### Abstract

Direct experimental investigations of the low-energy electronic structure of the $$\mathrm{Na_2 IrO_3}$$ iridate insulator are sparse and draw two conflicting pictures. One relies on flat bands and a clear gap, the other involves dispersive states approaching the Fermi level, pointing to surface metallicity. Here, by a combination of angle-resolved photoemission, photoemission electron microscopy, and x-ray absorption, we show that the correct picture is more complex and involves an anomalous band, arising from charge transfer from Na atoms to Ir-derived states. Bulk quasiparticles do exist, but in one of the two possible surface terminations the charge transfer is smaller and they remain elusive.

### Abstract

The complex antiferromagnetic orders observed in the honeycomb iridates are a double-edged sword in the search for a quantum spin-liquid: both attesting that the magnetic interactions provide many of the necessary ingredients, while simultaneously impeding access. Focus has naturally been drawn to the unusual magnetic orders that hint at the underlying spin correlations. However, the study of any particular broken symmetry state generally provides little clue about the possibility of other nearby ground states. Here we use magnetic fields approaching 100 tesla to reveal the extent of the spin correlations in $$\gamma$$-lithium iridate. We find that a small component of field along the magnetic easy-axis melts long-range order, revealing a bistable, strongly correlated spin state. Far from the usual destruction of antiferromagnetism via spin polarization, the high-field state possesses only a small fraction of the total iridium moment, without evidence for long-range order up to the highest attainable magnetic fields.

### Abstract

Magnetic honeycomb iridates are thought to show strongly spin-anisotropic exchange interactions which, when highly frustrated, lead to an exotic state of matter known as the Kitaev quantum spin liquid. However, in all known examples these materials magnetically order at finite temperatures, the scale of which may imply weak frustration. Here we show that the application of a relatively small magnetic field drives the three-dimensional magnet $$\beta-\mathrm{Li}_2\mathrm{IrO}_3$$ from its incommensurate ground state into a quantum correlated paramagnet. Interestingly, this paramagnetic state admixes a zig-zag spin mode analogous to the zig-zag order seen in other Mott-Kitaev compounds. The rapid onset of the field-induced correlated state implies the exchange interactions are delicately balanced, leading to strong frustration and a near degeneracy of different ground states.

### Abstract

We study pore nucleation in a model membrane system, a freestanding polymer film. Nucleated pores smaller than a critical size close, while pores larger than the critical size grow. Holes of varying size were purposefully prepared in liquid polymer films, and their evolution in time was monitored using optical and atomic force microscopy to extract a critical radius. The critical radius scales linearly with film thickness for a homopolymer film. The results agree with a simple model which takes into account the energy cost due to surface area at the edge of the pore. The energy cost at the edge of the pore is experimentally varied by using a lamellar-forming diblock copolymer membrane. The underlying molecular architecture causes increased frustration at the pore edge resulting in an enhanced cost of pore formation.

### Abstract

Dark Matter particles with inelastic interactions are ubiquitous in extensions of the Standard Model, yet remain challenging to fully probe with existing strategies. We propose a series of powerful searches at hadron and lepton colliders that are sensitive to inelastic dark matter dynamics. In representative models featuring either a massive dark photon or a magnetic dipole interaction, we find that the LHC and BABAR could offer strong sensitivity to the thermal relic dark matter parameter space for dark matter masses between ∼100 MeV and 100 GeV and fractional mass-splittings above the percent level; future searches at Belle II with a dedicated monophoton trigger could also offer sensitivity to thermal relic scenarios with masses below a few GeV. Thermal scenarios with either larger masses or splittings are largely ruled out; lower masses remain viable yet may be accessible with other search strategies.

### Abstract

We report on the capillary-driven leveling of a topographical perturbation at the surface of a freestanding liquid nanofilm. The width of a stepped surface profile is found to evolve as the square root of time. The hydrodynamic model is in excellent agreement with the experimental data. In addition to exhibiting an analogy with diffusive processes, this novel system serves as a precise nanoprobe for the rheology of liquids at interfaces in a configuration that avoids substrate effects.

### Abstract

In this paper, we propose a novel powerful strategy to perform searches for new electroweak states. Uncolored electroweak states appear in generic extensions of the Standard Model (SM) and yet are challenging to discover at hadron colliders. This problem is particularly acute when the lightest state in the electroweak multiplet is neutral and all multiplet components are approximately degenerate. In this scenario, production of the charged fields of the multiplet is followed by decay into nearly invisible states; if this decay occurs promptly, the only way to infer the presence of the reaction is through its missing energy signature. Our proposal relies on emission of photon radiation from the new charged states as a means of discriminating the signal from SM backgrounds. We demonstrate its broad applicability by studying two examples: a pure Higgsino doublet and an electroweak quintuplet field.

