Talks

Scalar-tensor theories with a screening mechanism can serve as a viable model of cosmic acceleration, while still passing existing local tests of gravity. The validity of different screening mechanisms, such as kinetic screening (or k-mouflage) has been studied extensively in static and weak field regimes. However, only recently have works started to focus on determining whether these would work as expected near extremely compact objects. In this talk I will discuss some recent efforts to characterise kinetic screening mechanisms in these highly dynamical and non-linear regimes using numerical relativity in the case of a single oscillating neutron star, as well as how this picture may change for a binary neutron star configuration in a quasi-equilibrium state.

Van der Waals (vdW) epitaxy is the growth mechanism of epitaxial layers on crystalline substrates, where the weak vdW forces govern epilayer-substrate interaction. This technique leads to heterointerfaces with negligible strain, despite large lattice mismatch and thermal expansion coefficients. With advances in the large area synthesis of 2D materials and heterostructures, it is important to comprehend the growth mechanism, prospects and challenges of vdW epitaxy to grow 2D materials on various crystalline substrates. In this talk, I will discuss the effects of various growth factors such as structure and symmetries of the substrate, growth temperature, interfacial strain and growth kinetics that ultimately controls the lateral coverage and thickness of the 2D/quasi-2D layers. Two specific cases of vdW epitaxial growth by chemical vapor deposition (CVD) will be discussed in detail: (1) quasi-2D system of Cr1+xTe2 and (2) monolayer WSe2, on crystalline substrates such as c-plane sapphir...

The fast-paced improvements in ultrafast radiation from X-ray, ultraviolet, visible, and infrared to terahertz frequencies is enabling simultaneous probing of electron, phonon, and spin dynamics on the ps-to-ns time scales, as well as sub-micrometer length scales. In this talk, I will present brief introduction to nonlinear optics, followed by examples of the discovery of new low symmetry phases with large property enhancements in decades old ferroelectrics using nonlinear optical microscopy. In a second example, I will show how an ultrafast laser pulse can create a complex polar supertextures with modulation periodicities of tens of nanometers in oxide thin film heterostructures that have built-in frustration by design.

Hydrodynamic flow of electrons in graphene has garnered significant attention over the past decade, emerging as a solid-state platform which can be used to probe the physics associated with relativistic plasma, black holes, and quantum gravity [1,2]. Particularly near the charge neutrality point, graphene is expected to behave like a “Dirac fluid” [3], with its shear viscosity per unit entropy density (η/s) reaching a universal holographic lower bound, ħ/4πk_B where ħ is the reduced Planck’s constant and kB is the Boltzmann’s constant. However, direct experimental evidence of this is still lacking. In this work [4], we have fabricated hBN-encapsulated ultraclean graphene devices with exceptionally high electron mobilities (~ 10^6 cm^2 V^-1 s^-1) and performed electrical and thermal transport from room temperature down to 20 K. We observed a giant violation of the Wiedemann-Franz Law near the charge neutrality point across a range of temperatures T >> T_F. We also computed t...

Recent experiments have revealed the possibility of achieving sizeable Coulomb interaction driven gapped phases in dual gated rhombohedral multilayer graphene devices where carrier densities and perpendicular electric fields can be simultaneously controlled. Of particular importance are the pentalayer and tetralayer devices whose spontaneous gaps of the order of a few tens of meV are attributable to layer pseudospin polarization of the states near the Dirac point in chiral 2DEG systems. The layer pseudospin polarization can take place in a variety of ways leading to different Hall conductivities depending on the signs of the mass terms for each one of the spin-valley flavors. By means of mean-field Hartree-Fock approach we examine and compare the self-consistent solutions corresponding to the different pseudospin magnetic phases. Addition of a spin-orbit coupling term or moire potentials are expected to sensitively alter the ground-state of the system, allowing for example to split the...

In my talk I will discuss the black hole spontaneous scalarization in scalar-Gauss-Bonnet gravity. Some of the basic ideas, results and astrophysical consequences will be presented. I will also discuss a new fully non-linear dynamical mechanism for the formation of scalarized black holes which is different from the spontaneous scalarization.

Classical energy conditions are used to provide restrictions on the matter fields present in the stress-energy tensor to avoid possible unphysical spacetimes. These classical energy conditions are imperative to the singularity theorems of Hawking and Penrose. However, we know that spacetime breaks down near said singularities and a quantum theory of gravity is needed. One insight into this area is semi-classical gravity where the spacetime is kept “classical” and the stress-energy tensor is quantized. In this regime one may ask what reasonable restrictions should be imposed on the quantum expectation of the stress-energy tensor? One such possibility is the smeared null energy conditions (SNEC). We will review motivation for the SNEC and explore its consequences in cosmological spacetimes that would otherwise violate the classical null energy condition, such as bouncing cosmologies.

Invertible disformal transformations are a useful tool to investigate ghost-free scalar-tensor theories. By performing a higher-derivative generalization of the invertible disformal transformation on Horndeski theories, we construct a novel class of ghost-free scalar-tensor theories, which we dub generalized disformal Horndeski theories. In the talk, I will clarify the basic idea for constructing the invertible disformal transformation with higher derivatives. I will also discuss some aspects of the generalized disformal Horndeski theories, including the consistency of matter coupling and cosmological perturbations.

Gravitational collapse shaped the cosmic large-scale structure and created a plethora of different density environments. For optimally probing gravity with galaxy surveys like Euclid and Rubin LSST, we need to dissect different density environments that are lumped together in traditional two-point statistics. I will explain how the one-point probability distribution of dark matter densities can be predicted analytically including signatures of modified gravity that match with cosmological simulations for nDGP and f(R) gravity. I will provide an outlook on how those predictions can be translated to galaxy clustering and weak lensing and observables.

There are now multiple direct probes of the region near black hole horizons, including direct imaging with the Event Horizon Telescope (EHT). As a result, it is now of considerable interest to identify what aspects of the underlying spacetime are constrained by these observations. For this purpose, we present a new formulation of an existing broad class of integrable, axisymmetric, stationary spinning black hole spacetimes, specified by four free radial functions, that makes manifest which functions are responsible for setting the location and morphology of the event horizon and ergosphere. We explore the size of the black hole shadow and high-order photon rings for polar observers, approximately appropriate for the EHT observations of M87*, finding analogous expressions to those for general spherical spacetimes. Of particular interest, we find that these are independent of the properties of the ergosphere, but does directly probe on the free function that defines the event horizon. Based on these, we extend the nonperturbative, nonparametric characterization of the gravitational implications of various near-horizon measurements to spinning spacetimes. Finally, we demonstrate this characterization for a handful of explicit alternative spacetimes.

Simulating non-linear scales of structure formation is essential to make use of frontier data from Stage IV galaxy surveys. Performing these simulations in modified gravity theories introduces additional challenges, and further forces us to make choices about which theories we deem ‘worth’ the computational investment. Horndeski Gravity is very helpful in this regard, as it encompasses a large swathe of models of major interest. I’ll introduce Hi-COLA, a software suite which simulates large-scale structure formation in the class of luminal Horndeski theories. Hi-COLA was designed to be: i) flexible — it avoids hard-coded models and instead receives a user-specified Lagrangian; ii) consistent — the background expansion history, linear growth and nonlinear screening are solved consistently with one another; iii) efficient — using the COLA method, large sets of simulations can be generated at low cost. I’ll explain how Hi-COLA can be used to make robust predictions for scalar-tensor theories on nonlinear scales. If time permits, we’ll also dip a toe into constraining the Horndeski framework with gravitational waves.