Showing 1–50 of 565 results for author: MacDonald, A H

Transition Metal Dichalcogenide Excitons in Periodic Electrostatic Potentials: Center-of-Mass Models

Authors: Jose M. Torres-Lopez, Sudipta Kundu, Felipe H. da Jornada, Tony Heinz, Allan H. MacDonald

Abstract: Two-dimensional (2D) van-der-Waals materials are a promising platform for exciton state engineering. In this paper, we study the properties of excitons in 2D group VI transition-metal dichalcogenide (TMD) semiconductors that are modified by a periodic electrostatic potential through the quadratic Stark effect. Using a model that retains only center-of-mass and valley degrees-of-freedom, we find th… ▽ More Two-dimensional (2D) van-der-Waals materials are a promising platform for exciton state engineering. In this paper, we study the properties of excitons in 2D group VI transition-metal dichalcogenide (TMD) semiconductors that are modified by a periodic electrostatic potential through the quadratic Stark effect. Using a model that retains only center-of-mass and valley degrees-of-freedom, we find that electrostatic potentials can drive optical valley splitting up to 10meVs and induce valley selective exciton dispersion. We explain why both properties are sensitive to the rotational symmetry of the electrostatic trapping potential using a combination of numerical results and analytical approximations. An important consequence of valley-splitting is that the lowest exciton band is non-degenerate and has a linear dispersion around $γ$ that is expected to suppress thermal excitations, allowing true Bose condensation and superfluidity of excitons in two space dimensions. △ Less

Submitted 6 May, 2026; v1 submitted 4 May, 2026; originally announced May 2026.

Comments: 10 pages, 10 figures

Half-quantized anomalous Hall conductance in topological insulator/ferromagnet van der Waals heterostructures

Authors: Shahid Sattar, Roman Stepanov, Alexander Tyner, M. F. Islam, A. H. MacDonald, C. M. Canali

Abstract: The half-quantized anomalous Hall conductance (AHC) in topological materials is a condensed matter physics realization of the parity anomaly of (2+1) quantum field theory and an important challenge for both theoretical and experimental research. A possible realization of this phenomenon may be achieved by interfacing a two-dimensional (2D) ferromagnetic (FM) layer with one surface of a thin slab o… ▽ More The half-quantized anomalous Hall conductance (AHC) in topological materials is a condensed matter physics realization of the parity anomaly of (2+1) quantum field theory and an important challenge for both theoretical and experimental research. A possible realization of this phenomenon may be achieved by interfacing a two-dimensional (2D) ferromagnetic (FM) layer with one surface of a thin slab of a topological insulator (TI), which breaks the otherwise conserved time-reversal symmetry, leading to a gap opening in the Dirac-like energy spectrum of the TI surface states. The resulting heterostructure can support chiral currents where only one spin channel contributes to transport, producing a half-quantized Hall conductance ($e^2/2h$). In this work, using first-principles methods together with tight-binding models, we investigate the magnetization-induced gap, the properties of the sidewalls states, and Hall conductance in three different FI/TI van der Waals heterostructures that are relevant for ongoing experiments. We also discuss the factors that can hinder the realization of exact half-quantization in a realistic system and their implication for the quantum anomalous Hall effect and the topological magnetoelectric effect. △ Less

Submitted 12 April, 2026; originally announced April 2026.

Comments: 9 pages, 5 figures

Observation of two-component exciton condensates in an excitonic insulator

Authors: Ruishi Qi, Qize Li, Jiahui Nie, Ruichen Xia, Haleem Kim, Hyungbin Lim, Jingxu Xie, Takashi Taniguchi, Kenji Watanabe, Michael F. Crommie, Allan H. MacDonald, Feng Wang

Abstract: Macroscopic quantum coherence emerges when bosons condense into a Bose-Einstein condensate (BEC). First observed as a single-component superfluid in helium, BECs later emerged in ultracold atomic gases at nanokelvin temperatures as weakly interacting quantum fluids, which can also host multicomponent spinor condensates with rich internal degrees of freedom. Excitons provide a promising solid-state… ▽ More Macroscopic quantum coherence emerges when bosons condense into a Bose-Einstein condensate (BEC). First observed as a single-component superfluid in helium, BECs later emerged in ultracold atomic gases at nanokelvin temperatures as weakly interacting quantum fluids, which can also host multicomponent spinor condensates with rich internal degrees of freedom. Excitons provide a promising solid-state platform for BECs that can combine strong interactions, electrical tunability, high transition temperatures, and multicomponent order. Yet, conclusive evidence for condensation has remained elusive. Here, we report evidence of two-component exciton BECs in MoSe2/hBN/WSe2 electron-hole bilayers by directly probing the spin susceptibility of constituent electrons and holes. This heterostructure hosts equilibrium exciton fluids with four spin-valley flavors. Using magneto-optical spectroscopy in a dilution refrigerator, we reveal three exciton condensate phases with distinct flavor polarizations. At zero magnetic field, the many-body ground state is a coherent superposition of two simultaneously condensed intravalley exciton flavors. Under a magnetic field, the intravalley exciton condensate first switches to a two-component intervalley exciton condensate via a first-order quantum phase transition at a weak critical field, and then turns into a fully-polarized single-component condensate at high fields. The two-component condensates persist up to ~1.8 K. Our results establish van der Waals electron-hole bilayers as a versatile platform for exploring strongly interacting, multicomponent exciton BECs. △ Less

Submitted 16 March, 2026; originally announced March 2026.

Magneto-optical Response of 5-SL MnBi$_2$Te$_4$ in Spin-Flip States

Authors: Shahid Sattar, Roman Stepanov, A. H. MacDonald, C. M. Canali

Abstract: Magneto-optical effects like Kerr and Faraday rotations provide a direct probe of topological order in thin films of the magnetic topological insulator MnBi$_2$Te$_4$ (MBT). Motivated by recent experimental studies of spin-flip/flop transitions in MBT thin films, we investigate the interplay between interlayer spin configurations, topological order, and magneto-optical response in five septuple-la… ▽ More Magneto-optical effects like Kerr and Faraday rotations provide a direct probe of topological order in thin films of the magnetic topological insulator MnBi$_2$Te$_4$ (MBT). Motivated by recent experimental studies of spin-flip/flop transitions in MBT thin films, we investigate the interplay between interlayer spin configurations, topological order, and magneto-optical response in five septuple-layer (5-SL) MBT using first-principles calculations and a simplified coupled-Dirac-cone model. Our results reveal that, despite possessing a non-zero out-of-plane magnetization, 5-SL MBT thin films can be either ${\cal C}=+1$ topological insulators or ${\cal C}=0$ topologically trivial insulators depending on the relative spin orientations of the top and bottom SLs. We evaluate the Faraday and Kerr rotation angles using tight-binding models derived from \textit{ab-initio} calculations and by comparing our results with those of a simplified coupled Dirac-cone model clarify the macroscopic mechanisms underlying the magneto-optical response of spin-flip states. These theoretical findings highlight the tunability of topological and magneto-optical properties in MBT thin films and provide microscopic insight into the emergence of complex topological order in layered antiferromagnetic materials. △ Less

Submitted 10 March, 2026; originally announced March 2026.

Comments: 10 pages, 7 figures, Supplementary Information

Charge-ordered states in twisted MoTe$_2$

Authors: Sparsh Mishra, Tobias M. R. Wolf, Allan H. MacDonald

Abstract: We analyze interaction-driven charge-density-wave (CDW) states in the spin-valley polarized first valence miniband of twisted MoTe$_2$ (tMoTe$_2$) using an adiabatic mapping from the continuum model to an effective Landau-level (LL) problem. When projected to the lowest LL, the leading spatial harmonic of the moiré-periodic potential changes sign at a magic twist angle $θ_c$ where the band reaches… ▽ More We analyze interaction-driven charge-density-wave (CDW) states in the spin-valley polarized first valence miniband of twisted MoTe$_2$ (tMoTe$_2$) using an adiabatic mapping from the continuum model to an effective Landau-level (LL) problem. When projected to the lowest LL, the leading spatial harmonic of the moiré-periodic potential changes sign at a magic twist angle $θ_c$ where the band reaches its minimum bandwidth. By solving self-consistent Hartree-Fock equations in a multi-LL Hilbert space, we find that triangular-lattice CDW states with density maxima on MX (or XM) sites or on MM sites are favored on opposite sides of the magic angle at most filling factors and that stripe order appears near $ν_h=1/2$. We show that CDW states at $ν_h >1/2$ can carry a nonzero total Chern number, providing a natural route to reentrant integer quantum Hall effects and discuss the energy competition between fractional Chern insulator and CDW states. △ Less

Submitted 9 March, 2026; v1 submitted 5 March, 2026; originally announced March 2026.

