Talks

Observational constraints on time-varying dark energy are commonly presented in terms of the two CPL parameters $w_0$ and $w_a$. Recent observations favor a sector of this parameter space in which $w_0 > -1$ and $w_0 + w_a < -1$, suggesting that the equation of state underwent a transition from violating the null energy condition (NEC) at early times to obeying it at late times. In this talk, I will demonstrate that this initial impression is misleading, by showing that simple quintessence models satisfying the NEC at all times predict an observational preference for the same sector. The upshot is that the CPL parameterization is simultaneously useful for detecting deviations from cosmological-constant dynamics ($w = -1$) but unreliable for predicting the true behavior of $w(z)$.

Exploring the structure of compact objects in modified theories of gravity is mandatory to parametrize the possible deviations w.r.t general relativity and confront these theories to the current and future observations. While important efforts have been devoted to understand the phenomenology of stars and black holes, it is still a challenging task to provide new exact analytical solutions describing rotating black hole in such theories. In this talk, I propose to recent efforts to construct such solutions. Concretely, I will review how one can mix the disformal field redefinitions affect the Petrov type of a given gravitational field and how this can be used to constrain the derivation of rotating black hole. Then, I will review the main properties of a new solution of a subset of Horndeski theories called the disformal Kerr black hole and comment on the most promising directions to derive exact rotating black hole solutions in scalar-tensor theories. This talk will be based on the two articles: https://inspirehep.net/literature/1800972, https://inspirehep.net/literature/1877661

I will show how to derive libraries of semi-analytic gravitational waveforms for coalescing “hairy” black hole binaries, focusing on the example of Einstein-scalar-Gauss-Bonnet gravity (ESGB). To do so, I will start from the state-of-the-art, effective-one-body waveform model “SEOBNRv5PHM” in general relativity, and deform it with ESGB corrections to infer inspiral-merger-ringdown waveform estimates.

In recent years, gravitational wave observations of compact objects have provided new opportunities to test our understanding of gravity in the strong-field, highly dynamical regime. To perform model-dependent tests of General Relativity with these observations, one needs accurate inspiral-merger-ringdown waveforms in alternative theories of gravity. In this talk, we will discuss the nonlinear dynamics of compact object mergers in a class of modified theories of gravity, as well as the challenges in numerically obtaining those solutions. The theory we focus on is Einstein-scalar-Gauss-Bonnet gravity, which is a representative example of a Horndeski gravity theory and is interesting because it admits scalar hairy black hole solutions.

Black holes in Horndeski theories of gravity are a perfect playground for exploring possible deviations from General Relativity in a theory-specific manner and studying their astrophysical manifestation. I will review the recent advances in constructing stationary hairy black hole models in Gauss-Bonnet theories. Special attention will be paid to their nonlinear dynamics when isolated or put in a binary, and the resulting astrophysical implications. The potential loss of hyperbolicity will also be discussed.

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.