### Abstract

We probe the viscous relaxation of structured liquid droplets in the partial wetting regime using a diblock copolymer system. The relaxation time of the droplets is measured after a step change in temperature as a function of three tunable parameters: droplet size, equilibrium contact angle, and the viscosity of the fluid. Contrary to what is typically observed, the late-stage relaxation time does not scale with the radius of the droplet-rather, relaxation scales with the radius squared. Thus, the energy dissipation depends on the contact area of the droplet, rather than the contact line.

### Abstract

We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments. The system uses a focused, vertically propagating continuous wave laser beam to capture and manipulate micro-targets by photon momentum transfer at much longer working distances than commonly used by optical tweezer systems. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution. Optical trapping of 10 ± 4 μm oil droplets in vacuum was demonstrated, over timescales of >1 h at extended distances of ~40 mm from the final focusing optic. The stability of the levitated droplet was such that it would stay in alignment with a ~7 μm irradiating beam focal spot for up to 5 min without the need for re-adjustment. The performance of the trap was assessed in a series of high-intensity ($$10^{17}$$ W cm) laser experiments that measured the X-ray source size and inferred free-electron temperature of a single isolated droplet target, along with a measurement of the emitted radio-frequency pulse. These initial tests demonstrated the use of optically levitated microdroplets as a robust target platform for further high-intensity laser interaction and pointsourcestudies.

# Recent Publications

Student authorFaculty author

21. L. Moreschini, I. Lo Vecchio, Nicholas P. Breznay, S. Moser, S. Ulstrup, R. Koch, J. Wirjo, C. Jozwiak, K. S. Kim, E. Rotenberg, A. Bostwick, J. G. Analytis, and A. Lanzara Quasiparticles and Charge Transfer At the Two Surfaces of the Honeycomb Iridate $$\mathrm{Na_2 IrO_3}$$ Physical Review B 96 (2017) 161116. K A Modic, B J Ramshaw, J B Betts, Nicholas P. Breznay, James G Analytis, Ross D McDonald, and Arkady Shekhter Robust Spin Correlations at High Magnetic Fields in the Harmonic Honeycomb Iridates Nature Communications 8 (2017) 2. Alejandro Ruiz, Alex Frano, Nicholas P. Breznay, Itamar Kimchi, Toni Helm, Iain Oswald, Julia Y. Chan, R. J. Birgeneau, Zahirul Islam, and James G. Analytis Correlated States in $$\beta-\mathrm{Li}_2\mathrm{IrO}_3$$ Driven by Applied Magnetic Fields Nature Communications 8 (2017) 961. Mark Ilton, Christian DiMaria, and Kari Dalnoki-Veress Direct Measurement of the Critical Pore Size in a Model Membrane Physics Review Letters 117 (2016) . Eder Izaguirre, Gordan Krnjaic, and Brian Shuve Discovering Inelastic Thermal Relic Dark Matter at Colliders Physical Review D 93 (2016) 063523. Mark Ilton, Miles M. P. Couchman, Cedric Gerbelot, Michael Benzaquen, Paul D. Fowler, Howard A. Stone, Elie Raphael, Kari Dalnoki-Veress, and Thomas Salez Capillary Leveling of Freestanding Liquid Nanofilms Physical Review Letters 117 (2016) . Brian Shuve and Michael E. Peskin Revision of the LHCb Limit on Majorana Neutrinos Physical Review D 94 (2016) 113007. Ahmed Ismael, Eder Izaguirre, and Brian Shuve Illuminating New Electroweak States at Hadron Colliders Physical Review D 94 (2016) 015001. Mark Ilton, Oliver Baeumchen, and Kari Dalnoki-Veress Onset of Area-Dependent Dissipation in Droplet Spreading Physical Review Letters 115 (2015) . C. J. Price, Thomas D. Donnelly, S. Giltrap, N. H. Stuart, S. Parker, S. Patankar, H. F. Lowe, D. Drew, E. T. Gumbrell, and R. A. Smith An in-vacuo optical levitation trap for high-intensity laser interaction experiments with isolated microtargets Review of Scientific Instruments 86 (2015) 033502.