Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P21: Focus Session: Advances in Scanned Probe Microscopy III - Novel SPM of Spin, Force & Conductance |
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Sponsoring Units: GIMS Chair: Yukio Hasegawa, University of Tokyo Room: D161 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P21.00001: A New Ultra-Low Temperature, High Magnetic Field STM in an Ultra-Quiet Laboratory Brian B. Zhou, Shashank Misra, Lukas Urban, Jungpil Seo, Andras Gyenis, SeJong Kahng, Ali Yazdani We report progress in the construction of a new UHV STM capable of operating at the extremes of temperature (25 mK) and magnetic field (14 T), allowing atomically resolved studies in previously unexplored areas of phase space. Our novel design is based on a bottom-loading dilution refrigerator in which the entire dilution stage and mounted microscope are moved between measurement and sample transfer positions. Pumping for the dilution fridge and large magnetic fields introduce demanding challenges in vibration isolation, which we have addressed with an ultra-quiet laboratory setting and rigid microscope design. Our system is situated inside both acoustic and RF-shielded enclosures in complement with various stages of isolation for both pump and ambient vibration sources. We will discuss unique aspects of the microscope design, such as a two-in-one double sample holder, and assess preliminary system performance. Supported by the W. M. Keck Foundation. Infrastructure at Princeton Nanoscale Microscopy Laboratory is also supported by grants from DOE, NSF, and ARO. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P21.00002: Low Temperature Scanning Tunneling Microscope for Spin Polarization Measurements Seong Heon Kim, Ryan Jaehne, LeuJen Chen, Alex de Lozanne We describe a new design for a 4K scanning tunneling microscope (STM) with an 8 tesla superconducting magnet to be used for spin polarized measurements. The novel aspects include a compact design for the chamber and the STM, the use of a secondary STM for in-situ tip characterization, and new ideas for vibration isolation. We developed a new STM head unit with 1 inch diameter and 3.2 inch length. This microscope is small enough to be installed within the small space available in the 2 inch diameter bore of our superconducting magnet. To achieve this small size, we modified the typical Pan-type z-approach walker. We also developed new simple and inexpensive electronics to control any stick-slip walker. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P21.00003: Compact probe design for Scanning Hall Probe Microscopy Neliza Leon-Brito, Seongsoo Kweon, Alex de Lozanne In the search for new materials with desirable magnetic properties for applications such as spintronics the study of magnetic properties at the micro and nanoscale is necessary. Magnetic Force Microscopy (MFM) has been the technique of choice for these types of studies, but its invasive nature makes it unsuitable for low coercivity materials like diluted magnetic semiconductors. Scanning Hall Probe Microscopy (SHPM) is an alternative technique which provides a magnetically non-invasive, calibrated measurement of the stray fields above the sample with good resolution ($\sim $1um). We have built a compact cryogenic variable-temperature (4 - 300K) SHPM with unique features such as an inverted tapered seal that also performs as a heat sink for the microscope body and a new coarse approach mechanism. Details of this design will be presented in this talk. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P21.00004: Realizing Spin Logic Atom by Atom Invited Speaker: Scanning tunneling microscopy (STM) has emerged as a leading technique which can address single atom magnetism with high energy and spatial resolution. With the development of sub-Kelvin high-magnetic field STM, two complementary methods, namely spin-polarized scanning tunneling spectroscopy (SP-STS) and inelastic STS (ISTS), can address the fundamental properties of individual magnetic impurities at surfaces [1,2]. We use a map of the distance-dependent RKKY interaction between Fe atoms on Cu(111) obtained by SP-STS to engineer complex magnetic nanostructures with tailored magnetic properties with atomic manipulation. By combining constructed anti-ferromagnetic structures with spin frustration, we realize an atomic-scale logic device which functions solely on the spin-degrees of freedom of its magnetic constituents. This work was done in collaboration with J. Wiebe, S. Lounis, B. Chilian, A. T. Costa, L. Zhou, D. L. Mills, and R. Wiesendanger. \\[4pt] [1] A. A. Khajetoorians, B. Chilian, J. Wiebe, S. Schuwalow, F. Lechermann, and R. Wiesendanger, Nature 467, 1084 (2010). \\[0pt] [2] A. A. Khajetoorians, S. Lounis, B. Chilian, A. T. Costa, L. Zhou, D. Mills, J. Wiebe, and R. Wiesendanger, arXiv:1010.1284v2 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P21.00005: Visualizing spin-dependent scattering in strong spin-orbit systems Anna Strozecka, Asier Eiguren, Jose Ignacio Pascual For surfaces which exhibit spin-orbit coupling, electrons originating from spin polarized surface bands are protected against backscattering by time reversal symmetry. Electron interference patterns observed in STM confirm the chiral spin texture of the surface Fermi contours of such materials and reveal the dominant role of spin in the scattering processes. Using a combined experimental and theoretical approach, we distinguish the role of spin in the electron scattering processes on Bi(110). Utilizing spectroscopic imaging of the local density of states, we studied the energy dependence of the interference patterns formed around single adsorbates. Simulations based on Green`s functions correctly reproduce the interference patterns, unveiling the role of spin in the interference process and allowing identification of the dominant scattering events. