D.A. Zanin

Scale invariance of a diode-like tunnel junction

We measure the current vs voltage (I-V) characteristics of a diodelike tunnel junction consisting of a sharp metallic tip placed at a variable distance d from a planar collector and emitting electrons via electric-field assisted emission. All curves collapse onto one single graph when I is plotted as a function of the single scaling variable Vd^{-\lambda}, d being varied from a few mm to a few nm, i.e., by about six orders of magnitude. We provide an argument that finds the exponent {\lambda} within the singular behavior inherent to the electrostatics of a sharp tip. A simulation of the tunneling barrier for a realistic tip reproduces both the scaling behavior and the small but significant deviations from scaling observed experimentally.

Fundamental Aspects of Near-Field Emission Scanning Electron Microscopy

Abstract In a previous publication (Kirk, 2010) the experimental technique of imaging near-field emission scanning electron microscopy (NFESEM) imaging was introduced. In NFESEM, a sharp tip in positioned at distances of a few 10nm from a metallic surface. Above a threshold voltage, electrons are field emitted from the tip. The field-emitted current is used, while scanning the tip across the surface at a well-defined, constant distance, to generate a topographic image of the surface with subnanometer vertical spatial resolution and a few-nanometer lateral spatial resolution. In this review, we discuss the fundamental physical processes that occur in NFESEM and provide some quantitative results. It is our goal to provide sufficient background information to allow NFESEM-based instruments to be developed in other laboratories.

Controlling vortex chirality in hexagonal building blocks of artificial spin ice

We exploit dipolar coupling to control the magnetic states in assemblies of single-domain magnetic nanoislands, arranged in one, two and three adjacent hexagonal rings. On tailoring the shape anisotropy of specific islands, and thus their switching fields, we achieve particular target states with near perfect reliability, and are able to control the chirality of the vortex target states. The magnetic states are observed during magnetization reversal with x-ray photoemission electron microscopy and our results are generally in excellent agreement with a numerical model based on point dipoles and realistic values of disorder. We conclude with a quantitative discussion of how our results depend on disorder and the chosen bias in shape anisotropy.

Improving the topografiner technology down to nanometer spatial resolution

In Scanning Tunnelling Microscopy (STM) the electrons are confined within the tunneling region, and this limitation has redirected scientists to alternative microscopy techniques, aimed at extracting the electrons away from the tunneling region. The topografiner - strictly speaking a precursor of STM, originally developed at the National Bureau of Standards - is an example. In this paper we report on the latest improvements of the topografiner technology that allow resolving topographic contrast with a lateral resolution down to 7 Å.

Scaling theory of electric-field-assisted tunnelling.

Recent experiments report the current (I) versus voltage (V) characteristics of a tunnel junction consisting of a metallic tip placed at a distance d from a planar electrode, d varying over six orders of magnitude, from few nanometres to few millimetres. In the ‘electric-field-assisted’ (or ‘field emission’) regime, as opposed to the direct tunnelling regime used in conventional scanning tunnelling microscopy, all I–V curves are found to collapse onto one single graph when d is suitably rescaled, suggesting that the current I=I(V,d) is in reality a generalized homogeneous function of one single variable, i.e. I=I(V⋅d−λ), where λ being some characteristic exponent and I(x) being a scaling function. In this paper, we provide a comprehensive explanation—based on analytical arguments, numerical simulations and further experimental results—for the scaling behaviour that we show to emerge for a variety of tip–plane geometries and thus seems to be a general feature of electric-field-assisted tunnelling.

Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy

We perform scanning tunnelling microscopy (STM) in a regime where primary electrons are field-emitted from the tip and excite secondary electrons out of the target—the scanning field-emission microscopy regime (SFM). In the SFM mode, a secondary-electron contrast as high as 30% is observed when imaging a monoatomic step between a clean W(110)- and an Fe-covered W(110)-terrace. This is a figure of contrast comparable to STM. The apparent width of the monoatomic step attains the 1 nm mark, i.e. it is only marginally worse than the corresponding width observed in STM. The origin of the unexpected strong contrast in SFM is the material dependence of the secondary-electron yield and not the dependence of the transported current on the tip–target distance, typical of STM: accordingly, we expect that a technology combining STM and SFM will highlight complementary aspects of a surface while simultaneously making electrons, selected with nanometre spatial precision, available to a macroscopic environment for further processing.

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.

Detecting the topographic, chemical, and magnetic contrast at surfaces with nm spatial resolution

Scanning tunnelling microscopy overshadowed other microscopy techniques owing to its unprecedented spatial resolution. However, the lack of secondary electrons in the experiment always motivated the quest for a complementary technique. The topografiner technology - a precursor of the STM - could not meet this task so far. Nevertheless, it still plays an important role in the arsenal of probing techniques. In this report, we present secondary-electron distributions of low-energy primary electrons directed at a cleaved GaAs(110) surface and preliminary measurements of single-energy surface imaging in a hybrid experiment.

The topografiner with energy analysis

The topografiner technology, developed originally at the National Bureau of Standards (now National Institute of Standards and Technology) with the aim of measuring the surface micro-topography, is less widespread than scanning tunneling microscopy but has a remarkable property: the electrons can escape the tip-surface junction. We have recently used topografiner imaging to map the surface of various metals and semiconductors with (almost) nanometer lateral spatial resolution. In this paper, we describe our attempt to endowing the NIST topografiner with an energy analysis of the electrons escaping the junction, with the aim of performing spectroscopy with nanometer spatial resolution.

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

Nature Communications 7: Article number: 13611 (2016); Published 5 December 2016; Updated 17 January 2017 The original version of this Article contained a typographical error in the spelling of the author S.A. Cannas, which was incorrectly given as S. Cannas. This has now been corrected in both the PDF and HTML versions of the Article.