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Effects of disorder in iron-based superconductors

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  1. Tez No: 400957
  2. Yazar: SULTAN DEMİRDİŞ
  3. Danışmanlar: DR. CORNELIS JACOMINUS VAN DER BEEK
  4. Tez Türü: Doktora
  5. Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2012
  8. Dil: İngilizce
  9. Üniversite: Ecole Polytechnique
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 261

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Özet (Çeviri)

In order to characterize the disorder in iron-based superconductors, several experimental techniques have been used in this thesis. The first one, introduced in Chapter 4, is the magneto-optical imaging technique which allows one to identify the spatial heterogeneity of the critical current density; this cannot be neglected for an accurate interpretation of further experimental results. Using the magneto-optical imaging technique in the differential mode, maps of the magnetic flux distribution were established from the low temperature to the critical temperature Tc. This work, performed on single crystals of Ba(Fe1􀀀xCox)2As2 , Ba(Fe1􀀀xNix)2As2 , Sr(Fe1􀀀xCox)2As2 and BaFe2(As1􀀀xPx)2, allows one to also establish the spatial distribution of Tc. The guiding principle in this manuscript is the characterization of flux pinning for the identification of the type of disorder. The Bitter decoration technique, described in Chapter 5, is used for the imaging of the vortex ensemble in different compounds of the 122-type family. In Chapter 6, a study of the flux pinning properties in charge-doped Ba(Fe1􀀀xCox)2As2 is presented. The highly disordered vortex ensembles observed in the Bitter decoration images is studied using a novel data analysis technique, that takes into account the interaction of individual vortices with their neighbors. In the latter, the pinning energies and pinning forces of each vortex are extracted from distribution the of vortices at small magnetic fields. We then correlated our Bitter decoration datas with critical current measurements. The spatial heterogeneity of Tc revealed in the DMO images of the decorated samples allows one to pinpoint the origin of the amorphous vortex structure in single crystalline Ba(Fe1􀀀xCox)2As2. Heterogeneity of the superconducting properties is found to explain these ensembles, with inhomogeneity on the nm scale the origin of the strong pinning mechanism at the origin of the low-field critical current. Namely, the low-field value of the critical current density can be explained as being due to the same material inhomogeneity that gives rise to the disordered vortex configurations [46; 206]. In Chapter 7, we present a systematic study of the flux pinning, using the same data analysis procedure, in the isovalently substituted BaFe2(As1􀀀xPx)2. Vortex ensembles with less vortex density fluctuations are observed in this compound. The pinning forces and pinning energies extracted from the Bitter decoration images are analyzed. It is found that they can be analyzed to yield a mean distance between effective pins of about 90 nm, that increases when one increasing the P content x. This result is found in nice agreement with the work of Shishido et. al. on the influence of the disorder o the normal state properties of the same material. Namely, they reported that the mean free path of the orbits increases from l  20 nm to 80 nm when the P content x varies from x = 0:41 to 1 [97]. The“weak collective pinning”contribution is also considered for all studied compounds. This pinning mechanism was proposed to arise from the quasiparticle in the vortex cores; it manifests itself as a second plateau, at magnetic fields above a few tenths of a T to 1 T. In the attempt to link the flux pinning properties in this regime to the quasi-particle scattering rate, a new experimental set-up based on the cavity perturbation technique described in Chapter 8 is established for the measurements of surface resistance Rs. In addition to this, additional atomic-sized point like defects are introduced using high-energy 2.5 MeV electron irradiation with the Pelletron accelerator SIRIUS [106] of the Laboratoire des Solides Irradiés. The introduction of such atomic sized point-like defects (vacancies,interstitials) by this kind of particle irradiation is shown to, indeed, enhance the weak collective pinning contribution to the critical current, at least in Ba(Fe1􀀀xCox)2As2. In Chapter 9, we present a study on the evolution of different superconducting properties under the effect of electron irradiation for Ba(Fe1􀀀xCox)2As2, Ba(Fe1􀀀xNix)2As2, and BaFe2(As1􀀀xPx)2 single crystals. An important annealing effect (70% of defects) is revealed for Ba(Fe1􀀀xCox)2As2 when the crystals are heated to 300 K after low-temperature irradiation. The critical temperature is found to decrease similarly as function of dose for all investigated materials, namely, 5% dpa induces a decrease of Tc by approximately 30 %. From the increase of the weak collective pinning contribution, the number of defects created by the irradiation (Fe vacancies) is estimated. In isovalently substituted BaFe2(As1􀀀xPx)2, the initially absent weak collective pinning contribution appears after irradiation. The measured surface impedance for Co and Ni-doped 122 compounds before and after electron irradiation increases . These results allows one to confirm the role of the atomic sized point-like defects as scatterers in iron-based superconductors, as well as the hypothesis that these defects are at the origin of the weak collective pinning contribution to jc.

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