Comments: 6 pages, 3 figures, Supplementary material 9 pages (typos corrected)

Microwave Imaging of Edge Conductivity in Graphene at Charge Neutrality and Quantum Hall States

Authors: Hongtao Yan, Chun-Chih Tseng, Anzhuoer Li, Manish Kumar, Kaile Wang, Shizai Chu, Kenji Watanabe, Takashi Taniguchi, Allan H. MacDonald, Matthew Yankowitz, Keji Lai

Abstract: We report local conductivity imaging of edge states in monolayer graphene by millikelvin microwave impedance microscopy (MIM). At the charge-neutrality point, as the magnetic field increases, the local conductivity at the edge drops to zero more slowly than in the bulk. This behavior is consistent with the calculated spatial profile of the charge gap in the canted antiferromagnetic phase. For comp… ▽ More We report local conductivity imaging of edge states in monolayer graphene by millikelvin microwave impedance microscopy (MIM). At the charge-neutrality point, as the magnetic field increases, the local conductivity at the edge drops to zero more slowly than in the bulk. This behavior is consistent with the calculated spatial profile of the charge gap in the canted antiferromagnetic phase. For comparison, we also perform microwave imaging of integer quantum Hall states away from neutrality, which host dissipationless chiral edge channels. The evolution of the edge signal as a function of the bulk gap is fundamentally different between the Landau level filling factor $ν= 0$ and $|ν| \ge 1$ integer quantum Hall states, which can be qualitatively explained by numerical simulations and theoretical analysis. Our results provide a comprehensive microscopic picture of the edge and bulk states as the Fermi level moves across the unique Landau-level spectrum of graphene. △ Less

Submitted 19 February, 2026; originally announced February 2026.

Journal ref: Phys. Rev. Lett. 136, 176603 (2026)

Stripe antiferromagnetism and chiral superconductivity in tWSe$_2$

Authors: Erekle Jmukhadze, Sam Olin, Allan H. MacDonald, Wei-Cheng Lee

Abstract: The layer-dependent Hamiltonians of parallel-stacked MoTe$_2$ and WSe$_2$ homobilayer moiré materials are topologically non-trivial, both in real space and in momentum space, and have been shown to support integer and fractional quantum anomalous Hall states, as well as antiferromagnetic and superconducting states. Here, we address the interplay between the antiferromagnetic and superconducting st… ▽ More The layer-dependent Hamiltonians of parallel-stacked MoTe$_2$ and WSe$_2$ homobilayer moiré materials are topologically non-trivial, both in real space and in momentum space, and have been shown to support integer and fractional quantum anomalous Hall states, as well as antiferromagnetic and superconducting states. Here, we address the interplay between the antiferromagnetic and superconducting states observed in tWSe$_2$ when the Fermi level is close to its $M$-point van Hove singularity and the displacement field is small. We combine DFT with path-integrals to construct a minimal moiré band model that accounts for lattice relaxation along the $c$-axis and perform Hartree-Fock calculations to identify competing charge and spin ordered states. For tWSe$_2$ at $θ=2.7^\circ$ and $θ=3.65^\circ$, we find that a layer antiferromagnet (AFM), a stripe spin-density-wave (SDW), and the ferromagnetic Chern insulator (FM) are the primary candidates for the ground state at zero displacement field, and argue that antiferromagnetic spin interactions on the next neighbor bond $J_2$ can induce a time-reversal symmetry breaking chiral superconducting state. △ Less

Submitted 8 July, 2026; v1 submitted 28 January, 2026; originally announced January 2026.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. B 114, 045110 (2026)

Polariton-induced superconductivity in two-dimensional metals

Authors: Riccardo Riolo, Frank H. L. Koppens, Pablo Jarillo-Herrero, Giacomo Mazza, Allan H. MacDonald, Marco Polini

Abstract: The electronic properties of two-dimensional (2D) metals are altered by changes in their three-dimensional dielectric environment. In this Letter we propose that superconductivity can be induced in a 2D metal by resonant coupling between its plasmonic collective modes and optical phonons in a nearby polar dielectric. Specifically, we predict that relatively high-temperature superconductivity can b… ▽ More The electronic properties of two-dimensional (2D) metals are altered by changes in their three-dimensional dielectric environment. In this Letter we propose that superconductivity can be induced in a 2D metal by resonant coupling between its plasmonic collective modes and optical phonons in a nearby polar dielectric. Specifically, we predict that relatively high-temperature superconductivity can be induced in bilayer graphene twisted to an angle somewhat larger than the magic value by surrounding it with a THz polar dielectric. Our conclusions are based on numerical solutions of Eliashberg equations for massless Dirac fermions with tunable Fermi velocities and Fermi energies, and can be understood qualitatively in terms of a generalized McMillan formula. △ Less

Submitted 1 November, 2025; originally announced November 2025.

Comments: 16 pages, 9 figures

Exciton-based sensing of remote electron correlations in 2D heterostructures

Authors: Tobias M. R. Wolf, Tian Xie, Chenhao Jin, Allan H. MacDonald

Abstract: Many monolayer transition metal dichalcogenides, including MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, are direct bandgap two-dimensional (2D) semiconductors with sharp optical resonances at excitonic bound state frequencies. Recent experiments have demonstrated that excitonic resonance frequencies in multilayer van der Waals stacks are altered by long-range Coulomb interactions with electrons in near… ▽ More Many monolayer transition metal dichalcogenides, including MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, are direct bandgap two-dimensional (2D) semiconductors with sharp optical resonances at excitonic bound state frequencies. Recent experiments have demonstrated that excitonic resonance frequencies in multilayer van der Waals stacks are altered by long-range Coulomb interactions with electrons in nearby but electrically isolated 2D materials. These modulations have been successfully used to detect transitions between distinct states of remote strongly correlated 2D electron fluids. In this Letter we provide a theory of these frequency shifts, enabling a more quantitative interpretation of excitonic-sensing experiments, and apply it as an example to WSe$_2$ that is proximate to graphene bilayers and multilayers. △ Less

Submitted 24 October, 2025; originally announced October 2025.

Comments: Main: 5 pages, 4 figures; Supmat: 6 pages, 5 figures. Comments are welcome!

Topological excitonic insulators in electron bilayers modulated by twisted hBN

Authors: Yongxin Zeng, Allan H. MacDonald, Nemin Wei

Abstract: Equilibrium interlayer exciton condensation is common in bilayer quantum Hall systems and is characterized by spontaneous phase coherence between isolated layers. It has been predicted that similar physics can occur in the absence of a magnetic field in some two-dimensional semiconductor bilayers. In this work we consider the case of two transition metal dichalcogenide (TMD) monolayers separated b… ▽ More Equilibrium interlayer exciton condensation is common in bilayer quantum Hall systems and is characterized by spontaneous phase coherence between isolated layers. It has been predicted that similar physics can occur in the absence of a magnetic field in some two-dimensional semiconductor bilayers. In this work we consider the case of two transition metal dichalcogenide (TMD) monolayers separated by a twisted hexagonal boron nitride (hBN) bilayer or multilayer. The hBN layers suppress tunneling between the TMD layers so that phase coherence is spontaneous when it is present. When twisted, the hBN layers also form a ferroelectric moiré pattern that applies opposite triangular-lattice modulation potentials to the two TMD layers. We show via mean-field theory that at total hole filling per moiré unit cell $ν=1$, this geometry can favor a chiral p-wave exciton condensate state in which the quantum anomalous Hall effect coexists with counter-flow superfluidity. We present a mean-field phase diagram for TMD hole bilayers modulated by twisted hBN, discuss the conditions needed for the realization of the p-wave condensate state, and propose experiments that could confirm its presence. △ Less

Submitted 13 September, 2025; originally announced September 2025.

Spontaneous Twirls and Structural Frustration in Moiré Materials

Authors: Jingtian Shi, Gaurav Chaudhary, Allan H. MacDonald, Ivar Martin

Abstract: Structural twirls form spontaneously in the domain wall networks of some moiré materials. We show that in heterobilayers, neighboring twirl chiralities tend to anti-align, forming staggered patterns that are well described by antiferromagnetic lattice $φ^4$ theories. In moiré systems with triangular domains, this leads to frustration in the chirality configuration of the structural twirls and to h… ▽ More Structural twirls form spontaneously in the domain wall networks of some moiré materials. We show that in heterobilayers, neighboring twirl chiralities tend to anti-align, forming staggered patterns that are well described by antiferromagnetic lattice $φ^4$ theories. In moiré systems with triangular domains, this leads to frustration in the chirality configuration of the structural twirls and to hysteresis with respect to variation of the average twist angle and possibly other control parameters. △ Less

Submitted 13 September, 2025; originally announced September 2025.

Journal ref: Phys. Rev. Lett. 136, 026101 (2026)

Valley Order in Moiré Topological Insulators

Authors: Bo Zou, Anzhuoer Li, Allan H. MacDonald

Abstract: Moiré materials with opposite non-zero miniband Chern numbers in time-reversal-partner valleys are two-dimensional topological insulators at band filling $ν=2$. We explore the possibility that in this class of moir'e materials intervalley coherence can sometimes be present in interaction induced insulators at band filling $ν=1$ , using Landau levels with opposite signs of the magnetic field as a c… ▽ More Moiré materials with opposite non-zero miniband Chern numbers in time-reversal-partner valleys are two-dimensional topological insulators at band filling $ν=2$. We explore the possibility that in this class of moir'e materials intervalley coherence can sometimes be present in interaction induced insulators at band filling $ν=1$ , using Landau levels with opposite signs of the magnetic field as a convenient generic model. In the absence of intravalley interactions the mean-field ground state at filling factor $ν=1$ is a gapless intervalley coherent state that maps under a particle-hole transformation of one valley to a strong-field superconducting vortex-lattice state that has been studied previously. When the ratio $λ$ of intravalley to intervalley interactions is increased, gapped states appear, one with broken time-reversal symmetry and a quantized Hall effect but no valley polarization and one with broken parity symmetry and zero Hall conductivity. We discuss the possibility that the latter state could be related to the fractional quantum spin Hall effect recently observed at an odd filling factor in a moir'e topological insulator and comment on related systems in which correlations between electrons in bands with opposite Chern numbers might play a key role. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Quantum Phases in Twisted Homobilayer Transition Metal Dichalcogenides

