eye towards the molecular rolling motion on surfaces. Assisted by scanning tunneling microscopy (STM), we have successfully observed the conformational changes through the chemomechincal interaction with the external photonic and tunneling electron excitations. In particular, we investigated a fullerene dimer structure which included an azo-based linkage. This system demonstrated a mechanical switching by incident irradiation, due to the azo transformation between the ``cis'' and ``trans'' states. This is particularly exciting because the hinge-like lateral motion is coupled to the surface through the axle and the rolling motion of the fullerene wheel. The results of these studies underscore the ability to control designed motion in molecular-sized nanostructures through specific macroscopic excitations. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B36.00010: Using a Geophone for Vibration Cancellation in a STM Alan Fang, Zhanybek Alpichshev, Aharon Kapitulnik We demonstrate a method for using a geophone (velocity-sensitive vibration sensor) for reducing the vibration-induced tunnel current noise in a Scanning Tunneling Microscope (STM). Some simple analog circuitry compensates for the transfer function of the geophone and STM head. This error signal is then fed back to the scan piezos. Although the geophone is placed one meter away from the STM head, (due to the low temperature and UHV requirements) we show a noise reduction performance of approximately 5x or better. Further improvements to the system are discussed. [Preview Abstract] |
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