Monday, 15 December 2025
This talk will review how composite fermions provide a unified framework for a broad range of emergent quantum phases in two dimensions. Composite fermions account not only for the “zoo’’ of fractional quantum Hall states, but also for metallic Fermi seas, Wigner crystals, stripes, superconductors, magneto-rotons, Abelian and non-Abelian anyons, and even Majorana modes. They further enable decisive progress on scaling in the FQHE, thereby extending the reach of T. V. Ramakrishnan’s seminal scaling theory of localization to a new class of metal-insulator transitions where both interaction and disorder appear in a non-perturbative fashion. Thanks to composite fermions, these 2D states have become arguably the best-understood strongly correlated electronic systems, with a simple organizing principle and as well as a remarkably accurate, systematically improvable quantitative theory.
Fractional quantum Hall (FQH) phases, emerging from strong electronic interactions, are characterized by anyonic quasiparticles with unique topological parameters, fractional charge, and statistics. In contrast, integer quantum Hall (IQH) effects arise from the band topology of non-interacting electrons. In this talk, I report a surprising super-universality in the critical behavior across all FQH and IQH transitions, revealing identical critical scaling exponent k = 0.41 ± 0.02, localization length exponent g = 2.4 ± 0.2 and the dynamical exponent z ≈ 1 for both. These results were experimentally obtained using ultra-high mobility trilayer graphene devices with a metallic screening layer close to the conduction channels. Previous studies on these global critical exponents were inconclusive due to significant sample-to-sample variations in measured values of k in conventional semiconductor heterostructures dominated by long-range correlated disorder. I will demonstrate that these robust scaling exponents are valid in the limit of short-range disorder correlations.
Additionally, I will discuss our recent studies on the effect of screening on the scaling exponents.
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.
Phase sensitive Andreev spectroscopy is widely used to characterize the nature of superconductors (SC). In flat band moire graphene SC, STM experiments in the Andreev and tunneling regimes show two distinct energy scales. Is this a signature of strongly fluctuating superconductivity? We develop a Green’s function formulation that allows us to include self-energy effects and go beyond the standard Blonder-Tinkham- Klapwijk framework. We first show that the two energy scales cannot be understood as a SC gap in the Andreev spectra and a pseudogap in tunneling. We next show that the high transparency Andreev reflection regime cannot be realized in moiré materials. The large mismatch between the Fermi velocity ($v_F$) of the flat band SC and the STM tip renormalizes a transparent interface into the tunneling regime. Finally, we model the Andreev experiment as a circular metallic disc embedded in an unconventional SC and show that it leads to tip-induced Andreev bound states (ABS). In regime of strong $v_F$ mismatch, tunneling into the ABS gives rise to the low-energy sub-gap scale in the conductance. Our analysis shows that the data supports the presence of an unconventional (non s-wave) SC in moire graphene.
Work done in collaboration with S. Biswas, S. Suman and M. Randeria and based on PNAS 122 (46) e2509881122
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.
It is well known that a bond disproportionated insulating ground state exists in all rare-earth nickelates except LaNiO 3, which remains metallic. The origin of bond disproportionation has been attributed to the existence of a negative charge-transfer energy, Δ, based on a model Hamiltonian approach. Additionally, it has also been suggested in past literature that the Ni-O-Ni angle, which controls the effective bandwidths in nickelates, is the controlling parameter. Using the ab initio results, we demonstrate that all nickelates fall within the regime of Δ > 0, thereby undermining the existing explanation. Additionally, our results show that LaNiO 3 has the lowest Δ among all nickelates, underscoring the absence of any disproportionation in this system, in contrast to all other nickelates, as anomalous. Parameterizing the inner potential at the Ni site in order to tune the Ni d level energy within the same, otherwise, ab initio approach, we show that the observed bond disproportionation is
controlled, not by the bare Δ, but by the effective charge transfer energy, Δ eff , which includes the effects of the oxygen p and Ni d bandwidths on the bare charge transfer energy. Our results indicate that bond disproportionation occurs only for a range of negative Δ eff . All nickelates except LaNiO 3 are found in this region. We also demonstrate that a negative Δ eff , below a critical threshold, gives rise to a homogeneous metallic state, with LaNiO 3 being part of this regime. While the Ni-O-Ni bond angles, controlled by the lanthanide ionic radii, have been thought to be responsible for the rich phase diagram of the nickelates, our results show that Δ eff depends significantly and nonmonotonically on the lanthanide ionic radii via structural distortions influencing electrostatic potentials on the Ni d orbitals, rather than the bandwidth, as suggested in the past, leading to the observed variations in the ground state properties. We find that the concept of Δ eff < 0 driving bond disproportionation is quite general and explains nearly all known cases of disproportionation, even those beyond the nickelates.
This work is based on an unpublished work by Sagar Sarkar, Basudeb Mandal, Shishir Kumar Pandey, Shinjini Paul, Priya Mahadevan, Cesare Franchini, A. J. Millis, and D. D.
Sarma.
