arXiv subjects

Patrick J. Ledwith

Publications and source records attributed to Patrick J. Ledwith.

17 recordsLinked to original sources

arXiv2026

Skyrmion Fractional Chern Insulator: An Intrinsically Multiband Route to Fractionalization in Rhombohedral Graphene

We propose an unconventional microscopic origin for the fractional quantum anomalous Hall (FQAH) effect in rhombohedral graphene moiré superlattices: skyrmion fractionalization. We view the state at filling $ν<1$ as a metal of skyrmion vacancies, charge $+e$ objects formed by removing layer-pseudospin skyrmions from the interaction-generated skyrmion lattice Chern insulator at $ν=1$. These vacancies are intrinsically multiband degrees of freedom, absent in single Chern band-projected studies. Building on a recently proposed ideal limit, we first develop an effective field theory showing that skyrmion vacancies can themselves fractionalize, thereby inducing charge fractionalization. Focusing on $ν=\frac{2}{3}$, we then construct explicit variational trial wavefunctions for the resulting skyrmion fractional Chern insulator and provide numerical evidence, together with general arguments, showing that this process is energetically favored. Our results establish a realistic route to the FQAH that does not rely on a partially filled Chern band, but instead arises from fractionalization of collective pseudospin textures.

cond-mat.mes-hall↗

arXiv2026

Nonlocal Moments in the Chern Bands of Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) has elements in common with two paradigmatic examples of strongly correlated physics: quantum Hall physics and Hubbard physics. On one hand, TBG hosts flat topological Landau-level-like bands which exhibits quantum anomalous Hall effects. On the other hand, these bands have concentrated charge density and show signs of extensive entropy resembling local moments. The combination of these features leads to a question: can decoupled moments emerge in an isolated topological band, despite the lack of exponentially localized Wannier states? In this work, we answer the question affirmatively by proposing a minimal model for these bands in TBG that combines topology and charge concentration at the AA sites, leading to analytic wavefunctions that closely approximate those of the BM model with realistic parameters. Importantly, charge concentration also leads to Berry curvature concentration at $Γ$, generating a small parameter $s$ that yields analytic tractability. We show that, rather surprisingly, the model hosts nearly decoupled flavor moments without any extra degrees of freedom. These moments are non-local due to topology-enforced power-law tails, yet have parametrically small overlap. We further develop a diagrammatic expansion in which the self energy can be computed exactly to leading order in $s^2$ in the fluctuating moment regime. At charge neutrality, we find a "Mott semimetal", with large flavor entropy and a Mott gap everywhere in the BZ except for the vicinity of the $Γ$ point. Away from neutrality, the Mott semimetal gaps out in a spectrally imbalanced manner, with one Mott band having zero $Z_k$ at the $Γ$ point. The model accurately reproduces results from finite temperature thermodynamic measurements, leads to new experimental predictions, and resolves the problem of the emergence of Hubbard physics in isolated topological bands.

cond-mat.str-el↗

arXiv2026

Gradient-based search of quantum phases: discovering unconventional fractional Chern insulators

The discovery and understanding of new quantum phases has time and again transformed both fundamental physics and technology, yet progress often relies on slow, intuition-based theoretical considerations or experimental serendipity. Here, we introduce a general gradient-based framework for targeted phase discovery. We define a differentiable function, dubbed "target-phase loss function", which encodes fingerprints of a quantum state, thereby recasting phase search as a tractable optimization problem in Hamiltonian space. The method is broadly applicable to a wide range of symmetry-broken and topological orders and can be interfaced with most many-body numerical solvers. As a demonstration, we apply it to spinless fermions on the kagome lattice using exact diagonalization and discover two distinctive fractional Chern insulators (FCIs): (i) at filling $ν= 1/3$, a "non-ideal" Abelian FCI whose band geometry lies far beyond the Landau-level mimicry paradigm and all recent generalizations; and (ii) at $ν= 1/2$, a non-Abelian FCI stabilized purely by finite-range two-body interactions. These results provide the first explicit realization of such types of FCIs and establish a versatile paradigm for systematic quantum-phase discovery.

cond-mat.str-el↗

arXiv2025

The ideal limit of rhombohedral graphene: Interaction-induced layer-skyrmion lattices and their collective excitations

We introduce an ideal limit of rhombohedral graphene multilayers. In this limit, we show analytically how short-range repulsion stabilizes a layer-pseudospin skyrmion lattice, which generates an effective magnetic field and gives rise to a Chern band. This establishes the real-space origin of interaction-driven topology in moiré rhombohedral graphene. The resulting interaction-induced skyrmion lattice is physically analogous to magnetic skyrmion crystals and hosts a hierarchy of collective excitations naturally described within the framework of skyrmion-lattice dynamics.

