Electron conduction through quasi-one-dimensional indium atomic wires on silicon

Electron conduction through quasi-one-dimensional (1D) indium atomic wires on silicon (the Si(111)-4x1-In reconstruction) is clarified with the help of local structural analysis using scanning tunneling microscopy. The reconstruction has a conductance per square as high as 100 uS, with global conduction despite numerous surface steps. A complete growth of indium wires up to both the surface steps and the lithographically printed electrodes is essential for the macroscopic transport. The system exhibits a metal-insulator transition at 130 K, consistent with a recent ultraviolet photoemission study [H. W. Yeom, S. Takeda, E. Rotenberg, I. Matsuda, K. Horikoshi, J. Schaefer, C. M. Lee, S. D. Kevan, T. Ohta, T. Nagao, and S. Hasegawa, Phys. Rev. Lett. 82, 4898 (1999)]Comment: 4 pages, 3 figure

Phase transition of the Si(111)-4x1-In surface reconstruction investigated by electron transport measurements

We measure the electron conductivity of the surface states and the subsurface space charge layer originating from the Si(111)-4x1-In reconstruction as a function of temperature. The conductivity of the surface states drops sharply around 130 K with decreasing temperature, revealing a metal-insulator phase transition of the surface reconstruction. In contrast, the influence of the phase transition on the conductivity of the space charge layer is limited to temperatures above 60 K. This means that the surface Fermi level remains strongly pinned despite the phase transition, indicating the presence of free carriers in the surface states down to rather low temperatures.Comment: 10 pages, 3 figures, submitted to Surface Scienc

Vectorial, non-destructive magnetic imaging with scanning tunneling microscopy in the field emission regime

When a scanning tunneling microscope is operated at tip-target distances ranging from few nanometers to few tens of nanometers (Fowler-Nordheim or field emission regime), a new electronic system appears, consisting of electrons that escape the tip-target junction. If the targetis ferromagnetic, this electronic system is spin polarized. Here, we use these spin polarized electrons to image magnetic domains in thin films.As two components of the spin polarization vector are detected simultaneously, the imaging of the local magnetization has vectorial charac-ter. The tip is nonmagnetic, i.e., the magnetic state of the target is not perturbed by the act of probing. We expect this spin polarized technol-ogy, which scales down scanning electron microscopy with polarization analysis by bringing the source of primary electrons in closeproximity to the target, to find its main applications in the imaging of noncollinear, weakly stable spin excitations.ISSN:0003-6951ISSN:1077-311

Quantitative measurement of the charge distribution along a tungsten nanotip using transmission electron holography

Off-axis electron holography can be used to measure the electron-optical phase shift associated with a charge density distribution in the transmission electron microscope (TEM). The charge density can then be recovered either by integrating the Laplacian of the reconstructed phase1 or, equivalently, by applying a loop integral2. Whichever approach is used, the perturbed reference wave3 does not affect the measurement of the projected charge density inside the specimen so long as it does not itself contain any charges. Here, we study a W nanotip, in which the charge density distribution is of interest for applications in field emission and atom probe tomography. We assess artefacts and noise in the measurements.Figure 1(a) shows an off-axis electron hologram of a W nanotip recorded at 300 kV using an FEI Titan 60-300 TEM. The interference fringe spacing is 0.318 nm, the nominal magnification is 140 000 and the voltage applied to the electrostatic biprism is 90 V. The apex of the nanotip has a diameter of approximately 5 nm and is covered with a layer of tungsten oxide. A voltage of 50 V was applied between the nanotip and a flat electrode positioned approximately 3 µm away from it. In order to remove the contribution to the phase shift from the mean inner potential, two holograms with and without a voltage applied to the nanotip were recorded. The difference between the two phase images was then evaluated after sub-pixel alignment. Figures 1(b) and (c) show the resulting unwrapped phase before and after adding phase contours of spacing 2π/3 radians. Figure 1(d) shows the charge distribution calculated by applying a Laplacian operator to a median-filtered version of the phase image. Figure 1(e) shows cumulative charge profiles along the nanotip determined both using a loop integral and by applying a Laplacian operator to either an unwrapped phase image or the original complex image wave. The integration region is marked by a green dashed rectangle in Fig. 1 (b). The measured charge profile is consistent between the three approaches. Figure 1(f) shows an evaluation of noise in the measurement obtained by performing a similar integration in a region of vacuum indicated by the red dashed rectangle in Fig. 1(b). Results such as those shown in Figs. 1(d) and (e) can be used to infer the electric field and electrostatic potential around the tip. Future work will involve comparing the present approaches with using a model-based technique for determining the charge density from a recorded phase image

Magnetic Analysis of Ultrathin Fe Films on W(011) with SFEMPA

An ultra-high vacuum Scanning Tunneling Microscope (STM) is converted into a lens-less low-energy Scanning Electron Microscope when the tip-target distance is some tens of nanometers and the tip acts as a source of field emitted electrons. This primary electron beam excites locally secondary electrons out of the sample. Those escaping the tip-target junction are analyzed according to their spin. We use this technology to measure the local magnetization versus applied magnetic field in ultrathin Fe films on W(011) at room temperature. The resulting hysteresis loop is square. The coercive field has its maximum strength between 2.2 monolayers (0.07 T) and 3 monolayers (0.025 T), being larger than 0.1 T at 2.7 monolayers and decreasing to 0.0075 T at 6 monolayers. Rotation of the magnetization, domain wall pinning at incomplete layers and lattice misfits within the Fe films are discussed as possible explanations of this “singular” behavior. © 2020 IEEE

Non-topographic contrast in constant-current Scanning Field-Emission Microscopy (SFEM)

Scanning Tunneling Microscopy is performed in the conventional (tunneling) and in the field-emission regime. Images of W(110)-surfaces with and without some carbon content are taken in the constant current mode, in which the tip-target vertical distance displaces to compensate for the changes of the tunneling, respectively, field emission current. In the field emission regime, we observe tip-target displacements that are not related to the topographic contrast. © 2020 IEEE

Critical exponents and scaling invariance in the absence of a critical point

The paramagnetic-to-ferromagnetic phase transition is classified as a critical phenomenon due to the power-law behaviour shown by thermodynamic observables when the Curie point is approached. Here we report the observation of such a behaviour over extraordinarily many decades of suitable scaling variables in ultrathin Fe films, for certain ranges of temperature T and applied field B. This despite the fact that the underlying critical point is practically unreachable because protected by a phase with a modulated domain structure, induced by the dipole–dipole interaction. The modulated structure has a well-defined spatial period and is realized in a portion of the (T, B) plane that extends above the putative critical temperature, where thermodynamic quantities do not display any singularity. Our results imply that scaling behaviour of macroscopic observables is compatible with an avoided critical point.ISSN:2041-172

Unexpected lateral contrast in topografiner imaging on sub-Ängstrom corrugations

ISSN:0420-019

Investigation of Fe/W(110) system with Field Emission Scanning Probe Microscopy

ISSN:0420-019