Che-Hui Lee

Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Effect of reduced dimensionality on the optical band gap of SrTiO3

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1–5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases.

Effect of stoichiometry on the dielectric properties and soft mode behavior of strained epitaxial SrTiO3 thin films on DyScO3 substrates

The effect of stoichiometry on the dielectric properties and soft mode behavior of strained epitaxial Sr1+xTiO3+δ films grown on DyScO3 substrates is reported. Direct comparisons between nominally stoichiometric and non-stoichiometric films have been performed through measurements of lattice parameters, temperature-dependent permittivities, second harmonic generation, and terahertz dielectric spectra. The nominally stoichiometric film shows dispersion-free low-frequency permittivity with a sharp maximum and pronounced soft mode behavior. Our results suggest that strained perfectly stoichiometric SrTiO3 films should not show relaxor behavior and that relaxor behavior emerges from defect dipoles that arise from non-stoichiometry in the highly polarizable strained SrTiO3 matrix.

Clean epitaxial MgB2 films fabricated by the ex situ annealing of chemical vapour deposition-grown B films in Mg vapour

Low residual resistivity in superconducting thin films is required for their applications in radio frequency (RF) cavities. Here we report on clean epitaxial MgB2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, in Mg vapour. The films show sharp superconducting transitions at about 40 K and a high residual resistivity ratio of about 10. The result indicates that a clean precursor B film and a contamination-free annealing procedure are important for a viable MgB2 film fabrication process for RF cavity applications.

Properties of MgB2 films grown at various temperatures by hybrid physical–chemical vapour deposition

A hybrid physical?chemical vapour deposition (HPCVD) system consisting of separately controlled Mg-source heater and substrate heater is used to grow MgB2 thin films and thick films at various temperatures. We are able to grow superconducting MgB2 thin films at temperatures as low as 350??C with a Tc0 of 35.5?K. MgB2 films up to 4??m in thickness grown at 550??C have Jc over 106?A?cm?2 at 5?K and zero applied field. The low deposition temperature of MgB2 films is desirable for all-MgB2 tunnel junctions and MgB2 thick films are important for applications in coated conductors.