@article{17678, abstract = {The spectra of several high-redshift (z>6) quasars have shown evidence for a Gunn-Peterson (GP) damping wing, indicating a substantial mean neutral hydrogen fraction (x_HI > 0.03) in the z ~ 6 intergalactic medium (IGM). However, previous analyses assumed that the IGM was uniformly ionized outside of the quasar's HII region. Here we relax this assumption and model patchy reionization scenarios for a range of IGM and quasar parameters. We quantify the impact of these differences on the inferred x_HI, by fitting the spectra of three quasars: SDSS J1148+5251 (z=6.419), J1030+0524 (z=6.308), and J1623+3112 (z=6.247). We find that the best-fit values of x_HI in the patchy models agree well with the uniform case. More importantly, we confirm that the observed spectra favor the presence of a GP damping wing, with peak likelihoods decreasing by factors of > few - 10 when the spectra are modeled without a damping wing. We also find that the Ly alpha absorption spectra, by themselves, cannot distinguish the damping wing in a relatively neutral IGM from a damping wing in a highly ionized IGM, caused either by an isolated neutral patch, or by a damped Ly alpha absorber (DLA). However, neutral patches in a highly ionized universe (x_HI < 0.01), and DLAs with the large required column densities (N_HI > few x 10^{20} cm^{-2}) are both rare. As a result, when we include reasonable prior probabilities for the line of sight (LOS) to intercept either a neutral patch or a DLA at the required distance of ~ 40-60 comoving Mpc away from the quasar, we find strong lower limits on the neutral fraction in the IGM, x_HI > 0.1 (at 95% confidence). This strengthens earlier claims that a substantial global fraction of hydrogen in the z~6 IGM is in neutral form.}, author = {Schroeder, Joshua and Mesinger, Andrei and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {3058--3071}, publisher = {Oxford University Press}, title = {{Evidence of Gunn–Peterson damping wings in high-z quasar spectra: Strengthening the case for incomplete reionization at z ∼ 6–7}}, doi = {10.1093/mnras/sts253}, volume = {428}, year = {2012}, } @article{17688, abstract = {Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low-redshift (z<1.5) compact supermassive (M>10^9 Msun) black hole (SMBH) binaries with orbital periods of approx. 0.1 - 10 yrs. Identifying the electromagnetic (EM) counterparts of these sources would provide confirmation of putative direct detections of GWs, present a rare opportunity to study the environments of compact SMBH binaries, and could enable the use of these sources as standard sirens for cosmology. Here we consider the feasibility of such an EM identification. We show that because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift z=0.2. At higher redshifts, the PTA error boxes are larger, and may contain as many as 100 massive-galaxy interlopers. The number of candidates, however, remains tractable for follow-up searches in upcoming wide-field EM surveys. We develop a toy model to characterize the dynamics and the thermal emission from a geometrically thin, gaseous disc accreting onto a PTA-source SMBH binary. Our model predicts that at optical and infrared frequencies, the source should appear similar to a typical luminous active galactic nucleus (AGN). However, owing to the evacuation of the accretion flow by the binary's tidal torques, the source might have an unusually low soft X-ray luminosity and weak UV and broad optical emission lines, as compared to an AGN powered by a single SMBH with the same total mass. For sources near z=1, the decrement in the rest-frame UV should be observable as an extremely red optical color. These properties would make the PTA sources stand out among optically luminous AGN, and could allow their unique identification.}, author = {Tanaka, Takamitsu and Menou, Kristen and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {1}, pages = {705--719}, publisher = {Oxford University Press}, title = {{Electromagnetic counterparts of supermassive black hole binaries resolved by pulsar timing arrays}}, doi = {10.1111/j.1365-2966.2011.20083.x}, volume = {420}, year = {2012}, } @article{17705, abstract = {Observations of high-redshift quasars at z>6 imply that supermassive black holes (SMBHs) with masses over a billion solar masses were in place less than 1 Gyr after the Big Bang. If these SMBHs assembled from "seed" BHs left behind by the first stars, then they must have accreted gas at close to the Eddington limit during a large fraction (>50%) of the time. A generic problem with this scenario, however, is that the mass density in million-solar-mass SMBHs at z=6 already exceeds the locally observed SMBH mass density by several orders of magnitude; in order to avoid this overproduction, BH seed formation and growth must become significantly less efficient in less massive protogalaxies, while proceeding uninterrupted in the most massive galaxies that formed first. Using Monte-Carlo realizations of the merger and growth history of BHs, we show that X-rays from the earliest accreting BHs can provide such a feedback mechanism. Our calculations paint a self-consistent picture of black-hole-made climate change, in which the first miniquasars---among them the ancestors of the z>6 quasar SMBHs---globally warm the IGM and suppress the formation and growth of subsequent generations of BHs. We present two specific models with global miniquasar feedback that provide excellent agreement with recent estimates of the z=6 SMBH mass function. For each of these models, we estimate the rate of BH mergers at z>6 that could be detected by the proposed gravitational-wave observatory eLISA/NGO.