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Events on Thursday, February 3rd, 2011

R. G. Herb Condensed Matter Seminar
Two-dimensional electron gas with universal subbands at the surface of SrTiO<sub>3</sub>
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Andres Santander, CSNSM - Université Paris-Sud, France
Abstract: Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO3) is the bedrock of the emerging field of oxide electronics. SrTiO3 is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. In this talk, we present our angle-resolved photoemission spectroscopy (ARPES) results showing that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3, independent of bulk carrier densities over more than seven decades, including the undoped insulating material [1]. Our data unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. The analysis of the data shows that this 2DEG is confined within a region of ~5 unit cells with a sheet carrier density of ~0.33 electrons per a2 (a is the cubic lattice parameter). The similarity of this 2DEG with those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices, and a novel route to generate 2DEGs at surfaces of functional oxides.

[1] A. F. Santander-Syro, O. Copie, T. Kondo, et al. "Two-dimensional electron gas with universal subbands at the surface of SrTiO3". Nature 469, 189 (2011).
Host: Natalia Perkins & Andrey Chubukov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Peculiar Velocity and its Effects on Cosmology with 21cm Tomography
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Yi Mao, UT Austin
Abstract: The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. We review how measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. A more careful treatment of the effects of peculiar velocity than previously attempted is necessary, however, to fulfill this promise. In this talk, we set out to account for peculiar velocity in every detail, and clarify the roles of thermal vs. velocity broadening and finite optical depth.
We will also find whether nonlinear effect of peculiar velocity may spoil future 21cm measurements. The discussion in this talk, although in the context of the Epoch
of Reionization, may affect the interpretation of the 21cm intensity mapping in the post-reionization epoch.
Host: Peter Timbie and Vernon Barger
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