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P21.00006: Cotunneling theory for STM spin-flip spectroscopy F. Delgado, J. Fernandez-Rossier Scanning Tunneling Spectroscopy of both magnetic atoms and molecules adsorbed on surfaces is analyzed from the theoretical point of view. We show that cotunneling is the leading mechanism that explains the spin assisted inelastic conductance reported in recent experiments [1-4]. We describe the electronic transport between the scanning tip and the conducting surface through the magnetic system (MS) with a generalized Anderson model. The correlations in the MS are calculated exactly and transport is considered to fourth order in the tip-MS and MS-surface coupling. Our theory accounts for the observed [2,4] asymmetric conductance and provides an explanation of the large inelastic contribution.\\[4pt] [1] A. J. Heinrich et. al, Science 306, 466 (2004)\\[0pt] [2] Xi Chen, et al, Phys. Rev. Lett. 101, 197208 (2008)\\[0pt] [3] A. A. Khajetoorians et al, Nature 467, 1084 (2010)\\[0pt] [4] X. Chen et al, Phys. Rev. Lett. 101, 197208 (2008) [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P21.00007: Analysis of Tunneling Spectra in Constant-Current Distance-Voltage Mode Daniel Dougherty, Alex Pronschinske, Daniel Mardit A technical challenge associated with the use of traditional constant height tunneling spectroscopy in current-voltage mode is that tunneling current increases very rapidly at even modest voltages. This can result in tip-induced damage or motion for soft or delicate materials like organic molecules. One solution to this problem is to measure tunneling spectra in constant current distance-voltage mode where the STM feedback loop maintains a constant small tunneling current. Using the standard integral expression for tunneling current with a WKB transmission function, it is possible to create a first order differential equation connecting distance-voltage spectra with sample density of electronic states. This can be used to experimentally extract density of states or to theoretically predict distance-voltage tunneling spectra from a known density of states. We illustrate the use of this approach with numerical and experimental examples. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P21.00008: Quantitative force measurements with intermodulation atomic force microscopy Daniel Platz, Daniel Forchheimer, Carsten Hutter, Erik Thol\'en, David Haviland Dynamic atomic force microscopy (dynamic AFM) is a key tool for surface characterization on the nanoscale. Operation close to a cantilever resonance increases sensitivity and allows for the measurement of the phase of the cantilever response. This phase is traditionally interpreted as a measure of the energy dissipation due to the tip-sample interaction. However, a full understanding of dissipative processes remains a challenge in dynamic AFM. To address this problem we have developed Intermodulation AFM. With this multi-frequency technique we can tremendously increase the number of information carrying signals close to resonance. Using Fourier analysis and linear algebra we combine the amplitudes and phases of these signals to separately reconstruct the conservative and non-conservative tip-sample interactions. We have tested this method both on simulated and on experimental data. The method works at one tip-surface separation, providing quantitative high resolution maps of surface properties while imaging at normal rates. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P21.00009: Nano-scale Strain Mapping using Near-field Microscopy Antonio Llopis, Arkadii Krokhin, Sergio Pereira, Ian Watson, Arup Neogi Advances in nanophotonics are beginning to allow for the creation of nano-scale light emitting devices. Improving the quality of these next-generation emitters requires similarly advanced methods for characterization. These techniques need to be capable of imaging operational prototypes with nanometric resolution. We demonstrate here a new method for mapping strain capable of meeting the demands of next-generation device characterization. This technique makes use of near-field spectroscopy along with theoretical modelling to achieve non-destructive strain mapping with a resolution on the order of 10-100nm. An InGaN ELOG MQW sample is mapped using a SNOM, producing near-field maps of the intensity and Huang-Rhys parameter. Theoretical calculations are then used to obtain the relation between the Huang-Rhys parameter and the biaxial strain $\varepsilon _{xx}$, thereby allowing the production of a near-field map of the biaxial strain in the sample. Finally, to verify the efficacy of the method, we compare the results with those obtained using high-resolution XRD. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P21.00010: Non-linear optical nano-structured probe for photonic force microscopy Aswini Kanneganti, Harshit Vallabh, Ninad Ingle, Xiao Zhang, Jing Li, Samarendra Mohanty Use of second-harmonic (SH) optical probes for imaging of microscopic samples has distinct advantages over fluorescence, which suffers from photobleaching. Further, SH nanoparticles can be optically trapped for probing interaction forces and even for topographic imaging of nanostructures. Here, we report SH generation in ZnS(pda)$_{1/2}$ (pda = propanediamine), a new class of nanostructured crystals. ZnS(pda)$_{1/2}$ is an isostructure of ZnTe(pda)$_{1/2}$ as confirmed by PXRD pattern. The SHG imaging of the nanocrystals was carried out by an ultrafast ($\sim $100fs) Ti: Sapphire laser beam (wavelength: 960 nm; repetition rate: 80 MHz) focused to a diffraction limited spot by use of a 100X microscope objective leading to very high peak power density. Dependence of SHG intensity as a function of laser power and axial position of the nanoparticle in the focused laser microbeam was quantitated for the purpose of photonic force microscopy. The suspended ZnS(pda)$_{1/2}$ nanocrystals could be trapped using the near-IR Ti: Sapphire laser microbeam. The SHG intensity was found to fall very rapidly as the nanocrystal is displaced from the focused spot, which led to highly sensitive height measurements. Non-linear optical characterization of the ZnS(pda)$_{1/2}$ nanocrystals and its use in photonic force microscopic imaging will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P21.00011: Design of a scanning gate microscope in a cryogen-free dilution refrigerator Matthew Pelliccione, Adam Sciambi, David Goldhaber-Gordon We report on our design of an ultra-low temperature scanning gate microscope housed in a system with no liquid helium bath. The recent increase in efficiency of pulse-tube cryocoolers and pending scarcity of liquid helium have made ``cryogen-free'' dewars popular in recent years. However, this new style of dewar presents challenges for performing scanning measurements, most notably the increased vibrations introduced by the cryocooler. We will highlight the tradeoffs made in choosing such a system to house a scanner, and describe our efforts to achieve a stability suitable for measurements on mesoscopic systems. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P21.00012: Electronic characterization of 1-D defects using scanning gate spectroscopy Steven R. Hunt, Brad L. Corso, Philip G. Collins Scanning gate microscopy (SGM) is a technique particularly useful for characterizing transport in electronic devices. We have extended the SGM technique into a spectroscopy by measuring the entire bias dependence of conductance at every position on a surface. Much as in current imaging tunneling spectroscopy (CITS), the resulting data set is a multidimensional, detailed map of the electronic behavior of a surface. We apply this scanning gate spectroscopy (SGS) technique to scattering in one-dimensional, carbon nanotube circuits. Transport in one-dimensional systems depends critically on inhomogeneities, including isolated point defects. The SGS technique enables straightforward investigation of low-dimensional transport physics at such sites. In our experiments, metallic single-walled carbon nanotubes are investigated before and after the electrochemical introduction of a point defect, in order to clearly establish the contribution of different defect types. SGS directly images the energy dependence of a defect's scattering, providing a way to distinguish between different defect chemistries and quantitatively model its energy levels and transmission. This research is partly supported by the NSF (DMR-0801271). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P21.00013: Scanning gate transconductance microscopy and spectroscopy of a mesoscopic ring Benoit Hackens, Frederico Martins, Sebastien Faniel, Vincent Bayot, Marco Pala, Hermann Sellier, Serge Huant, Ludovic Desplanque, Xavier Wallart In scanning gate microscopy (SGM), a dc voltage is applied to a sharp tip moving in the vicinity of a device. This alters the electrostatic potential seen by electrons inside the device, and consequently changes the device conductance [1]. Here, we superimpose a small ac voltage to the dc bias applied on the tip, and record the change of device conductance at the tip bias modulation frequency, i.e. the local transconductance. We first image the low temperature transconductance of a mesoscopic ring patterned in a two-dimensional electron system (2DES) hosted in an InGaAs/InAlAs heterostructure. The tranconductance images are decorated by concentric features that we associate with charging of electron traps located close to the 2DES. We perform spectroscopy of these traps by positioning the tip close to them, and recording the ring transconductance as a function of the tip dc voltage and the bias accross the quantum ring. We observe Coulomb diamonds in our spectroscopic data, which confirms that Coulomb blockade is at play. [1] B. Hackens et al., Nature Physics 2, 826 (2006). [Preview Abstract] |
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