}, author = {Tanaka, Takamitsu and Perna, Rosalba and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {2974--2987}, publisher = {Oxford University Press}, title = {{X-ray emission from high-redshift miniquasars: Self-regulating the population of massive black holes through global warming}}, doi = {10.1111/j.1365-2966.2012.21539.x}, volume = {425}, year = {2012}, } @article{1782, abstract = {Steering a quantum harmonic oscillator state along cyclic trajectories leads to a path-dependent geometric phase. Here we describe its experimental observation in an electronic harmonic oscillator. We use a superconducting qubit as a nonlinear probe of the phase, which is otherwise unobservable due to the linearity of the oscillator. We show that the geometric phase is, for a variety of cyclic paths, proportional to the area enclosed in the quadrature plane. At the transition to the nonadiabatic regime, we study corrections to the phase and dephasing of the qubit caused by qubit-resonator entanglement. In particular, we identify parameters for which this dephasing mechanism is negligible even in the nonadiabatic regime. The demonstrated controllability makes our system a versatile tool to study geometric phases in open quantum systems and to investigate their potential for quantum information processing.}, author = {Pechal, M and Berger, Stefan T and Abdumalikov, Abdufarrukh A and Johannes Fink and Mlynek, Jonas A and Steffen, L. Kraig and Wallraff, Andreas and Filipp, Stefan}, journal = {Physical Review Letters}, number = {17}, publisher = {American Physical Society}, title = {{Geometric phase and nonadiabatic effects in an electronic harmonic oscillator}}, doi = {10.1103/PhysRevLett.108.170401}, volume = {108}, year = {2012}, } @article{1783, abstract = {Nonlinearity and entanglement are two important properties by which physical systems can be identified as nonclassical. We study the dynamics of the resonant interaction of up to N=3 two-level systems and a single mode of the electromagnetic field sharing a single excitation dynamically. We observe coherent vacuum Rabi oscillations and their nonlinear √N speedup by tracking the populations of all qubits and the resonator in time. We use quantum state tomography to show explicitly that the dynamics generates maximally entangled states of the W class in a time limited only by the collective interaction rate. We use an entanglement witness and the 3-tangle to characterize the state whose fidelity F=78% is limited in our experiments by crosstalk arising during the simultaneous qubit manipulations which is absent in a sequential approach with F=91%.}, author = {Mlynek, Jonas A and Abdumalikov, Abdufarrukh A and Johannes Fink and Steffen, L. Kraig and Baur, Matthias P and Lang, C and Van Loo, Arjan F and Wallraff, Andreas}, journal = {Physical Review A - Atomic, Molecular, and Optical Physics}, number = {5}, publisher = {American Physical Society}, title = {{Demonstrating W-type entanglement of Dicke states in resonant cavity quantum electrodynamics}}, doi = {10.1103/PhysRevA.86.053838}, volume = {86}, year = {2012}, } @article{1784, abstract = {A localized qubit entangled with a propagating quantum field is well suited to study nonlocal aspects of quantum mechanics and may also provide a channel to communicate between spatially separated nodes in a quantum network. Here, we report the on-demand generation and characterization of Bell-type entangled states between a superconducting qubit and propagating microwave fields composed of zero-, one-, and two-photon Fock states. Using low noise linear amplification and efficient data acquisition we extract all relevant correlations between the qubit and the photon states and demonstrate entanglement with high fidelity.