### Overview

Our group works on a variety of problems in theoretical condensed matter Physics. The concept of quasiparticles, the long-lived excitations above the ground state of a many-body system, which remain particle-like even in the presence of strong electron-electron interactions, has been immensely successful in describing the phenomenology of electronic systems. However, recent decades have seen an increasing number of challenges to these ideas, following the discovery of phenomena such as the fractional quantum Hall effect, ‘strange’ metals and frustrated quantum magnetism. A central theme of our research is the Physics of such strongly correlated quantum systems, where the effects of interaction can lead to the emergence of dramatic new collective phenomena, not describable in terms of the standard quasiparticle-based framework. Many of these problems are inspired by our quest for understanding experiments on a variety of exotic materials, such as unconventional superconductors, quantum spin liquids and non-Fermi liquids, from a fundamental microscopic point of view.

### Research Focus

My research is focused primarily on the study of strongly correlated quantum matter, using techniques from quantum field theory and quantum information.

**Universal properties of incoherent metals:**

There are a plethora of materials, such as the cuprates, the pnictides, the ruthenates, and more recently, twisted bilayers of graphene and transition metal dichalcogenides, that display a strong departure from conventional Fermi liquid behavior. This includes the mysterious observation of an electrical resistivity that scales linearly with temperature over a broad range of energy scales, and often down to shockingly low temperatures. One of the most remarkable empirical facts related to transport across these microscopically distinct systems is their apparent universality of (“Planckian”) scattering rates, controlled only by the ratio of k_{B}T/ℏ. Given the ubiquitous nature of these observations, some of the fundamental questions that I am interested in are (i) Can we formulate a universal effective theory for a subset of these non-Fermi liquid metals?, (ii) What controls the emergent universality of the Planckian scattering rate?, and (iii) What is the relationship between chaotic properties of quantum many-body systems and transport, if any?

**Superconductivity and quantum criticality in moiré materials:**

The discovery of superconductivity and other competing orders in twisted bilayer graphene has ushered a new era of studying correlated quantum phenomena in a highly tunable platform. The key question of interest across these materials is tied to the low-energy fate of electronic interactions projected to the “flat” bands. The problem is inherently non-perturbative, without any “small” parameter and requires the development of new theoretical tools. Additionally, the observation of continuous metal-insulator transitions and other correlated phenomena in moiré transition metal dichalcogenides allows us to revisit many classic unsolved problems in the field from a fresh perspective. Interestingly, understanding the experimental phenomenology across these platforms often involves analyzing the effects of interaction, disorder and topology in a highly non-trivial setting. The three questions that fascinate me are (i) Can we put universal constraints on the superconducting and transport properties of interacting flat-band systems in these highly non-perturbative regimes?, (ii) What is the correct theoretical formulation going beyond the traditional Landau-Ginzburg-Wilson paradigm for the experimentally observed quantum phase transitions between metals and correlated insulators?, and (iii) How should we design new experiments that can help probe the frequency and momentum-resolved correlation functions in these materials in the absence of many conventional scattering techniques?

**Graduate Students**

Juan Felipe Mendez-Valderrama, Sunghoon Kim, Xuepeng Wang, Keiran Lewellen

**Postdocs**

Dan Mao (Bethe/KIC Fellow), Dimitri Pimenov

### Publications

- J.F. Mendez-Valderrama*, E. Tulipman*, E. Zhakina, A.P. Mackenzie, E. Berg and Debanjan Chowdhury, T-linear resistivity from magneto-elastic scattering: application to PdCrO2, arXiv:2301.10776.
- D. Mao and Debanjan Chowdhury, Diamagnetic response and phase stiffness for interacting isolated narrow bands, Proceedings of the National Academy of Sciences, 120 (11), e2217816120 (2023).
- S. Kim, A. Agarwala and Debanjan Chowdhury, Fractionalization and topology in amorphous electronic solids, Phys. Rev. Lett. 130, 026202 (2023).
- S. Kim, T. Senthil and Debanjan Chowdhury, Continuous Mott transition in moiré semiconductors: role of long-wavelength inhomogeneities, Phys. Rev. Lett. 130, 066301 (2023).
- Debanjan Chowdhury, A. Georges, O. Parcollet and S. Sachdev, Sachdev-Ye-Kitaev Models and Beyond: A Window into Non-Fermi Liquids, Rev. Mod. Phys. 94, 035004 (2022).
- T. Li, S. Jiang, L. Li, Y. Zhang, K. Kang, J. Zhu, K. Watanabe, T. Taniguchi, Debanjan Chowdhury, L. Fu, J. Shan, K.F. Mak, Continuous Mott transition in semiconductor moiré superlattices, Nature 597, 350 (2021).
- C. Lewandowski, S. Nadj-Perge and Debanjan Chowdhury, Does filling-dependent band renormalization aid pairing in twisted bilayer graphene?, npj Quantum Materials 6, 82 (2021).
- L. Zou and Debanjan Chowdhury, Deconfined metallic quantum criticality: a U(2) gauge theoretic approach, Phys. Rev. Research 2, 023344 (2020).
- J.S. Hofmann, E. Berg and Debanjan Chowdhury, Superconductivity, pseudogap and phase separation in topological flat bands, Phys. Rev. B 102, 201112(R) (2020).
- Y. Cao*, Debanjan Chowdhury*, D. Rodan-Legrain, O. Rubies-Bigorda, K. Watanabe, T. Taniguchi, T. Senthil and P. Jarillo-Herrero, Strange metal in magic-angle graphene with near Planckian dissipation, Phys. Rev. Lett. 124, 076801 (2020).
- Debanjan Chowdhury, Y. Werman, E. Berg and T. Senthil, Translationally invariant non-Fermi liquids with critical Fermi-surfaces: Solvable models, Phys. Rev. X 8, 031024 (2018).
- Debanjan Chowdhury, I. Sodemann and T. Senthil, Mixed-valence insulators with neutral Fermi-surfaces, Nature Communications 9, 1766 (2018).
- Debanjan Chowdhury and B. Swingle, Onset of many-body chaos in the O(N) model, Phys. Rev. D 96, 065005 (2017).
- B. Swingle and Debanjan Chowdhury, Slow scrambling in disordered quantum systems, Phys. Rev. B 95, 060201(R) (2017).

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