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## Acknowledgement 

We would like to acknowledge helpful discussion with L. Salasnich, S.R. Jain and G. Mus- sardo. This work has been funded by the Department of Science and Technology (DST), India under grant number SR/S2/CMP/0059(2007). BC thanks the International Center for Theoretical Physics (ICTP), Trieste for hospitality and financial support, where a part of the work was done. TKD acknowledges the University Grants Commission (UGC), India for the Emeritus Fellowship. AB acknowledges the UGC, India for the Research Fellowship in Sciences for Meritorious Students and the Council of Scientific and Industrial Research (CSIR), India for a Senior Research Fellowship. 

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$^{3}$K. Sakman et al , Phys. Rev. A 78 , 023615 (2008). 

