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Quantum Criticality in Quasi-One Dimensional Li 0.9 Mo 6 O 17 J.W. Allen, University of Michigan, DMR 0302825 (SRC, DMR-0084402). 180K. Time sequence of spectra at various T shows repeat-ability. 250K. 150K. Before scaling. 50K. 150K.

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Quantum Criticality in Quasi-One Dimensional Li0.9Mo6O17J.W. Allen, University of Michigan, DMR 0302825(SRC, DMR-0084402)



sequence of

spectra at various T





Before scaling



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

 0.2





 43 meV @ 50K

 215 meV @ 250K

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.

All data scaled


Quantum Criticality in Quasi-One Dimensional Li0.9Mo6O17J.W. Allen, University of Michigan, DMR Award # 0302825(SRC, DMR-0084402)

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.


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.