CURRENT RESEARCH INTERESTS
Entanglement in Quantum Many Body Systems
Non Equilibrium Dynamics of Quantum Systems
Van der Waals Materials
Strongly Fluctuating Superconductors
Entanglement in Many Body Systems
The information about a quantum state of a many body system is generally distributed among the degrees of freedom residing in different parts of the system, a phenomenon that is called quantum entanglement. Entanglement has played a major role in our understanding of the fundamental nature of quantum mechanics and is the driving force behind the idea that quantum computation can provide advantages over classical computation. When one loses access to the distributed information of a quantum state, one can still reproduce the results of local measurements on the remaining subsystem by introducing some classical randomness in the description. Entanglement entropy, which measures how much classical randomness we need to introduce, is a measure of the amount of entanglement in the original state. In condensed matter systems, the scaling of entanglement entropy with subsystem size can be used to fingerprint the underlying quantum state.
There are very few analytic or numerical techniques/approximation schemes to calculate entanglement entropy for interacting quantum systems, especially in spatial dimensions d>1. Our group has pioneered a new quantum field theory based technique to calculate entanglement entropy for large quantum systems. The key advantage of this technique is that it provides a relation between entanglement entropy and correlation functions in these systems. Since correlations are easier to calculate and measure experimentally, this opens the door for efficient calculation and measurement of entanglement entropy in large systems.
I am currently interested in:
Developing useful approximation schemes to calculate entanglement entropy in interacting systems. A key question is whether/ when can we get a good description of entanglement from few particle correlation functions (which is what we can measure in many body systems).
Understanding entanglement scaling in gapless states in d>1. An important question here is whether/how can entanglement distinguish between Fermi liquids ( ordinary metals) and non-Fermi liquids (exotic states of matter)? Does this depend on the details/mechanism by which we get a non-Fermi liquid, or is there some universal pattern?
Understanding dynamics of entanglement entropy and relating it to dynamics of correlations.
Developing methods to calculate other entanglement measures like negativity of entanglement, measures of multi-partite entanglement.
Some of our papers on this topic:
M. K. Sarkar, S. Moitra and R. Sensarma, arXiv: 2308.01964.
S. Moitra and R. Sensarma, arXiv: 2306.07963.
A. Chakraborty and R. Sensarma, Phys. Rev. Lett. 127, 200603 (2021).
Non Equilibrium Dynamics of Quantum Systems
A large class of phenomena that we see around us from switches to quantum gates to quantum sensing or detection are governed by dynamics of quantum many body systems out of thermal equilibrium. Non equilibrium quantum dynamics also has implications for fundamental questions in statistical mechanics like thermalization, realization of quantum chaos etc. Ultracold atoms provide us with a platform where this dynamics can be studied in a controlled way, but newer platforms like materials subjected to ultrashort laser pulses are also showing interesting phenomena.
A large class of non equilibrium dynamics from thermalization to glassy dynamics to quantum computers require keeping track of initial conditions (contrary to equilibrium physics). Our group has created a new formalism which can keep track of arbitrary initial conditions in the dynamics within a non-equilibrium quantum field theory. This has allowed us to look at dynamics in many body localized phases in ultracold atoms, where the system retains a memory of the initial conditions at long times (and hence does not thermalize). We have also studied quantum quenches in systems with dipole symmetry, where the steady state can show new symmetry broken phases not seen in thermal equilibrium.
Some broad questions of interest:
Can one systematically reduce the degrees of freedom one needs to track in non-equilibrium dynamics? To what extent can a procedure like renormalization work in non-equilibrium dynamics?
Can one have stable indicators of initial conditions in the short time dynamics of these systems, or does one need to fall back on long time steady states and conservation laws?
Developing a detailed theory of pump-probe experiments in condensed matter systems.