cond-mat.mes-hall↗

arXiv2025

Exotic Carriers from Concentrated Topology: Dirac Trions as the Origin of the Missing Spectral Weight in Twisted Bilayer Graphene

The nature of charge carriers in twisted bilayer graphene (TBG) near $ν= -2$, where superconductivity emerges, remains mysterious. While various symmetry-broken ground states have been proposed, experimental evidence of significant entropy persisting to low temperatures suggests that the disordered thermal state is a more natural starting point for understanding the normal state physics. Our previous work proposed that this thermal state in TBG hosts nearly decoupled nonlocal moments and an exotic Mott semimetal at charge neutrality. This evolves into a spectrally imbalanced Mott state at non-zero integer fillings. Notably, at $ν= -2$, the quasiparticle residue vanishes at the top of the valence band, precisely where superconductivity develops. The vanishing quasiparticle residue naturally leads to the following question: Which excitation accounts for the missing spectral weight, and how are they related to electrons? In this work, we demonstrate that the missing spectral weight corresponds to a trion excitation, which we explicitly construct and characterize. Remarkably, despite being composed of heavy particles away from the Gamma point, this trion is unexpectedly light. At charge neutrality, the electron and trion hybridize with a momentum-dependent phase that winds around zero, forming a massless Dirac cone. At finite doping $ν< 0$, the Dirac cone acquires a mass with the valence (conduction) band becoming trion (electron)-like near the $Γ$ point. Finally, we show that such trion excitations evolve into the quasiparticles of the intervalley Kekulé spiral (IKS) state at finite doping, where they represent electrons dressed by the IKS order parameter. More broadly, our work highlights the unusual spectral properties and excitations that emerge when fermions interact within topological bands with concentrated quantum geometry and topology.

cond-mat.str-el↗

arXiv2025

Modulation of superconductivity across a Lifshitz transition in alternating-angle twisted quadrilayer graphene

We report electric field-controlled modulation of the Fermi surface topology and explore its effects on the superconducting state in alternating-angle twisted quadrilayer graphene (TQG). The unique combination of flat and dispersive bands in TQG allows us to simultaneously tune the band structure through carrier density, $n$, and displacement field, $D$. From density-dependent Shubnikov-de Haas quantum oscillations and Hall measurements, we quantify the $D$-dependent bandwidth of the flat and dispersive bands and their hybridization. In the high $|D|$ regime, the increased bandwidth favors the single particle bands, which coincides exactly with the vanishing of the superconducting transition temperature $T_c$, showing that superconductivity in TQG is strongly bound to the symmetry-broken state. For a range of lower $|D|$ values, a Lifshitz transition occurs when the flat and dispersive band Fermi surfaces merge within the $ν=+2$ symmetry-broken state. The superconducting state correspondingly shows an enhanced $T_c$, suggesting that the superconducting condensate is strongly dependent on the Fermi surface topology and density of states within this symmetry-broken state.

cond-mat.str-el↗

arXiv2024

Constraints on real space representations of Chern bands

A Chern band is characterized by a Wannier obstruction indicating the absence of a basis of complete, orthogonal, and exponentially-localized states. Here, we study the properties of real space bases of a Chern band obtained by relaxing either exponential localization or orthogonality and completeness. This yields two distinct real space representations of a band with Chern number $C$: (i) a basis of complete orthogonal Wannier states which decay as power-law and (ii) a basis of exponentially-localized overcomplete non-orthogonal coherent states. For (i), we show that the power-law tail only depends on the Chern number and provide an explicit gauge choice leading to the universal asymptotic $w({\boldsymbol r}) \approx \frac{C e^{-i C φ_{\boldsymbol r}}}{2π|{\boldsymbol r}|^2}$ up to a normalized Bloch-periodic spinor. For (ii), we prove a rigorous lower bound on the spatial spread that can always be saturated for ideal bands. We provide an explicit construction of the maximally localized coherent state by mapping the problem to a dual Landau level problem where the Berry curvature and trace of the quantum metric take the roles of an effective magnetic field and scalar potential, respectively. Our coherent state result rigorously bounds the spatial spread of any localized state constructed as a linear superposition of wavefunctions within the Chern band. Remarkably, we find that such bound does not generically scale with the Chern number and provide an explicit example of an exponentially localized state in a Chern $C$ band whose size does not increase with $|C|$. Our results show that band topology can be encoded in a real space description and set the stage for a systematic study of interaction effects in topological bands in real space.

