Benjamin Ezekiel Feldman

All Publications

• Particle-Hole Asymmetric Ferromagnetism and Spin Textures in the Triangular Hubbard-Hofstadter Model PHYSICAL REVIEW X Ding, J. K., Yang, L., Wang, W. O., Zhu, Z., Peng, C., Mai, P., Huang, E. W., Moritz, B., Phillips, P. W., Feldman, B. E., Devereaux, T. P. 2024; 14 (4)

  View details for DOI 10.1103/PhysRevX.14.041025

  View details for Web of Science ID 001345204400001

• Anomalous Landau Level Gaps Near Magnetic Transitions in Monolayer WSe2 PHYSICAL REVIEW X Foutty, B. A., Calvera, V., Han, Z., Kometter, C. R., Liu, S., Watanabe, K., Taniguchi, T., Hone, J. C., Kivelson, S. A., Feldman, B. E. 2024; 14 (3)

  View details for DOI 10.1103/PhysRevX.14.031018

  View details for Web of Science ID 001284102100001

• Uncovering the spin ordering in magic-angle graphene via edge state equilibration. Nature communications Hoke, J. C., Li, Y., May-Mann, J., Watanabe, K., Taniguchi, T., Bradlyn, B., Hughes, T. L., Feldman, B. E. 2024; 15 (1): 4321

  Abstract

  The flat bands in magic-angle twisted bilayer graphene (MATBG) provide an especially rich arena to investigate interaction-driven ground states. While progress has been made in identifying the correlated insulators and their excitations at commensurate moiré filling factors, the spin-valley polarizations of the topological states that emerge at high magnetic field remain unknown. Here we introduce a technique based on twist-decoupled van der Waals layers that enables measurement of their electronic band structure and-by studying the backscattering between counter-propagating edge states-the determination of the relative spin polarization of their edge modes. We find that the symmetry-broken quantum Hall states that extend from the charge neutrality point in MATBG are spin unpolarized at even integer filling factors. The measurements also indicate that the correlated Chern insulator emerging from half filling of the flat valence band is spin unpolarized and suggest that its conduction band counterpart may be spin polarized.

  View details for DOI 10.1038/s41467-024-48385-z

  View details for PubMedID 38773076

  View details for PubMedCentralID 7069711

• Mapping twist-tuned multiband topology in bilayer WSe2. Science (New York, N.Y.) Foutty, B. A., Kometter, C. R., Devakul, T., Reddy, A. P., Watanabe, K., Taniguchi, T., Fu, L., Feldman, B. E. 2024; 384 (6693): 343-347

  Abstract

  Semiconductor moiré superlattices have been shown to host a wide array of interaction-driven ground states. However, twisted homobilayers have been difficult to study in the limit of large moiré wavelengths, where interactions are most dominant. In this study, we conducted local electronic compressibility measurements of twisted bilayer WSe2 (tWSe2) at small twist angles. We demonstrated multiple topological bands that host a series of Chern insulators at zero magnetic field near a "magic angle" around 1.23°. Using a locally applied electric field, we induced a topological quantum-phase transition at one hole per moiré unit cell. Our work establishes the topological phase diagram of a generalized Kane-Mele-Hubbard model in tWSe2, demonstrating a tunable platform for strongly correlated topological phases.

  View details for DOI 10.1126/science.adi4728

  View details for PubMedID 38669569

• Characterization of Two Fast-Turnaround Dry Dilution Refrigerators for Scanning Probe Microscopy JOURNAL OF LOW TEMPERATURE PHYSICS Barber, M. E., Li, Y., Gibson, J., Yu, J., Jiang, Z., Hu, Y., Ji, Z., Nandi, N., Hoke, J. C., Bishop-Van Horn, L., Arias, G. R., Van Harlingen, D. J., Moler, K. A., Shen, Z., Kou, A., Feldman, B. E. 2024

  View details for DOI 10.1007/s10909-023-03035-4

  View details for Web of Science ID 001190209400002

• Competing electron solids and electron fluids in the moire atomic limit NATURE PHYSICS Feldman, B. E., Yu, J. 2023; 19 (12): 1769-1770

  View details for DOI 10.1038/s41567-023-02197-y

  View details for Web of Science ID 001178645300018

• Hofstadter states and re-entrant charge order in a semiconductor moire lattice NATURE PHYSICS Kometter, C. R., Yu, J., Devakul, T., Reddy, A. P., Zhang, Y., Foutty, B. A., Watanabe, K., Taniguchi, T., Fu, L., Feldman, B. E. 2023; 19 (12): 1861-+

