Student Poster Abstracts

Quantum Bosonization of 2+1D Fermi Liquid

Developing controlled and efficient approaches to study compressible phases is an important challenge. We compute the non-analytic correction to the density response of a 2+1D Fermi liquid by bosonizing the Fermi surface using the co-adjoint orbit formalism. The non-analytic correction arises naturally as the one-loop correction within the bosonization framework, highlighting the simplicity and efficiency of this approach compared to the conventional fermionic methods, particularly for Fermi liquids with generic Landau parameters. We employ a patch prescription to evaluate the one-loop correction and match it with the result obtained from the fermionic approach for free Fermi gas. Our method can be extended to compute the response function in non-Fermi liquid systems, where strong quantum fluctuation dominates the low-energy regime, making the non-analytic the leading contribution.

Anyon Superconductivity from Topological Criticality in a Hofstadter-Hubbard Model

Anisotropic SU(4) Kondo Screening

We explore the possibility of anisotropic Kondo screening in generic J=3/2 Kondo and Anderson impurity models with tetragonal crystal fields, using a combination of poor man’s scaling and numerical renormalization group (NRG). With these techniques, we can examine the anisotropic screening of the magnetic and orbital susceptibilities at different temperatures and in different model regimes. Notably, we can explore the crossover between SU(4) and SU(2) Kondo physics, which at higher temperatures appears to be mainly driven by anisotropic hybridization rather than the crystal field itself. Our analysis helps establish a correspondence between anisotropic Kondo couplings and experimentally observed temperature scales while developing a better understanding of data obtained from numerical simulations.

Photon-like and roton-like excitations in the supersolid phase of the triangular lattice XXZ model

Inspired by recent experiments, we study the XXZ model on the triangular lattice close to the the Ising point, showcasing the signatures of a supersolid phase via it’s unconventional spin dynamics and dimer structure factors. By combining numerical matrix product state (MPS) simulations, with multiple analytical approaches we demonstrate that the ground state develops a three‐sublattice order accompanied by distinctive peaks in the static spin structure factor at the $K,K’$ points, while showcasing a ubiquitous roton-like energy minimum at the $M$ point. For finite magnetic field we show that perturbative quantum corrections about the Ising limit generate a graphene-like dispersion for hole excitations on an emergent honeycomb lattice and superexchange processes on a supertriangular lattice govern the particle branch. We postulate an effective staggered boson hopping model at zero field which matches the MPS calculated dynamical structure factor with one branch of gapless excitations at the $\Gamma$ and $K$ points and the roton-like minimum. Similar agreement is obtained from a Schwinger boson mean-field treatment where, in contrast, the fractionalized modes give rise to two branches of gapless excitations. Furthermore, we calculate the low energy excitations of a supersolid variational wave function from a quantum dimer model (QDM) formulation and a single mode approximation, showcasing quantitative agreement both with our MPS simulations and known neutron scattering data. Remarkably, we show that both the supersolid variational QDM wave function and our MPS ground state display very similar dimer structure factors with only transverse photon-like excitations. Our comprehensive theoretical and numerical calculations thus clarify the microscopic origin of supersolid order in the XXZ triangular lattice model, and it’s proximity to a spin liquid phase as discussed in recent experiments.

Enhanced Strange Metallicity due to Hubbard-U Coulomb Repulsion

We solve a model of electrons with Hubbard-Coulomb repulsion and a random Yukawa coupling to a two-dimensional bosonic bath, using an extended dynamical mean field theory scheme. Our model exhibits a quantum critical point, at which the repulsive component of the electron interactions strongly enhances the effects of the quantum critical bosonic fluctuations on the electrons, leading to a breakdown of Fermi liquid physics and the formation of a strange metal with “Planckian” quasiparticle decay rates at low temperatures (T -> 0) . Furthermore, the eventual Mott transition that occurs as the repulsion is increased seemingly bounds the maximum decay rate in the strange metal. Our results provide insight into low-temperature strange metallicity observed in proximity to a Mott transition, as is observed, for instance, in recent experiments on certain moiré materials.

Unconventional superconductivity near a nematic instability in a multi-orbital system

We analyze superconductivity in a multi-orbital fermionic system near the onset of a nematic order, using doped FeSe as an example.

First principle prediction of structural distortions in the cuprates and their impact on electronic structure

Describing the normal state of doped cuprates remains a challenge due to strong electronic correlations and structural complexity. While many-body methods have provided deep insights into the correlation effect, they often rely on simplified Hamiltonians that overlook key structural effects. Here, we demonstrate how density functional theory (DFT) can accurately capture essential structural, electronic, and magnetic properties of a prototype cuprate Bi₂Sr₂CaCu₂O_8+x (Bi-2212), and how the structural effects refine effective models.

