5.13.24
Oliver Morsch
Researchers at ETH Zurich have, for the first time, made visible how electrons form vortices in a material at room temperature. Their experiment used a quantum sensing microscope with an extremely high resolution.
Using a magnetic field sensor (red arrow) inside a diamond needle, researchers at ETH imaged electron vortices in a graphene layer (blue). (Illustration: Chaoxin Ding)
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In brief
In graphene, electrons behave like a liquid. This can lead to the formation of vortices.
Such electron vortices have now been made visible using a quantum magnetic field sensor with a high spatial resolution.
Typically, transport phenomena are more easily detected at low temperatures. Thanks to their highly sensitive sensor, the ETH researchers were able to observe vortices even at room temperature.
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When an ordinary electrical conductor – such as a metal wire – is connected to a battery, the electrons in the conductor are accelerated by the electric field created by the battery. While moving, electrons frequently collide with impurity atoms or vacancies in the crystal lattice of the wire, and convert part of their motional energy into lattice vibrations. The energy lost in this process is converted into heat that can be felt, for example, by touching an incandescent light bulb.
While collisions with lattice impurities happen frequently, collisions between electrons are much rarer. The situation changes, however, when graphene, a single layer of carbon atoms arranged in a honeycomb lattice, is used instead of a common iron or copper wire. In graphene, impurity collisions are rare and collisions between electrons play the leading role. In this case, the electrons behave more like a viscous liquid. Therefore, well-known flow phenomena such as vortices should occur in the graphene layer.
Reporting in the scientific journal Science, researchers at ETH Zurich in the group of Christian Degen have now managed to directly detect electron vortices in graphene for the first time, using a high-resolution magnetic field sensor.
Highly sensitive quantum sensing microscope
The vortices formed in small circular disks that Degen and his co-workers had attached during the fabrication process to a conducting graphene strip only one micrometre wide. The disks had different diameters between 1.2 and 3 micrometres. Theoretical calculations suggested that electron vortices should form in the smaller, but not in the larger disks.
To make the vortices visible the researchers measured the tiny magnetic fields produced by the electrons flowing inside the graphene. For this purpose, they used a quantum magnetic field sensor consisting of a so-called nitrogen-vacancy (NV) centre embedded in the tip of a diamond needle. Being an atomic defect, the NV centre behaves like a quantum object whose energy levels depend on an external magnetic field. Using laser beams and microwave pulses, the quantum states of the centre can be prepared in such a way as to be maximally sensitive to magnetic fields. By reading out the quantum states with a laser, the researchers could determine the strength of those fields very precisely.
“Because of the tiny dimensions of the diamond needle and the small distance from the graphene layer – only around 70 nanometres – we were able to make the electron currents visible with a resolution of less than a hundred nanometres”, says Marius Palm, a former PhD student in Degen’s group. This resolution is sufficient for seeing the vortices.
Inverted flow direction
In their measurements, the researchers observed a characteristic sign of the expected vortices in the smaller discs: a reversal of the flow direction. While in normal (diffusive) electron transport, the electrons in strip and disc flow in the same direction, in the case of a vortex, the flow direction inside the disc is inverted. As predicted by the calculations, no vortices could be observed in the larger discs.
“Thanks to our extremely sensitive sensor and high spatial resolution, we didn’t even need to cool down the graphene and were able to conduct the experiments at room temperature”, says Palm. Moreover, he and his colleagues not only detected electron vortices, but also vortices formed by hole carriers. By applying an electric voltage from below the graphene, they changed the number of free electrons in such a way that the current flow was no longer carried by electrons, but rather by missing electrons, also called holes. Only at the charge neutrality point, where there is a small and balanced concentration of both electrons and holes, the vortices disappeared completely.
“At this moment, the detection of electron vortices is basic research, and there are still lots of open questions”, says Palm. For instance, researchers still need to figure out how collisions of the electrons with the graphene’s borders influence the flow pattern, and what effects are occurring in even smaller structures. The new detection method used by the ETH researchers also permits taking a closer look at many other exotic electron transport effects in mesoscopic structures – phenomena that occur on length scales from several tens of nanometres up to a few micrometres.
See the full article here.
