51 research outputs found
Reciprocal transmittances and reflectances: An elementary proof
We present an elementary proof concerning reciprocal transmittances and
reflectances. The proof is direct, simple, and valid for the diverse objects
that can be absorptive and induce diffraction and scattering, as long as the
objects respond linearly and locally to electromagnetic waves. The proof
enables students who understand the basics of classical electromagnetics to
grasp the physical basis of reciprocal optical responses. In addition, we show
an example to demonstrate reciprocal response numerically and experimentally.Comment: 6 pages, 5 figures. RevTEX4. Improved wording. Physics Educatio
Theory of Josephson current on a lattice model of grain boundary in -wave superconductors
Identifying the origins of suppression of the critical current at grain
boundaries of high-critical-temperature superconductors, such as cuprates and
iron-based superconductors, is a crucial issue to be solved for future
applications with polycrystalline materials.
Although the dominant factor of current suppression might arise during
material fabrication and/or processing, investigating it due to an internal
phase change of the pair potential is an important issue in understanding the
threshold of the critical current.
In this paper, we study the Josephson current on a symmetric [001]-tilt grain
boundary (GB) of a -wave superconductor on a lattice model.
In addition to the suppression of the maximum Josephson current associated
with the internal phase change of the -wave pair potential which has been
predicted in continuum models, we find a unique phase interference effect due
to folding of the Fermi surface in the lattice model.
In particular, the resultant maximum Josephson current at low-tilting-angle
regions tends to be suppressed more than that in preexisting theories.
Because similar suppressions of the critical current at GBs have been
reported in several experimental works, the present model can serve as a guide
to clarify the complicated transport mechanism in GBs
Ambipolar suppression of superconductivity by ionic gating in optimally-doped BaFe2(As,P)2 ultrathin films
Superconductivity (SC) in the Ba-122 family of iron-based compounds can be
controlled by aliovalent or isovalent substitutions, applied external pressure,
and strain, the combined effects of which are sometimes studied within the same
sample. Most often, the result is limited to a shift of the SC dome to
different doping values. In a few cases, the maximum SC transition at optimal
doping can also be enhanced. In this work, we study the combination of charge
doping together with isovalent P substitution and strain by performing ionic
gating experiments on BaFe(AsP) ultrathin films. We
show that the polarization of the ionic gate induces modulations to the
normal-state transport properties that can be mainly ascribed to surface charge
doping. We demonstrate that ionic gating can only shift the system away from
the optimal conditions, as the SC transition temperature is suppressed by both
electron and hole doping. We also observe a broadening of the resistive
transition, which suggests that the SC order parameter is modulated
nonhomogeneously across the film thickness, in contrast with earlier reports on
charge-doped standard BCS superconductors and cuprates.Comment: 10 pages, 5 figure
Nanoscale Texture and Microstructure in a NdFeAs(O,F)/IBAD-MgO Superconducting Thin Film with Superior Critical Current Properties
This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y2O3/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the ab-plane as well as grain boundaries perpendicular to the ab-plane, good superconducting properties are measured: a critical temperature, T-c, of 46 K and a self-field critical current density, J(c), of 2 x 10(6) A/cm(2) at 4.2 K. Automated crystal orientation mapping by scanning precession electron diffraction in transmission electron microscope is employed to analyze the misorientation angles between adjacent grains in a large ensemble (247 grains), and 99% of the grain boundaries show in-plane misorientation angles (Delta gamma) less than the critical angle theta(c), which satisfies one of the necessary conditions for the high J(c). Comparing the columnar grain size distribution with the mean distance of the flux line lattice, the triple junctions of low-angle grain boundaries are found to be effective pinning centers, even at high temperatures (>= 35 K) and/or low magnetic fields
High J and low anisotropy of hydrogen doped NdFeAsO superconducting thin film
The recent realisations of hydrogen doped LnFeAsO (Ln = Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density J. The superconducting transition temperature T is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field J of over 10 MA/cm was recorded at 5 K, which is likely to be caused by a short London penetration depth. The anisotropic Ginzburg–Landau scaling for the angle dependence of J yielded temperature-dependent scaling parameters γ that decreased from 1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F). Additionally, γ of NdFeAs(O,H) is smaller than that of NdFeAs(O,F). Our results indicate that heavily electron doping by means of hydrogen substitution for oxygen in LnFeAsO is highly beneficial for achieving high J with low anisotropy without compromising T, which is favourable for high-field magnet applications
Decoupling of critical temperature and superconducting gaps in irradiated films of a Fe-based superconductor
Microstructure, pinning properties, and aging of CSD-grown SmBaCuO films with and without BaHfO nanoparticles
In order to improve the electrical transport properties of REBaCuO nanocomposite films, SmBaCuO films with and without BaHfO nanoparticles were grown by chemical solution deposition, and their microstructural and transport properties were investigated in a detailed study using transmission electron microscopy and transport measurements in magnetic fields up to 24 T. The optimization process of the crystallization step (temperature and oxygen partial pressure) as well as an aging effect, which is due to the release of trapped fluorine, are described. Critical temperature and critical current densities surprisingly improve initially during the aging. Due to the complex microstructure, the additional BaHfO3 nanoparticles have only a positive effect at low magnetic fields for our samples
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