Roles of intrinsic anisotropy and π-band pairbreaking effects on critical currents in tilted-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>c</mml:mi></mml:math>-axis<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Mg</mml:mi><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>films probed by magneto-optical and transport measurements

NdFeAs(O,F) thin films having different fluorine contents were grown on 5 • or 10 • vicinal cut MgO and CaF 2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis (ρ ab and ρ c ) were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than ρ ab , resulting from the anisotropic electronic structure. The resistivity anisotropy γ ρ = ρ c /ρ ab at 300 K was almost constant in the range of 50 ≤ γ ρ ≤ 90 irrespective of the fluorine content. On the other hand, γ ρ at 56 K showed a strong fluorine dependence: γ ρ was over 200 for the films with optimum fluorine contents (superconducting transition temperature T c around 50 K), whereas γ ρ was around 70 for the films in the under-doped regime (T c between 35 and 45 K). The mass anisotropy γ m = m * c /m * ab (m * c and m * ab are the effective masses along the c-axis and on the ab-plane) close to T c derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of ρ ab was in the range from 2 to 5. On the assumption γ 2 m = γ ρ , those values are small compared to the normal state anisotropy. arXiv:2003.09105v1 [cond-mat.supr-con]

Section: Introductionmentioning

confidence: 99%

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Iida¹,

Matsumoto²,

Kondo³

et al. 2020

“…Nonetheless, previously published works on iron-based superconducting thin films only deal with films on plane-cut (001)-oriented substrates; the growth on vicinal cut substrates has to the best of our knowledge not been studied. For the cuprate-based superconductors YBa 2 Cu 3 O 7 (YBCO) [10], Bi 2 Sr 2 CaCu 2 O 8 [11], Bi 2 Sr 2 Ca 2 Cu 3 O 10 [12], and HgBa 2 CaCu 2 O 6 [13] as well as MgB 2 [14], it has already been shown that vicinal cut substrates open access to many new experimental opportunities. By breaking the symmetry between substrate normal and crystallographic c-axis, it is possible to analyze the transport properties at a specific angle to the crystal lattice rather than just parallel to one of the crystallographic main axes, and therefore a new way to investigate electrical anisotropies is enabled.…”

Section: Introductionmentioning

confidence: 99%

Deposition and properties of Fe(Se,Te) thin films on vicinal CaF₂substrates

Bryja

Hühne

Iida

et al. 2017

We report on the growth of epitaxial Fe 1+δ Se 0.5 Te 0.5 thin films on 0°, 5°, 10°, 15°and 20°v icinal cut CaF 2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe 1+δ Se 0.5 Te 0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab-and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for H||ab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

“…Multi-band transport, connectivity, and grain boundary flux pinning are known major current limiting mechanisms in polycrystalline MgB 2 . First, in randomly oriented polycrystalline forms of MgB 2 , the different dimensionality of the σ and π bands should affect the intergranular current transport [3,26] but has not been well clarified yet. Second, in addition to intrinsic anisotropic multiband transport, current percolation due to electromagnetic anisotropy [27] and structural defects [28] restricts the macroscopic current transport, which is known as the connectivity problem [29].…”

Section: Introductionmentioning

confidence: 99%

Enhanced trapped field in MgB₂bulk magnets by tuning grain boundary pinning through milling

Sugino

Yamamoto

Shimoyama

et al. 2015

The effects of the grain size, lattice strain, and microstructure on the trapped field properties of ball-milled MgB 2 bulks were studied. A decrease in the in-plane lattice parameters and transition temperature and an increase in the in-plane x-ray diffraction peak widths occurred with milling as a result of grain refinement and carbon substitution. Microstructural analysis revealed that sub-micron-size fine MgB 2 grains were well connected, resulting in increased grain boundary density. The trapped field of the magnetized bulk samples exhibited a large improvement with milling and reached 3.72 T at 5 K, which is the highest among MgB 2 bulks prepared using pressureless synthesis. Excess milling resulted in degradation of the trapped field because of T c reduction. The increased grain boundary density and electron scattering are considered to contribute to the high trapped field through quantitatively and qualitatively enhancing the grain boundary flux pinning.