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Condensed Matter Seminars
Condensed Matter
Thursday, September 8, 2011
3:30 PM
Physics Building, Room 204
Note special time.
Note special room.

Seunghun Lee
[Host: Genya Kolomeisky]
UVA
"Hidden order and disorder in a spin glass state of a geometrically frustrated magnet"

ABSTRACT:
Frustration arises in magnetic materials when pairwise interactions between the magnetic moments cannot be satisfied simultaneously, leading to exotic properties such as spin liquid and spin ice at low temperatures. Some cases, however, exhibit spin glass behaviors, and it has been a theoretical and experimental challenge to understand whether or not the spin glass state is an intrinsic ground state for the materials. Here we have studied a quasitwo dimensional triangular lattice of bipyramids with dominant antiferromagnetic nearest neighbor interactions. We show that the magnetic problem can be mapped into a problem of two independent degrees of freedom, tricolor and binary sign: the color represents the director of the collinear spins of each bipyramid, while the binary sign represents spins within each bipyramid that are either parallel or antiparallel to the director. At the meanfield level, the tricolor orders long range in a 3 × 3 structure, while the binary sign has numerous degenerate ground states. When combined, the two degrees of freedom yield the collinear bipyramid spin ground states for the hybrid lattice. Infinite number of coplanar bipyramid spin states can be systematically generated by collective rotations of spins from the collinear states. For long range ordered collinear and coplanar states, we identify ‘partial but extensive’ spin zeroenergy modes of excitations that are qualitatively different from the ‘local’ zeroenergy excitations found in the spin liquid states of the pure twodimensional kagome and pure threedimensional pyrochlore lattices. We argue that due to the infinite ground state degeneracy and the unique characteristics of the zeroenergy excitations the ground state of the quasitwodimensional hybrid antiferromagnet becomes a spin glass. 



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