David R Reichman

Columbia University, New York City, NY, United States

Are you David R Reichman?

Claim your profile

Publications (126)581.72 Total impact

  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We have determined experimentally the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS2, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the non-local nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.
    03/2014;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We investigate the connection between local structure and dynamical heterogeneity in supercooled liquids. Through the study of four different models we show that the correlation between a particle's mobility and the degree of local order in nearby regions is highly system dependent. We suggest that the strength of the local structure-dynamics correlation in these models is connected to how closely a given liquid conforms to "mean-field" behavior. Finally, we investigate the connection between local ordering and that measured by "point-to-set" correlations, revealing the possibility of a structural origin for a previously observed growing structural length scale.
    02/2014;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We study the relaxation dynamics of a binary Lennard-Jones liquid in the presence of an amorphous wall generated from equilibrium particle configurations. In qualitative agreement with the results presented in Nature Phys. {\bf 8}, 164 (2012) for a liquid of harmonic spheres, we find that our binary mixture shows a saturation of the dynamical length scale close to the mode-coupling temperature $T_c$. Furthermore we show that, due to the broken symmetry imposed by the wall, signatures of an additional change in dynamics become apparent at a temperature well above $T_c$. We provide evidence that this modification in the relaxation dynamics occurs at a recently proposed dynamical crossover temperature $T_s > T_c$, which is related to the breakdown of the Stokes-Einstein relation. We find that this dynamical crossover at $T_s$ is also observed for a system of harmonic spheres as well as a WCA liquid, showing that it may be a general feature of glass-forming systems.
    02/2014;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We analyse, using Inhomogenous Mode-Coupling Theory, the critical scaling behaviour of the dynamical susceptibility at a distance epsilon from continuous second-order glass transitions. We find that the dynamical correlation length xi behaves generically as epsilon^{-1/3} and that the upper critical dimension is equal to six. More surprisingly, we find activated dynamic scaling, where xi grows with time as [ln(t)]^2 exactly at criticality. All these results suggest a deep analogy between the glassy behaviour of attractive colloids or randomly pinned supercooled liquids and that of the Random Field Ising Model.
    01/2014;
  • [show abstract] [hide abstract]
    ABSTRACT: Atomic-level details of dopant distributions can significantly influence the material properties. Using scanning tunneling microscopy, we investigate the distribution of substitutional dopants in nitrogen-doped graphene with regard to sublattice occupancy within the honeycomb structure. Samples prepared by chemical vapor deposition (CVD) using pyridine on copper exhibit well-segregated domains of nitrogen dopants in the same sublattice, extending beyond 100 nm. On the other hand, samples prepared by post-synthesis doping of pristine graphene exhibit a random distribution between sublattices. Based on theoretical calculations, we attribute the formation of sublattice domains to the preferential attachment of nitrogen to the edge sites of graphene during the CVD growth process. The breaking of sublattice symmetry in doped graphene can have important implications in its electronic applications, such as the opening of a tunable band-gap in the material.
    Journal of the American Chemical Society 01/2014; · 10.68 Impact Factor
  • Source
    Guy Cohen, David R. Reichman, Andrew J. Millis, Emanuel Gull
    [show abstract] [hide abstract]
    ABSTRACT: We present two methods for computing two-time correlation functions or Green's functions from real time bold-line continuous time quantum Monte Carlo. One method is a formally exact generalized auxiliary lead formalism by which spectral properties may be obtained from single-time observables. The other involves the evaluation of diagrams contributing to two-time observables directly on the Keldysh contour. Additionally, we provide a detailed description of the bold-line Monte Carlo method. Our methods are general and numerically exact, and able to reliably resolve high-energy features such as band edges. We compare the spectral functions obtained from real time methods to analytically continued spectral functions obtained from imaginary time Monte Carlo, thus probing the limits of analytic continuation.
    01/2014;
  • Source
    Guy Cohen, Emanuel Gull, David R. Reichman, Andrew J. Millis
    [show abstract] [hide abstract]
    ABSTRACT: The nonequilibrium spectral properties of the Anderson impurity model with a chemical potential bias are investigated within a numerically exact real time quantum Monte Carlo formalism. The two-time correlation function is computed in a form suitable for nonequilibrium dynamical mean field calculations. Additionally, the evolution of the model's spectral properties are simulated in an alternative representation, defined by a hypothetical but experimentally realizable weakly coupled auxiliary lead. The voltage splitting of the Kondo peak is confirmed and the dynamics of its formation after a coupling or gate quench are studied. This representation is shown to contain additional information about the dot's population dynamics. Further, we show that the voltage-dependent differential conductance gives a reasonable qualitative estimate of the equilibrium spectral function, but significant qualitative differences are found including incorrect trends and spurious temperature dependent effects.
    10/2013;
  • [show abstract] [hide abstract]
    ABSTRACT: We use scanning tunneling microscopy and x-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film.