  View details for DOI 10.1038/s41567-023-02195-0

  View details for Web of Science ID 001178645300033

• Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene. Nature communications Yu, J., Foutty, B. A., Kwan, Y. H., Barber, M. E., Watanabe, K., Taniguchi, T., Shen, Z. X., Parameswaran, S. A., Feldman, B. E. 2023; 14 (1): 6679

  Abstract

  The flat electronic bands in magic-angle twisted bilayer graphene (MATBG) host a variety of correlated insulating ground states, many of which are predicted to support charged excitations with topologically non-trivial spin and/or valley skyrmion textures. However, it has remained challenging to experimentally address their ground state order and excitations, both because some of the proposed states do not couple directly to experimental probes, and because they are highly sensitive to spatial inhomogeneities in real samples. Here, using a scanning single-electron transistor, we observe thermodynamic gaps at even integer moiré filling factors at low magnetic fields. We find evidence of a field-tuned crossover from charged spin skyrmions to bare particle-like excitations, suggesting that the underlying ground state belongs to the manifold of strong-coupling insulators. From the spatial dependence of these states and the chemical potential variation within the flat bands, we infer a link between the stability of the correlated ground states and local twist angle and strain. Our work advances the microscopic understanding of the correlated insulators in MATBG and their unconventional excitations.

  View details for DOI 10.1038/s41467-023-42275-6

  View details for PubMedID 37865663

  View details for PubMedCentralID 8099185

• 1/4 is the new 1/2 when topology is intertwined with Mottness. Nature communications Mai, P., Zhao, J., Feldman, B. E., Phillips, P. W. 2023; 14 (1): 5999

  Abstract

  In non-interacting systems, bands from non-trivial topology emerge strictly at half-filling and exhibit either the quantum anomalous Hall or spin Hall effects. Here we show using determinantal quantum Monte Carlo and an exactly solvable strongly interacting model that these topological states now shift to quarter filling. A topological Mott insulator is the underlying cause. The peak in the spin susceptibility is consistent with a possible ferromagnetic state at T = 0. The onset of such magnetism would convert the quantum spin Hall to a quantum anomalous Hall effect. While such a symmetry-broken phase typically is accompanied by a gap, we find that the interaction strength must exceed a critical value for this to occur. Hence, we predict that topology can obtain in a gapless phase but only in the presence of interactions in dispersive bands. These results explain the recent quarter-filled quantum anomalous Hall effects seen in moiré systems.

  View details for DOI 10.1038/s41467-023-41465-6

  View details for PubMedID 37752137

  View details for PubMedCentralID PMC10522641

• Tunable spin and valley excitations of correlated insulators in Γ-valley moiré bands. Nature materials Foutty, B. A., Yu, J., Devakul, T., Kometter, C. R., Zhang, Y., Watanabe, K., Taniguchi, T., Fu, L., Feldman, B. E. 2023

  Abstract

  Moiré superlattices formed from transition metal dichalcogenides support a variety of quantum electronic phases that are highly tunable using applied electromagnetic fields. While the valley degree of freedom affects optoelectronic properties in the constituent transition metal dichalcogenides, it has yet to be fully explored in moiré systems. Here we establish twisted double-bilayer WSe2 as an experimental platform to study electronic correlations within Γ-valley moiré bands. Through local and global electronic compressibility measurements, we identify charge-ordered phases at multiple integer and fractional moiré fillings. By measuring the magnetic field dependence of their energy gaps and the chemical potential upon doping, we reveal spin-polarized ground states with spin-polaron quasiparticle excitations. In addition, an applied displacement field induces a metal-insulator transition driven by tuning between Γ- and K-valley moiré bands. Our results demonstrate control over the spin and valley character of the correlated ground and excited states in this system.

  View details for DOI 10.1038/s41563-023-01534-z

  View details for PubMedID 37069292

• Interaction-driven spontaneous ferromagnetic insulating states with odd Chern numbers NPJ QUANTUM MATERIALS Mai, P., Huang, E. W., Yu, J., Feldman, B. E., Phillips, P. W. 2023; 8 (1)

  View details for DOI 10.1038/s41535-023-00544-z

  View details for Web of Science ID 000948370100001

• Topological Mott insulator at quarter filling in the interacting Haldane model PHYSICAL REVIEW RESEARCH Mai, P., Feldman, B. E., Phillips, P. W. 2023; 5 (1)

  View details for DOI 10.1103/PhysRevResearch.5.013162

  View details for Web of Science ID 000991768800002

• Correlated Hofstadter spectrum and flavour phase diagram in magic-angle twisted bilayer graphene NATURE PHYSICS Yu, J., Foutty, B. A., Han, Z., Barber, M. E., Schattner, Y., Watanabe, K., Taniguchi, T., Phillips, P., Shen, Z., Kivelson, S. A., Feldman, B. E. 2022

