Pairing of charge-ordered stripes in (La,Ca)MnO3

The propensity of systems of charge and spin to form, under certain conditions, 'stripe' phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors(1-5), A related phenomenon-static charge stripes-characterizes(6) the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering(7,8). Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1-xCaxMnO3 (x greater than or equal to 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn-Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn-Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.