Pseudogap state of high T_c cuprates: A predominant role of spin degrees of freedom

We map the field-temperature (H-T) diagram of the pseudogap state in Bi2Sr2CaCu2O8+y crystals in a wide range of hole doping. By probing the quasiparticle density-of-states with the c-axis interlayer resistivity measurements in fields up to 60 T we make a systematic evaluation of the pseudogap closing field Hpg that restores the superconductor to its ungapped state. In contrast to the characteristic fields of the superconducting state, Hpg(T) at low temperatures is temperature independent and scales with the pseudogap temperature T* by a simple Zeeman energy scale, indicating a predominant role of spins over the orbital effects in the formation of the pseudogap.