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Atomic force bio-analytics
PLTM Frederix, T Akiyamay, U Staufer, Ch Gerber, D Fotiadis, DJ Müller and A Engel
Current Opinion in Chemical Biology 2003, 7:641–647
The atomic force microscope (AFM) allows biomolecules to be observed and manipulated under native conditions. It operates in buffer solution, produces molecular images with outstanding signal-to-noise ratio, and addresses single molecules. Progress in sample preparation and instrumentation has led to topographs that reveal sub-nanometer details and surface dynamics of biomolecules. Antibodies or oligonucleotides immobilized on cantilevers induce bending upon binding of the cognate biomolecule, allowing sub-picomolar concentrations to be measured. Biomolecules tethered between support and retra ...
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Dephasing in sequential tunneling through a double-dot interferometer
Florian Marquardt and C. Bruder
Phys. Rev. B 68, 195305 (2003)
We analyze dephasing in a model system where electrons tunnel sequentially through a symmetric interference setup consisting of two single-level quantum dots. Depending on the phase difference between the two tunneling paths, this may result in perfect destructive interference. However, if the dots are coupled to a bath, it may act as a which-way detector, leading to partial suppression of the phase coherence and the reappearance of a finite tunneling current. In our approach, the tunneling is treated in leading order whereas coupling to the bath is kept to all orders [using P(E) theory]. We d ...
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Variable Electrostatic Transformer: Controllable Coupling of Two Charge Qubits
D. V. Averin and C. Bruder
Phys. Rev. Lett. 91, 057003 (2003)
We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling. We also discuss how to implement a phase gate making use of the controllable coupling. ...
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Kelvin probe spectroscopy of a two-dimensional electron gas below 300 mK
T. Vancura, S. Kicin, T. Ihn, K. Ensslin, M. Bichler, and W. Wegscheider
Applied Physics Letters 2003 Volume 83, Issue 13, pp. 2602-2604
A scanning force microscope with a base temperature below 300 mK is used for measuring the local electron density of a two-dimensional electron gas embedded in a Ga[Al]As heterostructure. At different separations between atomic force microscope tip and sample, a dc voltage is applied between the tip and the electron gas while simultaneously recording the frequency shift of the oscillating tip. Using a plate capacitor model, the local electron density can be extracted from the data. The result coincides within 10% with the data obtained from transport measurements. ...
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Singlet-Triplet Transition Tuned by Asymmetric Gate Voltages in a Quantum Ring
A. Fuhrer, T. Ihn, K. Ensslin, W. Wegscheider, and M. Bichler
Phys. Rev. Lett. 91, 206802 (2003)
Wave-function and interaction effects in the addition spectrum of a Coulomb-blockaded many-electron quantum ring are investigated as a function of asymmetrically applied gate voltages and magnetic field. Hartree and exchange contributions to the interaction are quantitatively evaluated at a crossing between states extended around the ring and states which are more localized in one arm of the ring. A gate tunable singlet-triplet transition of the two uppermost levels of this many-electron ring is identified at zero magnetic field. ...
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Is aluminum a suitable buffer layer for carbon nanotube growth?
T. de los Arcos, Z. M. Wu and P. Oelhafen
Chemical Physics Letters 380 (2003), 419 - 423
The possibility of using Al buffer layers for the catalytic chemical vapor deposition growth of carbon nanotubes has been investigated by in situ photoelectron spectroscopy. It was found that at the temperatures used for carbon nanotube growth, typically well above the eutectic temperature of the Al–Si system, the liquid Al–Si alloy formed getters efficiently the metallic catalyst away from the surface, thus precluding nanotube growth. ...
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Micellar Nanoreactors - Preparation and Characterization of Hexagonally Ordered Arrays of Metallic Nanodots
G. Kästle, H.-G. Boyen, F. Weigl, G. Lengl, T. Herzog, P. Ziemann, S. Riethmüller, O. Mayer, C. Hartmann, J.P. Spatz, M. Möller, M. Ozawa, F. Banhart, M.G. Garnier, P. Oelhafen
Adv. Funct. Mater. 2003, 13, No. 11, 853-861
The preparation of hexagonally ordered metallic nanodots was studied in detail with emphasis on the chemical state of the resulting particles. To obtain these dots, in a first step micellar structures were formed from diblock copolymers in solution. The reverse micelles themselves are capable of ligating defined amounts of a metal salt within their cores, acting as nanoreactors. After transfer of the metal-loaded reverse micelles onto a substrate, the polymer was removed by means of different plasmas (oxygen and/or hydrogen), which also allow the metal salt to be reduced to the metallic state. ...
