The fundamental properties of correlated electron materials such as their ability to conduct heat or electricity, whether they sustain static and localized moments or charges, and indeed how these complex systems approach their ultimate ground states reflect the degree to which their electrons are localized or delocalized. In f-electron systems, most notably heavy electron compounds, electrons initially localized on f-orbitals can be delocalized, expanding the Fermi surface. Of special interest is when this localization - delocalization occurs at a T=0 quantum critical point, which separates a magnetically ordered state where the electron is localized and does not participate in the Fermi surface, from a paramagnetic and strongly interacting metal where the f-electron is fully incorporated in the Fermi surface. This project will track this moment deconfinement transition away from the quantum critical point to higher temperatures and investigate its relationship to Kondo coherence, where f-electrons are thought to delocalize by the hybridization of individually Kondo compensated moments. The project will pursue this program in the YbTX compounds, where with an appropriate choice of transition metal T and main group element X we can span all regimes of the heavy electron phase diagram. It has also been suggested that strong quantum fluctuations associated with geometrical frustration may separate magnetic localization from the quantum critical point. This possibility will be investigated in the R2T2X (R=Ce,Yb) Shastry-Sutherland compounds. This program will combine the synthesis of new f-electron compounds, and their investigation using neutron scattering experiments as well as lab-based magnetometry, specific heat, and electrical transport measurements. The project makes extensive use of national research facilities, and the students trained will be well prepared to become effective future users. By learning a variety of different synthesis and characterization techniques, participating students will develop a valuable and very portable set of skills, and through the excitement of performing the first measurements on materials of their own invention, will gain the motivation to sustain them in their future careers as materials-inspired scientists.
Just as changing temperature can cause water to fundamentally change its properties from solid to liquid to vapor; other properties of materials can similarly be transformed by varying temperature, pressure, or magnetic field: metal to insulator, magnet to non-magnet, conductor to superconductor. The ability to control these phase transitions is central to implementing new generations of sensors, where for instance small magnetic fields transform a conductor into a nonconductor, or a small change in temperature can cause a magnet to become nonmagnetic. These effects become strongest when the phase transitions occur at the lowest temperatures, and it is the purpose of this research to study the most extreme of magnetic phase transitions, where magnetism is stable only at zero temperature. The goal of this research is to understand the underlying factors which control the stability of magnetism, information which will be used to design new generations of magnetic materials with improved functionality for applications as diverse as magnetic data storage and energy control. This project will develop new families of materials to enable this research, where the strength of the magnetism and its onset temperature will be varied compositionally. We will document the corresponding changes in the material's ability to conduct heat and electricity, and the strength of the magnetism induced in the material as we drive it ever closer to the composition where magnetism is no longer possible. Very near this magnetic instability itself, the magnetism exists only ephemerally, and over only short length scales. Neutrons can be used to microscopically probe these magnetic fluctuations. Scattering experiments will be performed at national neutron scattering facilities such as those at NIST in Gaithersburg MD and the Spallation Neutron Source in Oak Ridge TN. It is increasingly recognized that the dearth of scientists trained in synthesis techniques is placing US competitiveness in materials inspired research at risk. The project focus is to synthesize new materials with very specific functionality. Participating undergraduate and graduate students will learn a variety of different synthesis techniques, as well as the arsenal of experimental techniques required to certify the high quality of the samples. Students participating in this program develop a highly sought and very portable skill set, which has already proven of great value to themselves and their future employers.
Interactions among mobile electrons in metals are the source of their collective behaviors, which include magnetic order, superconductivity, and even the localization of charge to form an insulator. It remains a central challenge in condensed matter physics to construct a predictive framework that explicates relationships among these different ground states, with the eventual aim of prediction and control. Insight into the relationships among these phases is best gleaned at T=0, and in particular at quantum critical points (QCPs), where a phase like magnetic order is suppressed, while a second phase such as superconductivity is just emerging. While it is clear that QC fluctuations are decisive for determining the relative stabilities of competing phases, surprisingly little is known about the essential thermodynamics of these unusual phase transitions, and of their critical modes and fluctuations. Such information is crucial input to the development of theories, which are so far extremely sparse. This experimental project sought to identify new rare-earth based systems where the associated f-electrons could be localized, i.e. excluded from the Fermi surface, bearing a magnetic moment, and thus are potentially magnetically order, or alternatively the f-electron is delocalized, contained in the Fermi surface, and where the magnetic character is limited only to diffuse spin fluctuations. These experiments were pursued in a new family of R2T2X (R=Ce,Yb) compounds, where the Ce and Yb atoms lie on the geometrically frustrated Shastry-Sutherland lattice. This frustration is thought to suppress magnetic order, making it more straightforward to identify the presence or absence of electronic localization. Our research made extensive use of national research facilities, particularly the US neutron scattering centers at NIST and Oak Ridge National Laboratory. Both undergraduate and graduate students participated in all aspects of these experiments, receiving thorough training and broad experience in both measurement technique and applications which will prepare them to become effective future users of these national facilities. It is increasingly recognized that the dearth of scientists trained in synthesis techniques is placing US competitiveness in materials inspired research at risk. The synthesis of the intermetallic crystals needed to support our measurement program and our collaborations was a central theme of this research program, and the participating students learned a variety of different synthesis techniques, and were exposed as well as the arsenal of experimental techniques required to certify the high quality of the samples. Students participating in this program develop a valuable and very portable set of skills, and through the excitement of performing the first measurements on materials of their own invention, gain the motivation which will sustain them in their future careers as materials-inspired scientists.
