Seminar on Disorder & Strong Electron Correlations 2 Feb 2012

[ivanisse]

CONDENSED MATTER AND STATISTICAL PHYSICS SECTION

Seminar on Disorder and strong electron correlations


Thursday, 2 February 2012  -   11:30 a.m.


Luigi Stasi Seminar Room - ICTP Leonardo Bldg. - 1st floor


Yasir IQBAL, Universite de Toulouse


" Competing spin-disordered phases of the spin-1/2 Heisenberg 
antiferromagnet on the Kagome lattice "


Abstract:

Despite years of intense theoretical attack from different directions, 
the ground state of the S = 1/2 Kagome Heisenberg antiferromagnet has 
remained elusive. I will revisit this question within the framework of 
Gutzwiller projected fermionic wave functions studied using Variational 
quantum Monte Carlo technique. We found the so called U(1) Dirac state, 
an exotic algebraic spin liquid, to have the best variational energy. 
While there were doubts concerning its stability, experiments have 
hinted towards a gapless, algebraic spin liquid behavior. Indeed we show 
that the U(1) Dirac spin liquid is remarkably stable w.r.t dimerizing 
towards a large class of Valence bond crystal perturbations.

This stability is also preserved upon addition of a weak 2nd NN exchange 
couplings. However we find, that upon addition of a weak 2nd NN 
ferromagnetic coupling, a non-trivial valence bond crystal is 
stabilized, and has the lowest energy. This VBC possesses a non-trivial 
flux pattern and is a strong dimerization of another competing U(1) 
gapless spin liquid with a large spinon Fermi surface, the so called 
uniform RVB state. The U(1) Dirac state and the uniform RVB state are 
shown to be remarkably stable w.r.t. destabilizing into the class of Z2 
spin liquids.

I will also briefly touch upon my ongoing work dealing with a complete 
group theoretical classification of time-reversal invariant Valence bond 
crystals on the Kagome lattice, and present some results concerning the 
properties of the ground state on small clusters which are extracted 
using the Lanczos method on a given variational wave function.