Department of Physics
Condensed Matter Seminar

Using a quantum Monte Carlo method (SSE), we study the order-disorder phase transition for Heisenberg spin systems with both static and dynamic bond-dilutions. The quantum critical point is found to be sensitive to the nature of the bond configurations. A study of the ground state energy shows possible phase separation. Varying the interlayer coupling J\', the Neel temperature T_N and universal scalings are studied for weakly coupled Heisenberg antiferromagnetic layers consisting of coupled ladders. This system can be tuned to different two-dimensional scaling regimes for T > T_N: Renormalized Classical (RC), Quantum Critical (QC) and Quantum Disordered (QD) regimes. We also present results for a two-dimensional Heisenberg antiferromagnet with nearest-neighbor and four-spin ring exchange, which exhibits a quantum phase transition from the Valence Bond Solid (VBS) phase to Neel order. Using singlet-triplet basis, the triplon spectrum starting from the VBS phase is calculated and strong quantum fluctuations are found. The features are consistent with a critical point exhibiting so-called deconfined quantum criticality.