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Unusual electrons go with the flow

July 15, 2010
Courtesy of Princeton University
and World Science staff

Sci­en­tists on a quest to dis­cov­er new states of mat­ter have found that on the sur­faces of cer­tain ma­te­ri­als, charged sub­a­tom­ic par­t­i­cles known as elec­trons act like min­ia­ture su­per­heroes: they re­lent­lessly dodge cliff-like ob­sta­cles, some­times mov­ing straight through bar­ri­ers.

The find­ings rep­re­sent the first time such be­hav­ior by elec­trons has been tracked and recorded, sci­en­tists said, and hints at the pos­si­bil­i­ties of speed­ing up in­te­grat­ed cir­cuits that pro­cess in­forma­t­ion by flow of elec­trons be­tween dif­fer­ent de­vices. 

This view pro­vides a look in­to the heart of a scan­ning tun­nel­ing mi­cro­scope in the spe­cial­ly de­signed Prince­ton Nano­scale Mi­cros­co­py Lab­o­ra­to­ry, where pre­cise mea­sure­ments are pos­si­ble be­cause sounds and vi­bra­tions, through many tech­nolo­gies, are kept to a min­i­mum. (Pho­to: Bri­an Wil­son)


The new ma­te­ri­als might break the bot­tle­neck that oc­curs when me­tal­lic in­ter­con­nects get so small that even the ti­ni­est atom­ic im­per­fec­tion hin­ders their per­for­mance, the re­search­ers said. 

Prince­ton Uni­vers­ity phys­i­c­ist Ali Yaz­dani and collea­gues noted the odd be­hav­ior in a “topo­log­i­cal sur­face state” on a mi­cro­scop­ic wedge of the met­al an­ti­mony. The work is re­ported in the July 15 is­sue of the re­search jour­nal Na­ture.

Nor­mal­ly, elec­tron flow in ma­te­ri­als is hin­dered by im­per­fec­tions—seem­ingly slight edges and rifts act like cliffs and crevasses in this mi­cro­scop­ic world. Re­cent the­o­ries, how­ev­er, pre­dict that elec­trons on the sur­face of some com­pounds, con­tain­ing el­e­ments such as an­ti­mony, can be im­mune to such dis­rup­tions.

Elec­trons, like oth­er sub­a­tom­ic par­t­i­cles, have both par­t­i­cle-like and wave­like prop­er­ties, in a du­al­ity that has nev­er been sat­is­fac­torily ex­plained, yet shows it­self per­sist­ently in ex­pe­ri­ments. The elec­trons that de­fy block­ages in their flow, Yaz­dani said, do so be­cause of a spe­cial form of elec­tron wave that seem­ingly al­ters the pat­tern of flow around any im­per­fec­tion. 

Many of the “topo­log­i­cal” ma­te­ri­als, such as an­ti­mony, have been im­por­tant in the world econ­o­my, but their un­usu­al sur­face con­duc­tion had­n't pre­vi­ously been ex­am­ined. Part of the chal­lenge had been the dif­fi­cul­ty in meas­ur­ing the flow of elec­trons at the sur­face.

“Ma­te­rial im­per­fec­tions just can­not trap these sur­face elec­trons,” said Yaz­dani. “This demon­stra­t­ion sug­gests that sur­face con­duc­tion in these com­pounds may be use­ful for high-cur­rent trans­mis­sion even in the pres­ence of atom­ic-scale ir­reg­u­lar­i­ties.”

An elec­tron is a sub­a­tom­ic par­t­i­cle that car­ries a neg­a­tive elec­tric charge. It typ­ic­ally or­bits an at­om's nu­cle­us, but elec­trons can al­so hop be­tween at­oms in some ma­te­ri­als, such as crys­tals, and move freely in their in­te­ri­ors or sur­faces.

