In a quantum spin liquid, the spins remain disordered down to zero temperature, and yet, it displays topological order that is stable against local perturbations. The Kitaev model with anisotropic interactions on the bonds of a honeycomb lattice is a paradigmatic model for a quantum spin liquid. We explore the effects of a magnetic field and discover an intermediate gapless spin liquid sandwiched between the known gapped Kitaev spin liquid and a polarized phase. We show that the gapless spin liquid harbors fractionalized neutral fermionic excitations, dubbed spinons, that remarkably form a Fermi surface in a charge insulator.

Proceedings of the National Academy of Sciences, 201821406 (2019)

The main question we address is how to probe the fractionalized excitations of a quantum spin liquid (QSL), for example, in the Kitaev honeycomb model. By analyzing the energy spectrum and entanglement entropy, for antiferromagnetic couplings and a field along either [111] or [001], we find a gapless QSL phase sandwiched between the non-Abelian Kitaev QSL and polarized phases. Increasing the field strength towards the polarized limit destroys this intermediate QSL phase, resulting in a considerable reduction in the number of frequency modes and the emergence of a beating pattern in the local dynamical correlations, possibly observable in pump-probe experiments.

Phys. Rev. B 99, 140413(R) (2019)

We consider the electromagnetic response of a topological Weyl semimetal (TWS) with a pair of Weyl nodes in the bulk and corresponding Fermi arcs in the surface Brillouin zone. We compute the frequency-dependent complex conductivities σαβ(ω) and also take into account the modification of Maxwell equations by the topological θ-term to obtain the Kerr and Faraday rotations in a variety of geometries. For TWS films thinner than the wavelength, the Kerr and Faraday rotations, determined by the separation between Weyl nodes, are significantly larger than in topological insulators. In thicker films, the Kerr and Faraday angles can be enhanced by choice of film thickness and substrate refractive index. We show that, for radiation incident on a surface with Fermi arcs, there is no Kerr or Faraday rotation but the electric field develops a longitudinal component inside the TWS, and there is linear dichroism signal. Our results have implications for probing the TWS phase in various experimental systems.

Scientific Reports 5, 12683 (2015)

A quasi-two-dimensional honeycomb ruthenate has been synthesized by members of IRG-1.  Neutron diffraction shows antiferromagnetic ordering in each layer and between layers up to a temperature of 565 K.  At this critical temperature, the layers magnetically decouple due to the weak inter-layer coupling which we can understand through a combinations of density functional theory calculations and Monte-Carlo simulations. 

Phys. Rev. B 92, 100404(R)

“The Higgs mode in disordered superconductors close to a quantum phase transition,” was just published in Nature Physics! The recent paper is a theory-experiment collaboration where our theoretical predictions of a Higgs mode going soft at a quantum critical point in a disordered superconductor are put to the test in dynamical conductivity experiments. This is the first unequivocal observation of the Higgs mode in a superconductor. In contrast to previous attempts where there was considerable mixing of the Higgs mode with broken pairs, in the experiments reported here the energy scale for the Higgs mode could be reduced well below the pair breaking scale. Importantly, the Higgs mass was shown to vanish at the quantum critical point between a superconductor and an insulator leaving no doubt that its origin lay in the amplitude fluctuations of the superconducting order parameter. Our theory was first published in Swanson, Loh, Randeria, Trivedi Phys. Rev. X 4, 021007 (2014). Phil Anderson  has written a historical and insightful News and Views on our Nature Physics paper.