中等症・重症で入院を要する高齢発症潰瘍性大腸炎に対するステロイド大量静注療法の低有効性：多施設共同コホート研究

京都大学新制・課程博士博士(医学)甲第24492号医博第4934号新制||医||1063(附属図書館)京都大学大学院医学研究科医学専攻(主査)教授 今中 雄一, 教授 川上 浩司, 教授 妹尾 浩学位規則第4条第1項該当Doctor of Medical ScienceKyoto UniversityDFA

Disorder, spin-orbit, and interaction effects in dilute Ga1−xMnxAs{\rm Ga}_{1-x}{\rm Mn}_x{\rm As}

We derive an effective Hamiltonian for ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ in the dilute limit, where ${\rm Ga}_{1-x}{\rm Mn}_x {\rm As}$ can be described in terms of spin $F=3/2$ polarons hopping between the {\rm Mn} sites and coupled to the local {\rm Mn} spins. We determine the parameters of our model from microscopic calculations using both a variational method and an exact diagonalization within the so-called spherical approximation. Our approach treats the extremely large Coulomb interaction in a non-perturbative way, and captures the effects of strong spin-orbit coupling and Mn positional disorder. We study the effective Hamiltonian in a mean field and variational calculation, including the effects of interactions between the holes at both zero and finite temperature. We study the resulting magnetic properties, such as the magnetization and spin disorder manifest in the generically non-collinear magnetic state. We find a well formed impurity band fairly well separated from the valence band up to $x_{\rm active} \lesssim 0.015$ for which finite size scaling studies of the participation ratios indicate a localization transition, even in the presence of strong on-site interactions, where $x_{\rm active}<x_{\rm nom}$ is the fraction of magnetically active Mn. We study the localization transition as a function of hole concentration, Mn positional disorder, and interaction strength between the holes.Comment: 15 pages, 12 figure

Theoretical models of ferromagnetic III-V semiconductors

Recent materials research has advanced the maximum ferromagnetic transition temperature in semiconductors containing magnetic elements toward room temperature. Reaching this goal would make information technology applications of these materials likely. In this article we briefly review the status of work over the past five years which has attempted to achieve a theoretical understanding of these complex magnetic systems. The basic microscopic origins of ferromagnetism in the (III,Mn)V compounds that have the highest transition temperatures appear to be well understood, and efficient computation methods have been developed which are able to model their magnetic, transport, and optical properties. However many questions remain.Comment: 4 pages, 4 figures, review, to be published in Curr. Appl. Phy

Mechanism of carrier-induced ferromagnetism in magnetic semiconductors

Taking into account both random impurity distribution and thermal fluctuations of localized spins, we have performed a model calculation for the carrier (hole) state in Ga$_{1-x}$Mn$_x$As by using the coherent potential approximation (CPA). The result reveals that a {\it p}-hole in the band tail of Ga$_{1-x}$Mn$_x$As is not like a free carrier but is rather virtually bounded to impurity sites. The carrier spin strongly couples to the localized {\it d} spins on Mn ions. The hopping of the carrier among Mn sites causes the ferromagnetic ordering of the localized spins through the double-exchange mechanism. The Curie temperature obtained by using conventional parameters agrees well with the experimental result.Comment: 7 pages, 4 figure

Influence of band structure effects on domain-wall resistance in diluted ferromagnetic semiconductors

Intrinsic domain-wall resistance (DWR) in (Ga,Mn)As is studied theoretically and compared to experimental results. The recently developed model of spin transport in diluted ferromagnetic semiconductors [Van Dorpe et al., Phys. Rev. B 72, 205322 (2005)] is employed. The model combines the disorder-free Landauer-B\"uttiker formalism with the tight-binding description of the host band structure. The obtained results show how much the spherical 4x4 kp model [Nguyen, Shchelushkin, and Brataas, cond-mat/0601436] overestimates DWR in the adiabatic limit, and reveal the dependence of DWR on the magnetization profile and crystallographic orientation of the wall.Comment: 4 pages, 4 figures, submitted to Phys. Rev. B - Rapid Com

Electronic structure of In1−x_{1-x}Mnx_xAs studied by photoemission spectroscopy: Comparison with Ga1−x_{1-x}Mnx_xAs

We have investigated the electronic structure of the $p$-type diluted magnetic semiconductor In$_{1-x}$Mn$_x$As by photoemission spectroscopy. The Mn 3$d$ partial density of states is found to be basically similar to that of Ga$_{1-x}$Mn$_x$As. However, the impurity-band like states near the top of the valence band have not been observed by angle-resolved photoemission spectroscopy unlike Ga$_{1-x}$Mn$_x$As. This difference would explain the difference in transport, magnetic and optical properties of In$_{1-x}$Mn$_x$As and Ga$_{1-x}$Mn$_x$As. The different electronic structures are attributed to the weaker Mn 3$d$ - As 4$p$ hybridization in In$_{1-x}$Mn$_x$As than in Ga$_{1-x}$Mn$_x$As.Comment: 4 pages, 3 figure