@article{17649, abstract = {Weak lensing convergence peaks are a promising tool to probe nonlinear structure evolution at late times, providing additional cosmological information beyond second-order statistics. Previous theoretical and observational studies have shown that the cosmological constraints on Ωm and σ8 are improved by a factor of up to ~ 2 when peak counts and second-order statistics are combined, compared to using the latter alone. We study the origin of lensing peaks using observational data from the 154 deg2 Canada-France-Hawaii Telescope Lensing Survey. We found that while high peaks (with height κ >3.5 σκ, where σκ is the r.m.s. of the convergence κ) are typically due to one single massive halo of ~1015M⊙, low peaks (κ <~ σκ) are associated with constellations of 2-8 smaller halos (<~1013M⊙). In addition, halos responsible for forming low peaks are found to be significantly offset from the line-of-sight towards the peak center (impact parameter >~ their virial radii), compared with ~0.25 virial radii for halos linked with high peaks, hinting that low peaks are more immune to baryonic processes whose impact is confined to the inner regions of the dark matter halos. Our findings are in good agreement with results from the simulation work by Yang el al. (2011).}, author = {Liu, Jia and Haiman, Zoltán}, issn = {2470-0010}, journal = {Physical Review D}, number = {4}, publisher = {American Physical Society}, title = {{Origin of weak lensing convergence peaks}}, doi = {10.1103/physrevd.94.043533}, volume = {94}, year = {2016}, } @article{17654, abstract = {We explore the formation of massive high-redshift Population III (Pop III) galaxies through photoionization feedback. We consider dark matter haloes formed from progenitors that have undergone no star formation as a result of early reionization and photoevaporation caused by a nearby galaxy. Once such a halo reaches ≈109 M⊙, corresponding to the Jeans mass of the photoheated intergalactic medium at z ≈ 7, pristine gas is able to collapse into the halo, potentially producing a massive Pop III starburst. We suggest that this scenario may explain the recent observation of strong He ii 1640 Å line emission in CR 7, which is consistent with ∼107 M⊙ of young Pop III stars. Such a large mass of Pop III stars is unlikely without the photoionization feedback scenario, because star formation is expected to inject metals into haloes above the atomic cooling threshold (∼108 M⊙ at z ≈ 7). We use merger trees to analytically estimate the abundance of observable Pop III galaxies formed through this channel, and find a number density of ≈10−7 Mpc−3 at z = 6.6 (the redshift of CR 7). This is approximately a factor of 10 lower than the density of Ly α emitters as bright as CR 7.}, author = {Visbal, Eli and Haiman, Zoltán and Bryan, Greg L.}, issn = {1745-3925}, journal = {Monthly Notices of the Royal Astronomical Society: Letters}, number = {1}, pages = {L59--L63}, publisher = {Oxford University Press}, title = {{Formation of massive Population III galaxies through photoionization feedback: A possible explanation for CR 7}}, doi = {10.1093/mnrasl/slw071}, volume = {460}, year = {2016}, } @article{17656, abstract = {We explore the evolution of stellar mass black hole binaries (BHBs) which are formed in the self-gravitating discs of active galactic nuclei (AGN). Hardening due to three-body scattering and gaseous drag are effective mechanisms that reduce the semimajor axis of a BHB to radii where gravitational waves take over, on time-scales shorter than the typical lifetime of the AGN disc. Taking observationally motivated assumptions for the rate of star formation in AGN discs, we find a rate of disc-induced BHB mergers (⁠| $\mathcal {R} \sim 3\ {\rm yr}^{-1}\ {\rm Gpc}^{-3}$ |⁠, but with large uncertainties) that is comparable with existing estimates of the field rate of BHB mergers, and the approximate BHB merger rate implied by the recent Advanced LIGO detection of GW150914. BHBs formed thorough this channel will frequently be associated with luminous AGN, which are relatively rare within the sky error regions of future gravitational wave detector arrays. This channel could also possess a (potentially transient) electromagnetic counterpart due to super-Eddington accretion on to the stellar mass black hole following the merger.