Superconducting gap evolution in overdopedBaFe2(As1−xPx)2single crystals through nanocal

Abstract

We report on specific heat measurements on clean overdoped BaFe2(As1−xPx)2 single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature γr = C/T |T →0 is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave α model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of ∆0 ∼ 5.3 meV, corresponding to ∆0/kBTc ∼ 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of ∆0 ∼ 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on γr, all samples however show similar behavior [γr(H) − γr(H = 0) ∝ H n , with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

Microscopic parameters from high-resolution specific heat measurements on superoptimally substitutedBaFe2(As1−xPx)2Diao1, Campanini2, Fang3 et al. 2016Phys. Rev. BSelf Cite183150View full textAdd to dashboardCite

“…[25][26][27] Attributing ∆F (0) only to a fraction (γ n − γ r )/γ n of the samples would increase the average ∆(0) by 10%-20%, largely eliminating the difference between our results and ARPES measurements. 20 The ratios ∆(0)/k B T c are in line with measurements on Co-122. 22 ∆F (0) leads us to the thermodynamic critical field H c (0).…”

“…19 Figure 1 shows the electronic specific heat for three samples of different composition in magnetic fields H c. Comparing the zero-field temperature dependences of panel (a) through (c), as previously reported in Ref. [20], there seems to be a tendency of going from a fully gapped system for x = 0.32 to a system with pronounced gap anisotropy and possible nodes for x = 0.55. However, the magnetic field dependence also indicates a faster-than-linear increase of γ(H) for all samples.…”

“…Even for x = 0.32 there is also a significant residual specific heat coefficient γ r ∼ 5 mJ/molK 2 , constituting roughly 17% of the normal state Sommerfeld coefficient γ n . 20 Here, γ r is largely invariant with substitution, in contrast to Co-122 where an increase in γ r away from optimal substitution can be clearly identified. 22 The phonon background of the x = 0.32 and x = 0.50 crystals were obtained by applying a simple Debye law and entropy conservation on the partially field-suppressed states.…”

Decoupling of critical temperature and superconducting gaps in irradiated films of a Fe-based superconductor

“…However, studies of the same material (in the form of crystals) irradiated with electrons [2] indicate a much larger decrease in the critical temperature for a similar enhancement of residual resistivity [8], as expected for a nodal s±symmetry. Indeed, the presence of nodes in the gap of BaFe 2 (As 1−x P x ) 2 is suggested by various experiments that detect zero-energy quasiparticle states [11][12][13][14] even though the exact shape of the node lines and even their location on the Fermi surface is still debated [15][16][17][18].…”

Multi-band mass enhancement towards critical doping in a pnictide superconductor

“…The enhancement of the quasiparticle mass in Ba122 approaching optimal doping has been previously deduced from the jump in specific heat at the superconducting transition temperature, T c . However, this analysis depends on model assumptions that can only be justified in conventional superconductors, in which the relationship between the specific heat jump and T c is known [2,[9][10][11][14][15][16][17]. What has been missing is a direct measurement of the normal state density of states in hightemperature superconductors, from which the sum of quasiparticle masses from all Fermi pockets can be determined.…”