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Synthesis and Conformational Switching of Partially and Differentially Bridged Resorcin[4]arenes Bearing Fluorescent Dye Labels
Vladimir A. Azov, François Diederich, Yoriko Lill, Bert Hecht
Helv. Chim. Acta 2003, 86, No. 6, 2149-2155
We report the synthesis of modified Cram-type cavitands bearing one or two fluorescent labels for single-molecule spectroscopic studies of vase&bond;kite conformational switching (Scheme 3). Syntheses were performed by stepwise bridging of the four couples of neighboring H-bonded OH groups of resorcin[4]arene bowls (Schemes 2 and 3). The new substitution patterns enable the construction of a large variety of future functional architectures. 1H-NMR Investigations showed that the new partially and differentially bridged cavitands feature temperature- and pH-triggered vase&bond;kite conformationa ...
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Exceptional Redox and Photophysical Properties of a Triply Fused Diporphyrin-C₆₀ Conjugate: Novel Scaffolds for Multicharge Storage in Molecular Scale Electronic
Davide Bonifazi, Markus Scholl, Fayi Song, Luis Echegoyen, Gianluca Accorsi, Nicola Armaroli, François Diederich
Angew. Chem. Int. Ed. Engl. 2003, 42, 4966-4970
The expansion and functionalization of -conjugated molecular architectures to enhance the optoelectronic properties of the resulting chromophores is a topic of broad current interest.Fascinating examples are the molecular tapes consisting of triply fused porphyrins, reported by Osuka and co-workers, which exhibit exceptionally low-energy electronic states and hold great potential as molecular wires in nanoscale electronics devices... ...
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Nano-Structuring by Molecular Self-Assembly
Caterina Minelli, Nicolas Blondiaux, Myriam Losson, Martha Liley, Sylvia Jeney,Christian Hinderling, Raphaël Pugin, Derk Joester, François Diederich, Julius Vancso, Mark Hempenius, and Harry Heinzelmann
Chimia 2003, 57, 646-650
In contrast to conventional lithography techniques which will soon hit their limits in terms of feature size and fabrication cost, recent years have seen considerable progress in the development of selfassembling nano-structured surfaces. In the work reported here, surface structures on the submicron scale have been created from polymer, copolymer, nanoparticle, and dendrimer building blocks by employing phenomena such as self-assembly, self-organization and non-equilibrium processes. This low-cost approach is expected to result in a new generation of surfaces with novel physical and chemical ...
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Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)
S. Berner, M. de Wild, L. Ramoino, S. Ivan, A. Baratoff, H.-J. Güntherodt, H. Suzuki, D. Schlettwein, and T. A. Jung
Physical Review B 68, 115410 (2003)
The adsorption and the two-dimensional (2D) ordering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111) has been studied in detail by combined scanning tunneling microscopy and photoelectron spectroscopy at room temperature. SubPc is a polar, highly symmetric molecule, consisting of an extended aromatic system and a central B-Cl bond... ...
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Electron Spin Decoherence in Quantum Dots due to Interaction with Nuclei
Alexander V. Khaetskii, Daniel Loss, and Leonid Glazman
Phys. Rev. Lett. 88, 186802 (2002)
We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decay is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) lawlike. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by N/A, where N is the number of nuclei ...
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Decoherence of cold atomic gases in magnetic microtraps
C. Schroll, W. Belzig, and C. Bruder
Phys. Rev. A 68, 043618 (2003)
We derive a model to describe decoherence of atomic clouds in atom-chip traps taking the excited states of the trapping potential into account. We use this model to investigate decoherence for a single trapping well and for a pair of trapping wells that form the two arms of an atom interferometer. Including the discrete spectrum of the trapping potential gives rise to a decoherence mechanism with a decoherence rate that scales like ~1/r with the distance r0 from the trap minimum to the wire. ...
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Full counting statistics of incoherent Andreev transport
W. Belzig and P. Samuelsson
Europhys. Lett., 64 (2) , pp. 253-259 (2003)
We study the full counting statistics of heterostructures consisting of normal-metal parts connected to a superconducting terminal. Assuming that coherent superconducting correlations are suppressed in the normal metals, we show, using the Keldysh-Nambu Green's function approach, that the system can be mapped onto a purely normal system with twice the number of elements. For a superconducting beamsplitter with several normal terminals we obtain general results for the counting statistics. ...
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Noise of Spin-Polarized Currents at a Beam Splitter with Local Spin–Orbit Interaction
G. Burkard, J. C. Egues and D. Loss
Journal of Superconductivity: Incorporating Novel Magnetism, 16 (2): 237-240, April 2003
An electronic beam splitter with a local Rashba spin–orbit coupling can serve as a detector for spin-polarized currents. The spin–orbit coupling plays the role of a tunable spin rotator and can be controlled via a gate electrode on top of the conductor. We use spin-resolved scattering theory to calculate the zero-temperature current fluctuations (shot noise) for such a four-terminal device and show that the shot noise is proportional to the spin polarization of the source. Moreover, we analyze the effect of spin–orbit-induced intersubband coupling, leading to an additional spin rotation. ...
