| 1. |
- Bergvall, Anders, 1981, et al.
(författare)
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Basic Theory of Electron Transport Through Molecular Contacts
- 2016
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Ingår i: Handbook of Single-Molecule Electronics. - 9789814463393 ; , s. 31-78
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In this chapter, we will introduce a basic theoretical description of coherent electron transport through low-dimensional junctions and molecular devices. The description introduced is based on quantum transport theory using a tight-binding description of molecules and lead materials. We apply this theory in a few worked examples on junctions based on graphene and carbon nanotubes and on molecular-superconducting hybrid junctions.
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| 2. |
- Bergvall, Anders, 1981, et al.
(författare)
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Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:15
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Tidskriftsartikel (refereegranskat)abstract
- We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.
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| 3. |
- Bergvall, Anders, 1981, et al.
(författare)
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Influence of 0001 tilt grain boundaries on the destruction of the quantum Hall effect in graphene
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 91:24
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Tidskriftsartikel (refereegranskat)abstract
- The half-integer quantum Hall effect (QHE) is often suppressed in graphene grown by chemical vapor deposition on metals. The reason behind the suppression is unclear, and we hypothesize that it might be connected to extended defects in the material. In this paper we present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain-boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to nonzero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material.
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| 4. |
- Bergvall, Anders, 1981
(författare)
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Quantum Transport Theory in Graphene
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, we focus on different aspects of electron transport in nanostructured graphene (such as graphene nanoribbons). Wedevelop and implement numerical methods to study quantum coherent electron transport on an atomistic level, complemented by analytical calculations based on the Dirac approximation valid close to the points $\vec{K}$ and $\vec{K}^\prime$ in the graphene Brillouin zone. By simulating a graphene nanogap bridged with 1,4-phenylene-diamine molecules anchored via $C_{60}$ molecules, we show that a transistoreffect can be achieved by back-gating the system. By simulating STM-measurements on nanoribbons with single impurities, we investigatethe interplay between size quantization and the local scatterers, and show analytically how the features of the Fourier transformed local density of states can be explained by electrons scattering between different transverse modes of the ribbons. We extend the analys to also include analytical transport calculations, and explain the origin of characteristic dips found in the transmission and their relationsto quasi-bound states formed around the ribbon impurities. We construct and simulate graphene ribbons with transverse grain boundaries,and illustrate how such grain boundaries form metallic states bridging the two edges of the ribbon together. This is a plausible candidateto explain the attenuation (or even destruction) of the quantum Hall effect often seen in quantum Hall bar measurements, especiallywith graphene grown on metals (such as copper) where grain boundaries are common. The introductory chapters also present a basic introductionto the field of graphene and graphene ribbons, and we thoroughly present the tight-binding techniques used for simulation.
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| 5. |
- Bergvall, Anders, 1981, et al.
(författare)
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Spectral footprints of impurity scattering in graphene nanoribbons
- 2013
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:20
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Tidskriftsartikel (refereegranskat)abstract
- We report a detailed investigation of the interplay between size quantization and local scattering centers in graphene nanoribbons, as seen in the local density of states. The spectral signatures, obtained after Fourier transformation of the local density of states, include characteristic peaks that can be related to the transverse modes of the nanoribbon. In armchair ribbons, the Fourier transformed density of states of one of the two inequivalent sublattices takes a form similar to that of a quantum channel in a two-dimensional electron gas, modified according to the differences in band structure. After addition of the second sublattice contribution, a characteristic modulation of the pattern due to superposition is obtained, similar to what has been obtained in spectra due to single impurity scattering in large-area graphene. We present analytic results for the electron propagator in armchair nanoribbons in the Dirac approximation, including a single scattering center within a T-matrix formulation. For comparison, we have extended the investigation with numerics obtained with an atomistic recursive Green's function approach. The spectral signatures of the atomistic approach include the effects of trigonal warping. The impurity induced oscillations in the local density of states are not decaying at large distance in few-mode nanoribbons.
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| 6. |
- Slutskin, A.A., et al.
(författare)
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Non-Markovian decoherence of a two-level system weakly coupled to a bosonic bath
- 2011
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 96:4, s. 4003-
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Tidskriftsartikel (refereegranskat)abstract
- The Bloch-Redfield equation is a common tool for studying the evolution ofqubit systems weakly coupled to the environment. We investigate the accuracy of the Bornapproximation underlying this equation. We find that the high-order terms in the perturbativeexpansion contain accumulating divergences that make straightforward Born approximationinappropriate. We develop a diagrammatic technique to formulate, and solve the improvedself-consistent Born approximation. This more accurate treatment reveals an exponential timedependentprefactor in the non-Markovian contribution dominating the qubit long-time relaxationfound in Di Vincenzo D. P. and Loss D., Phys. Rev. B, 71 (2005) 035318. At the same time, theassociated dephasing is not affected and is described by the Born-Markov approximation.
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| 7. |
- Tesch, J., et al.
(författare)
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Impurity scattering and size quantization effects in a single graphene nanoflake
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 95:7
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Tidskriftsartikel (refereegranskat)abstract
- By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed.
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