Imaging Cooper pairing of heavy fermions in CeCoIn5

Abstract

§ These authors contributed equally to this research project. The Cooper pairing mechanism of heavy-fermion superconductors 1-4 , while long hypothesized as due to spin fluctuations 5-7 , has not been determined. It is the momentum space (k-space) structure of the superconducting energy gap (k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure (k) for any heavy-fermion superconductor. Bogoliubov quasiparticle interference (QPI) imaging 8-10 , a proven technique for measuring the energy gaps of high-T c superconductors 11-13 , has recently been proposed 14 as a new method to measure (k) in heavy-fermion superconductors, specifically CeCoIn 5 15 . By implementing this method, we immediately detect a superconducting energy gap whose nodes are oriented along k||()/a 0 directions 16-19 . Moreover, we determine the complete k-space structure of the (k) of a heavy-fermion superconductor. For CeCoIn 5 , this novel information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that highest magnitude (k) opens on a high-k band so that gap nodes occur at quite unanticipated k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with d x 2y 2 gap symmetry. The availability of such quantitative heavy band-and gap-structure data will be critical in identifying the microscopic mechanism of heavy fermion superconductivity in this material, and perhaps in general. 29 Yuan, T., Figgins, J., and Morr, D. K. Hidden order transition in URu 2 Si 2 and the emergence of a coherent Kondo lattice. Phys. Rev. B 86, 035129 (2012) 30 Petrovic C. et al., J. Phys. Condens. Matter 13, L337 (2001) 31 McCollam, A., et al. Anomalous de Haas-van Alphen Oscillations in CeCoIn 5 . Phys. Rev. Lett. 94, 186401 (2005) 32 Aynajian, P., et al. Visualizing heavy fermions emerging in a quantum critical Kondo lattice. Nature 486, 201 (2012) 33 Koitzsch, A., et al. Electronic structure of CeCoIn 5 from angle-resolved photoemission spectroscopy. Phys. Rev. B 79, 075104 (2009) 34 Jia, X. -W., et al, Growth characterization and Fermi surface of heavy-fermion CeCoIn 5 superconductor. Chinese Phys. Lett. 28 057401 (2011)

Unconventional superconductivity in the strong-coupling limit for the heavy fermion system CeCoIn5

“…However, in CeCoIn 5 this sort of data do not abound. Three previous STS studies claim nodal heavy fermion superconductivity in CeCoIn 5 from either fitting the quasiparticles' excitation spectrum [5], or infering information on the k-space structure of ∆, [6] or detecting close to surface atomic steps a spatial-evolution of low-energy excitations that is typical of nodal superconductivity. [7].…”

“…[7]. Nevertheless, performing multi-parameter modeling of the conductance depletion in order to extract spectroscopic information on the superconducting condensate from these experimentally rounded [5] or particle-hole asymmetric [6,7] STS spectra is a strategy that can be further improved in order to produce more confident results. In this work we measured low-temperature STS spectra of CeCoIn 5 with well defined features in the quasiparticle excitation spectra that are the signature of nodal superconductivity, mounted on a featureless background.…”

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

“…9b), Allan et al revealed Bogoliubov QPI patterns related to superconductivity within the energy range of AE1 meV. 81 The closing of the low temperature spectral gap of B550 meV at the bulk T c = 2.1 K, along with images of a square Abrikosov vortex lattice, consistent with that reported by neutron scattering experiments, 84 75 confirmed the observation of superconductivity in CeCoIn 5 . Furthermore, the V-shaped spectra suggested gap nodes, while the QPI patterns imaged (Fig.…”

“…81 Above T c , the normal state electronic properties of these materials are dominated by the hybridization of the conduction band (typically a light band of s, p, and/ or d character), with localized f orbitals, resulting in heavy, hybrid bands as sketched in Fig. 9a.…”

“…A cos(2y); A = 550 meV which is found to be consistent with the QPI data. 81 In order to address the surprising correlation, a theoretical study proposed the existence of a second set of interstitial O dopants (type-A oxygens) which would reside closer to the CuO 2 plane compared to type-B oxygens. 101 The type-A oxygens were predicted to have a greater effect on the CuO 2 plane and thus resolve the surprising weak positive correlation between type-B O locations and PG magnitude.…”