1,721,019 research outputs found
Smooth surfaces in very thin GdBa2Cu3O7 - δ films for application in superconducting tunnel junctions
No full textThis paper provides a systematic analysis of the morphology and the superconducting critical temperature
obtained in very thin GdBa2Cu3O7d films grown on (001) SrTiO3 substrates by DC sputtering. We
find that the use of a very thin SrTiO3 buffer layer (2 nm) modify the nucleation of GdBa2Cu3O7d on
the surface of the substrate reducing the formation of 3 dimensional clusters. Our results demonstrate
that 16 nm thick GdBa2Cu3O7d films with an average root-mean-square (RMS) smaller than 1 nm and
large surface areas (up 10 lm2) free of 3 dimensional topological defects can be obtained. In films thinner
than 24 nm the onset (zero resistance) of superconducting transition of the films is reduced, being close
to liquid nitrogen. This fact can be associated with stress reducing the orthorhombicity and slightly drop
in oxygen stoichiometry.1331sciescopu
Characterization of the insulator barrier and the superconducting transition temperature in GdBa2Cu3O72d/BaTiO3 bilayers for application in tunnel junctions
Full text linkThe optimization of the superconducting properties in a bottom electrode and the quality of an
insulator barrier are the first steps in the development of superconductor/insulator/superconductor
tunnel junctions. Here, we study the quality of a BaTiO3 tunnel barrier deposited on a 16 nm thick
GdBa2Cu3O7d thin film by using conductive atomic force microscopy. We find that the tunnel
current is systematically reduced (for equal applied voltage) by increasing the BaTiO3 barrier
thickness between 1.6 and 4 nm. The BaTiO3 layers present an energy barrier of 1.2 eV and an
attenuation length of 0.35.0.5 nm (depending on the applied voltage). The GdBa2Cu3O7d
electrode is totally covered by a BaTiO3 thickness above 3 nm. The presence of ferroelectricity was
verified by piezoresponse force microscopy for a 4 nm thick BaTiO3 top layer. The
superconducting transition temperature of the bilayers is systematically suppressed by increasing
the BaTiO3 thickness. This fact can be associated with stress at the interface and a reduction of the
orthorhombicity of the GdBa2Cu3O7d. The reduction in the orthorhombicity is expected by
considering the interface mismatch and it can also be affected by reduced oxygen stoichiometry
(poor oxygen diffusion across the BaTiO3 barrier). © 2015 AIP Publishing LLC1221sciescopu
Critical current densities and flux creep rates in near optimally doped BaFe2-xRuxAs2 (x≈0.7) single crystals
No full textWe present an investigation of the critical current densities Jc and flux creep rates in a near optimally doped BaFe2-xRuxAs2 (x≈0.7) single crystal by (measuring magnetization). The superconducting critical temperature is 18 K. The in-field dependences of the critical current density Jc are due to a mixed pinning scenario produced mainly by large precipitates and a less significant contribution of random disorder. Furthermore, a Maley analysis in the regime dominated by strong pinning centers (μ0H=0.1 T) is well described through a glassy exponent μ=1.9 and a collective pinning energy (U0) smaller than 100 K. © 2016 Elsevier Ltd1231sciescopu
Two-parameter scaling theory of the longitudinal magnetoconductivity in a Weyl metal phase: Chiral anomaly, weak disorder, and finite temperature
Full text linkIt is at the heart of modern condensed matter physics to investigate the role of a topological structure in anomalous transport phenomena. In particular, chiral anomaly turns out to be the underlying mechanism for the negative longitudinal magnetoresistivity in a Weyl metal phase. The existence of a dissipationless current channel causes enhancement of electric currents along the direction of a pair of Weyl points or applied magnetic fields (B). However, temperature (T) dependence of the negative longitudinal magnetoresistivity has not been understood yet in the presence of disorder scattering since it is not clear at all how to introduce effects of disorder scattering into the topological-in-origin transport coefficient at finite temperatures. The calculation based on the Kubo formula of the current-current correlation function is simply not known for this anomalous transport coefficient. Combining the renormalization group analysis with the Boltzmann transport theory to encode the chiral anomaly, we reveal how disorder scattering renormalizes the distance between a pair of Weyl points and such a renormalization effect modifies the topological-in-origin transport coefficient at finite temperatures. As a result, we find breakdown of B/T scaling, given by B/T1+η with 0<η<1. This breakdown may be regarded to be a fingerprint of the interplay between disorder scattering and topological structure in a Weyl metal phase. © 2016 American Physical Society1441sciescopu
Superconducting properties in heavily overdoped Ba(Fe0.86Co0.14)2As2 single crystals
No full textIn this work we report the influence of intrinsic superconducting parameters on the vortex
dynamics in an overdoped Ba(Fe1-xCox)2As2 (x=0.14) single crystal. We find a superconducting
critical temperature of 13.5 K, magnetic penetration depth λab (0) = 660 ± 50 nm, coherence
length ξab (0) = 5 nm, and the upper critical field anisotropy γT→Tc . 3.7. In fact, the Ginzburg-
Landau model may explain the angular dependent Hc2 for this anisotropic three-dimensional
superconductor. The vortex phase diagram, in comparison with the optimally doped compound,
presents a narrow collective creep regime. In addition, we found no sign of correlated pinning
