Showing 1–50 of 53 results for author: Chen, E

On the electronic and vibrational dimensionality of nanometer-scale silicon structures

Authors: Massimo V. Fischetti, Dallin O. Nielsen, Edward Chen

Abstract: We discuss the problem of assessing the electronic and vibrational dimensionality of a semiconductor nanostructure: How thin and/or wide must a nanostructure be in order to induce electron and phonon confinement? Clarifying the physical justification for common criteria found in the literature, we view the electron coherence length (defined as the electron and phonon inelastic mean free path) as t… ▽ More We discuss the problem of assessing the electronic and vibrational dimensionality of a semiconductor nanostructure: How thin and/or wide must a nanostructure be in order to induce electron and phonon confinement? Clarifying the physical justification for common criteria found in the literature, we view the electron coherence length (defined as the electron and phonon inelastic mean free path) as their `field of view' and argue (or, better yet, `speculate') that this sets the important length scale. Considering the example of Si nanosheets at room temperature, and drawing from results found in the literature, we estimate that the critical length below which electrons are subject to quantum confinement is of the order of (or smaller than) 8 nm, when their coherence length is determined by energy losses to phonons and remote phonons in gated structures. On the contrary, no single length-scale can be given for phonons: Taking their coherence length as determined by scattering with electrons and anharmonic three-phonon processes, short wavelength acoustic and optical phonons may be confined only by structures as small as 10 nm. Long-wavelength acoustic phonons, instead, may exhibit a coherence length of the order of 1 micrometer, so that they may be confined over much larger distances. △ Less

Submitted 18 August, 2026; originally announced August 2026.

Comments: 10 pages, 2 figures

Electron transport in a 1.6~nm-thick double-gated (100) silicon nanosheet: A theoretical study accounting for phonon confinement and remote-phonon scattering

Authors: Shoaib Mansoori, Bimin Cai, Edward Chen, Dallin O. Nielsen, Massimo V. Fischetti

Abstract: We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of phonons and the presence of interface hybrid plasmon-phonon excitations (IPPs or `remote phonons'). The band structure is calculated using local empirical pseudopotentials; an appr… ▽ More We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of phonons and the presence of interface hybrid plasmon-phonon excitations (IPPs or `remote phonons'). The band structure is calculated using local empirical pseudopotentials; an approximated elastic continuum model is used to consider the confinement of acoustic phonons; the dielectric continuum limit is used to deal with the IPPs. We find that the electron mobility is affected significantly by the boundary conditions chosen to deal with phonon confinement. The more realistic assumption of phonons clamped at the SiO2/HfO2 interfaces and optical phonons at the Si/SiO2 interfaces results in a room temperature mobility much smaller than what is obtained using the common assumption of bulk phonons in the elastic, high-temperature approximation. We also find that, as a result of the complicated structure of the primed subbands, the high-field saturated velocity is significantly lower than its bulk value, as it had been measured in the past in the case of Si inversion layers but never explained theoretically. Finally, we find that IPP scattering does depress the low-field mobility but to a small extent, thanks to the presence of the interfacial SiO2 layers and to the proximity of the metal gates. Moreover, by keeping electrons `cooler', IPP scattering results in a higher saturated velocity. Therefore, the presence of high-kappa materials in the gate-insulator stacks should not affect negatively the performance of field effect transistors based on Si nanosheets. △ Less

Submitted 15 August, 2026; v1 submitted 12 August, 2026; originally announced August 2026.

Comments: 28 pagews, 14 figures

On the static dielectric constant of thin dielectrics in extremely scaled silicon nanosheet transistors

Authors: Massimo V. Fischetti, Dallin O. Nielsen, Edward Chen

Abstract: We argue that the static dielectric constant of small (thin and/or narrow) semiconductor and insulator nanostructures depends strongly on the their environment. We do so by considering the electronic response simply reviewing, briefly but critically, the existing literature. Regarding the ionic response, in addition to reviewing the literature, we use a simple model to account for the confinement… ▽ More We argue that the static dielectric constant of small (thin and/or narrow) semiconductor and insulator nanostructures depends strongly on the their environment. We do so by considering the electronic response simply reviewing, briefly but critically, the existing literature. Regarding the ionic response, in addition to reviewing the literature, we use a simple model to account for the confinement of optical phonons in thin films and show that the reduction of their density of states has a negligible effect on the dielectric constant, in contrast to some claims found in the literature. In general, we argue that in realistic structures, such as double-gated Si nanosheets, the use of the bulk dielectric constants for both the channel and the gate insulators, is justified. △ Less

Submitted 1 August, 2026; v1 submitted 28 July, 2026; originally announced July 2026.

