A novel non-Fermi-liquid state in the iron-pnictide FeCrAs

We report transport and thermodynamic properties of stoichiometric single crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows an unusual “non-metallic” dependence on temperature T , rising continuously with decreasing T from ∼800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at TN ∼ 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, ρ(T ) = ρ0−AT x with x < 1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, γ ∼ 30 mJ/mol K. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T → 0 K properties suggest a new class of non-Fermi liquid. Introduction. – Metallic states that violate Landau’s Fermi liquid paradigm appear in many strongly correlated electron systems such as doped cuprates, quantum critical metals, and disordered Kondo lattices [1–3]. The new layered iron-pnictide high temperature superconductors also show non-Fermi-liquid behaviour, most notably in the undoped parent compounds which have a high, roughly constant resistivity at temperatures above a magnetic spin-density-wave transition that typically occurs around TSDW ∼ 150 K (see, for example, [4–7]). For T < TSDW , however, this incoherent charge transport gives way to a rapidly falling (i.e. “metallic”) resistivity. The basic structural units of the new iron-pnictide superconductors are layers in which iron atoms are tetrahedrally coordinated by arsenic. Some theoretical approaches to the incoherent T > TSDW state in these systems suggest that the physics is different from that of the cuprates, not least because this tetrahedral coordination produces only a small crystal-field splitting of the Fe 3dshell. Sawatzky et al. [8] argue that the electronic structure is dominated by narrow iron 3d bands coupled to a highly polarizable arsenic environment. Dynamical mean field theory (DMFT) treatments [9–11] find an incoherent metallic state that arises from strong on-site inter-orbital interactions in the 3d-shell that perhaps puts some [9] or all [10] of the weakly crystal field split 3d-orbitals close to a Mott transition. The local physics of this so-called “bad semiconducting” state may represent a qualitatively new kind of incoherent metal. In this paper we present magnetic, thermodynamic and transport properties of a ternary iron-arsenide, FeCrAs, whose behaviour shows intriguing similarities to the parent compounds of the layered iron-pnictide superconductors, including an antiferromagnetic transition in a similar temperature range (TN ∼ 125 K). However there are key differences: firstly, the crystal structure is hexagonal as opposed to tetragonal; secondly, although the iron atoms are tetrahedrally coordinated by arsenic, FeCrAs is threedimensional (it does not have insulating layers); thirdly, as shown below, in FeCrAs the magnetism for T < TN resides primarily on the Cr sites, whereas in the layered iron-pnictides the magnetism is on the iron sites; and finally, the behaviour of the electrical resistivity of FeCrAs is much more extreme in that it is “non-metallic” over a huge temperature range – the in-plane resistivity rises with decreasing temperature from ∼800 K to below 100 mK without any sign of saturation or a gap at low temperature. Moreover, there is a profound and novel incompat-