Laboratoire de Physique Théorique

de la Matière Condensée

Owen Benton (MPIPKS Dresden)

Exactly solvable spin-1/2 models with highly-degenerate, partially ordered, ground states

(Séminaire zoom: https://zoom.us/j/94742254686?pwd=YktiUHZTcHIySnBLZlgyblZMQ0VtUT09 ID de réunion : 947 4225 4686 Code secret : 769824 )

Magnetic ground states tend to be divided into ordered and spin liquid phases. Ordered phases have spontaneous symmetry breaking and sharp excitations, while spin liquids are symmetric, and have excitation continua. Increasingly, however, it is being realized that the sharp division between these two types of magnetic matter may make sometimes break down. This is particularly the case in certain classical magnets which exhibit the phenomenon of “moment fragmentation” in which antiferromagnetic order coexists with a type of spin liquid.
 
It is natural to ask whether similar physics, mixing ordered and spin liquid-like behaviour can occur in a quantum magnet? And, if so, are there simple, tractable models in which this occurs?
 
In this talk, I will introduce a family of exactly solvable S=1/2 anisotropic exchange models, with highly degenerate, partially ordered, ground states. This set of models involves only nearest neighbour interactions of a simple “XYZ” form. The ground states have a degeneracy which grows exponentially with system size, and are partially ordered, in the sense that infinite-range correlations of some spin components coexist with a macroscopic number of undetermined degrees of freedom. This new family of models thus establishes a starting point for the investigation of quantum systems in between spin liquids and magnetic order.