Microscopic origin of pressure-induced phase transitions in the iron pnictide superconductorsAFe2As2: Anab initiomolecular dynamics study

Abstract

Using ab initio molecular dynamics we investigate the electronic and lattice structure of AFe 2 As 2 ͑A =Ca,Sr,Ba͒ under pressure. We find that the structural phase transition ͑orthorhombic to tetragonal symmetry͒ is always accompanied by a magnetic phase transition in all the compounds while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe 2 As 2 and its absence in BaFe 2 As 2 . We argue that changes in the Fermi-surface nesting features dominate the phase transitions under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.The discovery of iron pnictide superconductors 1 with critical temperatures T c up to 57.4 K ͑Ref. 2͒ upon doping has strongly revived the interest in high-T c superconductivity. The undoped Fe-based parent compound undergoes at low temperatures a structural transition from tetragonal to orthorhombic symmetry accompanied by a magnetic phase transition to a stripe-type spin-density-wave state. 3-7 While the nature of these two transitions is different between LaFeAsO ͑1111 compound͒ and AFe 2 As 2 ͑122 compound͒ with A = ͑Ba, Sr, Ca͒, superconductivity appears in both material classes only when the lattice distortion and magnetic ordering are suppressed, indicating a strong competition between the structural distortion, magnetic ordering, and superconductivity in iron pnictides.Recently, superconductivity in the parent compounds 1111 and 122 was reported under application of pressure. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] In LaFeAsO, 8 resistivity measurements show superconductivity at Ϸ12 GPa with T c = 21 K. In BaFe 2 As 2 superconductivity is found to appear gradually with increasing pressure while in SrFe 2 As 2 the onset of superconductivity occurs abruptly. 22 In CaFe 2 As 2 , 12-14 detailed neutron-and x-ray diffraction analysis shows that the system undergoes a first-order phase transition from a magnetic orthorhombic to a nonmagnetic "collapsed" tetragonal phase under pressure. The possible appearance of superconductivity in this collapsed tetragonal phase is presently under debate. 15,16 While various experiments give different values of critical pressures 17-25 due to the fact that the phase transition is sensitive to possible nonhydrostatic pressure effects, Sn content in some samples, or the use of single crystals or polycrystalline material for structure determination, it is claimed that BaFe 2 As 2 and SrFe 2 As 2 do not manifest a collapsed tetragonal phase at elevated pressure. 13,17,18,23,24 Therefore, the fact that structurally similar compounds exhibit phase transitions of different nature urgently calls for a theoretical understanding. Moreover, it is still under intensive debate which is the driving mechanism of the collinear stripe-type antiferromagnetic ordering; whether the Fermi-surface nesting or the competition of exchange antiferromagnetic interactions between the ...

Review on origin of the unusual high-Tc superconductivity in Ca1−xRExFe2As2 (RE = La, Ce, Pr, Nd)

“…To summary up, the unusual features in CaRE122 are: (1) phase transition from tetragonal (T) to collapsed tetragonal (CT) phase induced by chemical pressure; (2) two non-bulk superconducting transitions with very small superconducting volume fraction; (3) annealing independent behavior; (4) extremely large magnetic anisotropy; and (5) almost pressure-independent high Tc. As is well known, vanishment of the hole pocket at the zone center in Brillouin zone takes place in the pressure-induced phase transition to CT phase in CaFe 2 As 2 [38,39], which thus results in the disappearance of the Fermi surface nesting between electron and hole Fermi surfaces. Since it is suggested the Fermi surface nesting is required for SC in IBSs, the transition to CT phase is not supportive for the formation of Cooper pair.…”

Unfolding of electronic structure through induced representations of space groups: Application to Fe-based superconductors

“…In previous studies [22][23][24], we simulated the application of pressure under different conditions by means of density functional theory calculations and were able to predict the appearance of the collapsed tetragonal phase at a critical pressure which is accompanied by the disappearance of the Fermi surface pockets centered around the Γ point. This feature has been recently confirmed by angle resolved photoemission experiments [25,26].…”

In-plane uniaxial stress effects on the structural and electronic properties of BaFe2As2and CaFe2AsTomić1, Jeschke2, Fernandes3 et al. 2013Phys. Rev. BSelf Cite285231View full textAdd to dashboardCite

“…Even though DFT calculations overestimate the value of the ordered Fe magnetic moment at P = 0 GPa, it is to be expected that the relative changes in magnetic moment should provide a reliable description of the situation of BaFe 2 As 2 under pressure as shown in previous studies. [20][21][22][23] Except for the pressures P = −0.22 GPa (for stress along a) and P = 0.22 GPa (for stress along b), stress always enforces a certain degree of orthorhombicity and the system remains magnetically ordered with a decreasing ordered moment as a function of compressive stress (Figure 2 (a)). Moreover, since the c axis continually expands from negative to positive pressures, h As increases accordingly as a function of stress (Figure 2 (e)).…”

“…In fact, the c axis expands with applied stress and at P = 0.67 GPa the c lattice parameter in CaFe 2 As 2 is too large for the formation of an interlayer As-As covalent bond, necessary for a transition to a collapsed tetragonal phase and suppression of magnetic moments as observed under hydrostatic or c-axis uniaxial pressure. [20][21][22]31 For tensile stress along the (shorter) b direction, the reversal of AFM and FM directions happens at P = −0.33 GPa followed by a jump in the orthorhombicity. The magnetic response of CaFe 2 As 2 is highly anisotropic as well, but contrary to the case of BaFe 2 As 2 , the magnetic moments in CaFe 2 As 2 are most effectively suppressed when pressure is applied along c. We measure a rate of about 0.1µ B /GPa 20 .…”