}, author = {Stone, Nicholas C. and Metzger, Brian D. and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {1}, pages = {946--954}, publisher = {Oxford University Press}, title = {{Assisted inspirals of stellar mass black holes embedded in AGN discs: Solving the ‘final au problem’}}, doi = {10.1093/mnras/stw2260}, volume = {464}, year = {2016}, } @article{17658, abstract = {We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio q ≡ Ms/Mp ∼ 0.04. For mass ratios above q = 0.04, solutions for the hydrodynamic flow transition from steady, to strongly fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3Bp. Using the disco code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The q = 0.04 transition has relevance for the evolution of massive black hole binary+disc systems at the centres of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs.}, author = {D'Orazio, Daniel J. and Haiman, Zoltán and Duffell, Paul and MacFadyen, Andrew and Farris, Brian}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {3}, pages = {2379--2393}, publisher = {Oxford University Press}, title = {{A transition in circumbinary accretion discs at a binary mass ratio of 1:25}}, doi = {10.1093/mnras/stw792}, volume = {459}, year = {2016}, } @article{17659, abstract = {The recent discovery of the gravitational wave source GW150914 has revealed a coalescing binary black hole (BBH) with masses of ∼30 M⊙. Previous proposals for the origin of such a massive binary include Population III (PopIII) stars. PopIII stars are efficient producers of BBHs and of a gravitational wave background (GWB) in the 10–100 Hz band, and also of ionizing radiation in the early Universe. We quantify the relation between the amplitude of the GWB (Ωgw) and the electron scattering optical depth (τe), produced by PopIII stars, assuming that fesc ≈ 10 per cent of their ionizing radiation escapes into the intergalactic medium. We find that PopIII stars would produce a GWB that is detectable by the future O5 LIGO/Virgo if τe ≳ 0.07, consistent with the recent Planck measurement of τe = 0.055 ± 0.09. Moreover, the spectral index of the background from PopIII BBHs becomes as small as dln Ωgw/dln f ≲ 0.3 at f ≳ 30 Hz, which is significantly flatter than the value ∼2/3 generically produced by lower redshift and less-massive BBHs. A detection of the unique flattening at such low frequencies by the O5 LIGO/Virgo will indicate the existence of a high-chirp mass, high-redshift BBH population, which is consistent with the PopIII origin. A precise characterization of the spectral shape near 30–50 Hz by the Einstein Telescope could also constrain the PopIII initial mass function and star formation rate.}, author = {Inayoshi, Kohei and Kashiyama, Kazumi and Visbal, Eli and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {3}, pages = {2722--2727}, publisher = {Oxford University Press}, title = {{Gravitational wave background from Population III binary black holes consistent with cosmic reionization}}, doi = {10.1093/mnras/stw1431}, volume = {461}, year = {2016}, } @article{17666, abstract = {We have investigated a recently proposed halo-based model, Camelus, for predicting weak-lensing peak counts, and compared its results over a collection of 162 cosmologies with those from N-body simulations. While counts from both models agree for peaks with S/N>1 (where S/N is the ratio of the peak height to the r.m.s. shape noise), we find ≈50% fewer counts for peaks near S/N=0 and significantly higher counts in the negative S/N tail. Adding shape noise reduces the differences to within 20% for all cosmologies. We also found larger covariances that are more sensitive to cosmological parameters. As a result, credibility regions in the {Ωm,σ8} are ≈30% larger. Even though the credible contours are commensurate, each model draws its predictive power from different types of peaks. Low peaks, especially those with 23). Our results confirm the importance of using a cosmology-dependent covariance with at least a 14\% improvement in parameter constraints. We identified the covariance estimation as the main driver behind differences in inference, and suggest possible ways to make Camelus even more useful as a highly accurate peak count emulator.