1. The Interaction Between Magnetism and Superconductivity in
Novel Uranium Compounds
M. Brian Maple, University of California, San Diego — DMR 0335173
Within the field of condensed matter physics
devoted to the study of correlated electron
materials, the metallic compound URu2Si2 is
known for its puzzling hidden order phase that
defies conclusive identification. Our laboratory has
developed techniques for growing large single
crystals of URu2Si2 to facilitate the study of this
material’s unusual physical properties.
Our current research effort involves chemical
substitution of Re for Ru, which suppresses the
formation of hidden order and may give rise to
quantum critical phenomena at intermediate
concentrations. We are also studying the effects
of pressure on the hidden order, ferromagnetic,
and superconducting phases of URu2Si2.
The crystal structure of URu2Si2 and a photograph of
a single crystal of URu1.8Re0.2Si2. This sample is
about 1 inch long, although larger samples are
routinely grown.
Low temperature - composition (T-x) phase diagram
for URu2-xRexSi2. Substitution of Re for Ru eventually
destroys hidden order and gives rise to
ferromagnetism at higher Re concentrations.
2. From left to right :
Wes Miller REU student 2006
Columbine Robinson UCSD undergraduate 2005
Colin McElroy UCSD undergraduate 2005
UCSD Undergraduates 2005-2006
Christopher Lee Columbine Robinson
Alex Dooraghi Keith Chan
Brian Maertz Yong-Chan Kim
Colin McElroy Ben Yukich
Martha Coakley
Summer REU Students 2006
Wes Miller Kevin Zeilnicki
Undergraduate Student Research:
Undergraduate students from UCSD as well as summer
REU students from other universities make up a
significant portion of our lab and are involved in many
research projects. Specifically, the undergraduates
participate in sample preparation and are trained to
perform some measurements such as powder x-ray
diffraction, electrical resistivity, and magnetic
susceptibility. Their involvement not only benefits the lab
but also prepares them for future careers in the sciences.
The Interaction Between Magnetism and Superconductivity in
Novel Uranium Compounds
M. Brian Maple, University of California, San Diego — DMR 0335173
Editor's Notes
Our laboratory studies the physics of correlated electron materials, a field concerned with materials that exhibit unusual physical properties, such as superconductors and unconventional magnets. A particularly interesting example of this type of material is the intermetallic compound URu2Si2, in which both magnetism and superconductivity (SC) appear to play important roles. Although it was discovered about 20 years ago, this compound is still the subject of a strong effort to identify the nature of its “hidden order” (HO) phase, into which the compound enters when it is cooled to temperatures less than 17 K ( -429 oF ). The hidden order state may be related to the heavy fermion behavior in URu2Si2, by which the compound’s constituent electrons behave as if they are tens of times heavier than they would be if they were in a conventional metal. The goal of our laboratory’s investigations is to find clues to the identity of the hidden order state and to better understand the interactions between the magnetic and superconducting phases in this compound. These results may offer a better understanding of superconductivity in materials like the technologically important high temperature superconducting cuprates.
To grow single crystals of URu2Si2 in our laboratory, depleted uranium (U), the transition metal ruthenium (Ru), and the semiconductor silicon (Si) are first combined in an electric arc furnace, where they are heated to a temperature of several thousand degrees. The single crystals are grown by “pulling” from the melt using the Czochralski technique. Samples of good quality of up to 2 inches long have been grown.
We are currently studying the effects of chemically substituting the transition metal rhenium (Re) for Ru in this material. Through a collaboration with the National High Magnetic Field Laboratory at Los Alamos National Laboratory, this study is offering clues to the nature of the hidden order phase in pure URu2Si2 as it is systematically destroyed by substitution of Re. We are also focusing on characterizing samples with Re concentrations around x=0.1 to x=0.2 (red area in the phase diagram) for hints of quantum critical behavior, which may be expected near a quantum phase transition. Yet another interesting aspect of the Re-doped system is the emergence of ferromagnetic (FM) order at high Re concentrations, which seems to coexist with non Fermi-liquid (NFL) behavior, wherein the low-temperature properties of a material differ markedly from those of a conventional metal. These latter investigations involve collaborations with several neutron scattering groups.
In parallel, our laboratory is also studying URu2Si2 under applied pressure and in magnetic fields down to very low temperatures, to probe the correlation between changes in the hidden order and superconducting states, a subject of some disagreement currently. This study is being extended to compounds with small Re concentrations.
