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

We argue that the combination of strong repulsive interactions and high magnetic fields can generate electron pairing and superconductivity. Inspired by the large lattice constants of moir’e materials, which make large flux per unit cell accessible at laboratory fields, we study the triangular lattice Hofstadter-Hubbard model at one-quarter flux quantum per plaquette, where previous literature has argued that a chiral spin liquid separates a weak-coupling integer quantum Hall phase and a strong-coupling topologically-trivial antiferromagnetic insulator at a density of one electron per site. We argue that topological superconductivity emerges upon doping in the vicinity of the integer quantum Hall to chiral spin liquid transition. We employ exact diagonalization and density matrix renormalization group methods to examine this theoretical scenario and find that electronic pairing indeed occurs on both sides of criticality over a remarkably broad range of interaction strengths. On the chiral spin liquid side, our results provide a concrete model realization of the long-hypothesized mechanism of anyon superconductivity. Our study thus establishes a beyond-BCS mechanism for electron pairing in a well-controlled limit, relying crucially on the interplay between electron correlations and band topology.

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.

Probing spin-orbit interactions near a topological phase transition using spin-ARPES

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique that enables the direct measurement of electronic band structure. Spin-resolved ARPES in addition directly probes the spin polarization of photoemitted electrons, providing deeper insight into the underlying physics of these materials, though the intrinsic inefficiency of spin detection has proven to be a significant barrier to harnessing the full power of this technique. Recent progress in creating increasingly efficient methods of spin detection have thus stimulated a wealth of spin-ARPES measurements across a wide range of materials. Leveraging the increasing capability of this technique, we have performed high-resolution spin-ARPES measurements to observe the spin texture of a material on the threshold of a topological phase transition, ZrTe5. Our work reveals spin-polarized electronic bands as the material approaches a topological phase transition and offers new perspectives on how spin-orbit interactions manifest in the topological character of this material. We highlight strong light polarization dependence of the spin texture, which further evidences coupling between the spin and orbital degrees of freedom. Beyond the material of study, our results underscore the growing capabilities of spin-resolved ARPES and its potential to uncover novel spin-dependent phenomena in complex materials.

Axion electrodynamics and giant magnetic birefringence in Weyl excitonic insulators

We show that in the excitonic insulator phase of time-reversal (TR) invariant Weyl semimetals (WSMs), the phase of the exciton condensate couples to the electromagnetic (EM) field as a massive dynamical axion. This is a consequence of the chiral anomaly and the chiral magnetic effect in the parent WSM.

The mass of the axion is generated by the inter-band matrix elements of electron-electron interactions and is much smaller than the gap in the single-particle spectrum.

Because of this, in the presence of magnetic field the axion coupling to EM fields results in giant anisotropic polarizability and birefringence. The photon-axion hybridization produces a magneto-induced polariton resonance near the axion gap.

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. We associate the nematic order with spontaneous polarization between d_zx and d_yz orbitals. We derive the pairing interaction, mediated by soft nematic fluctuations, and show that it is attractive, and that its strength depends on the position on the Fermi surface. As the consequence, right at the nematic quantum-critical point (QCP), superconducting gap opens up at T_c only at special points and extends into finite arcs at T < T_c.  In between the arcs the Fermi surface remains intact.   This gives rise to highly unconventional behavior of the specific heat, with no jump at T_c and an apparent finite offset at T = 0, when extrapolated from a finite T. We argue that this behavior is consistent with the specific heat data for FeSe_1-xS_x near critical x for the onset of a nematic order.  We discuss the behavior of the gap away from a QCP and the pairing symmetry, and apply the results to FeSe_1-xS_x and FeSe_1-xTe_x , which both show superconducting behavior near the QCP distinct from that in a pure FeSe. We also computed variation of the specific heat with a magnetic field, magnetic susceptibility, density of states, tunneling conductance, Raman intensity, superfluid stiffness and penetration depth without and with impurity scattering and for the latter computed also optical conductivity and T_c variation. We find good agreement with the existing data for FeSe_1-xS_x and FeSe_1-xTe_x and suggest new experiments.

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).

From Bose-Fermi Mixtures to Kinetic Magnetism in Moiré Heterostructures

Moiré heterostructures are fascinating experimental platforms for studying emergent quantum many-body phenomena. Here, we realize an equilibrium Bose-Fermi mixture in a bilayer electron system implemented in a WS2/WSe2 moiré heterobilayer with strong Coulomb coupling to a nearby moiré-free WSe2 monolayer. Absent the fermionic component, the underlying bosonic phase manifests as a dipolar excitonic insulator. By injecting excess charges into it, we show that the bosonic phase forms a stable mixture with added electrons but abruptly collapses upon hole doping. We develop a microscopic model to explain the unusual asymmetric stability.

Moreover, a similar setup can realize an approximate SU(4)-symmetric Hubbard model. Using matrix product states, we theoretically investigate the magnetic order upon hole doping, where several competing states emerge due to kinetic magnetism.

