Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2and BaFe2As

Milan Tomić

¹

,

Roser Valentí

²

,

Harald O. Jeschke

³

Abstract

We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2As2 within density functional theory and compare them with calculations under hydrostatic pressure as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to experimental observations. The discovery of superconductivity in iron pnictides [1] has initiated an enourmous amount of activities related to these materials. Superconductivity can be triggered either by chemical doping or by application of pressure on the undoped parent compounds. One of the families that has been intensively studied under pressure is the 122 family AEFe 2 As 2 (AE = Ca, Sr, and Ba). CaFe 2 As 2 at ambient pressure undergoes a first order phase transition from a tetragonal to an orthorhombic phase at 172 K accompanied by a magnetic transition. Initial reports on pressure experiments showed that at P ∼ 0.23 GPa the orthorhombic and antiferromagnetic phases are suppressed and the system superconducts at low temperatures [2,3]. Moreover, a compressed tetragonal phase -also called 'collapsed' tetragonal phase -was identified at higher pressures. Subsequent susceptibility and transport measurements under hydrostatic conditions showed at low temperatures and P ∼ 0.35 GPa a sharp orthorhombic to collapsed tetragonal phase but no signature of superconductivity [4]. In contrast, recent neutron diffraction experiments on CaFe 2 As 2 under uniaxial pressure along the c axis [5] indicate for pressures above 0.06 GPa and low temperatures the presence of an intermediate nonmagnetic tetragonal phase between the magnetic orthorhombic and the nonmagnetic collapsed tetragonal phases. This phase was identified by the authors as the phase responsible for superconductivity at T = 10 K. Other reports based on muon spin-relaxation measurements suggest the existence of superconductivity in the orthorhombic phase, raising the question whether superconductivity and magnetism can coexist [6].BaFe 2 As 2 shows an even more complex behavior under pressure. At ambient pressure it undergoes a phase transition from a metallic tetragonal phase to an orthorhombic antiferromagnetic phase at T = 140 K. Under pressure the gradual appearance of a superconducting dome has been observed by various groups [7] though the role of nonhydrostatic conditions is not yet well understood [8]. Recent synchrotron X-ray diffraction experiments ...

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

“…Utilizing Voigt and Reuss averages, 32 defined as B Voigt = 1 9 (C 11 + C 22 + C 33 + 2(C 12 + C 13 + C 23 )) B Reuss = (S 11 + S 22 + S 33 + 2(S 12 + S 13 + S 13 )) −1 it is possible to estimate the bulk modulus. Voigt and Reuss averages yield 61.9±5.1 GPa and 69.3±7.5 GPa, respectively, which is in good agreement with our previous estimate 20 and the experimental value of 59 ± 2 GPa. 33 For CaFe 2 As 2 , the elastic constant matrix is given by …”

“…All our structural relaxations were performed under constant stress using the Fast Inertial Relaxation Engine (FIRE). 19,20 For this purpose, we had to modify the algorithm accordingly (see Appendix A). Every 10 steps, we cycled through non-magnetic, ferromagnetic, antiferromagnetic-checkerboard, stripe-type antiferromagnetic (along unit cell axis a) and stripe-type antiferromagnetic (along unit cell axis b) spin configurations, and then we continued the relaxation with the lowest energy spin configuration.…”

“…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 .…”

Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

“…It is important to keep in mind that the orbital states are not the eigenstates of the electronic Bloch problem [20][21][22][23] . At first, one may think that this feature renders the qualitative distinction between the effects of the spin-orbit coupling and the nematic order less transparent.…”