Talks

Based on an idea of Kevin Costello, I will show how to construct a double cover of the twistor space of $\mathbb{R}^4$, $X = \pi^*(\mathcal{O}(1)\oplus\mathcal{O}(1))\to\Sigma$ where $\Sigma$ is an (hyper)elliptic curve. I then discuss how holomorphic theories such as BF and Chern-Simons theory on $X$ descend to theories on ordinary twistor space. Once on twistor space, compactifying along the $\mathbb{CP}^1$ direction of twistor space produces a corresponding 4d theory where we can study the algebra of collinear singularities. I will present my calculations which show that this algebra lives on the elliptic curve defining the double cover of twistor space.

We show that a 2D CFT consisting of a central charge c Liouville theory, a chiral level one, rank N Kac-Moody algebra and a weight −3/2 free fermion holographically generates 4D MHV leaf amplitudes associated to a single hyperbolic slice of flat space. Celestial amplitudes arise in a large-N and semiclassical large-c limit, according to the holographic dictionary, as a translationally-invariant combination of leaf amplitudes. A step in the demonstration is showing that the semiclassical limit of Liouville correlators are given by contact AdS3 Witten diagrams.

This talk is based on work in progress with Giuseppe Bogna. We consider the twistor description of classical self-dual Einstein gravity in the presence of a cosmological constant and a defect operator wrapping a certain $\mathbb{CP}^1$. The backreaction of this defect deforms the flat twistor space to that of quaternionic Taub-NUT space, a certain self-dual limit of a family of Kerr Taub-NUT AdS black holes. We discuss a 2-parameter family of Lie-algebras depending on the mass of the black hole and the cosmological constant. In various limits it reduces to algebras which were previously studied in the context of celestial holography and are closely related to $w_{1+\infty}$.

I will review an old puzzle related to the breakdown of the semiclassical description of the thermodynamics of very cold (ultraspinning) black holes. Then, I will discuss recent work where we resolved this puzzle by properly accounting for quantum corrections arising from graviton loops, which dominate the low-temperature thermodynamics.

In the last few years, a remarkable link has been established between the soft theorems and asymptotic symmetries of quantum field theories: soft theorems are Ward identities of the asymptotic symmetry generators. In particular, the tree-level subleading soft theorems are the Ward identities of the subleading asymptotic symmetries of the theory, for instance divergent gauge transformation in QED and superrotation in gravity. However, it is known that the subleading soft theorems receive quantum corrections with logarithmic dependence on the soft photon/graviton energy. It is therefore natural to ask how the quantum effects affect the classical (tree-level) symmetry interpretation. In this talk, we explore this question in the context of scalar QED and perturbative gravity. We show that the logarithmic soft theorems are the Ward identities of subleading asymptotic symmetries that arise from relaxed boundary conditions which take long-range interactions into account.

Magic-angle twisted bilayer graphene (MATBG) exhibits a wide variety of correlated phases, spanning from insulating to superconducting and magnetic states, favored by the flat bands. The degeneracy among closely competing ground states can be lifted by polarizing spin and valley degrees of freedom; hence, the four-fold degeneracy of the low-energy electrons has a significant impact on the underlying mechanism governing the correlated phases at different band fillings. The overall phase diagram of MATBG is remarkably sensitive to external perturbations such as carrier density, electromagnetic field, pressure, temperature, and dielectric environments. Despite this unprecedented tunability, a complete understanding of the observed phases has remained elusive. In our recent study, we conducted magneto-transport measurements on MATBG proximitized by a layer of tungsten diselenide, thereby introducing finite spin-orbit coupling into the system. Our findings unveiled an anomalous Hall effect ...

The recent advances in the 2D materials, in particular, the discovery of layered 2D magnet, have shown lot of promises in the field of low dimensional spin-based devices. More importantly, the possibility of creating heterostructure helps to incorporate multiple functionalities in the nanoscale devices as well as provides access to study interface induced physical phenomena. In this talk, I will discuss our electron transport and electron spin resonance (ESR) measurements in a 2D metallic Ferromagnet (Fe4GeTe2). The magnetization data exhibit a Ferromagnetic transition at ~270K with unusual temperature dependence compared to the conventional Ferromagnet. At ~100K, there is a spin reorientation transition (SRT) where the easy axis changes from in-plane to out of plane. Our transport data indicates that the Hall coefficient changes sign and the magnetoresistance, anomalous Hall magnitude become maximum near the SRT, providing the important role of SRT on the electron transport. The ESR d...

In this talk, we demonstrate the intriguing properties of three distinct materials: Sr2FeMoO6 thin films, tungsten (W)-doped Sr2FeMoO6 thin films, and the conducting interface of LaFeO3 and SrTiO3.
Leveraging pulsed laser deposition (PLD) as a versatile technique, we demonstrate precise control over the stoichiometry of Sr2FeMoO6 thin films, thereby achieving the highest spin-polarization within the system. Additionally, for W-doped Sr2FeMoO6 thin films, we observe fascinating transitions in both conductivity and magnetism, alongside the emergence of a substantial topological Hall effect, indicative of a non-trivial spin-texture arising from intricate ferro-antiferro interactions.
Transitioning from thin films to interfaces, we explore the crystallographic transition occurring at the LaFeO3-SrTiO3 interface, which induces a magnetic transition leading to a remarkable shift from non-spin polarized to spin-polarized conductivity. This novel phenomenon manifests as a room temperature sp...

Over the last few decades, a rapidly expanding field has dealt with emergent properties at various interfaces formed in heterostructured materials. Specifically, it has been shown that an atomically flat interface between two highly insulating oxide materials can exhibit properties not found in either of the bulk systems defining the interface, such properties covering realms of magnetic-nonmagnetic transitions, insulator-metal transitions, emergence of superconductivity, depending on specific systems and synthesis conditions.
Interestingly, there are few investigations to probe the nature of the charge carriers in such systems arising from difficulties inherent in the problem. It is generally challenging to investigate directly the nature of such interface states since these are typically buried under a depth and represent a tiny volume fraction of the entire sample.
Over the last few decades, we have established a way to extract layer-resolved electronic structure information with ...