Matter Created on Table-top as Graphene Enables Schwinger Effect

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Schwinger Effect

Researchers at the University of Manchester were able to observe the Schwinger effect, a process in which matter is created spontaneously in the presence of an electric field. The Schwinger effect normally occurs only in cosmic events because it requires a very strong electric field. But the research team was able to create matter from vacuum by applying high currents through specially designed graphene-based devices. All the properties of the newly created matter can be tested in metrology testing labs.

The Schwinger effect is a prediction of quantum electrodynamics (QED) in which matter (electron-positron pairs) is spontaneously created in the presence of an electric field, thereby causing the decay of the electric field. The Schwinger effect was first proposed by Fritz Sauter in 1931 and further work was carried out by other German physicists, though it was not until 1951 that Julian Schwinger gave a complete theoretical description.

Figure: Surface properties of graphene can be deduced by optical profilometry

The Schwinger effect can be conceptualized as vacuum decay in the presence of an electric field. Although the notion of vacuum decay suggests that something is created out of nothing, physical conservation laws are nevertheless obeyed. To conserve energy, the electric field loses energy when an electron-positron pair is created. Electric charge is conserved because an electron-positron pair is charge neutral. Linear and angular momentum are conserved because, in each pair, the electron and positron are created with opposite velocities and spins. 

The Schwinger effect has never been observed due to the extremely strong electric-field strengths required. Pair production takes place exponentially slowly when the electric field strength is much below the Schwinger limit, corresponding to approximately 108 V/m. Various mechanisms have been proposed to speed up the process. The foremost of these are large particle accelerators. 

For the experiment, the research team had to construct custom devices such as narrow constrictions and superlattices from graphene and test in metrology testing labs. The experimental setup in the metrology lab allowed the researchers to achieve exceptionally strong electric fields in a simple, table-top setup. The researchers observed spontaneous creation of electron (negative charge) and hole pairs (positive) and the process’ details agreed well with theoretical predictions.

The team also observed another unusual high-energy process that so far has no analogies in particle physics and astrophysics. They filled their simulated vacuum with electrons and accelerated them to the maximum velocity allowed by graphene’s vacuum, which is 1/300 of the speed of light. At this point, something seemingly impossible happened: electrons seemed to become superluminous, providing an electric current higher than allowed by general rules of quantum condensed matter physics. Optical profilometry is used for determining surface properties. The origin of this effect was explained as spontaneous generation of additional charge carriers (holes). Theoretical description of this process provided by the research team is rather different from the Schwinger one for the empty space.

“People usually study the electronic properties using tiny electric fields that allow easier analysis and theoretical description. We decided to push the strength of electric fields as much as possible using different experimental tricks not to burn our devices,” said the paper’s first author Dr Alexey Berduygin. Co-lead author Dr Na Xin added: “We just wondered what could happen at this extreme. To our surprise, it was the Schwinger effect rather than smoke coming out of our set-up.”

Dr Roshan Krishna Kumar, another leading contributor, said: “When we first saw the spectacular characteristics of our superlattice devices, we thought ‘wow … it could be some sort of new superconductivity’. Although the response closely resembles that routinely observed in superconductors, we soon found that the puzzling behavior was not superconductivity but rather something in the domain of astrophysics and particle physics. It is curious to see such parallels between distant disciplines.” The research team was led by Nobel laureate, Prof Sir Andre Geim in collaboration with colleagues from UK, Spain, US and Japan.

Journal Reference:

Alexey I. Berdyugin, Na Xin, Haoyang Gao, Sergey Slizovskiy, Zhiyu Dong, Shubhadeep Bhattacharjee, P. Kumaravadivel, Shuigang Xu, L. A. Ponomarenko, Matthew Holwill, D. A. Bandurin, Minsoo Kim, Yang Cao, M. T. Greenaway, K. S. Novoselov, I. V. Grigorieva, K. Watanabe, T. Taniguchi, V. I. Fal’ko, L. S. Levitov, Roshan Krishna Kumar, A. K. Geim. Out-of-equilibrium criticalities in graphene superlattices. Science, 2022; 375 (6579): 430 DOI: 10.1126/science.abi8627


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