Session E36: Advances in Scanned Probe Microscopy I

Scanning superconducting quantum interference device measurements of variations in superconducting transition temperature of two-dimensionally doped SrTiO$_{3}$

Mapping the spatial variation of the transition temperature, $T_{c}$, in unconventional superconducting materials can yield valuable insight into the nature of the superconducting state. In particular, many such materials have structural instabilities or transitions in their phase diagrams, and their superconducting state may therefore be particularly sensitive to local variations in the lattice. Such perturbations may manifest as local variations in $T_{c}$. I will discuss a recent application of scanning superconducting quantum interference device (SQUID) susceptometry to the study of superconductivity. By mapping the diamagnetic susceptibility of a superconductor as a function of temperature, we can observe the spatial distribution of $T_{c}$ on micron lengthscales. In two-dimensionally doped strontium titanate, we found that $T_{c}$ varies by $^{>}_{\sim}$10\% in a pattern set by twin structure. By comparing the magnitude of the variation in $T_{c}$ to quantities that could be tuned by the twinning, we inferred that $T_{c}$ was tuned by local variation in the dielectric constant. This new imaging modality, when combined with a controlled, symmetry-breaking field such as strain, will help us to study the interplay between structural inhomogeneity and the superconducting state, while recent improvements in the fabrication of our SQUID susceptometers will allow us to push the spatial resolution of such measurements to lengthscales below a micron in certain cases.   

Measuring exchange interactions between atomic spins using electron spin resonance STM

Exchange interactions between neighboring atoms give rise to magnetic order in magnetic materials. As the size of the electronic device is miniaturized toward the limit of single atoms, magnetic nanostructures such as coupled atomic dimers and clusters are explored more as prototypes for possible data storage, spintronics as well as quantum computing applications. Characterizing inter-atom exchange interactions calls for increasing spatial resolution and higher energy sensitivity to better understand this fundamental interaction. Here, using spin-polarized scanning tunneling microscopy (STM), we studied a magnetically coupled atomic dimer consisting of two 3d transition metal atoms, with one adsorbed on an insulating layer (MgO) and the other attached to the STM tip. We demonstrate the ability to measure the short-range exchange interaction between the two atomic spins with orders-of-magnitude variation ranging from milli-eV all the way to micro-eV. This is realized by the successful combination of inelastic electron tunneling spectroscopy (IETS) and electron spin resonance (ESR) techniques in STM implemented at different energy scales. We unambiguously confirm the exponential decay behavior of the direct exchange interaction.   

A kilobyte rewritable atomic memory

The ability to manipulate individual atoms by means of scanning tunneling microscopy (STM) opens op opportunities for storage of digital data on the atomic scale. Recent achievements in this direction include data storage based on bits encoded in the charge state (1), the magnetic state (2), or the local presence (3) of single atoms or atomic assemblies. However, a key challenge at this stage is the extension of such technologies into large-scale rewritable bit arrays. We demonstrate a digital atomic-scale memory of up to 1 kilobyte (8000 bits) using an array of individual surface vacancies in a chlorine terminated Cu(100) surface (4). The chlorine vacancies are found to be stable at temperatures up to 77 K. The memory, crafted using scanning tunneling microscopy at low temperature, can be read and re-written automatically by means of atomic-scale markers, and offers an areal density of 502 Terabits per square inch, outperforming state-of-the-art hard disk drives by three orders of magnitude. 1. J. Repp et al., Science 305, 493--5 (2004) 2. S. Loth et al., Science 335, 196--9 (2012) 3. R. Bennewitz et al., Nanotechnology 13, 499 (2002) 4. F. E. Kalff et al., Nature Nanotechnology 11, 926--9 (2016)   

Active current-noise cancellation for Scanning Tunneling Microscopy

The high sensitivity of the scanning tunneling microscope (STM) poses a barrier to its use in a noisy environment. Vibrational noise, whether structural or acoustic in source, manifests as relative motion between the probe tip and the sample, then appearing in the Z feedback that tries to cancel it. Here we describe an active noise cancellation process that nullifies this motion by adding a drive signal into the existing Z feedback loop. The drive is digitally calculated by actively monitoring vibrations measured by an accelerometer placed \textit{in-situ} close to the STM head. By transferring the vibration cancellation effort to this drive signal, vibration-created noise in the Z-feedback (during topography) or current (during spectroscopy) is significantly reduced. This inexpensive and easy solution, requiring no major instrumental modifications, is ideal for those looking to place their STM in a noisier environment, for example in the presence of active refrigeration systems (e.g. pulse tube cryocoolers) or coupled to high-vibration instrumentation.   

