M. Mucha-Kruczynski

University of Bath, Bath, England, United Kingdom

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Publications (29)142.5 Total impact

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    Xi Chen · J. R. Wallbank · M. Mucha-Kruczynski · E. McCann · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moire potential. Due to inversion-symmetry breaking characteristic for the moire potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of secondary Dirac points in the valence band, which we confirm by numerical simulations. Then, we analyze how single-particle gaps in the fractal energy spectrum produce a sequence of incompressible states observable under a variation of carrier density and magnetic field.
    Preview · Article · Mar 2016
  • M. Mucha-Kruczyński · J. R. Wallbank · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We identify features in the angle-resolved photoemission spectra (ARPES) arising from the periodic pattern characteristic for graphene heterostructure with hexagonal boron nitride (hBN). For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moiré superlattice in graphene/hBN systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and hBN. We also analyze how the presence of a moiré-periodic strain in graphene or scattering of photoemitted electrons off hBN can be distinguished from the miniband formation.
    No preview · Article · Feb 2016
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    M. Mucha-Kruczynski · J. R. Wallbank · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We model the angle-resolved photoemission spectra (ARPES) of graphene on hexagonal boron nitride (hBN) and show their characteristic features arising due to the formation of miniband structure for graphene electrons in the periodic moire pattern. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and hBN. We also show how the presence of a moire-periodic strain in graphene or scattering of photoemitted electrons off hBN can be distinguished from the miniband formation.
    Preview · Article · Nov 2015
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    [Show abstract] [Hide abstract] ABSTRACT: Bilayer graphene is a highly tunable material: not only can one tune the Fermi energy using standard gates, as in single-layer graphene, but the band structure can also be modified by external perturbations such as transverse electric fields or strain. We review the theoretical basics of the band structure of bilayer graphene and study the evolution of the band structure under the influence of these two external parameters. We highlight their key role concerning the ease to experimentally probe the presence of a Lifshitz transition, which consists in a change of Fermi contour topology as a function of energy close to the edges of the conduction and valence bands. Using a device geometry that allows the application of exceptionally high displacement fields, we then illustrate in detail the way to probe the topology changes experimentally using quantum Hall effect measurements in a gapped bilayer graphene system.
    Full-text · Article · Aug 2015 · Synthetic Metals
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    Marcin Mucha-Kruczyński · D. S. L. Abergel
    [Show abstract] [Hide abstract] ABSTRACT: We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely-aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for almost completely full first miniband is necessary to extract meaningful information about the moire characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moire parameters.
    Full-text · Article · Jul 2015 · Physical Review B
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    John R. Wallbank · Marcin Mucha-Kruczyński · Xi Chen · Vladimir I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moir\'e superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly.
    Full-text · Article · Nov 2014 · Annalen der Physik
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    Diana A. Cosma · Marcin Mucha-Kruczynski · Henning Schomerus · Vladimir I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We investigate the effects of homogeneous and inhomogeneous deformations and edge disorder on the conductance of gated graphene nanoribbons. Under increasing homogeneous strain the conductance of such devices initially decreases before it acquires a resonance structure, and finally becomes completely suppressed at larger strain. Edge disorder induces mode mixing in the contact regions, which can restore the conductance to its ballistic value. The valley-antisymmetric pseudo-magnetic field induced by inhomogeneous deformations leads to the formation of additional resonance states, which either originate from the coupling into Fabry-Perot states that extend through the system, or from the formation of states that are localized near the contacts, where the pseudo-magnetic field is largest. In particular, the n=0 pseudo-Landau level manifests itself via two groups of conductance resonances close to the charge neutrality point.
