The inclusion of the rightimpurities can make a high-temperature superconductor ofan insulator, but a thoroughunderstanding of the exoticproperties of these materialshas been lacking. A team ofresearchers from StanfordUniversity and the NationalInstitute of Standards andTechnology (NIST) has taken astep towards a quantitativeunderstanding of thesematerial properties [Science(2002) 295, 1691-1695]. Taking the example ofLa2CuO4, the parent of high-temperature superconductor(La,Ba)2CuO4 and a model two-dimensional quantum (spin-½) antiferromagnet,superconductivity arises nearthe antiferromagnetic phases.Investigation of theantiferromagnetic fluctuations,therefore, should shed light onthe exotic physics of thesematerials, say theresearchers.Martin Greven and his team atStanford grew crystal samplesof La2Cu1-z (Zn,Mg)zO4+d using

the traveling-solvent floatingzone method. By substitutingZn and Mg on the Cu site, theresearchers can remove amagnetic site withoutintroducing charge carriers. The crystals were then takento Peter Gehring and JeffreyLynn at NIST’s Center forNeutron Research for neutronscattering studies. By analyzinga range of samples with Znconcentration at approximately10% and varying Mg content,the researchers could pinpointthe moment at which randomimpurities disrupt the long-range magnetic order of thecrystals. Meanwhile, quantumMonte Carlo simulations werecarried out at Stanford, whichshow excellent agreement withthe experimental results.The crucial factor appears tobe the distance over whichmagnetic moments can passinformation – and this dependson temperature, says Greven.“At room temperature, in thecrystals we were looking at,they don’t ‘talk’ over a distancelarger than four or five

neighbors,” explains Greven.“But when you cool thesecrystals, the distance overwhich the magnetic momentscan exchange informationincreases in a nontrivial fashiondue to both their quantumnature as well as the presenceof the impurities, andeventually exceeds hundreds ofneighbors.” Quantum fluctuations erodethe magnetic order of thematerial with increased dilutionby impurities. When 40% ofthe magnetic atoms are

replaced with nonmagneticimpurities, the spins ofneighboring atoms becomedisordered and disconnectedthroughout the system.Looking to the future, saysGreven, “Once we trulyunderstand these and related materials and theirmagnetic properties as afunction of impurity content,we can hope to designbetter materials in the lab that have better propertiesfrom a technological point ofview.”

RESEARCH NEWS

Carbon nanotubes are allowingresearchers at the Universityof Illinois to investigate howsmall a wire can be fabricatedthat is still superconducting."The phenomenon ofsuperconductivity dependsupon the phase coherence ofthe condensate," explainsAlexey Bezryadin, whopresented the results at theAmerican Physical Societyconference. "For smallsystems, such as ultrathinwires, the phase is a quantum

variable which may or may nothave a definite value,corresponding to bothsuperconducting and insulatingstates."Bezryadin and his colleaguesfrom Harvard University usenanotubes as scaffolds ontowhich a uniform film ofsuperconducting molybdenum-germanium alloy can bedeposited. The nanowires areplaced on a silicon chip toapply a voltage and measurethe current. "The molybdenum-

germanium films have a sharpsuperconducting transition,and show no signs ofgranularity down to a thicknessof about 1 nm," saysBezryadin. "By changing howmuch material is deposited, wecan make wires of differentdiameters and study importantphase transitions betweensuperconducting and insulatingstates." To ensure that thenanotubes themselves do notskew the results, theresearchers used insulating

fluorinated carbon nanotubesfabricated by John Margraveat Rice University. "With these non-conductingnanotubes, we have no doubtthat the current we aremeasuring is flowing throughthe molybdenum-germaniumfilm and not through thecarbon scaffold," saysBezryadin. As an addedbenefit, the fluorotubes alsoappear to allow the fabricationof even smaller diameternanowires.

How low can you go?

Exotic magnetic properties

May 2002 9

Martin Greven, Patrick Mang, and Owen Vajk. (Credit: L.A. Cicero.)