Authors: Bohao Li, Wen-Xuan Qiu, Fengcheng Wu, A. H. MacDonald

Abstract: Twisted homobilayer transition metal dichalcogenides - specifically twisted bilayer MoTe$_2$ and twisted bilayer WSe$_2$ - have recently emerged as a versatile platform for strongly correlated and topological phases of matter. These two-dimensional systems host tunable flat Chern bands in which Coulomb interactions can dominate over kinetic energy, giving rise to a variety of interaction-driven ph… ▽ More Twisted homobilayer transition metal dichalcogenides - specifically twisted bilayer MoTe$_2$ and twisted bilayer WSe$_2$ - have recently emerged as a versatile platform for strongly correlated and topological phases of matter. These two-dimensional systems host tunable flat Chern bands in which Coulomb interactions can dominate over kinetic energy, giving rise to a variety of interaction-driven phenomena. A series of groundbreaking experiments have revealed a rich landscape of quantum phases, including integer and fractional quantum anomalous Hall states, quantum spin Hall states, anomalous Hall metals, zero-field composite Fermi liquids, and unconventional superconductors, along with more conventional topologically trivial correlated states including antiferromagnets. This review surveys recent experimental discoveries and theoretical progress in understanding these phases, with a focus on the key underlying mechanisms - band topology, electron interactions, symmetry breaking, and charge fractionalization. We emphasize the unique physics of twisted TMD homobilayers in comparison to other related systems, discuss open questions, and outline promising directions for future research. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Comments: 19 pages, 5 figures

Journal ref: National Science Review 13, nwaf570 (2026)

The Many-Body Ground State Manifold of Flat Band Interacting Hamiltonian for Magic Angle Twisted Bilayer Graphene

Authors: Kevin D. Stubbs, Michael Ragone, Allan H. MacDonald, Lin Lin

Abstract: At a magic relative twist angle, magic angle twisted bilayer graphene (MATBG) has an octet of flat bands that can host strong correlation physics when partially filled. A key theoretical discovery in MATBG is the existence of ferromagnetic Slater determinants as exact ground states of the corresponding flat band interacting (FBI) Hamiltonian. The FBI Hamiltonian describes the behavior of electrons… ▽ More At a magic relative twist angle, magic angle twisted bilayer graphene (MATBG) has an octet of flat bands that can host strong correlation physics when partially filled. A key theoretical discovery in MATBG is the existence of ferromagnetic Slater determinants as exact ground states of the corresponding flat band interacting (FBI) Hamiltonian. The FBI Hamiltonian describes the behavior of electrons that interact with each other in a high-dimensional space, and is constructed from the band structure of the non-interacting Bistritzer--MacDonald model at the chiral limit. A key property of the FBI Hamiltonian for MATBG is that it is frustration free and can be written as a sum of non-commuting terms. In this work, we provide a complete characterization of the ground state manifold of the FBI Hamiltonian, proving that it is precisely the linear span of such ferromagnetic Slater determinants. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Comments: 39 pages

Theory of magnetoroton bands in moiré materials

Authors: Bishoy M. Kousa, Nicolás Morales-Durán, Tobias M. R. Wolf, Eslam Khalaf, Allan H. MacDonald

Abstract: The recent realization of Hofstadter spectra and fractional Chern insulators in moiré materials has introduced a new ingredient, a periodic lattice potential, to the study of quantum Hall phases. While the fractionalized states in moiré systems are expected to be in the same universality class as their counterparts in Landau levels, the periodic potential can have qualitative and quantitative effe… ▽ More The recent realization of Hofstadter spectra and fractional Chern insulators in moiré materials has introduced a new ingredient, a periodic lattice potential, to the study of quantum Hall phases. While the fractionalized states in moiré systems are expected to be in the same universality class as their counterparts in Landau levels, the periodic potential can have qualitative and quantitative effects on physical observables. Here, we examine how the magnetoroton collective modes of fractional quantum Hall (FQH) states are altered by external periodic potentials. Employing a single-mode-approximation, we derive an effective Hamiltonian for the low-energy neutral excitations expressed in terms of three-point density correlation functions, which are computed using Monte Carlo. Our analysis is applicable to FQH states in graphene with a hexagonal boron nitride (hBN) substrate and also to fractional Chern insulator (FCI) states in twisted MoTe$_2$ bilayers. We predict experimentally testable trends in the THz absorption characteristics of FCI and FQH states and estimate the external potential strength at which a soft-mode phase transition occurs between FQH and charge density wave states. △ Less

Submitted 23 April, 2026; v1 submitted 24 February, 2025; originally announced February 2025.

Comments: 13 pages, 7 figures

Journal ref: Phys. Rev. Lett. 135, 246604 (2025)

Competition between excitonic insulators and quantum Hall states in correlated electron-hole bilayers

Authors: Ruishi Qi, Qize Li, Zuocheng Zhang, Zhiyuan Cui, Bo Zou, Haleem Kim, Collin Sanborn, Sudi Chen, Jingxu Xie, Takashi Taniguchi, Kenji Watanabe, Michael F. Crommie, Allan H. MacDonald, Feng Wang

Abstract: Excitonic insulators represent a unique quantum phase of matter, providing a rich ground for studying exotic quantum bosonic states. Strongly coupled electron-hole bilayers, which host stable dipolar exciton fluids with an exciton density that can be adjusted electrostatically, offer an ideal platform to investigate correlated excitonic insulators. Based on electron-hole bilayers made of MoSe2/hBN… ▽ More Excitonic insulators represent a unique quantum phase of matter, providing a rich ground for studying exotic quantum bosonic states. Strongly coupled electron-hole bilayers, which host stable dipolar exciton fluids with an exciton density that can be adjusted electrostatically, offer an ideal platform to investigate correlated excitonic insulators. Based on electron-hole bilayers made of MoSe2/hBN/WSe2 heterostructures, here we study the behavior of excitonic insulators in a perpendicular magnetic field. We report the observation of excitonic quantum oscillations in both Coulomb drag signals and electrical resistance at low to medium magnetic fields. Under a strong magnetic field, we identify multiple quantum phase transitions between the excitonic insulator phase and the bilayer quantum Hall insulator phase. These findings underscore the interplay between the electron-hole interactions and Landau level quantization that opens new possibilities for exploring quantum phenomena in composite bosonic insulators. △ Less

Submitted 30 January, 2025; originally announced January 2025.

Quantum oscillations in a dipolar excitonic insulator

Authors: Phuong X. Nguyen, Raghav Chaturvedi, Bo Zou, Kenji Watanabe, Takashi Taniguchi, Allan H. MacDonald, Kin Fai Mak, Jie Shan

Abstract: Quantum oscillations in magnetization or resistivity are a defining feature of metals subject to an external magnetic field. The phenomenon is generally not expected in insulators without a Fermi surface. The observations of quantum oscillations in Kondo insulating materials have provided a rare counterexample and attracted much theoretical interest. However, the magnetic oscillations in correlate… ▽ More Quantum oscillations in magnetization or resistivity are a defining feature of metals subject to an external magnetic field. The phenomenon is generally not expected in insulators without a Fermi surface. The observations of quantum oscillations in Kondo insulating materials have provided a rare counterexample and attracted much theoretical interest. However, the magnetic oscillations in correlated insulators remain poorly understood. Here we report the observations of resistivity quantum oscillations in an excitonic insulator realized in Coulomb-coupled electron-hole double layers with gate-tunability that allows the phenomenon to be explored in a more controllable fashion than in bulk materials. When the cyclotron energy of the electrons or holes is tuned to be comparable to or larger than the exciton binding energy, recurring transitions between excitonic insulators and electron-hole decoupled quantum Hall states are observed. Compressibility measurements show an oscillatory exciton binding energy as a function of magnetic field and electron-hole pair density. Coulomb drag measurements further reveal the formation of excitons with finite angular momentum. Our results are qualitatively captured by mean-field theory calculations. The study demonstrates a new platform for studying quantum oscillations in correlated insulators. △ Less

Submitted 29 January, 2025; originally announced January 2025.

Moiré band theory for M-valley twisted transition metal dichalcogenides

Authors: Chao Lei, Perry T. Mahon, Allan H. MacDonald

Abstract: We propose twisted bilayers of certain group IV and IVB trigonal transition metal dichalcogenides (TMDs) MX$_{2}$ (M$=$Zr, Hf, Sn and X$=$S, Se) as moiré materials. In monolayer form these TMDs have conduction band minima near the three inequivalent Brillouin zone $M$ points and negligible spin-orbit coupling, implying six flavors of low-energy conduction band states. The flavor sectors decouple a… ▽ More We propose twisted bilayers of certain group IV and IVB trigonal transition metal dichalcogenides (TMDs) MX$_{2}$ (M$=$Zr, Hf, Sn and X$=$S, Se) as moiré materials. In monolayer form these TMDs have conduction band minima near the three inequivalent Brillouin zone $M$ points and negligible spin-orbit coupling, implying six flavors of low-energy conduction band states. The flavor sectors decouple at the single-particle level and in twisted bilayers are accurately described by emergent moiré-periodic Hamiltonians that we derive from small-unit-cell density functional theory calculations. Because the valley-projected Hamiltonians have large valley-dependent mass anisotropies and are time-reversal invariant, spontaneous valley polarization is signaled in transport by anisotropy instead of by the anomalous Hall and magnetic circular dichroism signals commonly observed in graphene and $K$-valley TMD-based moiré multilayers. △ Less

Submitted 6 November, 2025; v1 submitted 27 November, 2024; originally announced November 2024.