  View details for DOI 10.1038/s41567-022-01589-w

  View details for Web of Science ID 000787131300001

• The preferred direction NATURE PHYSICS Feldman, B. E. 2021

  View details for DOI 10.1038/s41567-021-01456-0

  View details for Web of Science ID 000733695800010

• Quantum Hall valley nematics. Journal of physics. Condensed matter : an Institute of Physics journal Parameswaran, S. A., Feldman, B. 2019

  Abstract

  Two-dimensional electron gases subject to strong magnetic fields provide a canonical platform to explore a variety of exotic electronic phenomena. Here we review the physics of one intriguing example: quantum Hall valley nematics. In these phases of matter, the formation of a topologically insulating integer quantum Hall state is accompanied by the spontaneous breaking of a point-group symmetry that combines a spatial rotation with a permutation of valley indices. The resulting orientational order is particularly sensitive to quenched disorder, while quantum Hall physics links charge conduction to topological defects of this order. We discuss how these combine to yield a rich and intricate phase structure, and their implications for transport and spectroscopy measurements. We provide a brief survey of several relevant experimental systems and close with an outlook on future directions. .

  View details for PubMedID 30743251

• Interacting multi-channel topological boundary modes in a quantum Hall valley system. Nature Randeria, M. T., Agarwal, K., Feldman, B. E., Ding, H., Ji, H., Cava, R. J., Sondhi, S. L., Parameswaran, S. A., Yazdani, A. 2019

  Abstract

  Symmetry and topology are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties1,2. Domain walls between distinct broken-symmetry QHFM phases are predicted to host gapless one-dimensional modes-that is, quantum channels that emerge because of a topological change in the underlying electronic wavefunctions at such interfaces. Although various QHFMs have been identified in different materials3-8, interacting electronic modes at these domain walls have not been probed. Here we use a scanning tunnelling microscope to directly visualize the spontaneous formation of boundary modes at domain walls between QHFM phases with different valley polarization (that is, the occupation of equal-energy but quantum mechanically distinct valleys in the electronic structure) on the surface of bismuth. Spectroscopy shows that these modes occur within a topological energy gap, which closes and reopens as the valley polarization switches across the domain wall. By changing the valley flavour and the number of modes at the domain wall, we can realize different regimes in which the valley-polarized channels are either metallic or develop a spectroscopic gap. This behaviour is a consequence of Coulomb interactions constrained by the valley flavour, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting one-dimensional quantum wires.QHFM domain walls can be realized in different classes of two-dimensional materials, providing the opportunity to explore a rich phase space of interactions in these quantum wires.

  View details for PubMedID 30728501

• Squeezing strong correlations from graphene. Science (New York, N.Y.) Feldman, B. E. 2019; 363 (6431): 1035–36

  View details for PubMedID 30846582

• Ferroelectric quantum Hall phase revealed by visualizing Landau level wavefunction interference NATURE PHYSICS Randeria, M. T., Feldman, B. E., Wu, F., Ding, H., Gyenis, A., Ji, H., Cava, R. J., MacDonald, A. H., Yazdani, A. 2018; 14 (8): 796-+

  View details for DOI 10.1038/s41567-018-0148-2

  View details for Web of Science ID 000440583300011

• Visualizing heavy fermion confinement and Pauli-limited superconductivity in layered CeCoIn5 NATURE COMMUNICATIONS Gyenis, A., Feldman, B. E., Randeria, M. T., Peterson, G. A., Bauer, E. D., Aynajian, P., Yazdani, A. 2018; 9: 549

  Abstract

  Layered material structures play a key role in enhancing electron-electron interactions to create correlated metallic phases that can transform into unconventional superconducting states. The quasi-two-dimensional electronic properties of such compounds are often inferred indirectly through examination of bulk properties. Here we use scanning tunneling microscopy to directly probe in cross-section the quasi-two-dimensional electronic states of the heavy fermion superconductor CeCoIn5. Our measurements reveal the strong confined nature of quasiparticles, anisotropy of tunneling characteristics, and layer-by-layer modulated behavior of the precursor pseudogap gap phase. In the interlayer coupled superconducting state, the orientation of line defects relative to the d-wave order parameter determines whether in-gap states form due to scattering. Spectroscopic imaging of the anisotropic magnetic vortex cores directly characterizes the short interlayer superconducting coherence length and shows an electronic phase separation near the upper critical in-plane magnetic field, consistent with a Pauli-limited first-order phase transition into a pseudogap phase.

