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Register a new userPolarization Enhancement in Short Period Superlattices via Interfacial Intermixing
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The effect of intermixing at the interface of short period PbTiO$_3$/SrTiO$_3$ superlattices is studied using first-principles density functional theory. The results indicate that interfacial intermixing significantly enhances the polarization within the superlattice. This enhancement is directly related to the off-centering of Pb and Sr cations and can be explained through a discussion of interacting dipoles. This picture should be general for a wide range of multicomponent superlattices and may have important consequences for the design of ferroelectric devices.
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Heterostructures including the members of the 6.1{AA} semiconductor family (AlSb, GaSb, and InAs) are used in infrared optoelectronic devices as well as a variety of other applications. Short-period superlattices of these materials are also of interest for creating composite materials with designer infrared dielectric functions. The conditions needed to create sharp InAs/GaSb and InAs/AlSb interfaces are well known, but the AlSb/GaSb interface is much less well-understood. In this article, we test a variety of interventions designed to improve interface sharpness in AlSb/GaSb short-period superlattices. These interventions include substrate temperature, III:Sb flux ratio, and the use of a bismuth surfactant. Superlattices are characterized by high-resolution x-ray diffraction and infrared spectroscopy. We find that AlSb/GaSb short-period superlattices have a wide growth window over which sharp interfaces can be obtained.
A novel approach to reduce bulk conductance by the use of short period superlattices (SL) of two alternating topological insulator layers is presented. Evidence for a superlattice gap enhancement (SGE) was obtained from the observed reduction of bulk background doping by more than one order of magnitude, from 1.2x1020 cm-3 to 8.5x1018 cm-3 as the period of Bi2Se3/Sb2Te3 SLs is decreased from 12 nm to 5 nm, respectively. Tight binding calculations show that in the very thin period regime, a significant SGE can be achieved by the appropriate choice of materials. The ultrathin SL of alternating Bi2Se3 and Sb2Te3 layers behaves as a new designer material with a bulk bandgap as much as 60% larger than the bandgap of the constituent layer with the largest bandgap, while retaining topological surface features. Analysis of the weak antilocalization (WAL) cusp evident in the low temperature magneto-conductance of a very thin period SL sample grown confirms that the top and bottom surfaces of the SL structure behave as Dirac surface states. This approach represents a promising and yet to be explored platform for building truly insulating bulk TIs.
(LaNiO3)n/(LaMnO3)2 superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO3 is a paramagnetic metal and LaMnO3 is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO3 to the LaNiO3, inducing magnetism in LaNiO3. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO3 layer thickness is reduced from 5 unit cells to 2 unit cells and suggest a modulated magnetic structure within LaNiO3.
We report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices grown by low-temperature molecular beam epitaxy, which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in T$_C$ up to 130 K can be achieved using post-growth annealing at temperatures near the growth temperature. Pronounced thickness fringes in the high resolution X-ray diffraction spectra indicate good crystalline quality and sharp interfaces in the structures.
We report on growth and ferroelectric (FE) properties of superlattices (SLs) composed of the FE BaTiO3 and the paraelectric (PE) CaTiO3. Previous theories have predicted that the polarization in (BaTiO3)n/(CaTiO3)n SLs increases as the sublayer thickness (n) increases when the same strain state is maintained. However, our BaTiO3/CaTiO3 SLs show a varying lattice-strain state and systematic reduction in polarization with increasing n while coherently-strained SLs with n=1, 2 show a FE polarization of ca. 8.5 uC/cm^2. We suggest that the strain coupling plays more important role in FE properties than the electrostatic interlayer coupling based on constant dielectric permittivities.
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