I am an experimental condensed matter physicist, working in the general area of scattering studies of exotic ground states in new, mostly magnetic, materials. As described below, this means that we (my grad students, postdocs, collaborators and myself) make new materials which we think will have interesting and exotic ground states, and then take these materials to forefront neutron and x-ray scattering facilities in North America and around the world. We perform scattering experiments on these new materials and then work either independently or with our friends in theory to interpret the experiments, and thereby shed light on the exotic properties of the new materials.
How do we generate materials which exhibit exotic ground states? Well, we try to incorporate features into the crystal structure and the nature of the magnetic moments which encourage fluctuations, and thereby make it difficult for the material to find an ordered state at low temperatures. We have three features we can work with: we can make crystal architectures which are likely to show geometrical frustration; we can make magnetic crystals which have quantum magnetic moments in them, especially s=1/2 magnetic moments; and we can make three dimensional crystals which are made up of an assembly of low dimensional substructures, like stacks of quasi-two dimensional planes of atoms.
At present we have three themes to our work:
Geometrically frustrated magnets: these are magnetic materials which possess local geometries and magnetic interactions whose combination is incompatible with long range order. The easiest to appreciate occurrence of geometrical frustration happens with the combination of antiferromagnetism and triangular geometries. The tetrahedron is to three dimensions what the triangle is to two dimensions, so this is a common, but poorly understood occurance in three dimensional crystal structures made up of networks of interconnected tetrahedra.
We have been particularly active studying cubic “pyrochlore” magnets, which can be thought of in terms of magnetic moments decorating a network of corner sharing tetrahedra. Such materials have much difficult reaching a magnetically ordered state at low temperatures, and can display a host of exotic, disordered magnetic ground states such as spin liquid, spin glass, and spin ice states. We have devoted much effort to studying the rare earth titanate family of cubic pyrochores. You can look up our recent work on Yb2Ti2O7, Ho2Ti2O7, Er2Ti2O7, and Tb2Ti2O7 under publications. Grad students Jacob, Pat, Kate, and Katharina, and postdoc Jeremy have been leading these and related projects.
Quantum Magnets with singlet ground states: these are magnetic materials comprised of s=1/2 quantum magnetic moments decorating various lattices. One interesting and basic result of quantum mechanics is that while the classical picture of ferromagnetism corresponding to all spins in a solid pointing in the same direction is also valid when quantum mechanics is taken into account, the classical Neel state for antiferromagnetism is not correct when a fully quantum treatment of antiferromagnetism is necessary.
In these cases, which is really all cases as quantum mechanics applies to everything, antiferromagnetic interactions between the quantum moments can result in the formation of local singlets and an overall non-magnetic ground state. While the ground state is a non-magnetic singlet, the excited states, which we can probe by neutron spectroscopy, are triplet states and multi-triplet states. Application of a magnetic field Zeeman-splits the triplet state and a strong enough magnetic field can drive the energy of one of the triplet of excited states below the ground state. The net effect is that triplets can begin to condense into a “sea” of singlets and crystallize within the non-magnetic ground state in phenomena very closely related to Bose condensation.
We’ve focused on two types of quantum antiferromagnets which display singlet non-magetic ground states: the quasi-two dimensional Shastry-Sutherland system SrCu2(BO3)2 and quasi-one dimensional spin-Peierls systems. In the first case, the s=1/2 magnetic moments are already arranged into pairs which then form singlets at low enough temperatures. In the spin-Peierls case, uniform chains of s=1/2 magnetic moments must spontaneously “dimerized” to form a necklace of pairs of ions which then form singlets. In the latter case, the quantum magnetism and the lattice on which the s=1/2 moments reside are strongly coupled. You can look up some of our recent publications for both these materials. The SrCu2(BO3)2 was led by Sara Haravifard, who just finished her PhD and is now a postdoc at the University of Chicago. The spin-Peierls work on TiOBr and TiOCl is being led by Pat Clancy.
High temperature superconductors: Actually high temperature superconductors are also quantum magnets – doped quantum magnets, decorating a low dimensional structure, a two dimensional one in this case. So these materials combine a couple of the general themes we are interested in.
