34 research outputs found
Influence of strain on anisotropic thermoelectric transport of BiTe and SbTe
On the basis of detailed first-principles calculations and semi-classical
Boltzmann transport, the anisotropic thermoelectric transport properties of
BiTe and SbTe under strain were investigated. It was found that
due to compensation effects of the strain dependent thermopower and electrical
conductivity, the related powerfactor will decrease under applied in-plane
strain for BiTe_2_3_2_3$ suggests, that already in the single crystalline system
strong anisotropic scattering effects should play a role
Thermoelectric transport in strained Si and Si/Ge heterostructures
The anisotropic thermoelectric transport properties of bulk silicon strained
in [111]-direction were studied by detailed first-principles calculations
focussing on a possible enhancement of the power factor. Electron as well as
hole doping were examined in a broad doping and temperature range. At low
temperature and low doping an enhancement of the power factor was obtained for
compressive and tensile strain in the electron-doped case and for compressive
strain in the hole-doped case. For the thermoelectrically more important high
temperature and high doping regime a slight enhancement of the power factor was
only found under small compressive strain with the power factor overall being
robust against applied strain. To extend our findings the anisotropic
thermoelectric transport of an [111]-oriented Si/Ge superlattice was
investigated. Here, the cross-plane power factor under hole-doping was
drastically suppressed due to quantum-well effects, while under electron-doping
an enhanced power factor was found. With that, we state a figure of merit of
ZT and ZT at T=\unit[300]{K} and T=\unit[900]{K} for the
electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in
terms of band structure features
Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect
Magnetic tunnel junctions (MTJs) have attracted strong research interest
within the last decades due to their potential use as nonvolatile memory such
as MRAM as well as for magnetic logic applications. Half-metallic magnets
(HMMs) have been suggested as ideal electrode materials for MTJs to achieve an
extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR
ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode
effect, which was achieved in a magnetic tunnel junction concept based on HMMs
and type-II spin-gapless semiconductors (SGSs). The proposed concept has
recently been experimentally demonstrated using Heusler compounds. In the
present work, we investigate from first-principles MTJs based on type-II SGS
and HMM quaternary Heusler compounds FeVTaAl, FeVTiSi, MnVTiAl, and CoVTiSb.
Our quantum transport calculations based on a nonequilibrium
Green's function method have demonstrated that the MTJs under consideration
exhibit current rectification with relatively high on:off ratios. We show that,
in contrast to conventional semiconductor diodes, the rectification bias
voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of
the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that
the diode effect can be configured dynamically, i.e., depending on the relative
orientation of the magnetization of the electrodes, the MTJ allows the
electrical current to pass either in one or the other direction, which leads to
an inverse TMR effect. The combination of nonvolatility, reconfigurable diode
functionality, tunable rectification voltage window, and high Curie temperature
of the electrode materials makes the proposed MTJs very promising for
room-temperature spintronic applications and opens ways to magnetic memory and
logic concepts as well as logic-in-memory computing.Comment: 14+7 pages, 7+10 figure
Strong influence of the complex bandstructure on the tunneling electroresistance: A combined model and ab-initio study
The tunneling electroresistance (TER) for ferroelectric tunnel junctions
(FTJs) with BaTiO_{3} (BTO) and PbTiO}_{3} (PTO) barriers is calculated by
combining the microscopic electronic structure of the barrier material with a
macroscopic model for the electrostatic potential which is caused by the
ferroelectric polarization. The TER ratio is investigated in dependence on the
intrinsic polarization, the chemical potential, and the screening properties of
the electrodes. A change of sign in the TER ratio is obtained for both barrier
materials in dependence on the chemical potential. The inverse imaginary Fermi
velocity describes the microscopic origin of this effect; it qualitatively
reflects the variation and the sign reversal of the TER. The quantity of the
imaginary Fermi velocity allows to obtain detailed information on the transport
properties of FTJs by analyzing the complex bandstructure of the barrier
material.Comment: quality of figures reduce
Lorenz function of BiTe/SbTe superlattices
Combining first principles density functional theory and semi-classical
Boltzmann transport, the anisotropic Lorenz function was studied for
thermoelectric BiTe/SbTe superlattices and their bulk
constituents. It was found that already for the bulk materials BiTe
and SbTe, the Lorenz function is not a pellucid function on charge
carrier concentration and temperature. For electron-doped
BiTe/SbTe superlattices large oscillatory deviations
for the Lorenz function from the metallic limit were found even at high charge
carrier concentrations. The latter can be referred to quantum well effects,
which occur at distinct superlattice periods
The Computational 2D Materials Database: High-Throughput Modeling and Discovery of Atomically Thin Crystals
We introduce the Computational 2D Materials Database (C2DB), which organises
a variety of structural, thermodynamic, elastic, electronic, magnetic, and
optical properties of around 1500 two-dimensional materials distributed over
more than 30 different crystal structures. Material properties are
systematically calculated by state-of-the art density functional theory and
many-body perturbation theory (GW\!_0 and the Bethe-Salpeter Equation
for 200 materials) following a semi-automated workflow for maximal
consistency and transparency. The C2DB is fully open and can be browsed online
or downloaded in its entirety. In this paper, we describe the workflow behind
the database, present an overview of the properties and materials currently
available, and explore trends and correlations in the data. Moreover, we
identify a large number of new potentially synthesisable 2D materials with
interesting properties targeting applications within spintronics,
(opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily
accessible overview of the rapidly expanding family of 2D materials and forms
an ideal platform for computational modeling and design of new 2D materials and
van der Waals heterostructures.Comment: Add journal reference and DOI; Minor updates to figures and wordin
Thermoelectric transport in superlattices
The thermoelectric transport properties of
superlattices are analyzed on
the basis of first-principles calculations and semi-classical Boltzmann theory.
The anisotropy of the thermoelectric transport under electron and hole-doping
was studied in detail for different superlattice periods at changing
temperature and charge carrier concentrations. A clear preference for
thermoelectric transport under hole-doping, as well as for the in-plane
transport direction was found for all superlattice periods. At hole-doping the
electrical transport anisotropies remain bulk-like for all investigated
systems, while under electron-doping quantum confinement leads to strong
suppression of the cross-plane thermoelectric transport at several superlattice
periods. In addition, insights on the Lorenz function, the electronic
contribution to the thermal conductivity and the resulting figure of merit are
given