1.5.1 Properties of Polymeric o-A₁C₆₀

We now consider the o-A₁ phase in more detail. As would be expected from the simplistic rigid band model, in which the t_1u band is 1/6 full, o-A₁, as well as the high temperature cubic fcc-A₁, are metallic, as shown, for example by optical measurements[62,63]. However, NMR and ESR experiments indicate that the conduction electrons may be behaving in a quasi-1-dimensional fashion, i.e. the NMR T₁ relaxation rate is temperature independent[64] above about 100K and the ESR spin suscpetibility is large ($6\times 10^{-4}$ emu/mole) and temperature independent[65]. At low temperatures, a magnetic metal-insulator transition has been observed[65] as a rapid drop of the ESR spin susceptibility below about 50K (for A = Rb or Cs). Interestingly, this transition does not appear for A = K. It was originally suggested[65] that the magnetic phase is of the SDW type (see discussion above), although no conclusive evidence was provided. We note that high temperature NMR measurements[58] on the cubic phase of A₁ (for A = Rb and Cs) exhibit a Curie-like temperature dependence in the frequency shift, in contrast to the temperature independent Pauli shift expected for a normal metal. This temperature dependence has been interpreted as an indication that the conduction electrons are close to localization even above the structural transition. It is the structure of the low temperature magnetic phase that will be the main focus of the results on A₁ presented in this thesis.

Subsequent to the initial ESR measurements, neutron scattering experiments have been mounted by several groups but have shown no magnetic features[66]. The lack of magnetic neutron scattering indicates one or a combination of the following: i) the magnetic structure is highly disordered (such that even short-range magnetic correlations are not apparent in the neutron spectra), ii) the ordered are moments too small to resolve with neutrons (this is apparently the case for the SDW ordering in the TMTSF salts[23]), or iii) the ordering wavevector is in some unexpected direction. Progress in characterizing the nature of the magnetic state was made by NMR[64]. There are several key conclusions in this work:

• The T₁ relaxation rate above the magnetic transition is nearly temperature independent for all 3 nuclei (¹³C, ⁸⁷Rb or ¹³³Cs), and the ratio of the values of T₁T for the ¹³C and ¹³³Cs in Cs₁ is consistent with the different locations in the unit cell.
• In the magnetic state of Cs₁, the ¹³C line breaks into 2 components, one narrow as in the non-magnetic o-Cs₁ phase and one very broad. The ¹³³Cs line, however, broadens as a whole with no remnant narrow line. This indicates that the magnetic state of the sample is homogeneous, but for some unexplained reason, there are two kinds of carbon atoms: one which senses only small fields, and one which senses large fields.
• Using knowledge of the size of the isotropic hyperfine couplings, it is concluded that the broadening of the ¹³³Cs line is too large to be explained, unless it is due mainly to dipolar fields of moments which are aligned in a direction correlated with the applied field.
• Considering some models of the broadening, they conclude that the magnetic state is a spin-flop antiferromagnet with a relatively large magnetic moment ($0.5\mu_B$) which is ordered in the chains but not between them.

More recently, Antiferromagnetic Resonance (AFMR) has been observed in Rb₁ and Cs₁[68]. From measurement of the field and temperature dependence of this resonance, it is concluded[68] that: The magnetic state is well-ordered and is consistent with a spin-flop antiferromagnetic state (which they conclude is probably a 3d ordering of 1d SDWs). Furthermore, magnetic fluctuations are observed in the broad range 35K - 50K.

At this point we note that there is an analogous transition to the spin-flop of a local moment antiferromagnet for the SDW state. This transition is known as the spin-flip transition and has been observed in Cr, for example[21]. The transition in this case is from a longitudinal to transverse SDW. It differs from the spin-flop transition though in the fact that the phase boundary in the H-T plane intersects the H=0 axis.

Discussion of the several reported $\mu {\cal SR}$ measurements on the magnetic A₁ phase will be deferred to section 6.1 where they will be included with the discussion of the results presented in this thesis.