@inbook{14000, abstract = {This chapter presents an overview of the state of the art in attosecond time-resolved spectroscopy. The theoretical foundations of strong-field light–matter interaction and attosecond pulse generation are described. The enabling laser technologies are reviewed from chirped-pulse amplification and carrier-envelope-phase stabilization to the generation and characterization of attosecond pulses. The applications of attosecond pulses and pulse trains in electron- or ion-imaging experiments are presented, followed by attosecond electron spectroscopy in larger molecules. After this, high-harmonic spectroscopy and its applications to probing charge migration on attosecond time scales is reviewed. The rapidly evolving field of molecular photoionization delays is discussed. Finally, the applications of attosecond transient absorption to probing molecular dynamics are presented.}, author = {Baykusheva, Denitsa Rangelova and Wörner, Hans Jakob}, booktitle = {Molecular Spectroscopy and Quantum Dynamics}, editor = {Marquardt, Roberto and Quack, Martin}, isbn = {9780128172353}, pages = {113--161}, publisher = {Elsevier}, title = {{Attosecond Molecular Dynamics and Spectroscopy}}, doi = {10.1016/b978-0-12-817234-6.00009-x}, year = {2020}, } @unpublished{14028, abstract = {The present review addresses the technical advances and the theoretical developments to realize and rationalize attosecond-science experiments that reveal a new dynamical time scale (10−15-10−18 s), with a particular emphasis on molecular systems and the implications of attosecond processes for chemical dynamics. After a brief outline of the theoretical framework for treating non-perturbative phenomena in Section 2, we introduce the physical mechanisms underlying high-harmonic generation and attosecond technology. The relevant technological developments and experimental schemes are covered in Section 3. Throughout the remainder of the chapter, we report on selected applications in molecular attosecond physics, thereby addressing specific phenomena mediated by purely electronic dynamics: charge localization in molecular hydrogen, charge migration in biorelevant molecules, high-harmonic spectroscopy, and delays in molecular photoionization.}, author = {Baykusheva, Denitsa Rangelova and Wörner, Hans Jakob}, pages = {2002.02111}, title = {{Attosecond molecular spectroscopy and dynamics}}, doi = {10.48550/arXiv.2002.02111}, year = {2020}, } @unpublished{14095, abstract = {The Habitable Exoplanet Observatory, or HabEx, has been designed to be the Great Observatory of the 2030s. For the first time in human history, technologies have matured sufficiently to enable an affordable space-based telescope mission capable of discovering and characterizing Earthlike planets orbiting nearby bright sunlike stars in order to search for signs of habitability and biosignatures. Such a mission can also be equipped with instrumentation that will enable broad and exciting general astrophysics and planetary science not possible from current or planned facilities. HabEx is a space telescope with unique imaging and multi-object spectroscopic capabilities at wavelengths ranging from ultraviolet (UV) to near-IR. These capabilities allow for a broad suite of compelling science that cuts across the entire NASA astrophysics portfolio. HabEx has three primary science goals: (1) Seek out nearby worlds and explore their habitability; (2) Map out nearby planetary systems and understand the diversity of the worlds they contain; (3) Enable new explorations of astrophysical systems from our own solar system to external galaxies by extending our reach in the UV through near-IR. This Great Observatory science will be selected through a competed GO program, and will account for about 50% of the HabEx primary mission. The preferred HabEx architecture is a 4m, monolithic, off-axis telescope that is diffraction-limited at 0.4 microns and is in an L2 orbit. HabEx employs two starlight suppression systems: a coronagraph and a starshade, each with their own dedicated instrument.