arXiv subjects

A. D. Alvarenga

Publications and source records attributed to A. D. Alvarenga.

At least 19 recordsLinked to original sources

arXiv2019

Strong pinning in the hole-doped pnictide superconductor La$_{0.34}$Na$_{0.66}$Fe$_2$As$_2$

We present magnetization studies as a function of time, temperature and magnetic field for $H$ $\parallel$ c-axis, in a hole-doped pnictide superconductor, La$_{0.34}$Na$_{0.66}$Fe$_2$As$_2$, with, $T_c$ $\approx$ 27 K. The obtained vortex phase-diagram shows that the magnetic irreversibility line is very close to the mean-field superconducting transition line, similar to the low $T_c$ superconductors, evidencing a strong pinning behavior. The irreversibility line does not follow a power law behavior with ($T_c$-$T$), however, it is well described using an expression developed in the literature considering the effect of disorder in the system. The critical current density estimated using the Bean's critical-state model is found to be of the order of 10$^5$ A/cm$^2$ below 12 K in the limit of zero magnetic field. A plot of the normalized pinning force density as a function of the reduced magnetic field at different temperatures shows a good scaling and the analysis suggests that the vortex pinning is due to normal point like pinning centers. The temperature dependence of the critical current density suggests that the pinning due to the variation in charge carrier mean free path alone is not sufficient to explain the experimental data. Magnetic relaxation rate as a function of temperature and magnetic field is also studied.

cond-mat.supr-con↗

arXiv2018

Study of the second magnetization peak and the pinning behaviour in Ba(Fe$_{0.935}$Co$_{0.065}$)$_2$As$_2

Isothermal magnetic field dependence of magnetization and the magnetic relaxation measurements were performed for $H$$\parallel$c axis on single crystal of Ba(Fe$_{0.935}$Co$_{0.065}$)$_2$As$_2$ pnictide superconductor having $T_c$ = 21.7 K. The second magnetization peak (SMP) for each isothermal $M(H)$ was observed in a wide temperature range from $T_c$ to the lowest temperature of measurement (2 K). Magnetic field dependence of relaxation rate $R(H)$, shows a peak (H$_{spt}$) between H$_{on}$ (onset of SMP in $M(H)$) and H$_p$ (peak field of SMP in $M(H)$), which is likely to be related with a vortex-lattice structural phase transition, as suggested in literature for similar sample. In addition, the magnetic relaxation measured for magnetic fields near H$_{spt}$ show some noise which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley's criterion and the collective pinning theory suggests that the second magnetization peak (SMP) in the sample is due to the collective (elastic) to plastic creep crossover, which is also accompanied with a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggests single dominating pinning mechanism in the sample and is not showing the usual $δ$l and $δT_c$ nature of pinning. The critical current density ($J_c$) estimated using the Bean's critical state model is found to be 5 $\times$ 10$^5$ A/cm$^2$ at 2 K in the zero magnetic field limit. Surprisingly, the maximum in the pinning force density is not responsible for the maximum value of the critical current density in the sample.

cond-mat.supr-con↗

arXiv2017

Effects of proton irradiation on flux-pinning properties of underdoped Ba(Fe$_{0.96}$Co$_{0.04}$)$_2$As$_2$ pnictide superconductor

We study the effect of proton irradiation on Ba(Fe$_{0.96}$Co$_{0.04}$)$_2$As$_2$ superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy $U_0$ of the samples prior to and after irradiation, as well as comparison of the values of $U_0$ obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces $T_c$ modestly, but significantly reduces $U_0$ in both regimes: the critical current density $J_c$ is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of $J_c$ for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By consideration of a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in $U_0$ and $J_c$.

cond-mat.supr-con↗

arXiv2015

Experimental observation of an anomalous peak in isofield M(T) curves in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ resembling the fish-tail in $M$($H$) curves further suggesting a possible phase transition in the irreversible regime

We measure magnetization as a function of temperature, magnetic field and time in a BaFe$_2$(As$_{0.68}$P$_{0.32}$)$_2$ single crystal with $T_c$ = 27.6 K. The fish-tail observed on many $M$($H$) curves are used to construct isofield $M$($T$) curves which show an anomalous peak at some temperature $Tt$ suggesting a possible phase transition in the irreversible regime. A vortex dynamics study performed along the peaks evidences a minimum in the relaxation rate occurring at the same position $Tt$ of the minimum value of $M$ in these peaks. A vortex dynamics study performed on $M$($H$) curves show two distinct minimum in the relaxation rate: a first minimum ($H1$) for a lower field correlating with $Tt$ and a second ($H2$) correlating with the peak in the fish-tail. A phase diagram is constructed and the line corresponding to $Tt$ and $H1$ points obey an expression developed in the literature for a structural rhombic to square lattice phase transition, further supporting this view.

