Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case ofBaFe2−xNixAs2

Abstract

The characteristics of Fe-based superconductors are manifested in their electronic, magnetic properties, and pairing symmetry of the Cooper pair, but the latter remain to be explored. Usually in these materials, superconductivity coexists and competes with magnetic order, giving unconventional pairing mechanisms. We report on the results of the bulk magnetization measurements in the superconducting state and the low-temperature specific heat down to 0.4 K for BaFe 2−x Ni x As 2 single crystals. The electronic specific heat displays a pronounced anomaly at the superconducting transition temperature and a small residual part at low temperatures in the superconducting state. The normal-state Sommerfeld coefficient increases with Ni doping for x = 0.092, 0.096, and 0.10, which illustrates the competition between magnetism and superconductivity. Our analysis of the temperature dependence of the superconducting-state specific heat and the London penetration depth provides strong evidence for a two-band s-wave order parameter. Further, the data of the London penetration depth calculated from the lower critical field follow an exponential temperature dependence, characteristic of a fully gapped superconductor. These observations clearly show that the superconducting gap in the nearly optimally doped compounds is nodeless.

Electronic specific heat inBaFe2−xNixAs2

“…In some of the studies 6,7,10,14,16 , only parameter a is introduced to account for the experimental uncertainties such as the error of sample mass. Since such process does not involve changing the temperature dependence of the phonon specific heat, it may not introduce some of the artificial results discussed above.…”

“…The most important parameter is γ n = C/T , which is proportional to the density of states at the Fermi energy and the effective electron mass. The measurements in the iron-based superconductors have given a variety of values of γ n for different materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . A detailed study on the Ba(Fe 1−x Co x ) 2 As 2 shows that γ n becomes maximum at optimal doping 4,7 , suggesting the influence of the antiferromagnetic (AF) order that disappears at the same doping 17 .…”

“…To reveal the electronic specific heat, one may try to fit the data above T c with certain functions 2,3 or find a reference sample without superconductivity by assuming that the specific heat of phonons change little with doping 4,5 . In the latter case, the specific heat of the phonons of the reference sample may be simply adjusted as aC phonon (bT ) with a and b as the tuning parameters (a-b method) to account for the change of the phonon spectra with doping [6][7][8][10][11][12][13][14]16 . In most cases, a linear temperature dependence of the electronic specific heat below T c is assumed to account for the entropy conservation, i.e., γ n is temperature-independent.…”

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

“…To account this negligible variation, we use scaling coefficients close to unity. As an example, for Ba0.67K0.33Fe2As2 one may choose Ba(Fe0.847Co0.153)2As2 [46], Ba(Fe0.88Mn0.12)2As2 [47], BaFe1.75Ni0.25As2 [48].…”

Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO

“…The H c1 , i.e., the thermodynamic field at which the presence of vortices into the sample becomes energetically favorable, is a very useful parameter, providing key information regarding bulk thermodynamic properties and carrying information about the underlying pairing mechanism. Indeed, the gap properties of different families of pnictides have been investigated by tracking the H c1 and the magnetic penetration depth [24][25][26][27][28][29][30]. The gap properties of these different families display single to double gaps and even the presence of nodes.…”