@article{8481, abstract = {The copK gene is localized on the pMOL30 plasmid of Cupriavidus metallidurans CH34 within the complex cop cluster of genes, for which 21 genes have been identified. The expression of the corresponding periplasmic CopK protein is strongly upregulated in the presence of copper, leading to a high periplasmic accumulation. The structure and metal-binding properties of CopK were investigated by NMR and mass spectrometry. The protein is dimeric in the apo state with a dissociation constant in the range of 10- 5 M estimated from analytical ultracentrifugation. Mass spectrometry revealed that CopK has two high-affinity Cu(I)-binding sites per monomer with different Cu(I) affinities. Binding of Cu(II) was observed but appeared to be non-specific. The solution structure of apo-CopK revealed an all-β fold formed of two β-sheets in perpendicular orientation with an unstructured C-terminal tail. The dimer interface is formed by the surface of the C-terminal β-sheet. Binding of the first Cu(I)-ion induces a major structural modification involving dissociation of the dimeric apo-protein. Backbone chemical shifts determined for the 1Cu(I)-bound form confirm the conservation of the N-terminal β-sheet, while the last strand of the C-terminal sheet appears in slow conformational exchange. We hypothesize that the partial disruption of the C-terminal β-sheet is related to dimer dissociation. NH-exchange data acquired on the apo-protein are consistent with a lower thermodynamic stability of the C-terminal sheet. CopK contains seven methionine residues, five of which appear highly conserved. Chemical shift data suggest implication of two or three methionines (Met54, Met38, Met28) in the first Cu(I) site. Addition of a second Cu(I) ion further increases protein plasticity. Comparison of the structural and metal-binding properties of CopK with other periplasmic copper-binding proteins reveals two conserved features within these functionally related proteins: the all-β fold and the methionine-rich Cu(I)-binding site.}, author = {Bersch, Beate and Favier, Adrien and Schanda, Paul and van Aelst, Sébastien and Vallaeys, Tatiana and Covès, Jacques and Mergeay, Max and Wattiez, Ruddy}, issn = {0022-2836}, journal = {Journal of Molecular Biology}, keywords = {Molecular Biology}, number = {2}, pages = {386--403}, publisher = {Elsevier}, title = {{Molecular structure and metal-binding properties of the periplasmic CopK protein expressed in Cupriavidus metallidurans CH34 during copper challenge}}, doi = {10.1016/j.jmb.2008.05.017}, volume = {380}, year = {2008}, } @article{8482, abstract = {The SOFAST-HMQC experiment [P. Schanda, B. Brutscher, Very fast two-dimensional NMR spectroscopy for real-time investigation of dynamic events in proteins on the time scale of seconds, J. Am. Chem. Soc. 127 (2005) 8014–8015] allows recording two-dimensional correlation spectra of macromolecules such as proteins in only a few seconds acquisition time. To achieve the highest possible sensitivity, SOFAST-HMQC experiments are preferably performed on high-field NMR spectrometers equipped with cryogenically cooled probes. The duty cycle of over 80% in fast-pulsing SOFAST-HMQC experiments, however, may cause problems when using a cryogenic probe. Here we introduce SE-IPAP-SOFAST-HMQC, a new pulse sequence that provides comparable sensitivity to standard SOFAST-HMQC, while avoiding heteronuclear decoupling during 1H detection, and thus significantly reducing the radiofrequency load of the probe during the experiment. The experiment is also attractive for fast and sensitive measurement of heteronuclear one-bond spin coupling constants.}, author = {Kern, Thomas and Schanda, Paul and Brutscher, Bernhard}, issn = {1090-7807}, journal = {Journal of Magnetic Resonance}, keywords = {Nuclear and High Energy Physics, Biophysics, Biochemistry, Condensed Matter Physics}, number = {2}, pages = {333--338}, publisher = {Elsevier}, title = {{Sensitivity-enhanced IPAP-SOFAST-HMQC for fast-pulsing 2D NMR with reduced radiofrequency load}}, doi = {10.1016/j.jmr.2007.11.015}, volume = {190}, year = {2008}, } @article{8509, abstract = {The goal of this paper is to present to nonspecialists what is perhaps the simplest possible geometrical picture explaining the mechanism of Arnold diffusion. We choose to speak of a specific model—that of geometric rays in a periodic optical medium. This model is equivalent to that of a particle in a periodic potential in ${\mathbb R}^{n}$ with energy prescribed and to the geodesic flow in a Riemannian metric on ${\mathbb R}^{n} $.