Comments: 5+2+7 pages, 4+9 figures

Journal ref: Phys. Rev. Lett. 135, 196402 (2025)

Independently Tunable Flat Bands and Correlations in a Graphene Double Moiré System

Authors: Yimeng Wang, Jihang Zhu, G. William Burg, Anand Swain, Kenji Watanabe, Takashi Taniguchi, Yuebing Zheng, Allan H. MacDonald, Emanuel Tutuc

Abstract: We report on a double moiré system consisting of four graphene layers, where the top and bottom pairs form small-twist-angle bilayer graphene, and the middle interface has a large rotational mismatch. This system shows clear signatures of two sets of spatially separated flat bands associated with the top and bottom twisted bilayer graphene (TBG) subsystems, each independently tunable. Thermodynami… ▽ More We report on a double moiré system consisting of four graphene layers, where the top and bottom pairs form small-twist-angle bilayer graphene, and the middle interface has a large rotational mismatch. This system shows clear signatures of two sets of spatially separated flat bands associated with the top and bottom twisted bilayer graphene (TBG) subsystems, each independently tunable. Thermodynamic analysis reveals weak correlations between layers that allow the chemical potential to be measured as a function of carrier density for each constituent TBG. We find that correlated insulating states at integer number of electrons per moiré unit cell are most robust near magic angle, whereas gapped states at neutrality are more robust at larger twist angles. △ Less

Submitted 27 November, 2024; originally announced November 2024.

Comments: 6 pages, 4 figures, includes supplemental material

Journal ref: Phys. Rev. Lett. 134, 096204 (2025)

Vortex lattice states of bilayer electron-hole fluids in quantizing magnetic fields

Authors: Bo Zou, Allan H. MacDonald

Abstract: We show that the ground state of a weakly charged two-dimensional electron-hole fluid in a strong magnetic field is a broken translation symmetry state with interpenetrating lattices of localized vortices and antivortices in the electron-hole-pair field. The vortices and antivortices carry fractional charges of equal sign but unequal magnitude and have a honeycomb lattice structure that contrasts… ▽ More We show that the ground state of a weakly charged two-dimensional electron-hole fluid in a strong magnetic field is a broken translation symmetry state with interpenetrating lattices of localized vortices and antivortices in the electron-hole-pair field. The vortices and antivortices carry fractional charges of equal sign but unequal magnitude and have a honeycomb lattice structure that contrasts with the triangular lattices of superconducting electron-electron-pair vortex lattices. We predict that increasing charge density or weakening magnetic field drives a vortex delocalization transition that would be signaled experimentally by an abrupt increase in counterflow transport resistance. △ Less

Submitted 27 May, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Altermagnetism: an unconventional spin-ordered phase of matter

Authors: T. Jungwirth, R. M. Fernandes, E. Fradkin, A. H. MacDonald, J. Sinova, L. Smejkal

Abstract: The Pauli exclusion principle combined with interactions between fermions is a basic mechanism across condensed-matter systems giving rise to a spontaneous breaking of the spin-space rotation symmetry of spin-ordered phases. Ferromagnetism is a conventional manifestation of spin ordering which leads to numerous applications, e.g., in spintronic information technologies. Altermagnetism, whose recen… ▽ More The Pauli exclusion principle combined with interactions between fermions is a basic mechanism across condensed-matter systems giving rise to a spontaneous breaking of the spin-space rotation symmetry of spin-ordered phases. Ferromagnetism is a conventional manifestation of spin ordering which leads to numerous applications, e.g., in spintronic information technologies. Altermagnetism, whose recent discovery was largely motivated by spintronics, stands apart from conventional magnetism in the sense that it spontaneously breaks not only spin-space but also real-space rotation symmetries, while it preserves a symmetry combining spin-space and real-space rotations. This is realized on crystals by a collinear compensated ordering of spins with a characteristic d, g or i-wave symmetry. Our Perspective goes beyond the theory of spin arrangements on crystals by connecting altermagnetism to basic notions in condensed matter physics. Specifically, we reflect on the analogies and distinctions of altermagnetism as compared to superfluid 3He and theories of spin ordering in the momentum space generated by other higher-partial-wave instabilities of a Fermi-liquid. On one hand, all these physical systems have in common the extraordinary combination of spontaneous breaking of spin-space and real-space rotation symmetries. On the other hand, we point out that there are key differences, both at the symmetry level and, particularly, at the level of microscopic mechanisms of ordering. These explain the comparatively large abundance, robustness and utility of altermagnetism, as predicted by the symmetry-classification of spin arrangements on crystals and ab initio calculations, and supported by initial experiments. △ Less

Submitted 12 March, 2025; v1 submitted 1 November, 2024; originally announced November 2024.

Comments: 16 pages, 5 figures

Tunneling current-controlled spin states in few-layer van der Waals magnets

Authors: ZhuangEn Fu, Piumi I. Samarawickrama, John Ackerman, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Yuri Dahnovsky, Mingzhong Wu, TeYu Chien, Jinke Tang, Allan H. MacDonald, Hua Chen, Jifa Tian

Abstract: Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in… ▽ More Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Comments: 23 pages, 4 figures

Journal ref: Nature Communications 15 (2024) 3630

Emergent Trion Resonance Driven by Lattice Reconstruction in a Moiré Superlattice

Authors: Zhida Liu, Haonan Wang, Xiaohui Liu, Yue Ni, Hongtao Yan, Frank Y. Gao, Saba Arash, Hyunsue Kim, Dong Seob Kim, Xiangcheng Liu, Xiaoxiao Yu, Yongxin Zeng, Jiamin Quan, Di Huang, Kenji Watanabe, Takashi Taniguchi, Edoardo Baldini, Keji Lai, Allan H. MacDonald, Chih-Kang Shih, Jamie Warner, Li Yang, Xiaoqin Li

Abstract: We investigate how many-electron excited states emerge in twisted MoSe2 homobilayers when the lattice reconstructions evolve. Notably, we identify a new trion resonance that arises in the transition regime of lattice reconstruction, where gradual changes in atomic alignment between the layers occur. Magnetic field-dependent measurements, supported by first-principles calculations, indicate that th… ▽ More We investigate how many-electron excited states emerge in twisted MoSe2 homobilayers when the lattice reconstructions evolve. Notably, we identify a new trion resonance that arises in the transition regime of lattice reconstruction, where gradual changes in atomic alignment between the layers occur. Magnetic field-dependent measurements, supported by first-principles calculations, indicate that the exciton forms at the K valley while the doped hole resides in the Gamma valley. First-principles calculations further indicate that two nearly degenerate exciton resonances can arise, localized at different sites within the moiré supercell. We propose that the new trion resonance is a "charge-transfer" trion, in which the electron-hole pair is spatially separated from the doped hole. The emergence of these complex excited states stems from the distinct moiré potentials acting on holes and excitons, resulting in their different spatial distribution within the superlattice. △ Less

Submitted 1 July, 2026; v1 submitted 24 July, 2024; originally announced July 2024.

Comments: 7 pages, 4 figures

Experimental signature of layer skyrmions and implications for band topology in twisted WSe2 bilayers

Authors: Fan Zhang, Nicolás Morales-Durán, Yanxing Li, Wang Yao, Jung-Jung Su, Yu-Chuan Lin, Chengye Dong, Xiaohui Liu, Fu-Xiang Rikudo Chen, Hyunsue Kim, Kenji Watanabe, Takashi Taniguchi, Xiaoqin Li, Joshua A. Robinson, Allan H. Macdonald, Chih-Kang Shih

Abstract: Transition metal dichalcogenide (TMD) twisted homobilayers have been established as an ideal platform for studying strong correlation phenomena, as exemplified by the recent discovery of fractional Chern insulator (FCI) states in twisted MoTe2 and Chern insulators (CI) and unconventional superconductivity in twisted WSe2 (tWSe2). In these systems, nontrivial topology in the strongly layer-hybridiz… ▽ More Transition metal dichalcogenide (TMD) twisted homobilayers have been established as an ideal platform for studying strong correlation phenomena, as exemplified by the recent discovery of fractional Chern insulator (FCI) states in twisted MoTe2 and Chern insulators (CI) and unconventional superconductivity in twisted WSe2 (tWSe2). In these systems, nontrivial topology in the strongly layer-hybridized regime can arise from a spatial patterning of interlayer tunneling amplitudes and layer-dependent potentials that yields a lattice of layer skyrmions. Here we report on experimental signatures of skyrmion textures in the layer degree of freedom of Rhombohedral-stacked (R-stacked) tWSe2 homobilayers. This observation is based on scanning tunneling spectroscopy that separately resolves the Gamma-valley and K-valley moiré electronic states. We show that Gamma-valley states are subjected to a moiré potential with an amplitude of ~ 120 meV. At ~150 meV above the Gamma-valley, the K-valley states are subjected to a weaker moiré potential of ~30 meV. Most significantly, we reveal opposite layer polarizations of the K-valley at the MX and XM sites within the moiré unit cell, confirming the theoretically predicted layer skyrmion texture. The dI/dV mappings allow the parameters that enter the continuum model of moiré bands in twisted TMD bilayers to be determined experimentally, further establishing a direct correlation between the shape of the LDOS profile in real space and the topology of topmost moiré band. △ Less

Submitted 4 February, 2025; v1 submitted 28 June, 2024; originally announced June 2024.

