Muir J. Morrison

Unhappy vertices in artificial spin ice: new degeneracies from vertex frustration

In 1935, Pauling estimated the residual entropy of water ice with remarkable accuracy by considering the degeneracy of the ice rule solely at the vertex level. Indeed, his estimate works well for both the three-dimensional pyrochlore lattice and the two-dimensional six-vertex model, solved by Lieb in 1967. A similar estimate can be done for the honeycomb artificial spin. Indeed, its pseudo-ice rule, like the ice rule in Pauling and Liebs systems, simply extends to the global ground state a degeneracy which is already present in the vertices. Unfortunately, the anisotropy of the magnetic interaction limits the design of inherently degenerate vertices in artificial spin ice, and the honeycomb is the only degenerate array produced so far. In this paper we show how to engineer artificial spin ice in a virtually infinite variety of degenerate geometries built out of non-degenerate vertices. In this new class of vertex models, the residual entropy follows not from a freedom of choice at the vertex level, but from the nontrivial relative arrangement of the vertices themselves. In such arrays not all of the vertices can be chosen in their lowest energy configuration. They are therefore vertex-frustrated and contain unhappy vertices. This can lead to residual entropy and to a variety of exotic states, such as sliding phases, smectic phases and emerging chirality. These new geometries will finally allow for the fabrication of many novel, extensively degenerate versions of artificial spin ice.

Degeneracy and Criticality from Emergent Frustration in Artificial Spin Ice

Although initially introduced to mimic the spin-ice pyrochlores, no artificial spin ice has yet exhibited the expected degenerate ice phase with critical correlations similar to the celebrated Coulomb phase in the pyrochlore lattice. Here we study a novel artificial spin ice based on a vertex-frustrated rather than pairwise frustrated geometry and show that it exhibits a quasicritical ice phase of extensive residual entropy and, significantly, algebraic correlations. Interesting in its own regard as a novel realization of frustration in a vertex system, our lattice opens new pathways to study defects in a critical manifold and to design degeneracy in artificial magnetic nanoarrays, a task so far elusive.

Inelastic collisions of CaH with He at cryogenic temperatures

Using helium buffer gas cooling, we have prepared dense samples of ground-state molecular calcium monohydride (CaH X 2Σ) at cryogenic temperatures. We have used optical pumping to polarise the spin state of the CaH molecules and we have measured the inelastic collisions of molecular CaH with atomic helium at temperatures from 2 to 7 K. The measured CaH electronic spin depolarisation rate coefficient increases rapidly with increasing temperature, increasing from 2 × 10−13 cm3 s−1 to over 10−11 cm3 s−1. The strong dependence of rate coefficient on temperature is attributed to the CaH population in the first excited rotational state.

Proposed search for T-odd, P-even interactions in spectra of chaotic atoms

Violation of fundamental symmetries in atoms is the subject of intense experimental and theoretical interest. P-odd, T-even transitions have been observed and are in excellent agreement with electroweak theory. Searches for permanent electric dipole moments have placed bounds on T-odd, P-odd interactions, constraining proposed extensions to the Standard Model of elementary particles. Here we propose a new search for T-odd, P-even (TOPE) interactions in atoms. We consider open-shell atoms, such as the rare-earth atoms, which have dense, chaotic excitation spectra with strong level repulsion. The strength of the level repulsion depends on the underlying symmetries of the atomic Hamiltonian. TOPE interactions lead to enhanced level repulsion. We demonstrate how a statistical analysis of many chaotic spectra can determine the strength of level repulsion; in particular, the variance of the number of levels in an energy range has been shown to be a useful measure. We estimate that, using frequency comb spectroscopy, a sufficient number of chaotic levels could be measured to match or exceed the current experimental bounds on TOPE interactions.