Low temperature experimental techniques for detecting structural and magnetic transitions in (CH3)2NH2CuCl3

The discovery of several high critical temperature superconductors in the cuprate family has produced an elevated interest in low-dimensional systems (two dimensional planes or one dimensional chains) that retain the basic characteristics of these cuprate crystals in a simplified form. As a result of this link, an understanding of the structure and behavior of low-dimensional systems has become key in understanding the phenomenon of high temperature superconductivity. In order to study one such system, (CH3)2NH2CuCl3, also known as MCCL, for structural and magnetic phase transitions, samples of protonated and deuterated versions of the material in powder form are used as a dielectric in a copper parallel-plate capacitor, as a transition in the MCCL will produce a detectable change in the dielectric constant of the sample. The capacitor is housed in a sample cell and bolted to the base of a cryogenic probe capable of reaching 4.2 K. As the sample is cooled and warmed, a lock-in amplifier sends an excitation voltage to a capacitance bridge wired to the cell and the bridge is subsequently balanced to null the reading from the sample. Any change in capacitance disturbs the balance of the bridge and these deviations from null are monitored on separate X and Y channels, the magnitude and the phase of which are then graphed as functions of temperature to look for the transitions. The results suggest a structural transition in MCCL at 239.7 K, associated with the freezing of rotational degrees of freedom in the (CH3)2NH2 groups, and a magnetic transition at 16.6 K, which is conjectured to be associated with antiferromagnetic alignment.