@article{17690, abstract = {The ability of primordial gas to cool in proto-galactic haloes exposed to Lyman-Werner (LW) radiation is critically dependent on the self-shielding of H_2. We perform radiative transfer calculations of LW line photons, post-processing outputs from three-dimensional adaptive mesh refinement (AMR) simulations of haloes with T_vir > 10^4 K at redshifts around z=10. We calculate the optically thick photodissociation rate numerically, including the effects of density, temperature, and velocity gradients in the gas, as well as line overlap and shielding of H_2 by HI, over a large number of sight-lines. In low-density regions (n<10^4 cm^-3) the dissociation rates exceed those obtained using most previous approximations by more than an order of magnitude; the correction is smaller at higher densities. We trace the origin of the deviations primarily to inaccuracies of (i) the most common fitting formula (Draine & Bertoldi 1996) for the suppression of the dissociation rate and (ii) estimates for the effective shielding column density from local properties of the gas. The combined effects of gas temperature and velocity gradients are comparatively less important, typically altering the spherically averaged rate only by a factor of less than two. We present a simple modification to the DB96 fitting formula for the optically thick rate which improves agreement with our numerical results to within approx. 15 per cent, and can be adopted in future simulations. We find that estimates for the effective shielding column can be improved by using the local Sobolev length. Our correction to the H_2 self-shielding reduces the critical LW flux to suppress H_2-cooling in T_vir>10^4 K haloes by an order of magnitude; this increases the number of such haloes in which supermassive (approx. M=10^5 M_sun) black holes may have formed.}, author = {Wolcott-Green, J. and Haiman, Zoltán and Bryan, G. L.}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {2}, pages = {838--852}, publisher = {Oxford University Press}, title = {{Photodissociation of H2 in protogalaxies: Modelling self-shielding in three-dimensional simulations}}, doi = {10.1111/j.1365-2966.2011.19538.x}, volume = {418}, year = {2011}, } @article{17701, abstract = {Independent lines of evidence suggest that the first stars, which ended the cosmic dark ages, came in pairs, rather than singly. This could change the prevailing view that the early Universe had a Swiss-cheese-like appearance.}, author = {Haiman, Zoltán}, issn = {0028-0836}, journal = {Nature}, number = {7341}, pages = {47--48}, publisher = {Springer Science and Business Media LLC}, title = {{A smoother end to the dark ages}}, doi = {10.1038/472047a}, volume = {472}, year = {2011}, } @article{1775, abstract = {At optical frequencies the radiation produced by a source, such as a laser, a black body or a single-photon emitter, is frequently characterized by analysing the temporal correlations of emitted photons using single-photon counters. At microwave frequencies, however, there are no efficient single-photon counters yet. Instead, well-developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate first- and second-order correlation function measurements of a pulsed microwave-frequency single-photon source integrated on the same chip with a 50/50 beam splitter followed by linear amplifiers and quadrature amplitude detectors. We clearly observe single-photon coherence in first-order and photon antibunching in second-order correlation function measurements of the propagating fields.}, author = {Bozyigit, Deniz and Lang, C and Steffen, L. Kraig and Johannes Fink and Eichler, Christopher and Baur, Matthias P and Bianchetti, R and Leek, Peter J and Filipp, Stefan and Da Silva, Marcus P and Blais, Alexandre and Wallraff, Andreas}, journal = {Nature Physics}, number = {2}, pages = {154 -- 158}, publisher = {Nature Publishing Group}, title = {{Antibunching of microwave-frequency photons observed in correlation measurements using linear detectors}}, doi = {10.1038/nphys1845}, volume = {7}, year = {2011}, } @inproceedings{1776, abstract = {Superconducting circuits have been successfully established as systems to prepare and investigate microwave light fields at the quantum level. In contrast to optical experiments where light is detected using photon counters, microwaves are usually measured with well developed linear amplifiers. This makes measurements of correlation functions - one of the important tools in optics - harder to achieve because they traditionally rely on photon counters and beam splitters. Here, we demonstrate a system where we can prepare on demand single microwave photons in a cavity and detect them at the two outputs of the cavity using linear amplifiers. Together with efficient data processing, this allows us to measure different observables of the cavity photons, including the first-order correlation function. Using these techniques we demonstrate cooling of a thermal background field in the cavity.