Theory of point contact spectroscopy in correlated materials

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-01, Vol.112 (3), p.651-656
• Main Authors: Lee, Wei-Cheng, Park, Wan Kyu, Arham, Hamood Z., Greene, Laura H., Phillips, Philip
• Format: Article
• Language: English
• Subjects: Correlation analysis; defects; Electrodes; electronic nematicity; energy storage (including batteries and capacitors); INAUGURAL ARTICLE; iron-based superconductors; MATERIALS SCIENCE; Metals; Microscopy; non-Fermi liquid; phonons; Physical Sciences; Scattering; Spectrum analysis; spin dynamics; superconductivity; thermal conductivity
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1422509112

Significance Point-contact spectroscopy is a bulk spectroscopic probe that has been reliably used to map out bosonic and superconducting order parameter spectra via quasiparticle classical and Andreev scattering, respectively. We previously showed this technique to be exquisitely sensitive to an effective density of states specifically arising from non-Fermi liquid behavior, and in the case of the iron pnictides and chalcogenides, electronic nematicity manifesting as a zero bias conductance peak corresponds to an increased effective density of states at the Fermi level arising from orbital fluctuations. We developed a quantum mechanical theory to show how this technique reveals such effective density of states while being insensitive to gapless Fermi surface reconstructions and is therefore a valuable filter for detecting non-Fermi liquid behavior. We developed a microscopic theory for the point-contact conductance between a metallic electrode and a strongly correlated material using the nonequilibrium Schwinger-Kadanoff-Baym-Keldysh formalism. We explicitly show that, in the classical limit, contact size shorter than the scattering length of the system, the microscopic model can be reduced to an effective model with transfer matrix elements that conserve in-plane momentum. We found that the conductance dI / dV is proportional to the effective density of states, that is, the integrated single-particle spectral function A ( ω = eV ) over the whole Brillouin zone. From this conclusion, we are able to establish the conditions under which a non-Fermi liquid metal exhibits a zero-bias peak in the conductance. This finding is discussed in the context of recent point-contact spectroscopy on the iron pnictides and chalcogenides, which has exhibited a zero-bias conductance peak.