}, author = {Zorrilla Matilla, José Manuel and Haiman, Zoltán and Hsu, Daniel and Gupta, Arushi and Petri, Andrea}, issn = {2470-0010}, journal = {Physical Review D}, number = {8}, publisher = {American Physical Society}, title = {{Do dark matter halos explain lensing peaks?}}, doi = {10.1103/physrevd.94.083506}, volume = {94}, year = {2016}, } @article{17669, abstract = {Unprecedentedly precise cosmic microwave background (CMB) data are expected from ongoing and near-future CMB Stage-III and IV surveys, which will yield reconstructed CMB lensing maps with effective resolution approaching several arcminutes. The small-scale CMB lensing fluctuations receive non-negligible contributions from nonlinear structure in the late-time density field. These fluctuations are not fully characterized by traditional two-point statistics, such as the power spectrum. Here, we use N-body ray-tracing simulations of CMB lensing maps to examine two higher-order statistics: the lensing convergence one-point probability distribution function (PDF) and peak counts. We show that these statistics contain significant information not captured by the two-point function, and provide specific forecasts for the ongoing Stage-III Advanced Atacama Cosmology Telescope (AdvACT) experiment. Considering only the temperature-based reconstruction estimator, we forecast 9σ (PDF) and 6σ (peaks) detections of these statistics with AdvACT. Our simulation pipeline fully accounts for the non-Gaussianity of the lensing reconstruction noise, which is significant and cannot be neglected. Combining the power spectrum, PDF, and peak counts for AdvACT will tighten cosmological constraints in the Ωm-σ8 plane by ≈30%, compared to using the power spectrum alone.}, author = {Liu, Jia and Hill, J. Colin and Sherwin, Blake D. and Petri, Andrea and Böhm, Vanessa and Haiman, Zoltán}, issn = {2470-0010}, journal = {Physical Review D}, number = {10}, publisher = {American Physical Society}, title = {{CMB lensing beyond the power spectrum: Cosmological constraints from the one-point probability distribution function and peak counts}}, doi = {10.1103/physrevd.94.103501}, volume = {94}, year = {2016}, } @article{17672, abstract = {Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be common in galactic nuclei, as a result of frequent galaxy mergers. Hydrodynamical simulations of circum-binary discs predict strong periodic modulation of the mass accretion rate on time-scales comparable to the orbital period of the binary. As a result, SMBHBs may be recognized by the periodic modulation of their brightness. We conducted a statistical search for periodic variability in a sample of 35 383 spectroscopically confirmed quasars in the photometric data base of the Palomar Transient Factory (PTF). We analysed Lomb–Scargle periodograms and assessed the significance of our findings by modelling each individual quasar's variability as a damped random walk (DRW). We identified 50 quasars with significant periodicity beyond the DRW model, typically with short periods of a few hundred days. We find 33 of these to remain significant after a re-analysis of their periodograms including additional optical data from the intermediate-PTF and the Catalina Real-Time Transient Survey. Assuming that the observed periods correspond to the redshifted orbital periods of SMBHBs, we conclude that our findings are consistent with a population of unequal-mass SMBHBs, with a typical mass ratio as low as q ≡ M2/M1 ≈ 0.01.}, author = {Charisi, M. and Bartos, I. and Haiman, Zoltán and Price-Whelan, A. M. and Graham, M. J. and Bellm, E. C. and Laher, R. R. and Márka, S.