arXiv subjects

A. D. Alvarenga

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

26 records · Page 2Linked to original sources

arXiv2009

Superconducting fluctuations in the reversible magnetization of the iron-pnictide $Ba_{1-x}K_xFe_2As_2$

We report on isofield magnetization curves obtained as a function of temperature in two single crystals of $Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$=28K and 32.7 K. Results obtained for fields above 20 kOe show a well defined rounding effect on the reversible region extending 1-3 K above $T_c(H)$ masking the transition. This rounding appears to be due to three-dimensional critical fluctuations, as the higher field curves obey a well know scaling law for this type of critical fluctuations. We also analysed the asymptotic behavior of $\sqrt M$vs.T curves in the reversible region which probes the shape of the gap near $T_c(H)$. Results of the analysis suggests that phase fluctuations are important in $Ba_{1-x}K_xFe_2As_2$ which is consistent with nodes in the gap.

cond-mat.supr-con↗

arXiv2008

Onset of phase correlations in YBa2Cu3O{7-x} as determined from reversible magnetization measurements

Isofield magnetization curves are obtained and analyzed for three single crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as well as the BCS superconductor Nb, in the presence of magnetic fields applied both parallel and perpendicular to the $ab$ planes. Near Tc, the magnetization exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with recent theories, we associated Ta(H) with the onset of coherent phase fluctuations of the superconducting order parameter. For Nb and optimally doped YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H). The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis, suggesting that the magnetization is probing a region of temperatures above Tc where phase correlations persist. In this region, the fitting exponent falls in the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab planes. The results are interpreted in terms of an anisotropic pairing symmetry of the order parameter: d-wave along the ab planes and s-wave along the c axis.

cond-mat.supr-con↗

arXiv2007

The average kinetic energy density of Cooper pairs above T_c in YBa_2Cu_3O_{7-x}}, Bi_2Sr_2CaCu_2O_{8+δ}, and Nb}

We have obtained isofield curves for the square root of the average kinetic energy density of the superconducting state for three single crystals of underdoped YBa_2Cu_3O_{7-x}, an optimally doped single crystal of Bi_2Sr_2CaCu_2O_{8+δ}, and Nb. These curves, determined from isofield magnetization versus temperature measurements and the virial theorem of superconductivity, probe the order parameter amplitude near the upper critical field. The striking differences between the Nb and the high-T_c curves clearly indicate for the latter cases the presence of a unique superconducting condensate below and above T_c.

cond-mat.supr-con↗

arXiv2006

Dimensional crossover in the fluctuation magnetization in YBa_2Cu_3O_{7-x}and its evolution with the content of oxygen

We have studied the field-induced dimensional crossover in the fluctuation magnetization of three single crystals of $YBa_2Cu_3O_{7-x}$, with superconducting transition temperatures, $T_c$= 62.5, 52, and 41 K. The dimensional crossover is observed by studying the diamagnetic vortex fluctuations of the lowest-Landau-level type which occur in isochamps magnetization curves, $MvsT$, for temperatures close to the transition temperature $T_{c}(H)$. The study was accomplished by obtaining isochamps magnetization curves as a function of temperature for fields in the range of 0.4 T to 5 T. Magnetization curves for each sample when plotted together show two distinct well resolved crossing points, one formed by low field curves and located at a higher temperature than the other formed by high field curves. A lowest-Landau-level scaling analysis is applied to the curves forming the crossing points and it is verified that lower field curves obey the three-dimensional form of this scaling while the higher field curves obey the two-dimensional form. The results allow to observe the evolution of the dimensional crossover field $H_{cross}$, 3D-2D, with the content of oxygen in $YBa_2Cu_3O_{7-x}$. It is observed that the evolution of the field $H_{cross}$ with the content of oxygen in each sample qualitatively agrees with theoretical predictions and allow us to estimate the ratio of the anisotropy among the studied samples. \\pacs{74.40.+k}

cond-mat.supr-con↗

arXiv2006

Experimental observation of two-dimensional fluctuation magnetization in the vicinity of T_c for low values of the magnetic field in deoxygenated YBa_2Cu_3O_{7-x}

