Coexistence of spontaneous symmetry breaking and topological order in the frustrated ferromagnetic J1-J2 chain

DIPC Seminars

Speaker

Clio Efthimia Agrapidis, Institute for Theoretical Solid State Physics, Dresden

When

2019/05/07
14:00

Place

Donostia International Physics Center

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Frustrated one-dimensional (1D) magnets are known as ideal playgrounds for new exotic quantum phenomena to emerge. For instance, the interplay of frustration and fluctuations leads to unexpected condensed matter orders at low temperatures by spontaneously breaking of either a continuous or discrete symmetry, order by disorder. We consider an elementary frustrated 1D system: the spin-1 ferromagnetic (FM) 2 (J1) Heisenberg chain with next-nearest-neighbor antiferromagnetic (J2) interactions. On the basis of density-matrix renormalization group calculations we show, for the first time, the existence of a finite spin gap at J2/|J1| > 1/4. We determine the ground state in this region to be a valence bond solid (VBS) with spin-singlet dimer- ization between third-neighbor sites. The VBS is the consequence of spontaneous symmetry breaking through order by disorder. Quite interestingly, this VBS state has a Affleck-Kennedy- Lieb-Tasaki-type topological order. This is further proved by calculating string-order parameter and the entanglement spectrum of this system. Moreover, the phase transition at J2/|J1| > 1/4 consists of two steps: (i) first, translational symmetry is spontaneously broken due to FM dimerisation between first neighbors, (ii) the S = 1/2 pairs couple into effective spin-1 and the system is 2 mapped to a S = 1 Heisenberg chain, thus allowing for topological order as in the Haldane chain. This is an example of coexistence of spontaneous symmetry breaking and topological order. Host: Roman Orus