}, author = {Gaudi, B. Scott and Seager, Sara and Mennesson, Bertrand and Kiessling, Alina and Warfield, Keith and Cahoy, Kerri and Clarke, John T. and Shawn Domagal-Goldman, Shawn Domagal-Goldman and Feinberg, Lee and Guyon, Olivier and Kasdin, Jeremy and Mawet, Dimitri and Plavchan, Peter and Robinson, Tyler and Rogers, Leslie and Scowen, Paul and Somerville, Rachel and Stapelfeldt, Karl and Stark, Christopher and Stern, Daniel and Turnbull, Margaret and Amini, Rashied and Kuan, Gary and Martin, Stefan and Morgan, Rhonda and Redding, David and Stahl, H. Philip and Webb, Ryan and Oscar Alvarez-Salazar, Oscar Alvarez-Salazar and Arnold, William L. and Arya, Manan and Balasubramanian, Bala and Baysinger, Mike and Bell, Ray and Below, Chris and Benson, Jonathan and Blais, Lindsey and Booth, Jeff and Bourgeois, Robert and Bradford, Case and Brewer, Alden and Brooks, Thomas and Cady, Eric and Caldwell, Mary and Calvet, Rob and Carr, Steven and Chan, Derek and Cormarkovic, Velibor and Coste, Keith and Cox, Charlie and Danner, Rolf and Davis, Jacqueline and Dewell, Larry and Dorsett, Lisa and Dunn, Daniel and East, Matthew and Effinger, Michael and Eng, Ron and Freebury, Greg and Garcia, Jay and Gaskin, Jonathan and Greene, Suzan and Hennessy, John and Hilgemann, Evan and Hood, Brad and Holota, Wolfgang and Howe, Scott and Huang, Pei and Hull, Tony and Hunt, Ron and Hurd, Kevin and Johnson, Sandra and Kissil, Andrew and Knight, Brent and Kolenz, Daniel and Kraus, Oliver and Krist, John and Li, Mary and Lisman, Doug and Mandic, Milan and Mann, John and Marchen, Luis and Colleen Marrese-Reading, Colleen Marrese-Reading and McCready, Jonathan and McGown, Jim and Missun, Jessica and Miyaguchi, Andrew and Moore, Bradley and Nemati, Bijan and Nikzad, Shouleh and Nissen, Joel and Novicki, Megan and Perrine, Todd and Pineda, Claudia and Polanco, Otto and Putnam, Dustin and Qureshi, Atif and Richards, Michael and Riggs, A. J. Eldorado and Rodgers, Michael and Rud, Mike and Saini, Navtej and Scalisi, Dan and Scharf, Dan and Schulz, Kevin and Serabyn, Gene and Sigrist, Norbert and Sikkia, Glory and Singleton, Andrew and Shaklan, Stuart and Smith, Scott and Southerd, Bart and Stahl, Mark and Steeves, John and Sturges, Brian and Sullivan, Chris and Tang, Hao and Taras, Neil and Tesch, Jonathan and Therrell, Melissa and Tseng, Howard and Valente, Marty and Buren, David Van and Villalvazo, Juan and Warwick, Steve and Webb, David and Westerhoff, Thomas and Wofford, Rush and Wu, Gordon and Woo, Jahning and Wood, Milana and Ziemer, John and Arney, Giada and Anderson, Jay and Jesús Maíz-Apellániz, Jesús Maíz-Apellániz and Bartlett, James and Belikov, Ruslan and Bendek, Eduardo and Cenko, Brad and Douglas, Ewan and Dulz, Shannon and Evans, Chris and Faramaz, Virginie and Feng, Y. Katherina and Ferguson, Harry and Follette, Kate and Ford, Saavik and García, Miriam and Geha, Marla and Gelino, Dawn and Götberg, Ylva Louise Linsdotter and Hildebrandt, Sergi and Hu, Renyu and Jahnke, Knud and Kennedy, Grant and Kreidberg, Laura and Isella, Andrea and Lopez, Eric and Marchis, Franck and Macri, Lucas and Marley, Mark and Matzko, William and Mazoyer, Johan and McCandliss, Stephan and Meshkat, Tiffany and Mordasini, Christoph and Morris, Patrick and Nielsen, Eric and Newman, Patrick and Petigura, Erik and Postman, Marc and Reines, Amy and Roberge, Aki and Roederer, Ian and Ruane, Garreth and Schwieterman, Edouard and Sirbu, Dan and Spalding, Christopher and Teplitz, Harry and Tumlinson, Jason and Turner, Neal and Werk, Jessica and Wofford, Aida and Wyatt, Mark and Young, Amber and Zellem, Rob}, booktitle = {arXiv}, title = {{The habitable exoplanet observatory (HabEx) mission concept study final report}}, doi = {10.48550/arXiv.2001.06683}, year = {2020}, } @unpublished{14096, abstract = {A binary neutron star merger has been observed in a multi-messenger detection of gravitational wave (GW) and electromagnetic (EM) radiation. Binary neutron stars that merge within a Hubble time, as well as many other compact binaries, are expected to form via common envelope evolution. Yet five decades of research on common envelope evolution have not yet resulted in a satisfactory understanding of the multi-spatial multi-timescale evolution for the systems that lead to compact binaries. In this paper, we report on the first successful simulations of common envelope ejection leading to binary neutron star formation in 3D hydrodynamics. We simulate the dynamical inspiral phase of the interaction between a 12M⊙ red supergiant and a 1.4M⊙ neutron star for different initial separations and initial conditions. For all of our simulations, we find complete envelope ejection and final orbital separations of af≈1.3-5.1R⊙ depending on the simulation and criterion, leading to binary neutron stars that can merge within a Hubble time. We find αCE-equivalent efficiencies of ≈0.1-2.7 depending on the simulation and criterion, but this may be specific for these extended progenitors. We fully resolve the core of the star to ≲0.005R⊙ and our 3D hydrodynamics simulations are informed by an adjusted 1D analytic energy formalism and a 2D kinematics study in order to overcome the prohibitive computational cost of simulating these systems. The framework we develop in this paper can be used to simulate a wide variety of interactions between stars, from stellar mergers to common envelope episodes leading to GW sources.