ICTS

Superconductivity generally abhors magnetic fields while the quantum Hall effect usually requires strong magnetic fields. Recently the fractional quantum has been discovered in systems without the need for an applied magnetic field. I will discuss the problem of doping the resulting ``fractional quantum anomalous Hall state". I will argue that the doped state is a (unique) realization of a fluid of mobile anyons. Building on old ideas on ``anyon superconductivity", I will identify situations in which doping the prominent 2/3 fractional quantum anomalous Hall state in a Chern band may lead to a superconductor. I will describe the surprising effects of disorder on such a superconductor: at low doping, an ``Anomalous Vortex Glass" superconductor results with characteristics distinguishable from an ordinary superconductor. Time permitting, I will also consider the fate of a fluid of non-abelian anyons obtained by doping non-abelian quantum Hall states showing that a variety of interesting (super)conducting states arise naturally.

In this talk, I will discuss the importance of materials specific inputs in properties of strongly correlated electron systems. I will cite examples from High Tc cuprates, low dimensional quantum spin systems and metal-insulator transition in V2O3.

Modern condensed matter physics faces a challenge of understanding strange metals, insulators and superconductors. For seventy years, a standard model of condensed matter physics, built on band theory, quasiparticles, Landau Fermi liquid and BCS theory, has provided a firm foundation of our understanding of quantum materials. In the last three decades, many strange new classes of material have emerged that do not fit naturally into this traditional framework. In each of the main classes of quantum material, metals, insu-
lators and superconductors, we are faced with experimental anomalies that challenge our understanding to its core, and these are the strange metals, insulators and superconductors.

I will discuss these categories, with an emphasis on the role qualitative concepts topology, fractionalization and unconventional order, that may play a role in their future elucidation. As a concrete example of a challenging new developments, I will present some recent tunneling results from our collaboration[1], that demonstrate that the strange topological Kondo Insulator Samarium Hexaboride undergoes an axionic phase transition at low temperatures - with a surface spin magnetization proportional to an applied electric field[2].

[1] Work supported by the U.S. Department of Energy (DOE) under Contracts No. DE-FG02- 99ER45790 and DE-FG02-84ER45118.
[2] Saikat Banerjee, Anuva Aishwarya, Fei Lei, Lin Jiao, Vidya Madhavan, Eugene Mele and Piers Coleman, preprint (2025).

THz 2D coherent spectroscopy has emerged as a powerful new technique to measure novel quantum mechanical correlations. I will discuss my group's recent work using THz 2D coherent spectroscopy to measure collective modes in superconductors as well as give insight into strong interacting metals including the peculiar physics of the normal state of the cuprates in their "strange metal" regime. In the former we gain importance information about the amplitude mode in NbN and MgB2. In the latter we can characterize both the energy relaxation rate and higher order Fermi surface distortions that gives new insight into the cuprate normal state.

What is the fate of Dirac fermions that move in a background of disordered Z_2 gauge fields? Do they localize or freeze? In this talk, I will describe recent work addressing these questions by reporting a combination of numerical and analytical treatments of this problem. Working on a square lattice with a background $\pi$-flux such that the low-energy physics of the clean system is described by a Dirac theory, gauge field disorder induces flux defects with zero-flux plaquettes. I will demonstrate that the zero-energy states exhibit a multifractal character, determined by the concentration and correlations of flux defects. The key finding is that the multifractal spectrum is non-frozen, with a tendency towards termination. I will demonstrate that these critical states are characterized by a robust relation between the conductivity and the Lyapunov exponent, which is satisfied by the states irrespective of the concentration or the local correlations between the flux defects. I will discuss how renormalization group analysis, based on perturbing the Dirac point, fails to capture this new class of critical states.

