Talks

Circularly polarized lattice vibrations carry angular momentum and lead to magnetic responses in applied magnetic fields or when resonantly driven with ultrashort laser pulses. The phonons associated with such vibrations are known as chiral phonons. On the basis of purely circular ionic motion, these phonons are expected to carry a magnetic moment of the order of a few nuclear magnetons. However, some recent experiments have demonstrated a phonon magnetic moment of the order of a few Bohr magnetons. This kind of giant magnetic response points towards the electronic contribution to the magnetic moment of phonons. Many diverse mechanisms have been discovered for this enhanced magnetic response of chiral phonons. The orbital-lattice coupling is one such mechanism where low-energy electronic excitations on a magnetic ion hybridize with phonons and endow a large magnetic moment to phonons. In this talk, I'll present a microscopic model for the effective magnetic moments of chiral phonons ba...

We have studied 2DEG formed at the interface of two oxide perovskite insulators; one is LaVO3 (LVO) which is a Mott insulator and another one is KTaO3 (KTO) which is a band insulator. Our experimental collaborators in the group of Prof. Suvankar Chakraverty from INST, Mohali have created LVO/KTO interface for the first time and observed metallic behaviour at this interface with one order of magnitude higher electron density and very high mobility of the 2DEG. The reason for 2DEG formation at the interface of two insulators/semiconductors was not clear. Our DFT calculations showed that LVO and KTO bulk materials are insulating, but the LVO/KTO interface is metallic which is an emergent phenomena and existence of parabolic bands crossing the Fermi level indicates source of free electrons at the interface. In this LVO/KTO heterostructure, both the individual parts are polar, consisting of alternating charged layers. Our DFT calculations show, to avoid polar catastrophe, “electronic recons...

Bulk-boundary correspondence licenses us to probe the bulk topological order by studying the transport properties of the edge modes. However, edge modes in a fractional quantum Hall (FQH) state can undergo edge reconstruction and, on top of that, can be in the coherent regime or exhibit varying degrees of charge and thermal equilibrations, giving rise to a zoo of intriguing scenarios. This can happen even in many abelian cases (like ν = 2/3), as well as non-abelian cases (like ν = 5/2). 5/2 has been particularly a focal point of both theoretical and experimental studies as it hosts non-abelian quasiparticles, a proposed basis for topological quantum computation. I will discuss how shot noise can provide a path to resolution and how its application can go beyond the quantum Hall regime to other systems like graphene quantum Hall states, Kitaev magnets, fractional Chern insulators in Twisted Bilayer graphene, and more.

Orbital current, defined as the orbital character of Bloch states in solids, can ballistically travel with larger coherence length through a broader range of materials than its spin counterpart, facilitating a robust, higher density and energy efficient information transmission. Hence, active control of orbital transport plays a pivotal role in propelling the progress of the evolving field of quantum information technology. Unlike spin angular momentum, orbital angular momentum (OAM), couples to phonon angular momentum (PAM) efficiently via orbital-crystal momentum (L-k) coupling, giving us the opportunity to control orbital transport through crystal field potential mediated angular momentum transfer. Here, leveraging the orbital dependant efficient L-k coupling, we have experimentally demonstrated the active control of orbital current velocity using THz emission spectroscopy. Our findings include the identification of a critical energy density required to overcome collisions in orbita...

In this talk, I shall discuss the probing and strong tunability of inter-layer excitons that are trapped in a moire potential well created through a hetero-bilayer. I shall show that these excitons trapped at different energy states inside the moire well exhibit surprising anomalous Stark shift and strong dipolar repulsion. Time permitting, I shall also discuss about a novel excitonic state in a hetero-trilayer - a moire-trapped quadrapolar exciton and it's spectroscopic signatures.

I will describe shortly what the higher-spin charges are and why they are relevant from a holographic point of view. Besides, I will emphasize the recent result according to which they are realized as Noether charges in a non-linear regime.

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.