Spin singlets are useful

Authors: Silas Hoffman, Edward H. Chen, Matthew Brooks, Stephen Carr, Daniel Volya, Alan Tran, Tyler Keating, Thaddeus D. Ladd, Charles Tahan

Abstract: We evaluate the utility of the spin-zero manifold of an exchange-coupled array of $N$ spins for tasks in quantum computation and quantum simulation. Since pairs of electrons can be readily initialized into a product state of singlets in semiconducting quantum dot arrays, the full spin-zero manifold is available with exchange-only control, providing a Hilbert space of approximate dimension… ▽ More We evaluate the utility of the spin-zero manifold of an exchange-coupled array of $N$ spins for tasks in quantum computation and quantum simulation. Since pairs of electrons can be readily initialized into a product state of singlets in semiconducting quantum dot arrays, the full spin-zero manifold is available with exchange-only control, providing a Hilbert space of approximate dimension $2^N/(N/2)^{3/2}$, asymptotically close to the $2^N$ dimension of the full spin Hilbert space. Leveraging the spin-zero manifold enables larger computational space in a given array compared to traditional exchange-only control, in which spin arrays are organized into modular units of $n$ spins comprising $N/n$ encoded qubits, limiting to the exponentially smaller Hilbert dimension $2^{N/n}$. Here we focus on benchmarking metrics for this resource utilization by generalizing cross-entropy benchmarking, mirror benchmarking, and out-of-time-ordered correlators to this system. We show that operating in the spin-zero manifold can accelerate the realization of computational quantum advantage applications in semiconductor-based spin qubits. △ Less

Submitted 7 July, 2026; originally announced July 2026.

Comments: 8 pages, 6 figures

Resolving Finite-Size Errors in EOM-CCSD Band Gaps of Solids with Interacting-Bath Dynamical Embedding Theory

Authors: Jiachen Li, Christopher Hillenbrand, Christian Venturella, Enzhi Chen, Tianyu Zhu

Abstract: Periodic equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) has shown promise for quantitative calculations of band structures in solids. However, its steep computational scaling has limited calculations to relatively coarse $k$-point meshes, leading to sizable finite-size errors and discrepant estimates of thermodynamic-limit band gaps in recent benchmarks. In… ▽ More Periodic equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) has shown promise for quantitative calculations of band structures in solids. However, its steep computational scaling has limited calculations to relatively coarse $k$-point meshes, leading to sizable finite-size errors and discrepant estimates of thermodynamic-limit band gaps in recent benchmarks. In this work, we revisit EOM-CCSD band gaps for ten semiconductors and insulators using interacting-bath dynamical embedding theory (ibDET), a systematically improvable Green's function embedding framework that enables dense Brillouin-zone sampling at modest computational cost. By pushing the $k$-point sampling up to $10\times10\times10$, well beyond the system sizes accessible in canonical periodic EOM-CCSD calculations, we significantly reduce finite-size errors and obtain stable thermodynamic-limit extrapolations. We further compare $G_0W_0$@PBE, $G_0W_0$@HF, and EOM-CCSD on an equal footing using the same numerical settings in PySCF. We find that EOM-CCSD yields a mean absolute error of 0.32 eV relative to experimental band gaps for a test set of ten semiconductors and insulators, lower than that of $G_0W_0$@PBE. For ZnO, EOM-CCSD also accurately describes the Zn $3d$-band binding energy, despite overestimating the band gap. These results demonstrate that ibDET offers a practical route to high-accuracy many-body electronic structure calculations in periodic systems. △ Less

Submitted 17 August, 2026; v1 submitted 10 June, 2026; originally announced June 2026.

