Scientific Series

In this talk I will sketch the relation between unitary representations of the BMS3 group and three-dimensional, asymptotically flat gravity. More precisely, after giving an exact definition of the BMS group in three dimensions, I will argue that its unitary representations are classified by orbits of CFT stress tensors under conformal transformations. These stress tensors, in turn, can be interpreted as Bondi mass aspects for asymptotically flat metrics. I will also show how one can compute characters of the BMS3 group, which coincide with suitable gravitational one-loop partition functions

We present an effective Z2 gauge theory that captures various competing phases in spin-1/2 kagome lattice antiferromagnets: the topological Z2 spin liquid (SL) phase, and the 12-site and 36- site valence bond solid (VBS) phases. Our effective theory is a generalization of the recent Z2 gauge theory proposed for SL phases by Wan and Tchernyshyov. In particular, we investigate possible VBS phases that arise from vison condensations in the SL. In addition to the 12-site and 36-site VBS phases, there exists 6-site VBS that is closely related to the symmetry-breaking valence bond modulation patterns observed in the recent density matrix renormalization group simulations. We find that our results have remarkable consistency with a previous study using a different Z2 gauge theory. Motivated by the lattice geometry in the recently reported vanadium oxyfluoride kagome antiferromagnet, our gauge theory is extended to incorporate lowered symmetry by inequivalent up- and down-triangles. We investigate effects of this anisotropy on the 12-site, 36-site, and 6-site VBS phases. Particularly, interesting dimer melting effects are found in the 36-site VBS. We discuss the implications of our findings and also compare the results with a different type of Z2 gauge theory used in previous studies.

In recent years, entanglement has become a new frontier with applications across several fields in physics. Nevertheless, simple conceptual pictures and practical ways to quantify entanglement in many-body systems have remained elusive even for the simplest models. In this talk, I will consider entanglement and Renyi entropies as well as quantum (mutual, tripartite, etc.) information in a quantum field theory. For free field theories, I will show that quantum entropies and information can be computed and understood by analogy with the thermal Casimir effect in one higher dimension. Furthermore, I will introduce a geometrical picture for the quantum (mutual, tripartite) information as a sum over polymers establishing a connection to purely entropic effects that prove useful in deriving information inequalities.  Finally, I will show that similar ideas may be extended beyond free field theories.

In light of the upcoming Generation 2 (G2) direct-detection experiments attempting to record dark matter scattering with nuclei in underground detectors, it is timely to inquire about their ability to single out the correct theory of dark-matter-baryon interactions, in case a signal is observed. I will present a recent study in which we perform statistical analysis of a large set of direct-detection simulations, covering a wide variety of operators that describe scattering of fermionic dark matter with nuclei. I will show that a strong signal on G2 xenon and germanium targets has enough discrimination power to reconstruct the momentum dependence of the interaction, ruling out entire classes of models. However, zeroing in on a correct UV completion will critically depend on the availability of measurements from a wide variety of nuclear targets (including iodine and fluorine) and on the availability of low energy thresholds. This study quantifies complementarity amongst different experimental designs and targets, and provides a roadmap for future data analyses. It also highlights the critical need for bringing in information from all available probes in dark matter studies.

Many-body systems with both coherent dynamics and dissipation constitute a rich class of models which are nevertheless much less explored than their dissipationless counterparts. The advent of numerous experimental platforms that simulate such dynamics poses an immediate challenge to systematically understand and classify these models. In particular, nontrivial many-body states emerge as steady states under non-equilibrium dynamics. In this talk, I use a systematic approach based on the Keldysh formalism to study nonequilibrium phases and phase transitions in such models. I show that an effective thermal behavior generically emerges as a result of dissipation, and the universal behavior including the dynamics near the steady state is described by a universality class in equilibrium thermodynamics. In the end, I will also discuss possibilities that go beyond the paradigm of an effective thermal behaviour.

I will review the possible role in Geometric Langlands
of N=4 boundary conditions in four-dimensional supersymmetric Yang Mills theory.
The action of S-duality on such boundary conditions can be understood
in terms of symplectic duality.