}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {2}, pages = {2145--2171}, publisher = {Oxford University Press}, title = {{A population of short-period variable quasars from PTF as supermassive black hole binary candidates}}, doi = {10.1093/mnras/stw1838}, volume = {463}, year = {2016}, } @article{17693, abstract = {We perform one-dimensional radiation hydrodynamical simulations to solve accretion flows onto massive black holes (BHs) with a very high rate. Assuming that photon trapping limits the luminosity emerging from the central region to L≲LEdd, Inayoshi, Haiman & Ostriker (2016) have shown that an accretion flow settles to a "hyper-Eddington" solution, with a steady and isothermal (T≃8000 K) Bondi profile reaching ≳5000 times the Eddington accretion rate M˙Edd≡LEdd/c2. Here we address the possibility that gas accreting with finite angular momentum forms a bright nuclear accretion disc, with a luminosity exceeding the Eddington limit (1≲L/LEdd≲100). Combining our simulations with an analytic model, we find that a transition to steady hyper-Eddington accretion still occurs, as long as the luminosity remains below L/LEdd≲35 (MBH/10^4 M⊙)^3/2(n∞/10^5 cm^−3)(T∞/10^4 K)^−3/2(r⋆/10^14 cm)^−1/2, where n∞ and T∞ are the density and temperature of the ambient gas, and r⋆ is the radius of the photosphere, at which radiation emerges. If the luminosity exceeds this value, accretion becomes episodic. Our results can be accurately recovered in a toy model of an optically thick spherical shell, driven by radiation force into a collapsing medium. When the central source is dimmer than the above critical value, the expansion of the shell is halted and reversed by ram pressure of the collapsing medium, and by shell's weight. Our results imply that rapid, unimpeded hyper-Eddington accretion is possible even if the luminosity of the central source far exceeds the Eddington limit, and can be either steady or strongly episodic.}, author = {Sakurai, Yuya and Inayoshi, Kohei and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {4496--4504}, publisher = {Oxford University Press}, title = {{Hyper-Eddington mass accretion on to a black hole with super-Eddington luminosity}}, doi = {10.1093/mnras/stw1652}, volume = {461}, year = {2016}, } @article{17699, abstract = {We quantify the presence of Ly\alpha\ damping wing absorption from a partially-neutral intergalactic medium (IGM) in the spectrum of the z=7.08 QSO, ULASJ1120+0641. Using a Bayesian framework, we simultaneously account for uncertainties in: (i) the intrinsic QSO emission spectrum; and (ii) the distribution of cosmic HI patches during the epoch of reionisation (EoR). For (i) we use a new intrinsic Ly\alpha\ emission line reconstruction method (Greig et al.), sampling a covariance matrix of emission line properties built from a large database of moderate-z QSOs. For (ii), we use the Evolution of 21-cm Structure (EOS; Mesinger et al.) simulations, which span a range of physically-motivated EoR models. We find strong evidence for the presence of damping wing absorption redward of Ly\alpha\ (where there is no contamination from the Ly\alpha\ forest). Our analysis implies that the EoR is not yet complete by z=7.1, with the volume-weighted IGM neutral fraction constrained to x¯HI=0.40+0.21−0.19 at 1σ (x¯HI=0.40+0.41−0.32 at 2σ). This result is insensitive to the EoR morphology. Our detection of significant neutral HI in the IGM at z=7.1 is consistent with the latest Planck 2016 measurements of the CMB Thompson scattering optical depth (Planck Collaboration XLVII).}, author = {Greig, Bradley and Mesinger, Andrei and Haiman, Zoltán and Simcoe, Robert A.}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, publisher = {Oxford University Press}, title = {{Are we witnessing the epoch of reionization at z=7.1 from the spectrum of J1120+0641?}}, doi = {10.1093/mnras/stw3351}, year = {2016}, } @article{17700, abstract = {We report the formation of intermediate-mass black holes (IMBHs) in suites of numerical N-body simulations of Population III remnant black holes (BHs) embedded in gas-rich protogalaxies at redshifts z≳10. We model the effects of gas drag on the BHs' orbits, and allow BHs to grow via gas accretion, including a mode of hyper-Eddington accretion in which photon trapping and rapid gas inflow suppress any negative radiative feedback. Most initial BH configurations lead to the formation of one (but never more than one) IMBH in the center of the protogalaxy, reaching a mass of 10^3−5M⊙ through hyper-Eddington growth. Our results suggest a viable pathway to forming the earliest massive BHs in the centers of early galaxies. We also find that the nuclear IMBH typically captures a stellar-mass BH companion, making these systems observable in gravitational waves as extreme mass-ratio inspirals (EMRIs) with eLISA.