}, author = {Eichler, Christopher and Lang, C and Johannes Fink and Govenius, J and Filipp, Stefan and Wallraff, Andreas}, journal = {Physical Review Letters}, number = {24}, publisher = {American Physical Society}, title = {{Observation of entanglement between itinerant microwave photons and a superconducting qubit}}, doi = {10.1103/PhysRevLett.109.240501}, volume = {109}, year = {2012}, } @inproceedings{18000, abstract = {Measurement of electronics and mechanics of single molecules provides a fundamental understanding of conductance as well as bonding at the atomic scale. To study the mechanics at these length scales, we have built a conducting atomic force microscope (AFM) optimized for high displacement and force resolution. Here, we simultaneously measure conductance and force across single Au-molecule-Au junctions in order to obtain complementary information about the electronics and structure in these systems. First we show that single-atom Au contacts, which have a conductance of G0 (2e2/h), have a rupture force of about 1.4 nN, in excellent agreement with previous theoretical and experimental studies. For a series of amine and pyridine linked molecules which are bound to Au electrodes through an Au-N donor-acceptor bond, we observe that the rupture force depends on the backbone chemistry and can range from 0.5 to 0.8 nN. We also study junctions formed with molecules that bind through P-Au and S-Au interactions. We find that both the conductance signatures and junction evolution of covalent S-Au bond (thiolate) and a donor-acceptor S-Au bond (thiol) are dramatically different. Finally, we perform density functional theory based adiabatic molecular junction elongation and rupture calculations which give us an insight into the underlying mechanisms in these experiments.}, author = {Aradhya, Sriharsha V. and Frei, Michael and Hybertsen, Mark S. and Venkataraman, Latha}, booktitle = {Proceedings of the 2012 Annual Conference on Experimental and Applied Mechanics}, isbn = {9781461444350}, issn = {2191-5652}, location = {Costa Mesa, CA, United States}, pages = {75--84}, publisher = {Springer Nature}, title = {{Simultaneous measurement of force and conductance across single molecule junctions}}, doi = {10.1007/978-1-4614-4436-7_12}, volume = {6}, year = {2012}, } @article{18001, abstract = {We study the effects of molecular structure on the electronic transport and mechanical stability of single-molecule junctions formed with Au point contacts. Two types of linear conjugated molecular wires are compared: those functionalized with methylsulfide or amine aurophilic groups at (1) both or (2) only one of its phenyl termini. Using scanning tunneling and atomic force microscope break-junction techniques, the conductance of mono- and difunctionalized molecular wires and its dependence on junction elongation and rupture forces were studied. Charge transport through monofunctionalized wires is observed when the molecular bridge is coupled through a S–Au donor–acceptor bond on one end and a relatively weak Au−π interaction on the other end. For monofunctionalized molecular wires, junctions can be mechanically stabilized by installing a second aurophilic group at the meta position that, however, does not in itself contribute to a new conduction pathway. These results reveal the important interplay between electronic coupling through metal−π interactions and quantum mechanical effects introduced by chemical substitution on the conjugated system. This study affords a strategy to deterministically tune the electrical and mechanical properties through molecular wires.}, author = {Meisner, Jeffrey S. and Ahn, Seokhoon and Aradhya, Sriharsha V. and Krikorian, Markrete and Parameswaran, Radha and Steigerwald, Michael and Venkataraman, Latha and Nuckolls, Colin}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {50}, pages = {20440--20445}, publisher = {American Chemical Society}, title = {{Importance of direct metal−π coupling in electronic transport through conjugated single-molecule junctions}}, doi = {10.1021/ja308626m}, volume = {134}, year = {2012}, } @article{18002, abstract = {Using self-energy-corrected density functional theory (DFT) and a coherent scattering-state approach, we explain current–voltage (IV) measurements of four pyridine-Au and amine-Au linked molecular junctions with quantitative accuracy. Parameter-free many-electron self-energy corrections to DFT Kohn–Sham eigenvalues are demonstrated to lead to excellent agreement with experiments at finite bias, improving upon order-of-magnitude errors in currents obtained with standard DFT approaches. We further propose an approximate route for prediction of quantitative IV characteristics for both symmetric and asymmetric molecular junctions based on linear response theory and knowledge of the Stark shifts of junction resonance energies. Our work demonstrates that a quantitative, computationally inexpensive description of coherent transport in molecular junctions is readily achievable, enabling new understanding and control of charge transport properties of molecular-scale interfaces at large bias voltages.}, author = {Darancet, Pierre and Widawsky, Jonathan R. and Choi, Hyoung Joon and Venkataraman, Latha and Neaton, Jeffrey B.