}, author = {Bozyigit, Deniz and Lang, C and Steffen, L. Kraig and Johannes Fink and Eichler, Christopher and Baur, Matthias P and Bianchetti, R and Leek, Peter J and Filipp, Stefan and Wallraff, Andreas and Da Silva, Marcus P and Blais, Alexandre}, number = {1}, publisher = {IOP Publishing Ltd.}, title = {{Correlation measurements of individual microwave photons emitted from a symmetric cavity}}, doi = {10.1088/1742-6596/264/1/012024}, volume = {264}, year = {2011}, } @article{1777, abstract = {A wide range of experiments studying microwave photons localized in superconducting cavities have made important contributions to our understanding of the quantum properties of radiation. Propagating microwave photons, however, have so far been studied much less intensely. Here we present measurements in which we reconstruct the quantum state of itinerant single photon Fock states and their superposition with the vacuum by analyzing moments of the measured amplitude distribution up to fourth order. Using linear amplifiers and quadrature amplitude detectors, we have developed efficient methods to separate the detected single photon signal from the noise added by the amplifier. From our measurement data we have also reconstructed the corresponding Wigner function.}, author = {Eichler, Christopher and Bozyigit, Deniz and Lang, C and Steffen, L. and Fink, Johannes M and Wallraff, Andreas}, journal = {Physical Review Letters}, number = {22}, publisher = {American Physical Society}, title = {{Experimental state tomography of itinerant single microwave photons}}, doi = {10.1103/PhysRevLett.106.220503}, volume = {106}, year = {2011}, } @article{1778, abstract = {Creating a train of single photons and monitoring its propagation and interaction is challenging in most physical systems, as photons generally interact very weakly with other systems. However, when confining microwave frequency photons in a transmission line resonator, effective photon-photon interactions can be mediated by qubits embedded in the resonator. Here, we observe the phenomenon of photon blockade through second-order correlation function measurements. The experiments clearly demonstrate antibunching in a continuously pumped source of single microwave photons measured by using microwave beam splitters, linear amplifiers, and quadrature amplitude detectors. We also investigate resonance fluorescence and Rayleigh scattering in Mollow-triplet-like spectra.}, author = {Lang, C and Bozyigit, Deniz and Eichler, Christopher and Steffen, L. Kraig and Johannes Fink and Abdumalikov, Abdufarrukh A and Baur, Matthias P and Filipp, Stefan and Da Silva, Marcus P and Blais, Alexandre and Wallraff, Andreas}, journal = {Physical Review Letters}, number = {24}, publisher = {American Physical Society}, title = {{Observation of resonant photon blockade at microwave frequencies using correlation function measurements}}, doi = {10.1103/PhysRevLett.106.243601}, volume = {106}, year = {2011}, } @article{1780, abstract = {Continuous variable entanglement between two modes of a radiation field is usually studied at optical frequencies. Here we demonstrate experiments that show the entanglement between microwave photons of different energy in a broadband squeezed beam. We use a Josephson parametric amplifier to generate the two-mode correlated state and detect all four quadrature components simultaneously in a two-channel heterodyne setup using amplitude detectors. Analyzing two-dimensional phase space histograms for all possible pairs of quadratures allows us to determine the full covariance matrix, which is in good agreement with the one expected for a two-mode squeezed state.}, author = {Eichler, Christopher and Bozyigit, Deniz and Lang, C and Baur, Matthias P and Steffen, L. Kraig and Johannes Fink and Filipp, Stefan and Wallraff, Andreas}, journal = {Physical Review Letters}, number = {11}, publisher = {American Physical Society}, title = {{Observation of two-mode squeezing in the microwave frequency domain}}, doi = {10.1103/PhysRevLett.107.113601}, volume = {107}, year = {2011}, } @article{1781, abstract = {Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse interaction between qubits when they are nonresonant with the cavity but resonant with each other. We experimentally investigate the inverse scaling of the interqubit coupling with the detuning from a cavity mode and its proportionality to the qubit-cavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higher-harmonic cavity modes. Moreover, we observe dark states of the coupled qubit-qubit system and analyze their relation to the symmetry of the applied driving field at different frequencies.