51 research outputs found
Electronic and thermoelectric properties of Fe2VAl: The role of defects and disorder
Using first-principles calculations, we show that Fe2VAl is an indirect band
gap semiconductor. Our calculations reveal that its, sometimes assigned,
semimetallic character is not an intrinsic property but originates from the
antisite defects and site disorder, which introduce localized ingap and
resonant states changing the electronic properties close to band gap. These
states negatively affect the thermopower S and power factor PF=S^2\sigma,
decreasing the good thermoelectric performance of intrinsic Fe2VAl.Comment: 4 pages, 6 figures, thermoelectric properties, electronic structure
and transport properties, effect of antisite defects and disorder on
electronic and transport propertie
Low-Dimensional Transport and Large Thermoelectric Power Factors in Bulk Semiconductors by Band Engineering of Highly Directional Electronic States
Thermoelectrics are promising to address energy issues but their exploitation
is still hampered by low efficiencies. So far, much improvement has been
achieved by reducing the thermal conductivity but less by maximizing the power
factor. The latter imposes apparently conflicting requirements on the band
structure: a narrow energy distribution and a low effective mass. Quantum
confinement in nanostructures or the introduction of resonant states were
suggested as possible solutions to this paradox but with limited success. Here,
we propose an original approach to fulfill both requirements in bulk
semiconductors. It exploits the highly-directional character of some orbitals
to engineer the band-structure and produce a type of low-dimensional transport
similar to that targeted in nanostructures, while retaining isotropic
properties. Using first-principles calculations, the theoretical concept is
demonstrated in FeYZ Heusler compounds, yielding power factors 4-5 times
larger than in classical thermoelectrics at room temperature. Our findings are
totally generic and rationalize the search of alternative compounds with a
similar behavior. Beyond thermoelectricity, these might be relevant also in the
context of electronic, superconducting or photovoltaic applications.Comment: 6 pages, 2 figure
Resonant States in the Electronic Structure of the High Performance Thermoelectrics AgPb_{2+m}$ ; The Role of Ag-Sb Microstructures
Ab initio electronic structure calculations based on gradient corrected
density functional theory were performed on a class of novel quaternary
compounds AgPb_{2+m}$, which were found to be excellent high
temperature thermoelctrics with large figure of merit ZT ~2.2 at 800K. We find
that resonant states appear near the top of the valence and bottom of the
conduction bands of bulk PbTe when Ag and Sb replace Pb. These states can be
understood in terms of modified Te-Ag(Sb) bonds. Electronic structure near the
gap depends sensitively on the microstructural arrangements of Ag-Sb atoms,
suggesting that large ZT values may originate from the nature of these ordering
arrangements.Comment: Accepted in Physical Review Letter
First-principles modeling of the thermoelectric properties of SrTiO/SrRuO superlattices
Using a combination of first-principles simulations, based on the density
functional theory and Boltzmann's semiclassical theory, we have calculated the
transport and thermoelectric properties of the half-metallic two dimensional
electron gas confined in single SrRuO layers of SrTiO/SrRuO
periodic superlattices. Close to the Fermi energy we find that the
semiconducting majority spin channel displays a very large in-plane component
of the Seebeck tensor at room temperature, = 1500 V/K, and the
minority spin channel shows good in-plane conductivity = 2.5
(mcm). However, contrary to the expectation of Hicks and
Dresselhaus model about enhanced global thermoelectric properties due to the
confinement of the metallic electrons, we find that the total power factor and
thermoelectric figure of merit for reduced doping is too small for practical
applications. The reason for this failure can be traced back on the electronic
structure of the interfacial gas, which departs from the free electron
behaviour on which the model was based. The evolution of the electronic
structure, electrical conductivity, Seebeck coefficient, and power factor as a
function of the chemical potential is explained by a simplified tight-binding
model. We find that the electron gas in our system is composed by a pair of one
dimensional electron gases orthogonal to each other. This reflects the fact the
physical dimensionality of the electronic system can be even smaller than that
of the spacial confinement of the carriers.Comment: 9 pages, 7 figure
Highly-confined spin-polarized two-dimensional electron gas in SrTiO/SrRuO superlattices
We report first principles characterization of the structural and electronic
properties of (SrTiO)/(SrRuO) superlattices. We show
that the system exhibits a spin-polarized two-dimensional electron gas,
extremely confined to the 4 orbitals of Ru in the SrRuO layer. Every
interface in the superlattice behaves as a minority-spin half-metal
ferromagnet, with a magnetic moment of = 2.0 /SrRuO
unit. The shape of the electronic density of states, half metallicity and
magnetism are explained in terms of a simplified tight-binding model,
considering only the orbitals plus (i) the bi-dimensionality of the
system, and (ii) strong electron correlations.Comment: 5 pages, 4 figure
First-principles accurate prediction of the electronic and structural properties of ferroelectric oxide bulks and nanostructures
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