}, author = {Jamie A. P. Law-Smith, Jamie A. P. Law-Smith and Everson, Rosa Wallace and Enrico Ramirez-Ruiz, Enrico Ramirez-Ruiz and Mink, Selma E. de and Son, Lieke A. C. van and Götberg, Ylva Louise Linsdotter and Zellmann, Stefan and Alejandro Vigna-Gómez, Alejandro Vigna-Gómez and Renzo, Mathieu and Wu, Samantha and Schrøder, Sophie L. and Foley, Ryan J. and Tenley Hutchinson-Smith, Tenley Hutchinson-Smith}, booktitle = {arXiv}, title = {{Successful common envelope ejection and binary neutron star formation in 3D hydrodynamics}}, doi = {10.48550/arXiv.2011.06630}, year = {2020}, } @article{14125, abstract = {Motivation: Recent technological advances have led to an increase in the production and availability of single-cell data. The ability to integrate a set of multi-technology measurements would allow the identification of biologically or clinically meaningful observations through the unification of the perspectives afforded by each technology. In most cases, however, profiling technologies consume the used cells and thus pairwise correspondences between datasets are lost. Due to the sheer size single-cell datasets can acquire, scalable algorithms that are able to universally match single-cell measurements carried out in one cell to its corresponding sibling in another technology are needed. Results: We propose Single-Cell data Integration via Matching (SCIM), a scalable approach to recover such correspondences in two or more technologies. SCIM assumes that cells share a common (low-dimensional) underlying structure and that the underlying cell distribution is approximately constant across technologies. It constructs a technology-invariant latent space using an autoencoder framework with an adversarial objective. Multi-modal datasets are integrated by pairing cells across technologies using a bipartite matching scheme that operates on the low-dimensional latent representations. We evaluate SCIM on a simulated cellular branching process and show that the cell-to-cell matches derived by SCIM reflect the same pseudotime on the simulated dataset. Moreover, we apply our method to two real-world scenarios, a melanoma tumor sample and a human bone marrow sample, where we pair cells from a scRNA dataset to their sibling cells in a CyTOF dataset achieving 90% and 78% cell-matching accuracy for each one of the samples, respectively.}, author = {Stark, Stefan G and Ficek, Joanna and Locatello, Francesco and Bonilla, Ximena and Chevrier, Stéphane and Singer, Franziska and Aebersold, Rudolf and Al-Quaddoomi, Faisal S and Albinus, Jonas and Alborelli, Ilaria and Andani, Sonali and Attinger, Per-Olof and Bacac, Marina and Baumhoer, Daniel and Beck-Schimmer, Beatrice and Beerenwinkel, Niko and Beisel, Christian and Bernasconi, Lara and Bertolini, Anne and Bodenmiller, Bernd and Bonilla, Ximena and Casanova, Ruben and Chevrier, Stéphane and Chicherova, Natalia and D'Costa, Maya and Danenberg, Esther and Davidson, Natalie and gan, Monica-Andreea Dră and Dummer, Reinhard and Engler, Stefanie and Erkens, Martin and Eschbach, Katja and Esposito, Cinzia and Fedier, André and Ferreira, Pedro and Ficek, Joanna and Frei, Anja L and Frey, Bruno and Goetze, Sandra and Grob, Linda and Gut, Gabriele and Günther, Detlef and Haberecker, Martina and Haeuptle, Pirmin and Heinzelmann-Schwarz, Viola and Herter, Sylvia and Holtackers, Rene and Huesser, Tamara and Irmisch, Anja and Jacob, Francis and Jacobs, Andrea and Jaeger, Tim M and Jahn, Katharina and James, Alva R and Jermann, Philip M and Kahles, André and Kahraman, Abdullah and Koelzer, Viktor H and Kuebler, Werner and Kuipers, Jack and Kunze, Christian P and Kurzeder, Christian and Lehmann, Kjong-Van and Levesque, Mitchell and Lugert, Sebastian and Maass, Gerd and Manz, Markus and Markolin, Philipp