along the c axis. Our results show that vortex core to defect size ratio and λ play an important role
in the resulting vortex dynamics in materials with similar intrinsic thermal fluctuations.1111sciescopu
Upper critical magnetic field and vortex-free state in very thin epitaxial δ-MoN films grown by polymer-assisted deposition
No full textWe measured the thickness dependence of the superconducting properties in epitaxial δ-MoN thin films grown on α-Al2O3 (001) substrates by polymer-assisted deposition. Our results indicate that the superconducting properties such as the upper critical field (μ 0Hc2 ≈ 10 T) and the superconducting critical temperature (Tc = 12.5 K) are thickness independent for films thicker than ∼36 nm. By measuring the critical current density (Jc) in the vortex-free state, which coincides with the depairing current density (J0), we estimate that films thicker than ∼36 nm have a coherence length ξ(0) = 5.8 ± 0.2 nm and penetration depth λ(0) = 420 ± 50 nm. We found that it is possible to enhance the Hc2 (0) values to close to 10 T without any appreciable reduction in Tc. © 2013 IOP Publishing Lt
Charge ordering, ferroelectric, and magnetic domains in LuFe2O4 observed by scanning probe microscopy
Full text linkLuFe2O4 is a multiferroic system which exhibits charge order, ferroelectricity, and ferrimagnetism
simultaneously below 230 K. The ferroelectric/charge order domains of LuFe2O4 are imaged
with both piezoresponse force microscopy (PFM) and electrostatic force microscopy (EFM), while
the magnetic domains are characterized by magnetic force microscopy (MFM). Comparison of
PFM and EFM results suggests that the proposed ferroelectricity in LuFe2O4 is not of usual
displacive type but of electronic origin. Simultaneous characterization of ferroelectric/charge order
and magnetic domains by EFM and MFM, respectively, on the same surface of LuFe2O4 reveals
that both domains have irregular patterns of similar shape, but the length scales are quite different.
The domain size is approximately 100 nm for the ferroelectric domains, while the magnetic domain
size is much larger and gets as large as 1 lm. We also demonstrate that the origin of the formation
of irregular domains in LuFe2O4 is not extrinsic but intrinsic.9611sciescopu
Enhancement of the critical current density by increasing the collective pinning energy in heavy ion irradiated Co-doped BaFe2As2 single crystals
No full textWe investigate the effect of heavy ion irradiation (1.4 GeV Pb) on the vortex matter in Ba
(Fe0.92Co0.08)2As2 single crystals by superconducting quantum interference device (SQUID)
magnetometry. The defects created by the irradiation are discontinuous amorphous tracks,
resulting in an effective track density smaller than 25% of the nominal doses. We observe large
increases in the critical current density (Jc), ranging from a factor of ∼3 at low magnetic fields to
a factor of ∼10 at fields close to 1 T after irradiation with a nominal fluence of BΦ = 3.5 T. From
the normalized flux creep rates (S) and the Maley analysis, we determine that the Jc increase can
be mainly attributed to a large increment in the pinning energy, from <50 K to ≈500 K, while the
glassy exponent μ changes from ∼1.5 to <1. Although the enhancement of Jc is substantial in the
entire temperature range and S is strongly suppressed, the artificial pinning landscape induced by
the irradiation does not modify significantly the crossover to fast creep in the field-temperature
vortex phase diagram.112121sciescopu
Increment of the collective pinning energy in Na1-xCaxFe2As2 single crystals with random point defects introduced by proton irradiation
No full textWe study the influence of random point defects introduced by 3 MeV proton irradiation (doses 1 x 10(16) and 2 x 10(16) cm(2)) on the vortex dynamics of NaxCa1 - xFe2As2 (x = 0.5 and x = 0.75) single crystals. Our results indicate that the irradiation produces an enhancement of the critical current density and a reduction of the creep rate in vortex relaxation. The plateau in the temperature dependence of vortex creep rate initially present in as-grown single crystals disappears after irradiation. This fact can be associated with a large increment of the collective pinning energy (from <100 to 350-400 K). On the other hand, Maley analysis indicates that after irradiation both samples present a glassy exponent close to the one expected in the so-called large bundle regime (mu approximate to 7/9) for random point defects.1188sciescopu
Influence of random point defects introduced by proton irradiation on the flux creep rates and magnetic field dependence of the critical current density J(c) of co-evaporated GdBa2Cu3O7-delta coated conductors
No full textWe report the influence of random point defects introduced by 3 MeV proton irradiation (doses of 0.5 x 10(16), 1 x 10(16), 2 x 10(16) and 6 x 10(16) cm(-2)) on the vortex dynamics of co-evaporated 1.3 mu m thick, GdBa2Cu3O7-delta coated conductors. Our results indicate that the inclusion of additional random point defects reduces the low field and enhances the in-field critical current densities J(c). The main in-field J(c) enhancement takes place below 40 K, which is in agreement with the expectations for pinning by random point defects. In addition, our data show a slight though clear increase in flux creep rates as a function of irradiation fluence. Maley analysis indicates that this increment can be associated with a reduction in the exponent mu characterizing the glassy behavior.1197sciescopu
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