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Search Results: 1 - 10 of 348 matches for " Berndt Koslowski "
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Deconvolution of the density of states of tip and sample through constant-current tunneling spectroscopy
Holger Pfeifer,Berndt Koslowski,Paul Ziemann
Beilstein Journal of Nanotechnology , 2011, DOI: 10.3762/bjnano.2.64
Abstract: We introduce a scheme to obtain the deconvolved density of states (DOS) of the tip and sample, from scanning tunneling spectra determined in the constant-current mode (z–V spectroscopy). The scheme is based on the validity of the Wentzel–Kramers–Brillouin (WKB) approximation and the trapezoidal approximation of the electron potential within the tunneling barrier. In a numerical treatment of z–V spectroscopy, we first analyze how the position and amplitude of characteristic DOS features change depending on parameters such as the energy position, width, barrier height, and the tip–sample separation. Then it is shown that the deconvolution scheme is capable of recovering the original DOS of tip and sample with an accuracy of better than 97% within the one-dimensional WKB approximation. Application of the deconvolution scheme to experimental data obtained on Nb(110) reveals a convergent behavior, providing separately the DOS of both sample and tip. In detail, however, there are systematic quantitative deviations between the DOS results based on z–V data and those based on I–V data. This points to an inconsistency between the assumed and the actual transmission probability function. Indeed, the experimentally determined differential barrier height still clearly deviates from that derived from the deconvolved DOS. Thus, the present progress in developing a reliable deconvolution scheme shifts the focus towards how to access the actual transmission probability function.
The hydrogen molecule in a vice
Matthias Stocker,Simon R?ger,Berndt Koslowski
Physics , 2015,
Abstract: We conducted experiments on the role of molecular hydrogen physisorbed between two metallic surfaces. Such hydrogen leads to strong signatures in inelastic electron tunneling spectroscopy exceeding the amplitude of typical inelastic transitions by an order of magnitude and is accompanied by a step in the tunneling current leading under certain circumstances to a huge negative differential conductance. We show that the molecular vibration opens an inelastic channel as expected but simultaneously stalls the total elastic channel due to the forces exerted by the vibrating molecule on the electrodes. The required compliance of the stylus is surprisingly large but is possible in the wide spectrum of experimental conditions. Additionally, the inelastic channel experiences a saturation from which the lifetime of the molecular vibration can be deduced to be approximately 1 ns. This experiment allows resolving the forces corresponding to specific vibrational states of a single molecule.
Terthiophene on Au(111): A scanning tunneling microscopy and spectroscopy study
Berndt Koslowski,Anna Tschetschetkin,Norbert Maurer,Elena Mena-Osteritz
Beilstein Journal of Nanotechnology , 2011, DOI: 10.3762/bjnano.2.60
Abstract: Terthiophene (3T) molecules adsorbed on herringbone (HB) reconstructed Au(111) surfaces in the low coverage regime were investigated by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra-high vacuum conditions. The 3T molecules adsorb preferentially in fcc regions of the HB reconstruction with their longer axis oriented perpendicular to the soliton walls of the HB and at maximum mutual separation. The latter observation points to a repulsive interaction between molecules probably due to parallel electrical dipoles formed during adsorption. Constant-separation (I-V) and constant-current (z-V) STS clearly reveal the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, which are found at 1.2 eV and +2.3 eV, respectively. The HOMO–LUMO gap corresponds to that of a free molecule, indicating a rather weak interaction between 3T and Au(111). According to conductivity maps, the HOMO and LUMO are inhomogeneously distributed over the adsorbed 3T, with the HOMO being located at the ends of the linear molecule, and the LUMO symmetrically with respect to the longer axis of the molecule at the center of its flanks. Analysis of spectroscopic data reveals details of the contrast mechanism of 3T/Au(111) in STM. For that, the Shockley-like surface state of Au(111) plays an essential role and appears shifted outwards from the surface in the presence of the molecule. As a consequence, the molecule can be imaged even at a tunneling bias within its HOMO–LUMO gap. A more quantitative analysis of this detail resolves a previous discrepancy between the fairly small apparent STM height of 3T molecules (1.4–2.0 nm, depending on tunneling bias) and a corresponding larger value of 3.5 nm based on X-ray standing wave analysis. An additionally observed linear decrease of the differential tunneling barrier at positive bias when determined on top of a 3T molecule is compared to the bias independent barrier obtained on bare Au(111) surfaces. This striking difference of the barrier behavior with and without adsorbed molecules is interpreted as indicating an adsorption-induced dimensionality transition of the involved tunneling processes.
STM study on the self-assembly of oligothiophene-based organic semiconductors
Elena Mena-Osteritz,Marta Urdanpilleta,Erwaa El-Hosseiny,Berndt Koslowski
Beilstein Journal of Nanotechnology , 2011, DOI: 10.3762/bjnano.2.88
Abstract: The self-assembly properties of a series of functionalized regioregular oligo(3-alkylthiophenes) were investigated by using scanning tunneling microscopy (STM) at the liquid–solid interface under ambient conditions. The characteristics of the 2-D crystals formed on the (0001) plane of highly ordered pyrolitic graphite (HOPG) strongly depend on the length of the π-conjugated oligomer backbone, on the functional groups attached to it, and on the alkyl substitution pattern on the individual thiophene units. Theoretical calculations were performed to analyze the geometry and electronic density of the molecular orbitals as well as to analyze the intermolecular interactions, in order to obtain models of the 2-D molecular ordering on the substrate.
