Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of approximately 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

Inelastic neutron scattering measurements on the low energy spin waves in CaFe 2 As 2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic threedimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data. PACS: 75.30.Ds, 78.70.Nx, 75.30.Et, However, the parent phases of the iron-arsenides are not insulators. Rather, they are metallic and, for the AFe 2 As 2 compounds, the AF ordering is strongly coupled to a structural transition from a high-temperature tetragonal structure to a low temperature orthorhombic structure.[12] One other notable difference between the cuprates and iron arsenides concerns the conditions necessary for SC. While doping charge carriers does indeed suppress AF and lead to superconductivity in both systems, it has recently been shown that pressure alone can destroy the AF state in CaFe 2 As 2 and lead to SC. [13,14] 3 Despite these differences, the energy scale and dimensionality (or anisotropy) of the magnetic interactions may actually be quite similar, possibly leading to a common origin for SC in these two families of compounds. In order to move beyond qualitative comparisons and address the relevance of magnetic interactions to SC in the ironarsenides, direct measurements of the energy scale and anisotropy of the magnetic interactions are necessary. Here we report results from inelastic neutron scattering from CaFe 2 As 2 , both below and above the AF ordering temperature, and demonstrate that the magnetic exchange interactions are exceptionally large, with a similar energy scale as the cuprates. Although the magnetic exchange between the Fe layers is relatively small (> ~10% of the in-plane exchange), it is substantially larger than that found for the cuprates (~0.001%). This anisotropic 3D magnetism is supported by theoretical calculations of the spin dynamics. Despite the first-order magnetostructural transition observed in CaFe 2 As 2 , spin correlations are observed to persist above the AF ordering temperature, attesting to the strength of the magnetism and supportive of a model of frustrated magnetism in the high-temperature tetragonal phase.CaFe 2 As 2 is a non-superconducting parent compound that becomes superconducting by either doping [15] or the application of pressure.[14] CaFe 2 As 2 orders into a columnartype AF structure (as shown in Fig 1a)) with a simultaneous structural transition from a tetragonal (I4/mmm) to an orthorhombic (Fmmm) crystal structure below T s = 172 K with a = 5.51 Å, b = 5.45 Å, and c = 11.66 Å.[12] For the inelastic neutron scattering study, single crystals of CaFe 2 As 2 were grown out of Sn flux using conventional high temperature solution growth techniques described previously. scattering plane (in orthorhombic...

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the ''phonon-glass electron crystal'' (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg 3 Sb 2 and its derivative of isoelectronically Bi doped Bi; Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) alloys in Mg 3 Sb 2 . Single phase p-type Mg 3 Sb 2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg 3 Sb 2 . However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 6 10 20 /cm 3 for x ¢0.25. The substantial increase in ZT in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

We report the use of Boltzmann transport theory to investigate the electrical properties of thermoelectric Mg 2 Si, Mg 2 Sn, and a supercell model of the 50-50 alloy. The results are based on first-principles electronic structure calculations with the modified Becke-Johnson potential of Tran and Blaha, which yields band gaps in good accord with experiment. The calculated transport coefficients are discussed in relation to the thermoelectric performance of these materials. The results imply roughly symmetric behavior with respect to carrier type and the possibility of improvements in ZT , especially for p-type and lower temperatures.