Session H1: Focus Session: Advances in Scanned Probe Microscopy I: Novel Approaches to Complex Systems

Hybrid STM/AFM study of half-metallic surface states in cobaltates

Na$_{x}$CoO$_{2}$ is a well-known compound that has been studied at various Na concentrations, and has drawn much attention for its unconventional superconductivity and antiferromagnetic phase. In its stoichiometric concentration, NaCoO$_2$ has has recently been proposed as a system for observing topological superconductivity when mixed with a superconductor's electronic states through the proximity effect. We examine NaCoO$_2$ using an ultrahigh-vacuum low-temperature hybrid scanning tunneling and atomic force microscope at 4 K. We use the tuning-fork AFM mode to study the topography of this bulk insulating material when no tunneling is possible, and utilize a special electrical contact scheme to access the electronic surface states for spectroscopy.   

Trend and novel challenges in spectroscopic imaging scanning tunneling microscopy for correlated electron materials research

In this presentation we would like to discuss the recent progress of the spectroscopic imaging scanning tunneling microscopy (SI-STM) and how the real- and momentum-space sensitive spectroscopic tool is evolving into a more robust quantum theoretical tool for the phase transition study of the correlated electron materials. \\[4pt] [1] Jhinhwan Lee, et al., Science 325, 1099 (2009).\\[0pt] [2] Michael J. Lawler, et al., Nature 466, 374 (2010).\\[0pt] [3] Colin V. Parker, et al., Nature 468, 677 (2010).\\[0pt] [4] Ilija Zeljkovic, et al., arXiv:1104.4342v1 (2011).   

Unique vortex states on nanosize superconducting islands observed by LT-STM

With rapid progress of fabrication methods for nano-size superconductors, many novel properties of their superconductivity have been studied. Several vortex states unique to the nano superconductors, such as, vortex clustering, giant vortex, and anti-vortex, have been reported/predicted on specific size and shape of the islands. In this study we used Pb island structures with atomically flat surface formed on the Si(111) substrate under ultrahigh vacuum conditions as the superconducting sample. We investigate vortex states formed on the Pb islands under magnetic fields using low-temperature scanning tunneling microscopy (LT-STM), which enable us to observe the surface topograph and the superconducting gap (DOS) at atomic scale spatial resolutions simultaneously in real space. We performed precise tunneling spectroscopy on the Pb islands to take two dimensional DOS mapping at the Fermi level and succeeded in observing several kinds of vortex states (ex. multi vortex state an d giant vortex state) in real space. The details will be discussed in the presentation.   

Scanning SQUID microscopy: A powerful tool for probing magnetism and superconductivity in complex oxides

Magnetic measurements are useful in investigating novel materials because they probe the behavior of electrons and their interactions. Superconducting Quantum Interference Devices (SQUIDs) are ultra-sensitive flux magnetometers and, when used in imaging mode, they become a powerful tool for mapping magnetic fields above a sample. This talk will outline the basics of scanning SQUID microscopy and highlight our recent measurements on a new material system: complex oxide interfaces. Our scanning SQUID technique uncovered the coexistence of superconductivity and magnetism in the LAO/STO oxide system. These measurements highlight many key benefits of the scanning SQUID technique including a 3 micron imaging kernel, excellent flux sensitivity, and an on-chip field coil to simultaneously measure both the intrinsic magnetism and the sample's response to an applied magnetic field.   

Spatial Dependence of Kondo Screening and Magnetic Anisotropy on Saturated Copper Nitride Islands

Co adatoms on a copper nitride surface constitute a unique system for studying the interplay between Kondo physics and anisotropy in a high spin atom (S=3/2). By using scanning tunneling spectroscopy techniques, we can determine both the energy scales of the Kondo screening and the spin excitations on a single atom. Here we study the case of Co adatoms on large nitride islands that form as the copper surface is saturated with nitrogen. These islands present a rich spatial variation of their electronic structure. We observe how changes in the electronic structure of the insulator result in dramatic changes in the spectroscopy of the cobalt adatoms: a considerable increase of the anisotropy energy occurs as the Kondo resonance disappears. These results allow us to explore in detail the interplay between these two phenomena.   