cond-mat.str-el↗

arXiv2024

Vortexability: A Unifying Criterion for Ideal Fractional Chern Insulators

Fractional Chern insulators realize the remarkable physics of the fractional quantum Hall effect (FQHE) in crystalline systems with Chern bands. The lowest Landau level (LLL) is known to host the FQHE, but not all Chern bands are suitable for realizing fractional Chern insulators (FCI). Previous approaches to stabilizing FCIs focused on mimicking the LLL through momentum space criteria. Here instead we take a real-space perspective by introducing the notion of vortexability. Vortexable Chern bands admit a fixed operator that introduces vortices into any band wavefunction while keeping the state entirely within the same band. Vortexable bands admit trial wavefunctions for FCI states, akin to Laughlin states. In the absence of dispersion and for sufficiently short ranged interactions, these FCI states are the ground state -- independent of the distribution of Berry curvature. Vortexable bands are much more general than the LLL, and we showcase a recipe for constructing them. We exhibit diverse examples in graphene-based systems with or without magnetic field, and with any Chern number. A special class of vortexable bands is shown to be equivalent to the momentum space "trace condition" or "ideal band condition". In addition, we also identify a more general form of vortexability that goes beyond this criterion. We introduce a modified measure that quantifies deviations from general vortexability which can be applied to generic Chern bands to identify promising FCI platforms.

cond-mat.str-el↗

arXiv2023

Composite Fermi Liquid at Zero Magnetic Field in Twisted MoTe$_2$

The pursuit of exotic phases of matter outside of the extreme conditions of a quantizing magnetic field has been a long standing quest of solid state physics. Recent experiments have observed spontaneous valley polarization and fractional Chern insulators (FCIs) in zero magnetic field in twisted bilayers of MoTe$_2$, at partial filling of the topological valence band ($ν=-2/3$ and $-3/5$). We study the topological valence band at $\textit{half}$ filling, using exact diagonalization and density matrix renormalization group calculations. We discover a composite Fermi liquid (CFL) phase even at zero magnetic field that covers a large portion of the phase diagram centered around twist angle ${\sim}3.6^\circ$. The CFL is a non-Fermi liquid phase that shows metallic behavior despite the absence of Landau quasiparticles. We discuss experimental implications including the competition between the CFL and a Fermi liquid, which can be tuned with a displacement field. The topological valence band has excellent quantum geometry over a wide range of twist angles and a small bandwidth that is, remarkably, reduced by interactions for a range of angles. These key properties are responsible for stabilizing the exotic quantum Hall phases we find at zero field. Finally, we present an optical signature involving "extinguished" optical responses as a means to identify Chern bands with ideal quantum geometry.

cond-mat.str-el↗

arXiv2023

Strong-coupling topological states and phase transitions in helical trilayer graphene

Magic-angle helical trilayer graphene relaxes into commensurate moiré domains, whose topological and well-isolated set of narrow bands possess ideal characteristics for realizing robust correlated topological phases, compared with other graphene-based moiré heterostructures. Combining strong-coupling analysis and Hartree-Fock calculations, we investigate the ground states at integer fillings $ν$, and uncover a rich phase diagram of correlated insulators tuned by an external displacement field $D$. For small $D$, the system realizes several competing families of symmetry-broken generalized flavor ferromagnets, which exhibit various anomalous Hall signatures and Chern numbers as high as $|C|=6$. The interaction-induced dispersion renormalization is weak, so that the band flatness and the validity of strong-coupling theory are maintained at all integer fillings. For experimentally accessible displacement fields, the strong-coupling insulators at all $ν$ undergo topological phase transitions, which appear continuous or weakly first-order. For larger $D$, we also find translation symmetry-broken phases such as Kekulé spiral order. Our results demonstrate the robust capability of helical trilayer graphene to host gate-tunable topological and symmetry-broken correlated phases, and lay the groundwork for future theoretical studies on other aspects such as fractional topological states.

cond-mat.str-el↗

arXiv2023

Magic-angle helical trilayer graphene

We propose helical trilayer graphene (HTG), a helical structure featuring identical rotation angles $θ\approx 1.5^\circ$ between three consecutive layers of graphene, as a unique and experimentally accessible platform for realizing exotic correlated topological states of matter. While nominally forming a supermoiré (or moiré-of-moiré) structure, we show that HTG locally relaxes into large regions of a periodic single-moiré structure in which $C_{2z}$ is broken, giving rise to flat topological bands carrying valley-Chern numbers $C=\pm(1,-2)$. These bands feature near-ideal quantum geometry and are isolated from remote bands by a large gap $E_{\mathrm{gap}}\sim 100$ meV, making HTG a promising platform for experimental realization of correlated topological states such as integer and fractional quantum anomalous Hall states in $C=1$ and $2$ bands.