Topological Chiral Superconductivity in Flat Band Materials

Pseudogap in electron doped cuprates: thermal precursor to magnetism

We study pseudogap behavior in a metal near an antiferromagnetic instability and apply the results to electron-doped cuprates. We associate pseudogap behavior with thermal magnetic fluctuations and compute the fermionic self-energy along the Fermi surface beyond Eliashberg approximation. We analyze the spectral function as a function of frequency (energy distribution curves, EDC) and momentum (momentum distribution curves, MDC). We show that the EDC display pseudogap behavior with peaks at a finite frequency at all momenta. On the other hand, MDC peaks disperse within the pseudogap, ending at a gossamer Fermi surface. We analyze magnetically-mediated superconductivity and show that thermal fluctuations almost cancel out in the gap equation, even when the self-energy is obtained beyond the Eliashberg approximation. We favorably compare our results with recent ARPES study [K-J Xu et al, Nat. Phys. 19, 1834–1840 (2023)]

Exploring Electronic Ferroelectricity in BEDT-TTF based Organic Mott Insulators

The organic Mott insulators of the form κ-(BEDT-TTF)2X, where BEDT-TTF refers to bis(ethylenedithio)tetrathiafuvalene and X refers to a polyatomic anion, host a correlated electron system on a triangular lattice of (BEDT-TTF)2 dimers. Each dimer lattice site carries one hole and one S=1/2 spin which interacts antiferromagnetically between neighboring dimer lattice sites. Long range charge correlations between dimer lattice sites compete with intradimer hopping, leading to a charge degree of freedom and emergent electric dipoles within (BEDT-TTF)2 dimers. The interplay of strong antiferromagnetic interactions with Ising-like electric dipoles yields a rich phase diagram with quantum spin liquid candidates, quantum dipole liquid candidates, electronic ferroelectricity, and superconductivity. We use Raman scattering spectroscopy to study the insulating regions of the phase diagram, exploring the behavior of emergent electric dipoles with and without long range charge order. We examine the relationship between emergent electric dipoles with a quantum spin liquid candidate, revealing a coupling between the lattice, charge, and spin at lowest temperatures. We use uniaxial strain to tune interactions between (BEDT-TTF)2 dimers, which leads to an experimental mapping of the charge order phase boundary. Lastly, we calculate the electron-molecular-vibration coupling induced shifts to molecular vibrations, which leads to an experimental determination of the extended Hubbard model parameters that is unique. We find that exotic and disordered ground states emerge from the competition between intradimer and interdimer interactions, the coupling of multiple degrees of freedom, and the broad range of frequencies which correspond to dipole and spin fluctuations.

Effects of collective modes on nonlinear conductivity

Universal bounds in topological phases

Topology plays a fundamental role in condensed matter physics. Even when ground states share the same symmetries, they can still be topologically distinct, giving rise to distinct topological phases of matter. These phases are characterized by topological invariants, such as the Chern number. Topological invariants can manifest in physical observables, such as the quantized Hall conductance in Chern insulators.

In this work, we explore another fundamental aspect of topological phases: the existence of universal bounds on physical observables, which we term topological bounds. Specifically, we establish bounds on two physical observables in Chern insulators: the static structure factor and the energy gap. The bound on the static structure factor is determined solely by the Chern number, while the bound on the energy gap is determined by the Chern number and the electron density. Remarkably, these bounds apply universally to all insulators, including strongly correlated systems such as fractional Chern insulators. As an example, we apply these bounds to moiré transition metal dichalcogenides. We also interpret the topological bounds as geometric relations with quantum geometry.

Our results provide a new perspective on topological phases of matter, demonstrating that the topological invariants can universally manifest as topological bounds.

Spontaneous Pair Density Wave in a Fractionalized Kondo Lattice

We demonstrate that the discommensuration between the Fermi surfaces of a conduction sea and an underlying spin liquid provides a natural mechanism for the spontaneous formation of pair density waves. Using a recent formulation of the Kondo lattice model which incorporates a Yao Lee spin liquid proposed by the authors, we demonstrate that doping away from half-filling induces finite-momentum electron-Majorana pair condensation, resulting in amplitude-modulated PDWs. Our approach provides a precise, analytically tractable pathway for understanding the spontaneous formation of PDWs in higher dimensions and offers a natural mechanism for PDW formation in the absence of Zeeman splitting, and naturally accounts for the emergence of intertwined order.

Zero Flux Localization: How flat bands magically appear

I present a very simple and clear answer to the question of why the chiral limit of the bilayer graphene clicks and gives flat bands at specific twist angles. The solution is simple enough to be given as homework in a grad course.