and Mena, Julien and Menzel, Ulrike and Metzler, Julian M and Miglino, Nicola and Milani, Emanuela S and Moch, Holger and Muenst, Simone and Murri, Riccardo and Ng, Charlotte KY and Nicolet, Stefan and Nowak, Marta and Pedrioli, Patrick GA and Pelkmans, Lucas and Piscuoglio, Salvatore and Prummer, Michael and Ritter, Mathilde and Rommel, Christian and Rosano-González, María L and Rätsch, Gunnar and Santacroce, Natascha and Castillo, Jacobo Sarabia del and Schlenker, Ramona and Schwalie, Petra C and Schwan, Severin and Schär, Tobias and Senti, Gabriela and Singer, Franziska and Sivapatham, Sujana and Snijder, Berend and Sobottka, Bettina and Sreedharan, Vipin T and Stark, Stefan and Stekhoven, Daniel J and Theocharides, Alexandre PA and Thomas, Tinu M and Tolnay, Markus and Tosevski, Vinko and Toussaint, Nora C and Tuncel, Mustafa A and Tusup, Marina and Drogen, Audrey Van and Vetter, Marcus and Vlajnic, Tatjana and Weber, Sandra and Weber, Walter P and Wegmann, Rebekka and Weller, Michael and Wendt, Fabian and Wey, Norbert and Wicki, Andreas and Wollscheid, Bernd and Yu, Shuqing and Ziegler, Johanna and Zimmermann, Marc and Zoche, Martin and Zuend, Gregor and Rätsch, Gunnar and Lehmann, Kjong-Van}, issn = {1367-4811}, journal = {Bioinformatics}, keywords = {Computational Mathematics, Computational Theory and Mathematics, Computer Science Applications, Molecular Biology, Biochemistry, Statistics and Probability}, number = {Supplement_2}, pages = {i919--i927}, publisher = {Oxford University Press}, title = {{SCIM: Universal single-cell matching with unpaired feature sets}}, doi = {10.1093/bioinformatics/btaa843}, volume = {36}, year = {2020}, } @inproceedings{14186, abstract = {The goal of the unsupervised learning of disentangled representations is to separate the independent explanatory factors of variation in the data without access to supervision. In this paper, we summarize the results of Locatello et al., 2019, and focus on their implications for practitioners. We discuss the theoretical result showing that the unsupervised learning of disentangled representations is fundamentally impossible without inductive biases and the practical challenges it entails. Finally, we comment on our experimental findings, highlighting the limitations of state-of-the-art approaches and directions for future research.}, author = {Locatello, Francesco and Bauer, Stefan and Lucic, Mario and Rätsch, Gunnar and Gelly, Sylvain and Schölkopf, Bernhard and Bachem, Olivier}, booktitle = {The 34th AAAI Conference on Artificial Intelligence}, isbn = {9781577358350}, issn = {2374-3468}, location = {New York, NY, United States}, number = {9}, pages = {13681--13684}, publisher = {Association for the Advancement of Artificial Intelligence}, title = {{A commentary on the unsupervised learning of disentangled representations}}, doi = {10.1609/aaai.v34i09.7120}, volume = {34}, year = {2020}, } @inproceedings{14187, abstract = {We propose a novel Stochastic Frank-Wolfe (a.k.a. conditional gradient) algorithm for constrained smooth finite-sum minimization with a generalized linear prediction/structure. This class of problems includes empirical risk minimization with sparse, low-rank, or other structured constraints. The proposed method is simple to implement, does not require step-size tuning, and has a constant per-iteration cost that is independent of the dataset size. Furthermore, as a byproduct of the method we obtain a stochastic estimator of the Frank-Wolfe gap that can be used as a stopping criterion. Depending on the setting, the proposed method matches or improves on the best computational guarantees for Stochastic Frank-Wolfe algorithms. Benchmarks on several datasets highlight different regimes in which the proposed method exhibits a faster empirical convergence than related methods. Finally, we provide an implementation of all considered methods in an open-source package.