cond-mat.str-el↗

arXiv2022

Exact Many-Body Ground States from Decomposition of Ideal Higher Chern Bands: Applications to Chirally Twisted Graphene Multilayers

Motivated by the higher Chern bands of twisted graphene multilayers, we consider flat bands with arbitrary Chern number $C$ with ideal quantum geometry. While $C>1$ bands differ from Landau levels, we show that these bands host exact fractional Chern insulator (FCI) ground states for short range interactions. We show how to decompose ideal higher Chern bands into separate ideal bands with Chern number $1$ that are intertwined through translation and rotation symmetry. The decomposed bands admit an $SU(C)$ action that combines real space and momentum space translations. Remarkably, they also allow for analytic construction of exact many-body ground states, such as generalized quantum Hall ferromagnets and FCIs, including flavor-singlet Halperin states and Laughlin ferromagnets in the limit of short-range interactions. In this limit, the $SU(C)$ action is promoted to a symmetry on the ground state subspace. While flavor singlet states are translation symmetric, the flavor ferromagnets correspond to translation broken states and admit charged skyrmion excitations corresponding to a spatially varying density wave pattern. We confirm our analytic predictions with numerical simulations of ideal bands of twisted chiral multilayers of graphene, and discuss consequences for experimentally accessible systems such as monolayer graphene twisted relative to a Bernal bilayer.

cond-mat.mes-hall↗

arXiv2022

Family of ideal Chern flat bands with arbitrary Chern number in chiral twisted graphene multilayers

We consider a family of twisted graphene multilayers consisting of $n$-untwisted chirally stacked layers, e.g., AB, ABC, etc, with a single twist on top of $m$-untwisted {chirally stacked} layers. Upon neglecting both trigonal warping terms for the untwisted layers and the same sublattice hopping between all layers, the resulting models generalize several remarkable features of the chiral model of twisted bilayer graphene (CTBG). In particular, they exhibit a set of magic angles which are identical to those of CTBG at which a pair of bands (i) are perfectly flat, (ii) have Chern numbers in the sublattice basis given by $\pm (n,-m)$ or $\pm (n + m - 1, -1)$ depending on the stacking chirality, and (iii) satisfy the trace condition, saturating an inequality between the quantum metric and the Berry curvature, and thus realizing ideal quantum geometry. These are the first higher Chern bands that satisfy (iii) beyond fine-tuned models or combinations of Landau levels. We show that ideal quantum geometry is directly related to the construction of fractional quantum Hall model wavefunctions. We provide explicit analytic expressions for the flat band wavefunctions at the magic angle in terms of the CTBG wavefunctions. We also show that the Berry curvature distribution in these models can be continuously tuned while maintaining perfect quantum geometry. Similar to the study of fractional Chern insulators in ideal $C = 1$ bands, these models pave the way for investigating exotic topological phases in higher Chern bands for which no Landau level analog is available.

cond-mat.str-el↗

arXiv2022

Dirac cone spectroscopy of strongly correlated phases in twisted trilayer graphene

Mirror-symmetric magic-angle twisted trilayer graphene (MATTG) hosts flat electronic bands close to zero energy, and has been recently shown to exhibit abundant correlated quantum phases with flexible electrical tunability. However studying these phases proved challenging as these are obscured by intertwined Dirac bands. In this work, we demonstrate a novel spectroscopy technique, that allows to quantify the energy gaps and Chern numbers of the correlated states in MATTG by driving band crossings between Dirac cone Landau levels and the energy gaps in the flat bands. We uncover hard correlated gaps with Chern numbers of C = 0 at integer moire unit cell fillings of nu = 2 and 3 and reveal novel charge density wave states originating from van Hove singularities at fractional fillings of nu = 5/3 and 11/3. In addition, we demonstrate the existence of displacement field driven first-order phase transitions at charge neutrality and half fillings of the moire unit cell nu = 2, which is consistent with a theoretical strong-coupling analysis, implying the breaking of the C2T symmetry. Overall these properties establish the diverse and electrically tunable phase diagram of MATTG and provide an avenue for investigating electronic quantum phases and strong correlations in multiple-band moire systems.

