Pierre A. Pantaleón

Bio: Pierre A. Pantaleón is an academic researcher from IMDEA Nanoscience. The author has contributed to research in topics: Bilayer graphene & Graphene. The author has an hindex of 3, co-authored 6 publications receiving 34 citations.

Band structure of twisted bilayer graphene on hexagonal boron nitride

Abstract: The effect of a hexagonal boron nitride (hBN) layer closely aligned with twisted bilayer graphene (TBG) is studied. At sufficiently low angles between twisted bilayer graphene and hBN, ${\ensuremath{\theta}}_{hBN}\ensuremath{\lesssim}{2}^{\ensuremath{\circ}}$, the graphene electronic structure is strongly disturbed. The width of the low-energy peak in the density of states changes from $W\ensuremath{\sim}5--10$ meV for a decoupled system to $\ensuremath{\sim}20--30$ meV. Spikes in the density of states due to van Hove singularities are smoothed out. We find that for a realistic combination of the twist angle in the TBG and the twist angle between the hBN and the graphene layer the system can be described using a single moir\'e unit cell.

Tunable large Berry dipole in strained twisted bilayer graphene

Abstract: Twisted bilayer graphene is highly sensitive to external perturbations. Strains, and the presence of the substrate, break the symmetries of the central bands. The resulting changes in the Berry curvature lead to valley currents and to a nonlinear Hall effect. We show that these effects, described by a Berry dipole, can be very significant, such that the nonlinear effects surpass the linear response for moderate applied fields, $\ensuremath{\sim}0.1\phantom{\rule{0.28em}{0ex}}\mathrm{mV}/\ensuremath{\mu}\mathrm{m}$. The dependence of these effects on applied strain, coupling to the substrate, density of carriers, and temperature, makes them highly tunable.

Band structure and superconductivity in twisted trilayer graphene

Abstract: We study the symmetries of twisted trilayer graphene's band structure under various extrinsic perturbations, and analyze the role of long-range electron-electron interactions near the first magic angle. The electronic structure is modified by these interactions in a similar way to twisted bilayer graphene. We analyze electron pairing due to long-wavelength charge fluctuations, which are coupled among themselves via the Coulomb interaction and additionally mediated by longitudinal acoustic phonons. We find superconducting phases with either spin-singlet/valley-triplet or spin-triplet/valley-singlet symmetry, with critical temperatures up to a few Kelvin for realistic choices of parameters.

High transmission in twisted bilayer graphene with angle disorder

Abstract: Angle disorder is an intrinsic feature of twisted bilayer graphene and other moir\'e materials. Here, we discuss electron transport in twisted bilayer graphene in the presence of angle disorder. We compute the local density of states and the Landauer-B\"uttiker transmission through an angle disorder barrier with a width comparable to the moir\'e period, using a decimation technique based on a real-space description. We find that barriers which separate regions where the widths of the bands differ by $50%$ or more lead to a minor suppression of the transmission and that the transmission is close to 1 for normal incidence, which is reminiscent of Klein tunneling. These results suggest that transport in twisted bilayer graphene is weakly affected by twist angle disorder.

Narrow bands, electrostatic interactions and band topology in graphene stacks

Abstract: The occurrence of superconducting and insulating phases is well-established in twisted graphene bilayers, and they have also been reported in other arrangements of graphene layers. We investigate three such arrangements: untwisted AB bilayer graphene on an hBN substrate, two graphene bilayers twisted with respect to each other, and a single ABC stacked graphene trilayer on an hBN substrate. Narrow bands with different topology occur in all cases, producing a high density of states which enhances the role of interactions. We investigate the effect of the long range Coulomb interaction, treated within the self consistent Hartree-Fock approximation. We find that the on-site part of the Fock potential strongly modifies the band structure at charge neutrality. The Hartree part does not significantly modify the shape and width of the bands in the three cases considered here, in contrast to the effect that such a potential has in twisted bilayer graphene.

Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Part III: Scanning Probe Magnetometry

Abstract: Quantum anomalous Hall goes intrinsic Quantum anomalous Hall effect—the appearance of quantized Hall conductance at zero magnetic field—has been observed in thin films of the topological insulator Bi2Se3 doped with magnetic atoms. The doping, however, introduces inhomogeneity, reducing the temperature at which the effect occurs. Two groups have now observed quantum anomalous Hall effect in intrinsically magnetic materials (see the Perspective by Wakefield and Checkelsky). Serlin et al. did so in twisted bilayer graphene aligned to hexagonal boron nitride, where the effect enabled the switching of magnetization with tiny currents. In a complementary work, Deng et al. observed quantum anomalous Hall effect in the antiferromagnetic layered topological insulator MnBi2Te4. Science, this issue p. 900, p. 895; see also p. 848 Transport measurements indicate quantized Hall conductance without a magnetic field. The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field. We report the observation of a QAH effect in twisted bilayer graphene aligned to hexagonal boron nitride. The effect is driven by intrinsic strong interactions, which polarize the electrons into a single spin- and valley-resolved moiré miniband with Chern number C = 1. In contrast to magnetically doped systems, the measured transport energy gap is larger than the Curie temperature for magnetic ordering, and quantization to within 0.1% of the von Klitzing constant persists to temperatures of several kelvin at zero magnetic field. Electrical currents as small as 1 nanoampere controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory.

Charge order and broken rotational symmetry in magic-angle twisted bilayer graphene

Abstract: Bilayer graphene can be modified by rotating (twisting) one layer with respect to the other. The interlayer twist gives rise to a moiré superlattice that affects the electronic motion and alters the band structure1–4. Near a ‘magic angle’ of twist2,4, where the emergence of a flat band causes the charge carriers to slow down3, correlated electronic phases including Mott-like insulators and superconductors were recently discovered5–8 by using electronic transport. These measurements revealed an intriguing similarity between magic-angle twisted bilayer graphene and high-temperature superconductors, which spurred intensive research into the underlying physical mechanism9–14. Essential clues to this puzzle, such as the symmetry and spatial distribution of the spectral function, can be accessed through scanning tunnelling spectroscopy. Here we use scanning tunnelling microscopy and spectroscopy to visualize the local density of states and charge distribution in magic-angle twisted bilayer graphene. Doping the sample to partially fill the flat band, we observe a pseudogap phase accompanied by a global stripe charge order that breaks the rotational symmetry of the moiré superlattice. Both the pseudogap and the stripe charge order disappear when the band is either empty or full. The close resemblance to similar observations in high-temperature superconductors15–21 provides new evidence of a deeper link underlying the phenomenology of these systems.When scanning tunnelling spectroscopy is used to map the electronic structure of magic-angle twisted bilayer graphene, a pseudogap phase is found, accompanied by a global charge-ordered stripe phase.

Detecting topological currents in graphene superlattices

Abstract: Making use of graphene's valleys Graphene has two distinct valleys in its electronic structure, in which the electrons have the same energy. Theorists have predicted that creating an asymmetry between the two valleys will coax graphene into exhibiting the so-called valley Hall effect (VHE). In this effect, electrons from the two valleys move across the sample in opposite directions when the experimenters run current along the sample. Gorbachev et al. achieved this asymmetry by aligning graphene with an underlying layer of hexagonalboron nitride (hBN) (see the Perspective by Lundeberg and Folk). The authors measured the transport characteristics of the sample, which were consistent with the theoretical predictions for the VHE. The method may in the future lead to information processing using graphene's valleys. Science, this issue p. 448; see also p. 422 Graphene is aligned with a layer of hexagonal boron nitride to achieve the valley Hall effect. [Also see Perspective by Lundeberg and Folk] Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene’s two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several micrometers away from the nominal current path. Locally, topological currents are comparable in strength with the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by means of gate voltage can be exploited for information processing based on valley degrees of freedom.