A. Soukiassian

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing precursor metalorganic bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to clean films. The self-field Jc in the carbon doped film is lower than that in the clean film, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic-field applications.

Prediction of ferroelectricity in BaTiO3/SrTiO3 superlattices with domains

The phase transitions of superlattices into single- and multidomain states were studied using a mesoscale phase-field model incorporating structural inhomogeneity, micromechanics, and electrostatics. While the predictions of transition temperatures of BaTiO3∕SrTiO3 superlattices into multidomains show remarkably good, quantitative agreement with ultraviolet Raman spectroscopic and variable-temperature x-ray diffraction measurements, the single-domain assumption breaks down for superlattices in which the nonferroelectric layer thickness exceeds the characteristic domain size in the ferroelectric layers.

Interfacial coherency and ferroelectricity of BaTiO3∕SrTiO3 superlattice films

We studied the phase transitions, domain morphologies, and polarizations in BaTiO3∕SrTiO3 superlattices grown on SrTiO3 substrates. Using the phase field approach, we discovered the remarkable influence of film/substrate interfacial coherency on the ferroelectricity of the SrTiO3 layers within a superlattice: it is an orthorhombic ferroelectric for an incoherent interface while it exhibits only induced polarization by the adjacent BaTiO3 layers for a coherent interface. We presented the domain morphologies within individual BaTiO3 and SrTiO3 layers which have different ferroelectric symmetries. The results are compared to ultraviolet Raman spectroscopy and variable temperature x-ray diffraction measurements.

Effect of thermal strain on the ferroelectric phase transition in polycrystalline Ba0.5Sr0.5TiO3 thin films studied by Raman spectroscopy

We have applied Raman spectroscopy to study the influence of thermal strain on the vibrational properties of polycrystalline Ba0.5Sr0.5TiO3 films. The films were grown by rf magnetron sputtering on Pt∕SiO2 surface using different host substrates: strontium titanate, sapphire, silicon, and vycor glass. These substrates provide a systematic change in the thermal strain while maintaining the same film microstructure. From the temperature dependence of the ferroelectric A1 soft phonon intensity, the ferroelectric phase transition temperature, TC, was determined. We found that TC decreases with increasing tensile stress in the films. This dependence is different from the theoretical predictions for epitaxial ferroelectric films. The reduction of the ferroelectric transition temperature with increasing biaxial tensile strain is attributed to the suppression of in-plane polarization due to the small lateral grain size in the films.

Structural and thermoelectric properties of Bi2Sr2Co2Oy thin films on LaAlO3 (100) and fused silica substrates

We have grown Bi2Sr2Co2Oy thin films on LaAlO3 (100) and fused silica substrates by pulsed laser deposition. The films on LaAlO3 are c-axis oriented and partially in-plane aligned with multiple domains, while the films on fused silica are preferred c-axis oriented without in-plane alignment. The Seebeck coefficient and resistivity of films on both substrates are comparable to those of single crystals. An oxide p-n heterojunction was formed by depositing Bi2Sr2Co2Oy film on Nb-doped SrTiO3 single crystal, which showed a rectifying behavior. These thin films and heterostructures may be used for future thermoelectric applications.

Electrical and magnetic properties of (SrMnO3)n/(LaMnO3)2n superlattices

We report the magnetic and transport properties of [(SrMnO3)n∕(LaMnO3)2n]m superlattices grown by molecular-beam epitaxy on (100) SrTiO3 with periodicities n=1, 2, 3, 4, 5, 6, 8, and 16. Although the superlattice constituents, LaMnO3 and SrMnO3, are both antiferromagnetic insulators, for small n (n⩽2) the superlattices behave like the ferromagnetic conductor La0.67Sr0.33MnO3. As n increases, the magnetic properties become dominated by the LaMnO3 layers, but the electronic transport properties continue to be controlled by the interfaces.

Growth of nanoscale BaTiO 3 /SrTiO 3 superlattices by molecular-beam epitaxy

Abstract : Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002 deg). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.

Acoustic bragg mirrors and cavities made using piezoelectric oxides

The concept and design of acoustic Bragg mirrors and cavities made of multilayers of piezoelectric oxides with superior acoustic performance and potential applications in electronic and optical terahertz modulators are described. With these applications in mind the authors have grown phonon mirrors consisting of BaTiO3∕SrTiO3 superlattices on SrTiO3 substrates by reactive molecular-beam epitaxy and investigated their properties. Characterization of the superlattices by x-ray diffraction and high-resolution transmission electron microscopy reveals high structural quality with nearly atomically abrupt interfaces. The authors have observed folded acoustic phonons at the expected frequencies using uv Raman spectroscopy.

Thermodynamics and thin film deposition of MgB2 superconductors

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting microelectronics. Thermodynamics studies with the calculation of phase diagrams (CALPHAD) modelling technique show that due to the high volatility of Mg, MgB2 is only thermodynamically stable under fairly high Mg overpressures for likely in situ growth temperatures. This provides a helpful insight into the appropriate processing conditions for MgB2 thin films, including the identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature of 34 K and a zero-field critical current density of 1.3 × 106 A cm−2 were obtained. The qualities of these films are limited by the thermodynamic stability conditions, which favour deposition techniques that can maintain a high flux of Mg.