    Full-text · Article · Sep 2014 · Physical review. B, Condensed matter
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    [Show abstract] [Hide abstract] ABSTRACT: Fermi surface topology plays an important role in determining the electronic properties of metals1. In bulk metals, the Fermi energy is not easily tunable at the energy scale needed for reaching conditions for the Lifshitz transition - a singular point in the band structure where the connectivity of the Fermi surface changes. Bilayer graphene2,3 is a unique system where both Fermi energy and the low-energy electron dispersion can be tuned using the interplay between trigonal warping and a band gap opened by a transverse electric field3-7. Here, we drive the Lifshitz transition to experimentally controllable carrier densities by applying large transverse electric fields through a h-BN-encapsulated bilayer graphene structure, and detect it by measuring the degeneracies of Landau levels. These degeneracies are revealed by filling factor -3 and -6 quantum Hall effect states of holes at low magnetic fields reflecting the existence of three maxima on the top of the valence band dispersion. At high magnetic fields all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly-connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample, and this enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum.
    Full-text · Article · Mar 2014 · Physical Review Letters
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    Xi Chen · J. R. Wallbank · A. A. Patel · M. Mucha-Kruczynski · E. McCann · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We find a systematic reappearance of massive Dirac features at the edges of consecutive minibands formed at magnetic fields B_{p/q}= p\phi_0/(qS) providing rational magnetic flux through a unit cell of the moire superlattice created by a hexagonal substrate for electrons in graphene. The Dirac-type features in the minibands at B=B_{p/q} determine a hierarchy of gaps in the surrounding fractal spectrum, and show that these minibands have topological insulator properties. Using the additional $q$-fold degeneracy of magnetic minibands at B_{p/q}, we trace the hierarchy of the gaps to their manifestation in the form of incompressible states upon variation of the carrier density and magnetic field.
    Full-text · Article · Oct 2013 · Physical Review B
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    D. S. L. Abergel · J. R. Wallbank · Xi Chen · M. Mucha-Kruczyński · Vladimir I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We propose a theory of optical absorption in monolayer graphene-hexagonal boron nitride (hBN) heterostructures. In highly oriented heterostructures, the hBN underlay produces a long-range moir\'e superlattice potential for the graphene electrons which modifies the selection rules for absorption of incoming photons in the infrared to visible frequency range. The details of the absorption spectrum modification depend on the relative strength of the various symmetry-allowed couplings between the graphene electrons and the hBN, and the resulting nature of the reconstructed band structure.
    Preview · Article · Sep 2013 · New Journal of Physics
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    Diana A Gradinar · Marcin Mucha-Kruczyński · Henning Schomerus · Vladimir I Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: In inhomogeneously strained graphene, low-energy electrons experience a valley-antisymmetric pseudomagnetic field which leads to the formation of localized states at the edge between the valence and conduction bands, understood in terms of peculiar n=0 pseudomagnetic Landau levels. Here we show that such states can manifest themselves as an isolated quadruplet of low-energy conductance resonances in a suspended stretched graphene ribbon, where clamping by the metallic contacts results in a strong inhomogeneity of strain near the ribbon ends.
    Preview · Article · Jun 2013 · Physical Review Letters
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    J. R. Wallbank · M. Mucha-Kruczynski · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We present a phenomenological theory of the low energy moir\'e minibands of Dirac electrons in graphene placed on an almost commensurate hexagonal underlay with a unit cell pproximately three times larger than that of graphene.A slight incommensurability results in a periodically modulated intervalley scattering for electrons in graphene. In contrast to the perfectly commensurate Kekul\'e distortion of graphene, such supperlattice perturbation leaves the zero energy Dirac cones intact, but is able to open a band gap at the edge of the first moir\'e subbband, asymmetrically in the conduction and valence bands.
    Full-text · Article · Jun 2013 · Physical Review B
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    [Show abstract] [Hide abstract] ABSTRACT: Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter's butterfly) has been limited to the observation of new low-field oscillations and an internal structure within Landau levels. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate's moiré potential acts as a superlattice and leads to profound changes in the graphene's electronic spectrum. Second-generation Dirac points appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene's conduction and valence bands. Strong magnetic fields lead to Zak-type cloning of the third generation of Dirac points, which are observed as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures.
