Hua Pang

Magneto-structural coupling and harmonic lattice dynamics in CaFe2As2 probed by Mössbauer spectroscopy

In this paper we present a detailed M?ssbauer spectroscopy study of the structural and magnetic properties of the undoped parent compound CaFe2As2 single crystal. By fitting the temperature dependence of the hyperfine magnetic field we show that the magneto-structural phase transition is clearly first order in nature and we also deduce the compressibility of our sample to be 1.67 ? 10 ? 2?GPa ? 1. Within Landaus theory of phase transition, we further argue that the observed phase transition may stem from the strong magneto-structural coupling effect. The temperature dependence of the Lamb?M?ssbauer factor shows that the paramagnetic phase and the antiferromagnetic phase exhibit similar lattice dynamics in high-frequency modes with very close Debye temperatures, ?D ~ 270?K.

Charge redistribution at the antiferromagnetic phase transition in the SrFeAsF compound

Institute of Applied Magnetics, Key Lab for Magnetism and Magnetic Materials of the Ministry of Education,Lanzhou University, Lanzhou 730000, Gansu, P.R. China.(Dated: June 28, 2011)The triangle relationship between spin, electron and crystal structure has been one of the foremostissues in understanding the superconducting mechanism since the discovery of iron-based high-Tc super-conductors. Here, we report Mo¨ssbauer and first-principles calculations studies of the parent compoundSrFeAsF with the largest temperature gap (∼50K) between the structural and antiferromagnetic (AFM)transitions. Our results reveal that the structural transition has little effect on the electronic structure of thecompound SrFeAsF while the development of the AFM order induces a redistribution of the charges nearthe Fermi-level.PACS numbers: 76.80.+y, 74.10.+v, 71.15.MbINTRODUCTION

Mossbauer study of magnetic fluctuation in the iron-arsenic layer of Sr2VO3FeAs

Mossbauer spectra of the freshly prepared Sr2VO3FeAs compound have been measured under different temperatures. We observed a variation of the spectral shape with decreasing temperature. The obvious diminution of quadrupole splitting occurred at about 63K is ascribed to the weak influence of the ferromagnetic transition of the V-O layer to the spatial distribution of Fe-3d electrons, whereas the influence of the antiferromagnetic transition in the V-O layer is negligible. Below the superconducting transition temperature the distinct enhancement of the spectral area and the simultaneous increase of the line width suggest the emergence of magnetic fluctuations in the FeAs layer. Our experimental results strongly indicate the coexistence of magnetism and superconductivity in the freshly prepared Sr2VO3FeAs compound. Copyright (c) EPLA, 2014

Magnetic properties and spin dynamics in Ba(Fe1−xMnx)2As2 compounds studied by 57Fe Mössbauer spectroscopy

Ba(Fe(1-x)Mn(x))2As2 compounds with x = 0.016 and 0.064 have been studied by (57)Fe Mössbauer spectroscopy in the temperature range from 30 to 300 K. The unusual magnetic splitting spectra at lower temperatures have been analyzed using the distribution of hyperfine field. It is found that the influence of Mn dopant spreads beyond the nearest Fe magnetic moments, and the Fe 3d electrons behave more localized compared with those in the electron-doped compounds. This reduces the hyperfine interactions between iron nucleus and the sounding electrons. The shape of the spin density wave is near-rectangular at 6.4%-Mn doping, indicating quite different interband interactions compared with electron-doped compounds. A distinct broadening of the spectral linewidth around the spin density wave transition temperature has been observed and the spin correlation time is deduced according to the linewidth. The correlation time is further related to the spin-lattice relaxation rate by a simple model and the magnetic fluctuations can be explored effectively. It is found that the magnetic fluctuations of iron spins in Mn-doped compounds can be described well using a phenomenological two-component model and the resulting Curie-Weiss temperature is far from the quantum critical point at the present doping levels.

Phase separation in Ca1−x La x Fe2As2 superconductors: a 57Fe Mössbauer study

We report a detailed 57Fe Mossbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mossbauer spectra of Ca1−x La x Fe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s−1 and −0.32 mm s−1 at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1−y La y Fe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1−x La x Fe2As2 superconductors.

Phase separation in Ca(1-x)La(x)Fe₂As₂ superconductors: a ⁵⁷Fe Mössbauer study.

We report a detailed 57Fe Mossbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mossbauer spectra of Ca1−x La x Fe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s−1 and −0.32 mm s−1 at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1−y La y Fe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1−x La x Fe2As2 superconductors.

Phase separation in Ca1−xLaxFe2As2superconductors: a57Fe Mössbauer study

We report a detailed 57Fe Mossbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mossbauer spectra of Ca1−x La x Fe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s−1 and −0.32 mm s−1 at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1−y La y Fe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1−x La x Fe2As2 superconductors.