Showing papers by "Arthur J Freeman published in 2010"

Role of an interfacial FeO layer in the electric-field-driven switching of magnetocrystalline anisotropy at the Fe/MgO interface

Abstract: The electric-field-induced switching of magnetocrystalline anisotropy (MCA) between in-plane and out-of-plane orientations is investigated by first-principles calculations for the prototypical Fe on MgO(001) system. Our results predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, but the MCA switching by an applied electric field is found to be difficult to achieve. Instead, the existence of an interfacial FeO layer plays a key role in demonstrating the MCA switching that accompanies an electric-field-induced displacement of Fe atoms on the interfacial FeO layer.

Synthesis in Ionic Liquids: [Bi2Te2Br](AlCl4), a Direct Gap Semiconductor with a Cationic Framework

Abstract: The Lewis acidic ionic liquid EMIMBr-AlCl3 (EMIM = 1-ethyl-3-methylimidazolium) allows a novel synthetic route to the semiconducting layered metal chalcogenides halide [Bi2Te2Br](AlCl4) and its Sb analogue. [Bi2Te2Br](AlCl4) is a direct band gap, strongly anisotropic semiconductor and consists of cationic infinite layers of [Bi2Te2Br]+ and [AlCl4]− anions inserted between the layers.

Electronic structure and volume effect on thermoelectric transport in p-type Bi and Sb tellurides

Abstract: Thermoelectric transport properties (Seebeck coefficient, $S$, and electrical conductivity, $\ensuremath{\sigma}$) of $p$-type Bi and Sb tellurides are investigated using a first-principles all-electron density-functional approach. We demonstrate that the carrier concentration, band gap, and lattice constants have an important influence on the temperature behavior of $S$ and that the volume expansion by 5.5% in ${\text{Sb}}_{2}{\text{Te}}_{3}$ results in an increase in $S$ by $33\text{ }\ensuremath{\mu}\text{V}/\text{K}$ at 300 K. We argue that in addition to the electronic structure characteristics, the volume also affects the value of $S$ and hence should be considered as an origin of the experimental observations that $S$ can be enhanced by doping ${\text{Sb}}_{2}{\text{Te}}_{3}$ with Bi (which has a larger ionic size) in Sb sites or by the deposition of thick ${\text{Bi}}_{2}{\text{Te}}_{3}$ layers alternating with thinner ${\text{Sb}}_{2}{\text{Te}}_{3}$ layers in a superlattice, ${\text{Bi}}_{2}{\text{Te}}_{3}/{\text{Sb}}_{2}{\text{Te}}_{3}$. We show that the optimal carrier concentration for the best power factor of ${\text{Bi}}_{2}{\text{Te}}_{3}$ and ${\text{Sb}}_{2}{\text{Te}}_{3}$ is approximately ${10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$.

Interfacial Dirac cones from alternating topological invariant superlattice structures of Bi2Se3.

Abstract: When the three-dimensional topological insulators ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ have an interface with vacuum, i.e., a surface, they show remarkable features such as topologically protected and spin-momentum locked surface states. However, for practical applications, one often requires multiple interfaces or channels rather than a single surface. Here, for the first time, we show that an interfacial and ideal Dirac cone is realized by alternating band and topological insulators. The multichannel Dirac fermions from the superlattice structures open a new way for applications such as thermoelectric and spintronics devices. Indeed, utilizing the interfacial Dirac fermions, we also demonstrate the possible power factor improvement for thermoelectric applications.

First-principles investigation of magnetism and electronic structures of substitutional 3d transition-metal impurities in bcc Fe

Abstract: The magnetic and electronic structures of $3d$ impurity atoms from Sc to Zn in ferromagnetic body-centered-cubic iron are investigated using the all-electron full-potential linearized augmented plane-wave method based on the generalized gradient approximation (GGA). We found that, in general, the GGA results are closer to the experimental values than those of the local spin density approximation. The calculated formation enthalpy data indicate the importance of a systematic study on the ternary $\text{Fe-C-}X$ systems rather than the binary $\text{Fe-}X$ systems in steel design. The lattice parameters are optimized and the conditions for spin polarization at the impurity sites are discussed in terms of the local Stoner model. Our calculations, which are consistent with previous work, imply that the local spin polarizations at Sc, Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal atoms couple antiferromagnetically, while the late transition-metal atoms couple ferromagnetically to the host Fe atoms. The calculated total magnetization $(M)$ of bcc Fe is reduced by impurity elements from Sc to Cr as a result of the antiferromagnetic interaction, with the opposite effect for solutes which couple ferromagnetically. The changes in $M$ are attributed to nearest neighbor interactions, mostly between the impurity and host atoms. The atom averaged magnetic moment is shown to follow generally the well-known Slater-Pauling curve, but our results do not follow the linearity of the Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the trend of the local magnetic moments. The effect of spin-orbit coupling is found not to be significant although it comes into prominence at locations far from the impurity sites.