These free elec­trons are re­spon­si­ble for elec­tric cur­rent, play­ing cen­tral roles in many in­dus­t­ri­al, sci­en­tif­ic and med­i­cal ap­plica­t­ions. For most met­als, elec­trons in the in­te­ri­or car­ry most of the cur­rent, with the sur­face elec­trons not mov­ing much.

The in­tens­ity of elec­tric cur­rent de­pend on a giv­en ma­te­ri­al's “con­duc­ti­vity” at a giv­en tem­per­a­ture. Met­als such as cop­per and gold are good con­duc­tors, al­low­ing for rap­id flow of elec­trons. Ma­te­ri­als such as glass and Tef­lon, with struc­tures that hind­er elec­tron flow, are poor con­duc­tors. The at­oms of met­als have a struc­ture al­low­ing their elec­trons to be­have as if they were free, or not bound to the at­om.

The Prince­ton work is part of an in­quiry in­to ma­te­ri­als called topo­log­i­cal in­su­la­tors, which act as in­su­la­tors in their in­te­ri­or while let­ting charges move on their bound­a­ry. In a phe­nom­e­non known as the quan­tum Hall ef­fect, this be­hav­ior oc­curs when a mag­net­ic field is ap­plied to the ma­te­ri­al at a right an­gle. A type of topo­log­i­cal in­su­la­tor has al­so been found in which this be­hav­ior oc­curs with­out a mag­net­ic field.

The an­ti­mony crys­tal used in the new ex­pe­ri­ment is a met­al but shares the un­usu­al sur­face elec­tron char­ac­ter­is­tics with re­lat­ed in­su­lat­ing com­pounds.

Be­cause the elec­trons move freely on its sur­face re­gard­less of its shape, the ma­te­ri­al has a “topo­log­i­cal sur­face state,” Yaz­dani said. To­pol­o­gy is a math­e­mat­i­cal field con­cerned with spa­tial prop­er­ties pre­served de­spite de­forma­t­ion, like stretch­ing, of ob­jects. Ac­cord­ing­ly, a dough­nut and a cof­fee cup can be seen as top­o­log­ic­ally the same be­cause they both are basically ar­eas with holes in the mid­dle.

Yaz­dani's team was able to meas­ure how long elec­trons stay in a re­gion of the ma­te­ri­al and how many of them flow through to oth­er ar­eas. The re­sults showed a sur­pris­ing ef­fi­cien­cy by which sur­face elec­trons on an­ti­mony go through bar­ri­ers that typ­ic­ally stop oth­er sur­face elec­trons on the sur­face of most con­duct­ing ma­te­ri­als, such as cop­per, said the re­search­ers. They worked in the spe­cially de­signed Prince­ton Nanoscale Mi­cros­co­py Lab­o­r­a­to­ry, where pre­cise atom­ic-scale meas­urements are pos­si­ble be­cause sounds and vibra­t­ions are min­i­mized. 

The team used a pow­er­ful scan­ning tun­nel­ing mi­cro­scope to view elec­trons on the sur­face of the an­ti­mony sam­ple. In such a mi­cro­scope, an im­age is pro­duced by point­ing a fi­nely fo­cused elec­tron beam, as in a TV set, across a sam­ple. Re­search­ers gently scan the mi­cro­scope's single-at­om sharp met­al tip just above the sur­face of the ma­te­ri­al be­ing stud­ied. By mon­i­tor­ing how elec­trons flow from the nee­dle in­to the sam­ple, the in­stru­ment can pro­duce pre­cise im­ages of at­oms, as well as the flow of elec­tron waves.

The ex­pe­ri­ment, Cava said, “shows for the first time that the the­o­ret­ic­ally pre­dicted im­mun­ity of topo­log­i­cal sur­face states to death at the hands of the ever-present de­fects in the atom­ic ar­range­ments on crys­tal sur­faces is really true.”