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Lower Bound for Electron Spin Entanglement from Beam Splitter Current Correlations
Guido Burkard and Daniel Loss
Phys. Rev. Lett. 91, 087903 (2003)
We determine a lower bound for the entanglement of formation of pairs of electron spins injected into a mesoscopic conductor. The bound can be expressed in terms of experimentally accessible quantities, the zero-frequency current correlators (shot noise power or cross correlators) after transmission through an electronic beam splitter and can be used to gain information about the entanglement from experiment. Spin relaxation (T1 processes) and decoherence (T2) during the ballistic coherent transmission of carriers are taken into account within Bloch theory. A variable inhomogeneous magnetic fi ...
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Probing Entanglement via Rashba-Induced Shot Noise Oscillations
J. Carlos Egues; G. Burkard; D. Loss
Journal of Superconductivity: Incorporating Novel Magnetism 16(4): 711-718; Aug 2003
We have recently calculated shot noise for entangled and spin-polarized electrons in novel beam-splitter geometries with a local Rashba orbit–spin (s-o) interaction in the incoming leads. This interaction allows for a gate-controlled rotation of the incoming electron spins. Here we present an alternate simpler route to the shot noise calculation in the above work and focus on only electron pairs. Shot noise for these shows continuous bunching and antibunching behaviors. In addition, entangled and unentangled triplets yield distinctive shot noise oscillations. Besides allowing for a direct way ...
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Quantum computing with antiferromagnetic spin clusters
Florian Meier, Jeremy Levy and Daniel Loss
Phys. Rev. B 68, 134417 (2003)
We show that a wide range of spin clusters with antiferromagnetic intracluster exchange interaction allows one to define a qubit. For these spin cluster qubits, initialization, quantum gate operation, and readout are possible using the same techniques as for single spins. Quantum gate operation for the spin cluster qubit does not require control over the intracluster exchange interaction. Electric and magnetic fields necessary to effect quantum gates need only be controlled on the length scale of the spin cluster rather than the scale for a single spin. Here, we calculate the energy gap separa ...
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Anisotropic transport in a two-dimensional electron gas in the presence of spin-orbit coupling
John Schliemann and Daniel Loss
Phys. Rev. B 68, 165311 (2003)
In a two-dimensional electron gas as realized by a semiconductor quantum well, the presence of spin-orbit coupling of both the Rashba and Dresselhaus type leads to anisotropic dispersion relations and Fermi contours. We study the effect of this anisotropy on the electrical conductivity in the presence of fixed impurity scatterers. The conductivity also shows in general an anisotropy which can be tuned by varying the Rashba coefficient. This effect provides a method of detecting and investigating spin-orbit coupling by measuring spin-unpolarized electrical currents in the diffusive regime. Our ...
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Interaction Potential and Hopping Dynamics Governing Sliding Friction
E. Riedo, E. Gnecco, R. Bennewitz, E. Meyer, and H. Brune
Phys. Rev. Lett. 91 (2003) 084502.
The friction force on a nanometer-sized tip sliding on a surface is related to the thermally activated hopping of the contact atoms on an effective atomic interaction potential. A general analytical expression relates the height of this potential and the hopping attempt frequency to measurements of the velocity dependence of the friction force performed with an atomic force microscope. While the height of the potential is roughly proportional to the normal load, the attempt frequency falls in the range of mechanical eigenfrequencies of the probing tip in contact with the surface. ...
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Ripple formation induced in localized abrasion
A. Socoliuc, E.Gnecco, R.Bennewitz, and E.Meyer
Physical Review B 68 (2003), 115416
The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning force microscope in ultrahigh vacuum is reported. Wear of single atomic layers has been observed when the microscope tip is repeatedly scanned across a line. The initially flat surface is rearranged in a quasiperiodic pattern of mounds and pits. The distance between the pits is about 40 nm when normal forces of a few nanonewtons are applied, and it slowly increases with the load. If a square area is scanned, a pattern of ripples is formed. These features can be interpreted within an erosion process ...
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Sensitivity of single multiwalled carbon nanotubes to the environment
M. Krüger, I. Widmer, T. Nussbaumer, M. Buitelaar and C. Schönenberger
New Journal of Physics 5 (2003) 138
We report on electrical resistance measurements of single multiwalled carbon nanotubes (MWNTs) in different environments (ambient air, H2, O2 and the electrolytes LiClO4, KCl, KMnO4 and H3PO3). The gate dependence is studied using back-gating, electrochemical gating and gates evaporated directly onto the nanotubes (NTs). MWNTs at room temperature are p-doped. Upon changing the environment a change of the doping state of the MWNTs is inferred from the linear resistance. The effect of the environment on the contacts is negligible in our experiments. The p-doping is proposed to originate from the ...
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Intrinsic Thermal Vibrations of Suspended Doubly Clamped Single-Wall Carbon Nanotubes
B. Babic, J. Furer, S. Sahoo, Sh. Farhangfar, and C. Schönenberger
Nano Letters 3 (11), 1577 (2003)
We report the observation of thermally driven mechanical vibrations of suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition (CVD). Several experimental procedures are used to suspend carbon nanotubes. The vibration is observed as a blurring in images taken with a scanning electron microscope. The measured vibration amplitudes are compared with a model based on linear continuum mechanics. ...
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