Work done in collaboration with Hiranmay Das (IISc), Naba Nayak (IIT Powai), Soumya Bera (IIT Powai)

Both electron- and hole-doped cuprates exhibit strange metallic transport, encapsulated by a T-linear resistivity that extends down to low temperatures over an extended doping range and with a coefficient that scales with Tc. Such similarity in the charge response has led to a common assumption that both electron- and hole-doped cuprates are governed by the same physics. In this talk

Here, we show that the Hall effect and magnetoresistance (MR) in the two systems are in fact distinct. This distinction has profound consequences for our understanding of the origin and link between strange metallicity and high-temperature superconductivity in the two systems.

We investigate the hydrodynamic regime in metals with momentum-conserving electron-electron scattering. The conservation of momentum results in well-defined dynamics whose effects we investigate via the relevant continuity equations. We find anomalous contributions to the charge and heat transport currents arising from gradients of the velocity field in a semiclassical treatment with a Berry curvature. These contributions are non-vanishing for systems lacking inversion symmetry, and the corresponding transport coefficients do not obey the standard Onsager reciprocity relations. Instead, we show that the response coefficients relating the currents to the stress tensor obey independent reciprocity relations with the stress tensor and thus exhibit cross-tensor effects of charge and heat transport with the momentum transport. The Berry curvature contribution to the stress magnetization tensor is also obtained.

I will discuss recent theoretical work exploring correlated phases in heterostructures generated by twisting individual 2D layers whose low-energy electronic dispersions lie at the Brillouin zone M-point. The resulting moiré system has low-energy electrons with a valley-filtered anisotropy, such that each valley to good approximation is only dispersive in a single direction. Remarkably, the problem is accessible via sign-problem-free quantum Monte Carlo along two distinct parameter axes, providing a window into the physics of strong correlations in this new family of moiré materials.

Institute of Physics, École Polytechnique Fédéral de Lausanne, 1015 Switzerland
Twisted trilayer graphene (TTG) has emerged as a particularly intriguing platform for studying moiré superconductivity. Its flat-band physics closely resembles that of twisted bilayer graphene, yet TTG offers an additional degree of tunability in its band structure, providing a valuable handle for uncovering the mechanisms of moiré superconductivity. In addition, the interference between the two moiré lattices in mirror-symmetry-broken TTG gives rise to a supermoiré lattice, introducing a new degree of freedom for exploring correlated electronic phenomena.
In the first part, we report the first transport observation of gate-tunable double-dome superconductivity in MATTG. We found that superconductivity is supressed near v=-2.6 in a small displacement field region. Through temperature, magnetic field and current bias dependence, we reveal the distinct transport behavior of the right and left dome superconductivities, as well as their corresponding normal states. In the second part, we report the existence of the supermoiré lattice in the mirror-symmetry-broken TTG, elucidating its role in generating mini flat bands and mini Dirac bands. We also demonstrate interaction-induced symmetry-broken phases in the supermoiré mini flat bands alongside the cascade of multiple superconductor-insulator transitions enabled by the supermoiré lattice.
Our work provides new insights into moiré graphene superconductivity and highlights the importance of the supermoiré lattice as an additional degree of freedom for tuning the electronic properties of twisted multilayer systems.

Engineering artificial systems by twisting and stacking van der Waals (vdW) materials have proven to be an excellent platform for exploring emergent quantum phenomena that can be significantly different from the constituents. Recent
advances in the fabrication of high-quality twisted interfaces provide a unique opportunity to study the little-explored interfacial superconducting order in twisted cuprate superconductors, which was not possible till now. In our work, we fabricate superconducting quantum interference devices (SQUIDs) that utilize the twisted interface of Bi2Sr2CaCu2O8+δ (BSCCO), a high-Tc cuprate superconductor.

Quantum platforms are nearing a scale where they will allow the realization of a vast number of quantum circuits. Some of these will solve computational problems of interest to industry and commerce, others will answer questions about quantum systems that we normally ask. But this will still leave much room for searching for entirely new quantum phenomena. In this talk I will describe some thinking on one concrete possibility - that of looking for quantum Nash equilibria-and on a general method for using these platforms in "discovery mode" much like the use of particle accelerators for discovering new particle physics.