7,731 research outputs found
Tuning ferromagnetism at interfaces between insulating perovskite oxides
We use density functional theory calculations to show that the LaAlO3|SrTiO3
interface between insulating perovskite oxides is borderline in satisfying the
Stoner criterion for itinerant ferromagnetism and explore other oxide
combinations with a view to satisfying it more amply. The larger lattice
parameter of an LaScO3|BaTiO3 interface is found to be less favorable than the
greater interface distortion of LaAlO3|CaTiO3. Compared to LaAlO3|SrTiO3, the
latter is predicted to exhibit robust magnetism with a larger saturation moment
and a higher Curie temperature. Our results provide support for a "two phase"
picture of coexistent superconductivity and ferromagnetism.Comment: 5 pages, 4 figure
Large potential steps at weakly interacting metal-insulator interfaces
Potential steps exceeding 1 eV are regularly formed at metal|insulator
interfaces, even when the interaction between the materials at the interface is
weak physisorption. From first-principles calculations on metal|h-BN interfaces
we show that these potential steps are only indirectly sensitive to the
interface bonding through the dependence of the binding energy curves on the
van der Waals interaction. Exchange repulsion forms the main contribution to
the interface potential step in the weakly interacting regime, which we show
with a simple model based upon a symmetrized product of metal and h-BN wave
functions. In the strongly interacting regime, the interface potential step is
reduced by chemical bonding
Prediction of thickness limits of ideal polar ultrathin films
Competition between electronic and atomic reconstruction is a constantly
recurring theme in transition-metal oxides. We use density functional theory
calculations to study this competition for a model system consisting of a thin
film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a
nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar
structure to a chain-type thin film accompanied by substantial changes to the
electronic structure is predicted for a SrCuO2 film fewer than five unit cells
thick. An analytical model explains why atomic reconstruction becomes more
favorable than electronic reconstruction as the film becomes thinner, and
suggests that similar considerations should be valid for other polar films
Polarity-induced oxygen vacancies at LaAlO3|SrTiO3 interfaces
Using first-principles density functional theory calculations, we find a
strong position and thickness dependence of the formation energy of oxygen
vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an
analytical capacitor model. Oxygen vacancies are preferentially formed at
p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons
introduced by the oxygen vacancies reduce their energy by moving to the n-type
interface. This asymmetric behavior makes an important contribution to the
conducting (insulating) nature of n-type (p-type) interfaces while providing a
natural explanation for the failure to detect evidence for the polar
catastrophe in the form of core level shifts
Performance assessment of tariff-based air source heat pump load shifting in a UK detached dwelling featuring phase change-enhanced buffering
Using a detailed building simulation model, the amount of thermal buffering, with and without phase change material (PCM), needed to time-shift an air source heat pump's operation to off-peak periods, as defined by the UK 'Economy 10' tariff, was investigated for a typical UK detached dwelling. The performance of the buffered system was compared to the case with no load shifting and with no thermal buffering. Additionally, the load shifting of a population of buffered heat pumps to off-peak periods was simulated and the resulting change in the peak demand on the electricity network was assessed. The results from this study indicate that 1000 L of hot water buffering or 500 L of PCM-enhanced hot water buffering was required to move the operation of the heat pump fully to off-peak periods, without adversely affecting the provision of space heating and hot water for the end user. The work also highlights that buffering and load shifting increased the heat pump's electrical demand by over 60% leading to increased cost to the end user and increased CO2 emissions (depending on the electricity tariff applied and time varying CO2 intensity of the electricity generation mix, respectively). The study also highlights that the load-shifting of populations of buffered heat pumps wholly to off-peak periods using crude instruments such as tariffs increased the peak loading on the electrical network by over 50% rather than reducing it and that careful consideration is needed as to how the load shifting of a group of heat pumps is orchestrated
Band gaps in incommensurable graphene on hexagonal boron nitride
Devising ways of opening a band gap in graphene to make charge-carrier masses
finite is essential for many applications. Recent experiments with graphene on
hexagonal boron nitride (h-BN) offer tantalizing hints that the weak
interaction with the substrate is sufficient to open a gap, in contradiction of
earlier findings. Using many-body perturbation theory, we find that the small
observed gap is what remains after a much larger underlying quasiparticle gap
is suppressed by incommensurability. The sensitivity of this suppression to a
small modulation of the distance separating graphene from the substrate
suggests ways of exposing the larger underlying gap
Non-collinear Magnetoelectronics
The electron transport properties of hybrid ferromagnetic|normal metal
structures such as multilayers and spin valves depend on the relative
orientation of the magnetization direction of the ferromagnetic elements.
Whereas the contrast in the resistance for parallel and antiparallel
magnetizations, the so-called Giant Magnetoresistance, is relatively well
understood for quite some time, a coherent picture for non-collinear
magnetoelectronic circuits and devices has evolved only recently. We review
here such a theory for electron charge and spin transport with general
magnetization directions that is based on the semiclassical concept of a vector
spin accumulation. In conjunction with first-principles calculations of
scattering matrices many phenomena, e.g. the current-induced spin-transfer
torque, can be understood and predicted quantitatively for different material
combinations.Comment: 163 pages, to be published in Physics Report
Electronic structure induced reconstruction and magnetic ordering at the LaAlOSrTiO interface
Using local density approximation (LDA) calculations we predict
GdFeO-like rotation of TiO octahedra at the -type interface between
LaAlO and SrTiO. The narrowing of the Ti bandwidth which results
means that for very modest values of , LDA calculations predict charge
and spin ordering at the interface. Recent experimental evidence for magnetic
interface ordering may be understood in terms of the close proximity of an
antiferromagnetic insulating ground state to a ferromagnetic metallic excited
state
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