Tricritical Point in the "122" Iron-pnictide Superconductors
High temperature superconductivity (HTSc) in iron pnictides, with Tc as high as 55 K as it is for the case of 1111s (such as SmFeAsF(x)O(1-x) ), is certainly one of the most perplexing discoveries of the decade in the field of condensed matter physics. In the 122 series (with parents AFe(2)As(2), A = Ba, Sr, Ca, Eu) superconductivity (SC) is produced by doping the parent with electrons (e.g. by substitution of Co or Ni on the Fe sites), or with holes (e.g. by substitution of K, Na or Cs on the A sites). It was also found that even substitution of isovalent elements, for example, P for As or Ru , Ir, Pd or Rh for Fe, induce SC. A commonality of the 1111 and 122 series are the antiferromagnetic (AFM) (spin-density wave) and the structural (tetragonal to orthorhombic) transitions that are gradually (and concomitantly) suppressed with doping in order for the superconductivity to emerge.
Recent progress has been made on the quality of the 122 single crystals, which enabled the AFM transition for the parent to be unambiguously determined as first order . On the other hand, there are clear evidences for a second order AFM transition some doped 122s. Therefore, according to Landau’s theory of phase transitions, the evolution from first to second order AFM should be through a tricritical point. In electron-doped BaFe2(1-x)Co(2x)As2 high resolution X-ray  and magnetization  revealed indeed the existence of a magnetic tricritical point in the (x,T) plane at x_tr=0.022. Recent analysis of the thermodynamic and neutron data from literature shows that the tricritical point is present in superconducting hole doped and isovalent doped 122s as well  and that the doping corresponding to the emergence of superconductivity extrapolates to the magnetic tricritical point [3,4]. This concurs with recent theoretical work showing the presence of the tricritical point in the 1111 series as well and its relevance to SC phenomenon itself . A survey of the existent results published to date is presented.
 C. R. Rotundu et al., Heat capacity study of BaFe2As2: effects of annealing, Phys. Rev. B 82, 144525 (2010).
 M. G. Kim et al., Phys. Rev. B 83, 134522 (2011).
 C. R. Rotundu and R. J. Birgeneau, First and second order magnetic and structural transitions in BaFe2(1-x)Co(2x)As2, Phys. Rev. B 84, 092501 (2011).
 C. R. Rotundu et al., A neutron scattering study of the under-doped Ba(1-x)K(x)Fe2As2, x=0.09 and 0.17 self-ux grown single crystals and the universality of the tricritical point, Phys. Rev. B. 85, 144506 (2012).
 G. Giovannetti et al.., Proximity of iron pnictide superconductors to a quantum tricritical point, Nature Communications 2:398 (2011).