Measurements of the fluctuation-induced in-plane magnetoconductivity at high reduced temperatures and magnetic fields in the iron arsenide BaFe2−xNixAs2
Abstract:The superconducting fluctuations well inside the normal state of Fe-based superconductors were studied through measurements of the in-plane paraconductivity and magnetoconductivity in high quality BaFe 2−x Ni x As 2 crystals with doping levels from the optimal level (x = 0.10) up to highly overdoped (x = 0.20). These measurements, performed in magnetic fields up to 9 T perpendicular to the ab (Fe) layers, allowed a reliable check of the applicability to iron-based superconductors of Ginzburg-Landau approaches … Show more
“…To compare Eq. (3) with the data we used the T c and μ 0 H c 2 (0) values resulting from the scaling in the critical region, and used for the remaining parameters γ = 1.75 and c = 0.35 (in agreement with the one found in other iron-based superconductors 18,26,27 ). A good agreement with the experimental results is obtained, confirming the 3D nature of the x = 0.05 crystal.…”
Section: Resultsmentioning
confidence: 99%
“…In Fig. 1(d–f) it is presented an example of the normal-state background extraction procedure, that consist in a linear fit above 1.5 T c , a temperature above which fluctuation effects are expected to be negligible 14,20,26 .Figure 1( a – c ) Temperature dependence of resistivity near T c for NaFe 1− x Co x As ( x = 0.03, 0.07, 0.073) crystals under different magnetic fields up to 8 T applied perpendicular to the crystals’ ab- planes. ( d – f ) Example (for µ 0 H = 0 T) of the procedure used to determine the background contribution by a linear fit above 1.5 T c (lines).…”
Section: Resultsmentioning
confidence: 99%
“…As commented above, the data obtained with magnetic fields below 2 T (not shown) do not follow the Gaussian approach, probably due to a possible T c distribution. However, it has been also proposed that under these low field amplitudes phase fluctuations may play an important role 15,26 . Another possibility may be that H c 2 ( T ) is not linear at these field amplitudes (near T c ), an effect that some attribute to the multiband nature of these compounds 19,39,40 .…”
Section: Resultsmentioning
confidence: 99%
“…In the case of the x = 0.03 and 0.073 crystals, the Δ T c values (obtained from the FWHM of the d ρ /d T curve) are about 1.4 K. Taking into account that for these samples μ 0
∼ 55 T and, respectively, ∼29 T, according to the above argument it is expected that a 2 T field parallel to the c axis will allow to analyze fluctuation effects down to ∼0.6 K and ∼0.3 K above T c , respectively. The optimally doped ( x = 0.03) crystal is analyzed in terms of a GL approach for 3D anisotropic superconductors that includes an energy cutoff to extend the applicability of the GL approach to high reduced temperatures 26 . In the framework of this approach, that has been successfully applied in iron-based superconductors 20,26,27 , the fluctuation-induced conductivity in presence of a magnetic field with an arbitrary orientation is given bywhere is Planck’s constant and e the electron charge.…”
Section: Resultsmentioning
confidence: 99%
“…The bulk superconductivity is induced upon Co doping, the maximum T c occurring for x = 0.028 9 . We study both the critical and the Gaussian fluctuation regimes, by using the Ullah and Dorsey scaling and, respectively, the 3DGinzburg-Landau approach and the quasi-2D Lawrence-Doniach model 2,26 . This work extends a previous study of critical fluctuation effects in the magnetization of optimally doped NaFe 1− x Co x As ( x = 0.03) 16 , and will allow to explore the dependence of the superconducting parameters and of the dimensionality with the doping level.…”
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.