    Nano Letters 09/2013; · 13.03 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We have measured the single-molecule conductance of a family of bithiophene derivatives terminated with methyl-sulfide gold-binding linkers using a scanning tunneling microscope based break-junction technique. We find a broad distribution in the single-molecule conductance of bithiophene compared with that of a methyl-sulfide terminated biphenyl. Using a combination of experiments and calculations, we show that this increased breadth in the conductance distribution is explained by the dif-ference in 5-fold symmetry of thiophene rings as compared to the 6-fold symmetry of benzene rings. The reduced symmetry of thiophene rings results in a restriction on the torsion angle space available to these molecules when bound between two metal electrodes in a junction, causing each molecular junction to sample a different set of conformers in the conductance measurements. In contrast, the rotations of biphenyl are essentially unimpeded by junction binding, allowing each molecular junction to sample the similar conformers. This work demon-strates that the conductance of bithiophene displays a strong dependence on the conformational fluctua-tions accessible within a given junction configuration, and that the symmetry of such small molecules can significantly influence their conductance behaviors.
    Journal of the American Chemical Society 08/2013; · 10.68 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: In this work, we numerically investigate a new method for the characterization of growing length scales associated with spatially heterogeneous dynamics of glass-forming liquids. This approach, motivated by the formulation of the inhomogeneous mode-coupling theory (IMCT) [Biroli, G.; et al. Phys. Rev. Lett. 2006 97, 195701], utilizes inhomogeneous molecular dynamics simulations in which the system is perturbed by a spatially modulated external potential. We show that the response of the two-point correlation function to the external field allows one to probe dynamic correlations. We examine the critical properties shown by this function, in particular, the associated dynamic correlation length, that is found to be comparable to the one extracted from standardly employed four-point correlation functions. Our numerical results are in qualitative agreement with IMCT predictions but suggest that one has to take into account fluctuations not included in this mean-field approach to reach quantitative agreement. Advantages of our approach over the more conventional one based on four-point correlation functions are discussed.
    The Journal of Physical Chemistry B 07/2013; · 3.61 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We derive rigorous bounds for well-defined community structure in complex networks for a stochastic block model (SBM) benchmark. In particular, we analyze the effect of inter-community "noise" (inter-community edges) on any "community detection" algorithm's ability to correctly group nodes assigned to a planted partition, a problem which has been proven to be NP complete in a standard rendition. Our result does not rely on the use of any one particular algorithm nor on the analysis of the limitations of inference. Rather, we turn the problem on its head and work backwards to examine when, in the first place, well defined structure may exist in SBMs.The method that we introduce here could potentially be applied to other computational problems. The objective of community detection algorithms is to partition a given network into optimally disjoint subgraphs (or communities). Similar to k-SAT and other combinatorial optimization problems, "community detection" exhibits different phases. Networks that lie in the "unsolvable phase" lack well-defined structure and thus have no partition that is meaningful. Solvable systems splinter into two disparate phases: those in the "hard" phase and those in the "easy" phase. As befits its name, within the easy phase, a partition is easy to achieve by known algorithms. When a network lies in the hard phase, it still has an underlying structure yet finding a meaningful partition which can be checked in polynomial time requires an exhaustive computational effort that rapidly increases with the size of the graph. When taken together, (i) the rigorous results that we report here on when graphs have an underlying structure and (ii) recent results concerning the limits of rather general algorithms, suggest bounds on the hard phase.
    06/2013;
  • Seogjoo Jang, Timothy C. Berkelbach, David R. Reichman
    [show abstract] [hide abstract]
    ABSTRACT: The population transfer dynamics of model donor-bridge-acceptor systems is studied by comparing a recently developed polaron-transformed quantum master equation (PQME) with the well-known Redfield and Forster theories of quantum transport. We show that the PQME approach reduces to these two theories in their respective limits of validity and naturally interpolates between them as a function of the system-bath coupling strength. By exploring the parameter space of our model problem, we identify novel regimes of transport dynamics in bridged systems like those encountered in biological and organic energy transfer problems. Furthermore, we demonstrate that three-level systems like the ones studied herein represent ideal minimal models for the identification of quantum coherent transport as embodied in super-exchange phenomena that cannot be captured by Forster-like hopping approaches.
    New Journal of Physics 06/2013; · 4.06 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present a microscopic theory of neutral excitons and charged excitons (trions) in monolayers of transition metal dichalcogenides, including molybdenum disulfide. Our theory is based on an effective mass model of excitons and trions, parametrized by ab initio calculations and incorporating a proper treatment of screening in two dimensions. The calculated exciton binding energies are in good agreement with high-level many-body computations based on the Bethe-Salpeter equation. Furthermore, our calculations for the more complex trion species compare very favorably with recent experimental measurements, and provide atomistic insight into the microscopic features which determine the trion binding energy.
