Low temperature structural phase transition in BaCuSi{sub 2}O{sub 6}.
BaCuSi{sub 2}O{sub 6} is a quasi-2D compound composed of Cu{sub 2}Si{sub 4}O{sub 12} layers in which pairs of Cu{sup 2+} (s = 1/2) ions form vertical spin dimers. In zero magnetic field, the material has a singlet ground state, with a significant gap to the triplet states. Magnetic fields exceeding H{sub c1} {approx} 23 T close the spin gap, resulting in long range canted antiferromagnetic order at low temperature. Here we report results of x-ray scattering experiments performed at the Advanced Photon Source that reveal a weak structural phase transition at approximately 100 K. We discuss implications for the high-field ordered state. At room temperature, charge Bragg peaks of BaCuSi{sub 2}O{sub 6} were observed consistent with the I4{sub 1}/acd tetragonal structure previously reported. However, at 16 K, the even-order (H 0 0) peaks were found to have split along the longitudinal direction (Fig. 1). A careful set of measurements revealed that the splitting was consistent with transformation twins of an orthorhombic (or weakly monoclinic) lattice. The structural phase transition was accompanied by an incommensurate (IC) lattice modulation characterized by a reduced wavevector q{sub IC} = (0, 0.129 {+-} 0.001, 0), referred to the orthorhombic latttice, and its 2nd and 3rd harmonics. The temperature dependence of the IC peaks indicated a first order phase transition with T{sub c} {approx} 103K for warming cycles (Fig. 2). Further experiments are in progress to obtain a model for the IC modulation, which is likely associated with rotation of SiO{sub 4} tetrahedra in the Cu{sub 2}Si{sub 4}O{sub 12} layers. These observations imply that the spin Hamiltonian describing this system is more complex than originally thought. The incommensurate modulation will likely lead to inequivalent dimers, and the lower crystal symmetry means that anisotropic effects (including Dzyaloshinski-Moriya antisymmetric exchange and anisotropic exchange constants J{prime}) are not precluded. However, modifications to the spin Hamiltonian must be subtle given that BEC critical scaling exponents are observed for temperatures down to 30 mK.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Science Foundation (NSF)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 971924
- Report Number(s):
- ANL/XFD/CP-118965; TRN: US1001415
- Resource Relation:
- Conference: 5th International Conference on Synchrotron Radiation in Materials Science (SRMS 5); Jul. 30, 2006 - Aug. 2, 2006; Chicago, IL
- Country of Publication:
- United States
- Language:
- ENGLISH
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