David R Reichman

Columbia University, New York, New York, United States

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Publications (146)775.79 Total impact

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    ABSTRACT: We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation (PLE) spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton states are observed at energies of 2.24 eV and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV.
    Nano Letters 03/2015; DOI:10.1021/nl504868p · 13.03 Impact Factor
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    Glen M Hocky, Ludovic Berthier, David R. Reichman
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    ABSTRACT: Ultrastable glasses have risen to prominence due to their potentially useful material properties and the tantalizing possibility of a general method of preparation via vapor deposition. Despite the importance of this novel class of amorphous materials, numerical studies have been scarce because achieving ultrastability in atomistic simulations is an enormous challenge. Here we bypass this difficulty and establish that randomly pinning the position of a small fraction of particles inside an equilibrated supercooled liquid generates ultrastable configurations at essentially no numerical cost, while avoiding undesired structural changes due to the preparation protocol. Building on the analogy with vapor-deposited ultrastable glasses, we study the melting kinetics of these configurations following a sudden temperature jump into the liquid phase. In homogeneous geometries, we find that enhanced kinetic stability is accompanied by large scale dynamic heterogeneity, while a competition between homogeneous and heterogeneous melting is observed when a liquid boundary invades the glass at constant velocity. Our work demonstrates the feasibility of large-scale, atomistically resolved, and experimentally relevant simulations of the kinetics of ultrastable glasses.
    The Journal of Chemical Physics 09/2014; 141(22). DOI:10.1063/1.4903200 · 3.12 Impact Factor
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    ABSTRACT: We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.
    The Journal of Chemical Physics 08/2014; 141(7):074705. DOI:10.1063/1.4892793 · 3.12 Impact Factor
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    ABSTRACT: We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS_{2}, 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 nonlocal 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.
    Physical Review Letters 08/2014; 113(7):076802. · 7.73 Impact Factor
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    Yevgeny Bar Lev, Guy Cohen, David R. Reichman
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    ABSTRACT: We study the infinite temperature dynamics of a prototypical one-dimensional system expected to exhibit many-body localization. Using numerically exact methods, we establish the dynamical phase diagram of this system based on the statistics of its eigenvalues and its dynamical behavior. We show that the non-ergodic phase is re-entrant as a function of the interaction strength, illustrating that localization can be reinforced by sufficiently strong interactions even at infinite temperature. Surprisingly, within the accessible time range, the ergodic phase shows sub-diffusive behavior, suggesting that the diffusion coefficient vanishes throughout much of the phase diagram in the thermodynamic limit. Our findings strongly suggest that Wigner-Dyson statistics of eigenvalue spacings may appear in a class of ergodic but sub-diffusive systems.
    Physical Review Letters 07/2014; 114(10). DOI:10.1103/PhysRevLett.114.100601 · 7.73 Impact Factor
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    ABSTRACT: We have measured the single-molecule conductance of a family of oligothiophenes comprising one to six thiophene moieties terminated with methyl-sulfide linkers using the scanning tunneling microscope based break-junction technique. We find an anomalous behavior: the peak of the conductance histogram distribution does not follow a clear exponential decay with increasing number of thiophene units in the chain. The electronic properties of the materials were characterized by optical spectroscopy and electrochemistry to gain an understanding of the factors affecting the conductance of these molecules. We postulate that different conformers in the junction are a contributing factor to the anomalous trend in the observed conductance as a function of molecule length.
    Journal of the American Chemical Society 07/2014; 136(29). DOI:10.1021/ja505277z · 11.44 Impact Factor
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    ABSTRACT: Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10–100 ps) but significantly slower than pentacene (80–110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.
    Journal of the American Chemical Society 07/2014; 136(30):10654. DOI:10.1021/ja503980c · 11.44 Impact Factor
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    ABSTRACT: Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.
    Nano Letters 06/2014; DOI:10.1021/nl501077m · 13.03 Impact Factor
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    ABSTRACT: Excitons are studied experimentally and theoretically in atomically thin WS2 layers. We find a binding energy of 0.32eV as well as non-hydrogenic behavior of the exciton states due to the non-uniformity of the dielectric environment.
    CLEO: QELS_Fundamental Science; 06/2014
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    Guy Cohen, Emanuel Gull, David R Reichman, Andrew J Millis