By incorporating energy-lowering structural distortions with state-of-the-art exchange-correlation functionals, our DFT calculations achieve the following: (a) an accurate description of the insulating antiferromagnetic ground state in the undoped compound; (b) prediction of the lowest-energy hole-doped crystal structure consistent with scanning transmission electron microscopy; (c) identification of nearly degenerate competing spin and charge stripe orders in the hole-overdoped regime, indicating strong spin fluctuations; and (d) revealing the structural origin of shadow Fermi surface observed in angle-resolved photoemission spectroscopy (ARPES) measurements. Additionally, we show that capturing dynamic spin fluctuations through a many-body approach is essential for quantitatively reproducing ARPES spectra in overdoped cuprates, exposing the severe limitations of band theory methods such as DFT in this context. Finally, we show that the local structure of the CuO₂ layers, rather than dopant electrostatic effects, modulates the local charge-transfer gaps, local correlation strengths, and by extension the local superconducting gaps.

Reference: Zheting Jin and Sohrab Ismail-Beigi, Phys. Rev. X 14, 041053 (2024).

Correlated Insulators, Superconductors, and Quantum Criticality in Moiré Materials

Moiré transition metal dichalcogenide (TMD) materials provide a versatile platform for studying the interplay of strong correlations, disorder, and band topology in a highly tunable setting. In this poster, I will present my recent works that describe the role of topology, disorder, and quantum criticality on the experimental phenomenology of TMD bilayers. These include a theoretical explanation for the nontrivial electrical transport near a bandwidth-tuned metal-insulator transition in AA-stacked MoTe₂/WSe₂ [1], an investigation of the interaction-driven insulating phases at a commensurate filling in AB-stacked MoTe₂/WSe₂ [2], and a proposal for the continuous superconductor-insulator transition at a filling of one electron per moire unit cell in twisted bilayer WSe₂ [3].

[1] SK, Senthil, and Chowdhury, Phys. Rev. Lett. 130, 066301 (2023)
[2] Mendez-Valderrama*, SK*, and Chowdhury, Phys. Rev. B 110 (20), L201105 (2024)
[3] SK*, Mendez-Valderrama*, Wang*, and Chowdhury, Nat. Commun. 16 (1), 1701 (2025)

Investigating Amplitude Mode in Lanthanum Cuprates Using THz 2D Coherent Spectroscopy

Signatures of the amplitude mode has been reported in various conventional and unconventional superconductors. Many of these recent studies utilize optical techniques to probe the non-linear response of these systems, but most of them find it challenging to distinguish the response of the amplitude mode from that of quasi-particle excitations and other processes. Recently, THz 2D coherent spectroscopy has been shown to be uniquely sensitive to the amplitude mode in an s-wave superconductor NbN. Our study adapts this method to probe the presence of amplitude mode in electron-doped and hole-doped lanthanum cuprates. Utilizing controlled high-intensity broad-band and narrow-band THz pulses, we find the coherent 2D NL response from lanthanum class cuprates to be significantly different from that of conventional superconductors and attribute this mainly to the over-damping of the amplitude mode and the anisotropic d-wave superconducting gap.

Quantum dynamics of 2D randomly pinned charge density waves

The nature of dynamic charge correlations in randomly pinned charge density waves (CDW) is a long-standing theoretical problem. The inherent disorder present in real materials and the particular fragility of charge order to impurities makes this problem especially relevant to a host of systems currently at the forefront of modern condensed matter physics, including the cuprate high temperature superconductors and transition metal dichalcogenides. To address this question, we build on an exactly-solvable model of a 2D randomly pinned incommensurate CDW first introduced by us, and use the large-N technique to determine the phase diagram and CDW correlations. Our approach considers quantum and thermal fluctuations and disorder on equal footing by treating all effects non-perturbatively. We reveal a novel crossover between under-damped and over-damped dynamics of the fluctuations of the CDW order.

