Thermal activation energy of 3D vortex matter in NaFe1−x Co x As (x = 0.01, 0.03 and 0.07) single crystals

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We investigate changes in the vortex pinning mechanism caused by proton irradiation through the measurement of the in-plane electrical resistivity for H//c in a pristine and two proton-irradiated (total doses of 1 × 1015 and 1 × 1016 cm−2) SmBa2Cu3O7-δ (SmBCO) superconducting tapes. Even though proton irradiation has no effect on the critical temperature (Tc), the resulting artificial point defect causes an increase in normal state electrical resistivity. The electrical resistivity data around Tc shows no evidence of a phase transition to the vortex glass state but only broadens with increasing magnetic field due to the vortex depinning in the vortex liquid state. The vortex depinning is well interpreted by a thermally activated flux flow model in which the activation energy shows a nonlinear temperature change $${\boldsymbol{U}}{\boldsymbol{(}}{\boldsymbol{T}},{\boldsymbol{H}}{\boldsymbol{)}}{\boldsymbol{=}}{{\boldsymbol{U}}}_{{\boldsymbol{0}}}{\boldsymbol{(}}{\boldsymbol{H}}{\boldsymbol{)}}{{\boldsymbol{(}}{\bf{1}}-{\boldsymbol{T}}{\boldsymbol{/}}{{\boldsymbol{T}}}_{{\boldsymbol{c}}}{\boldsymbol{)}}}^{{\boldsymbol{q}}}$$U(T,H)=U0(H)(1−T/Tc)q (q = 2). The field dependence of activation energy shows a $${{\boldsymbol{U}}}_{{\bf{0}}}{\boldsymbol{ \sim }}{{\boldsymbol{H}}}^{-{\boldsymbol{\alpha }}}$$U0~H−α with larger exponents above 4 T. This field dependence is mainly due to correlated disorders in pristine sample and artificially created point defects in irradiated samples. Compared with the vortex pinning due to correlated disorders, the vortex pinning due to the appropriate amount of point defects reduces the magnitude of Uo(H) in the low magnetic field region and slowly reduces Uo(H) in high magnetic fields.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Effect of the proton irradiation on the thermally activated flux flow in superconducting SmBCO coated conductors

Choi

Ahmad

Seo

et al. 2020

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“…For magnetic fields perpendicular to the ab layers it readswhere is the polygamma function, , is so called Lawrence-Doniach parameter, S is the interlayer distance, and c is the total-energy cutoff constant that corresponds to ε -value at which Δ σ vanishes. To compare with the experimental data, we used T c = 16.3 K (as determined from the analysis of the critical region), c = 0.5 (according to the ε -value at which at which fluctuation effects vanish), and s = 0.74 nm 42 . As it is shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…After completion of heat treatment, typical dimensions of as-grown single crystals are 0.1 × 1 × 2 mm 3 for almost each of the three series of samples. We used single crystals from the same batches used in our recent paper 42 . During this study, XRD analyses revealed well defined (0 0 l ) peaks with FWHM of about 0.05°.…”

Section: Methodsmentioning

confidence: 99%

Anisotropy dependence of the fluctuation spectroscopy in the critical and gaussian regimes in superconducting NaFe1−xCo x As single crystals

Ahmad

Choi

Sóñora

et al. 2018

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We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1−xCoxAs single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x = 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x ≥ 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around Tc(H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above Tc may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level.

“…Following the treatment for different kinds of superconductors 9 , 41 – 43 the ac susceptibility data gives us the vortex activation barrier within the framework of the thermally activated vortex motion model. The insets to Fig.…”

Section: Discussionmentioning

confidence: 99%

The morphologic correlation between vortex transformation and upper critical field line in opal-based nanocomposites

Lee

Charnaya

Mühlbauer

et al. 2021

In this study, we investigate metallic nanocomposites to elucidate the properties of nanostructured conventional superconductors. Liquid tin, indium, and mercury are loaded into opal matrices by high pressure up to 10 kbar. The opal templates preserve the 3D dendritic morphology of confined superconducting metals to model a dendritic second phase with particular grain shape in bulk superconductors observed by a DualBeam microscope. We carry out measurements of the dc and ac magnetizations to study the superconducting phase diagrams, vortex dynamics, and impact of grain morphology in the opal composites. Besides, we apply the small-angle neutron scattering (SANS) to deny a regular vortex structure. The phase diagrams reveal an enhanced upper critical field Hc2(0) and curvature crossover in the upper critical field line. We also calculate the vortex activation barriers Ua and observe a transformation in the vortex system. According to the field dependence of Ua, the vortex structure transformation highly correlates with the curvature crossover in the upper critical field line. Our observations suggest that the similarity in the normalized phase diagrams and field dependences of Ua in the three nanocomposites is owing to their particular morphology of confinement.