ترغب بنشر مسار تعليمي؟ اضغط هنا

Local lattice distortions vs. structural phase transition in NdFeAsO1-xFx

119   0   0.0 ( 0 )
 نشر من قبل Maria Calamiotou
 تاريخ النشر 2015
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

The lattice properties at low temperatures of two samples of NdFeAsO1-xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superconducting sample from synchrotron XRD measurements. On the other hand, the atomic displacement parameters deviate from the expected behavior, in agreement with modifications in the phonon width, as obtained by Raman scattering. These deviations occur around 150 K for both F dopings, with distinct differences among the two compounds, i.e., they decrease at low doping and increase for the superconducting sample. The data do not support a hidden phase transition to an orthorhombic phase in the superconducting compound, but point to an isostructural lattice deformation. Based on the absence of magnetic effects in this temperature range for the superconducting sample, we attribute the observed lattice anomalies to the formation of local lattice distortions that, being screened by the carriers, can only acquire long-range coherence by means of a structural phase transition at low doping levels.



قيم البحث

اقرأ أيضاً

We have studied the structural and electronic phase diagrams of CeFeAsO1-xFx and SmFeAsO1-xFx by a detailed analysis of muon spin relaxation experiments, synchrotron X-ray diffraction, Mossbauer spectroscopy, electrical resistivity, specific heat, an d magnetic susceptibility measurements (Full abstract in the main document).
Local structure of NdFeAsO$_{1-x}$F$_{x}$ ($x$=0.0, 0.05, 0.15 and 0.18) high temperature iron pnictide superconductor system is studied using arsenic $K$-edge extended x-ray absorption fine structure measurements as a function of temperature. Fe-As bondlength shows only a weak temperature and F-substitution dependence, consistent with the strong covalent nature of this bond. The temperature dependence of the mean-square relative-displacements of the Fe-As bondlength are well described by the correlated-Einstein model for all the samples, but with different Einstein-temperatures for the superconducting and non-superconducting samples. The results indicate distinct local Fe-As lattice dynamics in the superconducting and non-superconducting iron-pnictide systems.
Electronic functionalities in materials from silicon to transition metal oxides are to a large extent controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their hig h superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the glue binding the ordered regions together. Here we use scanning X-ray microdiffraction (with beam 300 nm in diameter) to show that for La2CuO4+y, the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials.
269 - U. Welp , R. Xie , A. E. Koshelev 2008
We present heat capacity measurements of the upper critical fields of single-crystal NdFeAsO1-xFx. In zero-magnetic field a clear step in the heat capacity is observed at Tc = 47K . In fields applied perpendicular to the FeAs-layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO2-high-Tc superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical field slopes of dHc2,a = -0.72 T/K and dHc2,ab = -3.1 T/K. Zero-temperature coherence lengths of xiab = 3.7 nm and xic = 0.9 nm and a modest superconducting anisotropy of gamma ~ 4 can be deduced in a single-band model.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا