Physical Property Measurement System
The PPMS, operated by Luis Balicas, can measure the low temperature heat capacity, AC susceptibility, and resistivity of samples in applied fields of up to 9 T. This is a workhorse for our lab and in constant use.
Here are some of the projects that our group is currently working on:
(1) Geometrically frustrated systems
Geometrically frustrated magnets are materials which cannot form conventional magnetically ordered ground states at low temperatures. According to the third law of thermodynamics, all systems should (in theory) attempt to find a ground state which minimizes the free energy, which tends to be a low entropy state. In the lab, we can design materials which have problems ordering at low temperatures due to the topology of the spins and the nature of the interactions between the moments. For example, spins which tend to orient antiferromagnetically at low temperatures that reside on triangles cannot order into what is called a Neel state (an up-down-up-down configuration). The system is said to be “frustrated”.
The interesting physics in these systems comes about when an energetic compromise takes place between the spins to form some new state of magnetism. Some of these materials are spin glasses, for example, at low temperatures in the absence of disorder. Some materials enter a so-called “spin liquid” state where the spins are fluctuating down to zero Kelvin, and only have short-ranged order interactions. Others form a “spin ice” state where the spins freeze into a configuration that, spin for spin, can be calculated based upon how protons freeze out in water ice.
Our group is involved with the elucidation of some of these problems. In particular, we are interested in the physics of half-integer and integer spin systems on various lattices.
This work is an ongoing collaboration with J. Gardner (NIST), B. Gaulin (McMaster) and J. Greedan (McMaster).:
Figure 1: Lattices composed of triangles where one would expect frustration effects to be significant (A triangular lattice, B Kagome lattice, C Face centered cubic lattice, D - pyrochlore lattice)
(2) Heavy fermion physics
Heavy fermions are materials in which the constituent electrons have an extremely high “effective mass” at low temperatures. The physical reason for this is that the electrons are strongly interacting with one another, leading to a sluggish motion at low temperatures that arises as a higher mass (hence the name “heavy fermion”, since electrons are fermions). Most of these materials are cerium or uranium based intermetallic compounds. The interesting physics of these materials comes about when one starts to observe strange behaviour at low temperatures, such as the formation of “hidden order” states or exotic superconductivity. Many physicists have pointed out that the superconductivity in these materials looks very similar to the cuprate superconductors, and this suggests that these interactions between the electrons, which are magnetic in nature, are important in forming Cooper pairs.
In recent years, our group has concentrated upon the material URu2Si2. We are now doing an extensive series of neutron experiments to look for a mysterious “hidden order” phase. At 17.5 K, the electrons condense into some ordered state, but we still haven’t figured out (for the last 20 years) what this state is! This indicates either a huge gap in our understanding, or that 20 years of experiments have been looking in the wrong place. These experiments are being done in collaboration with B. Buyers (CNBC) and C. Broholm (NIST/Johns Hopkins).
Figure 2: Spin Excitations in URu2Si2 in the “hidden order” phase
(3) Exotic superconductors
Over the past five years I have become increasing involved with the physics of superconductivity (namely, the physics of unusual superconductors). High-Tc superconductivity has been an active field of physics for the last 20 years, but we have made little progress over the last decade towards our goal of designing a room temperature superconductor. The general consensus within the field is that we clearly do not have an understanding of the mechanism for how electrons form the Cooper pairs that make up the superconducting condensate.
Our group is using solid state chemistry to design new superconductors and test their properties. We are currently focusing on analogues of the osmium pyrochlore structures to investigate the intersection of magnetic frustration and superconductivity. We are also involved with active collaborations with the Luke and Uemura group (of McMaster and Columbia Universities respectively). Locally, we are involved with crystal growth of high quality superconducting oxides in collaborations with Prof. J. Brooks and G. Boebinger.
Figure 3: The Meissner effect, which causes a magnet to levitate over a superconductor.
(1) Low-dimensional magnetism
There are two main projects that we are involved with:
One dimensional chromates: In collaboration with Prof. N. Dalal, we are investigating the physics of a new series of one dimensional chromate oxides that have unusual heat capacity anomalies at low temperatures. The ability to tune the “anomaly temperature” through solid state chemistry techniques has made these materials attractive for organizations such as NASA, who wish to use these as refrigerants in interstellar space. It is currently unknown why these materials have these unusual transitions.
Two dimensional ruthenates: The ruthenate Sr2RuO4 has been of interest over the last few years due to it’s unusual superconductivity at low temperatures (being the first non layered cuprate discovered to have non-s wave pairing). In a collaboration with Z. Mao of Tulane, we are now looking at multilayered ruthenates to investigate how the electrons pair up at low temperatures using neutron diffraction and muon spin relaxation measurements.