A look at some of the topics that we will be exploring in the Ross Lab [This page is under construction!]
Emergence in Quantum Phases of Matter
We are using solid state materials with magnetic degrees of freedom to explore the topic of emergence. How does frustration (i.e. the competition between electronic interactions) in combination with quantum effects, generate new effective interactions? What are the fundamental excitations of the new quantum ground states that emerge? Can lattice disorder produce new types of emergent excitations? Some known examples are spinons in spin liquids (“half of an electron” that carries spins but no charge), magnetic monopoles in spin ice, or the theoretically predicted anyons arising from Kitaev spin liquids. We are exploring materials to find excitations like these. Neutron scattering is a fantastic probe of these excitations, with the ability to cover the full Brillouin zone as well as having access to a large dynamic range for spectroscopy (from 0.1 meV to 1 eV).
Using emergence in quantum phases of solid state materials, we can study analogies to the particles in our own universe (for example, through emergent electrodynamics in Quantum Spin Ice), or generate completely new types of particles with as-of-yet unknown applications. We can begin to dream about these applications already, such as topologically protected quantum computation using anyons, or controlling magnetic monopoles in spin ice for “magnetricity”.
Spin Liquids in Anisotropic Materials
At the root of most of these emergent excitations is a class of states called spin liquids. These are quantum entangled states that defy the usual ordered phases of matter already known to us (in magnetism, for example, a ferromagnet, antiferromagnet, or other “ordered” state). The spin liquid is disordered locally, but has a type of non-local (topological) order. Spin liquids were first proposed as a potential mechanism for high temperature superconductivity. He first envisioned a triangular lattice of isotropic spin 1/2 objects which could support a Resonating Valence Bond (RVB) state. Variations on the RVB have been explored theoretically and experimentally over the past few decades.
We are now beginning to understand that anisotropy arising from strong spin orbit coupling (SOC) can produce much more exotic types of disordered quantum states. The rare-earth titanate pyrochlores are a great example of how anisotropic exchange and single-ion anisotropy can lead to new phases of matter like quantum spin ice. A major focus of the Ross Lab will be to investigate other frustrated geometries based on rare-earth ions.
Quantum Phase Transitions
Thermal (classical) second order phase transitions are well-described through the Landau-Wilson-Ginzburg paradigm, which uses the concept of an order parameter to formulate a theory of the system’s behavior near a critical point. In quantum systems, tuning a parameter other than temperature, for example magnetic field, can produce a phase transition even at T=0 – this is called a Quantum Phase Transition (QPT). The theory of QPTs is still under development, and an exciting variation is a so-called deconfined QPT in which the order parameters of the two neighboring phases are not compatible. Near the critical point, new exotic types of behavior can emerge. We are interested in finding and exploring material systems that exemplify QPTs and deconfined QPTs. One goal is to explore the effect of rapid quenches through QPTs.
To fully understand the basic interactions in new materials, we need high quality single crystals. In the Ross Lab we will be producing single crystals using the optical floating zone technique. In collaboration with the Neilson Lab in Chemistry at CSU, we’ll also be exploring flux, vapor transport, and hydrothermal methods for crystal growth. We have a strong interest in exploring new techniques for crystal growth and envision projects focussed on experimental crystal growth. We will aim for a new understanding of the control of optical floating zone crystal growth.