Figure 1

Capacitance (scaled to the value at 4 K) (a) and loss tangent (b) vs temperature of Sul-Cu${}_2$Cl${}_4$ at a measurement frequency of 1 kHz for various external magnetic fields. The lines are guides to the eyes.Reuse & Permissions

Figure 2

Capacitance (scaled to the value at 14 T) vs external magnetic field of Sul-Cu${}_2$Cl${}_4$ at a measurement frequency of 10 kHz for several temperatures.Reuse & Permissions

Figure 3

(a) Capacitance (scaled to the value at 14 T) vs external magnetic field of Sul-Cu${}_2$Cl${}_4$ (second sample) at a measurement frequency of 10 kHz for several temperatures. (b) shows the corresponding loss-tangent curves.Reuse & Permissions

Figure 4

Critical field $H_c$ as a function of temperature. The shown data points were obtained from dielectric experiments (present work: $+$, from H dependence; $\times$, from T dependence), neutron-scattering [circles (Ref. 13)], and specific-heat measurements [triangles (Ref. 17) and squares (Ref. 18)]. The solid line represents a critical power law (Ref. 13) as described in the text. The inset shows a tentatively proposed QC scenario for the present material with the typical $\mathsf{V}$-shaped QC region. The dashed line corresponds to the same critical law as shown in the main frame.Reuse & Permissions