Temperature and magnetic field dependence of the lattice constant in spin-Peierls cuprate CuGeO₃ 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.