Many scientists have been interested in these materials for more than 20 years, mostly because they display superconductivity at temperatures as high as ~ 150 K. We have focused on the magnetism in the so-called “parent” materials in the High Tc family, and on how this magnetism is modified at relatively low doping. The “doping” means that we start out with a pure “parent” compound, La2CuO4, and then replace a small percentage (like 1 -10 %) of the La3+ with Ba2+. Its known that the effect of this doping is to remove s=1/2 quantum magnetic moments from the CuO2 plane in these materials. The magnetic phase diagram of these materials in this low doping regime is remarkable (at least to me!), in that the magnetism evolves from a strong, three dimensional, commensurate antiferromagnet through a couple of different, but related two dimensional, incommensurate antiferromagnetic states, one of which also displays the superconductivity. Our work on La(2-x)Ba(x)CuO4 is being led by grad students Greg, Jerod and Kate.
Geometrically frustrated magnets, Quantum Magnets with singlet ground states, High temperature superconductors
Bruce D. Gaulin
Professor and Brockhouse Chair in the Physics of Materials
Director, Brockhouse Institute for Materials Research
Dear Prospective Graduate Student;
Thank you for your interest in experimental condensed matter physics at McMaster. I am primarily interested in studying exotic phases of matter by neutron and x-ray scattering techniques. These phases are typically generated by unusual magnets which remain in entropy-dominated, disordered states down to very low temperatures; by quantum magnets where a non-magnetic singlet ground state occurs at low temperatures, and by high temperature superconductors which may display novel "stripe" phases in which magnetism and superconductivity co-exist in an inhomogeneous fashion.
My graduate students and postdoctoral fellows study these exotic materials by carrying out neutron and x-ray scattering experiments to determine the atomic and magnetic structure and dynamics of the materials, typically as a function of temperature. I presently have six grad students, Jacob Ruff, Pat Clancy, Kate Ross, Katharina Fritsch, Greg van Gastel, and Jerod Wagman, and one postdoctoral fellow, Dr. Jeremy Carlo. Jacob and Kate came to my group from the University of Waterloo, Pat from St. Francis Xavier University, Katharina from University of Leipzig in Germany, Greg and Jerod from the University of Toronto and Jeremy recently completed his PhD at Columbia University in the USA.
I have supervised or co-supervised about 25 very talented grad students and postdocs as a faculty member at McMaster. They have all gone on to exciting careers as faculty members themselves (at the U. of Toronto, Edinburgh U., Kent State U., the Technical University of Munich), as scientists at prestige government laboratories (at the Staecie Institute of NRC, NIST, Oak Ridge National Laboratory, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, Argonne National Laboratory, Helholtz Zentrum Berlin, Defence Research Establishment, Ottawa), and in high tech industry (at Seagate, JDS Uniphase, Read Rite Corp, and Moli Energy).
Neutron and x-ray scattering are very similar techniques, although they provide different and complementary information regarding the structure of materials. A student who has been trained in one of the techniques will have an easy time with the other, as they have so much in common with each other. I have built up and operate an x-ray scattering lab at McMaster, based on an 18 kW rotating anode x-ray generator, and we have the capability to carry out scattering studies over a range of temperatures from about 400 K to 0.3 K. We carry out our neutron scattering experiments at leading international facilities such as the Chalk River Laboratories in Canada, NIST, Oak Ridge, and Los Alamos in the USA, ISIS in the UK, the HZB in Germany, and the ILL in France.
A crucial first step in these studies is to produce the novel materials in pristine, single crystal form. McMaster has extensive facilities, the best in Canada and among the best in the world, for crystal growth and sample preparation. We have built up a new state-of-the-art floating zone optical image furnace laboratory, with two image furnaces, and these have been optimized for the growth of large single crystals of novel oxide materials, like frustrated pyrochlore magnets and high temperature superconductors.
McMaster has tremendous strengths in the study of new materials, and particularly exotic magnets, metals, and superconductors. I work closely with Professors Graeme Luke, Tom Timusk, Takashi Imai, and Maikel Rheinstadter in experimental physics, while it is a great advantage for us to have superb condensed matter theorists with related interests, as we do with Professors Berlinsky, Kallin, Sorensen, and Lee. All of us are part of the Canadian Institute for Advanced Research's (CIfAR) Program on Quantum Materials, which gives us an opportunity to interact with other outstanding scientists from Canada and around the world on a regular basis.
Please visit my website for much more detail. I am always happy to chat with students about my research interests or physics in general. Please feel free to contact me with any questions you may have! You can also contact my grad students and postdocs directly, if you'd like to get an insider's perspective on life as a Mac physics grad student. Their contact info is on my website, and they would also be happy to hear from you.
Sincerely yours;
Bruce Gaulin
