- Outline of Our Research
- Transition metal chemistry
- New Oxide and Intermetallic Superconductors
- Compounds with low dimensional or frustrated magnetic spin systems
- Research Motivation
Outline of Our Research
In our group, we search for and synthesize new compounds, grow crystals, determine their crystal structures, and characterize their electronic and magnetic properties. We work closely with colleagues in theoretical and experimental condensed matter physics at Princeton, Columbia University, Bell Labs and many other labs around the world. We interact with experts in other specialties in materials science, such as the neutron scattering group at the National Institute of Standards and Technologies.
Areas of interest to us include new superconductors, dielectric, thermoelectric, magnetoresistive, frustrated magnets, and transparent conducting materials. In all cases, we employ the principles and synthetic and analytical methods of solid state chemistry to try to find new materials with exceptional physical properties in the hope of driving the field of study in new or unexpected directions. Our research philosophy has probably best been summarized by NIKE: "just do it". Some examples of our current work are described briefly in the following.
Transition metal chemistry:
Of particular interest to us is the discovery of new intermetallic, oxide, pnictide, and chalcogenide compounds of the transition elements. Transition metal compounds, especially of the 3d elements, have been the subject of continuous interest in the solid state chemistry research community for many decades, due to the wide variety of electrical, magnetic and structural properties and phenomena which they display. The discovery of high temperature superconductivity in copper oxides, however, led to the general realization that the electronic and magnetic properties of the whole family of transition metal compounds, and the detailed relationship of those properties to crystal structure, were very poorly understood. Current research on these materials is at the frontiers of our knowledge of complex solids. Of particular interest are materials in which the charge carriers, instead of acting like non-interacting particles as in the classical picture of solids, are actually interacting with each other. The spin carriers and the interactions between the underlying lattice and the electronic and magnetic states play a critical role in the determination of the physical properties. Although such compounds might have been surveyed in the past, many of the major aspects of their chemistry and behavior remain unexplored from a modern perspective. With the motivation of understanding the origin of superconductivity in the cuprates as a wedge for re-opening old issues or introducing previously unimagined issues, the solid state chemistry and physics of transition metal compounds is currently undergoing an explosive rebirth of research activity which is driving the frontiers of the understanding of complex materials in new directions. This is being accomplished through the detailed consideration and characterization of new materials which only a decade ago would have seemed too complex to understand from chemical, theoretical, or physical perspectives.
New Oxide and Intermetallic Superconductors:
Simple intermetallic compounds were widely explored as potential superconducting materials in the 1950s and 1960s. The best intermetallic superconductors, based on niobium rich compounds with group IIIB and IVB elements (e.g., Nb3Ga and Nb3Sn), were discovered and developed during that time period. These are isotropic materials in which the conduction electrons travel in an electronic band made from niobium d states. The discovery of High Tc superconductivity in 1986 in structurally layered copper oxides startled the materials science and condensed matter physics communities. The highly anisotropic crystal structures, the evolution of superconductivity from magnetism by chemical doping of the 3d9 Cu2+ based host compounds, and the special aspects of copper-oxygen bonding make these materials of continuing interest. High Tc superconductivity has been a windfall for solid state chemists, with approximately 50 new superconducting copper oxide based compounds discovered, and hundreds of new materials found and characterized. In our group we have been very active in searching for new oxide superconductors. Our interest lies not only in cuprates, but also in potential analogous compounds based on other transition metals. Working our chemistry to yield compounds at the metal-insulator transition, or the localized-itinerant electron boundary, interesting electronic or magnetic properties often arise, even when superconductivity is elusive.
Although the method of synthesis is quite different, using the philosophy we developed to search for oxide superconductors we have also been searching for new intermetallic superconductors, encouraged by the recently discovered new quaternary intermetallic superconductors -- lanthanide nickel or palladium boro-carbides -- with superconducting transition temperatures rivaling those of the best intermetallics previously known. With layered crystal structures, an interplay of superconductivity and magnetism occurs between the magnetic lanthanide carbide layers and the superconducting transition metal boride layers in some variants. We believe that exploration of new, complex intermetallic compounds has great potential for yielding interesting superconducting and magnetic materials and continue to work in that area.
Compounds with low dimensional or frustrated magnetic spin systems:
Transition metals with unpaired electrons in their d orbitals display a variety of magnetic behaviors in solids, ranging from the familiar ferromagnetism and antiferromagnetism to much more exotic spin states. The study of these phenomena, especially in transition metal oxides, is an area of considerable current activity. Spin systems in which the geometrical spin arrangements are effectively low dimensional, such as chains, or ladders made from parallel chains, for example, are expected to show a variety of exotic magnetic properties. We have recently found and characterized a new copper oxide based chain compound in which the chains could be doped with holes (positive charge carriers) via chemical substitution over a wide range of composition. The magnetic behavior of the chain is changed profoundly as a function of hole concentration. Systems such as this are of interest as their low dimensionality makes their behavior treatable from a theoretical perspective, allowing a for a good test of theoretical models which in more complex geometries, such as the infinite planes in copper oxide superconductors, have proven difficult to model in sufficient depth. We have also synthesized and characterized new compounds with geometrically "frustrated" spin systems. These are based on placing antiferromagnetically interacting transition metal atoms at the nodes of triangular plane lattices. Long range ordering of antiferromagnetically interacting spins on triangular plane lattices is frustrated because there is no possible configuration where all spins can be surrounded by near neighbor spins aligned in the opposite direction. We are working closely with scientists at Bell Labs, Columbia University, NIST, and Johns Hopkins University in this area. The potential for new materials to impact the direction of this rapidly developing field is very significant.
Many of our modern technologies have changed in almost unimaginable ways in the past few decades. These changes have greatly improved our quality of life, often in ways which were at first unpredictable. Frequently, the direction of change has been to make existing technologies more sophisticated and complex, such as in the case of computing and communications, sometimes improving performance by many orders of magnitude and making the previously impossible into the ordinary. At the basis of all our sophisticated technologies are the materials - the chemical compounds - whose physical properties make the technologies possible. These are the unseen heroes of our modern age: without them the giant technologies would not function. Although they are largely taken for granted, without the continuing discovery and development of new materials, technology would not continue to grow.
As technologies evolve toward greater sophistication, and condensed matter physics moves toward greater understanding of complex systems, new materials very often play an important role in driving fields of research and development in new directions. The goal of the research in our group is to identify the technologies or scientific issues which crave the introduction of new electronic or magnetic materials, and to discover those materials using the methods of solid state chemistry.