    Full-text · Article · May 2013 · Nature
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    M. Mucha-Kruczynski · J. R. Wallbank · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: We study the superlattice minibands produced by the interplay between moir\'e pattern induced by hexagonal BN substrate on graphene layer and the interlayer coupling in bilayer graphene with Bernal stacking (BLG). We compare moir\'e miniband features in BLG, where they are affected by the interlayer asymmetry of BLG-hBN heterostructure and trigonal warping characteristic for electrons in Bernal-stacked bilayers with those found in monolayer graphene.
    Full-text · Article · Apr 2013 · Physical Review B
  • Marcin Mucha-Kruczynski
    [Show abstract] [Hide abstract] ABSTRACT: As discussed in Sect. 4.3, experimental measurements of the bilayer graphene Landau level structure with infrared absorption showed that tight-binding description for neutral bilayer is unable to describe all the important physics. Some theoretical explanations were suggested, based both on many-body effects and charging effects, but the issue has not yet been clarified. It would be therefore beneficial to have at one’s disposal another probe of the Landau level structure but with different selection rules. Then, electronic excitations between different pairs of levels would be measured. This could help gain more insight into the physics of the problem.
    No preview · Chapter · Jan 2013
  • Marcin Mucha-Kruczyński
    [Show abstract] [Hide abstract] ABSTRACT: With many models in physics, it is much easier to conceive a Gedankenexperiment and analyse it on paper, rather than prepare a real-life experiment. It is definitely the case when imagining a single plane of carbon atoms arranged in a honeycomb (hexagonal) pattern.
    No preview · Article · Jan 2013
  • Marcin Mucha-Kruczynski
    [Show abstract] [Hide abstract] ABSTRACT: The angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental tool based on the photoelectric effect, first observed by Hertz more than 120 years ago [1] and explained by Einstein at the beginning of the previous century with the help of the then novel idea of photons, quanta of electromagnetic radiation.
    No preview · Chapter · Jan 2013
  • Marcin Mucha-Kruczynski
    [Show abstract] [Hide abstract] ABSTRACT: In this chapter, we describe the crystal and reciprocal lattices of bilayer graphene. We also discuss briefly the symmetry of the crystal lattice. We then introduce the tight-binding model for \(\pi \) electrons in bilayer graphene. We start with a general formulation valid for all points in the Brillouin zone and the resulting electronic structure. Next, we concentrate on the linear approximation of that model around the corners of the Brillouin zone.
    No preview · Chapter · Jan 2013
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    J. R. Wallbank · A. A. Patel · M. Mucha-Kruczynski · A. K. Geim · V. I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: Using a general symmetry-based approach, we provide a classification of generic miniband structures for electrons in graphene placed on substrates with the hexagonal Bravais symmetry. In particular, we identify conditions at which the first moir\'e miniband is separated from the rest of the spectrum by either one or a group of three isolated mini Dirac points and is not obscured by dispersion surfaces coming from other minibands. In such cases the Hall coefficient exhibits two distinct alternations of its sign as a function of charge carrier density.
    Full-text · Article · Nov 2012 · Physical review. B, Condensed matter
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    M. Mucha-Kruczynski · V.I. Fal'ko
    [Show abstract] [Hide abstract] ABSTRACT: Combining the tight-binding approximation and linear elasticity theory for a planar membrane, we investigate stretching of a graphene flake assuming that two opposite edges of the sample are clamped by the contacts. We show that, depending on the aspect ratio of the flake and its orientation, gapped states may form in the membrane in the vicinity of the contacts. This gap in the pre-contact region should be the biggest for the armchair orientation of the flake and W/L less than or similar to 1. (c) 2012 Elsevier Ltd. All rights reserved.
    Full-text · Article · Jul 2012 · Solid State Communications