}, author = {Ryu, Taeho and Tanaka, Takamitsu L. and Perna, Rosalba and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {4122--4134}, publisher = {Oxford University Press}, title = {{Intermediate-mass black holes from Population III remnants in the first galactic nuclei}}, doi = {10.1093/mnras/stw1241}, volume = {460}, year = {2016}, } @article{17709, abstract = {We study very-high rate spherically symmetric accretion flows onto a massive black hole (BH; 10^2 < M_BH < 10^6 Msun) embedded in a dense gas cloud with a low abundance of metals, performing one-dimensional hydrodynamical simulations which include multi-frequency radiation transfer and non-equilibrium primordial chemistry. We find that rapid gas supply from the Bondi radius at a hyper-Eddington rate can occur without being impeded by radiation feedback when (n/10^5 cm^-3) > (M_BH/10^4Msun)^{-1}(T/10^4 K)^{3/2}, where n and T are the density and temperature of ambient gas outside of the Bondi radius. The resulting accretion rate in this regime is steady, and larger than 3000 times the Eddington rate. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is limited below the Eddington rate. For the hyper-Eddington case, the steady solution consists of two parts: a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T~8000 K. When the emergent luminosity is limited below the Eddington luminosity because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of T_vir> 10^4 K. Once a seed BH forms at the center of the galaxy, it can grow up to a maximum ~10^5 (T_vir/10^4 K) Msun via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lya emitters without X-rays and luminous infrared sources with hot dust emission, respectively.}, author = {Inayoshi, Kohei and Haiman, Zoltán and Ostriker, Jeremiah P.}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {3738--3755}, publisher = {Oxford University Press}, title = {{Hyper-Eddington accretion flows on to massive black holes}}, doi = {10.1093/mnras/stw836}, volume = {459}, year = {2016}, } @article{1794, abstract = {We consider Conditional random fields (CRFs) with pattern-based potentials defined on a chain. In this model the energy of a string (labeling) (Formula presented.) is the sum of terms over intervals [i, j] where each term is non-zero only if the substring (Formula presented.) equals a prespecified pattern w. Such CRFs can be naturally applied to many sequence tagging problems. We present efficient algorithms for the three standard inference tasks in a CRF, namely computing (i) the partition function, (ii) marginals, and (iii) computing the MAP. Their complexities are respectively (Formula presented.), (Formula presented.) and (Formula presented.) where L is the combined length of input patterns, (Formula presented.) is the maximum length of a pattern, and D is the input alphabet. This improves on the previous algorithms of Ye et al. (NIPS, 2009) whose complexities are respectively (Formula presented.), (Formula presented.) and (Formula presented.), where (Formula presented.) is the number of input patterns. In addition, we give an efficient algorithm for sampling, and revisit the case of MAP with non-positive weights.}, author = {Kolmogorov, Vladimir and Takhanov, Rustem}, journal = {Algorithmica}, number = {1}, pages = {17 -- 46}, publisher = {Springer}, title = {{Inference algorithms for pattern-based CRFs on sequence data}}, doi = {10.1007/s00453-015-0017-7}, volume = {76}, year = {2016}, } @article{17952, abstract = {The stability of chemical bonds can be studied experimentally by rupturing single molecule junctions under applied voltage. Here, we compare voltage-induced bond rupture in two Si–Si backbones: one has no alternate conductive pathway whereas the other contains an additional naphthyl pathway in parallel to the Si–Si bond. We show that in contrast to the first system, the second can conduct through the naphthyl group when the Si–Si bond is ruptured using an applied voltage. We investigate this voltage induced Si–Si bond rupture by ab initio density functional theory calculations and molecular dynamics simulations that ultimately demonstrate that the excitation of molecular vibrational modes by tunneling electrons leads to homolytic Si–Si bond rupture.