We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

We investigate the physical properties and electronic structure upon Cr-doping in the iron arsenide layers of BaFe 2 As 2 . This form of hole-doping leads to suppression of the magnetic/structural phase transition in BaFe 2-x Cr x As 2 for x > 0, but does not lead to superconductivity. For various x values, temperature dependence of the resistivity, specific heat, magnetic susceptibility, Hall coefficient, and single crystal x-ray diffraction data are presented. The materials show signatures of approaching a ferromagnetic state with x, including a metamagnetic transition for x as little as 0.36, an enhanced magnetic susceptibility, and a large Sommerfeld coefficient. Such results reflect renormalization due to spin fluctuations and they are supported by density functional calculations at x = 1.Calculations show a strong interplay between magnetic ordering and chemical ordering of Fe and Cr, with a ferromagnetic ground state. This ferromagnetic ground state is explained in terms of the electronic structure. The resulting phase diagram is suggestive that superconductivity does not derive simply from the suppression of the structural/magnetic transitions.2

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(Fe 1−x Cr x ) 2 As 2 as a function of Cr doping for 0 x 0.47. The average spin-density-wave moment is independent of concentration for x 0.2 and decreases rapidly for x 0.3. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials.The discovery of Fe-based superconductors 1 in multiple families of materials 2-4 with diverse doping possibilities enables systematic investigations which may ultimately lead to understanding the essential physics underlying superconductivity. The interplay of magnetism and superconductivity 5,6 has played a prominent role in the discussion based, partially, on the observation that superconductivity only emerges after sufficient suppression of the magnetically ordered state found in the parent compounds.The parent compounds of the so-called 122 family of materials, AFe 2 As 2 (A = Ba, Sr, Ca, and Eu), exhibit a structural phase transition, from tetragonal I 4/mmm to orthorhombic F mmm, 7 together with a simultaneous magnetic transition from a paramagnetic to antiferromagnetic spin-density-wave (SDW) state. Superconductivity only appears when these transitions are adequately suppressed. A key characteristic of the Fe-based materials is the variety of dopants which yield superconductivity. In particular, superconductivity obtained by doping on the transition-metal site is a unique property of the iron pnictides, i.e., by partial substitution of Fe by Co,8 Ni, 9 Rh, 10 Pd, 10 Ir, 11 Pt, 12 or Ru, 13,14 in BaFe 2 As 2 . From a chemical-doping perspective, the cases listed above are either isoelectronic substitution or electron doping on the Fe site. However, an exception occurs in the presence of hole doping; replacement of Fe with either Cr (Ref. 15) or Mn (Ref. 16) causes suppression of the SDW and structural transitions but does not result in superconductivity. The absence of superconductivity in these systems remains an unresolved issue.To address this question, we have selected the Ba(Fe 1−x Cr x ) 2 As 2 family of materials, for which bulk measurements have previously determined the phase diagram for x up to 0.18. 15 These studies showed that the magnetic and structural 17 transitions are much more resilient to doping than, for example, Co-doped BaFe 2 As 2 and, furthermore, do not yield superconductivity. The fact that these transitions are more robust in the case of Cr doping does not explain the lack of superconductivity. For example, doping with Ru causes the SDW state to persist to similar concentrations and, yet, superconductivity is still realized. 13,14 In this paper, we present neutron diffraction measurements of Ba(Fe 1−x Cr x ) 2 As 2 for 0 x 0.47. The SDW and structural transition temperatures are indistinguishabl...

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