An On/Off Berry Phase Switch in Circular Graphene Resonators

Berry phase is an example of anholonomy, where the phase of a quantum state may not return to its original value after its parameters cycle around a closed path; instead the quantum system's wavefunction may acquire a real measurable phase difference, referred to as a Berry phase. Berry phase is connected with the geometry of the quantum system, providing a measurable signature of system topology. In this talk, I will present the spectroscopic measurements of the quasi-bound resonances, originating from Klein scattering, in circular graphene resonators comprised of $p-n$ junction rings in the presence of a perpendicular magnetic field. Our results show the sudden appearance of new resonances which is manifested by a giant energy splitting of time-reversed angular momentum states, orders of magnitude larger than orbital and Zeeman shifts, when a small critical magnetic field is reached. This behavior results from turning on a $\pi $-Berry phase associated with the topological singularities at the Dirac points in graphene. The electronic states can be switched on and off with small magnetic field changes on the order of 5 mT, producing strong modulation of quantum state energies. The ability to modulate a Berry phase in graphene resonators with energy scales in the meV range may prove useful for electro-optical applications operating at THz frequencies   

Visualizing the condensation of graphene whispering gallery modes into Landau levels in real-space

Recent methods for the creation of circular p--n junctions in graphene have opened the door to investigating the effects of spatial confinement on Dirac fermions, such as the formation of whispering gallery modes (WGMs). These quasi-bound modes can be confined even further into highly-degenerate Landau levels by the application of a perpendicular magnetic field. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) to investigate the effects of increasing magnetic field on the graphene WGMs in graphene/boron nitride heterostructures. Using detailed differential conductance (dI/dV) mapping we directly visualize the condensation of the distinct WGMs into degenerate Landau levels. We further show that residual disorder allows for the imaging of cyclotron orbits and circular edge modes.   

Detection of overtone vibrations in single molecules by the scanning tunneling microscope

Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) is a powerful tool for studying molecules on surfaces. Vibrational overtone excitations are thought to be extremely weak or not detectable by this technique, although recent theoretical work has challenged these assumptions. Here we show that overtone excitations of single molecules on a metal surface can be detected and explore the variation in the overtone signal among different adsorption geometries. These results extend the capabilities of STM-IETS to the detection of new vibrational transitions, with the potential to yield novel information about the degree of anharmonicity of single-molecule binding potentials.   

Probing Intermolecular Coupled Vibrations by STM Inelastic Electron Tunneling Spectroscopy.

Intermolecular interactions can induce energy shifts and coupling of molecular vibrations. However, the detection of intermolecular coupled vibrations has not been reported at the single molecule level. Here we show the detection and identification of an intermolecular coupled vibration between two molecules, one on the substrate and the other one on the tip within the gap of a sub-Kelvin scanning tunneling microscope, by inelastic electron tunneling spectroscopy (IETS) and density functional calculations. Furthermore, the anisotropy of the molecular adsorption potential on the substrate is revealed by imaging the IETS intensity of the coupled vibration.   

A new way to make diamond tip hosting an atomic sized defect

The nitrogen-vacancy (NV) center in diamond has been fascinating people with its unique role in quantum information and magnetometry. NV magnetometry was used to investigate many fundamental physics studies and develop a number of industrial applications. One of the powerful aspects of NV magnetometry is the ability to scan in space to perform spatial magnetic field sensing with nano-meter resolution. As a new emerging scanning probe technique, it faces a huge challenge to be widely adopted due to its complexity in fabrication. Here, we report a new simple way of creating diamond tips with tools found in basic clean room facilities and mount the tips onto an experimental apparatus with common lab bench tools. Finally, scanning NV magnetometry was performed to demonstrate its application.   

A Field Emission Based Electromechanical System for Tunable, High-Resolution Position Sensing

Scanning Probe Microscopy (SPM) techniques like Atomic Force Microscopy and Scanning Tunneling Microscopy opened the doors to direct investigations of the nanoscale world. Pre-dating these technologies, however, was an instrument known as the topografiner, capable of imaging surface topography with nanometer scale vertical resolution. The topografiner utilized a field emission current originating from a sharp metal tip, and so possessed a distinct advantage over conventional SPM by not requiring contact with the sample (operating $\sim 20$~nm from the surface). In the end, the DC field emission techniques used in operation and the tip-geometry hindered the potential of the technique to reach atomic-scale resolution. By using a high-aspect-ratio multi-walled carbon nanotube as a field emitter, we achieve vertical displacement sensing with sub-atomic resolution. In our approach, we employ an AC electromechanical coupling technique and demonstrate a position sensitivity of $\eta = 5$~pm$/\sqrt{Hz}$ while the emitter is located $\sim 500$~nm from the surface. The sensitivity of our system has a strong dependence on both the vertical position and the oscillation amplitude of the mechanical resonator, and we discuss how our sensitivity may approach the femtometer regime.