Evolution of the Kondo Resonance for Screened Atoms on Metals and Thin Insulators

We study the magnetic anisotropy and the Kondo screening of the spin of Co atoms deposited on Cu$_{2}$N using scanning tunneling microscopy and spectroscopy. We find that for Co atoms placed on Cu$_{2}$N islands the Kondo screening is weaker when the atom is very close to the edge and at the same time has significant changes in the magnetic anisotropy. Furthermore we observe that Co atoms that are placed on the Cu surface but near the Cu$_{2}$N islands still show anisotropy and an unusually small Kondo temperature. At larger distances from the Cu$_{2}$N islands the usual large Kondo temperature recovers. We examine possible causes for these dramatic changes in the Kondo screening and magnetic anisotropy, including a possible extension of the electronic properties of the Cu$_{2}$N islands compared to the topographic influence.   

Atomic force microscopy as nano-stethoscope to study living organisms, insects

Atomic force microscopy (AFM) is a known method to study various surfaces. Here we report on the use of AFM to study surface oscillations (coming from the work of internal organs) of living organisms, like insects. As an example, ladybird beetles (Hippodamia convergens) measured in different parts of the insect at picometer level. This allows us to record a much broader spectral range of possible surface vibrations (up to several kHz) than the previously studied oscillations due to breathing, heartbeat cycles, coelopulses, etc. (up to 5 -10 Hz). The used here AFM method allows collecting signal from the area as small as $\sim $100nm2 (0.0001$\mu $m2) with an example of noise level of (2$\pm $0.2)$\times $10-3 nm r.m.s. at the range of frequencies $>$50Hz (potentially, up to a MHz). Application of this method to humans is discussed. The method, being a relatively non-invasive technique providing a new type of information, may be useful in developing of what could be called ``nanophysiology.''   

Scanning tunneling microscopy study of the assembly and structure of filamentous virus M13 bound to graphite

Viruses are an important class of biomaterials used for placing nano particles on inorganic substrates. To accomplish greater control over viral assembly on a substrate it is important to determine the in situ nanoscale structure of the viral protein coat. Scanning tunneling microscopy offers the unique potential for determining the structure and arrangement of the proteins of a virus adsorbed on a conducting substrate. In this work, I develop an experimental technique for isolating and studying M13 viruses that bind to graphite. Using scanning tunneling microscopy in ambient conditions I obtain the correct lateral dimension of the virus and the periodicity of its protein structure when it is bound to graphite. I also analyze the tunneling conductance fluctuations in these measurements and introduce a simple model for tunneling through an assembly of proteins to obtain an accurate estimation of the vertical dimension of a virus bound to a conducting substrate. I discuss broader implications of this scanning tunneling microscopy study for the in situ structure determination of other biomolecules.   

Nanomechanics of Murine Articular Cartilage Reveals the Effects of Chondroadherin Knockouts

With high resolution nanotechnology tools, quantification of cartilage biomechanical properties provides important insights into the role of low abundance matrix molecules on cartilage function and pathology. In this study, the role of chondroadherin (CHAD) on cartilage mechanical properties was assessed via atomic force microscopy-based nanoindentation (0.1-10 $\mu $m/s z-piezo displacement rates) of murine knee cartilage from wild type (WT) and CHAD knockout (KO) animals ages 1 year, 4 month, and 11 weeks (n$\ge $4 joints/age-group). A significant increase in indentation modulus, E, with indentation rate in all specimens (p$<$0.05, Friedman) suggested poro-viscoelastic behavior. For all age groups, CHAD KO significantly reduced E at all indentation rates (p$<$0.05, 2-way ANOVA); e.g., at 1-year, E was 0.77+/-0.1 MPa for WT (mean+/-SEM 1$\mu $m/s rate) and 0.25+/-0.07 MPa for CHAD KO cartilage. Lack of CHAD appears to delay development of load bearing extracellular matrix. This could affect the effective cross-link density of the tissue network and, hence, decrease local osmotic swelling while increasing the hydraulic permeability of the aggrecan-filled network. Ongoing studies are investigating the biochemical properties and nanostructure of CHAD KO joints.   

