Sul-Cu2Cl4
Sul-Cu2Cl4 is interesting for many reasons. First, it is a strongly frustrated chain system with complicated exchanges. Their topology is actually quite involved, being similar to a four-leg tube with diagonal interactions. Thus, it is gapped and incommensurate at the same time, and the gap is really tiny compared to the overall energy scale. Second, this material is a field-induced multiferroic (or, more precisely, improper ferroelectric). Polarization goes hand-in-hand with the spiral order stemming from the magnon condensation, and thanks to the high degree of one-dimensionality is is very sensitive to the perturbing electric fields. Finally, the material can also adopt some chemical disorder, that immediately ruins the ordered phase even at very high magnetic fields.
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