}, author = {Filipp, Stefan and Göppl, M and Johannes Fink and Baur, Matthias P and Bianchetti, R and Steffen, L. Kraig and Wallraff, Andreas}, journal = {Physical Review A - Atomic, Molecular, and Optical Physics}, number = {6}, publisher = {American Physical Society}, title = {{Multimode mediated qubit-qubit coupling and dark-state symmetries in circuit quantum electrodynamics}}, doi = {10.1103/PhysRevA.83.063827}, volume = {83}, year = {2011}, } @article{18012, abstract = {We report the first concurrent determination of conductance (G) and thermopower (S) of single-molecule junctions via direct measurement of electrical and thermoelectric currents using a scanning tunneling microscope-based break-junction technique. We explore several amine-Au and pyridine-Au linked molecules that are predicted to conduct through either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), respectively. We find that the Seebeck coefficient is negative for pyridine-Au linked LUMO-conducting junctions and positive for amine-Au linked HOMO-conducting junctions. Within the accessible temperature gradients (<30 K), we do not observe a strong dependence of the junction Seebeck coefficient on temperature. From histograms of thousands of junctions, we use the most probable Seebeck coefficient to determine a power factor, GS2, for each junction studied, and find that GS2 increases with G. Finally, we find that conductance and Seebeck coefficient values are in good quantitative agreement with our self-energy corrected density functional theory calculations.}, author = {Widawsky, Jonathan R. and Darancet, Pierre and Neaton, Jeffrey B. and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {1}, pages = {354--358}, publisher = {American Chemical Society}, title = {{Simultaneous determination of conductance and thermopower of single molecule junctions}}, doi = {10.1021/nl203634m}, volume = {12}, year = {2011}, } @article{18014, abstract = {We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au–C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au–xylylene–Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.}, author = {Chen, Wenbo and Widawsky, Jonathan R. and Vázquez, Héctor and Schneebeli, Severin T. and Hybertsen, Mark S. and Breslow, Ronald and Venkataraman, Latha}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {43}, pages = {17160--17163}, publisher = {American Chemical Society}, title = {{Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes}}, doi = {10.1021/ja208020j}, volume = {133}, year = {2011}, } @article{18015, abstract = {We investigate the binding and energy level alignment of 2,3,5,6-tetramethyl-1,4-benzenediamine (TMBDA) on Au(111) through a combination of helium atom scattering (HAS), X-ray photoemission (XPS), and scanning tunneling microscopy (STM). We show that TMBDA binds to step edges and to flat Au (111) terraces in a nearly flat-lying configuration. Through combination of HAS and STM data, we determine that the molecules are bound on step edges with an adsorption energy of about 1.2 eV, which is about 0.2 eV stronger than the adsorption energy we measure on flat surface. Preferential bonding to the under-coordinated Au atoms on step edges suggests that the molecules bind to Au through the nitrogen lone pair. Finally, STM measurements on TMBDA in these two different adsorption configurations show that the highest-occupied molecular orbital is deeper relative to Fermi for the more strongly bound molecules on step edges, confirming that the nitrogen bonds through charge donation to the Au.}, author = {Kamenetska, M. and Dell’Angela, M. and Widawsky, J.R. and Kladnik, G. and Verdini, A. and Cossaro, A. and Cvetko, D. and Morgante, A. and Venkataraman, Latha}, issn = {1932-7455}, journal = {The Journal of Physical Chemistry C}, number = {25}, pages = {12625--12630}, publisher = {American Chemical Society}, title = {{Structure and energy level alignment of tetramethyl benzenediamine on Au(111)}}, doi = {10.1021/jp202555d}, volume = {115}, year = {2011}, } @article{18016, abstract = {Understanding the electrical properties of semiconducting quantum dot devices have been limited due to the variability of their size/composition and the chemistry of ligand/electrode binding. Furthermore, to probe their electrical conduction properties and its dependence on ligand/electrode binding, measurements must be carried out at the single dot/cluster level. Herein we report scanning tunneling microscope based break junction measurements of cobalt chalcogenide clusters with Te, Se and S to probe the conductance properties. Our measured conductance trends show that the Co–Te based clusters have the highest conductance while the Co-S clusters the lowest. These trends are in very good agreement with cyclic voltammetry measurements of the first oxidation potentials and with density functional theory calculations of their HOMO–LUMO gaps.}, author = {Boardman, Brycelyn M. and Widawsky, Jonathan R. and Park, Young S. and Schenck, Christine L. and Venkataraman, Latha and Steigerwald, Michael L. and Nuckolls, Colin}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {22}, pages = {8455--8457}, publisher = {American Chemical Society}, title = {{Conductance of single cobalt chalcogenide cluster junctions}}, doi = {10.1021/ja201334s}, volume = {133}, year = {2011}, } @article{18017, abstract = {The conductance of individual 1,4-benzenediamine (BDA)–Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point-contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface.}, author = {Fatemi, V. and Kamenetska, M. and Neaton, J. B. and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {5}, pages = {1988--1992}, publisher = {American Chemical Society}, title = {{Environmental control of single-molecule junction transport}}, doi = {10.1021/nl200324e}, volume = {11}, year = {2011}, } @article{18018, abstract = {Controlling electron transport through a single-molecule device is key to the realization of nanoscale electronic components. A design requirement for single molecule electrical devices is that the molecule must be both structurally and electrically connected to the metallic electrodes. Typically, the mechanical and electrical contacts are achieved by the same chemical moiety. In this study, we demonstrate that the structural role may be played by one group (for example, a sulfide) while the electrical role may be played by another (a conjugated chain of C═C π-bonds). We can specify the electrical conductance through the molecule by modulating to which particular site on the oligoene chain the electrode binds. The result is a device that functions as a potentiometer at the single-molecule level.}, author = {Meisner, Jeffrey S. and Kamenetska, Masha and Krikorian, Markrete and Steigerwald, Michael L. and Venkataraman, Latha and Nuckolls, Colin}, issn = {1530-6992}, journal = {Nano Letters}, number = {4}, pages = {1575--1579}, publisher = {American Chemical Society}, title = {{A single-molecule potentiometer}}, doi = {10.1021/nl104411f}, volume = {11}, year = {2011}, } @article{18019, abstract = {We simultaneously measure conductance and force across nanoscale junctions. A new, two-dimensional histogram technique is introduced to statistically extract bond rupture forces from a large data set of individual junction elongation traces. For the case of Au point contacts, we find a rupture force of 1.4 ± 0.2 nN, which is in good agreement with previous measurements. We then study systematic trends for single gold metal−molecule−metal junctions for a series of molecules terminated with amine and pyridine linkers. For all molecules studied, single molecule junctions rupture at the Au−N bond. Selective binding of the linker group allows us to correlate the N−Au bond-rupture force to the molecular backbone. We find that the rupture force ranges from 0.8 nN for 4,4′ bipyridine to 0.5 nN in 1,4 diaminobenzene. These experimental results are in excellent quantitative agreement with density functional theory based adiabatic molecular junction elongation and rupture calculations.}, author = {Frei, Michael and Aradhya, Sriharsha V. and Koentopp, Max and Hybertsen, Mark S. and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {4}, pages = {1518--1523}, publisher = {American Chemical Society}, title = {{Mechanics and chemistry: Single molecule bond rupture forces correlate with molecular backbone structure}}, doi = {10.1021/nl1042903}, volume = {11}, year = {2011}, } @article{18020, abstract = {Understanding electron transport across π−π-stacked systems will help to answer fundamental questions about biochemical redox processes and benefit the design of new materials and molecular devices. Herein we employed the STM break-junction technique to measure the single-molecule conductance of multiple π−π-stacked aromatic rings. We studied electron transport through up to four stacked benzene rings held together in an eclipsed fashion via a paracyclophane scaffold. We found that the strained hydrocarbons studied herein couple directly to gold electrodes during the measurements; hence, we did not require any heteroatom binding groups as electrical contacts. Density functional theory-based calculations suggest that the gold atoms of the electrodes bind to two neighboring carbon atoms of the outermost cyclophane benzene rings in η2 fashion. Our measurements show an exponential decay of the conductance with an increasing number of stacked benzene rings, indicating a nonresonant tunneling mechanism. Furthermore, STM tip−substrate displacement data provide additional evidence that the electrodes bind to the outermost benzene rings of the π−π-stacked molecular wires.