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arXiv:0809.5069 · Physical Review Letters

Weakly Coupled s=1/2 Quantum Spin Singlets in Ba3Cr2O8

Maiko Kofu, J.-H. Kim, Sungdae Ji, S.-H. Lee, Hiroaki T. Ueda, Yiming Qiu, H. J. Kang, Mark Green, Y. Ueda

Published Jan 22, 2009• 43 citations
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Abstract

Using single crystal inelastic neutron scattering with and without the application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in ${\mathrm{Ba}}_{3}{\mathrm{Cr}}_{2}{\mathrm{O}}_{8}$. Even without a field, we found that there exist three singlet-to-triplet excitation modes in the ($h$, $h$, $l$) scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by lattice distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active ${\mathrm{Cr}}^{5+}(3{d}^{1})$. The strong intradimer coupling of ${J}_{0}=2.38(2)\text{ }\text{ }\mathrm{meV}$ and weak interdimer interactions ($|{J}_{\mathrm{inter}}|\ensuremath{\le}0.52(2)\text{ }\text{ }\mathrm{meV}$) makes ${\mathrm{Ba}}_{3}{\mathrm{Cr}}_{2}{\mathrm{O}}_{8}$ a good model system for weakly coupled $s=1/2$ quantum spin dimers.

DOI
10.1103/physrevlett.102.037206

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We have studied spin excitation spectra in the Shastry-Sutherland model compound SrCu2(BO3)2\mathrm{Sr}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}SrCu2​(BO3​)2​ in magnetic fields using far-infrared Fourier spectroscopy. The transitions from the ground singlet state to the triplet state at $24\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and to several bound triplet states are induced by the electric field component of the far-infrared light. To explain the light absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM) mechanism where light couples to a phonon mode, allowing the DM interaction. Two optical phonons couple light to the singlet to triplet transition in SrCu2(BO3)2\mathrm{Sr}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}SrCu2​(BO3​)2​. One is aaa-polarized and creates an intradimer dynamic DM along the ccc axis. The other is ccc-polarized and creates an intradimer dynamic DM interaction, it is in the (ab)(ab)(ab) plane and perpendicular to the dimer axis. Singlet levels at 21.5 and $28.6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ anticross with the first triplet as is seen in far-infrared spectra. We used a cluster of two dimers with a periodic boundary condition to perform a model calculation with scaled intra- and interdimer exchange interactions. Two static DM interactions are sufficient to describe the observed triplet state spectra. The static interdimer DM in the ccc-direction ${d}_{1}=0.7\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ splits the triplet state sublevels in zero field [C\'epas et al., Phys. Rev. Lett. 87, 167205 (2001)]. The static intradimer DM in the (ab)(ab)(ab) plane (perpendicular to the dimer axis) ${d}_{2}=1.8\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, allowed by the buckling of CuBO3\mathrm{Cu}\mathrm{B}{\mathrm{O}}_{3}CuBO3​ planes, couples the triplet state to the $28.6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ singlet as is seen from the avoided crossing.
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