For distribution - thanks

[CM ICTP - Trieste]

SEMINAR on   Disorder and strong electron correlations


Thursday, 27 September   -    11:00 a.m.



Seminar Room,  Main Bldg.- first floor



N. SURENDRAN   ( The Abdus Salam I.C.T.P. )

"Criticality in dimerized spin chains"

Abstract

	Antiferromagnetic Heisenberg spin chains with alternating bond 
strengths become gapless at specific values of the dimerization 
parameter, as predicted by the mapping to O(3) non-linear sigma(NLSM)  
with a topological term.  To find the location of the gapless points, 
we analytically and numerically study the system for various values of 
spin using a twisted boundary condition. A tunneling calculation 
reproduces the NLSM results obtained from more involved field theoretic 
methods.  Another approach involving a certain matrix element between 
two Neel states provides a set of gapless points which not only differ 
significantly from the tunneling/NLSM values,  but gives good agreement 
with the results of an exact diagonalization method.