Thermodynamic Study of Nodal Structure and Multiband Superconductivity of KFe₂As₂

Abstract

The temperature, field, and field-orientation dependences of the electronic specific heat C e of the ironpnictide superconductor KFe 2 As 2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and √ H linear dependences of C e /T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab plane, a fourfold oscillation is observed in C e with a sign change at 0.08T c . On the basis of the Doppler-shift analysis, the observed C e minima in H [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T C e /T exhibits an unusual upturn on cooling at moderate fields only for H ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity.

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…T < 0.1 K, are required to observe the signature of possible line nodes in KFe 2 As 2 . In accord with recent angleresolved heat-capacity experiments, 27 our results are compatible with either a d xy or a nodal s± state.…”

“…Actually, only the d x 2 −y 2 order parameter, with nodes located on the diagonals of the Brillouin zone, is excluded while d xy symmetry remains possible if the large gap effectively occurs on the sheet. This conclusion is corroborated by recent angleresolved heat-capacity measurements of Kittaka et al 27 …”

Evolution of London penetration depth with scattering in single crystals of K1−x Nax Kim1, Tanatar2, Liu3 et al. 2014Phys. Rev. B

“…Two recent heat capacity studies [33,34] observed a clear feature at around 0.7 K, with the general view of the curves very similar to the multiband MgB 2 [35]. Hardy et al [33] pointed out the importance of measurements with temperatures below 100 mK and were able to fit an experimental C e /T curve in the whole superconducting region, including the feature at 0.7 K, assuming four full-gap contributions, three of which have anomalously small gaps (lilliputian gap scenario).…”

Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1−xKx)FeLiu1, Tanatar2, Straszheim3 et al. 2014Phys. Rev. B6415722View full textAdd to dashboardCite

“…In configuration with magnetic field parallel to the plane, H ab, the upper critical field is Pauli limited, as suggested both by the difference in the shape of the phase diagram and quite sharp changes at H c2 [48]. Heat capacity study in H a configuration, however, had not found first order transition [49], but rather suggested multiband Fulde-Ferrel-Larkin-Ovchinnikov (FFLO [50,51]) state [52]. In slightly less doped material with x = 0.93, (T c ∼ 8 K) hysteresis is observed in the field-tuned resistive transition curves in H ab configuration at temperatures below 1 K, which can be attributed to a first-order superconducting transition due to paramagnetic effect [53].…”