Pilkwang Kim

Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices

Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum. Known as Hofstadter’s butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely. Here we demonstrate that moiré superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.C. R. Dean, L. Wang, P. Maher, C. Forsythe, F. Ghahari, Y. Gao, J. Katoch, M. Ishigami, P. Moon, M. Koshino, T. Taniguchi, K. Watanabe, K. L. Shepard, J. Hone, and P. Kim Department of Electrical Engineering, Columbia University, New York, NY Department of Mechanical Engineering, Columbia University, New York, NY Department of Physics, Columbia University, New York, NY Department of Physics and Nanoscience Technology Center, University of Central Florida, Orlando, FL 5 Department of Physics, Tohoku University, Sendai, Japan and National Institute for Materials Science, 1-1 Namiki, Tsukuba, Japan

Bright visible light emission from graphene

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.

Specular interband Andreev reflections at van der Waals interfaces between graphene and NbSe2

D. K. Efetov, ∗ L. Wang, C. Handschin, K. B. Efetov, 4 J. Shuang, R. Cava, T. Taniguchi, K. Watanabe, J. Hone, C. R. Dean, and P. Kim † Department of Physics, Columbia University, New York, NY 10027, USA Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA Theoretische Physik III, Ruhr-Universität Bochum, D-44780 Bochum, Germany National University of Science and Technology “MISiS”, Moscow, 119049, Russia Department of Chemistry, Princeton University, Princeton, NJ 08544, USA National Institute for Materials Science, Namiki 1-1, Ibaraki 305-0044, Japan (Dated: May 20, 2015)

Ising-Type Magnetic Ordering in Atomically Thin FePS3

Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order-disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.

Investigation of inelastic electron tunneling spectra of metal-molecule-metal junctions fabricated using direct metal transfer method

We measured the inelastic electron tunneling spectroscopy (IETS) characteristics of metal-molecule-metal junctions made with alkanethiolate self-assembled monolayers. The molecular junctions were fabricated using a direct metal transfer method, which we previously reported for high-yield metal-molecule-metal junctions. The measured IETS data could be assigned to molecular vibration modes that were determined by the chemical structure of the molecules. We also observed discrepancies and device-to-device variations in the IETS data that possibly originate from defects in the molecular junctions and insulating walls introduced during the fabrication process and from the junction structure.

A new approach for high-yield metal-molecule-metal junctions by direct metal transfer method.

The realization of high-yield, stable molecular junctions has been a long-standing challenge in the field of molecular electronics research, and it is an essential prerequisite for characterizing and understanding the charge transport properties of molecular junctions prior to their device applications. Here, we introduce a new approach for obtaining high-yield, vertically structured metal-molecule-metal junctions in which the top metal electrodes are formed on alkanethiolate self-assembled monolayers by a direct metal transfer method without the use of any additional protecting interlayers in the junctions. The fabricated alkanethiolate molecular devices exhibited considerably improved device yields (∼70%) in comparison to the typical low device yields (less than a few %) of molecular junctions in which the top metal electrodes are fabricated using the conventional evaporation method. We compared our method with other molecular device fabrication methods in terms of charge transport parameters. This study suggests a potential new device platform for realizing robust, high-yield molecular junctions and investigating the electronic properties of devices.

Universality of periodicity as revealed from interlayer-mediated cracks

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

The electronic structure and intervalley coupling of artificial and genuine graphene superlattices

The so-called artificial graphene is an artificial material whose low-energy carriers are described by the massless Dirac equation. Applying a periodic potential with triangular symmetry to a two-dimensional electron gas is one approach to make such a material. According to recent experimental results, it is now possible to realize artificial graphene in the lab and to even apply an additional lateral, one-dimensional periodic potential to it. We name the latter system an artificial graphene superlattice in order to distinguish it from a genuine graphene superlattice made from graphene. In this study, we investigate the electronic structure of artificial graphene superlattices, which exhibit the emergence of energy band gaps, merging and splitting of the Dirac points, etc. Then, from a similar investigation on genuine graphene superlattices, we show that many of these features originate from the coupling between Dirac fermions residing in two different valleys—the intervalley coupling. Furthermore, contrary to previous studies, we find that the effects of intervalley coupling on the electronic structure cannot be ignored, irrespective of the length of the spatial period of the superlattice.

Erratum to: The electronic structure and intervalley coupling of artificial and genuine graphene superlattices

The original version of this article unfortunately failed to cite four relevant papers (Refs. [53–56]). References [53, 54] report on a universal tight binding Hamiltonian of graphene. With an adjustable hopping parameter, the Hamiltonian successfully describes the merging of Dirac points and opening up of a band gap. References [55, 56] report that the energy spectrum of a twisted bilayer graphene exhibits merging and splitting of the Dirac points. The first sentence in the third paragraph of Section 3.2 in page 1109, instead of “... adopting single-valley approximation [45, 46, 50]”, should read “...adopting single-valley approximation [45, 46, 50], although there have been studies on universal tight-binding Hamiltonians on graphene and on twisted bilayer graphene predicting similar features [53–56].” The second last sentence of Section 3.2, instead of “... in the previous studies adopting single-valley approximation [46, 53]:”, should read “... in the previous studies adopting single-valley approximation [46, 57]:”. The last part of References should be, instead of