Neutron and X-Ray Scattering Studies of Low Dimensional Quantum Magnets

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

This experimental condensed matter physics project employs neutron and x-ray scattering techniques to investigate three classes of materials: (a) Sr-and excess Oxygen doped La2CuO4 high temperature superconductors (b) 1D and 2D quantum magnets (c) model materials with disorder and/or competing interactions. The research in La2-xSrxCuO4 and La2CuO4+y will focus on the incommensurate magnetic and charge ordering and the concomitant low energy spin dynamics, most especially in the under-doped regime. The data will be analyzed in the so-called stripe picture. Research areas (b) and (c) come together beautifully in the diluted spin-Peierls material Cu1-xMgxGeO3 where magnetic and structural superlattice order compete. This system will be studied as a function of x using both neutron and synchrotron x-ray scattering techniques. Experiments will be carried out both at zero field and at high fields where the structural order is respectively commensurate and incommensurate. The "order from disorder" phenomenon in the 2D material Sr2Cu3O4Cl2, including the novel effects expected from doping-induced disorder, will be explored. Finally, quantum Monte Carlo techniques will be used to model the 1D and 2D quantum spin systems. The materials studied have many practical applications varying from superconducting electronic devices to read heads in magnetic recorders. The students educated in this program will play a leadership role at our large national facilities. %%% This research program investigates the microscopic physics of materials that fall in the technical category of "highly correlated electronic materials" The best known examples are high temperature superconductors. The most important microscopic signature of such materials is that the way any electron in the material behaves depends in detail on what the electrons surrounding it are doing. This turns out to be a surprisingly difficult problem of fundamental interest but with many practical ramifications. The general strategy here is to begin at the atomic level and to look at how the properties evolve as the length scale is increased progressively from the microscopic to the macroscopic. As a specific example, materials such as La2CuO4 as a function of doping evolve continuously from two dimensional magnets to high temperature superconductors to normal metals. The research goal is to understand the nature of this evolution with emphasis on materials which are in the transition region between different states. As a result of this and related work by others we hope to arrive at a complete understanding of the various phases and their transitions in copper oxide superconducting materials. This will in turn enable the community to design better materials and to create new devices. Indeed, devices based on highly correlated electronic materials are already appearing in the marketplace. Students involved in this project typically pursue careers at research universities, national laboratories, technology-based industries and in management consulting and thereby contribute significantly to both U.S. science and the economy.