arXiv subjects

Parasharam M. Shirage

Publications and source records attributed to Parasharam M. Shirage.

24 records · Page 2Linked to original sources

arXiv2010

No anomalous spin fluctuation in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ probed by muon spin relaxation

Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluctuation but also the multiband character with several different orbital contributions at the Fermi surface may play an important role in the superconducting pairing mechanism of LaFeAsO$_{1-x}$F$_x$.

cond-mat.supr-con↗

arXiv2009

Inverse Iron Isotope Effect on the transition temperature of the (Ba,K)Fe2As2 superconductor

We report that (Ba,K)Fe2As2 superconductor (a transition temperature, Tc = 38 K) shows inverse Iron isotope effect (-0.18) (the sample including the larger atomic weight of Fe depicts higher Tc). Measurements of both temperature dependent magnetization and resistivity reveal a clear inverse shift by systematic studies on Tc using three types of Fe-isotopes (Fe-54, natural Fe and Fe-57). This indicates the first evidence of the inverse isotope effect in high-Tc superconductors. This atomic mass dependence on Tc implies the exotic coupling mechanism.

cond-mat.supr-con↗

arXiv2009

Three-dimensional electronic structure of superconducting iron pnictides observed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe2\_{-x}CoxAs2 (Ba122) (x=0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating the three-dimensional (3D) electronic structure. This observation suggests that the antiferromagnetism and superconductivity in the pnictides may have to be considered including the orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.

cond-mat.supr-con↗

arXiv2008

Superconductivity above 50K in LnFeAsO1-y (Ln= Nd, Sm, Gd, Tb and Dy) Synthesized by High-pressure Technique

We have succeeded in synthesizing single-phase polycrystalline samples of oxygen-deficient oxypnictide superconductors, LnFeAsO1-y (Ln: lanthanide elements) with Ln=La, Ce, Pr, Nd, Sm, Gd, Tb and Dy using high-pressure synthesis technique. It is found out that the synthesis pressure is the most important parameter for synthesizing single-phase samples, in particular for the heavier Ln?s, such as Tb and Dy. The lattice parameters systematically decrease with the atomic number of Ln, reflecting the shrinkage of Ln ionic radius. For the lighter Ln?s (La, Ce, Pr, Nd), Tc increases monotonously with decreasing the lattice parameters from 26K for Ln=La to 54K for Ln=Nd, then stays at the constant value around 53K for the heavier counterpart (Nd, Sm, Gd, Tb and Dy). The results suggest the intimate relationship between the crystal structural parameters and the superconductivity on the one hand, as well as the possible existence of the inherent maximum Tc on the other, which is located around 50 K in the LnFeAsO based materials.

cond-mat.supr-con↗

arXiv2008

Evidence of spin density wave in LaFeAsO, the parent material of the new Fe-based oxypnictide superconductors

The Fe-based Oxypnicide superconductors have generated a huge amount of interest; they are a high temperature superconductor, with a Tc of 55K, but do not have the two dimensional copper oxygen layer that was thought essential for superconductivity at these high temperatures. Initial studies have hinted towards the possibility of a spin density wave (SDW) in these compounds and how they could play an important role in the superconductivity. SDW's occur at low temperatures in low-dimensional materials with strong electron correlations or in metals with a high density of states at the Fermi surface. A transition to the SDW state has many similarities to the superconducting transition, driven by the condensation energy with an energy gap opening, and in many materials the SDW state occurs adjacent in the phase diagram to the superconducting state. In these materials it is believed that electron doping suppresses the SDW instability allowing superconductivity to emerge. In this paper we report on the first direct experimental evidence of a SDW in the LaFeAsO parent compound as observed with inelastic neutron scattering. We show that these excitation derive from a two-dimensional Fermi surface nesting with a nesting vector of (π,π,0).

cond-mat.supr-con↗

arXiv2008

Sheet Dependence on Superconducting Gap in Oxygen-Deficient Iron-based Oxypnictide Superconductors NdFeAs0.85

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point.

cond-mat.supr-con↗