Capacitive scheme to detect the topological magnetoelectric effect

Authors: Chao Lei, Perry T. Mahon, C. M. Canali, A. H. MacDonald

Abstract: The topological magnetoelectric effect (TME) is a defining property of three-dimensional $\mathbb{Z}_{2}$ topological insulators that was predicted on theoretical grounds more than a decade ago, but has still not been directly measured. In this Letter we propose a strategy for direct measurement of the TME and discuss the precision of the effect in real devices with charge and spin disorder. The topological magnetoelectric effect (TME) is a defining property of three-dimensional $\mathbb{Z}_{2}$ topological insulators that was predicted on theoretical grounds more than a decade ago, but has still not been directly measured. In this Letter we propose a strategy for direct measurement of the TME and discuss the precision of the effect in real devices with charge and spin disorder. △ Less

Submitted 13 December, 2024; v1 submitted 17 June, 2024; originally announced June 2024.

Comments: 5 + 1 pages, 3 figures

Journal ref: Phys. Rev. Lett. 133, 246607 (2024)

Intraband collective excitations in fractional Chern insulators are dark

Authors: Tobias M. R. Wolf, Yung-Chun Chao, Allan H. MacDonald, Jung Jung Su

Abstract: The low-energy collective excitations of semiconductors and insulators often couple strongly to light, allowing them to be probed optically. We argue here that in fractional Chern insulators intra-band collective excitations are dark in the sense that they couple anomalously weakly to light. This conclusion is based on a relationship between ideal quantum geometry and the structure factor of a Che… ▽ More The low-energy collective excitations of semiconductors and insulators often couple strongly to light, allowing them to be probed optically. We argue here that in fractional Chern insulators intra-band collective excitations are dark in the sense that they couple anomalously weakly to light. This conclusion is based on a relationship between ideal quantum geometry and the structure factor of a Chern band, and on a classical plasma analogy motivated by the vortexibility property of ideal Chern bands. △ Less

Submitted 15 June, 2024; originally announced June 2024.

Comments: Main text: 5 pages, 3 figures, Supmat: 5 pages, 1 figure; Comments are welcome

Optical Imaging of Flavor Order in Flat Band Graphene

Authors: Tian Xie, Tobias M. Wolf, Siyuan Xu, Zhiyuan Cui, Richen Xiong, Yunbo Ou, Patrick Hays, Ludwig F Holleis, Yi Guo, Owen I Sheekey, Caitlin Patterson, Trevor Arp, Kenji Watanabe, Takashi Taniguchi, Seth Ariel Tongay, Andrea F Young, Allan H. MacDonald, Chenhao Jin

Abstract: Spin and valley flavor polarization plays a central role in the many-body physics of flat band graphene, with fermi surface reconstructions often accompanied by quantized anomalous Hall and superconducting state observed in a variety of experimental systems. Here we describe an optical technique that sensitively and selectively detects flavor textures via the exciton response of a proximal transit… ▽ More Spin and valley flavor polarization plays a central role in the many-body physics of flat band graphene, with fermi surface reconstructions often accompanied by quantized anomalous Hall and superconducting state observed in a variety of experimental systems. Here we describe an optical technique that sensitively and selectively detects flavor textures via the exciton response of a proximal transition metal dichalcogenide layer. Through a systematic study of rhombohedral and rotationally faulted graphene bilayers and trilayers, we show that when the semiconducting dichalcogenide is in direct contact with the graphene, the exciton response is most sensitive to the large momentum rearrangement of the Fermi surface, providing information that is distinct from and complementary to electrical compressibility measurements. The wide-field imaging capability of optical probes allows us to obtain spatial maps of flavor orders with high throughput, and with broad temperature and device compatibility. Our work paves the way for optical probing and imaging of flavor orders in flat band graphene systems. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 29 pages, 4 figures, with supplementary materials

Adiabatic Approximation and Aharonov-Casher Bands in Twisted Homobilayer TMDs

Authors: Jingtian Shi, Nicolás Morales-Durán, Eslam Khalaf, Allan H. MacDonald

Abstract: Topological flat moiré bands with nearly ideal quantum geometry have been identified in homobilayer transition metal dichalcogenide moiré superlattices, and are thought to be crucial for understanding the fractional Chern insulating states recently observed therein. Previous work proposed viewing the system using an adiabatic approximation that replaces the position-dependence of the layer spinor… ▽ More Topological flat moiré bands with nearly ideal quantum geometry have been identified in homobilayer transition metal dichalcogenide moiré superlattices, and are thought to be crucial for understanding the fractional Chern insulating states recently observed therein. Previous work proposed viewing the system using an adiabatic approximation that replaces the position-dependence of the layer spinor with a nonuniform periodic effective magnetic field. When the local zero-point kinetic energy of this magnetic field cancels identically against that of an effective Zeeman energy, a Bloch-band version of Aharonov-Casher zero-energy modes, which we refer to as Aharonov-Casher band, emerges leading to ideal quantum geometry. Here, we critically examine the validity of the adiabatic approximation and identify the parameter regimes under which Aharonov-Casher bands emerge. We show that the adiabatic approximation is accurate for a wide range of parameters including those realized in experiments. Furthermore, we show that while the cancellation leading to the emergence of Aharonov-Casher bands is generally not possible beyond the leading Fourier harmonic, the leading harmonic is the dominant term in the Fourier expansions of the zero-point kinetic energy and Zeeman energy. As a result, the leading harmonic expansion accurately captures the trend of the bandwidth and quantum geometry, though it may fail to quantitatively reproduce more detailed information about the bands such as the Berry curvature distribution. △ Less

Submitted 29 June, 2024; v1 submitted 20 April, 2024; originally announced April 2024.

Journal ref: Phys. Rev. B 110, 035130 (2024)

Tuning Fermi Liquids with polaritonic Cavities

Authors: Riccardo Riolo, Andrea Tomadin, Giacomo Mazza, Reza Asgari, Allan H. MacDonald, Marco Polini

Abstract: The question of whether or not passive sub-wavelength cavities can alter the properties of quantum materials is currently attracting a great deal of attention. In this Article we show that the Fermi liquid parameters of a two-dimensional metal are modified by cavity polariton modes, and that these changes can be monitored by measuring a paradigmatic magneto-transport phenomenon, Shubnikov-de Haas… ▽ More The question of whether or not passive sub-wavelength cavities can alter the properties of quantum materials is currently attracting a great deal of attention. In this Article we show that the Fermi liquid parameters of a two-dimensional metal are modified by cavity polariton modes, and that these changes can be monitored by measuring a paradigmatic magneto-transport phenomenon, Shubnikov-de Haas oscillations in a weak perpendicular magnetic field. This effect is intrinsic, and totally unrelated to disorder. As an illustrative example, we carry out explicit calculations of the quasiparticle velocity of graphene in a planar van der Waals cavity formed by natural hyperbolic crystals and metal gates. The largest effects of the cavity occur when the phonon polariton modes of the former match energetically the graphene plasmon. For typical graphene carrier densities this occurs in the Terahertz spectral range. △ Less

Submitted 31 July, 2025; v1 submitted 29 March, 2024; originally announced March 2024.

Journal ref: Proc. Natl. Acad. Sci. USA 122, e2407995122 (2025)

Orbital Competition in Bilayer Graphene's Fractional Quantum Hall Effect

Authors: Bishoy M. Kousa, Nemin Wei, Allan H. MacDonald

Abstract: The lowest Landau level of bilayer graphene has an octet of internal degrees of freedom, composed from spin, valley and orbital two-level systems. Dominance of $n=0$ orbitals over $n=1$ orbitals in low energy quantum fluctuations leads to distinct fractional quantum Hall characteristics compared dominance of $n=1$ over $n=0$. The competition between $n=0$ and $n=1$ orbitals depends sensitively on… ▽ More The lowest Landau level of bilayer graphene has an octet of internal degrees of freedom, composed from spin, valley and orbital two-level systems. Dominance of $n=0$ orbitals over $n=1$ orbitals in low energy quantum fluctuations leads to distinct fractional quantum Hall characteristics compared dominance of $n=1$ over $n=0$. The competition between $n=0$ and $n=1$ orbitals depends sensitively on particle-hole asymmetry and on Lamb shifts due to exchange interactions with the negative energy sea, which must be accounted for simultaneously in assessing the orbital competition. We identify the circumstances under which $n=1$, which supports strong even-denominator FQH states with non-abelian quasiparticles, emerges robustly as the low-energy Landau level. △ Less

Submitted 15 February, 2024; originally announced February 2024.

Gate-tunable topological phases in superlattice modulated bilayer graphene

Authors: Yongxin Zeng, Tobias M. R. Wolf, Chunli Huang, Nemin Wei, Sayed Ali Akbar Ghorashi, Allan H. MacDonald, Jennifer Cano

Abstract: Superlattice potential modulation can produce flat minibands in Bernal-stacked bilayer graphene. In this work we study how band topology and interaction-induced symmetry-broken phases in this system are controlled by tuning the displacement field and the shape and strength of the superlattice potential. We use an analytic perturbative analysis to demonstrate that topological flat bands are favored… ▽ More Superlattice potential modulation can produce flat minibands in Bernal-stacked bilayer graphene. In this work we study how band topology and interaction-induced symmetry-broken phases in this system are controlled by tuning the displacement field and the shape and strength of the superlattice potential. We use an analytic perturbative analysis to demonstrate that topological flat bands are favored by a honeycomb-lattice-shaped potential, and numerics to show that the robustness of topological bands depends on both the displacement field strength and the periodicity of the superlattice potential. At integer fillings of the topological flat bands, the strength of the displacement field and the superlattice potential tune phase transitions between quantum anomalous Hall insulator, trivial insulator, and metallic states. We present mean-field phase diagrams in a gate voltage parameter space at filling factor $ν=1$, and discuss the prospects of realizing quantum anomalous Hall insulators and fractional Chern insulators when the superlattice potential modulation is produced by dielectric patterning or adjacent moiré materials. △ Less

Submitted 8 January, 2024; originally announced January 2024.

