Understanding of superconductivity may be closer

Understanding of superconductivity may be closer

Phys­i­cists have long de­bat­ed the causes of su­per­con­duc­tiv­ity, a phe­nom­e­non in which nor­mal re­sist­ance to a flow of elec­tri­cal cur­rent van­ishes in cer­tain ma­te­ri­als when ex­tremely cold. This al­lows hyper-efficient cur­rent trans­mis­sion—of­fer­ing the prom­ise of a new elec­tri­cal gold­en age with high-pow­ered com­put­ers, mag­net­ic­ally lev­i­tat­ing trains and super-efficient pow­er lines. But to put this ef­fect to prac­ti­cal use, sci­en­tists have to un­der­stand it bet­ter, es­pe­cially why it seems to oc­cur only in such cold and wheth­er that can be changed. A new study may help clar­i­fy these ques­tions, ac­cord­ing to re­search­ers who have found that su­per­con­duc­tiv­ity works dif­fer­ently in two slightly dif­fer­ent tem­per­a­ture ranges. Su­per­con­duc­tiv­ity was dis­cov­ered by the Dutch phys­i­cist Heike Kamer­lingh Onnes when in 1911 when he cooled mer­cu­ry to barely above ab­so­lute ze­ro, the low­est tem­per­a­ture the­o­ret­ic­ally pos­si­ble. Sci­en­tists lat­er con­clud­ed that su­per­con­duc­tiv­ity at such rock-bot­tom tem­per­a­tures oc­curs when vibra­t­ions of the grid-like atom­ic ar­range­ment of the ma­te­ri­al af­fects its elec­trons, suba­tom­ic par­t­i­cles that car­ry elec­tric charge. These, which nor­mally re­pel each oth­er be­cause they have the same charge, then join up as pairs that glide ef­fort­lessly through the ma­te­ri­al with­out scat­ter­ing off its at­oms. In 1986 came the dis­cov­ery of a class of ma­te­ri­als that al­low su­per­con­duc­tiv­ity at some­what less frig­id tem­per­a­tures: up to about 150 Kel­vin (mi­nus 253 F or mi­nus 123 C), con­sid­erably high­er than the 4 de­grees Kel­vin (mi­nus 452F or mi­nus 269 C) re­quired in the orig­i­nal Onnes tests. This ad­vance al­lowed the ma­te­ri­als to be cooled with liq­uid ni­tro­gen, which costs less than the liq­uid he­li­um needed to cool low­er-tem­per­a­ture su­per­con­duc­tivity. Since that find­ing, sci­en­tists have de­bat­ed wheth­er in these higher-tem­per­a­ture su­per­con­duc­tors—al­so called cop­per ox­ide su­per­con­duc­tors—elec­trons bond in the same ways as in the low­er-tem­per­a­ture su­per­con­duc­tors. The mech­an­ism turns out to be dif­fer­ent, ac­cord­ing to the new stu­dy. Rath­er than atom­ic vibra­t­ions driv­ing the elec­trons to join as pairs, the re­search­ers said, higher-tem­per­a­ture su­per­con­duc­tiv­ity de­pends on elec­trons’ abil­ity to take ad­van­tage of their nat­u­ral re­pul­sion in a com­plex situa­t­ion. This con­clu­sion, in­ves­ti­ga­tors said, was based on ex­pe­ri­ments show­ing that the places in a sam­ple where elec­trons form the most strongly bound pairs, are the same as where they show signs of stronger re­pul­sion at higher, non-su­per­con­duct­ing tem­per­a­tures. Sur­pris­ing­ly, in oth­er words, it seems “the elec­trons with the strongest re­pul­sion in one situa­t­ion are the most ad­ept at su­per­con­duc­tiv­ity in anoth­er,” said Prince­ton Uni­ver­s­ity phys­i­cist Ali Yaz­dani, one of the re­search­ers. That’s un­like the be­hav­ior of elec­trons in low­er-tem­per­a­ture su­per­con­duct­ive ma­te­ri­als, ac­cord­ing to the group, which stud­ied a com­pound made of stron­ti­um, bis­muth, cal­ci­um and cop­per ox­ide and re­ported the find­ings in the April 11 is­sue of the re­search jour­nal Sci­ence. Al­though much re­mains to be ex­plained, the re­search­ers said their work may be a a use­ful step. “The da­ta is a gold mine which we’re only be­gin­ning to ex­ploit,” agreed Prince­ton phys­i­cist Phil­ip An­der­son, who won a phys­ics No­bel in 1977 and was­n’t in­volved in the re­search. The in­ves­ti­ga­tors used a spe­cially rigged form of a de­vice known as scan­ning tun­nel­ing mi­cro­scope, which let them ex­am­ine a sin­gle at­om as elec­trons there went from re­pelling each oth­er to pair­ing up. The mi­cro­scope an­a­lyzes at­oms by meas­ur­ing cur­rent that flows be­tween the sur­face of a sam­ple, and a spe­cially de­signed probe on the mi­cro­scope. The probe, with a fi­ne tip just one at­om wide, is placed a hair’s breadth above the sam­ple, and can move in in­cre­ments smaller than an at­om over the sur­face to take mea­sure­ments. April 10, 2008,Courtesy Princeton University and World Science staff