Download NMR Studies on Magnetization Plateaus in Dimer Spin Systems

NMR Studies on Magnetization Plateaus in Dimer Spin Systems
M. Takigawaa , K. Kodamab , M. Horvatićc, C. Berthierc, H. Kageyamad, Y. Uedaa , and H. Tanakae
a
ISSP, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Neutron Science Research Center, JAERI, Tokai, Ibaragi, 319-1195, Japan
c
Grenoble High Magnetic Field Laboratory, CNRS, BP 166-38042 Grenoble, France
d
Department of Chemistry, Kyoto University, Kyoto 606-8502, Japan
e
Research Center for Low Temperature Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
b
A rich variety of quantum phase transitions can be induced in simple dimer-singlet spin systems by
applying magnetic field strong enough to suppress the energy gap to triplet excitations. While the
field-induced triplet bosons generally undergo Bose condensation, resulting in an antiferromagnetic order
perpendicular to the field, they may be localized into superstructure at certain commensurate filling due
to repulsive interaction, exhibiting magnetization plateaus. We report NMR results on SrCu 2 (BO3 )2 and
NH4 CuCl3 , the two best known materials for magnetization plateaus. Clear evidence for a symmetrybreaking first order transition was observed for the 1/8-plateau phase in SrCu 2 (BO3 )2 with oscillating
spin density. The Dzyaloshinski-Moriya interaction causing singlet-triplet mixing plays an essential role
to suppress Bose condensation. In contrast, double structural transitions in NH4 CuCl3 produce enough
number of inequivalent spins. Thus the 1/4-plateau state evolves without further symmetry breaking.
The 14 N NMR spectra, however, pose a puzzle about the dimer configuration. These results are in sharp
contrast to quantum critical behavior we found recently at the BEC transition in a spin 1 dimer system.
Sorting category: Ce Magnetism and properties of solids
Keywords: quantum spin system, magnetization plateau, field-induced transition, NMR
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