Temperature and magnetic field dependence of the lattice constant in   spin-Peierls cuprate CuGeO_3 studied by capacitance dilatometry in fields up   to 16 Tesla
  We present high resolution measurements of the thermal expansion coefficient and the magnetostriction along the a-axis of CuGeO_3 in magnetic fields up to 16 Tesla. From the pronounced anomalies of the lattice constant a occurring for both temperature and field induced phase transitions clear structural differences between the uniform, dimerized, and incommensurate phases are established. A precise field temperature phase diagram is derived and compared in detail with existing theories. Although there is a fair agreement with the calculations within the Cross Fisher theory, some significant and systematic deviations are present. In addition, our data yield a high resolution measurement of the field and temperature dependence of the spontaneous strain scaling with the spin-Peierls order parameter. Both the zero temperature values as well as the critical behavior of the order parameter are nearly field independent in the dimerized phase. A spontaneous strain is also found in the incommensurate high field phase, which is significantly smaller and shows a different critical behavior than that in the low field phase. The analysis of the temperature dependence of the spontaneous strain yields a pronounced field dependence within the dimerized phase, whereas the temperature dependence of the incommensurate lattice modulation compares well with that of the dimerization in zero magnetic field. 
T. Lorenz, U. Ammerahl, T. Auweiler, B. B\"uchner (II. Phys. Inst.,   Univ. K\"oln), A. Revcolevschi and G. Dhalenne (Lab. C...