}, author = {Kaloshin, Vadim and Levi, Mark}, issn = {0036-1445}, journal = {SIAM Review}, keywords = {Theoretical Computer Science, Applied Mathematics, Computational Mathematics}, number = {4}, pages = {702--720}, publisher = {Society for Industrial & Applied Mathematics}, title = {{Geometry of Arnold diffusion}}, doi = {10.1137/070703235}, volume = {50}, year = {2008}, } @article{8510, abstract = {In this paper, using the ideas of Bessi and Mather, we present a simple mechanical system exhibiting Arnold diffusion. This system of a particle in a small periodic potential can be also interpreted as ray propagation in a periodic optical medium with a near-constant index of refraction. Arnold diffusion in this context manifests itself as an arbitrary finite change of direction for nearly constant index of refraction.}, author = {Kaloshin, Vadim and Levi, Mark}, issn = {0273-0979}, journal = {Bulletin of the American Mathematical Society}, keywords = {Applied Mathematics, General Mathematics}, number = {3}, pages = {409--427}, publisher = {American Mathematical Society}, title = {{An example of Arnold diffusion for near-integrable Hamiltonians}}, doi = {10.1090/s0273-0979-08-01211-1}, volume = {45}, year = {2008}, } @article{895, abstract = {Background. The arginine vasopressin V1a receptor (V1aR) modulates social cognition and behavior in a wide variety of species. Variation in a repetitive microsatellite element in the 5′ flanking region of the V1aR gene (AVPR1A) in rodents has been associated with variation in brain V1aR expression and in social behavior. In humans, the 5′ flanking region of AVPR1A contains a tandem duplication of two ∼350 bp, microsatellite-containing elements located approximately 3.5 kb upstream of the transcription start site. The first block, referred to as DupA, contains a polymorphic (GT) 25microsatellite; the second block, DupB, has a complex (CT) 4-(TT)-(CT)8-(GT)24polymorphic motif, known as RS3. Polymorphisms in RS3 have been associated with variation in sociobehavioral traits in humans, including autism spectrum disorders. Thus, evolution of these regions may have contributed to variation in social behavior in primates. We examined the structure of these regions in six ape, six monkey, and one prosimian species. Results. Both tandem repeat blocks are present upstream of the AVPR1A coding region in five of the ape species we investigated, while monkeys have only one copy of this region. As in humans, the microsatellites within DupA and DupB are polymorphic in many primate species. Furthermore, both single (lacking DupB) and duplicated alleles (containing both DupA and DupB) are present in chimpanzee (Pan troglodytes) populations with allele frequencies of 0.795 and 0.205 for the single and duplicated alleles, respectively, based on the analysis of 47 wild-caught individuals. Finally, a phylogenetic reconstruction suggests two alternate evolutionary histories for this locus. Conclusion. There is no obvious relationship between the presence of the RS3 duplication and social organization in primates. However, polymorphisms identified in some species may be useful in future genetic association studies. In particular, the presence of both single and duplicated alleles in chimpanzees provides a unique opportunity to assess the functional role of this duplication in contributing to variation in social behavior in primates. While our initial studies show no signs of directional selection on this locus in chimps, pharmacological and genetic association studies support a potential role for this region in influencing V1aR expression and social behavior.}, author = {Donaldson, Zoe R and Fyodor Kondrashov and Putnam, Andrea S and Bai, Yaohui and Stoinski, Tara S and Hammock, Elizabeth A and Young, Larry}, journal = {BMC Evolutionary Biology}, number = {1}, publisher = {BioMed Central}, title = {{Evolution of a behavior-linked microsatellite-containing element in the 5′ flanking region of the primate AVPR1A gene}}, doi = {10.1186/1471-2148-8-180}, volume = {8}, year = {2008}, } @article{907, abstract = {The most common form of protein-coding gene overlap in eukaryotes is a simple nested structure, whereby one gene is embedded in an intron of another. Analysis of nested protein-coding genes in vertebrates, fruit flies and nematodes revealed substantially higher rates of evolutionary gains than losses. The accumulation of nested gene structures could not be attributed to any obvious functional relationships between the genes involved and represents an increase of the organizational complexity of animal genomes via a neutral process.