cond-mat.supr-con↗

arXiv2014

Measurements of the superconducting fluctuations in optimally-doped BaFe$_{2-x}$Ni$_x$As$_2$ under high magnetic fields: Probing the 3D anisotropic Ginzburg-Landau approach

The superconducting fluctuations well inside the normal state of Fe-based superconductors were experimentally studied through the in-plane paraconductivity in several high-quality optimally-doped BaFe$_{2-x}$Ni$_x$As$_2$ crystals. These measurements were performed in magnetic fields with amplitudes up to 14 T, and different orientations relative to the crystals c axis (0, 53 and 90 degrees). The results allowed a stringent check of the applicability of a recently proposed Ginzburg-Landau approach for the fluctuation electrical conductivity of 3D anisotropic materials in presence of finite applied magnetic fields.

cond-mat.supr-con↗

arXiv2014

Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ superconductors

The temperature dependence of the in-plane magnetic penetration depth $λ_{ab}$ of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals is determined directly from the shielding magnetic susceptibility, measured in the Meissner region with the field parallel to the ab layers. The doping levels studied cover the underdoped, optimally-doped and overdoped regimes. At temperatures below 0.5Tc a well-defined power-law behavior $λ_{ab}(T)-λ_{ab}(0)=AT^n$ (with n approximately 2.5) is observed. At lower temperatures (T<0.3Tc) the data are still consistent with n=2 and $A\propto T_c^{-3}$, as predicted by the strong pair-breaking scenario proposed by Gordon et al., Phys. Rev. B 81, 180501(R) (2010). The temperature dependence of the superfluid density $ρ_s\proptoλ_{ab}^{-2}$ presents a marked positive curvature just below Tc which is a sign of two-gap superconductivity. The analysis of $ρ_s(T)$ in terms of a two-gap model allowed to estimate parameters like the in-band and inter-band couplings, the relative weight of each band, and their dependence with the doping level. A comparison with $ρ_s(T)$ data obtained by using other techniques in compounds with a similar composition is also presented.

cond-mat.supr-con↗

arXiv2014

Critical and Gaussian conductivity fluctuations in BaFe1.9Ni0.1As2 superconductor

We study fluctuation conductivity in a single crystal of BaFe$_{1.9}$Ni$_{0.1}$As$_2$ superconductor ($T_c$ = 20 K) as a function of temperature and applied magnetic field. Magneto-conductivity curves, $Δσ$ vs. $T$, were analyzed in terms of -1/(dln($Δσ$)/d$T$) vs. $T$ plots, which allow to study different fluctuations regimes and to estimate exponent values and temperature widths of each regime. The analysis of magneto-conductivity curves evidence the existence of only two fluctuations regimes, a possible critical one (of glass-like type) going from the irreversible temperature to above $T_c(H)$ followed by Aslamazov-Larkin fluctuations in the Gaussian regime.

cond-mat.supr-con↗

arXiv2013

Three-dimensional anisotropic fluctuation diamagnetism around the superconducting transition of Ba(1-x)KxFe2As2 single crystals in the finite-field (or Prange) regime

The magnetization around the superconducting transition was recently measured in a high-quality Ba(1-x)KxFe2As2 single crystal with magnetic fields applied along and transverse to the crystal Fe-layers [J. Mosqueira et al., Phys. Rev. B 83, 094519 (2011)]. Here we extend this study to the finite field (or Prange) regime, in which the magnetic susceptibility is expected to be strongly dependent on the applied magnetic field. These measurements are analyzed in the framework of the three-dimensional anisotropic Ginzburg Landau (3D-aGL) approach generalized to the short wavelength regime through the introduction of a total-energy cutoff in the fluctuation spectrum. The results further confirm the adequacy of GL approaches to describe the fluctuation effects close to the superconducting transition of these materials.

cond-mat.supr-con↗

arXiv2013

Measurements of the fluctuation-induced in-plane magnetoconductivity at high reduced temperatures and magnetic fields in the iron arsenide BaFe(2-x)NixAs2

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)NixAs2 crystals with doping levels from the optimal (x=0.10) up to the 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 for 3D anisotropic compounds, even at high reduced temperatures and magnetic fields. Our results also allowed us to gain valuable insights into the dependence on the doping level of some central superconducting parameters (coherence lengths and anisotropy factor).