}, issn = {1530-6992}, journal = {Nano Letters}, number = {12}, pages = {6250--6254}, publisher = {American Chemical Society}, title = {{Quantitative current–voltage characteristics in molecular junctions from first principles}}, doi = {10.1021/nl3033137}, volume = {12}, year = {2012}, } @article{18003, author = {Roy, Xavier and Schenck, Christine L. and Ahn, Seokhoon and Lalancette, Roger A. and Venkataraman, Latha and Nuckolls, Colin and Steigerwald, Michael L.}, issn = {1521-3773}, journal = {Angewandte Chemie International Edition}, number = {50}, pages = {12473--12476}, publisher = {Wiley}, title = {{Quantum soldering of individual quantum dots}}, doi = {10.1002/anie.201206301}, volume = {51}, year = {2012}, } @article{18004, abstract = {We characterize electron transport across Au–molecule–Au junctions of heterogeneous carboxyl and methyl sulfide terminated saturated and conjugated molecules. Low-bias conductance measurements are performed using the scanning tunneling microscopy based break-junction technique in the presence of solvents and at room temperature. For a series of alkanes with 1–4 carbon atoms in the hydrocarbon chain, our results show an exponential decrease in conductance with increasing molecule length characterized by a decay constant of 0.9 ± 0.1 per methylene group. Control measurements in pH 11 solutions and with COOMe terminations suggest that the carboxylic acid group binds through the formation of a COO−–Au bond. Simultaneous measurements of conductance and force across these junctions yield a rupture force of 0.6 ± 0.1 nN, comparable to that required to rupture a Au–SMe bond. By establishing reliable, in situ junction formation, these experiments provide a new approach to probe electronic properties of carboxyl groups at the single molecule level.}, author = {Ahn, Seokhoon and Aradhya, Sriharsha V. and Klausen, Rebekka S. and Capozzi, Brian and Roy, Xavier and Steigerwald, Michael L. and Nuckolls, Colin and Venkataraman, Latha}, issn = {1463-9084}, journal = {Physical Chemistry Chemical Physics}, number = {40}, publisher = {Royal Society of Chemistry}, title = {{Electronic transport and mechanical stability of carboxyl linked single-molecule junctions}}, doi = {10.1039/c2cp41578j}, volume = {14}, year = {2012}, } @article{18006, abstract = {Understanding the role of intermolecular interaction on through-space charge transfer characteristics in π-stacked molecular systems is central to the rational design of electronic materials. However, a quantitative study of charge transfer in such systems is often difficult because of poor control over molecular morphology. Here we use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked π-systems held together by aliphatic chains. We study two systems, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), with inter-ring separations of 3.0 and 4.0 Å, respectively. We find that charge transfer across the π-coupled system of 44PCP is 20 times slower than in 22PCP. We attribute this difference to the decreased inter-ring electronic coupling in 44PCP. These measurements illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in π-stacked systems.}, author = {Batra, Arunabh and Kladnik, Gregor and Vázquez, Héctor and Meisner, Jeffrey S. and Floreano, Luca and Nuckolls, Colin and Cvetko, Dean and Morgante, Alberto and Venkataraman, Latha}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Quantifying through-space charge transfer dynamics in π-coupled molecular systems}}, doi = {10.1038/ncomms2083}, volume = {3}, year = {2012}, } @article{18007, abstract = {According to Kirchhoff's circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role1, as exemplified by the Aharonov–Bohm effect in metal rings2,3. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring4,5. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions6,7,8,9,10. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference.}, author = {Vazquez, H. and Skouta, R. and Schneebeli, S. and Kamenetska, M. and Breslow, R. and Venkataraman, Latha and Hybertsen, M.S.}, issn = {1748-3395}, journal = {Nature Nanotechnology}, number = {10}, pages = {663--667}, publisher = {Springer Nature}, title = {{Probing the conductance superposition law in single-molecule circuits with parallel paths}}, doi = {10.1038/nnano.2012.147}, volume = {7}, year = {2012}, } @article{18008, abstract = {The break-junction technique is widely used to measure electronic properties of nanoscale junctions including metal point-contacts and single-molecule junctions. In these measurements, conductance is measured as a function of electrode displacement yielding data that is analyzed by constructing conductance histograms to determine the most frequently observed conductance values in the nanoscale junctions. However much of the rich physics in these measurements is lost in this simple analysis technique. Conductance histograms cannot be used to study the statistical relation of distinct junction configurations, to distinguish structurally different configurations that have similar conductance values, or to obtain information on the relation between conductance