}, author = {Assis, Raquel and Kondrashov, Alexey S and Koonin, Eugene V and Fyodor Kondrashov}, journal = {Trends in Genetics}, number = {10}, pages = {475 -- 478}, publisher = {Elsevier}, title = {{Nested genes and increasing organizational complexity of metazoan genomes}}, doi = {10.1016/j.tig.2008.08.003}, volume = {24}, year = {2008}, } @article{7320, abstract = {A comparative, experimental diffusivity study of gas diffusion layer (GDL) materials for polymer electrolyte fuel cells (PEFC) is presented for the first time. The GDL plays an important role for electrochemical losses due to gas transport limitations at high current densities. Characterization and optimization of these layers is therefore essential to improve power density. A recently developed method which allows for fast diffusimetry is applied and data compared to the literature values. Measurements are made as a function of direction and compression and the effect of different binder structures and hydrophobic treatments on effective diffusivities are discussed. A better understanding of the results is gained by including novel GDL cross-section images and a meaningful unit cell model for the interpretation of the data. The diffusivity data is valuable for GDL manufacturers and future PEFC models. The study reveals that a binder–fiber ratio larger than 50% has a negative impact on the effective diffusion properties. The hydrophobic treatment which is necessary to improve the water management can impede diffusion and thus reduce the power density. Furthermore binder has an isotropic effect while compression pronounces the in-plane orientation of the fibers.}, author = {Flückiger, Reto and Freunberger, Stefan Alexander and Kramer, Denis and Wokaun, Alexander and Scherer, Günther G. and Büchi, Felix N.}, issn = {0013-4686}, journal = {Electrochimica Acta}, number = {2}, pages = {551--559}, publisher = {Elsevier}, title = {{Anisotropic, effective diffusivity of porous gas diffusion layer materials for PEFC}}, doi = {10.1016/j.electacta.2008.07.034}, volume = {54}, year = {2008}, } @article{7321, abstract = {Cell interaction phenomena in polymer electrolyte fuel cell stacks that arise from imbalance between adjacent cells are investigated in detail experimentally and theoretically. A specialized two-cell stack with advanced localized diagnostics was developed and used to analyze the mechanism and effect of cell-to-cell coupling as a result of operationally relevant variations in reactant feed flow. Contributions to overall and local voltage changes with respect to uniformly operated cells are scrutinized. Unequal operation of the cells causes in-plane current in the bipolar plate to redistribute current and result in inhomogeneous polarization. Both increasing and decreasing polarization along the air-flow path reduces cell power as compared to isopotential operation. A two-dimensional model based on a commercial computational fluid dynamics code is used to back and extend the experimental results to more general cases. Furthermore, the experimental setup presented allowed for the first time to perform simultaneous localized electrochemical impedance spectroscopy beyond the single-cell level. The mechanism of mutual cell interaction on local and integral spectra is revealed. Results show that virtually identical operation of the cells is essential to obtain meaningful integral spectra.}, author = {Freunberger, Stefan Alexander and Schneider, Ingo A. and Sui, Pang-Chieh and Wokaun, Alexander and Djilali, Nedjib and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {7}, publisher = {The Electrochemical Society}, title = {{Cell interaction phenomena in polymer electrolyte fuel cell stacks}}, doi = {10.1149/1.2913095}, volume = {155}, year = {2008}, } @article{7322, abstract = {The gas diffusion layers (GDLs) of a membrane electrode assembly (MEA) serve as link between flow field and porous electrode within a polymer electrolyte fuel cell. Beside ensuring sufficient electrical and thermal contact between the whole electrode area and the flow field, these typically 200–400 μm thick porous structures enable the access of educts to the electrode area which would be occluded by the flow field lands if the flow field is directly attached to the electrode. Hence, the characterisation of properties pertaining to mass transport of educts and products through these structures is indispensable whilst examining the contribution of the GDLs to the overall electrochemical