  View details for PubMedID 29416021

• High-resolution studies of the Majorana atomic chain platform NATURE PHYSICS Feldman, B. E., Randeria, M. T., Li, J., Jeon, S., Xie, Y., Wang, Z., Drozdov, I. K., Bernevig, B. A., Yazdani, A. 2017; 13 (3): 286-?

  View details for DOI 10.1038/NPHYS3947

  View details for Web of Science ID 000395814000022

• Observation of a nematic quantum Hall liquid on the surface of bismuth SCIENCE Feldman, B. E., Randeria, M. T., Gyenis, A., Wu, F., Ji, H., Cava, R. J., MacDonald, A. H., Yazdani, A. 2016; 354 (6310): 316-321

  Abstract

  Nematic quantum fluids with wave functions that break the underlying crystalline symmetry can form in interacting electronic systems. We examined the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope showed that a combination of single-particle effects and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We imaged the resulting anisotropic LL wave functions and found that they have a different orientation for each broken-symmetry state. The wave functions correspond to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase.

  View details for DOI 10.1126/science.aag1715

  View details for PubMedID 27846563

• Electron-hole asymmetric integer and fractional quantum Hall effect in bilayer graphene SCIENCE Kou, A., Feldman, B. E., Levin, A. J., Halperin, B. I., Watanabe, K., Taniguchi, T., Yacoby, A. 2014; 345 (6192): 55-57

  View details for DOI 10.1126/science.1250270

  View details for Web of Science ID 000338284400046

• Fractional Quantum Hall Phase Transitions and Four-Flux States in Graphene PHYSICAL REVIEW LETTERS Feldman, B. E., Levin, A. J., Krauss, B., Abanin, D. A., Halperin, B. I., Smet, J. H., Yacoby, A. 2013; 111 (7)

  Abstract

  Graphene and its multilayers have attracted considerable interest because their fourfold spin and valley degeneracy enables a rich variety of broken-symmetry states arising from electron-electron interactions, and raises the prospect of controlled phase transitions among them. Here we report local electronic compressibility measurements of ultraclean suspended graphene that reveal a multitude of fractional quantum Hall states surrounding filling factors ν=-1/2 and -1/4. Several of these states exhibit phase transitions that indicate abrupt changes in the underlying order, and we observe many additional oscillations in compressibility as ν approaches -1/2, suggesting further changes in spin and/or valley polarization. We use a simple model based on crossing Landau levels of composite fermions with different internal degrees of freedom to explain many qualitative features of the experimental data. Our results add to the diverse array of many-body states observed in graphene and demonstrate substantial control over their order parameters.

  View details for DOI 10.1103/PhysRevLett.111.076802

  View details for Web of Science ID 000323333800007

  View details for PubMedID 23992076

• Unconventional Sequence of Fractional Quantum Hall States in Suspended Graphene SCIENCE Feldman, B. E., Krauss, B., Smet, J. H., Yacoby, A. 2012; 337 (6099): 1196-1199

  Abstract

  Graphene provides a rich platform to study many-body effects, owing to its massless chiral charge carriers and the fourfold degeneracy arising from their spin and valley degrees of freedom. We use a scanning single-electron transistor to measure the local electronic compressibility of suspended graphene, and we observed an unusual pattern of incompressible fractional quantum Hall states that follows the standard composite fermion sequence between filling factors ν = 0 and 1 but involves only even-numerator fractions between ν = 1 and 2. We further investigated this surprising hierarchy by extracting the corresponding energy gaps as a function of the magnetic field. The sequence and relative strengths of the fractional quantum Hall states provide insight into the interplay between electronic correlations and the inherent symmetries of graphene.

  View details for DOI 10.1126/science.1224784

  View details for Web of Science ID 000308414000029

  View details for PubMedID 22955829

• Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene PHYSICAL REVIEW LETTERS Martin, J., Feldman, B. E., Weitz, R. T., Allen, M. T., Yacoby, A. 2010; 105 (25)

  Abstract

  Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. Here we report local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors ν= ± 4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. These results indicate the existence of a zero-field ordered state and are consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at ν=0, ± 1, and ± 2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayer graphene.

  View details for DOI 10.1103/PhysRevLett.105.256806

  View details for Web of Science ID 000286752100005

  View details for PubMedID 21231612

• Broken-symmetry states and divergent resistance in suspended bilayer graphene NATURE PHYSICS Feldman, B. E., Martin, J., Yacoby, A. 2009; 5 (12): 889-893

  View details for DOI 10.1038/NPHYS1406

  View details for Web of Science ID 000273086700016