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    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.
    Physical Review Letters 04/2014; 112(14):146802. DOI:10.1103/PhysRevLett.112.146802 · 7.73 Impact Factor
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    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.
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    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.
    Physical Review Letters 02/2014; 113(15). DOI:10.1103/PhysRevLett.113.157801 · 7.73 Impact Factor
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    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.
    Physical Review E 02/2014; 89(5-1). DOI:10.1103/PhysRevE.89.052311 · 2.33 Impact Factor
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    Yevgeny Bar Lev, David R. Reichman
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    ABSTRACT: Following the field theoretic approach of Basko et al., Ann. Phys. 321, 1126 (2006), we study in detail the real-time dynamics of a system expected to exhibit many-body localization. In particular, for time scales inaccessible to exact methods, we demonstrate that within the second-Born approximation that the temporal decay of the density-density correlation function is non-exponential and is consistent with a finite value for $t\to\infty$, as expected in a non-ergodic state. This behavior persists over a wide range of disorder and interaction strengths. We discuss the implications of our findings with respect to dynamical phase boundaries based both on exact diagonalization studies and as well as those established by the methods of Ref. 1.
    Physical Review B 02/2014; 89(22). DOI:10.1103/PhysRevB.89.220201 · 3.66 Impact Factor
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    Liesbeth M. C. Janssen, Peter Mayer, David R. Reichman
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    ABSTRACT: We consider a generalized hierarchical formulation of schematic kinetic equations in which the full basis of multipoint density correlations is taken into account. By varying the parameters that control the effective contributions of higher-order correlations, we show that infinite hierarchies can give rise to both sharp and avoided glass transitions. Moreover, small changes in the form of the coefficients result in different scaling behaviors of the structural relaxation time, providing a means to tune the fragility in glass-forming materials. This demonstrates that the infinite-order construct of generalized mode-coupling theory constitutes a powerful and unifying framework for kinetic theories of the glass transition.
    Physical Review E 01/2014; 90(5-1). DOI:10.1103/PhysRevE.90.052306 · 2.33 Impact Factor
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    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.
    Physical Review Letters 01/2014; 113(24). DOI:10.1103/PhysRevLett.113.245701 · 7.73 Impact Factor
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    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; 136(4). DOI:10.1021/ja408463g · 11.44 Impact Factor
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    Guy Cohen, David R. Reichman, Andrew J. Millis, Emanuel Gull
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    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.
    Physical Review B 01/2014; 89(11). DOI:10.1103/PhysRevB.89.115139 · 3.66 Impact Factor
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    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(10). DOI:10.1021/nl401781d · 13.03 Impact Factor
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    R. Härtle, G. Cohen, D. R. Reichman, A. J. Millis
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    ABSTRACT: The interplay between interference effects and electron-electron interactions in electron transport through an interacting double quantum dot system is investigated using a hierarchical quantum master equation approach which becomes exact if carried to infinite order and converges well if the temperature is not too low. Decoherence due to electron-electron interactions is found to give rise to pronounced negative differential resistance, enhanced broadening of structures in current-voltage characteristics and an inversion of the electronic population. Dependence on gate voltage is shown to be a useful method of distinguishing decoherence-induced phenomena from effects induced by other mechanisms such as the presence of a blocking state. Comparison of results obtained by the hierarchical quantum master equation approach to those obtained from the Born-Markov approximation to the Nakajima-Zwanzig equation and from the non-crossing approximation to the nonequilibrium Green's function reveals the importance of an inter-dot coupling that originates from the energy dependence of the conduction bands in the leads and the need for a systematic perturbative expansion.
    Physical Review B 09/2013; 88(23). DOI:10.1103/PhysRevB.88.235426 · 3.66 Impact Factor

Publication Stats

5k Citations
775.79 Total Impact Points

Institutions

  • 2005–2014
    • Columbia University
      • Department of Chemistry
      New York, New York, United States
  • 2009
    • Boston University
      Boston, Massachusetts, United States
  • 2001–2007
    • Harvard University
      • Department of Chemistry and Chemical Biology
      Cambridge, MA, United States
  • 2000
    • University of Utah
      Salt Lake City, Utah, United States
  • 1996–1997
    • Massachusetts Institute of Technology
      • • Department of Materials Science and Engineering
      • • Department of Chemistry
      Cambridge, MA, United States