characteristics of a MEA. A fast and cost effective method to measure the effective diffusivity of a GDL is presented. Electrochemical impedance spectroscopy is applied to measure the effective ionic conductivity of an electrolyte-soaked GDL. Taking advantage of the analogy between Ficks and Ohms law, this provides a measure for the effective diffusivity. The method is described in detail, including experimental as well as theoretical aspects, and selected results, highlighting the anisotropy and dependence on the degree of compression, are shown. Moreover, a two-dimensional model consisting of regularly spaced ellipses is developed to represent the porous structure of the GDL, and by using conformal maps, the agreement between this model and experiment with respect to the sensitivity of the effective diffusivity towards compression is shown.}, author = {Kramer, Denis and Freunberger, Stefan Alexander and Flückiger, Reto and Schneider, Ingo A. and Wokaun, Alexander and Büchi, Felix N. and Scherer, Günther G.}, issn = {1572-6657}, journal = {Journal of Electroanalytical Chemistry}, number = {1}, pages = {63--77}, publisher = {Elsevier}, title = {{Electrochemical diffusimetry of fuel cell gas diffusion layers}}, doi = {10.1016/j.jelechem.2007.09.014}, volume = {612}, year = {2008}, } @inproceedings{7425, abstract = {The propagation of single cell performance losses to adjacent cells in a polymer electrolyte fuel cell stack is studied by means of local current density measurements in a two cell stack. In this stack, the working conditions of adjacent cells can be controlled independently in order to deliberately change the performance of one cell (inducing cell) and study the coupling effects to the adjacent cell (response cell), while keeping the working conditions of the later one unchanged. The experiments have shown that changes in the current density distribution caused by lowering of the air stoichiometry in the inducing cell cause changes in the current density distribution of the response cell in the order of 60% of the change of the inducing cell, even when the air stoichiometry of the response cell is kept constant. The losses in cell voltage of the inducing cell cause losses in cell voltage of the response cell in a magnitude between 30 and 50%.}, author = {Santis, Marco and Freunberger, Stefan Alexander and Papra, Matthias and Büchi, Felix N.}, booktitle = {3rd International Conference on Fuel Cell Science, Engineering and Technology}, isbn = {0791837645}, location = {Ypsilanti, MI, United States}, pages = {763--765}, publisher = {ASMEDC}, title = {{Experimental investigation of the propagation of local current density variations to adjacent cells in PEFC stacks}}, doi = {10.1115/fuelcell2005-74116}, year = {2008}, } @inproceedings{753, abstract = {This paper addresses the following question: what is the minimum-sized synchronous window needed to solve consensus in an otherwise asynchronous system? In answer to this question, we present the first optimally-resilient algorithm ASAP that solves consensus as soon as possible in an eventually synchronous system, i.e., a system that from some time GST onwards, delivers messages in a timely fashion. ASAP guarantees that, in an execution with at most f failures, every process decides no later than round GST + f + 2, which is optimal.}, author = {Alistarh, Dan-Adrian and Gilbert, Seth and Guerraoui, Rachid and Travers, Corentin}, pages = {32 -- 46}, publisher = {Springer}, title = {{How to solve consensus in the smallest window of synchrony}}, doi = {10.1007/978-3-540-87779-0_3}, volume = {5218 LNCS}, year = {2008}, } @article{7752, author = {Robinson, Matthew Richard and Pilkington, Jill G. and Clutton-Brock, Tim H. and Pemberton, Josephine M. and Kruuk, Loeske. E.B.}, issn = {0960-9822}, journal = {Current Biology}, number = {10}, pages = {751--757}, publisher = {Elsevier}, title = {{Environmental heterogeneity generates fluctuating selection on a secondary sexual trait}}, doi = {10.1016/j.cub.2008.04.059}, volume = {18}, year = {2008}, } @article{1717, abstract = {Two key processes are in the basis of morphogenesis: the spatial allocation of cell types in fields of naïve cells and the regulation of growth. Both are controlled by morphogens, which activate target genes in the growing tissue in a concentration-dependent manner. Thus the morphogen model is an intrinsically quantitative concept. However, quantitative studies were performed only in recent years on two morphogens: Bicoid and Decapentaplegic. This review covers quantitative aspects of the formation and precision of the Decapentaplegic morphogen gradient. The morphogen gradient concept is transitioning from a soft definition to a precise idea of what the gradient could really do.