● The EXAFS technique (a technique that uses x-rays as an indirect nanoscope) has been used to probe how distortions in the Mn-O bond length ( the parameter  2 ) of La 1.2 Sr 1.8 Mn 2 O 7 change as temperature is increased from 3K to 300K ● Results show an incredibly sharp increase in the amount of distortion as the crystal demagnetizes at about 120K (the temperature at which the system demagnetizes as temperature increases). It is the sharpest local structure transition of this type ever observed (see above figure). ● This distortion is highly anisotropic; the magnitude of the increase in the ab-plane is approximately two times larger than in the c-axis ● Results also show that the magnetization of the system and the distortion are strongly related—this gives indirect evidence for a new quasi-particle, which we call the Dimeron, recently proposed for the quasi-cubic system (La 1-x Ca x MnO 3 ). Local Distortion Study of La 1.2 Sr 1.8 Mn 2 O 7 Through the Ferromagnetic Transition c b a Mn O La/Sr ab-plane c-axis Frank Bridges, UCSC, DMR

The EXAFS technique (Extended X-Ray Absorption Fine Structure: a technique that uses x-rays as an indirect nanoscope) has been used to probe how distortions in the layered structure of La1.2Sr1.8Mn2O7 change as temperature is increased from 3-300K (-270°C to 25°C). Specifically we use EXAFS to examine the statistical distribution of Mn-O bond- lengths (the Mn are the white atoms in the figure and the O are the blue atoms) throughout the material. The figures in this view graph display the width of the distribution (a measure of distortion) versus temperature. The lower red lines in the figures indicate the predicted increase in the width of the distribution due to temperature effects. As the temperature is increased through 120K (-153°C), the above figures show a steep increase in the distortion: this is the sharpest increase in distortion that has ever been observed in a non-structural transition, by EXAFS. 120K is also the temperature at which the system demagnetizes as temperature is increased. The results show that there is 2 times more distortion in Mn-O pairs that lie in the ab-plane than in those along the c-axis (see the atomic figure) Results also show that the magnetization of the system and the distortion are strongly related—this gives indirect evidence for a new quasi-particle also in this system, which we call the Dimeron. Our group recently proposed the concept of the Dimeron in a paper by Lisa Downward et. al. in Physical Review Letters vol. 95, (2005). Frank Bridges, UCSC, DMR

Layered, Giant Magneto-Resistance (GMR) materials are presently used in read-heads in magnetic media such as hard drives and floppy drives, however the underlying physics behind them is not fully understood. Studies of relatives to these materials (e.g. La 1.2 Sr 1.8 Mn 2 O 7 ) are providing invaluable insight in to the mechanism behind GMR/CMR (Colossal Magneto- Resistance). Also it is believed that the physics behind CMR is related to the physics behind high temperature super- conductivity, and hence may also provide valuable insight into this phenomenon as well. Broader Impact In the course of this study, 4 graduate students and 7 undergraduate students have had the opportunity to learn the experimental and theoretical sides of the EXAFS technique. As a result, all 11 students have had an invaluable and practical introduction (or more in some cases!) to quantum mechanics, group theory, solid state, and data analysis. In addition, the EXAFS experiments have involved taking x-ray data at Stanford Synchrotron Radiation Laboratory, at Brookhaven, and at Argonne National Laboratory; these are particularly precious opportunities for undergraduates to gain rare experience at world- class facilities. The group also maintains a public webpage that provides movies of the type of motion probed by EXAFS, images (2D and stereographic) of crystal structures, and descriptions of the research aimed at a high school level audience 1. Education Local Distortion Study of La 1.2 Sr 1.8 Mn 2 O 7 Through the Ferromagnetic Transition