Contains table of contents for Section 4, and a report on one research project.Joint Services Electronics Program Grant DAAL04-95-1-003

Birgeneau, Robert J.

Greven, Martin

Kim, Hyoung-June

Martin, Debra L.

DSpace@MIT

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Mesoscopic Quantum Magnetic ConductorsChapter 1. Mesoscopic Quantum Magnetic ConductorsAcademic and Research StaffProfessor Robert J. Birgeneau, Dr. Martin GrevenGraduate StudentsYoung-June KimTechnical and Support StaffDebra L. Martin1.1 Order Driven by DisorderSponsorJoint Services Electronics ProgramGrant DAAL04-95-1-0038Over the past several years, a new class of novelmesoscopic materials characterized by "intermedi-ate" dimensionality has been discovered. Thesederive from materials research in the field of high-temperature superconductivity. However, the physicsissues, including especially the magnetic and elec-tronic properties of these "intermediate" dimensionalmaterials, are quite distinct from those of the "two-dimensional" high-temperature superconductors.Specifically, a variety of new materials have beendiscovered which are composed of either isolated orcoupled copper oxide chains. The behavior of thesesystems as a function of n, the number of coupledchains, is quite remarkable. Coupled chain systemsin which one of the constituents in the rows is mag-netic (e.g., Cu 2+) are referred to as "spin ladders." Byvarying the chemistry, one can dope these "spin lad-ders," producing novel quantum magnetic conduc-tors.Ladders with an odd number of chains have gaplessspin excitations and, on doping, should behave likeLuttinger liquids. Even chain ladders, on the otherhand, have a gap in their spin excitation spectrumand, therefore, on doping could exhibit fundamentallydifferent behavior. Replacing the Cu2 + atoms whichhave S = 1/2, for example, by Zn 2+, which has S = 0,will cause the spin chain to break into finite size seg-ments. Length scales from nanometers to micronscan easily be achieved in this fashion. Recently,Azuma and coworkers discovered that in the n = 2spin ladder material, SrCu 203, doping with Zn, that is,synthesizing Sr(Cul-Snx) 203, causes the system toexhibit long-range antiferromagnetic order. In otherwords, disordering the Cu2+(S=1/2) ladders by addingZn 2- (S =0) causes the material to change from aone-dimensional short ranged ordered state to threedimensional long-range order. This is counter-intui-tive since one might think that putting in disorder viathe Zn" would cause the spin system to becomeeven more disordered. It turns out that this phenome-non derives from a pure quantum mechancial effectoccuring in one-dimensional spin systems. Usingquantum Monte Carlo techniques, we have eluci-dated the mechanisms by which the spinless Zn2+dopants cause the order to grow within the one-dimensional spin ladders. By careful consideration ofthe actual structure of SrCu 203, we deduce that thereshould be successive crossovers of the ladder spinordering from one to two and ultimately to threedimensions. This model will be testable by neutronscattering techniques once suitably large crystalshave been grown.1.1.1 PublicationGreven M., and R.J. Birgeneau. "Correlations andNeel Order of Randomly Diluted Quantum SpinLadders." Submitted to Phys. Rev Lett.170 RLE Progress Report Number 140

Mesoscopic Quantum Magnetic Conductors

http://dspace.mit.edu/bitstream/1721.1/57413/1/RLE_PR_140_01_04s_01.pdf

Mesoscopic Quantum Magnetic Conductors

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