4 research outputs found
Magnetic and structural transitions in layered FeAs systems: AFe2As2 versus RFeAsO compounds
Resistivity, specific heat and magnetic susceptibility measurements performed
on SrFe2As2 samples evidence a behavior very similar to that observed in
LaFeAsO and BaFe2As2 with the difference that the formation of the SDW and the
lattice deformation occur in a pronounced first order transition at T_0=205K.
Comparing further data evidences that the Fe-magnetism is stronger in SrFe2As2
and in EuFe2As2 than in the other layered FeAs systems investigated up to now.
Full potential LDA band structure calculations confirm the large similarity
between the compounds, especially for the relevant low energy Fe 3d states. The
relation between structural details and magnetic order is analyzed.Comment: 4 pages, 3 figure
Strong coupling between magnetic and structural order parameters in SrFe2As2
X-ray and Neutron diffraction as well as muon spin relaxation and M\"ossbauer
experiments performed on SrFeAs polycrystalls confirm a sharp first
order transition at ,K corresponding to an orthorhombic phase
distortion and to a columnar antiferromagnetic Fe ordering with a propagation
vector (1,0,1), and a larger distortion and larger size of the ordered moment
than reported for BaFeAs. The structural and the magnetic order
parameters present an remarkable similarity in their temperature dependence
from down to low temperatures, showing that both phenomena are intimately
connected. Accordingly, the size of the ordered Fe moments scale with the
lattice distortion when going from SrFeAs to BaFeAs.
Full-potential band structure calculations confirm that the columnar magnetic
order and the orthorhombic lattice distortion are intrinsically tied to each
other.Comment: 10 pages, 4 figure
Feshbach resonances and mesoscopic phase separation near a quantum critical point in multiband FeAs-based superconductors
High Tc superconductivity in FeAs-based multilayers (pnictides), evading
temperature decoherence effects in a quantum condensate, is assigned to a
Feshbach resonance (called also shape resonance) in the exchange-like interband
pairing. The resonance is switched on by tuning the chemical potential at an
electronic topological transition (ETT) near a band edge, where the Fermi
surface topology of one of the subbands changes from 1D to 2D topology. We show
that the tuning is realized by changing i) the misfit strain between the
superconducting planes and the spacers ii) the charge density and iii) the
disorder. The system is at the verge of a catastrophe i.e. near a structural
and magnetic phase transition associated with the stripes (analogous to the 1/8
stripe phase in cuprates) order to disorder phase transition. Fine tuning of
both the chemical potential and the disorder pushes the critical temperature Ts
of this phase transition to zero giving a quantum critical point. Here the
quantum lattice and magnetic fluctuations promote the Feshbach resonance of the
exchange-like anisotropic pairing. This superconducting phase that resists to
the attacks of temperature is shown to be controlled by the interplay of the
hopping energy between stripes and the quantum fluctuations. The
superconducting gaps in the multiple Fermi surface spots reported by the recent
ARPES experiment of D. V. Evtushinsky et al. arXiv:0809.4455 are shown to
support the Feshbach scenario.Comment: 31 pages, 7 figure