Elementary Excitations Due to Orbital Degrees of Freedom in Iron-Based Superconductors

Abstract

One central issue under intense debate in the study of the iron based superconductors is the origin of the structural phase transition that changes the crystal lattice symmetry from tetragonal to orthorhombic. This structural phase transition, occurring universally in almost every family of the iron-based superconductors, breaks the lattice C 4 rotational symmetry and results in an anisotropy in a number of physical properties. Due to the unique topology of the Fermi surface, both orbital-and spin-based scenarios have been proposed as the driving force. In this review, we focus on theories from the orbitalbased scenario and discuss several related experiments. It is pointed out that although both scenarios lead to the same macroscopic phases and are not distinguishable in bulk measurements of the thermodynamic properties, the elementary excitations could be fundamentally different, and provide us with the possibility to resolve this long-standing debate between orbital-and spin-based theories.

Superconductivity in Fe-chalcogenides

“…χ nem , on the other hand, is a consequence of degenerate spin fluctuations around momenta (π,0) and (0,π), and the ground state of the system breaks this degeneracy due to the strong quadrupole spin-spin interactions [113,114,125]. The main difficulty in resolving this debate experimentally lies in the fact that whenever one of these order parameters is non-zero, all others become non-zero simultaneously, for all of them break exactly the same C 4 rotational symmetry [126,127]. Therefore, typical thermodynamic properties used to characterize symmetry-breaking phases are blind to the orbital and the spin scenarios, and further experimental identifications have been proposed and measured to differentiate them.…”

Nematic Quantum Critical Fluctuations inBaFe2−xNixAs2Liu1, Gu2, Zhang3 et al. 2016Phys. Rev. Lett.496751View full textAdd to dashboardCite

“…In an itinerant ferromagnetic system, both the quasiparticle properties and the dynamics of order parameters may be altered fundamentally around the QCP [32,33]. It has been argued that nematic order may fall into the same theory as a zero-momentum order [34]. A recent Monte Carlo simulation in a metal suggests that the quantum critical nematic fluctuations has a high degree of isotropy at the nematic QCP [19].…”

Theory of point contact spectroscopy in correlated materials

“…ΣðeÞ =Σ sys ðeÞ +Σ L ðeÞ +Σ R ðeÞ; [6] withΣ sys ðeÞ being the self-energy due to the interactions inside the system H I , andΣ L;R ðeÞ being the self-energy due to the contacts.Γ L;R ðeÞ is the imaginary part ofΣ L;R ðeÞ. All of the functions here are matrices with indices ðm; nÞ standing for the degrees of freedom in H sys .…”

“…H eavy fermion systems (1, 2), high-T c cuprates (3,4), and very recently the iron-based superconductors (5,6) all exhibit symptoms of quantum criticality. The most striking feature of quantum criticality is that the quantum fluctuations associated with the quantum critial point (QCP) couple strongly to itinerant electrons, giving rise to drastic changes in the electronic properties.…”

“…In the heavy-fermions, the Fano background could be accounted for via multichannel tunneling models (10,(23)(24)(25). Our focus here is on establishing a clear link between the suggestive relationship that excess zero-bias signal measured in PCS is a direct measure of the effective density of states arising from electron correlations (6,19,21,26,27).…”