Nanoindenter Stiffness Measurements on a MEMS Sound Sensor

We demonstrate a novel technique to extract the various components of the stiffness (or compliance) measured along the surface of a MEMS directional sound sensor. Because the sensor comprises a cantilever beam mounted on torsion springs, the overall stiffness consists of various compliance components added in series. Stiffness measurements made using a nanoindenter are found to agree with an analytical model and a finite element model (FEM) of the sensor. Moreover, by exploiting the differing power-law characteristics of the individual compliance components, we demonstrate extraction of the separate components from a logarithmic plot of the overall stiffness. Finally, we measure the ultimate (failure) strength of the sensor, from which we obtain the maximum acoustic intensity the sensor can tolerate.   

Primary electron beam generation in Near-Field-Emission SEM

Due to low electron energies used in Near-Field-Emission SEM (NFESEM), the understanding of the physical phenomena governing the primary electron beam generation is of fundamental relevance. The geometry and the chemical composition of the ultra-sharp field emitter have been therefore investigated by using different well-known electron microscopy techniques. The last hundreds of nanometers of such a field emitter, produced by electrochemical etching of a tungsten wire, can be macroscopically approximated by a cone with angle of aperture of about $6^\circ \pm 1^\circ$. The shape of the very apex is strongly dependent on the preparation conditions. Moreover, the only remaining contamination after the annealing procedure of the tungsten tips is a tungsten-oxide coating, uniformly distributed on the surface.   

Chemical contrast in Near-Field-Emission SEM

In Near-Field-Emission SEM the primary electron beam, of some tens of eV, is generated by cold field emission from a polycrystalline W-tip. Recently, topography images have been obtained by scanning a W(110) sample with a tip at constant height, typically of tens of nm, recording the secondary electron yield and the emission current. We report on the observation of a chemical contrast of a W(110) surface covered by submonolayer of Fe achieved with the NFESEM technique. The chemical contrast is caused by a significant lower secondary electron yield for Fe with respect to W. The Fe islands with a diameter of 2 nm to 5 nm are clearly distinguishable, giving a direct indication of the microscope lateral resolution. The adsorbate position, size and shape are confirmed by STM. Moreover, this technique shows the presence of Fe growing along the step edges of the substrate, which can not be identified with STM.   

Electrostatic characterization of Near-Field-Emission SEM

The properties of the primary electron beam in Near-Field-Emission SEM (NFESEM) are uniquely determined by the actual geometry and position of the conducting components in the experimental apparatus. The reciprocal dependence of the accessible quantities, namely the voltage applied to the emitting tip with respect to the conducting sample surface ($V$), the relative distance between the tip and the sample ($d$) and the field-emission current ($I$), has been thoroughly characterized. In particular, the voltage $V$ needed to produce a given current $I$ has been measured as a function of $d$; the values of $I$ have been chosen in the range from 0.05 nA to 1.5 nA, while $d$ has been varied from 4 to 1500 nm. For values of $d$ smaller than a certain threshold $\bar d$, dependent on the tip, $V$ turns out to be directly proportional to the distance between the tip and the sample surface. At larger distances, $d > \bar d$, we found $V \propto I^a \cdot d^b$, with $a$ and $b$ generally varying from one tip to the another. These results are supported by preliminary theoretical calculations which assume electrostatic geometries directly inspired by the NFESEM setup, such as an hyperboloid-shaped emitter with a conduction plane lying at a generic $d$.