Journal ref: Phys. Rev. B 109, 195406 (2024)

Weak Coupling Theory of Magic-Angle Twisted Bilayer Graphene

Authors: Jihang Zhu, Iacopo Torre, Marco Polini, Allan H. MacDonald

Abstract: Strong correlations occur in magic-angle twisted bilayer graphene (MATBG) when the octet of flat moiré minibands centered on charge neutrality (CN) is partially occupied. The octet consists of a single valence band and a single conduction band for each of four degenerate spin-valley flavors. Motivated by the importance of Hartree electrostatic interactions in determining the filling-factor depende… ▽ More Strong correlations occur in magic-angle twisted bilayer graphene (MATBG) when the octet of flat moiré minibands centered on charge neutrality (CN) is partially occupied. The octet consists of a single valence band and a single conduction band for each of four degenerate spin-valley flavors. Motivated by the importance of Hartree electrostatic interactions in determining the filling-factor dependent band structure, we use a time-dependent Hartree approximation to gain insight into electronic correlations. We find that the electronic compressibility is dominated by Hartree interactions, that paramagnetic states are stable over a range of density near CN, and that the dependence of energy on flavor polarization is strongly overestimated by mean-field theory. △ Less

Submitted 16 October, 2024; v1 submitted 5 January, 2024; originally announced January 2024.

Journal ref: PhysRevB.110.L121117 (2024)

Weak localization as a probe of intervalley coherence in graphene multilayers

Authors: Nemin Wei, Yongxin Zeng, A. H. MacDonald

Abstract: Spontaneous intervalley coherence is suspected in several different graphene multilayer systems, but is difficult to confirm because of a paucity of convenient experimental signatures. Here we suggest that magneto-conductance features associated with quantum corrections to Drude conductivity can serve as a smoking gun for intervalley coherence that does not break time-reversal symmetry. In this cl… ▽ More Spontaneous intervalley coherence is suspected in several different graphene multilayer systems, but is difficult to confirm because of a paucity of convenient experimental signatures. Here we suggest that magneto-conductance features associated with quantum corrections to Drude conductivity can serve as a smoking gun for intervalley coherence that does not break time-reversal symmetry. In this class of ordered multilayer quantum transport corrections can produce weak localization or weak antilocalization, depending on whether the valley order belongs to the orthogonal or symplectic symmetry class. Our analysis motivates low-temperature weak-field magnetoresistance measurements in graphene multilayers in which time-reversal invariant intervalley coherent order is conjectured. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: Main text: 5 pages, 3 figures; Supplementary materials: 12 page, 4 figures

Bulk versus surface: Nonuniversal partitioning of the topological magnetoelectric effect

Authors: Chao Lei, Perry T. Mahon, Allan H. MacDonald

Abstract: The electronic ground state of a three-dimensional (3D) band insulator with time-reversal ($Θ$) symmetry or time-reversal times a discrete translation ($ΘT_{1/2}$) symmetry is classified by a $\mathbb{Z}_{2}$-valued topological invariant and characterized by quantized magnetoelectric response. Here we demonstrate by explicit calculation in model $\mathbb{Z}_{2}$ topological insulator thin-films th… ▽ More The electronic ground state of a three-dimensional (3D) band insulator with time-reversal ($Θ$) symmetry or time-reversal times a discrete translation ($ΘT_{1/2}$) symmetry is classified by a $\mathbb{Z}_{2}$-valued topological invariant and characterized by quantized magnetoelectric response. Here we demonstrate by explicit calculation in model $\mathbb{Z}_{2}$ topological insulator thin-films that whereas the magnetoelectric response is localized at the surface in the $Θ$ symmetry (nonmagnetic) case, it is nonuniversally partitioned between surface and interior contributions in the $ΘT_{1/2}$ (antiferromagnetic) case, while remaining quantized. Within our model the magnetic field induced polarization arises entirely from an anomalous $\mathscr{N}=0$ Landau level subspace within which the projected Hamiltonian is a generalized Su-Schrieffer-Heeger model whose topological properties are consistent with those of the starting 3D model. We identify a new connection between the ground-state geometry of that 3D model and surface-interior partitioning in thin films. △ Less

Submitted 21 October, 2024; v1 submitted 29 November, 2023; originally announced November 2023.

Comments: 5+16 pages, 4 figures

Journal ref: Phys. Rev. B 110, 165148 (2024)

Ab-initio study of the energy competition between Γand K valleys in bilayer transition metal dichalcogenides

Authors: Sam Olin, Erekle Jmukhadze, Allan H. MacDonald, Wei-Cheng Lee

Abstract: Moiré engineering in two-dimensional van der Waals bilayer crystals has emerged as a flexible platform for controlling strongly correlated electron systems. The competition between valleys for the band extremum energy position in the parent layers is crucial in deciding the qualitative nature of the moiré Hamiltonian since it controls the physics of the moiré minibands. Here we use density functio… ▽ More Moiré engineering in two-dimensional van der Waals bilayer crystals has emerged as a flexible platform for controlling strongly correlated electron systems. The competition between valleys for the band extremum energy position in the parent layers is crucial in deciding the qualitative nature of the moiré Hamiltonian since it controls the physics of the moiré minibands. Here we use density functional theory to examine the competition between K and $Γ$ for the valence band maximum in homo- and hetero-bilayers formed from the transition metal dichalcogenides (TMD), MX\{_2} where M=Mo,W and X=S,Se,Te. We shed light on how the competition is influenced by interlayer separation, which can be modified by applying pressure, by external gate-defined electric fields, and by transition metal atom d-orbital correlations. Our findings are related to several recent experiments, and contribute to the development of design rules for moiré materials. △ Less

Submitted 30 October, 2023; v1 submitted 26 October, 2023; originally announced October 2023.

Journal ref: Phys. Rev. B 109, 165101 (2024)

Atomistic Control in Molecular Beam Epitaxy Growth of Intrinsic Magnetic Topological Insulator MnBi2Te4

Authors: Hyunsue Kim, Mengke Liu, Lisa Frammolino, Yanxing Li, Fan Zhang, Woojoo Lee, Chengye Dong, Yi-Fan Zhao, Guan-Yu Chen, Pin-Jui Hsu, Cui-Zu Chang, Joshua Robinson, Jiaqiang Yan, Xiaoqin Li, Allan H. MacDonald, Chih-Kang Shih

Abstract: Intrinsic magnetic topological insulators have emerged as a promising platform to study the interplay between topological surface states and ferromagnetism. This unique interplay can give rise to a variety of exotic quantum phenomena, including the quantum anomalous Hall effect and axion insulating states. Here, utilizing molecular beam epitaxy (MBE), we present a comprehensive study of the growth… ▽ More Intrinsic magnetic topological insulators have emerged as a promising platform to study the interplay between topological surface states and ferromagnetism. This unique interplay can give rise to a variety of exotic quantum phenomena, including the quantum anomalous Hall effect and axion insulating states. Here, utilizing molecular beam epitaxy (MBE), we present a comprehensive study of the growth of high-quality MnBi2Te4 thin films on Si (111), epitaxial graphene, and highly ordered pyrolytic graphite substrates. By combining a suite of in-situ characterization techniques, we obtain critical insights into the atomic-level control of MnBi2Te4 epitaxial growth. First, we extract the free energy landscape for the epitaxial relationship as a function of the in-plane angular distribution. Then, by employing an optimized layer-by-layer growth, we determine the chemical potential and Dirac point of the thin film at different thicknesses. Overall, these results establish a foundation for understanding the growth dynamics of MnBi2Te4 and pave the way for the future applications of MBE in emerging topological quantum materials. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Comments: 20 pages, 4 figures

Itinerant ferromagnetism in transition metal dichalcogenides moiré superlattices

Authors: Pawel Potasz, Nicolás Morales-Durán, Nai Chao Hu, Allan H. MacDonald

Abstract: Moiré materials are artificial crystals formed at van der Waals heterojunctions that have emerged as a highly tunable platform to realize much of the rich quantum physics of electrons in atomic scale solids, also providing opportunities to discover new quantum phases of matter. Here we use finite-size exact diagonalization methods to explore the physics of single-band itinerant electron ferromagne… ▽ More Moiré materials are artificial crystals formed at van der Waals heterojunctions that have emerged as a highly tunable platform to realize much of the rich quantum physics of electrons in atomic scale solids, also providing opportunities to discover new quantum phases of matter. Here we use finite-size exact diagonalization methods to explore the physics of single-band itinerant electron ferromagnetism in semiconductor moiré materials. We predict where ferromagnetism is likely to occur in triangular-lattice moiré systems, and where it is likely to yield the highest Curie temperatures. △ Less

Submitted 23 January, 2024; v1 submitted 11 September, 2023; originally announced September 2023.

