Graphene Nanobubbles as Valley Filters and Beam SplittersSettnes, Mikkel; Power, Stephen R.; Brandbyge, Mads; Jauho, Antti-Pekka
doi: 10.1103/PhysRevLett.117.276801pmid: 28084750
The energy band structure of graphene has two inequivalent valleys at the K and K ′ points of the Brillouin zone. The possibility to manipulate this valley degree of freedom defines the field of valleytronics, the valley analogue of spintronics. A key requirement for valleytronic devices is the ability to break the valley degeneracy by filtering and spatially splitting valleys to generate valley polarized currents. Here, we suggest a way to obtain valley polarization using strain-induced inhomogeneous pseudomagnetic fields (PMFs) that act oppositely on the two valleys. Notably, the suggested method does not involve external magnetic fields, or magnetic materials, unlike previous proposals. In our proposal the strain is due to experimentally feasible nanobubbles, whose associated PMFs lead to different real space trajectories for K and K ′ electrons, thus allowing the two valleys to be addressed individually. In this way, graphene nanobubbles can be exploited in both valley filtering and valley splitting devices, and our simulations reveal that a number of different functionalities are possible depending on the deformation field.
Honeycomb-Lattice Heisenberg-Kitaev Model in a Magnetic Field: Spin Canting, Metamagnetism, and Vortex CrystalsJanssen, Lukas; Andrade, Eric C.; Vojta, Matthias
doi: 10.1103/PhysRevLett.117.277202pmid: 28084771
The Heisenberg-Kitaev model is a paradigmatic model to describe the magnetism in honeycomb-lattice Mott insulators with strong spin-orbit coupling, such as A 2 IrO 3 ( A = Na , Li ) and α - RuCl 3 . Here, we study in detail the physics of the Heisenberg-Kitaev model in an external magnetic field. Using a combination of Monte Carlo simulations and spin-wave theory, we map out the classical phase diagram for different directions of the magnetic field. Broken SU(2) spin symmetry renders the magnetization process rather complex, with sequences of phases and metamagnetic transitions. In particular, we find various large-unit-cell and multi- Q phases including a vortex-crystal phase for a field in the ( 111 ) direction. We also discuss quantum corrections in the high-field phase.
Si Nanoribbons on Ag(110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain StructurePrévot, Geoffroy; Hogan, Conor; Leoni, Thomas; Bernard, Romain; Moyen, Eric; Masson, Laurence
doi: 10.1103/PhysRevLett.117.276102pmid: 28084766
We report a combined grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and density-functional theory (DFT) study which clearly elucidates the atomic structure of the Si nanoribbons grown on the missing-row reconstructed Ag(110) surface. Our study allows us to discriminate between the theoretical models published in the literature, including the most stable atomic configurations and those based on a missing-row reconstructed Ag(110) surface. GIXD measurements unambiguously validate the pentamer model grown on the reconstructed surface, obtained from DFT. This pentamer atomistic model accurately matches the high-resolution STM images of the Si nanoribbons adsorbed on Ag(110). Our study closes the long-debated atomic structure of the Si nanoribbons grown on Ag(110) and definitively excludes a honeycomb structure similar to that of freestanding silicene.
Shape Coexistence in Ni 78 as the Portal to the Fifth Island of InversionNowacki, F.; Poves, A.; Caurier, E.; Bounthong, B.
doi: 10.1103/PhysRevLett.117.272501pmid: 28084779
Large-scale shell-model calculations predict that the region of deformation which comprises the heaviest chromium and iron isotopes at and beyond N = 40 will merge with a new one at N = 50 in an astonishing parallel to the N = 20 and N = 28 case in the neon and magnesium isotopes. We propose a valence space including the full p f shell for the protons and the full s d g shell for the neutrons, which represents a comeback of the the harmonic oscillator shells in the very neutron- rich regime. The onset of deformation is understood in the framework of the algebraic SU(3)-like structures linked to quadrupole dominance. Our calculations preserve the doubly magic nature of the ground state of Ni 78 , which, however, exhibits a well-deformed prolate band at low excitation energy, providing a striking example of shape coexistence far from stability. This new IOI adds to the four well-documented ones at N = 8 , 20, 28, and 40.
Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First PrinciplesDewandre, Antoine; Hellman, Olle; Bhattacharya, Sandip; Romero, Aldo H.; Madsen, Georg K. H.; Verstraete, Matthieu J.
doi: 10.1103/PhysRevLett.117.276601pmid: 28084752
The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures.
Charge Transport by Superexchange in Molecular Host-Guest SystemsSymalla, Franz; Friederich, Pascal; Massé, Andrea; Meded, Velimir; Coehoorn, Reinder; Bobbert, Peter; Wenzel, Wolfgang
doi: 10.1103/PhysRevLett.117.276803pmid: 28084749
Charge transport in disordered organic semiconductors is generally described as a result of incoherent hopping between localized states. In this work, we focus on multicomponent emissive host-guest layers as used in organic light-emitting diodes (OLEDs), and show using multiscale ab initio based modeling that charge transport can be significantly enhanced by the coherent process of molecular superexchange. Superexchange increases the rate of emitter-to-emitter hopping, in particular if the emitter molecules act as relatively deep trap states, and allows for percolation path formation in charge transport at low guest concentrations.
Weaving Knotted Vector Fields with Tunable HelicityKedia, Hridesh; Foster, David; Dennis, Mark R.; Irvine, William T. M.
doi: 10.1103/PhysRevLett.117.274501pmid: 28084747
We present a general construction of divergence-free knotted vector fields from complex scalar fields, whose closed field lines encode many kinds of knots and links, including torus knots, their cables, the figure-8 knot, and its generalizations. As finite-energy physical fields, they represent initial states for fields such as the magnetic field in a plasma, or the vorticity field in a fluid. We give a systematic procedure for calculating the vector potential, starting from complex scalar functions with knotted zero filaments, thus enabling an explicit computation of the helicity of these knotted fields. The construction can be used to generate isolated knotted flux tubes, filled by knots encoded in the lines of the vector field. Lastly, we give examples of manifestly knotted vector fields with vanishing helicity. Our results provide building blocks for analytical models and simulations alike.