Two-dimensional resonant magnetic excitation inBaFe1.84Co0.16As2

Abstract

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC . This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.PACS numbers: 78.70.Nx, 74.20.Mn Understanding the physics of superconductivity in high-T c cuprates and other unconventional superconductors remains a central unresolved problem at the forefront of condensed matter physics. One widespread school of thought maintains that magnetic fluctuations are intimately involved in the pairing mechanism. This view is supported by a growing number of neutron scattering investigations showing the appearance of a magnetic excitation coincident with the onset of superconductivity [1,2,3,4,5,6,7,8]. The spectrum shows a resonance at a wavevector related to the antiferromagnetic order in the non-superconducting parent compounds. The apparent resonance energy scales with T C for different cuprate materials exhibiting a wide range of superconducting transition temperatures [9], providing tantalizing evidence for a common mechanism related to magnetic fluctuations.The discovery of a new family of Fe-based high temperature superconductors with T C as high as 55 K [10,11,12,13,14,15,16] presents an exciting opportunity to examine the relationship of spin excitations to the superconducting condensate in unconventional superconductors. The new materials are composed of Fe containing planes (FeAs or FeSe). Both theory and experiment indicate that simple electron-phonon coupling cannot describe superconductivity in these materials [17,18]. Furthermore, the superconducting state exists in close proximity to magnetism as the parent compounds exhibit spin-density wave order [19,20]. These observations have been put forth as evidence that the superconductivity in the Fe-based materials is unconventional. The presence of the Fe planes suggests quasi-two-dimensionality, as observed in the cuprates. However, neutron scattering investigations of the spin waves in the undoped parent compounds SrFe 2 As 2 [21], BaFe 2 As 2 [22], and CaFe 2 As 2 [23], indicate anisotropic exchange that cannot be classified as two dimensional. Band structure calculations [24,25] indicate that doping should enhance the twodimensionality of the Fermi surface, favoring superconductivity [25]. Directly probing the magnetic fluctuations in superconducting Fe-based systems is crucial for further progress.Recent measurements on a polycrystalline sample of Ba 0.6 K 0.4 Fe 2 As 2 found a spin excitation that appears at the onset...

Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

“…In AFe 2 As 2 (A=Ba, Ca and Sr) electron/hole doping or pressure suppresses these phase transitions and the related magnetic ordering and induces superconductivity through electron pairing at lower temperatures, supporting the idea of a possible coupling between spin degrees of freedom and superconductivity in the iron pnictides. Inelastic neutron scattering measurements have evidenced a resonant spin excitation in polycrystalline Ba 0.6 K 0.4 Fe 2 As 2 [12] as well as in single crystals of Ba(Fe 0.92 Co 0.08 ) 2 As 2 [13] and BaFe 1.9 Ni 0.10 As 2 [14]. Recent experiments have established that the role of phonons in the electron pairing mechanism cannot be entirely discarded.…”

Anisotropic neutron spin resonance in superconductingBaFe1.9Ni0.1As2

“…[1][2][3][4] Since its initial discovery in optimal hole-doped YBa 2 Cu 3 O 6+x , [1][2][3][4] the resonance has been found in electron-doped cuprates, 5 heavy fermion, 6,7 and iron arsenide superconductors. [8][9][10][11][12][13] Below the superconducting transition temperature T c , the intensity of the resonance increases like the superconducting order parameter and its energy scales with T c . 5 Although the resonance appears to be a ubiquitous property of unconventional superconductors, [1][2][3][4][5][6][7][8][9][10][11][12] its microscopic origin and relationship with superconductivity are still debated.…”

“…[8][9][10][11][12][13] Below the superconducting transition temperature T c , the intensity of the resonance increases like the superconducting order parameter and its energy scales with T c . 5 Although the resonance appears to be a ubiquitous property of unconventional superconductors, [1][2][3][4][5][6][7][8][9][10][11][12] its microscopic origin and relationship with superconductivity are still debated. 14 In all these materials, the resonance occurs at the antiferromagnetic ͑AF͒ wave vector Q of the parent compound.…”

Absence of localized-spin magnetism in the narrow-gap semiconductor FeSb2

“…[16][17][18] Many of the unique electronic properties which are of great current and future technological interest are intertwined with the emerging magnetism in these materials. Recent discovery of the new variety of the high-temperature superconductivity in layered transition metal pnictides, which is also closely related with magnetism, [19][20][21][22][23][24][25] has generated new surge of interest in the physical properties of these compounds.…”