}, author = {Négiar, Geoffrey and Dresdner, Gideon and Tsai, Alicia and Ghaoui, Laurent El and Locatello, Francesco and Freund, Robert M. and Pedregosa, Fabian}, booktitle = {Proceedings of the 37th International Conference on Machine Learning}, location = {Virtual}, pages = {7253--7262}, title = {{Stochastic Frank-Wolfe for constrained finite-sum minimization}}, volume = {119}, year = {2020}, } @inproceedings{14188, abstract = {Intelligent agents should be able to learn useful representations by observing changes in their environment. We model such observations as pairs of non-i.i.d. images sharing at least one of the underlying factors of variation. First, we theoretically show that only knowing how many factors have changed, but not which ones, is sufficient to learn disentangled representations. Second, we provide practical algorithms that learn disentangled representations from pairs of images without requiring annotation of groups, individual factors, or the number of factors that have changed. Third, we perform a large-scale empirical study and show that such pairs of observations are sufficient to reliably learn disentangled representations on several benchmark data sets. Finally, we evaluate our learned representations and find that they are simultaneously useful on a diverse suite of tasks, including generalization under covariate shifts, fairness, and abstract reasoning. Overall, our results demonstrate that weak supervision enables learning of useful disentangled representations in realistic scenarios.}, author = {Locatello, Francesco and Poole, Ben and Rätsch, Gunnar and Schölkopf, Bernhard and Bachem, Olivier and Tschannen, Michael}, booktitle = {Proceedings of the 37th International Conference on Machine Learning}, location = {Virtual}, pages = {6348–6359}, title = {{Weakly-supervised disentanglement without compromises}}, volume = {119}, year = {2020}, } @article{14195, abstract = {The idea behind the unsupervised learning of disentangled representations is that real-world data is generated by a few explanatory factors of variation which can be recovered by unsupervised learning algorithms. In this paper, we provide a sober look at recent progress in the field and challenge some common assumptions. We first theoretically show that the unsupervised learning of disentangled representations is fundamentally impossible without inductive biases on both the models and the data. Then, we train over 14000 models covering most prominent methods and evaluation metrics in a reproducible large-scale experimental study on eight data sets. We observe that while the different methods successfully enforce properties “encouraged” by the corresponding losses, well-disentangled models seemingly cannot be identified without supervision. Furthermore, different evaluation metrics do not always agree on what should be considered “disentangled” and exhibit systematic differences in the estimation. Finally, increased disentanglement does not seem to necessarily lead to a decreased sample complexity of learning for downstream tasks. Our results suggest that future work on disentanglement learning should be explicit about the role of inductive biases and (implicit) supervision, investigate concrete benefits of enforcing disentanglement of the learned representations, and consider a reproducible experimental setup covering several data sets.}, author = {Locatello, Francesco and Bauer, Stefan and Lucic, Mario and Rätsch, Gunnar and Gelly, Sylvain and Schölkopf, Bernhard and Bachem, Olivier}, journal = {Journal of Machine Learning Research}, publisher = {MIT Press}, title = {{A sober look at the unsupervised learning of disentangled representations and their evaluation}}, volume = {21}, year = {2020}, } @inproceedings{14326, abstract = {Learning object-centric representations of complex scenes is a promising step towards enabling efficient abstract reasoning from low-level perceptual features. Yet, most deep learning approaches learn distributed representations that do not capture the compositional properties of natural scenes. In this paper, we present the Slot Attention module, an architectural component that interfaces with perceptual representations such as the output of a convolutional neural network and produces a set of task-dependent abstract representations which we call slots. These slots are exchangeable and can bind to any object in the input by specializing through a competitive procedure over multiple rounds of attention. We empirically demonstrate that Slot Attention can extract object-centric representations that enable generalization to unseen compositions when trained on unsupervised object discovery and supervised property prediction tasks. }, author = {Locatello, Francesco and Weissenborn, Dirk and Unterthiner, Thomas and Mahendran, Aravindh and Heigold, Georg and Uszkoreit, Jakob and Dosovitskiy, Alexey and Kipf, Thomas}, booktitle = {34th International Conference