“…To compare Eq. (3) with the data we used the T c and μ 0 H c 2 (0) values resulting from the scaling in the critical region, and used for the remaining parameters γ = 1.75 and c = 0.35 (in agreement with the one found in other iron-based superconductors 18,26,27 ). A good agreement with the experimental results is obtained, confirming the 3D nature of the x = 0.05 crystal.…”
Section: Resultsmentioning
confidence: 99%
“…In Fig. 1(d–f) it is presented an example of the normal-state background extraction procedure, that consist in a linear fit above 1.5 T c , a temperature above which fluctuation effects are expected to be negligible 14,20,26 .Figure 1( a – c ) Temperature dependence of resistivity near T c for NaFe 1− x Co x As ( x = 0.03, 0.07, 0.073) crystals under different magnetic fields up to 8 T applied perpendicular to the crystals’ ab- planes. ( d – f ) Example (for µ 0 H = 0 T) of the procedure used to determine the background contribution by a linear fit above 1.5 T c (lines).…”
Section: Resultsmentioning
confidence: 99%
“…As commented above, the data obtained with magnetic fields below 2 T (not shown) do not follow the Gaussian approach, probably due to a possible T c distribution. However, it has been also proposed that under these low field amplitudes phase fluctuations may play an important role 15,26 . Another possibility may be that H c 2 ( T ) is not linear at these field amplitudes (near T c ), an effect that some attribute to the multiband nature of these compounds 19,39,40 .…”
Section: Resultsmentioning
confidence: 99%
“…In the case of the x = 0.03 and 0.073 crystals, the Δ T c values (obtained from the FWHM of the d ρ /d T curve) are about 1.4 K. Taking into account that for these samples μ 0
∼ 55 T and, respectively, ∼29 T, according to the above argument it is expected that a 2 T field parallel to the c axis will allow to analyze fluctuation effects down to ∼0.6 K and ∼0.3 K above T c , respectively. The optimally doped ( x = 0.03) crystal is analyzed in terms of a GL approach for 3D anisotropic superconductors that includes an energy cutoff to extend the applicability of the GL approach to high reduced temperatures 26 . In the framework of this approach, that has been successfully applied in iron-based superconductors 20,26,27 , the fluctuation-induced conductivity in presence of a magnetic field with an arbitrary orientation is given bywhere is Planck’s constant and e the electron charge.…”
Section: Resultsmentioning
confidence: 99%
“…The bulk superconductivity is induced upon Co doping, the maximum T c occurring for x = 0.028 9 . We study both the critical and the Gaussian fluctuation regimes, by using the Ullah and Dorsey scaling and, respectively, the 3DGinzburg-Landau approach and the quasi-2D Lawrence-Doniach model 2,26 . This work extends a previous study of critical fluctuation effects in the magnetization of optimally doped NaFe 1− x Co x As ( x = 0.03) 16 , and will allow to explore the dependence of the superconducting parameters and of the dimensionality with the doping level.…”
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.
Detailed measurements of the in-plane resistivity were performed in a high-quality Ba($$\hbox {Fe}_{1-x}\hbox {Co}_{{x}}$$
Fe
1
-
x
Co
x
)$$_2\hbox {As}_2$$
2
As
2
($$x=0.065$$
x
=
0.065
) single crystal, in magnetic fields up to 9 T and with different orientations $$\theta$$
θ
relative to the crystal c axis. A significant $$\rho (T)_{H,\theta }$$
ρ
(
T
)
H
,
θ
rounding is observed just above the superconducting critical temperature $$T_c$$
T
c
due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, $$H_{c2}(\theta )$$
H
c
2
(
θ
)
. In spite of the relatively small anisotropy of this compound, it is found that $$H_{c2}(\theta )$$
H
c
2
(
θ
)
presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large $$\theta$$
θ
(typically above $$\sim 60^o$$
∼
60
o
). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar $$H_{c2}(\theta )$$
H
c
2
(
θ
)
anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near $$T_c$$
T
c
, a result that differs from the ones obtained by using a single-band model.
We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa$$_2$$
2
Fe$$_4$$
4
As$$_4$$
4
F$$_2$$
2
, with $$T_c\;$$
T
c
$$\simeq$$
≃
34 K, for $$H$$
H
$$\parallel$$
‖
$$c$$
c
-axis and $$H$$
H
$$\parallel$$
‖
$$ab$$
ab
-planes. A second magnetization peak (SMP) was observed in the isothermal M(H) curves measured below 16 K for $$H$$
H
$$\parallel$$
‖
$$ab$$
ab
-planes. A peak in the temperature variation of the critical current density, $$J_{c}$$
J
c
(T), at 16 K, strongly suggests the emergence of Josephson vortices at lower temperatures, which leads to the SMP in the sample. In addition, it is noticed that the appearance of Josephson vortices below 16 K renders easy magnetic flux penetration. A detailed vortex dynamics study suggests that the SMP can be explained in terms of elastic pinning to plastic pinning crossover. Furthermore, contrary to the common understanding, the temperature variation of the first peak field, $$H_1$$
H
1
, below and above 16 K, behaves non-monotonically. A highly disordered vortex phase, governed by plastic pinning, has been observed between 17 and 23 K, within a field region around an extremely large first peak field. Pinning force scaling suggests that the point defects are the dominant source of pinning for H$$\parallel$$
‖
$$ab$$
ab
-planes, whereas, for H $$\parallel$$
‖
$$c$$
c
-axis, point defects in addition to surface defects are at play. Such disorder contributes to the pinning due to the variation in charge carrier mean free path, $$\delta l$$
δ
l
-pinning. Moreover, the large $$J_c$$
J
c
observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.
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