Character of the structural and magnetic phase transitions in the parent and electron-doped BaFe2As2compounds

Abstract

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At TS = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at TN = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe1−xCox)2As2 and Ba(Fe1−xRhx)2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve towards split secondorder transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co-doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magneto-elastic coupling in the form of an emergent Ising-nematic degree of freedom.

Order parameter fluctuation and ordering competition in Ba1−xKxFe2As2

“…Extensive experiments have confirmed that nematic order exists in almost all FeSCs [2][3][4][5][13][14][15][16][17]. In most cases, the nematic order sets in at a temperature T n slightly higher than the critical temperature of magnetic order T m [4,6,10,18,19].…”

Anomalous surface lattice dynamics in the low-temperature phase of Ba(Fe 1− x Co x ) 2 As 2

“…Fig. 2B displays the T dependence of the loss spectra for Ba (Fe 0.95 Co 0.05 ) 2 As 2 after removal of the background. All of the modes soften and broaden as temperature increases.…”

“…Fig. 2A displays an EELS spectrum from Ba(Fe 0.95 Co 0.05 ) 2 As 2 single crystals taken at an incident electron energy E 0 = 20 eV and a temperature of 46 K. Three phonon peaks are identified as Zω 1 , Zω 2 , and Zω 3 with energies 33.5, 26.5, and 14.0 meV, respectively. Based on the previous studies for the bulk (10-14, 17), we can identify these modes ( Fig.…”

“…Fig. 1A shows the phase diagram for the compound of interest in this paper, Ba(Fe 1−x Co x ) 2 As 2 (3), where doping the parent compound (Co for Fe) lowers the transition temperatures and at x ∼ 2.2%, there seems to be a tricritical point beyond which the magnetic transition becomes second order (2,4). The strong spin-lattice coupling in these systems (4-7), along with the presence of a tricritical point, creates an environment where either strongly firstorder or nearly second-order phase transitions may be observed.…”

“…One of the most intriguing aspects of these new compounds is the intimate coupling between spin and lattice, offering a wonderful platform to study and manipulate their relationship. The parent compounds (no disorder induced by doping) of the 122 family [Alkaline earth (A) Fe 2 As 2 ] exhibit a coupled magnetic and structural transition from the lowtemperature (LT) antiferromagnetic orthorhombic phase to a high-temperature (HT) paramagnetic tetragonal phase (2)(3)(4), which has the signature of being first order in the bulk. Fig.…”