The curse of Midas: when touching triangular and kagome 2d magnets turn them into 1d spin ladders and tubes.

Cs₂CoBr₄ is a new member of an old and famous family of distorted triangular-lattice systems. It differs from its thoroughly studied Cu-based counterparts by the presence of strong bond-dependent XY anisotropy. In applied fields it shows as may as 5 different ordered phase, including a m=1/3 plateau state [1], an incommensurate spin density wave with a field-dependent period [2], an incommensurate fan-structure with a period defined entirely by the frustration ratio, and two distinct commensurate phases. The most spectacular excitation spectrum is observed actually in zero field. Here semiclassical spin wave theory fails spectacularly. Instead, the spectrum is a series of bound fractional excitations [3]. These are reminiscent of Zeeman ladders in Ising spin chains, but have a far more complex structure and are able to propagate in two dimensions.

But is the distorted-triangular-lattice model indeed a good starting point for understanding this material? The latest high-field neutron and THz spectroscopy experiments suggest that a zig-zag spin ladder description may actually be mode adequate.

Much excitement was recently caused by the discovery of a new family of rare earth boratotungstates, believed to realize the breathing-kagome antiferromagnetic model. Of these materials, Nd₃BWO₉, is highly frustrated, but orders magnetically at very low temperatures (TN/qW<12)and has a bewilderingly complex phase diagram in applied fields [4]. On the other hand, Pr₃BWO₉ does not order down to at least 50 mK. Yet it shows power-law magnetic specific heat and is thus a prime candidate for being a two-dimensional spin liquid candidate.

But is the breathing-kagome-lattice model a indeed good starting point for understanding these materials? The latest neutron spectroscopy experiments suggest a quasi one dimensional frustrated "hexagonal spin-tube" description may actually be mode adequate.