Theoretical Study on multi-band effect on the ferromagnetism of itinerant electrons

1.用密度矩阵重整化群方法研究了具有两重轨道简并的一维Hubbard模型，得到了基态相图。我们得到了n&lt；1，n=1和n&gt；1的理想铁磁基态，其中n表示每个格位的电子密度。N&lt；1的铁磁性相对于能带色散是脆弱的，而n&gt；1的铁磁性是健壮的。利用动力学平均场理论研究了无限维双简并Habbard模型，得到了基态位图。对于n=1和n&gt；1，我们得到了理想的铁磁基态，而对于n&lt；1，我们没有发现铁磁。我们还发现，对于n&lt；相似或相等的相分离，n&lt；小于或等于。尤其是所谓的双交换机制稳定了n&gt；1.3的金属铁磁性。我们构造了双交换模型中单粒子格林函数的单位近似。利用该格林函数计算了磁化率和电导率。考虑自旋的全量子效应，得到了无限维双交换模型的基态相图。

1. We studied the 1D Hubbard model with two-fold orbital degeneracy by using density matrix renormalization group method, and obtained ground state phase diagram. We obtained perfectly ferromagnetic ground state for n<1, n=1 and n>1 where n denotes the electron density per site. The ferromagnetism for n<1 was shown to be fragile against chay the band dispersion, while that for n>1 is robust.2. We studied the doubly degenerate Habbard model in infinite dimensions by using the dynamical mean field theory, and to obtained ground stats place diagram. We obtained perfectly ferromagnetic ground state for n=1 and n>1 while we did not find ferromagnetic for n<1. We also found phase separation for n<similar or equal>1 and n<less than oFrom above results we established that the Hand's rule coupling which accompanies orbital degeneracy is quite effective in realizing ferromagnetism. Especially the so-called double-exchange mechanism stabilizes metallic ferromagnetism for n>1.3. We constructed a single-site approximation for the single particle Green's function in the double exchange model. By using this Green's function are calculated the magnetic susceptibility and the electrical conductivity.4. We obtained the ground state phase diagram of the double exchange model in infinite dimensions taking into account of full quantum effects of spins.