@article{386, abstract = {We present a detailed study of the local density of states (LDOS) associated with the surface-state band near a step edge of the strong topological insulator Bi2Te3 and reveal a one-dimensional bound state that runs parallel to the step edge and is bound to it at some characteristic distance. This bound state is clearly observed in the bulk gap region, while it becomes entangled with the oscillations of the warped surface band at high energy, and with the valence-band states near the Dirac point. We obtain excellent fits to theoretical predictions [Alpichshev, 2011] that properly incorporate the three-dimensional nature of the problem to the surface state. Fitting the data at different energies, we can recalculate the LDOS originating from the Dirac band without the contribution of the bulk bands or incoherent tunneling effects. }, author = {Alpichshev, Zhanybek and Analytis, J G and Chu, J H and Fisher, I R and Kapitulnik, A}, journal = {Physical Review B - Condensed Matter and Materials Physics}, number = {4}, publisher = {American Physical Society}, title = {{STM imaging of a bound state along a step on the surface of the topological insulator Bi2Te3}}, doi = {10.1103/PhysRevB.84.041104}, volume = {84}, year = {2011}, } @article{3965, abstract = {The elevation function on a smoothly embedded 2-manifold in R-3 reflects the multiscale topography of cavities and protrusions as local maxima. The function has been useful in identifying coarse docking configurations for protein pairs. Transporting the concept from the smooth to the piecewise linear category, this paper describes an algorithm for finding all local maxima. While its worst-case running time is the same as of the algorithm used in prior work, its performance in practice is orders of magnitudes superior. We cast light on this improvement by relating the running time to the total absolute Gaussian curvature of the 2-manifold.}, author = {Wang, Bei and Edelsbrunner, Herbert and Morozov, Dmitriy}, journal = {Journal of Experimental Algorithmics}, number = {2.2}, pages = {1 -- 13}, publisher = {ACM}, title = {{Computing elevation maxima by searching the Gauss sphere}}, doi = {10.1145/1963190.1970375}, volume = {16}, year = {2011}, } @article{1723, abstract = {The emergence of differences in the arrangement of cells is the first step towards the establishment of many organs. Understanding this process is limited by the lack of systematic characterization of epithelial organisation. Here we apply network theory at the scale of individual cells to uncover patterns in cell-to-cell contacts that govern epithelial organisation. We provide an objective characterisation of epithelia using network representation, where cells are nodes and cell contacts are links. The features of individual cells, together with attributes of the cellular network, produce a defining signature that distinguishes epithelia from different organs, species, developmental stages and genetic conditions. The approach permits characterization, quantification and classification of normal and perturbed epithelia, and establishes a framework for understanding molecular mechanisms that underpin the architecture of complex tissues.}, author = {Escudero, Luis M and Costa, Luciano and Anna Kicheva and Briscoe, James and Freeman, Matthew and Babu, Madan M}, journal = {Nature Communications}, number = {1}, publisher = {Nature Publishing Group}, title = {{Epithelial organisation revealed by a network of cellular contacts}}, doi = {10.1038/ncomms1536}, volume = {2}, year = {2011}, } @article{1724, abstract = {Morphogens, such as Decapentaplegic (Dpp) in the fly imaginal discs, form graded concentration profiles that control patterning and growth of developing organs. In the imaginal discs, proliferative growth is homogeneous in space, posing the conundrum of how morphogen concentration gradients could control position-independent growth. To understand the mechanism of proliferation control by the Dpp gradient, we quantified Dpp concentration and signaling levels during wing disc growth. Both Dpp concentration and signaling gradients scale with tissue size during development. On average, cells divide when Dpp signaling levels have increased by 50%. Our observations are consistent with a growth control mechanism based on temporal changes of cellular morphogen signaling levels. For a scaling gradient, this mechanism generates position-independent growth rates.