on Neural Information Processing Systems}, isbn = {9781713829546}, issn = {1049-5258}, location = {Virtual}, pages = {11525--11538}, publisher = {Neural Information Processing Systems Foundation}, title = {{Object-centric learning with slot attention}}, volume = {33}, year = {2020}, } @misc{14592, abstract = {Cryo-electron microscopy (cryo-EM) of cellular specimens provides insights into biological processes and structures within a native context. However, a major challenge still lies in the efficient and reproducible preparation of adherent cells for subsequent cryo-EM analysis. This is due to the sensitivity of many cellular specimens to the varying seeding and culturing conditions required for EM experiments, the often limited amount of cellular material and also the fragility of EM grids and their substrate. Here, we present low-cost and reusable 3D printed grid holders, designed to improve specimen preparation when culturing challenging cellular samples directly on grids. The described grid holders increase cell culture reproducibility and throughput, and reduce the resources required for cell culturing. We show that grid holders can be integrated into various cryo-EM workflows, including micro-patterning approaches to control cell seeding on grids, and for generating samples for cryo-focused ion beam milling and cryo-electron tomography experiments. Their adaptable design allows for the generation of specialized grid holders customized to a large variety of applications.}, author = {Schur, Florian KM}, publisher = {Institute of Science and Technology Austria}, title = {{STL-files for 3D-printed grid holders described in Fäßler F, Zens B, et al.; 3D printed cell culture grid holders for improved cellular specimen preparation in cryo-electron microscopy}}, doi = {10.15479/AT:ISTA:14592}, year = {2020}, } @article{14694, abstract = {We study the unique solution m of the Dyson equation \( -m(z)^{-1} = z\1 - a + S[m(z)] \) on a von Neumann algebra A with the constraint Imm≥0. Here, z lies in the complex upper half-plane, a is a self-adjoint element of A and S is a positivity-preserving linear operator on A. We show that m is the Stieltjes transform of a compactly supported A-valued measure on R. Under suitable assumptions, we establish that this measure has a uniformly 1/3-Hölder continuous density with respect to the Lebesgue measure, which is supported on finitely many intervals, called bands. In fact, the density is analytic inside the bands with a square-root growth at the edges and internal cubic root cusps whenever the gap between two bands vanishes. The shape of these singularities is universal and no other singularity may occur. We give a precise asymptotic description of m near the singular points. These asymptotics generalize the analysis at the regular edges given in the companion paper on the Tracy-Widom universality for the edge eigenvalue statistics for correlated random matrices [the first author et al., Ann. Probab. 48, No. 2, 963--1001 (2020; Zbl 1434.60017)] and they play a key role in the proof of the Pearcey universality at the cusp for Wigner-type matrices [G. Cipolloni et al., Pure Appl. Anal. 1, No. 4, 615--707 (2019; Zbl 07142203); the second author et al., Commun. Math. Phys. 378, No. 2, 1203--1278 (2020; Zbl 07236118)]. We also extend the finite dimensional band mass formula from [the first author et al., loc. cit.] to the von Neumann algebra setting by showing that the spectral mass of the bands is topologically rigid under deformations and we conclude that these masses are quantized in some important cases.}, author = {Alt, Johannes and Erdös, László and Krüger, Torben H}, issn = {1431-0643}, journal = {Documenta Mathematica}, keywords = {General Mathematics}, pages = {1421--1539}, publisher = {EMS Press}, title = {{The Dyson equation with linear self-energy: Spectral bands, edges and cusps}}, doi = {10.4171/dm/780}, volume = {25}, year = {2020}, } @article{179, abstract = {An asymptotic formula is established for the number of rational points of bounded anticanonical height which lie on a certain Zariski dense subset of the biprojective hypersurface x1y21+⋯+x4y24=0 in ℙ3×ℙ3. This confirms the modified Manin conjecture for this variety, in which the removal of a thin set of rational points is allowed.