Study on CuGe_0.9885Si_0.0115O₃ in high magnetic fields by specific heat measurements

Specific heat is measured for a single crystal of CuGe_1-xSi_xO₃ (x = 0.0115) in magnetic fields up to 14.5 T. In zero field two peaks at approximately 3 and approximately 10 K were observed, suggesting that the coexistence of the antiferromagnetic and spin-Peierls orders are realized. In our previous measurements of specific heat in high-field region around 15 T a small but clear anomaly was observed at approximately 4.2 K for x = 0.01, while a sharp peak was observed at approximately 5 K for x = 0.015, which corresponds to the transition in the magnetic-field-induced phases between M phase and M′ phase. They appear very different from each other. In the present study for x = 0.0115 we observe a clear peak at 14.5 T which is just intermediate between those for x = 0.01 and 0.015 in temperature and in size. This systematic change in the transition with varying x shows that these transitions in the high-field phases are of the same kind in this range of x, though it also shows that small change in Si concentration affects the magnetic-field-induced states to a large extent.