Mott Transition, i.e., the interaction-driven metal-to-insulator transition (MIT), is a fundamental issue in the physics of strongly correlated electron systems. Conversely, searching for new a playground bestowing a truly archetypical Mott MIT to test our understanding of it seems to be and incessant effort in the condensed matter physics community. Here we present a broken-symmetry manifested Mott MIT at the surface of a prototype Mott system: Ca1.9Sr0.1RuO4. In the bulk crystal of

Ca1.9Sr0.1RuO4, a first-order MIT with an abrupt structural

transition occurs at TC = 154 K on cooling. In contrast, both

scanning tunneling spectroscopy (STS) and electron energy loss spectroscopy (EELS) show that a surface MIT without simultaneous structural transition is found at a temperature of TC,S = 130 K, remarkably lower than that in the bulk

crystal. Furthermore, a surface phonon anomaly is observed at TC,S indicating a strong electron-phonon

coupling. Our structural study by using low energy electron diffraction (LEED) I-V refinement and also first principle calculations finally discovered that the broken symmetry at the surface simply restrains the structure from the lattice distortion which is intimately involved in the bulk MIT, thus persisting the metallic phase at surface to lower temperature and preventing any structural transition across the surface MIT. Our results clearly show that this unique surface provides a unique opportunity to gain insight into the precise nature of Mott transition with no participation of structural transition.

CAREER: New Phases at the Surfaces/Interfaces of Transition-Metal OxidesJiandi Zhang, Florida International University, DMR-0346826

(top) The T-dependence of STS and EELS spectra of Ca1.9Sr0.1RuO4

showing the surface MIT associated with a phonon anomaly at 130 K. (bottom) An LEED image and schematic view of the surface lattice relaxation of Ca1.9Sr0.1RuO4 which does not change across the observed surface MIT.

Societal Impact: The technological impact by the studies of new phenomena at the proximity of surfaces & interfaces of correlated electron materials like doped transition-metal oxides could eventually be enormous, because most of the devices one conceives of making from these novel materials involve surfaces and interfaces, such as spin valves, spin transistors, hard disk read/write heads, many types of sensors, memory devices, etc. If these materials are going to find their way into the marketplace it is obvious that the properties of the surface and interfaces must be understood and controlled at a level that is now commonplace for devices made of more conventional materials.

EDUCATION: Four graduate students (Anne Cai, Fernanda Foetter who is now at Univ. of Florida, Yanxin Liu, and Rajendra Joshi), two undergraduates (Sarah Bryan and Dalgis Mesa), and a postdoctoral fellow (Lei Cai) are involved in this research program. This program also provided the independent studies of few undergraduate students in Physics Department. This project is also a partnership between Florida International University, University of Tennessee, and Oak Ridge National Lab. The collaborations being set up will lead a unique opportunity for the materials research and education, especially the training of minority students for their careers in this technology-driven world.

Several education outreach activities have been accomplished through this funded program. These included the open house and lectures to local public. Shown on the right is the picture taken during a visiting of local high school students and teachers.

CAREER: New Phases at the Surfaces/Interfaces of Transition-Metal OxidesJiandi Zhang, Florida International University, DMR-0346826