The electronic properties of solid-state materials used in today's electronic devices are governed by properties of valence electrons. One such property is the spin of the electron, which, in layman's terms, is the sense of rotation of the spinning motion of the electron. As realized almost ninety years ago by German physicist Friedrich Hund (1896 – 1997), the electrons in a given atom all tend to spin with the same sense of rotation, a rule of thumb which is now called Hund's rule (see Figure).
Since the electron current in electronic devices consists precisely of these electrons hopping from one atom to the neighboring atom, the consolidation of the spinning motion of electrons due to Hund's rule may have profound consequences for the electronic properties of the device. Metallic materials, in which the electron motion is governed by Hund's rule, are called Hund's metals. Indeed, theorists have argued that the electrons in a recently intensively studied class of superconductors behave like Hund's metals. In superconductors, the electrons are hopping without any resistance and thus can flow through the material without any loss of energy. So far, the superconducting electron flow survives only at very temperatures. The research community is intensively searching for new materials, hopefully showing superconductivity under ambient conditions, which would solve many of the most pressing problems of the current era of information technology. For a target-oriented search of such materials, the electron properties of the basic constituents of Hund's metals need to be understood in detail, and this requirement was lacking so far.
Such a basic constituent, called Hund's impurity, was realized by depositing iron-hydrogen molecules on the surface of platinum. The experimentalists were able to intentionally remove hydrogen from the Hund's impurity by using the tip of a scanning tunneling microscope as a tool. It was found that attaching or removing the hydrogen has profound consequences for the electronic properties of the Hund's impurity, which was studied in great detail by comparing the experimental data to cutting edge computer simulations. In a next step, it is planned to couple many Hund's impurities by moving them closer, again by using the tip of a scanning tunneling microscope as a tool. This would enable a bottom-up assembly of a Hund's metal and its study will hopefully give relevant insight for the targeted development of novel high-temperature superconducting materials.
Left panel: Occupation of five electron orbitals in an atom (boxes) with five or six spin up (magenta colored arrows) or spin down (cyan colored arrows) electrons according to Hund's rules. For adding the sixth electron to the orbitals, the energy of UCoulomb has to be paid due to the mutual electrostatic repulsion of the negatively charged electrons. However, if one of the electrons changes its spin from up to down, an energy also has to be paid (JHund). Center panel: Scanning tunneling microscope image of an iron atom (cone with red tip) and three iron-hydrogen molecules (cones with yellow tips) on the surface of platinum. Right panel: The hydrogen of the bottom right iron-hydrogen molecule has been removed by using the tip of the scanning tunneling microscope as a tool
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A. A. Khajetoorians, M. Valentyuk, M. Steinbrecher, T. Schlenk, A. Shick, J. Kolorenč, A. I. Lichtenstein, T. O. Wehling, R. Wiesendanger and J. Wiebe: Tuning emergent magnetism in a Hund's impurity,
Nature Nanotechnology 10, 958–964 (2015) 