Polymer extension at stagnation points governs flow thickening of polymer solutions in ordered porous media

Authors: Emily Y. Chen, Simon J. Haward, Amy Q. Shen, Sujit S. Datta

Abstract: Polymer solutions exhibit anomalous flow thickening -- marked by an abrupt increase in the macroscopic flow resistance -- above a threshold flow rate in a porous medium, but not in bulk solution. This phenomenon has evaded a mechanistic description for over half a century. Here, we develop a model that quantitatively links pore-scale flow fields and fluid rheology to macroscopic flow thickening, a… ▽ More Polymer solutions exhibit anomalous flow thickening -- marked by an abrupt increase in the macroscopic flow resistance -- above a threshold flow rate in a porous medium, but not in bulk solution. This phenomenon has evaded a mechanistic description for over half a century. Here, we develop a model that quantitatively links pore-scale flow fields and fluid rheology to macroscopic flow thickening, and validate it in experiments in two- and three-dimensional (2D and 3D) porous media. We find that flow thickening in ordered media is governed by polymer extension at stagnation points -- in contrast to disordered media, where viscous dissipation by unsteady flow fluctuations also contributes substantially. Our results provide a foundation to predict and control such flows in energy, environmental, industrial, and microfluidic applications. △ Less

Submitted 26 May, 2026; originally announced May 2026.

Dominant scattering mechanisms in the low/high electric field transport in cryogenic 2D confinement in Silicon (110) with high-$κ$ oxides

Authors: Hsin-Wen Huang, Xi-Jun Fang, Edward Chen, Yuh-Renn Wu

Abstract: The performance of silicon nano-devices at cryogenic temperatures is critical for quantum qubit control circuits and space applications. Using multi-valley Monte Carlo simulations, we investigate electron transport in Si~(110) systems. At low electric fields, phonon absorption becomes negligible, and mobility is governed by competition between remote Coulomb scattering~(RCS) at low inversion charg… ▽ More The performance of silicon nano-devices at cryogenic temperatures is critical for quantum qubit control circuits and space applications. Using multi-valley Monte Carlo simulations, we investigate electron transport in Si~(110) systems. At low electric fields, phonon absorption becomes negligible, and mobility is governed by competition between remote Coulomb scattering~(RCS) at low inversion charge density and surface roughness scattering~(SRS) at high density, leading to a mobility peak. High-$κ$ dielectrics such as $\mathrm{HfO_2}$ introduce remote phonon scattering~(RPS), which suppresses mobility. Under high electric fields, phonon emission dominates at 4~K, limiting velocity enhancement and resulting in limited current improvement △ Less

Submitted 1 July, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Comments: 5 pages, 7 figures

Journal ref: J. Phys. D: Appl. Phys. 59 245107, 2026

Elastic instability of wormlike micelle solution flow in serpentine channels

Authors: Emily Y. Chen, Sujit S. Datta

Abstract: Wormlike micelle (WLM) solutions are abundant in energy, environmental, and industrial applications, which often rely on their flow through tortuous channels. How does the interplay between fluid rheology and channel geometry influence the flow behavior? Here, we address this question by experimentally visualizing and quantifying the flow of a semi-dilute WLM solution in millifluidic serpentine ch… ▽ More Wormlike micelle (WLM) solutions are abundant in energy, environmental, and industrial applications, which often rely on their flow through tortuous channels. How does the interplay between fluid rheology and channel geometry influence the flow behavior? Here, we address this question by experimentally visualizing and quantifying the flow of a semi-dilute WLM solution in millifluidic serpentine channels. At low flow rates, the base flow is steady and laminar, with strong asymmetry and wall slip. When the flow rate exceeds a critical threshold, the flow exhibits an elastic instability, producing spatially-heterogeneous, unsteady three-dimensional (3D) flow characterized by two notable features: (i) the formation and persistence of stagnant but strongly-fluctuating and multistable "dead zones" in channel bends, and (ii) intermittent 3D "twists" throughout the bulk flow. The geometry of these dead zones and twisting events can be rationalized by considering the minimization of local streamline curvature to reduce flow-generated elastic stresses. Altogether, our results shed new light into how the interplay between solution rheology and tortuous boundary geometry influences WLM flow behavior, with implications for predicting and controlling WLM flows in a broad range of complex environments. △ Less

Submitted 3 April, 2025; originally announced April 2025.

