Electronic correlations in the iron pnictides

Abstract

In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy 1 . The hightransition-temperature (T c ) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-T c superconductivity in the iron pnictides 2-4 puts a spotlight on the relevance of correlation effects in these materials 5 . Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.The recent discovery of superconductivity in the iron pnictides promises to be an important milestone in condensed-matter physics 2,3 . Here is a new class of materials with a layered structure and relatively high superconducting T c values 3,4 rivalling the doped cuprates. Electronic conduction is believed to occur in the ironpnictogen layers 6 , similar to the cuprates where the charge carriers are delocalized in the copper-oxygen planes. Two decades of research on the cuprates have established that a proper account of the exotic normal-state properties is a prerequisite for the understanding of the superconducting instability 7 .The correlated metallic state of the superconducting cuprates is derived through chemical doping of the parent compounds, which are strongly correlated (Mott) insulators. However, the parent compounds of the iron pnictides are metallic, albeit highly dissipative, bad metals 5 . Magnetic ordering in the parent iron arsenides at low temperatures leads to partial gapping of the Fermi surface but does not initiate an insulating state. Moreover, there is no evidence of long-range magnetic ordering in the 1111-iron phosphide LaFePO (ref. 8). Recent theoretical work on the normal state of the iron pnictides suggests that despite the apparent itinerant behaviour, Mott physics is relevant to charge dynamics and magnetic properties 5,9,10 . Thus motivated, we investigated the normal state of the 1111-iron phosphide superconductor LaFePO with infrared and optical spectroscopy, focusing on charge dynamics in the conducting planes.An optical experiment measures the dynamical response of the electron subjected to an external electromagnetic field and facilitates monitoring of many-body effects experienced by the electron in a material. These many-body effects include the interaction of the electron with other electrons, phonons as well as ordered or fluctuating spins. Figure 1a shows the real part of the ab-plane optical conductivity σ 1 (ω) of LaFePO over a broad frequency range. Sample growth and characterization procedures, and...

Quantum oscillations probe the normal electronic states of novel superconductors

“…This value suggests that LaFePO is a moderately correlated system with a Fermi surface which can be described by band structure calculations. Similar mass enhancement, when compared with the bare band mass, was found also in ARPES and optical studies (Lu et al 2008;Qazilbash et al 2009). Despite the absence of a Mott transition the electronic many-body effects are believed to be important in iron pnictides.…”

Nanoscale chemical phase separation in FeTe0.55Se0.45as seen via scanning tunneling spectroscopy

“…Although there are some similarities to cuprates -layered structure and proximity to magnetism, Fe-based superconductors are chemically much more flexible, where superconductivity can be induced by either external pressure, or isovalent substitution (chalcogen), or partial replacement for the Fe by other transition metals. Thus these materials offer a new platform to explore collective behavior in complex transition metal compounds, including the pairing mechanism for superconductivity, the relevance of electron correlations 5,6 to the functionality, and the possible relationship between quantum critical behavior and superconductivity 7 .…”

Spin dynamics of aJ1−J2antiferromagnet and its implications for iron pnictides

“…The experimentally observed "bad metal" behavior, the Drude-weight suppression 15,16 and the temperature-induced spectral-weight transfer [16][17][18] place these materials near to a Mott transition 4,9,19,20 ; a Mott insulator can emerge when the iron square lattice either expands 21 or contains ordered vacancies 22 . In a metallic system close to a Mott transition, quasi-local moments are expected to arise; this picture is further supported by the experimental observation of zone boundary spin wave excitations in the magnetically ordered state at low temperatures 23 .…”