About the Workshop

Scope

This Workshop is designed as a tool to present the current state of affairs and facilitate discussion focused on the dual nature of f electrons in multitude of systems. In particular the workshop will enable discussion of data and theory in the following research areas: (i) understanding the physical properties of correlated systems and heavy fermions and (ii) unraveling the physical principles of f-electron materials with special emphasis on Ce, U and Pu compounds.
 

The History 

The First International Workshop on Dual Nature of f-Electrons (Jul. 16 - Jul. 18, 2006 @ Santa Fe, USA)
The Third International Workshop on Dual Nature of f-Electrons (May 25 -May 28, 2010 @ Dresden, Germany)
 

Motivation

When atoms form a solid, the fate of their outer-shell electrons is determined qualitatively by a tradeoff between kinetic and potential energies that leads to the formation of dispersive bands. This simple picture, combined with Pauli's exclusion principle, forms the basis for understanding why solids are metallic, semiconducting or insulating and why solids crystallize in particular structures. Electrons also interact with each other and with their chemical environment. These interactions give rise to phase transitions, such as crystallographic changes, magnetic order, and conventional superconductivity. By the early 1980's, substantial success of these concepts invoked claims that the physics of solids was rapidly becoming a solved problem. At that time, there were, however, a few 'anomalous' solids; some were insulators or non-magnetic when they should have been metals or magnetic. We now understand that these 'anomalies' are the rule for classes of materials. They arise from very strong repulsive Coulomb and spin-orbit interactions that must be treated on an equal footing with kinetic and potential energies. The physics of these anomalous d-and f-electron materials is exceptionally non-trivial. The Coulomb and spin-orbit interactions create a complex and highly correlated electronic state whose response is controlled collectively by all of the outer-shell electrons. This complexity generates a rich spectrum of physical states and exotic behaviors that pose the grandest challenging questions for today's condensed matter physics.

Minimal models that include strong electron-electron interactions are crucial to describe the physics of transition metals (3d systems) and lanthanide (4f systems) compounds. A multi-orbital Hubbard-like model captures the basic physics of 3d systems. Recent solutions of the Periodic Anderson Model (PAM) confirm that it contains at least part of the essential physics of 4f systems. The solutions of these models thus account for gross consequences of electronic correlations, i.e., the aforementioned 'anomalous' behaviors in 3d and 4f systems. The modeling for the 4f materials, however, appears incomplete and, for the 5f systems (actinides), inadequate. In neither model, do outer-shell electrons assume the dual roles of being simultaneously localized and itinerant (band-like). Nevertheless, a set of experiments on correlated 4f- and 5f-electron materials during the past four years argues precisely for this seemingly contradictory duality. Particularly compelling experiments (e.g., Phys. Rev. Lett. 92, 016401 (2004)) on cerium compounds show that the single 4f-electron of cerium is localized at room temperature and partially 'dissolves' with decreasing temperature into a highly correlated band state but retains some fraction of its localized nature. Similarly, low-temperature experiments on correlated uranium compounds (e.g. Nature 410, 340 (2001)) find that electrons in uranium's 5f-orbital simultaneously participate in magnetic ordering of localized electrons and in forming a correlated band of itinerant electrons out of which unconventional superconductivity emerges. These experiments imply that duality imposed by strong correlations drives states in which low energy spin and charge degrees of freedom cannot be disentangled. The Fermi surface of these unconventional metals will reflect these unusual properties and may also be composed of more than one component. Such duality could provide a natural explanation for the homogeneous coexistence of multiple broken symmetries, e.g., magnetism and superconductivity, found in some cerium and uranium compounds, for the reconciliation of experimental observations with theories of quantum criticality, and for the unexplained relationships between photoemission spectra and ordered magnetic moments.

Plutonium is particularly germane. The ad hoc assumption that its 5f electrons are simultaneously localized and itinerant gives the correct equilibrium volume for d-Pu, but why this assumption seems to be correct and why the localized fraction of 5f electrons should pair in a special configuration are not understood.

Recent theoretical effort is available, where the dual nature of f electrons is applied in explaining the ground state properties for Pu metal and a limited number of heavy fermion superconductors. We also have some initial indications from experiment allowing the explanation of the electronic structure features by assuming duality of the f electrons. It seems necessary, at this initial stage of development, to confront both sides: theory and experiment, and attempt to discuss both the present level of understanding and - more importantly - the directions to be explored in the future.