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The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH) is a public research university in the city of Zürich, Switzerland. Founded by the Swiss Federal Government in 1854 with the stated mission to educate engineers and scientists, the school focuses exclusively on science, technology, engineering and mathematics. Like its sister institution The Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne](CH) , it is part of The Swiss Federal Institutes of Technology Domain (ETH Domain)) , part of the The Swiss Federal Department of Economic Affairs, Education and Research [EAER][Eidgenössisches Departement für Wirtschaft, Bildung und Forschung] [Département fédéral de l’économie, de la formation et de la recherche] (CH).
The university is an attractive destination for international students thanks to low tuition fees of 809 ₣ per semester, PhD and graduate salaries that are amongst the world’s highest, and a world-class reputation in academia and industry. There are currently students from over 120 countries, many of which are pursuing doctoral degrees. In the QS World University Rankings ETH Zürich is ranked very highly in the world and very highly by the Times Higher Education World Rankings. In the QS World University Rankings by subject it is ranked very highly in the world for engineering and technology, earth & marine science.
Nobel laureates, Fields Medalists, Pritzker Prize winners, and Turing Award winners have been affiliated with the Institute, including Albert Einstein. Other notable alumni include John von Neumann and Santiago Calatrava. It is a founding member of the IDEA League and the International Alliance of Research Universities (IARU) and a member of the CESAER network.
ETH Zürich was founded on 7 February 1854 by the Swiss Confederation and began giving its first lectures on 16 October 1855 as a polytechnic institute (eidgenössische polytechnische schule) at various sites throughout the city of Zurich. It was initially composed of six faculties: architecture, civil engineering, mechanical engineering, chemistry, forestry, and an integrated department for the fields of mathematics, natural sciences, literature, and social and political sciences.
It is locally still known as Polytechnikum, or simply as Poly, derived from the original name eidgenössische polytechnische schule, which translates to “federal polytechnic school”.
ETH Zürich is a federal institute (i.e., under direct administration by the Swiss government), whereas The University of Zürich [Universität Zürich ] (CH) is a cantonal institution. The decision for a new federal university was heavily disputed at the time; the liberals pressed for a “federal university”, while the conservative forces wanted all universities to remain under cantonal control, worried that the liberals would gain more political power than they already had. In the beginning, both universities were co-located in the buildings of the University of Zürich.
From 1905 to 1908, under the presidency of Jérôme Franel, the course program of ETH Zürich was restructured to that of a real university and ETH Zürich was granted the right to award doctorates. In 1909 the first doctorates were awarded. In 1911, it was given its current name, Eidgenössische Technische Hochschule. In 1924, another reorganization structured the university in 12 departments. However, it now has 16 departments.
ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École polytechnique fédérale de Lausanne](CH), and four associated research institutes form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.
Reputation and ranking
ETH Zürich is ranked among the top universities in the world. Typically, popular rankings place the institution as one of the best universities in continental Europe and ETH Zürich is consistently ranked among the top universities in Europe, and among the best universities of the world.
Historically, ETH Zürich has achieved its reputation particularly in the fields of chemistry, mathematics and physics. Nobel laureates are associated with ETH Zürich, the most recent of whom is Richard F. Heck, awarded the Nobel Prize in chemistry in 2010. Albert Einstein is perhaps its most famous alumnus.
The QS World University Rankings placed ETH Zürich very high in the world. ETH Zürich has ranked very highly in the world in Engineering, Science and Technology, just behind The Massachusetts Institute of Technology, Stanford University and The University of Cambridge (UK). ETH Zürich also ranked very highly in the world in Natural Sciences, and in Earth & Marine Sciences.
The Times Higher Education World University Rankings has ranked ETH Zürich very highly in the world in the field of Engineering & Technology, just behind
The Massachusetts Institute of Technology, Stanford University, The California Institute of Technology, Princeton University, The University of Cambridge (UK),
Imperial College London (UK) and
The University of Oxford (UK).
In a comparison of Swiss universities by swissUP Ranking and in rankings published by CHE comparing the universities of German-speaking countries, ETH Zürich traditionally is ranked very highly in natural sciences, computer science and engineering sciences.
In the survey CHE Excellence Ranking on the quality of Western European graduate school programs in the fields of biology, chemistry, physics and mathematics, ETH Zürich was assessed as one of the institutions to have excellent programs in all the considered fields, the other two being Imperial College London (UK) and the University of Cambridge (UK), respectively.