Evidence for a Nodal-Line Superconducting State in LaFePO

Abstract

In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual s± state where there the gap changes sign between the electron and hole Fermi surface sheets. Here we report measurements of the penetration depth λ(T ) in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to ∼ 100 mK. We find that λ(T ) varies almost perfectly linearly with T strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function may not be generic to all pnictide superconductors.

Quantum oscillations probe the normal electronic states of novel superconductors

“…In LaFePO, SrFe 2 P 2 and CaFe 2 P 2 , the mean free paths of electrons are at least a factor of 2 larger than those of the holes, which may be caused by differences in the orbital character between the electron and hole sheets (Analytis et al 2009b;Coldea et al 2009). Furthermore, for the same electron sheet, the mean free path varies along k z , suggesting possibly anisotropy in scattering, which may be relevant for understanding the presence of line nodes in some of the iron pnictides (Fletcher et al 2009). The Hall effect data in the paramagnetic (PM) state of BaFe 2 As 2 also suggest that electron bands have a higher mobility than the hole bands (Fang et al 2009), and at low temperatures, in the magnetic state, hole mobility is suppressed when compared with that of electrons, suggesting that hole localization may be promoted by spin fluctuations (Fang et al 2009).…”

“…There is also a small three-dimensional pocket centred at Z (figure 3) not observed experimentally in torque measurements and it is not clear whether it plays any role (Coldea et al 2008;Carrington et al 2009). Moreover, the symmetry of the order parameters in LaFePO is expected to have line nodes (Fletcher et al 2009), suggesting that the predicted s ± -wave type of symmetry (Mazin et al 2008a,b) may not be applicable to all iron-based superconductors.…”

“…Such an SC state has an extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets, as the magnetic fluctuations, while too broad to induce a magnetic instability, are instrumental in stabilizing superconductivity (Chubukov et al 2008;Kuroki et al 2008;Mazin et al 2008a,b). So far experiments show evidence for a more complex picture where the gap order parameter is not simply an s ± -wave but may also have line nodes (Fletcher et al 2009) and thus the knowledge of the exact details of the Fermi surface that may favour one type of pairing over another is essential.…”

Probing the pairing symmetry of the iron pnictides with electronic Raman scattering

“…Due to backflow, the Raman response from each separate band vanishes identically for the constant vertices in Eq. (15), but due to the change in sign, the entire Raman spectra arises solely from the mixing terms. The positions of the cusp-like feature at ω = 2∆ 0 are the same as that shown in Fig.…”

“…Nuclear magnetic resonance (NMR) studies [5][6][7][8] showed a low temperature T 3 spin lattice relaxation rate typical of a gap with nodes. However, penetration depth measurements [9][10][11][12][13][14][15] have been fit both to exponential activated T -dependence, indicative of a fully gapped state, and low-T power laws. ARPES measurements on single crystals of 122-type materials [16][17][18][19][20][21] measured the spectral gap reporting isotropic or nearly isotropic gaps on all Fermi surface sheets.…”