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arXiv2012

Anomalous metamagnetic-like transition in a FeRh/Fe$_3$Pt interface occurring at T120 K in the field-cooled-cooling curves for low magnetic fields

We report on the magnetic properties of a special configuration of a FeRh thin film. An anomalous behavior on the magnetisation vs. temperature was observed when low magnetic fields are applied in the plane of a thin layer of FeRh deposited on ordered Fe$_3$Pt. The anomalous effect resembles a metamagnetic transition and occur only in the field-cooled-cooling magnetisation curve at temperatures near 120 K in samples without any heat treatment.

cond-mat.mtrl-sci↗

arXiv2012

Vortex dynamics as a function of field orientation in BaFe1.9Ni0.1As2

Vortex dynamics in a multiband anisotropic superconducting such as the Fe based superconductors, is interesting and potentially important for applications. In this study we examine flux-creep data for fields along the second magnetization peak observed in M(H) curves of BaFe$_{1.9}$Ni$_{0.1}$As$_2$ for H//c-axis, H//ab$-planes and H forming a 45$^o$ angle with ab-planes. We find that the M-H loops taken from the different field directions can be collapsed onto a single universal curve at all temperatures with a simple scaling factor equivalent to the superconducting anisotropy, showing not only that the vortex pinning is isotropic, three dimensional and most likely related to point like defects. The resulting critical currents however, taken from the Bean model appears to show enhanced low field pinning for H//c. The features in the vortex- dynamics also differ in different field orientations and show no direct correlation with the second magnetization peak Hp as is the case with a direct crossover in pinning regimes. Isofield plots of the scaled activation energy obtained from flux-creep data is found to be a smooth function of temperature as the Hp(T) line is crossed consistent with a single type of pinning regime operating at this field, independent of field orientation. The functional form of the Hp(T) lines in the resulting phase diagrams also support this view.

cond-mat.supr-con↗

arXiv2012

Flux-creep in the second magnetization peak of BaFe{1.9}Ni{0.1}As2 superconductor

Flux-creep data was obtained for fields along the second magnetization peak observed in M(H) curves of BaFe{1.9}Ni{0.1}As2 for H||c-axis. H||ab-planes and H forming a 45 degrees angle with ab-planes. The M-H loops from the different field directions can be collapsed onto a universal curve with a scaling factor equivalent to the superconducting anisotropy, showing that the pinning is three dimensional, although with remarkable differences in the vortex-dynamics as a function of field orientation. The resulting relaxation rate, R, when plotted as a function of field and temperature does not show any specific feature in the vicinity of the second magnetization peak field Hp, the relaxation shows a maximum at a field H_2 well above Hp for H||c and a minimum at H^* for fields well below Hp for H||ab$ and H$-45-ab. Isofield plots of the scaled activation energy obtained from flux-creep data at several different temperatures also do not show any evidence of a change in the pinning mechanism as the peak field is crossed. The Hp lines in the resulting phase diagrams do not appear to be consistent with a description-terms of a collective-plastic pinning crossover.

cond-mat.supr-con↗

arXiv2012

Magnetic phase diagram of the layered superconductor Bi{2+x}Sr{2-x}CuO{6+δ} (Bi2201) with Tc~7 K

We report on magnetization measurements performed on a single crystal of Bi{2+x}Sr{2-x}CuO{6+δ} with Tc\sim7 K for H||c-axis. The isofield M(T) curves show a large reversible region, with a pronounced rounding effect as M approaches zero which prevents the determination of Tc(H). Deviations from the linear behavior of magnetisation near Tc(H) are studied through the asymptotic behavior of $\sqrt{-M(T)}\sim(T-Ta)^m$, where m is an exponent and Ta an apparent temperature transition. Values of m deviates from the expected mean field value, 1/2, suggesting the importance of phase fluctuations. Resulting values of Ta, interpreted as the onset of phase correlations, decrease as field increases showing an upward curvature. Values of Tc(H) are obtained through a two-dimensional critical scaling analysis obeyed by many M(T) curves. The resulting phase diagram do not show upward curvature and lies below the Ta(H) line. The value of H{c2}(0) estimated from the initial slope $H_{c2}/dT is twice the value suggested by the phase diagram. Amplitude fluctuations above Ta(H) are explained in terms of a Ginzburg-Landau approach extended to high reduced temperatures and magnetic fields by the introduction of a total-energy cutoff in the fluctuation spectrum.

cond-mat.supr-con↗

arXiv2011

Fish-tail and vortex dynamics in Ni doped iron-pnictide BaFe{1.82}Ni_{0.18}As_2

We study the vortex-dynamics of a BaFe{1.82}Ni_{0.18}As_2 crystal with Tc = 8 K, by measuring flux-creep over the second magnetisation (or fish-tail) peak for both H//c-axis and H//ab planes. Magnetic relaxation data show an anomalously long initial stage of relaxation, lasting approximately10 minutes for H//c-axis and 2-3 min for H//ab, resembling a transient effect with a lower relaxation rate, which is followed by the usual log(time) relaxation. Interestingly, study of the relaxation rate R vs H for both stages of relaxation and for both field directions, are featureless over the field range associated with the fish-tail. The same trend was confirmed by plotting R vs T obtained from flux-creep data measured as a function of temperature for a fixed field (H//$c-axis). A plot of the activation energy U(M,T) calculated from the time relaxation of the magnetisation at a fixed field also shows a smooth behavior further supporting the view that the fish-tail peak is not associated with a crossover in vortex pinning regime within the collective pinning scenario.

