Temperature driven orbital redistribution in La1-xSr1+xMnO4

We present high-resolution measurements of the thermal expansion coefficient α along the a-axis and the c-axis of La1-xSr1+xMnO4(0⩽x⩽0.5). For x⩽0.125, where a long-range spin ordered ground state is present, the antiferromagnetic ordering transition causes pronounced anomalies of the lattice constants a and c. In addition, even at T⪢TN anomalous contributions to αa,c are present. The data suggest a change of the orbital occupation of the eg-levels, i.e. an increase of the in-plane dx2-y2-character of the electrons, driven by temperature. At higher doping 14⩽x⩽12 no anomalous behavior is observed.