High Frequency Inductive Measurements of Organic Conductors with the Application of High Magnetic Fields and Low Temperatures

Organic conductors are interesting to study due to their low dimensionality that leads to a number of competing low temperature ground states. Comprised of a number of different molecules that can be varied by the substitution of one atom for another, organic systems also provide a large number of similar compounds that lend themselves to comparison studies. Two such low-dimensional organic conductors, Per2[Pt(mnt)2] and (TMTSF)2ClO4, which are members of large families of compounds, are the topic of this dissertation. Both materials are considered quasi-one-dimensional and have a number of low temperature transitions, some of which can be studied via changes in the magnetic properties of the systems. The Per2[M(mnt)2] family of compounds provides a system for exploring the similarities and differences of the system's properties when the metal M has a localized spin (M = Pt, Ni, and Fe) versus when the metal is diamagnetic (M = Au, Cu, and Co). In the case of Per2[Pt(mnt)2] - one of the compounds of focus in this dissertation - the metallic perylene chains undergo a metal- insulator transition due to the formation of a charge density wave at Tc ~ 8 K, which also occurs in Per2[Au(mnt)2] at 12 K. However, unlike in the M = Au compound, an additional transition occurs in the M = Pt compound due to the localized Pt spins (S = 1/2) on the insulating Pt(mnt)2 chains - the spin chains of Per2[Pt(mnt)2] undergo a spin-Peierls transition at 8 K. One focus of the experimental work of this dissertation focuses on the magnetic properties of the spin chains in Per2[Pt(mnt)2], via inductive susceptibility measurements at temperatures down to 0.5 K and fields up to 60 T. The experimental results show a coupling of the spin-Peierls and charge density wave states below 8 K and 20 T, above which both states are suppressed. Further measurements show a second spin state transition occurs above 20 T that coincides with a field induced insulating state in the perylene chains. These results support a strong coupling between the charge density wave and spin-Peierls state even at high magnetic fields, which are discussed in the context of other experimental results and theories. Additionally, a simple model is developed to explore the possible mechanisms behind the coupling of the two segregated chains. The other experimental part of this dissertation focuses on one member of the (TMTSF)2X family of compounds, where the anion molecule (X) can have an octahedral symmetry (X = PF6, SbF6, AsF6) or a tetrahedral symmetry (X = ClO4, ReO4, BF4). All of these compounds undergo metal-insulator transitions with the formation of a spin density wave, which can be suppressed and replaced by a transition to a superconducting state with the application of pressure in all but the X = ClO4 compound. In (TMTSF)2ClO4, which is the other compound of focus in this dissertation, both the spin density wave state and the superconducting state can be realized at ambient pressure; however, the determination of the state is dependent on the rate at which the material is cooled through an anion ordering temperature. If the sample is cooled too quickly it remains disordered and the sample enters the spin density wave state; on the other hand, if it cools slowly and the anions are allowed to order, superconductivity is realized at 1.2 K. This superconducting state has been experimentally studied with a wide variety of experimental techniques and much is known about its properties. However, nanoparticles of (TMTSF)2ClO4 have recently been realized, which opens up a new avenue of research on this compound, since the bulk properties of a material are often modified when its size approaches the length scale of the ground state order parameter. As such, the experimental work on (TMTSF)2ClO4 in this dissertation focuses on the critical temperature and fields of the superconducting state of the nanoparticles using the same inductive susceptibility technique mentioned above. The experiments on an assembly of (TMTSF)2ClO4 nanoparticles show that the nanoparticles exhibit bulk-like properties similar to those of randomly oriented crystals of the parent compound; possible explanations for this observation and future plans are discussed in this context.