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Register a new userAnisotropic electrical resistivity of LaFeAsO: evidence for electronic nematicity
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Single crystals of LaFeAsO were successfully grown out of KI flux. Temperature dependent electrical resistivity was measured with current flow along the basal plane, rho_perpend(T), as well as with current flow along the crystallographic c-axis, rho_parallel(T), the latter one utilizing electron beam lithography and argon ion beam milling. The anisotropy ratio was found to lie between rho_parallel/rho_perpend = 20 - 200. The measurement of rho_perpend(T) was performed with current flow along the tetragonal [1 0 0] direction and along the [1 1 0] direction and revealed a clear in-plane anisotropy already at T leq 175 K. This is significantly above the orthorhombic distortion at T_0 = 147 K and indicates the formation of an electron nematic phase. Magnetic susceptibility and electrical resistivity give evidence for a change of the magnetic structure of the iron atoms from antiferromagnetic to ferromagnetic arrangement along the c-axis at T^ast = 11 K.
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Orbital ordering has recently emerged as another important state in iron based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.
We expose the theoretical mechanisms underlying disorder-induced nematicity in systems exhibiting strong fluctuations or ordering in the nematic channel. Our analysis consists of a symmetry-based Ginzburg-Landau approach and associated microscopic calculations. We show that a single featureless point-like impurity induces nematicity locally, already above the critical nematic transition temperature. The persistence of fourfold rotational symmetry constrains the resulting disorder-induced nematicity to be inhomogeneous and spatially average to zero. Going beyond the single impurity case, we discuss the effects of finite disorder concentrations on the appearance of nematicity. We identify the conditions that allow disorder to enhance the nematic transition temperature, and we provide a concrete example. The presented theoretical results can explain a large series of recent experimental discoveries of disorder-induced nematic order in iron-based superconductors.
We present measurements of the thermal expansion coefficient alpha of polycrystalline LaFeAsO1-xFx (x <= 0.1). The magnetic and structural transitions of the samples with x <= 0.04 give rise to large anomalies in alpha(T), while the onset of superconductivity in the crystals with x >= 0.05 is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for x = 0.05 is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping.
This paper has been withdrawn by the authors due to errors in the X-ray diffraction data. Other measured data are not affected; however, the errors significantly change the interpretation and conclusions, and thus warrant withdrawal and later resubmission.
Anisotropic resistivities of Bi_2Sr_2Ca_{1-x}Er_xCu_2O_8 single crystals were measured and analyzed from 4.2 to 500 K with special interest in the parent antiferromagnetic insulator of x=1.0. Although the resistivity is semiconducting along both the in- and out-of-plane directions, the temperature dependence is found to be significantly different. As a result, the resistivity ratio for x=1.0 takes a broad maximum near room temperature. The electric conduction in parent antiferromagnetic insulators is different from other semiconductors, and is as unconventional as that in high-temperature superconductors.
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