}, author = {Li, Haixing and Kim, Nathaniel T. and Su, Timothy A. and Steigerwald, Michael L. and Nuckolls, Colin and Darancet, Pierre and Leighton, James L. and Venkataraman, Latha}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {49}, pages = {16159--16164}, publisher = {American Chemical Society}, title = {{Mechanism for Si–Si bond rupture in single molecule junctions}}, doi = {10.1021/jacs.6b10700}, volume = {138}, year = {2016}, } @article{17954, abstract = {Guidelines to predict trends in the electrical conductance of molecules have been developed for the π-system of conjugated systems. Little is known, however, about the conductance of the underlying σ-systems because the π-system usually dominates the transport. Here we study a family of bipyridine-based molecules using STM-break junction experiments and density functional theory transport calculations. We use different lengths and substitution patterns to probe the role of both the σ-system and the π-system in controlling conductance. By exploiting the destructive interference feature found in the π-system of the meta-coupled six-membered aromatic rings, we show that the conductance of the σ-system of a meta-coupled molecule can be probed directly and can even exceed that of its para-coupled analog. These results add to the understanding of the conductance through the chemically hidden σ-electrons.}, author = {Borges, Anders and Fung, E-Dean and Ng, Fay and Venkataraman, Latha and Solomon, Gemma C.}, issn = {1948-7185}, journal = {The Journal of Physical Chemistry Letters}, number = {23}, pages = {4825--4829}, publisher = {American Chemical Society}, title = {{Probing the conductance of the σ-system of bipyridine using destructive interference}}, doi = {10.1021/acs.jpclett.6b02494}, volume = {7}, year = {2016}, } @article{17955, abstract = {The development of molecular components functioning as switches, rectifiers or amplifiers is a great challenge in molecular electronics. A desirable property of such components is functional robustness, meaning that the intrinsic functionality of components must be preserved regardless of the strategy used to integrate them into the final assemblies. Here, this issue is investigated for molecular diodes based on N-phenylbenzamide (NPBA) backbones. The transport properties of molecular junctions derived from NPBA are characterized while varying the nature of the functional groups interfacing the backbone and the gold electrodes required for break-junction measurements. Combining experimental and theoretical methods, it is shown that at low bias (<0.85 V) transport is determined by the same frontier molecular orbital originating from the NPBA core, regardless of the anchoring group employed. The magnitude of rectification, however, is strongly dependent on the strength of the electronic coupling at the gold–NPBA interface and on the spatial distribution of the local density of states of the dominant transport channel of the molecular junction.}, author = {Koepf, Matthieu and Koenigsmann, Christopher and Ding, Wendu and Batra, Arunbah and Negre, Christian F. A. and Venkataraman, Latha and Brudvig, Gary W. and Batista, Victor S. and Schmuttenmaer, Charles A. and Crabtree, Robert H.}, issn = {2040-3372}, journal = {Nanoscale}, number = {36}, pages = {16357--16362}, publisher = {Royal Society of Chemistry}, title = {{Controlling the rectification properties of molecular junctions through molecule–electrode coupling}}, doi = {10.1039/c6nr04830g}, volume = {8}, year = {2016}, } @article{17956, abstract = {A highly conducting electronic contact between a strained disilane and Au is demonstrated through scanning tunneling microscope-based single-molecule measurements. Conformationally locked cis diastereomers of bis(sulfide)-anchor-equipped 1,2-disilaacenaphthenes readily form high-conducting junctions in which the two sulfide anchors bind in a bipodal fashion to one gold electrode, providing enough stability for a stable electrical contact between the Si–Si σ bond and the other electrode.}, author = {Kim, Nathaniel T. and Li, Haixing and Venkataraman, Latha and Leighton, James L.