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

Abstract

In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe 1−x Co x ) 2 As 2 , through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron-phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type, equivalent to what is observed in the bulk by applying a uniaxial pressure. Such equivalence indicates that the surface mirrors the bulk under extreme conditions. spin-lattice coupling | structure transition | magnetic ordering | surface versus bulk T he recent discovery of high-temperature superconductivity in layered iron-based compounds (1) has created enormous activity in the scientific community. 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 (2-4), which has the signature of being first order in the bulk. 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. Creating a surface by cleaving these layered materials is a controlled way to tip the balance between competing phases, thus providing a unique opportunity to study the subtle aspects of the interactions between lattice and spin through charge. Important for this study is the fac...

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“…Recent observations suggest that alternative mechanisms may also contribute to the numeral integrity and may be context-specific [65]–[66]. Although we cannot formally exclude that the centrioles do indeed duplicate during the endo-S but simply cannot be resolved due to their proximity or tight engagement, that we were able to observe supernumerary centrioles in the SPD-2 S545E variant would suggest that this residue may be a target of the licensing mechanism per se in the intestinal cells.…”

Role of Antiferromagnetic Ordering in the (1×2) Surface Reconstruction ofCa(Fe1−xLi1, Liang2, Li3 et al. 2014Phys. Rev. Lett.Self Cite7260View full textAdd to dashboardCite

“…The discovery of high-temperature superconductivity (SC) in layered Fe-based compounds has brought to the attention of materials community a very interesting class of materials [1][2][3][4][5][6]. One of the most intriguing aspects of these materials is the coupling of structure, magnetism, and superconductivity.…”

“…For the AFe 2 As 2 materials, the presence of a surface stabilizes the orthorhombic phase [2,12,13] and presumably, the AFM ordering. For example, the surface of BaðFe 1−x Co x Þ 2 As 2 exhibits a higher structural transition temperature than observed in the bulk [2], opposite to the effect of hydrostatic pressure to the bulk [8].…”

Coupled structural and magnetic antiphase domain walls on BaFe2As2

“…This is further supported by the recent measurements of the temperature dependence of the phonon modes at the surface of BaFe 2 As 2 , which display a gigantic enhancement in the spin/lattice coupling in the low temperature AFM orthorhombic phase. 27 …”