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SUMMARY:Order by disorder and phase transitions  - Laura Messio\, LPTMC Pa
 ris
DTSTART:20220120T140000Z
DTEND:20220120T150000Z
UID:TALK161536@talks.cam.ac.uk
CONTACT:Jan Behrends
DESCRIPTION:In most phase transitions\, the low temperature phase possesse
 s a broken symmetry\, that is restored above a critical temperature (with 
 the notable exception of topological phase transitions). This broken symme
 try is associated to a non-zero order parameter\, and the ground state(s) 
 (at T=0) usually maximizes(s) this order parameter. This is not the case i
 n the presence of order by disorder\, where entropic effects are responsib
 le for the enhancement of the order parameter. This situation occurs in th
 e Vesignieite compound BaCu3V2O8(OH)2\, a Mott insulator modeled by a J1-J
 3 Heisenberg model on the kagome lattice (with first and third neighbor in
 teractions). The nature of the classical ground state and the possible pha
 se transitions are investigated through analytical and numerical calculati
 ons. This model has many similarities with the J1-J2 square lattice\, know
 n to develop a collinear phase for J2>J1|/2 through the order by disorder 
 mechanism. In both model\, the limit of large J2 or J3 gives several decou
 pled sublattices and J1 does not lift the ground state degeneracy. On both
  lattices\, collinear spin configurations are favored by fluctuations\, gi
 ving rise to a discrete order parameter and to a finite temperature phase 
 transition. On the kagome lattice\, a q=4 Potts parameter emerges\, agains
 t a Z2 on the square lattice. Effect of quantum fluctuations are studied t
 hrough linear spin wave approximation and high temperature expansions of t
 he S=1/2 model.\n\nReference: V. Grison et al\, Phys.Rev.B 102\, 214424 (2
 020)
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