}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {36}, pages = {11505--11508}, publisher = {American Chemical Society}, title = {{High-conductance pathways in ring-strained disilanes by qay of direct σ-Si–Si to Au coordination}}, doi = {10.1021/jacs.6b07825}, volume = {138}, year = {2016}, } @article{17957, abstract = {While the single-molecule conductance properties of π-conjugated and σ-conjugated systems have been well-studied, little is known regarding the conductance properties of mixed σ–π backbone wires and the factors that control their transport properties. Here we utilize a scanning tunneling microscope-based break-junction technique to study a series of molecular wires with π–σ–π backbone structures, where the π-moiety is an electrode-binding thioanisole ring and the σ-moiety is a triatomic α–β–α chain composed of C, Si, or Ge atoms. We find that the sequence and composition of group 14 atoms in the α–β–α chain dictates whether electronic communication between the aryl rings is enhanced or suppressed. Placing heavy atoms at the α-position decreases conductance, whereas placing them at the β-position increases conductance: for example, the C–Ge–C sequence is over 20 times more conductive than the Ge–C–Ge sequence. Density functional theory calculations reveal that these conductance trends arise from periodic trends (i.e., atomic size, polarizability, and electronegativity) that differ from C to Si to Ge. The periodic trends that control molecular conductance here are the same ones that give rise to the α and β silicon effects from physical organic chemistry. These findings outline a new molecular design concept for tuning conductance in single-molecule electrical devices.}, author = {Su, Timothy A. and Li, Haixing and Klausen, Rebekka S. and Widawsky, Jonathan R. and Batra, Arunabh and Steigerwald, Michael L. and Venkataraman, Latha and Nuckolls, Colin}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {24}, pages = {7791--7795}, publisher = {American Chemical Society}, title = {{Tuning conductance in π–σ–π single-molecule wires}}, doi = {10.1021/jacs.6b04394}, volume = {138}, year = {2016}, } @article{17958, abstract = {Charge transport phenomena in single-molecule junctions are often dominated by tunneling, with a transmission function dictating the probability that electrons or holes tunnel through the junction. Here, we present a new and simple technique for measuring the transmission functions of molecular junctions in the coherent tunneling limit, over an energy range of 1.5 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different exposed areas, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction’s transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.}, author = {Capozzi, Brian and Low, Jonathan Z. and Xia, Jianlong and Liu, Zhen-Fei and Neaton, Jeffrey B. and Campos, Luis M. and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {6}, pages = {3949--3954}, publisher = {American Chemical Society}, title = {{Mapping the transmission functions of single-molecule junctions}}, doi = {10.1021/acs.nanolett.6b01592}, volume = {16}, year = {2016}, } @article{17959, abstract = {Single-molecule conductance measurements have focused primarily on organic molecular systems. Here, we carry out scanning tunneling microscope-based break-junction measurements on a series of metal chalcogenide Co6Se8 clusters capped with conducting ligands of varying lengths. We compare these measurements with those of individual free ligands and find that the conductance of these clusters and the free ligands have different decay constants with increasing ligand length. We also show, through measurements in two different solvents, 1-bromonaphthalene and 1,2,4-trichlorobenzene, that the conductance decay of the clusters depends on the solvent environment. We discuss several mechanisms to explain our observations.}, author = {Choi, Bonnie and Capozzi, Brian and Ahn, Seokhoon and Turkiewicz, Ari and Lovat, Giacomo and Nuckolls, Colin and Steigerwald, Michael L. and Venkataraman, Latha and Roy, Xavier}, issn = {2041-6539}, journal = {Chemical Science}, number = {4}, pages = {2701--2705}, publisher = {Royal Society of Chemistry}, title = {{Solvent-dependent conductance decay constants in single cluster junctions}}, doi = {10.1039/c5sc02595h}, volume = {7}, year = {2016}, }