| abstract: |
The superconductivity in LaFeAsOF with transition temperature (Tc) ∼ 26 K was discovered by Kamihara et al in 2008 [1]. After the discovery, several types of iron-pnictide superconductors were found, having similar Fe and pnictide layers, such as Ba(Fe,Co)2As2, LiFeAs and Fe(Se,Te). A par- ent compound BaFe2As2 shows structural transition from tetragonal to or- thorhombic symmetry and antiferromagnetic transition near T = 140 K [2,3]. By a variety of the parameters as electron- or hole-doping, pressure and iso- valent element substitution, the two transition are suppressed and supercon- ductivity is induced [4]. In the almost all iron-pnictides superconductors, an antiferromagnetic phase is adjacent to the superconducting phase as in BaFe2As2. The high Tc superconducting mechanism is now under debate. The conventional theory of phonon-mediated superconductivity has difficulty to explain the higher superconducting transition temperatures [5–7]. It was proposed that the pairing interaction originates from antiferromagnetic spin fluctuations [6,8]. The orbital fluctuations [9–12], excitons [13,14] are other candidates of the intermediate bosons as well as spin fluctuations. The bal- ance of the bosons will determine if the system takes the superconductivity, structural distortion or antiferromagnetic order as the ground state. There- fore, the detail information of the magnetic correlations will help to illuminate the superconducting mechanism. We focused the doping dependence of the magnetic correlations in BaFe2As2 one of the iron pnictides by the neutron scattering. The study consists of three topics: 1) Doping dependence of the magnetic fluctuations in the normal state of the Ba(Fe, Co)2 As2 2) Doping dependence of the in-plane anisotropy of the magnetic correlations in the normal state of the Ba(Fe, Co)2As2 3) Doping dependence of the static mag- netic moment size and structure in the newly discovered “isovalent” doped BaFe2(As1−x Px )2. The three doping dependence study captured the marked contribution of orbital character to the magnetic correlations.
In chapter 3, we investigated the doping dependence of magnetic fluctu- ations in the normal state of Ba(Fe,Co)2As2. In optimally electron-doped superconductor Ba(Fe0.94Co0.06)2As2, it was found that magnetic fluctuations are enhanced in the superconducting phase [15], which agree with the the- ory of the superconductivity mediated by antiferromagnetic fluctuations [16]. However, the enhancement is also expected also for the superconducting state mediated by orbital fluctuations [17]. Thus, measuring magnetic correlations only in the superconducting state is not adequate to distinguish the glue of the pairing. Then, we investigated spin dynamics in single crystals by inelas- tic neutron scattering over the composition range from undoped to the over- doped regime in electron-doped Ba(Fe1−x Cox )2As2 (x = 0, 0.06 and 0.24). Damped magnetic fluctuations in the paramagnetic state of the parent com- pound x = 0 share a remarkable similarity with those in the normal state of the optimally doped compound x = 0.06. Moreover, the vanishment of the magnetic correlations in the heavily overdoped compound x = 0.24, in which superconductivity disappears, was confirmed. Having the disappearance of the hole pockets in the heavily overdoped regime in mind, these results sug- gest that the magnetic correlations in the former two compounds originates from Fermi surface nesting between hole and electron pockets. Secondly, we found that the magnetic fluctuations do not slow down sufficiently before the antiferromagnetic order in the parent compound. This indicates that the magnetic order is not directly driven by the spin-spin interaction, but by an interaction with spin and another degrees of freedom. In chapter 4, we focused the in-plane anisotropy of magnetic correlations in Ba(Fe,Co)2As2. In Ba(Fe0.935Co0.065)2As2, the low energy antiferromag- netic spin correlations measured by neutron scattering [18] shows different in-plane peak widths in the transverse and longitudinal directions even in the tetragonal normal state. Together with the anisotropic behavior observed in electronic resistivity, elastic constant, SI-STM, etc. [19], the anisotropic spin fluctuation spectra were sometimes referred to as an outcome of spontaneous rotational symmetry breaking into C2 due to spin or electronic nematicity. On the other hand, it was suggested [20, 21] that a proper Fermi surface nesting calculation with multiband character of the bands can reproduce the spectra without taking account of the extra symmetry breaking. It was predicted that the in-plane distortion is smaller in the parent compound BaFe2As2. This is in striking contrast to the larger distortion expected for the nematic order. To clarify the origin of the in-plane anisotropy, we in- vestigated the doping dependence of the low energy spin fluctuations in the wide composition of Ba(Fe1−x Cox )2As2 (x = 0, 0.045 and 0.06) crystals. The anisotropy was investigated by the longitudinal (Q // (1, 1, 0)) and trans- verse (Q // (-1, 1, 0)) scans through the inelastic magnetic peak at Q = (1/2, 1/2, 0) and the energy transfer of h ̄ω = 10 meV, and at Q = (3/2, 3/2, 0) and h ̄ω = 28 meV in the normal states. From the measurements, three characteristic results were obtained: a) The distortion is larger in the doped (x = 0.045 and 0.06) compounds at T = 180 K than the parent compound (x = 0) b) The distortion is larger at higher energies. c) The distortion is only weakly temperature dependent and is preserved even at the high tem- perature T = 300 K. a) and c) support the Fermi surface nesting picture with multiband character of the bands. b) would be explained by the same picture. The results indicate that the in-plane distortion of the spin fluctua- tions are well explained by the Fermi surface nesting picture with multiband character of the electronic bands. In chapter 5, we measured the doping dependence of the static mag- netic moment size and structure in the newly discovered isovalent doped BaFe2(As1−x Px )2 by the powder Xray diffraction and neutron diffraction to reveal the relationship between the suppression of magnetic order and struc- tural change without changing the number of the electric charge. The lattice constants become small with P doping. The static moment size is suppressed by doping and no moment was found in the x = 0.35 superconducting sam- ple. Considered with the fact that the uniaxiality of physical pressure along c assists the suppression of the magnetic order and inducing the superconduc- tivity, the shrink of c may be a key when the number of the electric charge is fixed. This implies that 3d orbitals of yz and zx, which have the character of the l dispersion, is responsible for the magnetic order and superconductivity.
By focusing on the difference between the samples with various doping levels, antiferromagnetic correlations in the iron pnictide BaFe2As2 become clear by neutron scattering technique. While the origin of the superconduc- tivity was not elucidated, our study emphasised that the multiband character of the bands cannot be ignored in the magnetic correlations. The multiband character may play a key role also in the paring interaction of the supercon- ductivity. |