Comments: 17 pages, 15 figures

Journal ref: Phys. Rev. B 109, 045144(2024)

Electrical Control of Two-Dimensional Electron-Hole Fluids in the Quantum Hall Regime

Authors: Bo Zou, Yongxin Zeng, A. H. MacDonald, Artem Strashko

Abstract: We study the influence of quantizing perpendicular magnetic fields on the ground state of a bilayer with electron and hole fluids separated by an opaque tunnel barrier. In the absence of a field, the ground state at low carrier densities is a condensate of s-wave excitons that has spontaneous interlayer phase coherence. We find that a series of phase transitions emerge at strong perpendicular fiel… ▽ More We study the influence of quantizing perpendicular magnetic fields on the ground state of a bilayer with electron and hole fluids separated by an opaque tunnel barrier. In the absence of a field, the ground state at low carrier densities is a condensate of s-wave excitons that has spontaneous interlayer phase coherence. We find that a series of phase transitions emerge at strong perpendicular fields between condensed states and incompressible incoherent states with full electron and hole Landau levels. When the electron and hole densities are unequal, condensation can occur in higher angular momentum electron-hole pair states and, at weak fields, break rotational symmetry. We explain how this physics is expressed in dual-gate phase diagrams, and predict transport and capacitively-probed thermodynamic signatures that distinguish different states. △ Less

Submitted 27 December, 2023; v1 submitted 8 September, 2023; originally announced September 2023.

Symmetry, topology, and geometry: The many faces of the topological magnetoelectric effect

Authors: Perry T. Mahon, Chao Lei, Allan H. MacDonald

Abstract: A delicate tension complicates the relationship between the topological magnetoelectric effect (TME) in three-dimensional (3D) $\mathbb{Z}_2$ topological insulators (TIs) and time-reversal symmetry (TRS). TRS underlies a particular $\mathbb{Z}_2$ topological classification of the electronic ground state of crystalline band insulators and the associated quantization of the magnetoelectric response… ▽ More A delicate tension complicates the relationship between the topological magnetoelectric effect (TME) in three-dimensional (3D) $\mathbb{Z}_2$ topological insulators (TIs) and time-reversal symmetry (TRS). TRS underlies a particular $\mathbb{Z}_2$ topological classification of the electronic ground state of crystalline band insulators and the associated quantization of the magnetoelectric response coefficient calculated using bulk linear response theory but, according to standard symmetry arguments, simultaneously forbids a nonzero magnetoelectric coefficient in any physical finite-size system. This contrast between theories of magnetoelectric response in formal bulk models and in real finite-sized materials originates from the distinct approaches required to introduce notions of (electronic) polarization and orbital magnetization in these fundamentally different environments. In this work we argue for a modified interpretation of the bulk linear response calculations in non-magnetic $\mathbb{Z}_2$ TIs that is more plainly consistent with TRS, and use this interpretation to discuss the effect's observation - still absent over a decade after its prediction. Motivated by analytical results, we conjecture a type of microscopic bulk-boundary correspondence: a bulk insulator with (generalized) TRS supports a magnetoelectric coefficient that is purely itinerant (which is generically related to the geometry of the ground state) if and only if magnetic surface dopants are required for the TME to manifest in finite samples thereof. We conclude that in non-magnetic $\mathbb{Z}_2$ TIs the TME is activated by magnetic surface dopants, that the charge density response to a uniform dc magnetic field is localized at the surface and specified by the configuration of those dopants, and that the TME is qualitatively less robust against disorder than the integer quantum Hall effect. △ Less

Submitted 17 June, 2024; v1 submitted 17 August, 2023; originally announced August 2023.

Comments: 33 pages, 5 figures

Journal ref: Phys. Rev. Research 6, 023289 (2024)

Magic Angles and Fractional Chern Insulators in Twisted Homobilayer TMDs

Authors: Nicolás Morales-Durán, Nemin Wei, Jingtian Shi, Allan H. MacDonald

Abstract: We explain the appearance of magic angles and fractional Chern insulators in twisted K-valley homobilayer transition metal dichalcogenides by mapping their continuum model to a Landau level problem. Our approach relies on an adiabatic approximation for the quantum mechanics of valence band holes in a layer-pseudospin field that is valid for sufficiently small twist angles and on a lowest Landau le… ▽ More We explain the appearance of magic angles and fractional Chern insulators in twisted K-valley homobilayer transition metal dichalcogenides by mapping their continuum model to a Landau level problem. Our approach relies on an adiabatic approximation for the quantum mechanics of valence band holes in a layer-pseudospin field that is valid for sufficiently small twist angles and on a lowest Landau level approximation that is valid for sufficiently large twist angles. It simply explains why the quantum geometry of the lowest moiré miniband is nearly ideal at particular flat-band twist angles, predicts that topological flat bands occur only when the valley-dependent moiré potential is sufficiently strong compared to the interlayer tunneling amplitude, and provides a powerful starting point for the study of interactions △ Less

Submitted 12 December, 2023; v1 submitted 6 August, 2023; originally announced August 2023.

Comments: 14 pages, 5 figures

Journal ref: Phys. Rev. Lett. 132, 096602 (2024)

Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures

Authors: Ruishi Qi, Andrew Y. Joe, Zuocheng Zhang, Yongxin Zeng, Tiancheng Zheng, Qixin Feng, Emma Regan, Jingxu Xie, Zheyu Lu, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Michael F. Crommie, Allan H. MacDonald, Feng Wang

Abstract: Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton superfluids. The interlayer excitons can also interact strongly with excess charge carriers when electron a… ▽ More Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton superfluids. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~$0.8\times {10}^{12} \mathrm{cm}^{-2}$ and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems. △ Less

Submitted 22 June, 2023; originally announced June 2023.

Pressure tunable quantum anomalous Hall states in a topological antiferromagnet

Authors: Su Kong Chong, Chao Lei, Jie Li, Yang Cheng, David Graf, Seng Huat Lee, Masaki Tanabe, Ting-Hsun Yang, Zhiqiang Mao, Allan H. MacDonald, Kang L. Wang

Abstract: Mechanical modulation of the lattice parameter can modify the electronic structure and manipulate the magnetic coupling of a material without introducing impurities. Inspired by success in pressure-controlled magnetism, we investigate the effect of hydrostatic pressure on quantized Chern states in the antiferromagnetic topological insulator MnBi2Te4, using transport as a probe. We show that pressu… ▽ More Mechanical modulation of the lattice parameter can modify the electronic structure and manipulate the magnetic coupling of a material without introducing impurities. Inspired by success in pressure-controlled magnetism, we investigate the effect of hydrostatic pressure on quantized Chern states in the antiferromagnetic topological insulator MnBi2Te4, using transport as a probe. We show that pressure can enhance the robustness of quantum anomalous Hall (QAH) phases that are otherwise delicate in 7SL MnBi2Te4 and in the spin-flop (SF) state of 8SL MnBi2Te4. We explain our findings using a coupled Dirac cone model of MnBi2Te4, which identifies stronger hybridization between van der Waals layers as the driver of topological states. We further demonstrate that moderate pressures readily available in laboratory systems can provide reversible control of magnetic and topological phases. Our results reveal a strong connection between the mechanical engineering of band topology and magnetism. △ Less

Submitted 17 June, 2023; originally announced June 2023.

Comments: 11 pages, 4 figures

Strong magnetic proximity effect in Van der Waals heterostructures driven by direct hybridization

Authors: C. Cardoso, A. T. Costa, A. H. MacDonald, J. Fernández-Rossier

Abstract: We propose a new class of magnetic proximity effects based on the spin dependent hybridization between the electronic states at the Fermi energy in a non-magnetic conductor and the narrow spin split bands of a ferromagnetic insulator. Unlike conventional exchange proximity, we show this hybridization proximity effect has a very strong influence on the non-magnetic layer and can be further modulate… ▽ More We propose a new class of magnetic proximity effects based on the spin dependent hybridization between the electronic states at the Fermi energy in a non-magnetic conductor and the narrow spin split bands of a ferromagnetic insulator. Unlike conventional exchange proximity, we show this hybridization proximity effect has a very strong influence on the non-magnetic layer and can be further modulated by application of an electric field. We use DFT calculations to illustrate this effect in graphene placed next to a monolayer of CrI$_3$, a ferromagnetic insulator. We find strong hybridization of the graphene bands with the narrow conduction band of CrI$_3$ in one spin channel only. We show that our results are robust with respect to lattice mismatch and twist angle variations. Furthermore, we show that an out-of-plane electric field can be used to modulate the hybridization strength, paving the way for applications. △ Less

Submitted 26 May, 2023; originally announced May 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 108, 184423 (2023)

Magnetic States of Graphene Proximitized Kitaev Materials

Authors: Jingtian Shi, A. H. MacDonald

Abstract: Single layer $α$-ruthenium trichloride ($\rmα-RuCl_3$) has been proposed as a potential quantum spin liquid. Graphene/$\rm RuCl_3$ heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of $\rm RuCl_3$ layer and graphene electronic properties. We perform self-c… ▽ More Single layer $α$-ruthenium trichloride ($\rmα-RuCl_3$) has been proposed as a potential quantum spin liquid. Graphene/$\rm RuCl_3$ heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of $\rm RuCl_3$ layer and graphene electronic properties. We perform self-consistent Hartree-Fock calculations on a Hubbard-Kanamori model of the $4d^5$ $t_{2g}$ electrons of $\rmα-RuCl_3$ and confirm that out-of-plane ferromagnetic and zigzag antiferromagnetic states are energetically competitive. We show that the influence of hybridization between graphene and $\rmα-RuCl_3$ bands is strongly sensitive to the magnetic configuration of $\rm RuCl_3$ and the relative orientations of the two layers. We argue that strong hybridization leads to graphene magneto-resistance and that it may tilt the balance between closely competing magnetic states. Our analysis can be applied to any van der Waals heterobilayer system with weak interlayer hybridization and allows for arbitrary lattice constant mismatch and relative orientation. △ Less

Submitted 19 July, 2023; v1 submitted 20 May, 2023; originally announced May 2023.

