Field-dependent differential susceptibility studies on tetrathiafulvalene-Au${\mathrm{S}}_4$${\mathrm{C}}_4$${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$: Universal aspects of the spin-Peierls phase diagram

An applied magnetic field is known to produce novel effects in the phase behavior of magnetoelastic spin-Peierls systems. Hence we report measurements of the differential susceptibility ($\chi$) and magnetization ($M$) in fields up to 40 kOe (4 T) on the spin-Peierls compound tetrathiafulvalene (TTF)-Au${\mathrm{S}}_4$ ${\mathrm{C}}_4$ ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$ in the temperature region ($1.1\mathrm{K}<~T<~4.2\mathrm{K}$). This range of field and temperature encompasses an interesting phase region, including the zero-field spin-Peierls transition temperature $T_c\mathrm{}\left(0\right)=2.03\mathrm{}$ K. The measurements of the differential (ac) susceptibility provide a more sensitive probe of the transition behavior than magnetization measurements. The first definitive evidence for significant deviations from mean-field critical behavior appear in these measurements, and the appropriate criteria for determining the precise location of the transitions are thus provided by the thermodynamic theory of $\lambda$ transitions. Using the new criteria, qualitative and even quantitative agreement is obtained with current theories of the field dependence of spin-Peierls transitions. A novel contour plot of ${\chi }_{\mathrm{ac}}$ in the $H-T$ plane is shown to be useful for the delineation of the global phase-transition behavior. An investigation of the role of relaxation effects in ${\chi }_{\mathrm{ac}}$ relative to the nature of the phase boundaries is conducted. A major feature is the observation of a striking degree of "universality" in the phase behavior of three spin-Peierls systems, TTF-Au${\mathrm{S}}_4$ ${\mathrm{C}}_4$ ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$, TTF-Cu${\mathrm{S}}_4$ ${\mathrm{C}}_4$ ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$, and methylethylmorpholinium di-tetracyanoquinodimethane [MEM-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$]. These universal features are preserved through considerable differences in lattice structure and a variation in $T_c\mathrm{}\left(0\right)\mathrm{}$ of a factor of 10.