The mergers of black hole-neutron star and neutron star-neutron star binaries are one of the primary targets for Advanced LIGO and other gravitational wave detectors now coming online. In addition, these events may source a number of electromagnetic counterparts, including short gamma-ray bursts and ejecta powered transients. Particularly in the case of binaries that are dynamically-assembled in dense stellar regions like globular clusters, these mergers may involve neutron stars with non-negligible spin. I will present results from simulations of such events that show that neutron star spin can have important consequences, including altering the amount of unbound material that may power a transient, and determining whether or not a hypermassive neutron star forms following a binary neutron star merger. I will also discuss how in some cases, this resulting hypermassive neutron star becomes dominated by the one-arm spiral instability, and how this affects the gravitational signal from such events.

I will present a generalization of the spinor approach of Euclidean loop quantum gravity to the 3D Lorentzian case, where the gauge group is the noncompact SU(1,1). The key tool of this generalization is the recoupling theory between unitary infinite-dimensional representations and non-unitary finite-dimensional ones, needed to generalize the Wigner-Eckart theorem to tensor operators for SU(1,1).
SU(1,1) tensor operators are used to build observables and a quantum Hamiltonian constraint, analogous to the one introduced by Bonzom and Livine in the Euclidean case. I will show that the Lorentzian Ponzano-Regge amplitude is a solution of the Hamiltonian constraint, by making use of the Biedenharn-Elliott relations (generalized to the case where unitary and non-unitary SU(1,1) representations are coupled to each other).

Recent experiments show that charge-density wave correlations are prevalent in underdoped cuprate superconductors. The correlations are short-ranged at weak magnetic fields but their intensity and spatial extent increase rapidly at low temperatures beyond a crossover field. Here we consider the possibility of long-range charge-density wave order in a model of a layered system where such order competes with superconductivity. We show that in the clean limit, low-temperature long-range order is stabilized by arbitrarily weak magnetic fields. This apparent discrepancy with the experiments is resolved by the presence of disorder. Like the field, disorder nucleates halos of charge-density wave, but unlike the former it also disrupts inter-halo coherence, leading to a correlation length that is always finite. Our results are compatible with various experimental trends, including the onset of longer range correlations induced by inter-layer coupling above a characteristic field scale.

With the completion of the Planck satellite, in order to continue collecting cosmological information it is
important to gain a precise understanding of the formation of Large Scale Structures (LSS) of the universe.
The Effective Field Theory of LSS (EFTofLSS) offers a consistent theoretical framework that aims to develop
an analytic understanding of LSS at long distances, where inhomogeneities are small. We present the recent
developments in the field covering topics of biased tracers in the EFTofLSS including the effects of baryonic
physics and primordial non-Gaussianities, finding that new bias coefficients are required. We discuss the EFT
framework for dark matter clustering in Lagrangian formalism and present its consequences on baryon acoustic
oscillations (BAO). We present analytic results and compare them with the output of N-body simulations.

The iron chalcogenide FeSe has attracted much recent interest due to a high superconducting transition in monolayer samples. In bulk samples, nematic order is seen without the presence of magnetic order, hinting at the importance of nematic order in determining the monolayer properties. More generally, there has been growing evidence of the importance of nematic fluctuations in a variety of strongly correlated high-temperature superconductors. We study an effective two band model of the iron-pnictides with interactions that capture the nematic ordering arising from spontaneous symmetry breaking between the two orbitals. These models are sign-problem free and can be simulated in an unbiased fashion using Determinant Quantum Monte Carlo. We find a variety of unexpected orders and consider the effects of the nematic fluctuations on superconductivity. 

In quantum gravity, observables must be diffeomorphism-invariant. Such observables are nonlocal, in contrast with the standard assumption of locality in flat spacetime quantum field theory. I will show how to construct 'gravitationally dressed' observables in linearized gravity that become local in the weak gravity limit, and whose corrections to exact locality are characterized by the Newtonian potential. One can attempt to make these observables more local by concentrating gravitational field lines into a smaller solid angle. In AdS_3 gravity I will show that nonperturbatively there are sharp limits to how much the gravitational dressing can be concentrated.

Based on arXiv:1507.07921 with Steve Giddings, and arXiv:1510.00672 with Don Marolf and Eric Mintun.