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Search Results: 1 - 10 of 338686 matches for " S?ren Ulstrup "
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Nonequilibrium electron-vibration coupling and conductance fluctuations in a C60-junction
Sren Ulstrup,Thomas Frederiksen,Mads Brandbyge
Physics , 2012, DOI: 10.1103/PhysRevB.86.245417
Abstract: We investigate chemical bond formation and conductance in a molecular C60-junction under finite bias voltage using first-principles calculations based on density functional theory and nonequilibrium Green's functions (DFT-NEGF). At the point of contact formation we identify a remarkably strong coupling between the C60-motion and the molecular electronic structure. This is only seen for positive sample bias, although the conductance itself is not strongly polarity dependent. The nonequilibrium effect is traced back a sudden shift in the position of the voltage drop with a small C60-displacement. Combined with a vibrational heating mechanism we construct a model from our results that explain the polarity-dependent two-level conductance fluctuations observed in recent scanning tunneling microscopy (STM) experiments [N. N\'eel et al., Nano Lett. 11, 3593 (2011)]. These findings highlight the significance of nonequilibrium effects in chemical bond formation/breaking and in electron-vibration coupling in molecular electronics.
Extracting the Temperature of Hot Carriers in Time- and Angle-Resolved Photoemission
Sren Ulstrup,Jens Christian Johannsen,Marco Grioni,Philip Hofmann
Physics , 2014, DOI: 10.1063/1.4863322
Abstract: The interaction of light with a material's electronic system creates an out-of-equilibrium (non-thermal) distribution of optically excited electrons. Non-equilibrium dynamics relaxes this distribution on an ultrafast timescale to a hot Fermi-Dirac distribution with a well-defined temperature. The advent of time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiments has made it possible to track the decay of the temperature of the excited hot electrons in selected states in the Brillouin zone, and to reveal their cooling in unprecedented detail in a variety of emerging materials. It is, however, not a straightforward task to determine the temperature with high accuracy. This is mainly attributable to an a priori unknown position of the Fermi level and the fact that the shape of the Fermi edge can be severely perturbed when the state in question is crossing the Fermi energy. Here, we introduce a method that circumvents these difficulties and accurately extracts both the temperature and the position of the Fermi level for a hot carrier distribution by tracking the occupation statistics of the carriers measured in a TR-ARPES experiment.
High-temperature behaviour of supported graphene: electron-phonon coupling and substrate-induced doping
Sren Ulstrup,Marco Bianchi,Richard Hatch,Dandan Guan,Alessandro Baraldi,Dario Alfè,Liv Hornek?r,Philip Hofmann
Physics , 2012, DOI: 10.1103/PhysRevB.86.161402
Abstract: One of the salient features of graphene is the very high carrier mobility that implies tremendous potential for use in electronic devices. Unfortunately, transport measurements find the expected high mobility only in freely suspended graphen. When supported on a surface, graphene shows a strongly reduced mobility, and an especially severe reduction for temperatures above 200 K. A temperature-dependent mobility reduction could be explained by scattering of carriers with phonons, but this is expected to be weak for pristine, weakly-doped graphene. The mobility reduction has therefore been ascribed to the interaction with confined ripples or substrate phonons. Here we study the temperature-dependent electronic structure of supported graphene by angle-resolved photoemission spectroscopy, a technique that can reveal the origin of the phenomena observed in transport measurements. We show that the electron-phonon coupling for weakly-doped, supported graphene on a metal surface is indeed extremely weak, reaching the lowest value ever reported for any material. However, the temperature-dependent dynamic interaction with the substrate leads to a complex and dramatic change in the carrier type and density that is relevant for transport. Using ab initio molecular dynamics simulations, we show that these changes in the electronic structure are mainly caused by fluctuations in the graphene-substrate distance.