and junction elongation. Here, we give a detailed introduction to a novel statistical analysis method based on the two-dimensional cross-correlation histogram (2DCH) analysis of conductance traces and show that this method provides new information about the relation of different junction configurations that occur during the formation and evolution of metal and single-molecule junctions. We first illustrate the different types of correlation effects by using simulated conductance traces. We then apply this analysis method to several different experimental examples. We show from break-junction measurements of different metal point-contacts that in aluminum, the first conductance histogram peak corresponds to two different junction structures. In tantalum, we identify the frequent absence of adhesive instability. We show that conductance plateaus shift in a correlated manner in iron and vanadium junctions. Finally, we highlight the applicability of the correlation analysis to single-molecule platinum–CO–platinum and gold–4,4′-bipyridine–gold junctions.}, author = {Makk, Péter and Tomaszewski, Damian and Martinek, Jan and Balogh, Zoltán and Csonka, Szabolcs and Wawrzyniak, Maciej and Frei, Michael and Venkataraman, Latha and Halbritter, András}, issn = {1936-086X}, journal = {ACS Nano}, number = {4}, pages = {3411--3423}, publisher = {American Chemical Society}, title = {{Correlation analysis of atomic and single-molecule junction conductance}}, doi = {10.1021/nn300440f}, volume = {6}, year = {2012}, } @article{18009, abstract = {Bulk silicon, the bedrock of information technology, consists of the deceptively simple electronic structure of just Si–Si σ bonds. Diamond has the same lattice structure as silicon, yet the two materials have dramatically different electronic properties. Here we report the specific synthesis and electrical characterization of a class of molecules, oligosilanes, that contain strongly interacting Si–Si σ bonds, the essential components of the bulk semiconductor. We used the scanning tunneling microscope-based break-junction technique to compare the single-molecule conductance of these oligosilanes to those of alkanes. We found that the molecular conductance decreases exponentially with increasing chain length with a decay constant β = 0.27 ± 0.01 Å–1, comparable to that of a conjugated chain of C═C π bonds. This result demonstrates the profound implications of σ conjugation for the conductivity of silicon.}, author = {Klausen, Rebekka S. and Widawsky, Jonathan R. and Steigerwald, Michael L. and Venkataraman, Latha and Nuckolls, Colin}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {10}, pages = {4541--4544}, publisher = {American Chemical Society}, title = {{Conductive molecular silicon}}, doi = {10.1021/ja211677q}, volume = {134}, year = {2012}, } @article{1801, abstract = {Brain circuits are assembled from a large variety of morphologically and functionally diverse cell types. It is not known how the intermingled cell types of an individual adult brain region differ in their expressed genomes. Here we describe an atlas of cell type transcriptomes in one brain region, the mouse retina. We found that each adult cell type expressed a specific set of genes, including a unique set of transcription factors, forming a 'barcode' for cell identity. Cell type transcriptomes carried enough information to categorize cells into morphological classes and types. Several genes that were specifically expressed in particular retinal circuit elements, such as inhibitory neuron types, are associated with eye diseases. The resource described here allows gene expression to be compared across adult retinal cell types, experimenting with specific transcription factors to differentiate stem or somatic cells to retinal cell types, and predicting cellular targets of newly discovered disease-associated genes.}, author = {Sandra Siegert and Cabuy, Erik and Scherf, Brigitte G and Kohler, Hubertus and Panda, Satchidananda and Le, Yunzheng and Fehling, Hans J and Gaidatzis, Dimos and Stadler, Michael B and Roska, Botond M}, journal = {Nature Neuroscience}, number = {3}, pages = {487 -- 495}, publisher = {Nature Publishing Group}, title = {{Transcriptional code and disease map for adult retinal cell types}}, doi = {10.1038/nn.3032}, volume = {15}, year = {2012}, } @article{18010, abstract = {Electronic factors in molecules such as quantum interference and cross-conjugation can lead to dramatic modulation and suppression of conductance in single-molecule junctions. Probing such effects at the single-molecule level requires simultaneous measurements of independent junction properties, as conductance alone cannot provide conclusive evidence of junction formation for molecules with low conductivity. Here, we compare the mechanics of the conducting para-terminated 4,4′-di(methylthio)stilbene and moderately conducting 1,2-bis(4-(methylthio)phenyl)ethane to that of insulating meta-terminated 