Nonequilibrium field theory of open spin systems: from semiclassical to nonperturbative quantum dynamics

Open systems of many interacting spins—as realized by localized magnetic moments in a spintronic device, or qubits in a quantum computer—pose a formidable challenge for presently available theoretical methods, especially when the memory effects induced by the surrounding environment are relevant. Even archetypical examples like the spin-boson model, in which a single spin interacts with a continuum of bosonic modes requires switching between specialized methods for different choice of system-bath parameters. Here, we present a field theory (FT) of open quantum spin systems based on the Schwinger-Keldysh (SK) functional integral, which serves as the starting point for both semiclassical and fully quantum descriptions of the dynamics. In the semiclassical regime, we obtain corrections to the Landau-Lifshitz-Gilbert equations, conventionally employed in spintronics and magnonics, accounting for, e.g., nonlocal magnetic damping. On the other hand, the fully quantum regime is probed by combining SKFT with the two-particle irreducible (2PI) action resumming a class of Feynman diagrams to an infinite order. Remarkably, our SKFT+2PI closely tracks numerically exact benchmarks for the spin-boson and spin-chain-boson models, even in the nonperturbative and non-Markovian regime. The favorable numerical cost of solving integro-differential equations produced by SKFT+2PI framework with increasing number of spins, time steps or spatial dimensionality makes it a promising route for simulation of driven-dissipative systems in quantum computing or quantum spintronics and magnonics in the presence of a single or multiple dissipative environments.

Origin or interaction-induced Chern band in rhombohedral multilayer graphene

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.

Regularizing 3D conformal field theories via anyons on the fuzzy sphere

In the recently introduced “fuzzy sphere” method, numerical regularization of certain 3D conformal field theories (CFTs) is achieved through the non-commutative geometry of the lowest Landau level filled by electrons; here, the charge is trivially gapped due to the Pauli exclusion principle at filling factor v=1, while the electron spins encode the desired CFT. Successful applications of the fuzzy sphere to paradigmatic CFTs, such as the 3D Ising model, raise an important question: how finely tuned does the underlying electron system need to be? In this work, we will show that the 3D Ising CFT can also be realized at fractional electron fillings, unaffected by the exchange statistics of the particles or the nature of topological order in the charge sector. In such cases, the CFT spectrum is intertwined with the charge-neutral spectrum of the underlying fractional quantum Hall (FQH) state – a feature that is trivially absent in the previously studied v=1 case. Remarkably, the mixing between the CFT and the FQH spectra is strongly suppressed within the numerically-accessible system sizes.

Shot noise in strongly correlated metals

The nature of charge carriers in strange metals is a subject of considerable interest. Recent measurements in the quantum-critical heavy-fermion metal YbRh₂Si₂ motivate an investigation of shot noise across various models. We first establish that F=√3/4 is a universal property of all Fermi liquids, analogous to the Wiedemann-Franz law. This result underscores the absence of quasiparticles in YbRh₂Si₂ . Next, we examine shot noise in a system of electrons coupled to critical bosons and demonstrate the emergence of electron-boson drag, which equalizes the local temperatures of both components. Results at the Kondo-destruction quantum critical point will also be presented.

Pauli 'unlimited': magnetic field induced-superconductivity in UTe2

Inspired by the observation of extreme field-boosted superconductivity in uranium ditelluride, we present a scenario in which superconductivity can be induced by a strong Zeeman field, rather than destroyed by it, as is the case in Pauli-limited superconductivity. The resulting superconducting state has an upper critical field far greater than the superconducting transition temperature, and with spin-orbit coupling, it is sensitive to the field orientation. We demonstrate the interplay between superconductivity and metamagnetism in a simple effective theory.

Universal wave function statistics and superconductivity in quasiperiodic twisted trilayer graphene

Twisted multilayer graphene (TMG) was recently discovered to host superconductivity and offers a much richer playground to study moiré physics, with more knobs to tune including multiple twist angles. The low-energy physics of TMG is described by multi-cone Dirac fermions coupled with matrix-value fields, which are effectively topological when inter-valley scattering is suppressed. The topology protection prevents the wave functions from being Anderson (or Wannier) localized by quasiperiodicity; the latter is natural in TMG with 3 or more layers due to inevitable interlayer twist-angle differences. Here we focus on quasiperiodic twisted trilayer graphene (TTG). We employ two different models, with and without a collinear approximation for the moiré wavevectors [1]. We show that quasiperiodic TTG behaves like a dirty surface of a bulk class-AIII topological insulator, and exhibits “spectrum-wide quantum criticality” [2] that is robust in the absence of exotic surface-coupling scenarios [3]. We show that superconductivity can be enhanced without fine-tuning to magic angles, similar to previous results in a model of quasiperiodic TBLG [4]. We also explore how the chiral symmetry breaking terms affects the wave function statistics and superconductivity.

[1] Ziyan Zhu, Stephen Carr, Daniel Massatt, Mitchell Luskin, and Efthimios Kaxiras, PRL 125, 116404 (2020)
[2] Björn Sbierski, Jonas F. Karcher, and Matthew S. Foster, PRX 10, 021025 (2020)
[3] Alexander Altland, Piet W. Brouwer, Johannes Dieplinger, Matthew S. Foster, Mateo Moreno-Gonzalez, and Luka Trifunovic, PRX 14, 011057 (2024)
[4] Xinghai Zhang, Justin H. Wilson, and Matthew S. Foster, PRB 111, 024207 (2025)

vortices and vortex lattices in the bilayer exciton superfluids in the quantum Hall regime