Topological charge excitations and Green's function zeros in paramagnetic Mott insulator

We investigate the emergence of topological features in the charge excitations of Mott insulators in the Chern-Hubbard model. In the strong correlation regime, treating electrons as the sum of holons and doublons excitations, we compute the topological phase diagram of Mott insulators at half-filling using composite operator formalism. The Green function zeros manifest as the tightly bound pairs of such elementary excitations of the Mott insulators. Our analysis examines the winding number associated with the occupied Hubbard bands and the band of Green’s function zeros. We show that both the poles and zeros show gapless states and zeros, respectively, in line with bulk-boundary correspondence. The gapless edge states emerge in a junction geometry connecting a topological Mott band insulator and a topological Mott zeros phase. These include an edge electronic state that carries a charge and a charge-neutral gapless zero mode. Our study is relevant to several twisted materials with flat bands where interactions play a dominant role.

Fermion Statistics Induced Spin Chirality and Young's Interference Patterns of Spin Chirality in Topological Superconductors

We demonstrate that in both normal and superconducting metals with broken time-reversal symmetry (TRS), orbital currents in the ground state can induce spin-chirality. Remarkably, non-zero chirality can emerge purely due to Fermion statistics, without the need for spin-dependent interactions, even when the ground state remains spin-unpolarized. This chirality in the carrier band generates a chiral three-spin RKKY interaction between localized spins coupled to the carriers via the s-d Hamiltonian, an effect detectable by local probes like spin-sensitive STM. The spatial distribution of chirality exhibits Young’s interference patterns near localized magnetic adatoms. The interference arises because Bogoliubov quasiparticles are coherent superposition of electrons and holes with opposite spins, allowing coherent and reversible particle-to-hole conversion within the superconductor bulk. Magnetic adatoms act as beam splitters, enabling interference between particle and hole states, leading to spatial patterns similar to those observed in Young’s double-slit experiment. In topological superconductors, these interference patterns are further modulated by nodal lines that encode the winding numbers of the superconducting gap function’s phase. In systems such as topological superconductors where detecting TRS breaking by conventional means is challenging, local detection of spin chirality provides a reliable diagnostic of superconducting topological phases and also provides insight about the nature of pairing.

Fermi surface reconstruction in the doped Hubbard model

We study the validity of Luttinger’s theorem in the 2D repulsive Hubbard model, the parent Hamiltonian for cuprate superconductors, as a function of doping. Using determinant quantum Monte Carlo (DQMC) simulations, we compute the single-particle spectral functions and from its zero energy contour in momentum space, obtain the Fermi surface of the interacting system. This reveals the following: (1) With only nearest neighbor hopping, there is a Lifshitz transition at a critical doping, followed by continuous deviation from the Luttinger volume as one approaches the Mott Insulating limit, with deviations being maximal at the antinodal points. We will discuss the relation between Luttinger breaking Fermi surface and the T linear resistivity in transport. (2) Inclusion of a next nearest neighbor hopping that breaks particle-hole symmetry changes the continuous Fermi surface into Fermi pockets around the hot spot regions. It is important to note that we find Fermi surface restructuring at intermediate temperatures, where there is no spontaneous symmetry breaking. Deviation from the Luttinger count is also accompanied by an anomalous change in Seebeck coefficient.

Designed cleaving planes in ruthenium dioxide for ARPES experiments

Ruthenium dioxide has a complex band structure, underpinning a variety of phenomena including superconductivity under strain and a Dirac nodal line network. It has also been proposed as a candidate altermagnet, and although recent studies suggest it lacks the requisite magnetic order, it has been shown to host unusual spin-polarised states in its band structure. These phenomena motivate the need for further studies into its electronic structure. Angle-resolved photoemission spectroscopy (ARPES) would be an ideal probe for this, and while there have been several pioneering studies to date, the three-dimensional structure of ruthenium dioxide makes it difficult to prepare the requisite clean and flat surfaces with conventional methods. We have therefore investigated a fabrication method based on Focused Ion Beam (FIB) structuring to stimulate sample cleavage along desired crystallographic planes. With this method, we were able to obtain high quality surfaces, on which we performed ARPES measurements. This capability to tailor the sample cleavage leads to a significant increase in ARPES data quality, allowing new resolution of subtle band splitting and additional resolution of bulk vs. surface states in this system.

A Closed Band-Projected Density Algebra Must be Girvin-MacDonald-Platzman

The band-projected density operators in a Landau level obey the Girvin-MacDonald-Platzman (GMP) algebra, and a large amount of effort in the study of fractional Chern insulators has been directed towards approximating this algebra in a Chern band. In this paper, we prove that the GMP algebra, up to form factors, is the only closed algebra that projected density operators can satisfy in two and three dimensions, highlighting the central place it occupies in the study of Chern bands in general. A number of interesting corollaries follow.

Phase Transitions between Obstructed Atomic Insulators

I will present critical theories that describe the phase transitions between different types of trivial atomic insulators.