Some of our papers on this topic:
M. M. Islam, K. Sengupta and R. Sensarma, arXiv:2305.13372 .
M. M. Islam and R. Sensarma, Phys. Rev. B 106, 024306 (2022).
A. Chakraborty, P. Gorantla, and R. Sensarma, Phys. Rev. B 99, 054306 (2019).
Strongly Fluctuating Superconductivity
Superconductivity is a low temperature phase of certain materials where the dc resistivity of the system vanishes and the system behaves like a perfect diamagnet, expelling magnetic fields from its bulk. This is a paradigmatic system in condensed matter physics which shows spontaneous symmetry breaking. The basic microscopic understanding of the phenomenon of superconductivity is provided in terms of Bose Einstein Condensation of Cooper pairs (pairs of electrons) by the famous Bardeen Cooper Schrieffer (BCS) theory of superconductivity. Similar phenomena is also present in heavy nuclei and core of compact stars.
While the BCS theory describes a large class of superconductors qualitatively and quantitatively, strongly fluctuating superconductors like a unitary Fermi gas, underdoped high Tc cuprates, strongly disordered films of superconductors close to a superconductor-insulator transition requires a "beyond BCS" description, which takes into account the large fluctuations of the superconducting order parameter. We have worked on understanding tunneling spectra and other experiments in high Tc cuprates, thermodynamics and vortices in ultracold atomic gases and collective modes in disordered superconductors.
Broadly, I am interested in understanding
How do the strong fluctuations affect the phenomenology in the superconducting phase? what are experimental signatures to track them unambiguously? Do non-equilibrium experiments like tunneling or Josephson junctions provide a better handle on fluctuations than standard equilibrium measurements (linear response)?
What kind of interesting non-superconducting phases of matter (insulator, metal, pseudogap, strange metal ...) can result when the fluctuations manage to kill superconductivity?
What is the nature of the phase transitions between the superconductor and these states of matter.
Some of our papers on this topic:
P. P. Poduval, A. Samanta, P. Gupta, N. Trivedi and R. Sensarma, Phys. Rev. B 106, 064512 (2022).
A. Samanta, A. Ratnakar, N. Trivedi, and R. Sensarma, Phys. Rev. B 101, 024507 (2020).
D. Chakraborty, R. Sensarma, and A. Ghosal, Phys. Rev. B 95, 014516 (2017).
Van der Waals Materials
Two dimensional materials provide a dizzying array of platforms where the ability to change charge carrier density allows us to drive the system from weakly interacting to strongly interacting regimes. Additionally the topology of the band structure often plays an important role in determining properties of these material. The ability to stack sheets of 2d materials on top of each other has led to formation of a zoo of quantum heterostructures, with widely varying phenomenology, which are collectively called Van der Waals materials.
An important discovery of recent years is the fact that when sheets of materials have slightly misaligned crystal axes, their band structures sensitively depend on twist angles. In graphene, this leads to the idea of a magic angle where the bandwidth of the system is minimal, leading to a strongly interacting system, which shows variety of phenomena like orbital magnetism and superconductivity. I have worked on electronic correlations in graphene. More recently we have explored electronic correlations in twisted double bilayer graphene, where correlations lead to formation of non-Fermi liquids.
Questions of interest:
What is the mechanism of superconductivity in twisted graphene systems and how can we use experimental measurements to narrow down the possibilities.
Is the superconductor in a twisted graphene system described by a BCS theory or by a strongly fluctuating superconductivity.
Does other competing ordered states play a big role in determining the fate of the superconductor?
Some of our papers on this topic:
U. Ghorai, A. Ghosh, S. Chakraborty, A. Das and R. Sensarma, Phys. Rev. B 108, 045117 (2023).
P. Mohan, U. Ghorai and R. Sensarma, Phys. Rev. B 103, 155149 (2021).
B. Datta, H. Agarwal, A. Samanta, A. Ratnakar, K. Watanabe, T. Taniguchi, R. Sensarma, and M. M. Deshmukh, Phys. Rev. Lett. 121, 056801 (2018).