Refereed Articles
Geometrical Frustration, Spin Ice and Negative Thermal Expansion – The Physics of Underconstraint
  The idea that some systems could have a thermodynamically large number of accessible ground states was presaged in the work of Pauling on ice (Pauling, Cornell University Press, Ithaca, NY, 1945) [1]. With the advent of spin glasses, the methodology for describing ground states changed dramatically, and in particular it was realized that the observed slow dynamics were due to relaxation among a large number of nearly degenerate ground states. Now the accepted wisdom is that both `frustrationa, as well as structural disorder, is responsible for spin glass behavior. However, well before spin-glasses were identi"ed as a distinct class of systems, it had been appreciated that even for structurally periodic systems, bond frustration could lead to a thermodynamically large number of states. There is now a well-de"ned class of magnets which display e!ects of macroscopic ground state degeneracy. This class of geometrically frustrated magnets presents some new paradigms with which to view condensed matter systems } marginal underconstraint and downward shift of spectral weight. We discuss possible realizations of these phenomena in both in spin ice and also outside the context of local-moment magnetism  
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    Geometrical Frustration, Spin Ice and Negative Thermal Expansion – The Physics of Underconstraint.  A. P. Ramirez, C. L. Broholm, R. J. Cava, G. R. Kowach, Physica B 2000, 280, 290.