}, author = {Anna Kicheva and González-Gaitán, Marcos A}, journal = {Current Opinion in Cell Biology}, number = {2}, pages = {137 -- 143}, publisher = {Elsevier}, title = {{The Decapentaplegic morphogen gradient a precise definition}}, doi = {10.1016/j.ceb.2008.01.008}, volume = {20}, year = {2008}, } @article{1719, abstract = {We study the mechanics of tissue growth via cell division and cell death (apoptosis). The rearrangements of cells can on large scales and times be captured by a continuum theory which describes the tissue as an effective viscous material with active stresses generated by cell division. We study the effects of anisotropies of cell division on cell rearrangements and show that average cellular trajectories exhibit anisotropic scaling behaviors. If cell division and apoptosis balance, there is no net growth, but for anisotropic cell division the tissue undergoes spontaneous shear deformations. Our description is relevant for the study of developing tissues such as the imaginal disks of the fruit fly Drosophila melanogaster, which grow anisotropically.}, author = {Bittig, Thomas and Wartlick, Ortrud and Anna Kicheva and González-Gaitárr, Marcos and Julicher, Frank}, journal = {New Journal of Physics}, publisher = {IOP Publishing Ltd.}, title = {{Dynamics of anisotropic tissue growth}}, doi = {10.1088/1367-2630/10/6/063001}, volume = {10}, year = {2008}, } @article{1749, abstract = {Scanning probe microscopy; Semiconductor quantum dots; Composition gradients; Composition profiles; Nanotomography; Single quantum dots; Strained sige/si; Three-dimensional (3D); Wet-chemical etchings; X-ray scattering measurements; quantum dot; methodology; nanotechnology; optical tomography; scanning probe microscopy; three dimensional imaging; Imaging, Three-Dimensional; Materials Testing; Microscopy, Scanning Probe; Nanotechnology; Quantum Dots; Tomography,}, author = {Rastelli, Armando and Stoffel, Mathieu and Malachias, Ângelo S and Merdzhanova, Tsvetelina and Georgios Katsaros and Kern, Klaus and Metzger, Till H and Schmidt, Oliver G}, journal = {Nano Letters}, number = {5}, pages = {1404 -- 1409}, publisher = {American Chemical Society}, title = {{Three-dimensional composition profiles of single quantum dots determined by scanning-probe-microscopy-based nanotomography}}, doi = {10.1021/nl080290y}, volume = {8}, year = {2008}, } @article{1751, abstract = {When strained Stranski-Krastanow islands are used as "self-assembled quantum dots," a key goal is to control the island position. Here we show that nanoscale grooves can control the nucleation of epitaxial Ge islands on Si(001), and can drive lateral motion of existing islands onto the grooves, even when the grooves are very narrow and shallow compared to the islands. A position centered on the groove minimizes energy. We use as prototype grooves the trenches which form naturally around islands. During coarsening, the shrinking islands move laterally to sit directly astride that trench. In subsequent growth, we demonstrate that islands nucleate on the "empty trenches" which remain on the surface after complete dissolution of the original islands.}, author = {Georgios Katsaros and Tersoff, Jerry and Stoffel, Mathieu and Rastelli, Armando and Acosta-Diaz, P and Kar, Gouranga S and Costantini, Giovanni and Schmidt, Oliver G and Kern, Klaus}, journal = {Physical Review Letters}, number = {9}, publisher = {American Physical Society}, title = {{Positioning of strained islands by interaction with surface nanogrooves}}, doi = {10.1103/PhysRevLett.101.096103}, volume = {101}, year = {2008}, } @article{1763, abstract = {The field of cavity quantum electrodynamics (QED), traditionally studied in atomic systems, has gained new momentum by recent reports of quantum optical experiments with solid-state semiconducting and superconducting systems. In cavity QED, the observation of the vacuum Rabi mode splitting is used to investigate the nature of matter-light interaction at a quantum-mechanical level. However, this effect can, at least in principle, be explained classically as the normal mode splitting of two coupled linear oscillators. It has been suggested that an observation of the scaling of the resonant atom-photon coupling strength in the Jaynes-Cummings energy ladder with the square root of photon number n is sufficient to prove that the system is quantum