* * *

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Scientists on a quest to discover new states of matter have found that on the surfaces of certain materials, charged subatomic particles known as electrons act like miniature superheroes: they relentlessly dodge cliff-like obstacles, sometimes moving straight through barriers. The findings represent the first time such behavior by electrons has been tracked and recorded, scientists said, and hints at the possibilities of speeding up integrated circuits that process information by flow of electrons between different devices. The new materials might break the bottleneck that occurs when metallic interconnects get so small that even the tiniest atomic imperfection hinders their performance, the researchers said. Princeton University Physics professor Ali Yazdani and his team observed the odd behavior in a “topological surface state“ on a microscopic wedge of the metal antimony. The work is reported in the July 15 issue of the research journal Nature. Normally, electron flow in materials is hindered by imperfections—seemingly slight edges and rifts act like cliffs and crevasses in this microscopic world. Recent theories, however, predict that electrons on the surface of some compounds containing elements such as antimony can be immune to such disruptions. Electrons, like other subatomic particles, have both particle-like and wavelike properties, in a duality that has never been satisfactorily explained, yet shows itself persistently in experiments. The electrons that defy blockages in their flow, Yazdani said, do so because of a special form of electron wave that seemingly alters the pattern of flow around any imperfection. Many of the “topological“ materials, such as antimony, have been important in the world economy, but their unusual surface conduction hadn't previously been examined. Part of the challenge had been the difficulty in measuring the flow of electrons at the surface. “Material imperfections just cannot trap these surface electrons,“ said Yazdani. “This demonstration suggests that surface conduction in these compounds may be useful for high-current transmission even in the presence of atomic-scale irregularities.“ An electron is a subatomic particle that carries a negative electric charge. It typically orbits an atom's nucleus, but electrons can also hop between atoms in some materials, such as crystals, and move freely in their interiors or surfaces. These free electrons are responsible for electric current, playing central roles in many industrial, scientific and medical applications. For most metals, electrons in the interior carry most of the current, with the surface electrons not moving much. The intensity of electric current depend on a given material's “conductivity“ at a given temperature. Metals such as copper and gold are good conductors, allowing for rapid flow of electrons. Materials such as glass and Teflon, with structures that hinder electron flow, are poor conductors. The atoms of metals have a structure allowing their electrons to behave as if they were free, or not bound to the atom. The Princeton team's work is part of an inquiry into materials called topological insulators—substances that act as insulators in their interior while letting charges move on their boundary. In a phenomenon known as the quantum Hall effect, this behavior occurs when a magnetic field is applied to the material at a right angle. A type of topological insulator has also been uncovered in which this behavior occurs even without a magnetic field. The antimony crystal used in the new experiment is a metal but shares the unusual surface electron characteristics with related insulating compounds. Because the electrons move freely on its surface regardless of its shape, the material has a “topological surface state,“ Yazdani said. Topology is a mathematical field concerned with spatial properties that are preserved despite deformation, like stretching, of objects. Accordingly, a doughnut and a coffee cup can be seen as topologically the same because they both are essentially areas with holes in the middle. Yazdani's team was able to measure how long electrons stay in a region of the material and how many of them flow through to other areas. The results showed a surprising efficiency by which surface electrons on antimony go through barriers that typically stop other surface electrons on the surface of most conducting materials, such as copper, said the researchers. They worked in the specially designed Princeton Nanoscale Microscopy Laboratory, where precise atomic-scale measurements are possible because sounds and vibrations are minimized. The team used a powerful scanning tunneling microscope to view electrons on the surface of the antimony sample. In such a microscope, an image is produced by pointing a finely focused electron beam, as in a TV set, across a sample. Researchers gently scan the microcope's single-atom sharp metal tip just above the surface of the material being studied. By monitoring how electrons flow from the needle into the sample, the instrument can produce precise images of atoms, as well as the flow of electron waves. The experiment, Cava said, “shows for the first time that the theoretically predicted immunity of topological surface states to death at the hands of the ever-present defects in the atomic arrangements on crystal surfaces is really true.“