The Department of Physics and Astronomy invites you to a colloquium featuring Jonathan E Spanier, Department of Materials Science & Engineering, Department of Physics, A J Drexel Institute for Energy & the Environment, Drexel University. Dr. Spanier will present his talk, “Light-matter interactions enabled by symmetry-breaking: surfaces, interfaces and bulk oxide perovskites”, Tuesday, March 22, at 4:10 p.m. in Webster B17.
Meet for refreshments before the lecture at 3:45 – 4:10 p.m. in the foyer on floor G above the lecture hall.
Abstract: Symmetry-breaking and chemical potential mismatch between constituent materials at a surface or interface result in novel phenomena inaccessible in the bulk, e.g., adsorbate-induced ferroelectric ordering in thin films and nanostructures, and the onset of high carrier mobility and of ferromagnetic and superconductive ordering at the interface of the band insulators LaAlO3 and SrTiO3. I will discuss our recent findings that involve coupling of surface chemistry with sub-surface optoelectronic response in this archetypal complex oxide 2DEG. Intense, narrow-linewidth, surface chemisorption-induced and switchable ultraviolet photoluminescence from radiative recombination of 2DEG electrons with photo-excited holes is a phenomenon that has no analogue in a conventional semiconductor 2DEG. These results are promising for future probing of collective electronic behavior in this 2DEG and for reconfigurable nanoscale optoelectronics-on-demand through local surface chemical control.
In contrast, symmetry-breaking in the bulk can enable phenomena that are absent either on surfaces or at interfaces. For example in classic solid-state photovoltaic (PV) devices photo-generated charges are separated by the field at a p-n junction or due to the Dember effect. These PV effects involve an inhomogeneous excitation and/or a spatially-inhomogeneous medium; and the photovoltage does not exceed the band gap Eg. However the bulk photovoltaic effect (BPVE) in non-centrosymmetric crystals is striking: the photovoltage generated by the BPVE can greatly exceed Eg, but the direct conversion of light energy to electricity is extremely low. Also known as the anomalous PV effect, the BPVE can be observed in crystals that belong to twenty point groups lacking a center of inversion symmetry. I will discuss recent advances in understanding the BPVE light-matter interaction, and optical absorption, charge carrier separation and energy conversion in ferroelectric perovskite oxides.