mechanical in nature. Here we report a direct spectroscopic observation of this characteristic quantum nonlinearity. Measuring the photonic degree of freedom of the coupled system, our measurements provide unambiguous spectroscopic evidence for the quantum nature of the resonant atom-field interaction in cavity QED. We explore atom-photon superposition states involving up to two photons, using a spectroscopic pump and probe technique. The experiments have been performed in a circuit QED set-up, in which very strong coupling is realized by the large dipole coupling strength and the long coherence time of a superconducting qubit embedded in a high-quality on-chip microwave cavity. Circuit QED systems also provide a natural quantum interface between flying qubits (photons) and stationary qubits for applications in quantum information processing and communication.}, author = {Johannes Fink and Göppl, M and Baur, Matthias P and Bianchetti, R and Leek, Peter J and Blais, Alexandre and Wallraff, Andreas}, journal = {Nature}, number = {7202}, pages = {315 -- 318}, publisher = {Nature Publishing Group}, title = {{Climbing the Jaynes-Cummings ladder and observing its √n nonlinearity in a cavity QED system}}, doi = {10.1038/nature07112}, volume = {454}, year = {2008}, } @article{1764, abstract = {Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states.}, author = {Fragner, A and Göppl, M and Johannes Fink and Baur, Matthias P and Bianchetti, R and Leek, Peter J and Blais, Alexandre and Wallraff, Andreas}, journal = {Science}, number = {5906}, pages = {1357 -- 1360}, publisher = {American Association for the Advancement of Science}, title = {{Resolving vacuum fluctuations in an electrical circuit by measuring the lamb shift}}, doi = {10.1126/science.1164482}, volume = {322}, year = {2008}, } @article{1765, abstract = {High quality on-chip microwave resonators have recently found prominent new applications in quantum optics and quantum information processing experiments with superconducting electronic circuits, a field now known as circuit quantum electrodynamics (QED). They are also used as single photon detectors and parametric amplifiers. Here we analyze the physical properties of coplanar waveguide resonators and their relation to the materials properties for use in circuit QED. We have designed and fabricated resonators with fundamental frequencies from 2 to 9 GHz and quality factors ranging from a few hundreds to a several hundred thousands controlled by appropriately designed input and output coupling capacitors. The microwave transmission spectra measured at temperatures of 20 mK are shown to be in good agreement with theoretical lumped element and distributed element transmission matrix models. In particular, the experimentally determined resonance frequencies, quality factors, and insertion losses are fully and consistently explained by the two models for all measured devices. The high level of control and flexibility in design renders these resonators ideal for storing and manipulating quantum electromagnetic fields in integrated superconducting electronic circuits.}, author = {Göppl, M and Fragner, A and Baur, Matthias P and Bianchetti, R and Filipp, Stefan and Johannes Fink and Leek, Peter J and Puebla, G and Steffen, L. Kraig and Wallraff, Andreas}, journal = {Journal of Applied Physics}, number = {11}, publisher = {American Institute of Physics}, title = {{Coplanar waveguide resonators for circuit quantum electrodynamics}}, doi = {10.1063/1.3010859}, volume = {104}, year = {2008}, } @article{1826, abstract = {Proliferating cell populations at steady-state growth often exhibit broad protein distributions with exponential tails. The sources of this variation and its universality are of much theoretical interest. Here we address the problem by asymptotic analysis of the population balance equation. We show that the steady-state distribution tail is determined by a combination of protein production and cell division and is insensitive to other model details. Under general conditions this tail is exponential with a dependence on parameters consistent with experiment. We discuss the conditions for this effect to be dominant over other sources of variation and the relation to experiments.}, author = {Tamar Friedlander and Brenner, Naama}, journal = {Physical Review Letters}, number = {1}, publisher = {American Physical Society}, title = {{Cellular properties and population asymptotics in the population balance equation}}, doi = {10.1103/PhysRevLett.101.018104}, volume = {101}, year = {2008}, }