Who said there is no zero point?

An inter­na­tional research team led by The Uni­ver­sity of Man­ches­ter has suc­ceeded in observ­ing the so-​called Schwinger effect, an elu­sive process that nor­mally occurs only in cos­mic events. By apply­ing high cur­rents through spe­cially designed graphene-​based devices, the team — based at the National Graphene Insti­tute — suc­ceeded in pro­duc­ing particle-​antiparticle pairs from a vacuum.

A vac­uum is assumed to be com­pletely empty space, with­out any mat­ter or ele­men­tary par­ti­cles. How­ever, it was pre­dicted by Nobel lau­re­ate Julian Schwinger 70 years ago that intense elec­tric or mag­netic fields can break down the vac­uum and spon­ta­neously cre­ate ele­men­tary particles.

This requires truly cosmic-​strength fields such as those around mag­ne­tars or cre­ated tran­si­to­rily dur­ing high-​energy col­li­sions of charged nuclei. It has been a long­stand­ing goal of par­ti­cle physics to probe these the­o­ret­i­cal pre­dic­tions exper­i­men­tally and some are cur­rently planned for high-​energy col­lid­ers around the world.

Now an inter­na­tional, Manchester-​led research team – headed by another Nobel lau­re­ate, Prof Andre Geim, in col­lab­o­ra­tion with col­leagues from UK, Spain, US and Japan — has used graphene to mimic the Schwinger pro­duc­tion of elec­tron and positron pairs.

Excep­tion­ally strong elec­tric fields

In the Jan­u­ary 2022 issue of Sci­ence, they report spe­cially designed devices such as nar­row con­stric­tions and super­lat­tices made from graphene, which allowed the researchers to achieve excep­tion­ally strong elec­tric fields in a sim­ple table-​top setup. Spon­ta­neous pro­duc­tion of elec­tron and hole pairs was clearly observed (holes are a solid-​state ana­logue of sub­atomic par­ti­cles called positrons) and the process’s details agreed well with the­o­ret­i­cal predictions.

The sci­en­tists 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. 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 elec­tronic prop­er­ties using tiny elec­tric fields that allows 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, a post-​doctoral researcher in The Uni­ver­sity of Manchester’s Depart­ment of Physics and Astronomy.

Co-​lead author from the same depart­ment 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.”

Another lead­ing con­trib­u­tor, Dr Roshan Krishna Kumar from the Insti­tute of Pho­tonic Sci­ences in Barcelona, 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 those rou­tinely observed in super­con­duc­tors, we soon found that the puz­zling behav­iour 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 is also impor­tant for the devel­op­ment of future elec­tronic devices based on two-​dimensional quan­tum mate­ri­als and estab­lishes lim­its on wiring made from graphene that was already known for its remark­able abil­ity to sus­tain ultra-​high elec­tric currents.

Main illus­tra­tion by Mat­teo Cec­ca­nti and Simone Cassandra.

Advanced mate­ri­als is one of The Uni­ver­sity of Manchester’s research bea­cons — exam­ples of pio­neer­ing dis­cov­er­ies, inter­dis­ci­pli­nary col­lab­o­ra­tion and cross-​sector part­ner­ships that are tack­ling some of the biggest ques­tions fac­ing the planet. #ResearchBeacons