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PNAS published latest research of Alexey Kavokin, Westlake University
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Titled The Nernst Effect in Corbino Geometry, Prof. Kavokin’s latest research appeared recently in Proceedings of the National Academy of Sciences of the United States of America (PNAS). Alexey Kavokin, Chair Professor of Physics at Westlake University, is the first author, and is joined by correspondent author Boris Altshuler from Colombia University.
Kavokin and collaborators found that a Corbino disk geometry offers a precious opportunity for the observation of the specific Nernst effect having a purely thermodynamic nature. They predict, that, with an applied temperature difference between the outer and inner edges of the disk and a magnetic field perpendicular to the plane of the disk, there will be strong oscillations of the induced magnetic field in the center of the disk as a function of the external field.
The inspiration kicked in when Professor Boris Altshuler visited Westlake University in 2019. By the end of the three-week visit, Kavokin and Altshuler formed the concept and model. “In the following weeks, we completed the entire calculations, with only pens and paper. It is beautiful,” said Kavokin.
Fig. 1: Alexey Kavokin (third from the right) and Boris Altshuler (second from the left) and the team at Hangzhou’s West Lake.
The sparkle between two classical research fields
Research in one field may unexpectedly play an even more important role in another field, as it is often the case in science. And sometimes bringing two seemingly irrelevant phenomena together leads to remarkable findings. The Nernst effect in the Corbino disk is a good illustration to this rule.
To better explain this work, we’d better go back to 1911. An Italian physicist named Corbino attached two electrodes on the inner and outer edge of a disk and placed a magnetic field normal to the disk. The induced Lorentz force gives rise to magnetization currents (as shown in Fig. 2). Corbino ran the test with different metals and showed that the radial resistivity of the disk becomes stronger with the increase of the magnetic field. This is how the effect of magnetoresistance was found. This geometry setting is later named the "Corbino disk".
Over the years, scientist have studied the Hall effect in the Corbino geometry, in both classical and quantum limits. In contrast, the most important thermomagnetic effect, namely the Nernst effect, remains poorly explored in this disk geometry.
Fig. 2: A Corbino disk: In Corbino’s experiment, the inner and outer edges of the disk are connected to electrodes. A magnetic field perpendicular to the plane of the disk gives rise to magnetization currents.
The Nernst Effect is, as shown in Fig. 3, the induction of an electric current (in the y-direction) by an external magnetic ﬁeld (in the z-direction) and a temperature gradient (in the x-direction). It was discovered in the 19th century by German scientist Walter Nernst and his supervisor Albert von Ettingshausen. Recently, the giant Nernst or Nernst–Ettingshausen effects have been observed in graphene, in a pseudogap phase of quasi–two-dimensional, high-temperature superconductors, and in conventional superconducting ﬁlms being in the ﬂuctuation regime.
How can we understand the Nernst effect? Generally speaking, the Nernst signal consists of two parts: a kinetic and a thermodynamic one. The former is governed by the conductivity of the sample and the derivative of the chemical potential of the carriers over temperature. The latter is related to the stationary magnetization currents induced by the temperature gradient. These currents are independent from the conductivity of the sample, which makes them unique and extremely interesting from a fundamental point of view. The research community started to gain some understandings of the thermodynamic part in 1964, whereas this problem has been readdressed in almost every decade due to its importance for the quantum Hall effect and superconductivity researches. However, so far, there hasn´t been any concrete experimental validation.
Fig. 3: The Nernst Effect: An electrical field (Ey) is induced by a magnetic field normal to the plane (Hz) and a temperature gradient in the x-direction (red arrow)
Fig. 4: Nobel laureate in Chemistry Walter Nernst (first from the left in the back) and his advisor Albert von Ettingshausen (second from the left in the front)
This time, Kavokin and his collaborators were onto some breakthrough.
The key to the much awaited validation is to prove the existence of magnetization currents. Magnetization currents are too elusive in ordinary geometries, e.g. rectangle bars, etc. That was when Kavokin and his colleagues looked into the Corbino disk.
"When there's a strong external magnetic field, the current does not propagate between the inner and outer edges of the disk, and therefore we can safely neglect the kinetic currents. In the presence of the magnetic field and temperature gradient, the Corbino disk would produce significant magnetization currents. We can therefore view the total currents in the sample as the magnetization currents," said Kavokin.
They calculated the magnetic field induced by the currents and found that the field experiences pronounced unharmonic oscillations. "We compared Corbino disks made of normal metal and of graphene and discovered that their oscillation behaviors are differ strongly different. The behavior predicted by our microscopic model can be experimentally measured by state-of-art technology, e.g. with a SQUID magnetometer in the center of the disk."
Towards new electronic devices and materials
Thanks to the Corbino disk, we finally have a chance to peek into the pure thermodynamic part of the Nernst effect. Kavokin said, "It has been a subject of debate for many years: the contribution of the diamagnetic (or magnetization) currents to the Nernst effect. Our work sheds light on it."
With the universal link established in this work, one could in turn extract information on carriers, which offer a powerful tool for the experimental studies of transport phenomena in 2D crystals. This is, by no doubt, great news for experimentalists. It may help developing new electronic devices and materials, and all together a brand-new electronic world.
PNAS is the official journal of the National Academy of Sciences of the United States of America. Founded in 1914, it is one of the most quoted multidisciplinary journals
Introduction to the author:
Professor Alexey Kavokin received his master’s degree in physics with honor from St-Petersburg State Technical University and his Ph.D. in Physical and Mathematical Sciences at A.F. Ioffe Institute, Russian Academy of Sciences in St Petersburg. He was the Chair of Nanoscience and Photonics and Professor of Physics and Astronomy School at University of Southampton. Prof. Kavokin accepted the offer as the Director of the International Center of Polaritonics and Chair Professor at Westlake University in June 2018. He has invited professors and researchers to join the center and work together on “quantum fluids of light”. He has also invited many international scholars to visit Westlake University for in-depth collaboration, including the co-author of the paper mentioned above, Boris Altshuler, who is a member of the American Academy of Arts and Sciences and Professor at Columbia University. Professor Kavokin establishing and managing a world-leading research center in Hangzhou, Zhejiang Province.
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