Exploring QCD at small sea quark masses with improved Wilson-type quarks
  We explore the region of small sea quark masses below $m_{PS}/m_V=0.5$ in two-flavor QCD using a mean-field improved clover quark action and an RG-improved gauge action at $a \simeq 0.2$ fm on $12^3 \times 24$ and $16^3 \times 24$ lattices. We find that instability of the standard BiCGStab algorithm at small quark masses can be mostly removed by the BiCGStab(DS-$L$) algorithm, which employs $L$-th minimal residual polynomials with a dynamical selection of $L$. We also find singular spikes of $\Delta H$ in the HMC algorithm at moderate values of $\Delta\tau$. Nature of the spike is studied. We also study finite-size effects and chiral properties of meson masses. 
CP-PACS Collaboration: Y. Namekawa, S. Aoki, M. Fukugita, K-I.   Ishikawa, N. Ishizuka, Y. Iwasaki, K. Kanaya, T. Kaneko, Y....