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Journal Articles

First-principles study of Ca-Fe-Pt-As-type iron-based superconductors

Nakamura, Hiroki; Machida, Masahiko

Physica C, 484, p.39 - 42, 2013/01

 Times Cited Count:10 Percentile:44.02(Physics, Applied)

We investigate electronic states and phonon structure of recently discovered iron-based superconductor (CaFeAs)$$_{10}$$Pt$$_4$$As$$_8$$ by first-principles calculations. Though Pt electrons affect the band structure and density of states around the Fermi energy, the Fermi surface produced by Fe $$d$$-electrons is similar to those of the usual iron-based superconductors. Phonon density of states formed by Fe-atom vibrations is also consistent with those of usual. Thus, we expect that this material has the same superconducting mechanism as other iron-based superconductors.

Journal Articles

Orbital ordering and charge profile deformation in iron-based superconductors

Machida, Masahiko; Nakamura, Hiroki

Physica C, 484, p.43 - 45, 2013/01

 Times Cited Count:1 Percentile:5.06(Physics, Applied)

We investigate electronic structures of a typical iron-based superconductor LaFeAsO by first-principles calculations in order to clarify the most stable ground-state of the mother compound. With use of around mean field (AMF) for LDA + $$U$$ formalism, we obtain an orbital ordered state showing relatively low magnetic moment widely for intermediate $$U$$ values. In the low moment state, we find that electric quadrupole moment develops around iron atom, i.e., the charge profile deforms from a spherical to a collapsed one in contrast to the high moment state with almost zero quadrupole moment. Moreover, we examine as height dependence of the low moment state and find that the low moment state is also widely stable for the variation of As height compared to the high moment one.

Journal Articles

Effects of irradiation-particle energy on critical current density in Co-doped BaFe$$_{2}$$As$$_{2}$$

Taen, Toshihiro*; Yagyuda, Hidenori*; Nakajima, Yasuyuki*; Tamegai, Tsuyoshi*; Okayasu, Satoru; Kitamura, Hisashi*; Murakami, Takeshi*; Laviano, F.*; Ghigo, G.*

Physica C, 484, p.62 - 65, 2013/01

 Times Cited Count:8 Percentile:37.59(Physics, Applied)

We report effects of particle energy on physical properties of Co-doped BaFe$$_{2}$$As$$_{2}$$ in the case of Xe and Au irradiations. The irradiation-induced suppression of $$T$$$$_{rm c}$$ is found to be related with several factors, such as defect structures and the thickness of the crystal. Heavy-ion irradiation induces enhancement of critical current density $$J$$$$_{rm c}$$, and a weak dip in the hysteresis loop. The fish-tail effect is completely suppressed by a modest density of defects. The enhancement of $$J$$$$_{rm c}$$ is closely related to the defect structure. Namely, splayed structure in columnar defects, inevitably introduced at lower energies, results in a strong enhancement of $$J$$$$_{rm c}$$ by forced entanglement of vortices.

Journal Articles

Spectral-density polynomial expansion for two-particle Green's function in BdG framework

Nagai, Yuki; Ota, Yukihiro*; Machida, Masahiko

Physics Procedia, 27, p.72 - 75, 2012/00

 Times Cited Count:4 Percentile:80.42

We develop an efficient numerical method on the basis of the spectral-density polynomial expansion to calculate two-particle Green's function in Bogoliubov-de Gennes (BdG) framework for inhomogeneous superconducting systems. With use of this method, one can systematically and effectively evaluate various physical quantities such as spin or charge susceptibility, nuclear magnetic relaxation rate, optical or thermal conductivity even in magnetic fields. We demonstrate its remarkable feature, i.e. high efficiency with large-scale parallel computing techniques.

Journal Articles

Direct numerical simulation for non-equilibrium transport phenomena in superconducting detectors

Ota, Yukihiro*; Kobayashi, Keita; Machida, Masahiko; Koyama, Tomio*; Nori, F.*

Physics Procedia, 27, p.352 - 355, 2012/00

 Times Cited Count:3 Percentile:74.04

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