http://hdl.handle.net/1813/3129 2013-05-26T08:26:38Z http://hdl.handle.net/1813/33260 Title: Ultrafast thermoelectric properties of gold under conditions of strong electron-phonon nonequilibrium Authors: Hopkins, Patrick; Bauer, Matthew; Duda, John; Smoyer, Justin; English, Timothy; Norris, Pamela; Beechem, Thomas; Stewart, Derek Abstract: The electronic scattering rates in metals after ultrashort pulsed laser heating can be drastically different than those predicted from free electron theory. The large electron temperature achieved after ultrashort pulsed absorption and subsequent thermalization can lead to excitation of subconduction band thermal excitations of electron orbitals far below the Fermi energy. In the case of noble metals, which all have a characteristic flat d-band several electron volts well below the Fermi energy, the onset of d-band excitations has been shown to increase electron-phonon scattering rates by an order of magnitude. In this paper, we investigate the effects of these large electronic thermal excitations on the ultrafast thermoelectric transport properties of gold, a characteristic noble metal. Under conditions of strong electron-phonon nonequilibrium (relatively high electron temperatures and relatively low lattice temperatures, T_{e} >> T_{L}), we find that the Wiedemann–Franz law breaks down and the Seebeck coefficient is massively enhanced. Although we perform representative calculations for Au, these results are expected to be similar for the other noble metals (Ag and Cu) due to the characteristic large d-band separation from the Fermi energy. 2010-11-23T00:00:00Z http://hdl.handle.net/1813/30570 Title: Thermal conductivity of bulk and nanowire Mg2Si_{x}Sn_{1-x} alloys from first principles Authors: Li, Wu; Lindsay, Lucas; David, Broido; Stewart, Derek; Mingo, Natalio Abstract: The lattice thermal conductivity (κ) of the thermoelectric materials, Mg2Si, Mg2Sn, and their alloys, are calculated for bulk and nanowires, without adjustable parameters. We find good agreement with bulk experimental results. For large nanowire diameters, size effects are stronger for the alloy than for the pure compounds. For example, in 200 nm diameter nanowires κ is lower than its bulk value by 30%, 20%, and 20% for Mg2Si0.6Sn0.4, Mg2Si, and Mg2Sn, respectively. For nanowires less than 20 nm thick, the relative decrease surpasses 50%, and it becomes larger in the pure compounds than in the alloy. At room temperature, κ of Mg2Si_{x}Sn_{1−x} is less sensitive to nanostructuring size effects than Si_{x}Ge_{1−x}, but more sensitive than PbTe_{x}Se_{1−x}. This suggests that further improvement of Mg2Si_{x}Sn_{1−x} as a nontoxic thermoelectric may be possible. 2012-11-29T00:00:00Z http://hdl.handle.net/1813/29688 Title: Chemical ordering in Cr3Al and relation to semiconducting behavior Authors: Boekelheide, Zoe; Stewart, Derek; Hellman, Frances Abstract: Cr3Al shows semiconductor-like behavior which has been attributed to a combination of antiferromagnetism and chemical ordering of the Cr and Al atoms on the bcc sublattice. This article presents a detailed theoretical and experimental study of the chemical ordering in Cr3Al. Using density functional theory within the Korringa-Kohn-Rostoker (KKR) formalism, we consider five possible structures with the Cr3Al stoichiometry: a bcc solid solution, two-phase C11b Cr2Al+Cr, off-stoichiometric C11b Cr3Al, D03 Cr3Al, and X-phase Cr3Al. The calculations show that the chemically ordered, rhombohedrally distorted X-phase structure has the lowest energy of those considered and should, therefore, be the ground state found in nature, while the D03 structure has the highest energy and should not occur. While KKR calculations of the X phase indicate a pseudogap in the density of states, additional calculations using a full potential linear muffin-tin orbital approach and a plane-wave technique show a narrow band gap. Experimentally, thin films of Cr(1−x)Alx were grown and the concentration, growth temperature, and substrate were varied systematically. The peak resistivity (2400 μΩ-cm) is found for films with x=0.25, grown epitaxially on a 300 ∘C MgO substrate. At this x, a transition between nonmetallic and metallic behavior occurs at a growth temperature of about 400 ∘C, which is accompanied by a change in chemical ordering from X phase to C11b Cr3Al. These results clarify the range of possible structures for Cr3Al and the relationship between chemical ordering and electronic transport behavior. 2012-08-15T00:00:00Z http://hdl.handle.net/1813/29066 Title: Antiferromagnetism in Cr3Al and relation to semiconducting behavior Authors: Boekelheide, Zoe; Saerbeck, Thomas; Stampfl, Anton; Robinson, Robert; Stewart, Derek; Hellman, Frances Abstract: Antiferromagnetism and chemical ordering have both been previously suggested as causes of the observed semiconductorlike behavior in Cr3Al. Two films of Cr3Al(001)/MgO(001) were grown under different conditions to achieve different types of chemical ordering and electronic properties: one X-phase structure (semiconducting) and one C11b structure (metallic). The films were investigated by x-ray and neutron diffraction. Both films show commensurate antiferromagnetic order, with a high Néel temperature greater than 578 K, showing that the antiferromagnetism in Cr3Al is quite robust. Density-functional theory calculations were performed and it was shown that the well-known antiferromagnetic pseudogap in the density of states occurs for all types of chemical ordering considered. The conclusion of these studies is that the antiferromagnetism causes a pseudogap in the density of states, which is a necessary condition for the semiconductorlike transport behavior; however, that antiferromagnetism is seen in both metallic and semiconducting Cr3Al samples shows that antiferromagnetism is not a sufficient condition for semiconducting behavior. Chemical ordering is equally important. 