Browsing by Author "Park, Jitae"

Filter results by typing the first few letters
Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    Spin dynamics in 122-type iron-based superconductors
    (2012) Park, Jitae; Keimer, Bernhard (Prof. Dr.)
    In this thesis, we present the experimental data on four different iron-based SC materials. It is mainly about the magnetic-dynamics study in the FeSC that is assumed to be among the most crucial ingredients for superconductivity in this system. Thus, the main goal of this thesis is to figure out the exact relationship between spin dynamics and superconductivity, and then further to realize what is the contribution of magnetic fluctuations for superconductivity by providing experimental data for modeling a microscopic mechanism of electron pairing in the FeSC system. In Chap. 2, we first discuss basic characteristics of FeSC, such as crystal structure and electron band-structure by briefly reviewing the relevant literature. Then, an introduction about magnetic and SC phases will follow based on the generic phase diagram. Details about current understanding of magnetic ground state in the parent compounds will be discussed in terms of spin-wave excitations which would be important when we are considering the spin dynamics in doped materials. To study magnetic dynamics in FeSC, we employed the inelastic-neutron-scattering (INS) method which can uniquely probe the underlying spin dynamics in the four dimensional energy and momentum space in a wide range. By taking advantage of the well developed theory for the magnetic neutron-scattering process, one can quantify the imaginary part of spin susceptibility that is an essential physical quantity the description of elementary magnetic excitations and can be compared with theoretical calculations directly. Moreover, the technique’s energy-resolving scale spans over the most relevant energy range of magnetic fluctuations (from 0 to 100 meV). For these reasons, neutron scattering is a very powerful technique for magnetism study, and we introduce how neutron-scattering experiment works theoretically and practically in Chap. 3. For a slightly underdoped Ba1-xKxFe2As2 compound, we report the phase separation between magnetically ordered and SC phases at low temperatures, which was confirmed by complementary experimental techniques such as neutron and X-ray scattering, muon-spin relaxation, and magnetic-force microscopy measurements. Based on our experimental data, we discuss the possibility of this phase separation being an intrinsic property of the Ba1-xKxFe2As2 system. However, this view has been recently challenged by several new measurements performed on the next generation of single crystals, which apparently exhibit a much more homogeneous behavior. These results are presented and discussed in Chap. 4. The most prominent feature in the spin-excitation spectrum of the SC state is the magnetic resonant mode that is characterized as spin-1 excitonic collective mode below the edge of the particle-hole continuum. Our experimental observations of magnetic resonant modes in BaFe1.85Co0.15As2, BaFe1.91Ni0.09As2, and Rb0.8Fe1.6Se2 compounds will be presented and a discussion about their physical implications will follow in Chap. 4. In addition, we will show that the temperature-dependent resonance energy displays an order-parameter-like behavior in the same manner as the SC energy gap that is expected within the conventional Fermi-liquid approaches for the magnetic resonant mode. As most theories of superconductivity are based on a pairing boson of sufficient spectral weight in the normal state, detailed knowledge of the spin-excitation spectrum above the SC transition temperature is fundamentally required to assess the viability of magnetically mediated Cooper pairing. Thus, in Chap. 4, we present the results of normal-state spin-fluctuation spectra in absolute units and find that the normal-state spectrum carries a weight comparable to that in the underdoped cuprates, while the spectrum agrees well with predictions of the theory of nearly antiferromagnetic metals. In the following, we show that the first-principles calculations can remarkably well reproduce our INS data, especially for anisotropic shape of in-plane spin fluctuations, implying that the spin dynamics for paramagnetic state in this system can be well described within the itinerant approach. Finally, in Chap. 5, we collect all the reported resonant mode data in various materials and doping levels of FeSC, and compare them after putting in the same plot. A linear relation between resonance energy and Tc is realized with a ratio of ωres/kBTc ≈ 4.8, which is slightly lower than the respective value for cuprates. A certain correlation between the resonance energy and SC energy gap is also found, and its physical implications will be further discussed.