}, author = {Browning, Timothy D and Heath Brown, Roger}, issn = {0012-7094}, journal = {Duke Mathematical Journal}, number = {16}, pages = {3099--3165}, publisher = {Duke University Press}, title = {{Density of rational points on a quadric bundle in ℙ3×ℙ3}}, doi = {10.1215/00127094-2020-0031}, volume = {169}, year = {2020}, } @article{17902, abstract = {Probing structural changes of a molecule induced by charge transfer is important for understanding the physicochemical properties of molecules and developing new electronic devices. Here, we interrogate the structural changes of a single diketopyrrolopyrrole (DPP) molecule induced by charge transport at a high bias using scanning tunneling microscope break junction (STM-BJ) techniques. Specifically, we demonstrate that application of a high bias increases the average nonresonant conductance of single Au–DPP–Au junctions. We infer from the increased conductance that resonant charge transport induces planarization of the molecular backbone. We further show that this conformational planarization is assisted by thermally activated junction reorganization. The planarization only occurs under specific electronic conditions, which we rationalize by ab initio calculations. These results emphasize the need for a comprehensive view of single-molecule junctions which includes both the electronic properties and structure of the molecules and the electrodes when designing electrically driven single-molecule motors.}, author = {Zang, Yaping and Fung, E-Dean and Fu, Tianren and Ray, Suman and Garner, Marc H. and Borges, Anders and Steigerwald, Michael L. and Patil, Satish and Solomon, Gemma and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {1}, pages = {673--679}, publisher = {American Chemical Society}, title = {{Voltage-induced single-molecule junction planarization}}, doi = {10.1021/acs.nanolett.0c04260}, volume = {21}, year = {2020}, } @article{17903, abstract = {Light emission from tunnel junctions are a potential photon source for nanophotonic applications. Surprisingly, the photons emitted can have energies exceeding the energy supplied to the electrons by the bias. Three mechanisms for generating these so-called overbias photons have been proposed, but the relationship between these mechanisms has not been clarified. In this work, we argue that multielectron processes provide the best framework for understanding overbias light emission in tunnel junctions. Experimentally, we demonstrate for the first time that the superlinear dependence of emission on conductance predicted by this theory is robust to the temperature of the tunnel junction, indicating that tunnel junctions are a promising candidate for electrically driven broadband photon sources.}, author = {Fung, E-Dean and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {12}, pages = {8912--8918}, publisher = {American Chemical Society}, title = {{Too cool for blackbody radiation: Overbias photon emission in ambient STM due to multielectron processes}}, doi = {10.1021/acs.nanolett.0c03994}, volume = {20}, year = {2020}, } @article{17904, abstract = {The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC–CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.}, author = {Doud, Evan A. and Starr, Rachel L. and Kladnik, Gregor and Voevodin, Anastasia and Montes, Enrique and Arasu, Narendra P. and Zang, Yaping and Zahl, Percy and Morgante, Alberto and Venkataraman, Latha and Vázquez, Héctor and Cvetko, Dean and Roy, Xavier}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {47}, pages = {19902--19906}, publisher = {American Chemical Society}, title = {{Cyclopropenylidenes as strong carbene anchoring groups on Au surfaces}}, doi = {10.1021/jacs.0c10743}, volume = {142}, year = {2020}, } @article{17905, abstract = {A series of four oligomers of cyclopentadithiophene-vinylenes end capped with pyridine groups was prepared and their optical and electronic properties studied. Treatment with trifluoroacetic acid (TFA) leads to the bisprotonation of the nitrogens of the pyridine, which has an important impact on the optical properties. Excess treatment with TFA provokes the oxidation of the conjugated core, generating radical cations and dications. The ease of the TFA treatment in solution was extended to protonation in the solid-state where further characterization of the neutral and TFA-treated samples was carried out in electrically active substrates in organic field-effect transistors. Finally, the new molecules were found to be excellent conductors in single-molecule junctions thanks to strong electron delocalization and resonance orbital mediated transport. These studies show the opening of a spectrum of possibilities by suitable terminal substitution of π-cores.