cond-mat.mes-hall↗

arXiv2021

Strong Coupling Theory of Magic-Angle Graphene: A Pedagogical Introduction

We give a self contained review of a recently developed strong coupling theory of magic-angle graphene. An advantage of this approach is that a single formulation can capture both the insulating and superconducting states, and with a few simplifying assumptions, can be treated analytically. We begin by reviewing the electronic structure of magic angle graphene's flat bands, in a limit that exposes their peculiar band topology and geometry. We highlight how similarities between the flat bands and the lowest Landau level give insight into the effect of interactions. For example, at certain fractional fillings, we note the promise for realizing fractional Chern states. At integer fillings, this approach points to flavor ordered insulators, which can be captured by a sigma-model in its ordered phase. Unexpectedly, topological textures of the sigma model carry electric charge which allows us to extend the same theory to describe the doped phases away from integer filling. We show how this approach can lead to superconductivity on disordering the sigma model, and estimate the T$_c$ for the superconductor. We highlight the important role played by an effective super-exchange coupling both in pairing and in setting the effective mass of Cooper pairs. Seeking to enhance this coupling helps predict new superconducting platforms, including the recently discovered alternating twist trilayer platform. We also contrast our proposal from strong coupling theories for other superconductors.

cond-mat.str-el↗

arXiv2021

TB or not TB? Contrasting properties of twisted bilayer graphene and the alternating twist $n$-layer structures ($n=3, 4, 5, \dots$)

The emergence of alternating twist multilayer graphene (ATMG) as a generalization of twisted bilayer graphene (TBG) raises the question - in what important ways do these systems differ? Here, we utilize ab-initio relaxation and single-particle theory, analytical strong coupling analysis, and Hartree-Fock to contrast ATMG with $n=3,4,5,\ldots$ layers and TBG. $\textbf{I}$: We show how external fields enter in the decomposition of ATMG into TBG and graphene subsystems. The parallel magnetic field has little effect for $n$ odd due to mirror symmetry, but surprisingly also for any $n > 2$ if we are are the largest magic angle. $\textbf{II}$: We compute the relaxation of the multilayers leading to effective parameters for each TBG subsystem. We find that the second magic angle for $n=5$, $θ= 1.14$ is closest to the "chiral" model and thus may be an optimal host for fractional Chern insulators. $\textbf{III}$: We integrate out non-magic subsystems and reduce ATMG to its magic angle TBG subsystem with a screened interaction. $\textbf{IV}$: We perform an analytic strong coupling analysis of the external fields and corroborate it with Hartree-Fock. An in-plane magnetic field in TBG drives a transition to a valley Hall state or a gapless "magnetic semimetal." A displacement field $V$ has little effect on TBG, but induces a gapped phase in ATMG for small $V$ for $n = 4$ and above a critical $V$ for $n = 3$. For $n\geq 3$, we find the superexchange coupling - believed to set the scale of superconductivity in the skyrmion mechanism - increases with $V$ at angles near and below the magic angle. $\textbf{V}$: We complement our strong coupling approach with a weak coupling theory of ATMG pair-breaking. While for $n=2$ orbital effects of the in-plane magnetic field can give a critical field of the same order as the Pauli field, for $n>2$ we expect the critical field to exceed the Pauli limit.

cond-mat.str-el↗

arXiv2020

Fractional Chern Insulator States in Twisted Bilayer Graphene: An Analytical Approach

Recent experiments on bilayer graphene twisted near the magic angle have observed spontaneous integer quantum Hall states in the presence of an aligned hexagonal boron nitride (hBN) substrate. These states arise from valley ferromagnetism, and the complete filling of Chern bands. A natural question is whether fractional filling of the same bands would lead to fractional quantum Hall states, i.e. to fractional Chern insulators (FCIs). Here, we argue that the magic angle graphene bands have favorable quantum geometry for realizing FCI phases. We show that in the tractable `chiral' limit, the flat bands wavefunctions are an analytic function of the crystal momentum. This remarkable property fixes the quantum metric up to an overall momentum dependent scale factor, the local Berry curvature, whose variation is itself small. Thus the three conditions associated with FCI stability (i) narrow bands (ii) quantum metric satisfying the isotropic ideal droplet condition and (iii) relatively uniform Berry curvature are all satisfied. Our work emphasizes continuum real space approaches to FCIs in contrast to earlier works which mostly focused on tight binding models. This enables us to construct a Laughlin wavefunction on a real-space torus that is a zero energy ground state of pseudopotential-like interactions. The latter can be realized for Coulomb interaction in the limit of very short screening length. We also map the problem onto an equivalent one of Dirac particles in an inhomogeneous but periodic magnetic field and no crystal potential. Finally we discuss evolution of the band geometry on tuning away from the chiral limit and show numerically that some of the desirable properties continue to hold at a quantitative level for realistic parameter values.

cond-mat.str-el↗