}, author = {Schneebeli, Severin T. and Kamenetska, Maria and Cheng, Zhanling and Skouta, Rachid and Friesner, Richard A. and Venkataraman, Latha and Breslow, Ronald}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {7}, pages = {2136--2139}, publisher = {American Chemical Society}, title = {{Single-molecule conductance through multiple π−π-stacked benzene rings determined with direct electrode-to-benzene ring connections}}, doi = {10.1021/ja111320n}, volume = {133}, year = {2011}, } @article{18021, abstract = {Charge transport across metal–molecule interfaces has an important role in organic electronics1. Typically, chemical link groups such as thiols2 or amines3 are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal–carbon coupling has been shown through C60, benzene and π-stacked benzene4,5,6,7, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe3)-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique2,3. Gold atoms at the electrode displace the SnMe3 linkers, leading to the formation of direct Au–C bonded single-molecule junctions with a conductance that is ∼100 times larger than analogous alkanes with most other terminations. The conductance of these Au–C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au–C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal–organic contacts.}, author = {Cheng, Z.-L. and Skouta, R. and Vazquez, H. and Widawsky, J. R. and Schneebeli, S. and Chen, W. and Hybertsen, M. S. and Breslow, R. and Venkataraman, Latha}, issn = {1748-3395}, journal = {Nature Nanotechnology}, number = {6}, pages = {353--357}, publisher = {Springer Nature}, title = {{In situ formation of highly conducting covalent Au–C contacts for single-molecule junctions}}, doi = {10.1038/nnano.2011.66}, volume = {6}, year = {2011}, } @article{1815, abstract = {Many membrane channels and receptors exhibit adaptive, or desensitized, response to a strong sustained input stimulus, often supported by protein activity-dependent inactivation. Adaptive response is thought to be related to various cellular functions such as homeostasis and enlargement of dynamic range by background compensation. Here we study the quantitative relation between adaptive response and background compensation within a modeling framework. We show that any particular type of adaptive response is neither sufficient nor necessary for adaptive enlargement of dynamic range. In particular a precise adaptive response, where system activity is maintained at a constant level at steady state, does not ensure a large dynamic range neither in input signal nor in system output. A general mechanism for input dynamic range enlargement can come about from the activity-dependent modulation of protein responsiveness by multiple biochemical modification, regardless of the type of adaptive response it induces. Therefore hierarchical biochemical processes such as methylation and phosphorylation are natural candidates to induce this property in signaling systems.}, author = {Tamar Friedlander and Brenner, Naama}, journal = {Mathematical Biosciences and Engineering}, number = {2}, pages = {515 -- 526}, publisher = {Arizona State University}, title = {{Adaptive response and enlargement of dynamic range}}, doi = {10.3934/mbe.2011.8.515}, volume = {8}, year = {2011}, } @article{18362, abstract = {Maximally stable component detection is a very popular method for feature analysis in images, mainly due to its low computation cost and high repeatability. With the recent advance of feature-based methods in geometric shape analysis, there is significant interest in finding analogous approaches in the 3D world. In this paper, we formulate a diffusion-geometric framework for stable component detection in non-rigid 3D shapes, which can be used for geometric feature detection and description. A quantitative evaluation of our method on the SHREC’10 feature detection benchmark shows its potential as a source of high-quality features.}, author = {Litman, Roee and Bronstein, Alexander and Bronstein, Michael M.}, issn = {0097-8493}, journal = {Computers & Graphics}, number = {3}, pages = {549--560}, publisher = {Elsevier}, title = {{Diffusion-geometric maximally stable component detection in deformable shapes}}, doi = {10.1016/j.cag.2011.03.011}, volume = {35}, year = {2011}, } @article{18363, abstract = {Natural objects can be subject to various transformations yet still preserve properties that we refer to as invariants. Here, we use definitions of affine-invariant arclength for surfaces in in order to extend the set of existing non-rigid shape analysis tools. We show that by re-defining the surface metric as its equi-affine version, the surface with its modified metric tensor can be treated as a canonical Euclidean object on which most classical Euclidean processing and analysis tools can be applied. The new definition of a metric is used to extend the fast marching method technique for computing geodesic distances on surfaces, where now, the distances are defined with respect to an affine-invariant arclength. Applications of the proposed framework demonstrate its invariance, efficiency, and accuracy in shape analysis.}, author = {Raviv, Dan and Bronstein, Alexander and Bronstein, Michael M. and Kimmel, Ron and Sochen, Nir}, issn = {0097-8493}, journal = {Computers & Graphics}, number = {3}, pages = {692--697}, publisher = {Elsevier}, title = {{Affine-invariant geodesic geometry of deformable 3D shapes}}, doi = {10.1016/j.cag.2011.03.030}, volume = {35}, year = {2011}, }