Today's Fundamental Discoveries for
Tomorrow's Technology
We lay fundamental scientific groundwork that will be essential for determining the microscopic properties of quantum materials. These new materials are typically new quantum magnets and superconductors, which we synthesize and characterize using neutron scattering techniques. The remarkable properties of quantum materials could form the technological basis for future generations of faster and more powerful information technologies and devices.
Recent News
Evan Smith receives 2024 Neutron Scattering Society of America (NSSA) Outstanding Student Research Prize
Mitch DiPasquale receives Thode Postdoc Fellowship
Evan Smith receives APS Outstanding Dissertation Award
Making Fundamental Discoveries, from Crystal Growth to Characterization
Our research group makes fundamental discoveries in quantum materials – materials whose unique properties arise from their exotic quantum properties, often involving quantum magnetism, superconductivity, and topology. The mysteries of quantum materials are fertile ground for Nobel Prize-worthy discoveries. Many of these materials have remarkable properties that could form the technological basis for major breakthroughs in information technology—if the unique properties of quantum materials can be better understood and then harnessed. Such breakthroughs that could be based on superconducting electronics, spintronics, and quantum computing, for example.
To achieve such understanding, our team determines elementary excitations and structure-property relationships in new, mostly magnetic, materials. We make crystals of new materials that we predict will have exotic ground states and take them to forefront neutron and x-ray scattering facilities around the world to characterize them. We often collaborate with leading theorists 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? We 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: (1) we make crystal architectures that are likely to show geometrical frustration; (2) we make magnetic crystals that have quantum magnetic moments in them, especially s=1/2 magnetic moments; and (3) we make three dimensional crystals that are made up of an assembly of low dimensional substructures, like stacks of quasi-two-dimensional planes of atoms.
World-leading Research on New Quantum Materials
Our Research Themes
Our research effort is currently focussed on three subject areas and one instrument development area, as described below.
Geometrically Frustrated Magnets and Quantum Spin Liquids:
Geometric frustration is a common phenomena in magnetism wherein the pair-wise interactions between magnetic moments in solids cannot all be simultaneously satisfied, due to the geometry of the crystalline lattices on which the magnetic moments reside. Well known examples are triangular lattice antiferromagnets, and the result is that these materials cannot easily find a low temperature ordered state. Of course real materials are quantum mechanical in nature, and the quantum version of this problem can result in a Quantum Spin Liquid ground state, which is a disordered, highly entangled quantum state.
We and others have been synthesizing new quantum magnets with a variety of crystalline architectures with the goal of providing definitive evidence for such a Quantum Spin Liquid state in a real material. Our focus has tended to be on the synthesis and characterization of cubic pyrochlore insulators, which have the potential to display a Quantum Spin Ice ground state. Quantum Spin Ice is a particularly interesting type of Quantum Spin Liquid as it is theoretically known to possess an emergent Quantum Electrodynamics, and its elementary excitations are not traditional spin waves, but emergent magnetic and electric monopoles and emergent photons!
Quantum Magnetism and High Temperature Superconductivity
Cuprate high temperature superconductors are now making their way into our technology, especially as vehicles for producing high magnetic fields. But their fundamental properties and the mechanism by which electrons form Cooper-pairs in these materials remain elusive, despite years of study. Cuprate superconductors are in fact also two dimensional quantum magnets – they achieve a superconducting ground state by doping mobile holes or electrons into a Mott insulating, antiferromagnetic background. This leads to intertwined spin and charge stripes – a fascinating, inhomogeneous state which has been proposed to both co-exist with and compete with superconductivity.
Our group can routinely grow large single crystals of several families of cuprate superconductors, with controlled doping. These ~ 5 gram single crystals are the subject for our own neutron scattering experiments at world leading neutron scattering sources around the world. We also collaborate with a broad group of colleagues who perform complementary advanced characterization. The ultimate goal is a detailed understanding of how two dimensional stripe physics, and the structure of the lattice itself contribute to the superconducting ground state and its instabilities.
Multipolar Order and Fluctuations in New Quantum Materials
The best known and understood quantum magnets are characterized by assemblies of interacting S=1/2 magnetic dipoles on different lattices, as occurs in Cu2+ and Ti2+ – based magnets. However, a combination of spin-orbit coupling and crystalline electric field effects can give rise to assemblies of “pseudo S=1/2” degrees of freedom which carry either octupolar or quadrupole moments. These interacting multipolar moments can form both ordered and disordered phases, but they are more difficult to study than dipolar quantum magnets, as they produce weaker magnetic fields and do not couple strongly to neutron scattering as the neutron itself is a dipole probe of matter.
Nonetheless neutron scattering can probe elementary excitations of materials with multipolar states, and it does possess a weak sensitivity to octupolar correlations at high momentum transfer. High resolution x-ray scattering can also be used to study structural phase transitions induces by quadrupolar order.
Certain 4d, 5d, and 4f quantum materials possess the necessary combination of SO coupling and CEF effects to produce quantum multipolar moments in solids. The nature of these ground states is poorly explored or understood at present. We have been focussing on synthesizing 4d and 5d double perovskite materials based on Os, Re, Ru which show strong evidence for multipolar ordering. So called “dipolar-octupolar” pseudo-spin 1/2 degrees of freedom also show up in the context of Ce3+ based pyrochlores and we have led the study of these fascinating and exotic quantum materials.
New neutron scattering instrumentation
Prof. Gaulin has a longstanding interest in new instrumentation for neutron scattering. He has served as PI on three separate CFI awards that have built state-of-the-art neutron diffractometers and spectrometers at the Spallation Neutron Source and at the McMaster Nuclear Reactor. He served on the Instrument Development Teams for both SEQUOIA, a high resolution, direct geometry chopper time-of-flight instrument, and VULCAN a diffractometer optimized for engineering materials applications. These instruments have been complete and operational since ~ 2008.
More recently, Prof. Gaulin has led two successful CFI IF awards, which will build out a suite of 5 neutron diffractometers at the McMaster Nuclear Reactor (MNR). The MNR is a medium flux nuclear reactor. One of five diffractometers (MAD, shown in the picture) is complete and has been in operation for some time. Another, a small angle neutron scattering (SANS) diffractometer is essentially complete and entering commissioning. The remaining three diffractometers are a powder diffractometer, a reflectometer and an engineering diffractometer and stress scanner. Together, these instruments and associated ancillary equipment will constitute the Canadian Neutron Beam Laboratory, which is planned to be in full operation by 2029.
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