    Physical Review B. 05/2013; 88(4).
  • Kuljit S. Virk, Mark S. Hybertsen, David R. Reichman
    [show abstract] [hide abstract]
    ABSTRACT: We present a self-contained theoretical and computational framework for dynamics following photoexcitation in quantum dots near planar interfaces. A microscopic Hamiltonian parametrized by first-principles calculations is merged with a reduced density matrix formalism that allows for the prediction of time-dependent charge and energy transfer processes between the quantum dot and the electrode. While treating charge and energy transfer processes on an equal footing, the nonperturbative effects of sudden charge transitions on the Fermi sea of the electrode are included. We illustrate the formalism with calculations of an InAs quantum dot coupled to the Shockley state on an Au[111] surface and use it to concretely discuss the wide range of kinetics possible in these systems and their implications for photovoltaic systems and tunnel junction devices. We discuss the utility of this framework for the analysis of recent experiments.
    Physical review. B, Condensed matter 05/2013; 87(20).
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.
    Nature Material 05/2013; · 35.75 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline tetracene lattice satisfactorily accounts for the experimental observations. It also reveals the critical roles of the charge transfer states and the high dephasing rates in ensuring the ultrafast formation of multiexciton states. In addition, we address the origins of microscopic relaxation and dephasing rates, and adopt these rates in a quantum master equation description. We show the need to take the theoretical effort one step further in the near future by combining high-level electronic structure calculations with accurate quantum relaxation dynamics for large systems.
    Accounts of Chemical Research 04/2013; · 20.83 Impact Factor
  • Source
    Glen M Hocky, David R Reichman
    [show abstract] [hide abstract]
    ABSTRACT: In this work, we study the nature of transitions between inherent structures of a two-dimensional model supercooled liquid. We demonstrate that these transitions occur predominately along a small number of directions on the energy landscape. Moreover, we show that the number of such directions decreases as the temperature of the liquid is decreased in the mildly supercooled regime, in concert with earlier studies on an athermal jamming system. We show that this decrease happens in parallel with a change in character of the transitions as dynamics in the system become more heterogeneous and localized. We investigate the origin of these trends, which suggests interesting connections between jamming and thermal glassy phenomena.
    The Journal of Chemical Physics 03/2013; 138(12):12A537. · 3.16 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.
    The Journal of Chemical Physics 03/2013; 138(11):114102. · 3.16 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We apply our theoretical formalism for singlet exciton fission, introduced in the previous paper [T. C. Berkelbach, M. S. Hybertsen, and D. R. Reichman, J. Chem. Phys. 138, 114102 (2013)] to molecular dimers of pentacene, a widely studied material that exhibits singlet fission in the crystal phase. We address a longstanding theoretical issue, namely whether singlet fission proceeds via two sequential electron transfer steps mediated by charge-transfer states or via a direct two-electron transfer process. We find evidence for a superexchange mediated mechanism, whereby the fission process proceeds through virtual charge-transfer states which may be very high in energy. In particular, this mechanism predicts efficient singlet fission on the sub-picosecond timescale, in reasonable agreement with experiment. We investigate the role played by molecular vibrations in mediating relaxation and decoherence, finding that different physically reasonable forms for the bath relaxation function give similar results. We also examine the competing direct coupling mechanism and find it to yield fission rates slower in comparison with the superexchange mechanism for the dimer. We discuss implications for crystalline pentacene, including the limitations of the dimer model.
    The Journal of Chemical Physics 03/2013; 138(11):114103. · 3.16 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Recent progress in large-area chemical vapor deposition (CVD) synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined CVD synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 micrometers in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. The knowledge gained about grain structure enables systematic studies of the optical, mechanical, and electronic properties of grain boundaries.
    03/2013;

Publication Stats

2k Citations
581.72 Total Impact Points

Institutions

  • 2005–2013
    • Columbia University
      • • Department of Chemistry
      • • Department of Physics
      New York City, NY, United States
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, CA, United States
  • 2012
    • Stanford University
      • Department of Chemistry
      Stanford, CA, United States
  • 2011
    • University of Toronto
      • Department of Chemistry
      Toronto, Ontario, Canada
  • 2002–2011
    • Tel Aviv University
      • Department of Chemistry
      Tel Aviv, Tel Aviv, Israel
  • 2007–2010
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2009
    • Boston University
      Boston, Massachusetts, United States
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
  • 2001–2009
    • Harvard University
      • • Department of Physics
      • • Department of Chemistry and Chemical Biology
      Cambridge, MA, United States
  • 2008
    • Sapienza University of Rome
      • Department of Physics
      Roma, Latium, Italy
  • 2004
    • Massachusetts Institute of Technology
      • Department of Chemistry
      Cambridge, MA, United States