}, author = {Wartlick, Ortrud and Mumcu, Peer and Anna Kicheva and Bittig, Thomas and Seum, Carole and Jülicher, Frank and González-Gaitán, Marcos A}, journal = {Science}, number = {6021}, pages = {1154 -- 1159}, publisher = {American Association for the Advancement of Science}, title = {{Dynamics of Dpp signaling and proliferation control}}, doi = {10.1126/science.1200037}, volume = {331}, year = {2011}, } @article{1754, abstract = {We report on a technique enabling electrical control of the contact silicidation process in silicon nanowire devices. Undoped silicon nanowires were contacted by pairs of nickel electrodes and each contact was selectively silicided by means of the Joule effect. By a realtime monitoring of the nanowire electrical resistance during the contact silicidation process we were able to fabricate nickel-silicide/silicon/nickel- silicide devices with controlled silicon channel length down to 8 nm. }, author = {Mongillo, Massimo and Spathis, Panayotis and Katsaros, Georgios and Gentile, Pascal and Sanquer, Marc and De Franceschi, Silvano}, journal = {ACS Nano}, number = {9}, pages = {7117 -- 7123}, publisher = {American Chemical Society}, title = {{Joule-assisted silicidation for short-channel silicon nanowire devices}}, doi = {10.1021/nn202524j}, volume = {5}, year = {2011}, } @article{1755, abstract = {Spin-selective tunneling of holes in SiGe nanocrystals contacted by normal-metal leads is reported. The spin selectivity arises from an interplay of the orbital effect of the magnetic field with the strong spin-orbit interaction present in the valence band of the semiconductor. We demonstrate both experimentally and theoretically that spin-selective tunneling in semiconductor nanostructures can be achieved without the use of ferromagnetic contacts. The reported effect, which relies on mixing the light and heavy holes, should be observable in a broad class of quantum-dot systems formed in semiconductors with a degenerate valence band.}, author = {Georgios Katsaros and Golovach, Vitaly N and Spathis, Panayotis N and Ares, Natalia and Stoffel, Mathieu and Fournel, Frank and Schmidt, Oliver G and Glazman, Leonid I and De Franceschi, Silvano}, journal = {Physical Review Letters}, number = {24}, publisher = {American Physical Society}, title = {{Observation of spin-selective tunneling in sige nanocrystals}}, doi = {10.1103/PhysRevLett.107.246601}, volume = {107}, year = {2011}, } @article{17647, abstract = {We study the effects of a thin gaseous accretion disk on the inspiral of a stellar--mass black hole into a supermassive black hole. We construct a phenomenological angular momentum transport equation that reproduces known disk effects. Disk torques modify the gravitational wave phase evolution to detectable levels with LISA for reasonable disk parameters. The Fourier transform of disk-modified waveforms acquires a correction with a different frequency trend than post-Newtonian vacuum terms. Such inspirals could be used to detect accretion disks with LISA and to probe their physical parameters.}, author = {Yunes, Nicolás and Kocsis, Bence and Loeb, Abraham and Haiman, Zoltán}, issn = {0031-9007}, journal = {Physical Review Letters}, number = {17}, publisher = {American Physical Society}, title = {{Imprint of accretion disk-induced migration on gravitational waves from extreme mass ratio inspirals}}, doi = {10.1103/physrevlett.107.171103}, volume = {107}, year = {2011}, } @article{17661, abstract = {Recent studies have shown that the number counts of convergence peaks N(kappa) in weak lensing (WL) maps, expected from large forthcoming surveys, can be a useful probe of cosmology. We follow up on this finding, and use a suite of WL convergence maps, obtained from ray-tracing N-body simulations, to study (i) the physical origin of WL peaks with different heights, and (ii) whether the peaks contain information beyond the convergence power spectrum P_ell. In agreement with earlier work, we find that high peaks (with amplitudes >~ 3.5 sigma, where sigma is the r.m.s. of the convergence kappa) are typically dominated by a single massive halo. In contrast, medium-height peaks (~0.5-1.5 sigma) cannot be attributed to a single collapsed dark matter halo, and are instead created by the projection of multiple (typically, 4-8) halos along the line of sight, and by random galaxy shape noise. Nevertheless, these peaks dominate the sensitivity to the cosmological parameters w, sigma_8, and Omega_m. We find that the peak height distribution and its dependence on cosmology differ significantly from predictions in a Gaussian random field. We directly compute the marginalized errors on w, sigma_8, and Omega_m from the N(kappa) + P_ell combination, including redshift tomography with source galaxies at z_s=1 and z_s=2. We find that the N(kappa) + P_ell combination has approximately twice the cosmological sensitivity compared to P_ell alone. These results demonstrate that N(kappa) contains non-Gaussian information complementary to the power spectrum.