390 research outputs found
Quartett formation at (100)/(110)-interfaces of d-wave superconductors
Across a faceted (100)/(110) interface between two d-wave-superconductors the
structure of the superconducting order parameter leads to an alternating sign
of the local Josephson coupling. Describing the Cooper pair motion along and
across the interface by a one-dimensional boson lattice model, we show that a
small attractive interaction between the bosons boosts boson binding at the
interface -- a phenomenon, which is intimately tied to the staggered sequence
of 0- and Pi-junction contacts along the interface. We connect this finding to
the recently observed h/4e oscillations in (100)/(110) SQUIDS of cuprate
superconductors.Comment: 13 pages, 2 figure
Chemical control of orbital polarization in artificially structured transition-metal oxides: La2NiXO6 (X=B, Al, Ga, In) from first principles
The application of modern layer-by-layer growth techniques to
transition-metal oxide materials raises the possibility of creating new classes
of materials with rationally designed correlated electron properties. An
important step toward this goal is the demonstration that electronic structure
can be controlled by atomic composition. In compounds with partially occupied
transition-metal d shells, one important aspect of the electronic structure is
the relative occupancy of different d orbitals. Previous work has established
that strain and quantum confinement can be used to influence orbital occupancy.
In this paper we demonstrate a different modality for orbital control in
transition-metal oxide heterostructures, using density-functional band
calculations supplemented by a tight-binding analysis to show that the choice
of nontransition-metal counterion X in transition-metal oxide heterostructures
composed of alternating LaNiO3 and LaXO3 units strongly affects orbital
occupancy, changing the magnitude and in some cases the sign of the orbital
polarization
Metal-Insulator Transition of the LaAlO3-SrTiO3 Interface Electron System
We report on a metal-insulator transition in the LaAlO3-SrTiO3 interface
electron system, of which the carrier density is tuned by an electric gate
field. Below a critical carrier density n_c ranging from 0.5-1.5 * 10^13/cm^2,
LaAlO3-SrTiO3 interfaces, forming drain-source channels in field-effect devices
are non-ohmic. The differential resistance at zero channel bias diverges within
a 2% variation of the carrier density. Above n_c, the conductivity of the ohmic
channels has a metal-like temperature dependence, while below n_c conductivity
sets in only above a threshold electric field. For a given thickness of the
LaAlO3 layer, the conductivity follows a sigma_0 ~(n - n_c)/n_c characteristic.
The metal-insulator transition is found to be distinct from that of the
semiconductor 2D systems.Comment: 4 figure
A possible solution of the grain boundary problem for applications of high-Tc superconductors
It is shown that the critical current density of high-Tc wires can be greatly
enhanced by using a threefold approach, which consists of grain alignment,
doping, and optimization of the grain architecture. According to model
calculations, current densities of 4x10^6 A/cm2 can be achieved for an average
grain alignment of 10 degree at 77K. Based on this approach, a road to
competitive high-Tc cables is proposed.Comment: 3 pages, 5 figure
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
Polar catastrophe and electronic reconstructions at the LaAlO3/SrTiO3 interface: evidence from optical second harmonic generation
The so-called "polar catastrophe", a sudden electronic reconstruction taking
place to compensate for the interfacial ionic polar discontinuity, is currently
considered as a likely factor to explain the surprising conductivity of the
interface between the insulators LaAlO3 and SrTiO3. We applied optical second
harmonic generation, a technique that a priori can detect both mobile and
localized interfacial electrons, to investigating the electronic polar
reconstructions taking place at the interface. As the LaAlO3 film thickness is
increased, we identify two abrupt electronic rearrangements: the first takes
place at a thickness of 3 unit cells, in the insulating state; the second
occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for
which the samples become conducting. Two possible physical scenarios behind
these observations are proposed. The first is based on an electronic transfer
into localized electronic states at the interface that acts as a precursor of
the conductivity onset. In the second scenario, the signal variations are
attributed to the strong ionic relaxations taking place in the LaAlO3 layer
Focussing quantum states
Does the size of atoms present a lower limit to the size of electronic
structures that can be fabricated in solids? This limit can be overcome by
using devices that exploit quantum mechanical scattering of electron waves at
atoms arranged in focussing geometries on selected surfaces. Calculations
reveal that features smaller than a hydrogen atom can be obtained. These
structures are potentially useful for device applications and offer a route to
the fabrication of ultrafine and well defined tips for scanning tunneling
microscopy.Comment: 4 pages, 4 figure
Chemical control of polar behavior in bicomponent short-period superlattices
Using first-principles density functional calculations, we study the
interplay of ferroelectricity and polar discontinuities in a range of 1-1 oxide
superlattices, built out of ferroelectric and paraelectric components. Studies
have been carried out for a varied choice of chemical composition of the
components. We find that, when polar interfaces are present, the polar
discontinuities induce off- centric movements in the ferroelectric layers, even
though the ferroelectric is only one unit cell thick. The distortions yield
non-switchable polarizations, with magnitudes comparable to those of the
corresponding bulk ferroelectrics. In contrast, in superlattices with no polar
discontinuity at the interfaces, the off-centric movements in the ferroelectric
layer are usually suppressed. The details of the behavior and functional
properties are, however, found to be sensitive to epitaxial strain, rotational
instabilities and second-order Jahn-Teller activity, and are therefore strongly
in uenced by the chemical composition of the paraelectric layer.Comment: 7 pages, 2 figure
- …