Transformation between spin-Peierls and incommensurate fluctuating phases of Sc-doped TiOCl

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Zhang, Jian ; Wölfel, Alexander ; Bykov, Maxim ; Schönleber, Andreas ; van Smaalen, Sander ; Kremer, Reinhard K. ; Williamson, Hailey L. (2014)
  • Publisher: APS
  • Journal: (issn: 0556-2805)
  • Related identifiers: doi: 10.1103/PhysRevB.90.014415
  • Subject:
    • ddc: ddc:530
    arxiv: Condensed Matter::Strongly Correlated Electrons

Single crystals of ScxTi1−xOCl(x=0.005) have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4–200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ScxTi1−xOCl gives broadened diffraction maxima at incommensurate positions between Tc1=61.5(3) and ∼90 K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ScxTi1−xOCl(x=0.005) is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of ∼10 K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound CuGeO3. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz–Thouless-type phases in two-dimensional systems.
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