First- and second-order magnetic and structural transitions in BaFe2(1−x)Co2xAs

Abstract

We present here high-resolution magnetization measurements on high-quality BaFe 2(1−x) Co 2x As 2 , 0 x 0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the (x,T ) plane at x m tr ≈ 0.022 and reveal the emergence of the heat-capacity anomaly associated with the onset of the structural transition at x s ≈ 0.0064. We show that the extrapolated T c onset doping could be close to the magnetic tricritical point x m tr .The 122 series (AFe 2 As 2 , A = Ba, Sr, Ca, Eu) is one of the most studied among the newly discovered iron arsenide high-temperature superconductors. One intriguing fact is that in this series, superconductivity can be induced by doping in any of the three atomic sites; the cobalt doped system BaFe 2−2x Co 2x As 2 1,2 is, for instance, one of the most studied systems. The antiferromagnetic (spin-density wave) (at T N ) and structural (tetragonal to orthorhombic) (at T = T s ) transitions that are near coincident in the parent compounds 3 are concomitantly and gradually suppressed upon doping. A large number of papers have been written on the thermodynamic nature of the transitions in 122s arguing for either firstorder or second-order phase changes. If we solely consider the BaFe 2 As 2 system, reports range from both second-order structural and magnetic phase transitions, 3 to both transitions second-order but with the possibility of a magnetic first-order transition within 0.5 K of T N , 4 to only a first-order magnetic transition, 5 to indiscernibility between the two scenarios for the magnetic phase transition (neither the abrupt change at T N of the magnetic order parameter that is expected for a first-order transition nor the divergence of the correlation length at T N that would suggest a second-order transition were seen). 6 More recent combined high-resolution x-ray diffraction and heat-capacity measurements on exceptionally high-quality BaFe 2 As 2 crystals revealed a first-order magnetic transition preceded by a structural transition that starts as a second-order transition at a slightly higher temperature but with a first-order jump in the orthorhombic distortion coincident with the firstorder magnetic transition. 7 Since data on some doped Ba122 samples show clear second-order magnetic and structural transitions, 8-10 it has been theoretically suggested that the magnetostructural transition in the parent is close to a tricritical point 11 that is tunable through doping. The only exception appears to be the case of the hole-doped Ba 1−x K x Fe 2 As 2 for which both the magnetic and structural transitions seem to be first order over the entire phase diagram. 12 Further, ARPES measurements on cobalt-doped Ba122 evidenced a Lifshitz transition at the onset of superconductivity, 13 while high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) pointed to a magnetic tricritical point at x ≈ 0.022, but with a structural transition whose onset is second order across the whole cobalt doping range. 14 Magnetic and structural o...

Magnetoelastically coupled structural, magnetic, and superconducting order parameters in BaFe2(As1−xP<

“…When BaFe 2 As 2 is electron, hole or isovalently 'doped' 1,[5][6][7] , these transitions are suppressed and superconductivity (SC) emerges, with optimal T c appearing when magnetism is completely absent, indicating that AFM and SC order parameters compete. While for electron doped materials the structural (T S ) and magnetic (T N ) transitions separate 5,8,9 with T S > T N , in the present isovalently substituted BaFe 2 (As 1−x P x ) 2 materials, no such splitting is observed at any composition.…”

“…In contrast, our previous mechanical strain studies on Ba(Fe 1−x Co x ) 2 As 2 did not show significant changes in T N , and changes have only been observed for pressures ∼5× those used here 21 , though a recent neutron study in the parent compound could detect changes in T N at much smaller pressures 22 . Nevertheless, as a function of Co doping the difference between T N and T S grows 5,8 , which could be interpreted in terms of a decreasing g 3,10 . Furthermore, even though the lattice softens as a function of temperature, the average value of C 0 s in fact increases as a function of Co doping 4,23 .…”

Neutron scattering study of underdoped Ba1−xKxFe2As

“…Complementary high resolution X-ray diffraction and heat capacity measurements on high quality BaFe 2 As 2 crystals revealed a 1 st order magnetic transition preceded by a structural transition that starts as a 2 nd order transition at a slightly higher temperature but with a first order jump in the orthorhombic distortion coincident with the first order magnetic transition 26,27 . For the electron-doped BaFe 2(1−x) Co 2x As 2 it has been shown recently that the magnetic transition order changes upon doping from 1 st to 2 nd through a tricritical point 27,28 , which is believed to be relevant to the superconductivity phenomenon itself 29 . For this series the structural transition is 2 nd order.…”

Antiferromagnetic and nematic phase transitions inBaFe2(As1−xPxXiao-Shu1, Stanev2, Shen3 et al. 2015Phys. Rev. B10570View full textAdd to dashboardCite

“…What is particularly interesting and sets FeSCs apart from other unconventional superconductors is that the suppression of the AFM parent state by doping is preceded or coincident with a structural transition of the lattice from tetragonal to orthorhombic symmetry 2 . The interplay of the magnetic and structural transitions generates rich physics [3][4][5][6][7] .…”