cond-mat.supr-con↗

arXiv2010

Non-centro-symmetric superconductors Li2Pd3B and Li2(Pd0.8Pt0.2)3B: amplitude and phase fluctuations analysis of the experimental magnetization data

We report on magnetization data obtained as a function of temperature and magnetic field in Li2 (Pd0.8Pt0.2)3B and Li2Pd3B non-centro-symmetric superconductors. Reversible magnetization curves were plotted as M1/2 vs. T. This allows study of the asymptotic behavior of the averaged order parameter amplitude (gap) near the superconducting transition. Results of the analysis show, as expected, a mean field superconducting transition for Li2Pd3B. On contrary, a large deviation from the mean field behavior is revealed for Li2(Pd0.8Pt0.2)3B. This is interpreted as due to the strength of the non s-wave spin-triplet pairing in this Pt-containing compound which produces nodes in the order parameter and consequently, phase fluctuations. The diamagnetic signal above Tc(H) in Li2Pd3B is well explained by superconducting Gaussian fluctuations, which agrees with the observed mean field transition. For Li2(Pd0.8Pt0.2)3B the diamagnetic signal above Tc(H) is much higher than the expected Gaussian values and appears to be well explained by three dimensional critical fluctuations of the lowest-Landau-level type, which somehow agrees with the scenario of a phase mediated transition.

cond-mat.supr-con↗

arXiv2010

Flux-dynamics associated with the Second Magnetisation Peak in iron-pnictide Ba_{1-x}K_xFe_2As_2

We report on isofield magnetic relaxation data on a single crystal of $Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$= 32.7 K which exhibit the so called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M(t), was used to obtain the activation energy U(M) in order to study different vortex dynamics regimes. Results of this analysis along with time dependent measurements as a function of field and temperature extended to the reversible region of some M(H) curves demonstrate that the irreversibility as well the second magnetization peak position, $H_p(T)$, are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon (well below $H_p$), and $H_p$, the vortex dynamics is controlled by collective pinning. For fields below Hon the activation energy, $U_0$, increases with field as expected for collective pinning, but the pinning mechanism is likely to be in the single vortex limit.

cond-mat.supr-con↗

arXiv2010

Anomalous magnetisation in a Fe_3Pt-FeRh film occurring at T~120 K in the field-cooled-cooling curves for low magnetic fields

We report on an anomalous magnetization observed with temperature for low magnetic fields applied in the plane of a film formed by a thin layer of Fe-Rh deposited on a thin foil of ordered Fe$_3$Pt. The anomalous effect resembles a metamagnetic transition and occur only in the field-cooled-cooling magnetization curve at temperatures near 120 K. We also observe an aging effect which broads the metamagnetic-like transition suggesting the existence of Fe-Rh antiferromagnetic clusters which apparently are too small to be detected by the x-ray diffraction.The X-Ray diffractogram obtained in the Fe-Rh surface of the film do not show the Fe-Rh antiferromagnetic phase (a=2.98 $Å$) but peaks at same locations of those found for the pure ordered Fe$_3$Pt film (a=3.72 $Å$)

cond-mat.mtrl-sci↗

arXiv2009

Study of diamagnetic fluctuations in YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}: Possible observation of phase correlations persisting above T_c

We report on isofield curves of $\sqrt{-M}$ vs. T, where M is the reversible magnetization, of YBa_2Cu_3O_{6.95}, YBa_2Cu_3O_{6.65}, and Bi_2Sr_2CaCu_2O_{8+x} with the magnetic field, H, applied parallel to the c-axis of the samples (and also parallel to the ab- planes for YBaCuO). For temperatures close to the critical temperature, T_c, the quantity \sqrt{-M} is proportional to the order parameter amplitude |ψ|. Curves of \sqrt{-M} vs. T allowed to study the asymptotic behavior of the form (T_a-T)^m of |ψ| near T_c, as a function of field. Results for the studied samples produced values of T_a(H) lying above T_c, suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above T_c. The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudo-gap region of YBaCuO. \sqrt{-M} vs. T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above T_a(H) which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a {total-energy} cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5 < m < 0.7, are interpreted as due to phase fluctuations of the d-wave pairing symmetry of the order parameter in the ab- planes.

cond-mat.supr-con↗