Detecting the local transport properties and the dimensionality of transport of epitaxial graphene by a multi-point probe approach
Lucas Barreto,Edward Perkins,Jens Johannsen,Sren Ulstrup,Felix Fromm,Christian Raidel,Thomas Seyller,Philip Hofmann
Physics , 2012, DOI: 10.1063/1.4789508
Abstract: The electronic transport properties of epitaxial monolayer graphene (MLG) and hydrogen-intercalated quasi free-standing bilayer graphene (QFBLG) on SiC(0001) are investigated by micro multi-point probes. Using a probe with 12 contacts, we perform four-point probe measurements with the possibility to effectively vary the contact spacing over more than one order of magnitude, allowing us to establish that the transport is purely two-dimensional. Combined with the carrier density obtained by angle-resolved photoemission spectroscopy, we find the room temperature mobility of MLG to be (870+-120)cm2/Vs. The transport in QFBLG is also found to be two-dimensional with a mobility of (1600+-160) cm2/Vs.
High Crystallinity and Decoupling of Graphene on a Metal: Reduced Coulomb Screening and Tunable pn-Junctions
Sren Ulstrup,Mie Andersen,Marco Bianchi,Lucas Barreto,Bj?rk Hammer,Liv Hornek?r,Philip Hofmann
Physics , 2014,
Abstract: High quality epitaxial graphene films can be applied as templates for tailoring graphene-substrate interfaces that allow for precise control of the charge carrier behavior in graphene through doping and many-body effects. By combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy and density functional theory we demonstrate that oxygen intercalated epitaxial graphene on Ir(111) has high structural quality, is quasi free-standing, and shows signatures of many-body interactions. Using this system as a template, we show that tunable pn-junctions can be patterned by adsorption and intercalation of rubidium, and that the n-doped graphene regions exhibit a reduced Coulomb screening via enhanced electron-plasmon coupling. These findings are central for understanding and tailoring the properties of graphene-metal contacts e.g. for realizing quantum tunneling devices.
Electronic Structure of Epitaxial Single-Layer MoS$_2$
Jill A. Miwa,Sren Ulstrup,Signe G. S?rensen,Maciej Dendzik,Antonija Grubi?i? ?abo,Marco Bianchi,Jeppe Vang Lauritsen,Philip Hofmann
Physics , 2014, DOI: 10.1103/PhysRevLett.114.046802
Abstract: The electronic structure of epitaxial single-layer MoS$_2$ on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39$\pm$0.05)~eV direct band gap at $\bar{K}$ with the valence band top at $\bar{\Gamma}$ having a significantly higher binding energy than at $\bar{K}$. The moir\'e superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at $\bar{K}$ is found to be lifted by the spin-orbit interaction, leading to a splitting of (145$\pm$4)~meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the strength of the potassium doping is $k$-dependent, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.
Synthesis of Epitaxial Single-Layer MoS$_2$ on Au(111)
Signe S. Gr?nborg,Sren Ulstrup,Marco Bianchi,Maciej Dendzik,Charlotte E. Sanders,Jeppe V. Lauritsen,Philip Hofmann,Jill A. Miwa
Physics , 2015,
Abstract: We present a method for synthesizing large area epitaxial single-layer MoS$_2$ on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling microscopy and low energy electron diffraction, the evolution of the growth is followed from nanoscale single-layer MoS$_2$ islands to a continuous MoS$_2$ layer. An exceptionally good control over the MoS$_2$ coverage is maintained using an approach based on cycles of Mo evaporation and sulfurization to first nucleate the MoS$_2$ nano-islands and then gradually increase their size. During this growth process the native herringbone reconstruction of Au(111) is lifted as shown by low energy electron diffraction measurements. Within these MoS$_2$ islands, we identify domains rotated by 60$^{\circ}$ that lead to atomically sharp line defects at domain boundaries. As the MoS$_2$ coverage approaches the limit of a complete single-layer, the formation of bilayer MoS$_2$ islands is initiated. Angle-resolved photoemission spectroscopy measurements of both single and bilayer MoS$_2$ samples show a dramatic change in their band structure around the center of the Brillouin zone. Brief exposure to air after removing the MoS$_2$ layer from vacuum is not found to affect its quality.
