Coexistence of spontaneous symmetry breaking and topological order in the frustrated ferromagnetic J1J2 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 onedimensional (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 spin1 ferromagnetic (FM)
2
(J1) Heisenberg chain with nextnearestneighbor antiferromagnetic (J2)
interactions. On the basis of densitymatrix 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 spinsinglet dimer ization between thirdneighbor
sites. The VBS is the consequence of spontaneous symmetry breaking through
order by disorder. Quite interestingly, this VBS state has a AffleckKennedy
LiebTasakitype topological order. This is further proved by calculating
stringorder 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 spin1 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