Transport and band structure studies of crystalline ZnRh2 O4

Abstract: We report the synthesis and characterization of non-${d}^{10}$ $p$-type transparent conducting oxides of the normal spinel ${\text{ZnRh}}_{2}{\text{O}}_{4}$. Undoped ${\text{ZnRh}}_{2}{\text{O}}_{4}$ was successfully prepared by means of bulk solid-state synthesis. The conduction mechanism and bulk defect chemistry of polycrystalline sintered pellets of ${\text{ZnRh}}_{2}{\text{O}}_{4}$ were studied through electrical conductivity and Seebeck coefficient measurements, in defect equilibrium at elevated temperature under controlled atmospheres. Optical diffuse reflectance measurements were also carried out to evaluate band gap. The data were analyzed in terms of an activated mobility (small polaron conduction), with a hopping energy of 0.25 eV. Results from band structure calculations by $\text{LDA}+\text{U}$ and optical band-gap measurement by UV-visible spectrometry are in good agreement with literature data.

Continuously tunable band gap in GaN/AlN (0001) superlattices via built-in electric field

Abstract: Based on all-electron density-functional theory calculations using the generalized gradient approximation, we demonstrate the continuous tunability of the band gap and strength of the built-in electric field in GaN/AlN 0001 superlattices by control of the thickness of both the well GaN and barrier AlN regions. The effects of strain for these quantities are also studied. Calculations taking into account the self-interaction correction exhibit the same dependence on thickness. The calculated electric field strength values are in good agreement with recent experiments. Spontaneous polarization dominates the contribution to the electric field and the strain-induced piezoelectric polarization is estimated to contribute only about 5‐10%.

A Systematic Study on Iron Carbides from First-Principles

Abstract: The formation enthalpy of a series of Fe-C carbides has been estimated using a first-principles approach. The Fe to C ratio dependence of the formation enthalpy is reasonable, but it is revealed that - and -carbides require an extraordinary environment to be able to form. Furthermore, an addition of substitutional solutes other than Fe and C should promote other carbides with different crystal structures. The analysis suggests further studies to discover the critical concentrations of alloying which stimulate the other carbides to become more stable.

Magneto-optical spectra of Mn-Ge films

Abstract: Experimental magneto-optical Kerr effect spectra of Mn-implanted Ge samples are interpreted using an ab-initio theoretical model based on density functional theory. The presence of Mn in the Ge matrix is accounted for considering either Mn precipitation as Mn5Ge3 nanoparticles or as perfect dilution. The Mn concentration profile in the sample is also considered in the theoretical model. The computational results are in good qualitative agreement if Mn5Ge3 nanocluster formation is assumed. The effect of partial etching of the implanted region is also studied by comparing our experimental and theoretical MOKE spectra data.

Electric field-induced change in magnetocrystalline anisotropy in ferromagnetic transition-metal thin films

Abstract: In a search for promising ferromagnets exhibiting a giant magnetocrystalline anisotropy (MCA) change by an external electric field, the MCA in the Fe-Co(001) alloy monolayers is determined by means of full-potential linearized augmented plane-wave method. A large MCA change by the electric field is found to appear in the Fe0.75Co0.25 monolayer, and a reasonable origin from a band structural change in the minority-spin d states is obtained, where a position of Fermi level relative to the d band level is a key factor for designing the giant MCA change.

The Tellurophosphate K4P8Te4: Phase-Change Properties, Exfoliation, Photoluminescence in Solution and Nanospheres.

Abstract: We describe the inorganic polymer K(4)P(8)Te(4) which is soluble, giving solutions that exhibit white emission upon 355 nm laser irradiation. An indirect band gap semiconductor (E(g) approximately 1.4 eV), K(4)P(8)Te(4) crystallizes in the space group P2(1)/m, with a = 6.946(1) A, b = 6.555(1) A, c = 9.955(2) A, and beta = 90.420(3) degrees at 173(2) K. The compound features infinite chains of [P(8)Te(4)(4-)] with covalent P-Te bonding and exhibits reversible crystal-glass phase-change behavior. When deposited from solution, the material forms highly crystalline K(4)P(8)Te(4) nanospheres. The thermal analysis, FT-IR, UV-vis diffuse reflectance, (31)P magic angle spinning solid-state NMR spectroscopy, and pair distribution function (PDF) analysis for the crystal and glass forms and ab initio electronic structure calculations by the screened-exchange local density function approximation are also reported. Speciation of K(4)P(8)Te(4) in solution studied with (31)P solution-state NMR spectroscopy, electrospray ionization mass spectrometry, and PDF analysis indicate exfoliation of [P(8)Te(4)(4-)] chains followed by rearrangement into molecular species.