Title : 
An analysis of the t2 V (or extreme anisotropic nextnearestneighbor Heisenberg) model 

Speaker  :  Y. Sudhakar, SINP 
Date  :  April 12, 2013 
Time  :  12:00 PM 
Venue  :  Theory Seminar Room 3307 
Abstract  : 
The t2  V model (involving nextnearestneighbor hopping and nearestneighbor repulsion) has been shown to be the strong electronphonon limiting case of a molecular chain with cooperative breathing mode [Phys. Rev. B 86, 035453 (2012)]. We derive microscopically, using Green�s functions, the exact instability conditions in the two limiting cases of the t2  V model for hardcorebosons: the twoparticle system and the halffilled system. We show explicitly that the critical repulsion Vc for the twoparticle case is Vc /t2 = 4 for a ring of any size while for the halffilled case the Vc /t2 = 2 2 in the thermodynamic limit. Our t2  V model can be mapped onto an extreme anisotropic Heisenberg model (with nearestneighbor XY interaction and nextnearestneighbor ising interaction) and is of the form J? i (Si+ Si+2 + H.c.) + J i Siz Si+1; our spin model correspondingly lends itself to exact instability solutions (by the Green�s function method) in the two limiting cases of twomagnons (the nontrivial highest excited state) and N � eel antiferromagnet ground state. Using finite size scaling, at other fillings of the t2  V model (or other magnetizations of the spin model), we also obtain the critical repulsion numerically by using modified Lanczos. During the phase transition, away from halffilling (zero magnetization), the system undergoes a striking discontinuous transition from a superfluid to a supersolid [i.e., a superfluid homogeneously coexisting with a perioddoubling chargedensitywave (antiferromagnetic) state]. At halffilling, the CDW (N eel) state and the superfluid state are mutually exclusive. 