Talks

The pyrochlore magnet Yb2Ti2O7 has the remarkable property that it orders magnetically, but has no propagating magnons over wide regions of reciprocal space. Using inelastic neutron scattering we observe that at high magnetic fields, in addition to dispersive magnons there is also a two-magnon continuum, which grows in intensity upon reducing field, overlaps with one-magnon states at intermediate fields leading to strong dispersion renormalizations and magnon decays. We re-evaluate the Hamiltonian finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong quantum fluctuation effects observed at low fields.

Harboring interpenetrating lattices of corner-sharing tetrahedra, materials with the pyrochlore structure type dominate exploration of the physics of quantum spin ices. Recent synthetic advances in the control of point defects, such as in Yb2Ti2O7 and Pr2Zr2O, have demonstrated that even sub-percent changes in the type and/or number of defects radically modulates the low temperature physics of these materials. This sensitivity to disorder is driven not only by the geometrically frustrated nature of the lattice, but also by the propensity of a corner sharing tetrahedral framework to undergo displacive motions, in a manner analogous to that of beta-crystabolite (quartz), either statically, or dynamically (as soft phonons). An outlook for continued improvements in our ability to control the chemistry behind quantum spin ices will also be presented.

We aim to provide a concise review on theoretical background on emergent quantum electrodynamics in pyrochlore quantum spin ice. We first introduce elementary excitations in quantum spin ice using a simple model and then extend the discussion to more realistic systems. Implications to experiments are also discussed.

In this tutorial, we will review the microscopic aspects of rare-earth magnets relevant for quantum spin ice. We first discuss the single-ion properties of the variety of rare-earth atoms that appear in quantum spin ice candidate materials. Second, we consider the origin of the two-ion exchange interactions, including electric and magnetic multipolar interactions, super-exchange and virtual crystal field mediated interactions. We provide a detailed microscopic basis for the super-exchange interaction and discuss the implications of its multipolar structure for models of rare-earth materials. Finally, we introduce the generic symmetry allowed anisotropic exchange model for rare-earth pyrochlore magnets.

In this work we consider a recent proposal in which gravitational interactions are mediated via the exchange of classical information and apply it to a quantized Friedman-Robertson-Walker (FRW) universe with the assumption that any test particles must feel a classical metric. We show that such a model results in decoherence in the FRW state that manifests itself as a dark energy fluid that fills the spacetime. Motivated by quantum-classical interactions this model is yet another example of theories with violation of energy-momentum conservation whose signature could have significant consequences for the observable universe.

Causal dynamical triangulations (CDT) is a sum-over-histories approach to quantum gravity which leverages the techniques developed in lattice quantum field theory. In this talk, I discuss the thick sandwich problem in CDT: Given initial and final spacelike hypersurfaces, each with a fixed geometry, what is the transition amplitude for one transitioning into the other? And what geometries dominate the associated path integral? I discuss preliminary studies performed in this direction. I also highlight open problems and interesting directions for future research.

I will discuss two ways in which revising the notion of time at the Big Bang will lead to testable predictions. I will then contrast these predictions against standard ΛCDM scenario, and cosmological observations. The first model, Holographic Cosmology, is based on a 3d quantum field theory without time, suggesting the possibility of nonperturbative effects on large angles (l<30) in the CMB sky. The second model, Periodic Time Cosmology, relates (past and future) cosmic expansion history to the spectrum of cosmic perturbations by demanding consistency with an exactly periodic notion of time. Comparing this model to observations leads to surprising implications for dark energy and/or neutrino masses in cosmology.

During past few decades, string theory has been used as a source of conjectural dualities in various areas of physics and mathematics. We have extended these applications of string dualities to the study of chiral algebras in 2d CFT. In this talk, I will sketch how to use S-duality of D3-D5-NS5 systems to shed some light on already known dual constructions of chiral algebras and generate huge amount of new dualities.

Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural and covariant manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the usual hypersurface data on which definitions of entanglement typically rely is not available. Instead, we appeal to a more global expression which, for a gaussian scalar field, expresses the entropy of a spacetime region in terms of the field’s correlation function within that region. In this talk I will present results from evaluating this entropy for causal sets sprinkled into a 1 + 1-dimensional causal diamond in flat spacetime, and specifically for a smaller causal diamond within a larger concentric one. In the first instance we find an entropy that obeys a (spacetime) volume law instead of the expected (spatial) area law. We find, however, that one can obtain the expected area law by following a prescription for truncating the eigenvalues of a certain “Pauli-Jordan” operator and the projections of their eigenfunctions on the Wightman function that enters into the entropy formula.

One of the defining features of holography is the geometerization of the renormalization group scale. This means that when a quantum field theory is holographically dual to a bulk gravity theory, then the direction normal to the boundary in the bulk (the `radial' direction) is to be interpreted as the energy scale of the dual quantum field theory. So this direction can be seen to be `emergent', and the evolution of bulk fields along this direction is equated with the renormalization group flow of sources or couplings of boundary operators. Given that gravitational theories are generally covariant, this emergent direction must be treated on equal footing as those of the space on which the boundary field theory lives. I will describe the precise integrability condition the renormalization group flow need satisfy which encodes this peculiar response of the quantum field theory under coarse graining so as to respect this property of covariance. In other words, this condition is `dual' to general covariance itself.

Using Picard-Lefschetz theory we show that the Lorentzian path integral forms a good starting point for quantum cosmology which avoids the conformal factor problem present in Euclidean gravity. We study the Lorentzian path integral for a homogeneous and isotropic model with a positive cosmological constant. Applied to the “no-boundary” proposal, we show that this leads to the inverse of the result obtained by Hartle and Hawking. Including a inflation field, the Lorentzian path integral prefers to start at the 'top of the hill' leading to good initial conditions for slow roll inflation. However, when including gravitons the fluctuations seem to be unstable.