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 spin 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 these exotic 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, envisioned on 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 is to investigate other frustrated geometries based on rare-earth ions.
Quantum Phase Transitions and Quantum Quenches
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, particularly how non-equilibrium quantum phenomena are generated after quenching through them.
To fully understand the basic interactions in new materials, we need high quality single crystals. In the Ross Lab we produce single crystals using the optical floating zone technique. In collaboration with the Neilson Lab in Chemistry at CSU, we also make use of flux, vapor transport, and hydrothermal methods for crystal growth.
We have investigated the effect of magnetic dilution (substituting Ga for Fe) on the partially ordered helical antiferromagnetic ground state of Fe3PO4O3. The helical state in this compound is known to form in needle-like domains, with short range correlations in the ab plane. Our most recent results, published in PRB, show that the helical pitch length and the ab plane correlation length are intrinsically related to each other – the pitch length appears to set the size of the correlations in the ab plane. This is reminiscent of Skyrmions, whose size is set by the helical pitch length.
Our paper on the single ion properties of Cobalt-based S=1/2 pyrochlores NaCaCo2F7 and NaSrCo2F7 was published in PRB as an editors suggestion. Based on inelastic neutron scattering, from which we can extract the single ion angular momentum wavefunctions and thus their g-tensor anisotropies, we show that these materials are good examples of the S=1/2 XY antiferromagnetic pyrochlore model with exchange disorder. This model is currently being discussed in the context of “order by disorder” and these pyrochlores appear to be a counter-example to this phenomenon.
Our first paper from a collaboration with the Neilson Lab was published in PRB as an editor’s suggestion. Fe3PO4O3 is a frustrated antiferromagnet with triangular motifs that decorate a simple rhombohedral lattice. The geometric frustration produces an anti-ferromagnetic helical magnetic structure with unusual needle-like domains. The domains are restricted to 70 Å in the hexagonal ab plane, while they are unrestricted along the c-axis, producing a very unusual neutron powder diffraction profile (see figure below) with broad and flat-topped magnetic peaks coexisting with a sharp magnetic peak. Due to the short correlation length of the domains in the ab plane, the material…