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Register a new userElectronic Evidence of Unusual Magnetic Ordering in a Parent Compound of FeAs-Based Superconductors
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High resolution angle-resolved photoemission measurements have been carried out on BaFe2As2, a parent compound of the FeAs-based superconductors. In the magnetic ordering state, there is no gap opening observed on the Fermi surface. Instead, dramatic band structure reorganization occurs across the magnetic transition. The appearance of the singular Fermi spots near (pi,pi) is the most prominent signature of magnetic ordering. These observations provide direct evidence that the magnetic ordering state of BaFe2As2 is distinct from the conventional spin-density-wave state. They reflect the electronic complexity in this multiple-orbital system and necessity in involving the local magnetic moment in describing the underlying electron structure.
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We report the first comprehensive high-resolution angle-resolved photoemission measurements on CeFeAsO, a parent compound of FeAs-based high temperature superconductors with a mangetic/structural transition at $sim$150 K. In the magnetic ordering state, four hole-like Fermi surface sheets are observed near $Gamma$(0,0) and the Fermi surface near M(+/-$pi$,+/-$pi$) shows a tiny electron-like pocket at M surrounded by four Dirac cone-like strong spots. The unusual Fermi surface topology deviates strongly from the band structure calculations. The electronic signature of the magnetic/structural transition shows up in the dramatic change of the quasiparticle scattering rate. A dispersion kink at $sim$ 25meV is for the first time observed in the parent compound of Fe-based superconductors.
A new compound with the FeAs-layers, namely (Sr_3Sc_2O_5)Fe_2As_2 (abbreviated as FeAs-32522), was successfully fabricated. It has a layered structure with the space group of I4/mmm, and with the lattice constants a = 4.069 $AA$ and c = 26.876 $AA$. The in-plane Fe ions construct a square lattice which is close to that of other FeAs-based superconductors, such as REFeAsO (RE = rare earth elements) and (Ba,Sr)Fe_2As_2. However the inter FeAs-layer spacing in the new compound is greatly enlarged. The temperature dependence of resistivity exhibits a weak upturn in the low temperature region, but a metallic behavior was observed above about 60 K. The magnetic susceptibility shows also a non-monotonic behavior. Interestingly, the well-known resistivity anomaly which was discovered in all other parent compounds, such as REFeAsO, (Ba,Sr)Fe_2As_2 and (Sr,Ca,Eu)FeAsF and associated with the Spin-Density-Wave (SDW)/structural transition has not been found in the new system either on the resistivity data or the magnetization data. This could be induced by the large spacing distance between the FeAs-planes, therefore the antiferromagnetic correlation between the moments of Fe ions in neighboring FeAs-layers cannot be established. Alternatively it can also be attributed to the self-doping effect between Fe and Sc ions. The Hall coefficient R_H is negative but strongly temperature dependent in wide temperature region, which indicates the dominance of electrical conduction by electron-like charge carriers and probably a multi-band effect or a spin related scattering effect. It is found that the magnetoresistance cannot be described by the Kohlers rule, which gives further support to above arguments.
In addition to higher Tc compared with the ubiquitous cuprates for a material composed of a single electronically active layer, the newly discovered LnFeAsO superconductors offer additional compositional variation. In a similar fashion to the CuO2 layers in cuprates, the FeAs layers now dominate the electronic states that produce superconductivity. Cuprate superconductors distinguish themselves structurally by adopting different stacking of the Cu-O and electronically inactive spacer layers. Using the same structural philosophy, materials with the formula (A,K)Fe2As2,A=Ba or Sr have been reported and possess a Tc~38 K. Here, we report the neutron diffraction studies of BaFe2As2 that shows, in contrast to previous studies on the LnFeAsO materials, an antiferromagnetic transition which concurs with first-order structural transition. Although the magnetic and structural transitions occur differently in the AFe2As2 and LnFeAsO-type materials, this work clearly demonstrates that the complete evolution to a low symmetry structure is a pre-requirement for the magnetic order.
The recent discovery of iron ferropnictide superconductors has received intensive concerns on magnetic involved superconductors. Prominent features of ferropnictide superconductors are becoming apparent: the parent compounds exhibit antiferromagnetic (AFM) ordered spin density wave (SDW) state; the magnetic phase transition is always accompanied to a crystal structural transition; superconductivity can be induced by suppressing the SDW phase via either chemical doping or applied external pressure to the parent state. These features generated considerable interests on the interplay between magnetism and structure in chemical doped samples, showing crystal structure transitions always precedes to or coincide with magnetic transition. Pressure tuned transition on the other hand would be more straightforward to superconducting mechanism studies since there are no disorder effects caused by chemical doping; however, remarkably little is known about the interplay in the parent compounds under controlled pressure due to the experimental challenge of in situ measuring both of magnetic & crystal structure evolution at high pressure & low temperatures. Here we show from combined synchrotron Mossbauer and x-ray diffraction at high pressures that the magnetic ordering surprisingly precedes the structural transition at high pressures in the parent compound BaFe2As2, in sharp contrast to the chemical doping case. The results can be well understood in terms of the spin fluctuations in the emerging nematic phase before the long range magnetic order that sheds new light on understanding how parent compound evolves from a SDW state to a superconducting phase, a key scientific inquiry of iron based superconductors.
We investigate the transient electronic structure of BaFe2As2, a parent compound of iron-based superconductors, by time- and angle-resolved photoemission spectroscopy. In order to probe the entire Brillouin zone, we utilize extreme ultraviolet photons and observe photoemission intensity oscillation with the frequency of the A1g phonon which is antiphase between the zone-centered hole Fermi surfaces (FSs) and zone-cornered electron FSs. We attribute the antiphase behavior to the warping in one of the zone-centered hole FSs accompanying the displacement of the pnictogen height, and find that this displacement is the same direction as that induced by substitution of P for As, where superconductivity is induced by a structural modification without carrier doping in this system.
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