3,3′-di(methylthio)stilbene single-molecule junctions. We simultaneously measure force and conductance across single-molecule junctions and use force signatures to obtain independent evidence of junction formation and rupture in the meta-linked cross-conjugated molecule even when no clear low-bias conductance is measured. By separately quantifying conductance and mechanics, we identify the formation of atypical 3,3′-di(methylthio)stilbene molecular junctions that are mechanically stable but electronically decoupled. While theoretical studies have envisaged many plausible systems where quantum interference might be observed, our experiments provide the first direct quantitative study of the interplay between contact mechanics and the distinctively quantum mechanical nature of electronic transport in single-molecule junctions.}, author = {Aradhya, Sriharsha V. and Meisner, Jeffrey S. and Krikorian, Markrete and Ahn, Seokhoon and Parameswaran, Radha and Steigerwald, Michael L. and Nuckolls, Colin and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {3}, pages = {1643--1647}, publisher = {American Chemical Society}, title = {{Dissecting contact mechanics from quantum interference in single-molecule junctions of stilbene derivatives}}, doi = {10.1021/nl2045815}, volume = {12}, year = {2012}, } @article{18011, abstract = {We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au–S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au–S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.}, author = {Frei, Michael and Aradhya, Sriharsha V. and Hybertsen, Mark S. and Venkataraman, Latha}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {9}, pages = {4003--4006}, publisher = {American Chemical Society}, title = {{Linker dependent bond rupture force measurements in single-molecule junctions}}, doi = {10.1021/ja211590d}, volume = {134}, year = {2012}, } @article{18013, abstract = {Van der Waals (vdW) interaction, and its subtle interplay with chemically specific interactions and surface roughness at metal/organic interfaces, is critical to the understanding of structure–function relations in diverse areas, including catalysis, molecular electronics and self-assembly1,2,3. However, vdW interactions remain challenging to characterize directly at the fundamental, single-molecule level both in experiments and in first principles calculations with accurate treatment of the non-local, London dispersion interactions. In particular, for metal/organic interfaces, efforts so far have largely focused on model systems consisting of adsorbed molecules on flat metallic surfaces with minimal specific chemical interaction4,5,6,7,8,9. Here we show, through measurements of single-molecule mechanics, that pyridine derivatives10,11 can bind to nanostructured Au electrodes through an additional binding mechanism beyond the chemically specific N–Au donor–acceptor bond. Using density functional theory simulations we show that vdW interactions between the pyridine ring and Au electrodes can play a key role in the junction mechanics. These measurements thus provide a quantitative characterization of vdW interactions at metal/organic interfaces at the single-molecule level.}, author = {Aradhya, Sriharsha V. and Frei, Michael and Hybertsen, Mark S. and Venkataraman, Latha}, issn = {1476-4660}, journal = {Nature Materials}, number = {10}, pages = {872--876}, publisher = {Springer Nature}, title = {{Van der Waals interactions at metal/organic interfaces at the single-molecule level}}, doi = {10.1038/nmat3403}, volume = {11}, year = {2012}, } @article{18201, abstract = {Owing to thermal fluctuations, two-dimensional (2D) systems cannot undergo a conventional phase transition associated with the breaking of a continuous symmetry1. Nevertheless they may exhibit a phase transition to a state with quasi-long-range order via the Berezinskii–Kosterlitz–Thouless (BKT) mechanism2. A paradigm example is the 2D Bose fluid, such as a liquid helium film3, which cannot condense at non-zero temperature although it becomes superfluid above a critical phase space density. The quasi-long-range coherence and the microscopic nature of the BKT transition were recently explored with ultracold atomic gases4,5,6. However, a direct observation of superfluidity in terms of frictionless flow is still missing for these systems. Here we probe the superfluidity of a 2D trapped Bose gas using a moving obstacle formed by a micrometre-sized laser beam. We find a dramatic variation of the response of the fluid, depending on its degree of degeneracy at the obstacle location.}, author = {Desbuquois, Rémi and Chomaz, Lauriane and Yefsah, Tarik and Leonard, Julian and Beugnon, Jérôme and Weitenberg, Christof and Dalibard, Jean}, issn = {1745-2481}, journal = {Nature Physics}, number = {9}, pages = {645--648}, publisher = {Springer Nature}, title = {{Superfluid behaviour of a two-dimensional Bose gas}}, doi = {10.1038/nphys2378}, volume = {8}, year = {2012}, }