2012-03-09T00:00:00Z http://hdl.handle.net/1813/29065 Title: Thermal conductivity of diamond nanowires from first principles Authors: Li, Wu; Mingo, Natalio; Lindsay, Lucas; Broido, David; Stewart, Derek; Katcho, Nebil Abstract: Using ab initio calculations we have investigated the thermal conductivity (k) of diamond nanowires, unveiling unusual features unique to this system. In sharp contrast with Si, k(T) of diamond nanowires as thick as 400 nm still increase monotonically with temperature up to 300 K, and room-temperature size effects are stronger than for Si. A marked dependence of k on the crystallographic orientation is predicted, which is apparent even at room temperature. [001] growth direction always possesses the largest k in diamond nanowires. The predicted features point to a potential use of diamond nanowires for the precise control of thermal flow in nanoscale devices. 2012-05-17T00:00:00Z http://hdl.handle.net/1813/23577 Title: Role of light and heavy embedded nanoparticles on the thermal conductivity of SiGe alloys Authors: Kundu, Anupam; Mingo, Natalio; Broido, David; Stewart, Derek Abstract: We have used an atomistic ab initio approach with no adjustable parameters to compute the lattice thermal conductivity of Si0.5Ge0.5 with a low concentration of embedded Si or Ge nanoparticles of diameters up to 4.4 nm. Through exact Green's function calculation of the nanoparticle scattering rates, we find that embedding Ge nanoparticles in Si0.5Ge0.5 provides 20% lower thermal conductivities than embedding Si nanoparticles. This contrasts with the Born approximation, which predicts an equal amount of reduction for the two cases, irrespective of the sign of the mass difference. Despite these differences, we find that the Born approximation still performs remarkably well, and it permits investigation of larger nanoparticle sizes, up to 60 nm, not feasible with the exact approach. 2011-09-09T00:00:00Z http://hdl.handle.net/1813/22944 Title: Thermal conductivity of indium arsenide nanowires with wurtzite and zinc blende phases Authors: Zhou, Feng; Moore, Arden; Bolinsson, Jessica; Persson, Ann; Froberg, Linus; Pettes, Michael; Kong, Huijun; Rabenberg, Lew; Caroff, Philippe; Stewart, Derek; Mingo, Natalio; Dick, Kimberly; Samuelson, Lars; Linke, Heiner; Shi, Li Abstract: The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron microscopy. Nanowires of both phases showed a reduction of thermal conductivity by a factor of 2 or more compared to values reported for zinc blende indium arsenide bulk crystals within the measured temperature range. Theoretical models were developed to analyze the measurement results and determine the effect of phase on phonon transport. Branch-specific phonon dispersion data within the discretized first Brillouin zone were calculated from first principles and used in numerical models of volumetric heat capacity and thermal conductivity. The combined results of the experimental and theoretical studies suggest that wurtzite indium arsenide possesses similar volumetric heat capacity, weighted average group velocity, weighted average phonon-phonon scattering mean free path, and anharmonic scattering-limited thermal conductivity as the zinc blende phase. Hence, we attribute the differing thermal conductivity values observed in the indium arsenide nanowires of different phases to differences in the surface scattering mean free paths between the nanowire samples. 2011-05-19T00:00:00Z http://hdl.handle.net/1813/19474 Title: Band Gap and Electronic structure of an Epitaxial, Semiconducting Cr0.80Al0.20 Thin Film Authors: Boekelheide, Z.; Gray, A. X.; Papp, C.; Balke, B.; Stewart, D. A.; Ueda, S.; Kobayashi, K.; Hellman, F.; Fadley, C. S. Abstract: Cr(1-x)Alx exhibits semiconducting behavior for x=0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr0.80Al0.20 thin film show several features in the band region, including a gap at the Fermi energy (Ef) for which the valence band edge is 95 +- 14 meV below Ef. Theory agrees well with the valence band measurements, and shows an incomplete gap at Ef due to the hole band at M shifting almost below Ef. 2010-12-03T00:00:00Z http://hdl.handle.net/1813/15842 Title: Cluster scattering effects on phonon conduction in graphene Authors: Mingo, Natalio; Esfarjani, Keivan; Broido, David; Stewart, Derek Abstract: The phonon-scattering cross section associated with isotopic clusters is evaluated from first principles and used to estimate the reduction in thermal conductance of wide graphene samples. A strong sensitivity of the thermal conductivity toward clustering is predicted for micrometer-sized samples at low-temperatures. Important differences are obtained between the atomistically computed cross section, and existing analytical approximations, emphasizing the importance of atomistic investigations of thermal transport. Finally, possible techniques are suggested for synthesizing graphene containing isotopic clusters. 2010-01-13T00:00:00Z http://hdl.handle.net/1813/13697 Title: Ab initio theory of the lattice thermal conductivity in diamond Authors: Ward, Alistair; Broido, David; Stewart, Derek; Deinzer, Gernot Abstract: We present a first-principles theoretical approach to calculate the lattice thermal conductivity of diamond based on an exact solution of the Boltzmann transport equation. Density-functional perturbation theory is employed to generate the harmonic and thir-order anharmonic interatomic force constants that are required as input. A central feature of this approach is that it provides accurate representations of the interatomic forces and at the same time introduced no adjustable parameters. The calculated lattice thermal conductivities for isotopically enriched and naturally occurring diamond are both in very good agreement with experimental data. The role of the scattering of heat-carrying acoustic phonons by optic branch phonons is also investigated. We show that inclusion of this scattering channel is indispensable in properly describing the thermal conductivity of semiconductors and insulators. The accurate adjustable-parameter free results obtained herein highlight the promise of this approach in providing predictive descriptions of the lattice thermal conductivity of materials. 2009-09-16T00:00:00Z