David S. Parker

Extended s± scenario for the nuclear spin-lattice relaxation rate in superconducting pnictides

D. Parker 1,2,6, O.V. Dolgov3, M.M. Korshunov1,4, A.A. Golubov5, and I.I. Mazin6 1Max-Planck-Institut für Physik komplexer Systeme, D-011 87 Dresden, Germany 2Max-Planck Institut für Chemische Physik fester Stoffe, D -01187 Dresden, Germany 3Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany 4L.V. Kirensky Institute of Physics, Siberian Branch of RAS, 660036 Krasnoyarsk, Russia 5Faculty of Science and Technology, University of Twente, 75 00 AE Enschede, The Netherlands and 6Naval Research Laboratory, 4555 Overlook Ave. SW, Washingt on, DC 20375 (Dated: September 10, 2008)

First principles investigations of the thermoelectric behavior of tin sulfide

We present the results of density functional theory and Boltzmann transport calculations suggesting that the commonly available and inexpensive material SnS may show reasonable thermoelectric performance at temperatures suitable for exhaust waste heat recovery, if the material can be heavily p-doped and if the mobility is not greatly degraded by high doping.

Seebeck and Figure of Merit Enhancement in Nanostructured Antimony Telluride by Antisite Defect Suppression through Sulfur Doping

Antimony telluride has a low thermoelectric figure of merit (ZT < ∼0.3) because of a low Seebeck coefficient α arising from high degenerate hole concentrations generated by antimony antisite defects. Here, we mitigate this key problem by suppressing antisite defect formation using subatomic percent sulfur doping. The resultant 10-25% higher α in bulk nanocrystalline antimony telluride leads to ZT ∼ 0.95 at 423 K, which is superior to the best non-nanostructured antimony telluride alloys. Density functional theory calculations indicate that sulfur increases the antisite formation activation energy and presage further improvements leading to ZT ∼ 2 through optimized doping. Our findings are promising for designing novel thermoelectric materials for refrigeration, waste heat recovery, and solar thermal applications.

Magnetic phase transition in single crystals of the chiral helimagnet Cr1/3NbS2

The chiral helimagnet Cr1/3NbS2 has been investigated by magnetic, transport and thermal properties measurements on single crystals and by first principles electronic structure calculations. From the measured field and temperature dependence of the magnetization for fields applied perpendicular to the c axis, the magnetic phase diagram has been constructed in the vicinity of the phase transitions. A transition from a paramagnetic to a magnetically ordered phase occurs near 120 K. With increasing magnetic field and at temperatures below 120 K, this material undergoes transitions from a helimagnetic to a soliton-lattice phase near 900 Oe, and then to a ferromagnetic phase near 1300 Oe. The transitions are found to strongly affect the electrical transport. The resistivity decreases sharply upon cooling near 120 K, and the spin reorientation from the helimagnetic ground state to the commensurate ferromagnetic state is evident in the magnetoresistance. At high fields a large magnetoresistance (55 % at 140 kOe) is observed near the magnetic transition temperature. Heat capacity and electronic structure calculations show the density of states at the Fermi level is low in the magnetically ordered state. Effects of spin fluctuations are likely important in understanding the behavior of Cr1/3NbS2 near and above the magnetic ordering transitions.

Very heavily electron-doped CrSi2 as a high-performance high-temperature thermoelectric material

We analyze the thermoelectric behavior, using first principles and Boltzmann transport calculations, of very heavily electron-doped CrSi2 and find that at temperatures of 900?1250?K and electron dopings of 1?4???1021?cm?3, thermopowers as large in magnitude as 200??V?K?1 may be found. Such high thermopowers at such high carrier concentrations are extremely rare, and suggest that excellent thermoelectric performance may be found in these ranges of temperature and doping.

Electronic and transport properties of zintl phase AeMg2Pn2, Ae = Ca,Sr,Ba, Pn = As,Sb,Bi in relation to Mg3Sb2

First principles calculations of the electronic structure and transport properties of Zintl phase AeMg2Pn2, Ae = Ca,Sr,Ba, Pn = As,Sb,Bi compounds and Mg3Sb2 are reported. These are discussed in relation to the thermoelectric performance of the compounds and specifically the optimization of carrier concentration. It is found that there are several promising compositions and that the materials that have been studied to date are not fully optimized in terms of carrier concentration and may not ultimately be the best thermoelectrics in this family. We additionally report optical properties and show that there are significant differences among the compounds arising from differences in electronic structure and that these differences should be accessible to experiment. This provides a way to test the trends observed here.

Ferromagnetism of Fe3Sn and alloys.

Hexagonal Fe3Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K1 of 1.8 MJ/m3. However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. However, transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe3Sn is reported, as are some are magnetic and structural properties for the Fe3Sn2, and Fe5Sn3 compounds, which could be prepared as mm-sized single crystals.

Thermoelectric properties of n-type PbSe revisited

It was recently predicted [D. Parker D. J. Singh, Phys. Rev. B 82, 035204 (2010)], contrary to then-prevalent opinion, that p-type PbSe would be a good thermoelectric material. This prediction was confirmed in Wang et al. [Adv. Mater. 23, 1367 (2011)], and recent experimental work (Zhang et al. Energy Env. Sci. 5, 5246 (2012)] now indicates that n-type PbSe may show good thermoelectric performance. In light of this work, we now re-examine the thermoelectric performance of n-type PbSe with a revised approximation (not available at the time of the original work) which improves band gap accuracy. We now find that n-type PbSe has large thermopowers characteristic of a high performance material, with values as high in magnitude as 250 μV/K at 1000 K and 300 μV/K at 800 K. We further find that optimal 1000 K n-type doping ranges are between 2 × 1019 cm−3 and 7 × 1019 cm−3, while at 800 K, the corresponding range is from 7 × 1018 to 4 × 1019 cm−3.

Acoustic impedance and interface phonon scattering in Bi2Te3 and other semiconducting materials

We present first principles calculations of the phonon dispersions of \BiTe and discuss these in relation to the acoustic phonon interface scattering in ceramics. The phonon dispersions show agreement with what is known from neutron scattering for the optic modes. We find a difference between the generalized gradient approximation and local density results for the acoustic branches. This is a consequence of an artificial compression of the van der Waals bonded gaps in the \BiTe structure when using the generalized gradient approximation. As a result local density approximation calculations provide a better description of the phonon dispersions in Bi