Graceful exit via monopoles in a theory with O'Raifeartaigh type supersymmetry breaking

We study the stability of supersymmetry breaking metastable vacua and supersymmetric vacua in the presence of solitons. The metastable vacua of supersymmetric QCD and those found elsewhere such as in models based on the SU(5) grand unified group support the existence of topological solitons. The vacua containing such topological defects can become unstable against decay into lower energy configurations. We show for a specific model that a finite region of the available parameter space of couplings becomes disallowed due to the presence of monopoles. In a manner similar to previous studies based on cosmic strings, it is shown that soliton solutions arising in supersymmetric theories can put constraints on the range of allowed values of the couplings arising in the theories. Implications for cosmology are discussed.Comment: 20 pages, 7 figure

Quantum to classical transition in the ground state of a spin-SS quantum antiferromagnet

We study a frustrated spin-$S$ staggered-dimer Heisenberg model on square lattice by using the bond-operator representation for quantum spins, and investigate the emergence of classical magnetic order from the quantum mechanical (staggered-dimer singlet) ground state for increasing $S$. Using triplon analysis, we find the critical couplings for this quantum phase transition to scale as $1/S(S+1)$. We extend the triplon analysis to include the effect of quintet dimer-states, which proves to be essential for establishing the classical order (N\'eel or collinear in the present study) for large $S$, both in the purely Heisenberg case and also in the model with single-ion anisotropy.Comment: Revised and improved versio

SrCu2(BO3)2Sr Cu_2 (BO_3)_2: A Unique Mott Hubbard Insulator

We discuss the recently discovered system $Sr Cu_2 (BO_3)_2$, a realization of an exactly solvable model proposed two decades earlier. We propose its interpretation as a Mott Hubbard insulator. The possible superconducting phase arising from doping is explored, and its nature as well as its importance for testing the RVB theory of superconductivity are discussed.Comment: 18 pages,7 figures, Based on Invited Talk at 16th Nishinomiya Yukawa memorial symposium, Nov 13-14, Nishinomiya. To appear in Progress in Theoretical Physics, Suppliment 145(2002