2,156 research outputs found
Calculations of spin-disorder resistivity from first principles
Spin-disorder resistivity of Fe and Ni is studied using the noncollinear
density functional theory. The Landauer conductance is averaged over random
disorder configurations and fitted to Ohm's law. The distribution function is
approximated by the mean-field theory. The dependence of spin-disorder
resistivity on magnetization in Fe is found to be in excellent agreement with
the results for the isotropic s-d model. In the fully disordered state,
spin-disorder resistivity for Fe is close to experiment, while for fcc Ni it
exceeds the experimental value by a factor of 2.3. This result indicates strong
magnetic short-range order in Ni at the Curie temperature.Comment: 3 pages, 3 figure
Oxide tunnel junctions supporting a two-dimensional electron gas
The discovery of a two-dimensional electron gas (2DEG) at the interface
between insulating oxides has led to a well-deserved level of excitement due to
possible applications as "in-plane" all-oxide nanoelectronics. Here we expand
the range of possibilities to the realm of "out-of-plane" nanoelectronics by
examining such all-oxide heterostructures as barriers in tunnel junctions. As
an example system we perform first-principles electronic structure and
transport calculations of a tunnel junction with a [SrTiO3]4/[LaO]1/[SrTiO3]4
heterostructure tunneling barrier embedded between SrRuO3 electrodes. The
presence of the LaO atomic layer induces the formation of a 2DEG within the
tunneling barrier which acts as an extended defect perpendicular to the
transport direction, providing a route for resonant tunneling. Our calculations
demonstrate that the tunneling conductance in this system can be strongly
enhanced compared to a pure SrTiO3 barrier due to resonant tunneling, but that
lattice polarization effects play a significant role in determining this
behavior. In addition we find that this resonant tunneling is highly selective
of the orbital symmetry of the tunneling states due to the "orbital
polarization" of the 2DEG. We also discuss how the properties of the 2DEG are
affected by the presence of metal electrodes.Comment: 8 pages, 5 figures, 1 tabl
Spin-polarized two-dimensional electron gas at GdTiO3/SrTiO3 interfaces: Insight from first-principles calculations
Two-dimensional electron gases (2DEGs) at oxide interfaces have been a topic of intensive research due to their high carrier mobility and strong confinement. Additionally, strong correlations in the oxide materials can give rise to new and interesting physics, such as magnetism and metal-insulator transitions at the interface. Using first-principles calculations based on density functional theory, we demonstrate the presence of a highly spin-polarized 2DEG at the interface between the Mott insulator GdTiO3 and a band insulator SrTiO3. The strong correlations in the dopant cause ferromagnetic alignment of the interface Ti atoms and result in a fully spin-polarized 2DEG. The 2DEG consists of two types of carriers distinguished by their orbital character. The majority of the interface charge is strongly localized on the Ti dxy orbitals at the interface and a smaller fraction resides on the delocalized Ti dxz,yz states
Resonant tunneling across a ferroelectric domain wall
Motivated by recent experimental observations, we explore electron transport properties of a ferroelectric tunnel junction (FTJ) with an embedded head-to-head ferroelectric domain wall, using first-principles density-functional theory calculations. We consider a FTJ with La0.5Sr0.5MnO3 electrodes separated by a BaTiO3 barrier layer and show that an in-plane charged domain wall in the ferroelectric BaTiO3 can be induced by polar interfaces. The resulting V-shaped electrostatic potential profile across the BaTiO3 layer creates a quantum well and leads to the formation of a two-dimensional electron gas, which stabilizes the domain wall. The confined electronic states in the barrier are responsible for resonant tunneling as is evident from our quantum-transport calculations. We find that the resonant tunneling is an orbital selective process, which leads to sharp spikes in the momentum- and energy-resolved transmission spectra. Our results indicate that domain walls embedded in FTJs can be used to control the electron transport
The Origin of Tunneling Anisotropic Magnetoresistance in Break Junctions
First-principles calculations of electron tunneling transport in Ni and Co
break junctions reveal strong dependence of the conductance on the
magnetization direction, an effect known as tunneling anisotropic
magnetoresistance (TAMR). The origin of this phenomenon stems from resonant
states localized in the electrodes near the junction break. The energy and
broadening of these states is strongly affected by the magnetization
orientation due to spin-orbit coupling, causing TAMR to be sensitive to bias
voltage on a scale of a few mV. Our results bear a resemblance to recent
experimental data and suggest that TAMR driven by resonant states is a general
phenomenon typical for magnetic broken contacts and other experimental
geometries where a magnetic tip is used to probe electron transport.Comment: 4 pages, 3 figure
Interface states in CoFe2O4 spin-filter tunnel junctions
Spin-filter tunneling is a promising way to generate highly spin-polarized
current, a key component for spintronics applications. In this work we explore
the tunneling conductance across the spin-filter material CoFe2O4 interfaced
with Au electrodes, a geometry which provides nearly perfect lattice matching
at the CoFe2O4/Au(001) interface. Using density functional theory calculations
we demonstrate that interface states play a decisive role in controlling the
transport spin polarization in this tunnel junction. For a realistic CoFe2O4
barrier thickness, we predict a tunneling spin polarization of about -60%. We
show that this value is lower than what is expected based solely on
considerations of the spin-polarized band structure of CoFe2O4, and therefore
that these interface states can play a detrimental role. We argue this is a
rather general feature of ferrimagnetic ferrites and could make an important
impact on spin-filter tunneling applications.Comment: 5 pages, 4 Figures plus 1 page supplemen
Polarization discontinuity induced two-dimensional electron gas at ZnO/Zn(Mg)O interfaces: A first-principles study
The discovery of a high-mobility two-dimensional electron gas (2DEG) in wurtzite ZnO/Zn(Mg)O heterostructures is promising for applications due to the high mobility of the carriers. In this paper, we study the formation and properties of the 2DEG at ZnO/Zn(Mg)O interfaces using first-principles calculations based on hybrid density functional theory. The 2DEG arises from the polarization discontinuity at the interface between the two materials. The uncompensated bound charge at the interface gives rise to an electric field in the bulk of ZnO which confines free carriers close to the interface. We find that the type of the confined carriers is determined by the interface termination, while the amount of charge and the confinement width could be controlled by the Mg doping and the device dimensions
Ferroelectric Dead Layer Driven by a Polar Interface
Based on first-principles and model calculations we investigate the effect of
polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As
a representative model, we consider a TiO2-terminated BaTiO3 film with LaO
monolayers at the two interfaces that serve as doping layers. We find that the
polar interfaces create an intrinsic electric field that is screened by the
electron charge leaking into the BaTiO3 layer. The amount of the leaking charge
is controlled by the boundary conditions which are different for three
heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and
SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements
in BaTiO3 to produce the electric polarization directed into the interior of
the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces
that is non-switchable and thus detrimental to ferroelectricity. Our
first-principles and model calculations demonstrate that the effect is stronger
for a larger effective ionic charge at the interface and longer screening
length due to a stronger intrinsic electric field that penetrates deeper into
the ferroelectric. The predicted mechanism for a ferroelectric dead layer at
the interface controls the critical thickness for ferroelectricity in systems
with polar interfaces.Comment: 33 Pages, 5 figure
- …