New ordered states and response to dynamical field of strongly fluctuating quantum systems

我们主要用量子蒙特卡罗方法研究了由晶格不均匀引起的量子自旋系统的局域磁结构。特别是，S=1/2掺杂的S=1反铁磁海森伯模型(霍尔丹系统)，键随机S=1/2反铁磁海森伯模型(无间隙系统)和键交替S=1/2反铁磁海森伯模型(二聚体系统)。我们还研究了巡游电子系统(Hubbard模型)中的铁磁性机制，发现了一种新的机制。我们还发展了一种新的直接数值方法来研究强相互作用量子自旋系统中ESR的线形，并研究了由于偶极相互作用、各向异性和DM相互作用引起的动力学位移。我们研究了以下现象，其中量子动力学起着重要的作用。从非绝热跃迁的角度研究了纳米分子磁体如Mn12、Fe8和V15的量子磁滞现象。我们研究了声子、偶极相互作用和超精细相互作用等环境对量子动力学的影响。特别是，我们阐明了极低温度下的欺骗性非绝热跃迁、由于热激发态引起的宇称效应和由于热流入而产生的量子烟现象的机制。

We have investigated the local magnetic structures of quantum spin systems induced by inhomogeneity of the lattice mainly by the quantum Monte Carlo method. In particularly, the S=1/2 doped S=1 antiferromagnetic Heisenberg model (Haldane system), the bond random S=1/2 antiferromagnetic Heisenberg model (gapless system) and the bond alternate S=1/2 antiferromagnetic Heisenberg model (dimer system) with a defect of the alternation. We also investigated the mechanism of ferromagnetism in the itinerant electron system (Hubbard model) and found a new type of mechanism. We also developed a new direct numerical method to study the line shape of ESR in strongly interacting quantum spin systems, and investigate the dynamical shift due to the dipole interaction, anisotropy and DM interactions. We have studied the following phenomena in which the quantum dynamics plays an important role. We investigated the quantum hysteresis phenomena in nanoscale molecular magnets such as Mn12, Fe8 and V15 in the view point of non adiabatic transition. We studied the effects of the environments, such as the phonon, the dipole interaction and hyperfine interactions with the nuclear spins, on the quantum dynamics. In particular, we clarified mechanisms of deceptive non-adiabatic transition at very low temperature, of the parity effect due to the thermally excited states and of the quantum foehn phenomena due to inflow of the heat.