Tuesday, 16 December 2025
The mixed-valent spinel LiV2O4 is known as the first oxide heavy-fermion system. There is a consensus that a subtle interplay of charge, spin, and orbital degrees of freedom of correlated electrons plays a crucial role in the enhancement of quasi-particle mass, but the specific mechanism has remained yet elusive. A charge-ordering (CO) instability of V3+ and V4+ ions that is geometrically frustrated by the V pyrochlore sublattice from forming a long-range CO down to T =0 K has been proposed as a prime candidate for the mechanism. In this talk, we uncover the hidden CO instability by applying epitaxial strain on single-crystalline LiV2O4 thin films. We find a crystallization of heavy fermions in a LiV2O4 film on MgO, where a charge-ordered insulator comprising of a stack of V3+ and V4+ layers along [001], the historical Verwey-type ordering, is stabilized by the in-plane tensile and out-of-plane compressive strains from the substrate. Our discovery of the [001] Verwey type CO, together with previous realizations of a distinct [111] CO, evidence the proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the V pyrochlore lattice. Our recent transport (Hall coefficient RH and thermopower S) studies on LiV2O4 single crystals at low temperatures below 2K points to a semimetallic ground state with almost equally heavy electrons and holes, which gives us a hint for the origin of CO instability in k-space. We note that the semimetallic ground state is the natural consequence of the even number of electrons in the primitive cell containing 4 V ions (1.5 x 4 =6). The volume of semimetallic Fermi surfaces inferred from transport appears to respect roughly the LDA semimetallic Fermi surfaces, though the mass is two orders of magnitude enhanced. We argue that the coupling of itinerant electrons in the almost half-filled two eg bands and almost Mott-localized electrons in the narrow a1g bands below the Fermi level gives rise to the formation of extremely narrow semimetallic quasi-particle bands at the Fermi level. LiV2O4 may bridge the 4f Kondo heavy fermion physics with the correlated d-electron physics.
U. Niemann, Y.-M. Wu, R. Oka, D. Hirai, Y. Wanga, Y. E. Suyolcua, M. Kim, P. A. van Aken, and H. Takagi, Proceedings of the National Academy of Sciences 120, e2215722120 (2023).
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.
Experimental advances in nonlinear spectroscopy provide access to out of equilibrium dynamics of quantum materials. This opens new possibilities for interrogating material systems, and offers insights inaccessible to conventional probes. However, fully harnessing these new capabilities requires developing a deeper theoretical understanding of nonlinear spectroscopy and its capabilities. I will present some recent work on this front.
Wednesday, 17 December 2025
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.
Thursday, 18 December 2025
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.
Magic-angle twisted bilayer graphene (MATBLG), a promising platform for exploring strongly correlated electron physics, exhibits a rich phase diagram unveiled by various transport and thermodynamic probes, yet the microscopic nature or "granularity" of its charge carriers remains unexplored. Shot noise provides a direct window into this aspect by revealing the effective charge, dynamics, statis- tics, and interactions of the carriers. We report a comprehensive shot noise investigation of MATBLG as a function of carrier density across the flatband, temperature, bias energy, and magnetic field. In the superconducting (SC) phase of the flatband, a doubling of the Fano factor (F ≃ 2) is observed,
signifying charge transport via Cooper pairs, providing unambiguous evidence of electron pairing. In the normal state of the flatband, a universal Fano factor (F ≃ 0.43) is measured, highlighting the dominance of electron-electron interactions in the flatband. Most strikingly, around the SC dome, we observe a strong suppression of the Fano factor (F ≃ 0.15) above the critical current. Considering various possibilities, our results suggest that a lack of granular nature of the quasiparticles can explain this observed suppression.
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.
The second order nonlinear Hall effect is the dominant Hall response in nonmagnetic systems. We will discuss the different origin of the second order charge Hall response, and its symmetry properties. Building on this, we will explore nonlinear thermal Hall and nonlinear Nernst responses in different systems. We will end by discussing the origin of the recently observed giant nonlinear Nernst effect in ABA trilayer graphene.
Friday, 19 December 2025
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.
The chiral central charge is a topological invariant characterizing the boundary modes of a bulk chiral topological phase in two spatial dimensions. The bulk-boundary correspondence relates the chiral central charge to the quantized thermal Hall response at low temperatures, but the measurement of the latter is challenging and often controversial. Thermal Hall data at higher temperatures can be potentially useful but here the bulk-boundary correspondence no longer holds and the interplay of fermionic and gauge boson bulk excitations complicates the connection to the chiral central charge. Using large-N matter Chern-Simons field theory as well as numerical approaches, we analyse the finite temperature thermal Hall response for two microscopic spin models: a Kagome system with abelian semionic topological order and a perturbed honeycomb Kitaev model with non-abelian Ising topological order and comment on possible experimental signatures of underlying chiral topological order. At the low-temperature end, we extract the chiral central charge for the above microscopic models from the thermal Hall response and find that the chiral central charge obtained from transport calculations is in excellent agreement with that obtained from a recently developed wavefunction based lattice modular commutator method.