Journal ref: Phys. Rev. B 108, 064401 (2023)

Partial condensation of mobile excitons in graphene multilayers

Authors: Igor V. Blinov, Chunli Huang, Nemin Wei, Qin Wei, Tobias Wolf, Allan H. MacDonald

Abstract: At a large displacement field, in rhomboedral and Bernal-stacked graphene a normal paramagnetic state transitions to a correlated state. Recent experiments showed that such systems have several phase transitions as a function of the carrier density. The phase adjacent to a paramagnetic state has anomalously high resistance and reduced degeneracy of the Fermi sea. We show that both phenomena can be… ▽ More At a large displacement field, in rhomboedral and Bernal-stacked graphene a normal paramagnetic state transitions to a correlated state. Recent experiments showed that such systems have several phase transitions as a function of the carrier density. The phase adjacent to a paramagnetic state has anomalously high resistance and reduced degeneracy of the Fermi sea. We show that both phenomena can be explained through a concept of partial intervalley exciton condensation: a fraction of particles condenses into excitons, and another forms an intervalley coherent Fermi liquid. The exciton part of the system do not contribute to the electrical current thus increasing the resistance. Within this paradigm, the increase in the resistance has entirely geometrical origin. We check validity of the phenomenological theory through numerical calculations. We also show that the quantum oscillation data should not be very different between the partial excitonic state and the intervalley coherent states suggested by other authors. Further, we suggest STM/AFM or Raman spectroscopy to have a conclusive evidence for the occurrence of the partial exciton condensation that we suggest in this paper. △ Less

Submitted 30 March, 2023; originally announced March 2023.

Kerr, Faraday, and Magnetoelectric Effects in MnBi$_2$Te$_4$ Thin Films

Authors: Chao Lei, Allan H. MacDonald

Abstract: The topological magneto-electric effect (TME) is a characteristic property of topological insulators. In this article, we use a simplified coupled-Dirac-cone electronic structure model to theoretically evaluate the THz and far infrared Kerr and Faraday responses of thin films of MnBi$_2$Te$_4$ with up to $N=10$ septuple layers with the goal of clarifying the relationship between these convenient m… ▽ More The topological magneto-electric effect (TME) is a characteristic property of topological insulators. In this article, we use a simplified coupled-Dirac-cone electronic structure model to theoretically evaluate the THz and far infrared Kerr and Faraday responses of thin films of MnBi$_2$Te$_4$ with up to $N=10$ septuple layers with the goal of clarifying the relationship between these convenient magneto-optical observables and the TME. We find that for even $N$ the linear Kerr and Faraday responses to an electric field vanish in the low-frequency limit, even though the magnetoelectric response is large and approximately quantized. △ Less

Submitted 9 July, 2024; v1 submitted 26 March, 2023; originally announced March 2023.

Comments: 5+7 pages

Journal ref: Phys. Rev. B 108, 125424 (2023)

Giant spin Hall effect in AB-stacked MoTe2/WSe2 bilayers

Authors: Zui Tao, Bowen Shen, Wenjin Zhao, Nai Chao Hu, Tingxin Li, Shengwei Jiang, Lizhong Li, Kenji Watanabe, Takashi Taniguchi, Allan H. MacDonald, Jie Shan, Kin Fai Mak

Abstract: The spin Hall effect (SHE), in which electrical current generates transverse spin current, plays an important role in spintronics for the generation and manipulation of spin-polarized electrons. The phenomenon originates from spin-orbit coupling. In general, stronger spin-orbit coupling favors larger SHEs but shorter spin relaxation times and diffusion lengths. To achieve both large SHEs and long-… ▽ More The spin Hall effect (SHE), in which electrical current generates transverse spin current, plays an important role in spintronics for the generation and manipulation of spin-polarized electrons. The phenomenon originates from spin-orbit coupling. In general, stronger spin-orbit coupling favors larger SHEs but shorter spin relaxation times and diffusion lengths. To achieve both large SHEs and long-range spin transport in a single material has remained a challenge. Here we demonstrate a giant intrinsic SHE in AB-stacked MoTe2/WSe2 moiré bilayers by direct magneto optical imaging. Under moderate electrical currents with density < 1 A/m, we observe spin accumulation on transverse sample edges that nearly saturates the spin density. We also demonstrate long-range spin Hall transport and efficient non-local spin accumulation limited only by the device size (about 10 um). The gate dependence shows that the giant SHE occurs only near the Chern insulating state, and at low temperatures, it emerges after the quantum anomalous Hall breakdown. Our results demonstrate moiré engineering of Berry curvature and large SHEs for potential spintronics applications. △ Less

Submitted 22 March, 2023; originally announced March 2023.

Magnetism and Quantum Melting in Moiré-Material Wigner Crystals

Authors: Nicolás Morales-Durán, Pawel Potasz, Allan H. MacDonald

Abstract: Recent experiments have established that semiconductor-based moiré materials can host incompressible states at a series of fractional moiré-miniband fillings. These states have been identified as generalized Wigner crystals in which electrons localize on a subset of the available triangular-lattice moiré superlattice sites. In this article, we use momentum-space exact diagonalization to investigat… ▽ More Recent experiments have established that semiconductor-based moiré materials can host incompressible states at a series of fractional moiré-miniband fillings. These states have been identified as generalized Wigner crystals in which electrons localize on a subset of the available triangular-lattice moiré superlattice sites. In this article, we use momentum-space exact diagonalization to investigate the many-body ground state evolution at rational fillings from the weak-hopping classical lattice gas limit, in which only spin degrees-of-freedom are active at low energies, to the strong-hopping metallic regime where the Wigner crystals melt. We specifically address the nature of the magnetic ground states of the generalized Wigner crystals at fillings $ν$ = 1/3 and $ν$ = 2/3. △ Less

Submitted 19 July, 2023; v1 submitted 27 October, 2022; originally announced October 2022.

Comments: 12 pages, 8 figures

Anomalous Landau quantization in intrinsic magnetic topological insulators

Authors: Su Kong Chong, Chao Lei, Seng Huat Lee, Jan Jaroszynski, Zhiqiang Mao, Allan H. MacDonald, Kang L. Wang

Abstract: The intrinsic magnetic topological insulators, Mn(Bi1-xSbx)2Te4, in their spin-aligned strong field configuration have been identified as a Weyl semimetal with single pair of Weyl nodes1-4. A direct consequence of the Weyl state is the layer-dependent Chern number (C) in thin film quantization. Previous reports in MnBi2Te4 thin films revealed the higher C states in the spin alignment by either inc… ▽ More The intrinsic magnetic topological insulators, Mn(Bi1-xSbx)2Te4, in their spin-aligned strong field configuration have been identified as a Weyl semimetal with single pair of Weyl nodes1-4. A direct consequence of the Weyl state is the layer-dependent Chern number (C) in thin film quantization. Previous reports in MnBi2Te4 thin films revealed the higher C states in the spin alignment by either increasing the film thickness5 or controlling chemical potential into electron doping6-8. A clear picture of the higher Chern states is still missing as the situation is complicated by the emerging of surface band Landau levels (LLs) in magnetic field. Here, we report a tunable layer-dependent of C= 1 state with the Sb substitutions by performing a detailed analysis of the quantization states in Mn(Bi1-xSbx)2Te4 dualgated devices, consistent with the calculations of the bulk Weyl point separations in the compounds. The observed Hall quantization plateaus for our thicker Mn(Bi1-xSbx)2Te4 films under strong magnetic fields can be interpreted from a theory of surface and bulk spin-polarized Landau levels spectrum in thin film magnetic topological insulators. Our results demonstrate that Mn(Bi1-xSbx)2Te4 thin films provide a highly tunable platform for probing the physics of the anomalous Landau quantization that is strongly sensitive to magnetic order. △ Less

Submitted 28 August, 2022; originally announced August 2022.

Comments: 23 pages, 7 figures

Bistritzer-MacDonald dynamics in twisted bilayer graphene

Authors: Alexander B. Watson, Tianyu Kong, Allan H. MacDonald, Mitchell Luskin

Abstract: The Bistritzer-MacDonald (BM) model, introduced in \cite{Bistritzer2011}, attempts to capture the electronic properties of twisted bilayer graphene (TBG), even at incommensurate twist angles, by an effective periodic model over the bilayer moiré pattern. Starting from a tight-binding model, we identify a regime where the BM model emerges as the effective dynamics for electrons modeled as wave-pack… ▽ More The Bistritzer-MacDonald (BM) model, introduced in \cite{Bistritzer2011}, attempts to capture the electronic properties of twisted bilayer graphene (TBG), even at incommensurate twist angles, by an effective periodic model over the bilayer moiré pattern. Starting from a tight-binding model, we identify a regime where the BM model emerges as the effective dynamics for electrons modeled as wave-packets spectrally concentrated at the monolayer Dirac points, up to error that can be rigorously estimated. Using measured values of relevant physical constants, we argue that this regime is realized in TBG at the first "magic" angle. △ Less

Submitted 13 October, 2022; v1 submitted 27 July, 2022; originally announced July 2022.

Comments: 50 pages, 9 figures. Fixed wave-packet scaling, bounded some higher-order terms in the residual, and added reference to Bal-Cazeaux-Massatt-Quinn. Bounding the higher-order terms required strengthening our assumption on the regularity of initial data