"Long before it's in the papers"
January 28, 2015


Understanding of superconductivity may be closer

April 10, 2008
Courtesy Princeton University
and World Science staff

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. 

The two fig­ures show re­sults ob­tained with a spe­cial­ized scan­ning mi­cro­scope at tem­per­a­tures well above and well be­low when the elec­trons pair up in high-tem­per­a­ture su­per­con­duc­tivity. At top is an atom­ic-scale map of the  pair­ing strength of elec­trons while su­per­con­duct­ing, in which red shows the strongest pair­ing re­gions and blue the weak­est. The bot­tom fig­ure shows a meas­ure­ment re­lat­ed to elec­t­ron-elec­t­ron in­ter­ac­tion on the same sites at the high­er tem­per­a­ture, when elec­trons re­pel each other. The con­nec­tion be­tween these two meas­ure­ments is the main find­ing of the Prince­ton study. (Cred­it: Yaz­dani Group)

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.

* * *

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Physicists have long debated just what causes superconductivity, a phenomenon in which normal resistance to the 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 the 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 superconductors. 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 materials. 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 superconducting materials, according to the group, who 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 as superconductors. The microscope analyzes atoms by measuring current that flows between the surface of a sample and specially designed probe attached to 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.