Re­veal­ing the mys­ter­ies of su­per­con­duc­tors

Chemical Industry Digest - - Science Pages -

The U.S. De­part­ment of En­ergy’s Ames Lab­o­ra­tory has suc­cess­fully demon­strated that a new type of op­ti­cal mag­ne­tome­ter, the NV mag­ne­to­scope, can map a unique fea­ture of su­per­con­duc­tive ma­te­ri­als that along with zero re­sis­tance de­fines the su­per­con­duc­tiv­ity it­self. That unique fea­ture is the Meiss­ner ef­fect, which is the ex­pul­sion of the mag­netic field dur­ing a ma­te­rial’s tran­si­tion to a su­per­con­duct­ing state.

Rus­lan Pro­zorov, an Ames Lab­o­ra­tory physi­cist who is an ex­pert in su­per­con­duc­tiv­ity and mag­netism at low tem­per­a­tures, said “The Meiss­ner ef­fect is the hall­mark sig­na­ture of a true su­per­con­duc­tor, which sep­a­rates it from a hy­po­thet­i­cal per­fect metal with zero re­sis­tance. That is fine in text­books and in prin­ci­ple, but in real su­per­con­duct­ing ma­te­ri­als the Meiss­ner ef­fect is quite com­pli­cated. Ro­bust screen­ing of a mag­netic field by a su­per­con­duct­ing sam­ple and Meiss­ner ex­pul­sion upon cool­ing in a mag­netic field can be con­fused. This ef­fect is ac­tu­ally very weak and frag­ile and dif­fi­cult to ob­serve.”

Un­til now, physi­cists were un­able to vi­su­al­ize its spa­tial dis­tri­bu­tion in the ma­te­rial and how that might vary be­tween dif­fer­ent su­per­con­duct­ing com­pounds. Now it is pos­si­ble to map unique and distin­guish­ing fea­tures of the Meiss­ner ef­fect, us­ing a very sen­si­tive mag­ne­to­scope that takes ad­van­tage of the quan­tum state of a par­tic­u­lar kind of an atomic de­fect, called ni­tro­gen-vacancy (NV) cen­ters, in di­a­mond.

“This tech­nique, which is min­i­mally in­va­sive and ex­tremely sen­si­tive, is im­ple­mented in an op­ti­cal de­vice that op­er­ates suc­cess­fully while sam­ples are at the low tem­per­a­tures (4 de­grees above ab­so­lute zero), which is nec­es­sary for quan­tum ma­te­ri­als ex­plo­ration. This was no triv­ial un­der­tak­ing,” said Pro­zorov. While the sci­ence be­hind us­ing NV cen­ters as sen­sors has been known, sci­en­tists at Ames Lab­o­ra­tory wanted to know if the tech­nol­ogy could be har­nessed for prob­ing mag­netic fields with un­prece­dented sen­si­tiv­ity and good spa­tial res­o­lu­tion and ap­ply it to study­ing var­i­ous mag­netic and su­per­con­duct­ing ma­te­ri­als.

Ames Lab­o­ra­tory sci­en­tist and mem­ber of Pro­zorov’s group, Naufer Nus­ran, led the devel­op­ment of this unique set-up, and cur­rent work used di­a­mond film with NV-cen­ters im­planted right be­neath the sur­face to mea­sure larger-scale vari­a­tion of the mag­netic fields. This is the first sci­en­tific pa­per pub­lished mea­sur­ing the spa­tial dis­tri­bu­tion of the Meiss­ner ef­fect us­ing an NV mag­ne­to­scope, prov­ing that the tech­nique works and is ready to be de­ployed to study even more com­plex prob­lems. Nus­ran also part­nered with the Cen­ter for Nanoscale Ma­te­ri­als, a DOE Of­fice of Sci­ence user fa­cil­ity at Ar­gonne Na­tional Lab­o­ra­tory, to de­sign and fab­ri­cate the nanoscale pil­lars of di­a­mond, each with a sin­gle NV cen­ter, for the con­struc­tion of the mag­ne­to­scope, which took three years. De­ploy­ment of these sen­sors, now housed in Ames Lab­o­ra­tory’s ul­tra-low noise Sen­si­tive In­stru­men­ta­tion Fa­cil­ity (SIF), is the next step in re­search for the Pro­zorov group in the new lab.

The re­search is fur­ther dis­cussed in the pa­per,” Spa­tially-re­solved study of the Meiss­ner ef­fect in su­per­con­duc­tors us­ing NV-cen­ters-in-di­a­mond op­ti­cal mag­ne­tom­e­try,” au­thored by N.M. Nus­ran, K. R. Joshi, K Cho, M. A. Tanatar, W.R. Meier, S. L. Bud’ko, P.C. Can­field, Y. Liu, T.A. Lo­grasso, and R. Pro­zorov; and pub­lished in the New Jour­nal of Physics.

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