Tailoring the electronic texture of a topological insulator via its surface orientation
Lucas Barreto,Wendell Simoes e Silva,Malthe Stensgaard,Sren Ulstrup,Xie-Gang Zhu,Marco Bianchi,Maciej Dendzik,Philip Hofmann
Physics , 2013, DOI: 10.1088/1367-2630/15/10/103011
Abstract: Three dimensional topological insulator crystals consist of an insulating bulk enclosed by metallic surfaces, and detailed theoretical predictions about the surface state band topology and spin texture are available. While several topological insulator materials are currently known, the existence and topology of these metallic states have only ever been probed for one particular surface orientation of a given material. For most topological insulators, such as Bi$_{1-x}$Sb$_{x}$ and Bi$_2$Se$_3$, this surface is the closed-packed (111) surface and it supports one topologically guaranteed surface state Dirac cone. Here we experimentally realise a non closed-packed surface of a topological insulator, Bi$_{1-x}$Sb$_{x}$(110), and probe the surface state topology by angle-resolved photoemission. As expected, this surface also supports metallic states but the change in surface orientation drastically modifies the band topology, leading to three Dirac cones instead of one, in excellent agreement with the theoretical predictions but in contrast to any other experimentally studied TI surface. This illustrates the possibility to tailor the basic topological properties of the surface via its crystallographic direction. Here it introduces a valley degree of freedom not previously achieved for topological insulator systems.
Electron-phonon coupling in quasi free-standing graphene
Jens Christian Johannsen,Sren Ulstrup,Marco Bianchi,Richard Hatch,Dandan Guan,Federico Mazzola,Liv Hornek?r,Felix Fromm,Christian Raidel,Thomas Seyller,Philip Hofmann
Physics , 2012, DOI: 10.1088/0953-8984/25/9/094001
Abstract: Quasi free-standing monolayer graphene can be produced by intercalating species like oxygen or hydrogen between epitaxial graphene and the substrate crystal. If the graphene is indeed decoupled from the substrate, one would expect the observation of a similar electronic dispersion and many-body effects, irrespective of the substrate and the material used to achieve the decoupling. Here we investigate the electron-phonon coupling in two different types of quasi free-standing monolayer graphene: decoupled from SiC via hydrogen intercalation and decoupled from Ir via oxygen intercalation. Both systems show a similar overall behaviour of the self-energy and a weak renormalization of the bands near the Fermi energy. The electron-phonon coupling is found to be sufficiently weak to make the precise determination of the coupling constant lambda through renormalization difficult. The estimated value of lambda is 0.05(3) for both systems.
Observation of Ultrafast Free Carrier Dynamics in Single Layer MoS$_2$
Antonija Grubi?i? ?abo,Jill A. Miwa,Signe S. Gr?nborg,Jonathon M. Riley,Jens C. Johannsen,Cephise Cacho,Oliver Alexander,Richard T. Chapman,Emma Springate,Marco Grioni,Jeppe V. Lauritsen,Phil D. C. King,Philip Hofmann,Sren Ulstrup
Physics , 2015, DOI: 10.1021/acs.nanolett.5b01967
Abstract: The dynamics of excited electrons and holes in single layer (SL) MoS$_2$ have so far been difficult to disentangle from the excitons that dominate the optical response of this material. Here, we use time- and angle-resolved photoemission spectroscopy for a SL of MoS$_2$ on a metallic substrate to directly measure the excited free carriers. This allows us to ascertain a direct quasiparticle band gap of 1.95 eV and determine an ultrafast (50 fs) extraction of excited free carriers via the metal in contact with the SL MoS$_2$. This process is of key importance for optoelectronic applications that rely on separated free carriers rather than excitons.
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