}, author = {Guijarro, Fernando G. and Medina Rivero, Samara and Gunasekaran, Suman and Arretxea, Iratxe and Ponce Ortiz, Rocío and Caballero, Rubén and Cruz, Pilar de la and Langa, Fernando and Venkataraman, Latha and Casado, Juan}, issn = {2046-2069}, journal = {RSC Advances}, number = {68}, pages = {41264--41271}, publisher = {Royal Society of Chemistry}, title = {{Synthesis and electronic properties of pyridine end-capped cyclopentadithiophene-vinylene oligomers}}, doi = {10.1039/d0ra08220a}, volume = {10}, year = {2020}, } @article{17906, abstract = {One-dimensional sp-hybridized carbon wires, including cumulenes and polyynes, can be regarded as finite versions of carbynes. They are likely to be good candidates for molecular-scale conducting wires as they are predicted to have a high-conductance. In this study, we first characterize the single-molecule conductance of a series of cumulenes and polyynes with a backbone ranging in length from 4 to 8 carbon atoms, including [7]cumulene, the longest cumulenic carbon wire studied to date for molecular electronics. We observe different length dependence of conductance when comparing these two forms of carbon wires. Polyynes exhibit conductance decays with increasing molecular length, while cumulenes show a conductance increase with increasing molecular length. Their distinct conducting behaviors are attributed to their different bond length alternation, which is supported by theoretical calculations. This study confirms the long-standing theoretical predictions on sp-hybridized carbon wires and demonstrates that cumulenes can form highly conducting molecular wires.}, author = {Zang, Yaping and Fu, Tianren and Zou, Qi and Ng, Fay and Li, Hexing and Steigerwald, Michael L. and Nuckolls, Colin and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {11}, pages = {8415--8419}, publisher = {American Chemical Society}, title = {{Cumulene wires display increasing conductance with increasing length}}, doi = {10.1021/acs.nanolett.0c03794}, volume = {20}, year = {2020}, } @article{17907, abstract = {Carbyne is a linear allotrope of carbon that is composed of a chain of sp-hybridized carbon atoms. Through appropriate engineering of the chain termination, carbyne can harbor helical states where the π-electron delocalization twists along the axis of the chain. Herein, we present a comprehensive analysis of these helical states at the tight-binding level. We demonstrate that, in general, the molecular orbital coefficients of the helical states trace out an ellipse, in analogy to elliptically polarized light. Helical states can be realized in a model, inspired by the structure of cumulene, which considers a chain terminated by sp2-hybridized atoms oriented at a nontrivial dihedral angle. We provide a complete analytic solution for this model. Additionally, we present a variation of the model that yields perfect helical states that trace out a circle as opposed to an ellipse. Our results provide a deeper understanding of helical states and lay a foundation for more advanced levels of theory.}, author = {Gunasekaran, Suman and Venkataraman, Latha}, issn = {1089-7690}, journal = {The Journal of Chemical Physics}, number = {12}, publisher = {AIP Publishing}, title = {{Tight-binding analysis of helical states in carbyne}}, doi = {10.1063/5.0021146}, volume = {153}, year = {2020}, } @article{17908, abstract = {Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by 2 orders of magnitude as compared to a fully conjugated analogue, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects of the Fano type that arise from the hybridization of localized metal-based d-orbitals and the delocalized ligand-based π-system. By rotation of the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally.}, author = {Camarasa-Gómez, María and Hernangómez-Pérez, Daniel and Inkpen, Michael S. and Lovat, Giacomo and Fung, E-Dean and Roy, Xavier and Venkataraman, Latha and Evers, Ferdinand}, issn = {1530-6992}, journal = {Nano Letters}, number = {9}, pages = {6381--6386}, publisher = {American Chemical Society}, title = {{Mechanically tunable quantum interference in ferrocene-based single-molecule junctions}}, doi = {10.1021/acs.nanolett.0c01956}, volume = {20}, year = {2020}, }