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

本文主要用量子蒙特卡罗方法研究了晶格不均匀性引起的量子自旋系统的局域磁结构。特别是S=1/2掺杂S=1反铁磁海森堡模型（霍尔丹系统）、键随机S=1/2反铁磁海森堡模型（无间隙系统）和键交替S=1/2反铁磁海森堡模型（二聚体系统），但存在交替缺陷。我们还研究了巡游电子系统（Hubbard模型）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谱线的形状，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。我们研究了量子动力学起重要作用的下列现象。从非绝热相变的角度研究了Mn_（12）、Fe_（8）和V_（15）等纳米分子磁体中的量子磁滞现象。我们研究了声子、偶极相互作用以及与核自旋的超精细相互作用等环境对量子动力学的影响。特别是，我们澄清了欺骗性的非绝热过渡在很低的温度下，由于热激发态和量子焚风现象，由于流入的热量的宇称效应的机制。

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.