A quantum Monte Carlo study on the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a triangular lattice

We study the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a two-dimensional triangular lattice using auxiliary field quantum Monte Carlo method for system sizes up to $12\times 12$ sites. Combining three methods to analyze the numerical data, we find, for the attractive interaction of $U=-4t$, that the transition temperature stays almost constant within the band filling range of $1.0 < n < 1.4$, while it is found to be much lower in the $n<1$ region.Comment: RevTeX 6 page

低次元電子系における多体論的アプローチによる超伝導臨界温度の評価

電気通信大学200

Raman spectroscopic studies on the ferroelectric soft mode in SnxSr1-xTiO3

The Raman spectra of novel ferroelectric ceramics SnxSr1-xTiO3 (x = 0.1, 0.05 and 0.02) were obtained to clarify the mechanism of their ferroelectric phase transitions. Two transverse-optic modes in the ferroelectric phase showed softening toward the ferroelectric transition temperature. A comparison of the spectra obtained for SnxSr1-xTiO3 with the spectrum of PbxSr1-xTiO3 facilitated the assignment of the observed modes under the assumption of the ferroelectric phase in C4v1 symmetry. However, several peaks violating the Raman selection rules were observed, suggesting the emergence and growth of polar regions even in the paraelectric phase

特異なバンド構造を持つ強相関電子系における非従来型超伝導の発現機構

The band structure strongly governs the electronic properties of solids. This fact holdseven in strongly correlated electron systems, where the focus tends to be laid on theelectron-electron interaction. In the present study, we investigate unconventional superconductivityof two strongly correlated materials. We adopt models which have realisticband structures that reproduce the first principles band calculations, and apply a manybodymethod called FLEX, which takes into account the spin fluctuations appropriately.The first one is a pressure-induced superconductor β -(BEDT-TTF)2ICl2, which hasthe highest Tc among organic superconductors. This material is essentially a two-bandsystem due to the dimerization of molecules. Applying FLEX to the two band model withrealistic band structure, superconductivity with Tc = 10K is obtained in a certain pressureregime, which is consistent with the experimental results. Also, this result is similar tothat of the previous single band model study, where strong dimerization is assumed. Wefind that although the dimerization is actually not so strong, the characteristic bandstructure peculiar to this material makes the single band theory work well. Moreover,this very character of the band structure is found to be the origin of the high Tc of thismaterial.The second one is Pr2Ba4Cu7O15－δ, which exhibits superconductivity with Tc = 15Kfrom δ = 0.2 to 0.5. Unlike other cuprates, the superconductivity occurs in the CuOdouble-chain structure, which has a double-well band structure. A previous purely onedimensionalstudy has suggested that as the band filling is increased with increasing δ, thenumber of Fermi points changes from two to four due to the double-well band structure,and this induces superconductivity. However, based on a detailed analysis of the bandstructure, we find that the number of Fermi points is four and does not change due to theslight two dimensionality present in the actual material. Thus, the reason of the disappearanceof superconductivity in the low δ regime is not clear, and also the application ofa purely one-dimensional theory is questionable. Here we adopt a quasi-one-dimensionalmodel that takes into account the two dimensional band structure, and apply FLEX thattakes into account the nearest neighbor repulsion. We find that the superconductivitywith Tc = 10K occurs in a certain band filling range due to spin fluctuations arisingfrom the double well band structure, while a suppression of the superconductivity occursaround 1/4 filling due to the effect of the nearest neighbor repulsion. Assuming δ = 0corresponds to 1/4 filling, this result explains the experimentally observed δ dependenceof the Tc. Also, auxiliary field quantum Monte Carlo, which is an exact numerical method,is performed on a simplified one-dimensional model to verify the FLEX results.電気通信大学200