Quantum Criticality in Quasi-One Dimensional Li0.9Mo6O17

J.W. Allen, University of Michigan, DMR 0302825(SRC, DMR-0084402)

For many — indeed most — physical systems there are characteristic energy scales set by the various forces that act. For example, the ferromagnetism of iron disappears if the temperature T is greater than 1043K (1418 °F) because thermal energy then exceeds the characteristic energy of the magnetic forces in iron.

Quantum critical systems are strikingly different in having no energy scale except temperature itself. Quantum criticality (QC) is predicted in theories of quasi-one dimensional systems. QC may be important in nano-technology. Studies of systems in nature are just beginning.

Here we use a technique called photoemission spectroscopy to observe QC in the spectra of the energy distribution of the electrons of a quasi-one dimensional chemical compound. The spectra have the QC scaling property, that their shape depends only the ratio of energy to temperature.

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-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10

E-EF(eV)

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E-EF(eV) 0

Before scaling

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Timesequence ofspectra at various Tshowsrepeat-ability.

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-10 -5 0 5 10E/kBT

T150_sT T120_sT T100_sT T80_sT T50_sT T80u_sT T100u_sT T120u_sT T150u_sT T180u_sT T200u_sT T220u_sT T250_sT T220d_sT T200d_sT T180d_sT

normalized @ -4.2

43 meV @ 50K 215 meV @ 250K

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QC spectra all have the same shape when plotted vs the ratio of energy to temperature.

(E-EF)/kBT

All data scaled

Education:This grant provides partial support for two graduate students, Sung-Kwan Mo and Feng Wang. Both have passed their Ph.D. candidacy exams.

The grant also supports summer research opportunities for undergraduates like Spencer Dowdall, a UM double major in physics and math. Spencer has joined the experiments at the Wisconsin SRC.

Training for Multi-Institutional Research This work is a good example of the multi-institutional and internationally collaborative style of research for which science students must now be trained.

The photoemission experiments are performed by UM researchers at the NSF-funded Wisconsin Synchrotron Radiation Laboratory using samples prepared at the Oak Ridge National Laboratory (David Mandrus group), and LEPES-CNRS, Grenoble, France (C. Schlenker,J. Dumas).

Theoretical guidance has been provided by S. Moukouri (UM) and José Alvarez, formerly a UM postdoc, now returned to his native Spain, at the University of Madrid.

Quantum Criticality in Quasi-One Dimensional Li0.9Mo6O17

J.W. Allen, University of Michigan, DMR Award # 0302825 (SRC, DMR-0084402)