We measured isofield magnetization curves as a function of temperature in two single crystal of deoxygenated YBaCuO with T_c = 52 and 41.5 K. Isofield MvsT were obtained for fields running from 0.05 to 4 kOe. The reversible region of the magnetization curves was analyzed in terms of a scaling proposed by Prange, but searching for the best exponent $\upsilon$. The scaling analysis carried out for each data sample set with $\upsilon$=0.669, which corresponds to the 3D-xy exponent, did not produced a collapsing of curves when applied to MvsT curves data obtained for the lowest fields. The resulting analysis for the Y123 crystal with T_c = 41.5 K, shows that lower field curves collapse over the entire reversible region following the Prange's scaling with $\upsilon$=1, suggesting a two-dimensional behavior. It is shown that the same data obeying the Prange's scaling with $\upsilon$=1 for crystal with T_c = 41.5 K, as well low field data for crystal with $T_c$ = 52 K, obey the known two-dimensional scaling law obtained in the lowest-Landau-level approximation.

cond-mat.supr-con↗

arXiv2005

A comparative study of high-field diamagnetic fluctuations in deoxygenated YBa2Cu3O(7-x) and polycrystalline (Bi-Pb)2Sr2Ca3O(10)

We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41 K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c axis. The reversible data were interpreted in terms of 2D lowest-Landau-level fluctuation theory. The data were fit well by the 2D LLL expression for magnetization obtained by Tesanovic etal., producing reasonable values of kappa but larger values of dHc2/dT. Universality was studied by obtaining a simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis LLL scaling factor used to obtain the simultaneous scaling was extracted from theory, and compared with the experimental values. The comparison between the values of the x-axis produced a deviation of 40% which suggests that the hypothesis of universality of the 2D-LLL fluctuations is not supported by the studied samples. We finaly observe that Y123 magnetization data for temperatures above $T_c$ obbey a universal scaling obtained for the diamagnetic fluctuation magnetization from a theory considering non-local field effects. The same scaling was not obbeyed by the corresponding magnetization calculated from the two-dimensional lowest-Landau-level theory.

cond-mat.supr-con↗

arXiv2004

Vortex dynamics differences in YBaCuO at low temperatures for H||ab planes due to twin boundary pinning anysotropy

We measured magnetization, M, of a twin-aligned single crystal of YBa2Cu 3Ox (YBaCuO), with Tc = 91 K, as a function of temperature, T, and magnetic field, H, with H applied along the ab planes. Isothermal M-vs.-H and M-vs.-time curves were obtained with H applied parallel and perpendicular to the twin boundary, TB, direction. M-vs.-H curves exhibited two minimums below 38 K, which resembled similar curves that have been obtained in YBaCuO for H parallel to c axis. Above 12 K, the field positions of the minimums for H||TB and HperpTB were quite similar. Below 12 K, the position of the second minimum, Hmin, occurred at a higher field value with H||TB. Below 6 K, only one minimum appeared for both field directions. At low temperatures, these minimums in the M-vs.-H curves produced maximums in the critical current. It was determined that vortex lines were expelled more easily for H||TB than for HperpTB, and, therefore, below a certain field value, that Jc(HperpTB) was larger than Jc(H||TB) . At T<12 K with H||TB, the relaxation rate for flux lines leaving the crystal was found to be different from that for flux entering the crystal. We also observed flux jumps at low temperatures, with their sizes depending on the orientation of magnetic field with respect to the TBs.

cond-mat.supr-con↗

arXiv2002

Experimental observation of high field diamagnetic fluctuations in Niobium

We have performed a magnetic study of a bulk metallic sample of Nb with critical temperature $T_{c}=8.5$ K. Magnetization versus temperature (M {\it vs} T) data obtained for fixed magnetic fields above 1 kOe show a superconducting transition which becomes broader as the field is increased. The data are interpreted in terms of the diamagnetic lowest Landau level (LLL) fluctuation theory. The scaling analysis gives values of the superconducting transition temperature $T_{c}(H)$ consistent with $H_{c2}(T)$% . We search for universal 3D LLL behavior by comparing scaling results for Nb and YBaCuO, but obtain no evidence for universality.

cond-mat.supr-con↗