Instituciones de mercado, reforma en Europa del Este y teoría económica Market institutions, reforms in eastern Europe and economic theory
Koslowski Rey
Cuadernos de Economía , 1994,
Abstract: El derrumbe de los estados totalitarios ha renovado el interés por las instituciones. Este trabajo muestra que la transición del socialismo a economías de mercado exige desarrollar muchas instituciones, incluido el mercado mismo. Se requiere establecer un nuevo sistema de derechos de propiedad para construir un mercado donde estos se transfieran en forma rutinaria. La rutinización requiere una estructura legal del Estado para regular las transferencias y resolver los conflictos entre las partes. La experiencia de Europa de Este muestra la pobreza del enfoque neoclásico que supone dadas a las instituciones: no puede considerar en forma adecuada la estructura legal que provee el Estado para establecer y legitimar los derechos de propiedad, ni la regulación estatal generalizada inherente al capitalismo del siglo veinte. Al suponer el comportamiento económico puede separarse de las especifidades de cada sociedad, cae en errores conceptuales que llevan a un optimismo infundado sobre la reforma económica y a una visión ingenua de la privatización. En las fases iniciales, la construcción del Estado puede ser la mejor estrategia de una reforma económica exitosa. Irónicamente, la mejor ayuda que puedan prestar los gobiernos occidentales es apoyar el desarrollo de las instituciones estatales que fijen las reglas para organizar el intercambio. The crumbling of the totalitarian States has renewed interest in the institutions. This article shows that the transition from socialism to a market economy entails the development of a number of institutions, including the market itself. A new system of ownershíp has to be worked out to form a market where ownership can be easily transferred. The ease of transfer is a matter for the State to provide a legal structure to regulate transfer and resolve disputes. Experience in Eastern Europe has shown the poverty of the neo-classical approach which the institutions are supposed to possess. There is no proper legal structure enabling the Sta te to establish and legitimize the rights of ownership, or the overall regulation by the State inherent in 20th century capitalismo By supposing that the performance of an economy can be independent of the specifics of each society, there have been serious conceptual errors which have led to unfounded optimism on economic reforms and a simplistic view of privatization. In the initíal stages, the constructíon of the State could be the best strategy for successful economíc reformo ironically, the best form of assístance whích the governments of the west could províde would be to support the de
Reduction of a Quantum Theory
Tim Koslowski
Physics , 2006,
Abstract: This paper serves as a preparation of work that focuses on extracting cosmological sectors from Loop Quantum Gravity. We start with studying the extraction of subsystems from classical systems. A classical Hamiltonian system can be reduced to a subsystem of ''relevant observables'' using the pull-back under the Poisson-embedding of the ''relevant part of phase space'' into full phase space. Since a quantum theory can be thought of as a noncommutative phase space, one encounters the problem of embedding noncommutative spaces. We solve this problem for a physically interesting set of quantum systems and embeddings by constructing the noncommutative analogue of the construction of an embedding as the projection to the base space of an embedding of fibre bundles over the involved spaces. This paper focuses on the physical ideas that enter our programme of reduction of quantum theories and tries to explain these on examples rather than abstractly, which will be the focus of a forthcoming paper.
Shape Dynamics
Tim Koslowski
Physics , 2011, DOI: 10.1088/1742-6596/360/1/012059
Abstract: General Relativity can be reformulated as a geometrodynamical theory, called Shape Dynamics, that is not based on spacetime (in particular refoliation) symmetry but on spatial diffeomorphism and local spatial conformal symmetry. This leads to a constraint algebra that is (unlike General Relativity) a Lie algebra, where all local constraints are linear in momenta and may thus be quantized as vector fields on the geometrodynamic configuration space. The Hamiltonian of Shape Dynamics is complicated but admits simple expressions whenever spatial derivatives are negligible.
Observable Equivalence between General Relativity and Shape Dynamics
Tim Koslowski
Physics , 2012,
Abstract: In this conceptual paper we construct a local version of Shape Dynamics that is equivalent to General Relativity in the sense that the algebras of Dirac observables weakly coincide. This allows us to identify Shape Dynamics observables with General Relativity observables, whose observables can now be interpreted as particular representative functions of observables of a conformal theory at fixed York time. An application of the observable equivalence of General Relativity and Shape Dynamics is to define the quantization of General Relativity through quantizing Shape Dynamics and using observable equivalence. We investigate this proposal explicitly for gravity in 2+1 dimensions.
Loop Quantization of Shape Dynamics
Tim Koslowski
Physics , 2013,
Abstract: Loop Quantum Gravity (LQG) is a promising approach to quantum gravity, in particular because it is based on a rigorous quantization of the kinematics of gravity. A difficult and still open problem in the LQG program is the construction of the physical Hilbert space for pure quantum gravity. This is due to the complicated nature of the Hamilton constraints. The Shape Dynamics description of General Relativity (GR) replaces the Hamilton constraints with spatial Weyl constraints, so the problem of finding the physical Hilbert space reduces to the problem of quantizing the Weyl constraints. Unfortunately, it turns out that a loop quantization of Weyl constraints is far from trivial despite their intuitive physical interpretation. A tentative quantization proposal and interpretation proposal is given in this contribution.
Shape Dynamics and Effective Field Theory
Tim Koslowski
Physics , 2013, DOI: 10.1142/S0217751X13300172
Abstract: Shape Dynamics is a gauge theory based on spatial diffeomorphism- and Weyl-invariance which is locally indistinguishable form classical General Relativity. If taken seriously, it suggests that the spacetime--geometry picture that underlies General Relativity can be replaced by a picture based on spatial conformal geometry. This classically well understood trading of gauge symmetries opens new conceptual avenues in many approaches to quantum gravity. I focus on the general implications for quantum gravity and effective field theory and consider the application of the Shape Dynamics picture in the exact renormalization group approaches to gravity, loop- and polymer- quantization approaches to gravity and low energy effective field theories. I also discuss the interpretation of known results through in the Shape Dynamics picture, in particular holographic renormalization and the problem of time in canonical quantum gravity.
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