Events at Physics |
Events on Friday, September 25th, 2009
- Physics Department Colloquium
- New Phenomena at Oxide Interfaces
- Time: 4:00 pm
- Place: 2241 Chamberlin Hall (coffee and cookies at 3:30 pm)
- Speaker: Jean-Marc Triscone, University of Geneva
- Abstract: At interfaces between complex oxides, electronic systems with unusual properties can be generated [see for instance 1,2]. A striking example is the interface between LaAlO3 and SrTiO3, two good insulating perovskite oxides, which was found in 2004 to be conducting with a high mobility [3]. We discovered that the ground state of this system is a superconducting condensate, with a critical temperature of about 200 mK [4]. The characteristics observed for the superconducting transitions are consistent with a two-dimensional superconducting sheet a few nanometers thick. Recent field effect experiments revealed the sensitivity of the normal and superconducting states to the carrier density. In particular, the electric field allows the tuning of the critical temperature between 200 mK and 0 K and thus the on-off switching of superconductivity, revealing a complex phase diagram and a superconductor to insulator transition[5]. Recent results suggest that this phase diagram is linked to the large interfacially generated spin-orbit coupling. I will discuss the perspectives opened by this new field of research sometimes called "oxide interface engineering".
[1] "When oxides meet face to face." E. Dagotto, Science 318, 1076 (2007).
[2] "Enter the oxides." J. Heber, Nature 459, 28 (2009).
[3] "A high mobility electron gas at the LaAlO3/SrTiO3 heterointerface." A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).
[4] "Superconducting interfaces between insulating oxides." N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[5] "Electric field control of the LaAlO3/SrTiO3 interface ground state." A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
- Host: Rzchowski
- Math Colloquia
- Complexity Theory --- The World of P and NP
- Time: 4:00 pm
- Place: B239 Van Vleck
- Speaker: Jin-Yi Cai, UW Madison CS Dept.
- Abstract: The study of computational complexity presents challenging mathematical problems. In Complexity Theory computational problems are classified into complexity classes, the best known include P, NP and Valiant's class #P for counting problems. A central problem in Valiant's theory is the permanent vs. determinant problem. We will report some latest progress on this problem. Graph homomorphism was introduced by Lovasz over 40 years ago, and it is also called the partition functions in Statistical Physics, and can encode a rich class of counting problems: Given an $m imes m$ symmetric matrix $A$ over the complex numbers, compute the function $Z_A(cdot)$, where for an arbitrary input graph $G$, [ Z_A(G) = sum_{xi:V(G)
ightarrow [m]} prod_{(u,v)in E(G)} A_{xi(u),xi(v)}.] Our foucs is the computational complexity of $Z_A(cdot)$. With Xi Chen and Pinyan Lu, we have achieved a complete classification theorem for the complexity of $Z_A(cdot)$. The classification proof is too complicated to present, but we will present the proof of a lemma. It states that in order to be computable in polynomial time, the matrix $A$ must possess a group structure. Another component of the proof uses Gauss sums. (In a subsequent Number Theory Seminar I will present some related work.) No prior knowledge of complexity theory is assumed. <br>