}, author = {Yang, Xiuyuan and Kratochvil, Jan M. and Wang, Sheng and Lim, Eugene A. and Haiman, Zoltán and May, Morgan}, issn = {1550-7998}, journal = {Physical Review D}, number = {4}, publisher = {American Physical Society}, title = {{Cosmological information in weak lensing peaks}}, doi = {10.1103/physrevd.84.043529}, volume = {84}, year = {2011}, } @article{17676, abstract = {It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic halos at high redshift. The resulting radiative cooling by HD can lower the gas temperature to that of the cosmic microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD-cooling by UV irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of both H_2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen as well as the shielding of HD by H_2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below 100 K. The main reason for the lack of HD cooling for J>J_crit is the partial photo-dissociation of H_2, which prevents the gas from reaching sufficiently low temperatures (T<150K) for HD to become the dominant coolant; direct HD photo-dissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD-cooling requires only partial H_2 photo-dissociation, the critical flux J_crit is modest, and is below the UV background required to reionize the universe at redshift z=10-20. We conclude that HD-cooling can reduce the masses of typical stars only in rare halos forming well before the epoch of reionization.}, author = {Wolcott-Green, J. and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {2603--2616}, publisher = {Oxford University Press}, title = {{Suppression of HD cooling in protogalactic gas clouds by Lyman-Werner radiation}}, doi = {10.1111/j.1365-2966.2010.18080.x}, volume = {412}, year = {2011}, } @article{17681, abstract = {The gravitational waves (GWs) emitted by inspiralling binary black holes, expected to be detected by the Laser Interferometer Space Antenna (LISA), could be used to determine the luminosity distance to these sources with the unprecedented precision of ≲1 per cent. We study cosmological parameter constraints from such standard sirens, in the presence of gravitational lensing by large-scale structure. Lensing introduces magnification with a probability distribution function (PDF) whose shape has significant skewness and kurtosis, and depends on cosmological parameters. We use Monte Carlo simulations to generate mock samples of standard sirens, including a small intrinsic scatter, as well as the additional, larger scatter from lensing, in their inferred distances. We derive constraints on cosmological parameters, by simultaneously fitting the mean and the distribution of the residuals on the distance versus redshift (dL–z) Hubble diagram. We find that for standard sirens at redshift z≈ 1, the sensitivity to a single cosmological parameter, such as the matter density Ωm, or the dark energy equation of state w, is ∼50–80 per cent tighter when the lensing PDF is used, compared to the sensitivity derived from a Gaussian PDF with the same variance. When these two parameters are constrained simultaneously, the non-Gaussian shape yields a further enhanced improvement (by ∼120 per cent), owing to the correlation between the parameters. The sensitivity to the amplitude of the matter power spectrum, σ8 from the cosmological dependence of the PDF alone, however, is ∼20 per cent worse than that from the Gaussian PDF. The improvements for Ωm and w arise purely from the non-Gaussian shape of the lensing PDF and can be attributed specifically to the sharpness of the peak of this PDF (i.e. to a finite kurtosis); the dependence of the PDF on these parameters does not improve constraints relative to those available from the mean dL–z relation. At higher redshifts, the PDF resembles a Gaussian more closely, and the effects of the non-Gaussianities become less prominent. These results highlight the importance of obtaining an accurate and reliable PDF of the lensing convergence, in order to realize the full potential of standard sirens as cosmological probes.}, author = {Shang, Cien and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {1}, pages = {9--22}, publisher = {Oxford University Press}, title = {{Cosmology with standard sirens: The importance of the shape of the lensing magnification distribution}}, doi = {10.1111/j.1365-2966.2010.17607.x}, volume = {411}, year = {2011}, } @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}, }