Quasi-aperiodic grain boundary phases of Σ5 tilt grain boundaries in refractory metals

Authors: Enze Chen, Timofey Frolov

Abstract: We report new ground-state structures and phase transitions in $Σ$5[001] tilt grain boundaries (GBs) in body-centered cubic (BCC) refractory metals Nb, Ta, Mo, and W. $Σ$5 tilt GBs have been extensively investigated over the past several decades, with their ground-state structure -- composed of kite-shaped structural units -- previously thought to be well understood. By performing a rigorous GB st… ▽ More We report new ground-state structures and phase transitions in $Σ$5[001] tilt grain boundaries (GBs) in body-centered cubic (BCC) refractory metals Nb, Ta, Mo, and W. $Σ$5 tilt GBs have been extensively investigated over the past several decades, with their ground-state structure -- composed of kite-shaped structural units -- previously thought to be well understood. By performing a rigorous GB structure search that optimizes the number of atoms in the boundary core, we predict new quasi-aperiodic "split kite" phases analogous to those previously found in GBs in face-centered cubic metals. Our results suggest that complex aperiodic phases of GBs appear to be a general phenomenon, as validated through density functional theory calculations. Moreover, the atoms in the split kite phase demonstrate distinct collective diffusion dynamics. Phase-contrast image simulations of split kites show better agreement with experimental observations, offering an alternative explanation for previous microscopy results and motivating future atomically resolved imaging of the GB structure. △ Less

Submitted 6 July, 2025; v1 submitted 26 January, 2025; originally announced January 2025.

Journal ref: Physical Review B, vol. 112, (6), 2025, pg. L060101

Stagnation points at grain contacts generate an elastic flow instability in 3D porous media

Authors: Emily Y. Chen, Christopher A. Browne, Simon J. Haward, Amy Q. Shen, Sujit S. Datta

Abstract: Many environmental, energy, and industrial processes involve the flow of polymer solutions in three-dimensional (3D) porous media where fluid is confined to navigate through complex pore space geometries. As polymers are transported through the tortuous pore space, elastic stresses accumulate, leading to the onset of unsteady flow fluctuations above a threshold flow rate. How does pore space geome… ▽ More Many environmental, energy, and industrial processes involve the flow of polymer solutions in three-dimensional (3D) porous media where fluid is confined to navigate through complex pore space geometries. As polymers are transported through the tortuous pore space, elastic stresses accumulate, leading to the onset of unsteady flow fluctuations above a threshold flow rate. How does pore space geometry influence the development and features of this elastic instability? Here, we address this question by directly imaging polymer solution flow in microfabricated 3D ordered porous media with precisely controlled geometries consisting of simple-cubic (SC) or body-centered cuboid (BC) arrays of spherical grains. In both cases, we find that the flow instability is generated at stagnation points arising at the contacts between grains rather than at the polar upstream/downstream grain surfaces, as is the case for flow around a single grain. The characteristics of the flow instability are strongly dependent on the unit cell geometry: in SC packings, the instability manifests through the formation of time-dependent, fluctuating 3D eddies, whereas in BC packings, it manifests as continual fluctuating 'wobbles' and crossing in the flow pathlines. Despite this difference, we find that characteristics of the transition from steady to unsteady flow with increasing flow rate have commonalities across geometries. Moreover, for both packing geometries, our data indicate that extensional flow-induced polymeric stresses generated by contact-associated stagnation points are the primary contributor to the macroscopic resistance to flow across the entire medium. Altogether, our work highlights the pivotal role of inter-grain contacts -- which are typically idealized as discrete points and therefore overlooked, but are inherent in most natural and engineered media -- in shaping elastic instabilities in porous media. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Typical Quantum States of the Universe are Observationally Indistinguishable

Authors: Eddy Keming Chen, Roderich Tumulka

Abstract: We establish three impossibility results regarding our knowledge of the quantum state of the universe. Suppose the universal quantum state is a typical unit vector in a high-dimensional subspace $\mathscr{H}_0$ of Hilbert space $\mathscr{H}$, such as the low-entropy subspace defined by the Past Hypothesis. We show that: (1) Any particular observation is incapable of identifying the universal state… ▽ More We establish three impossibility results regarding our knowledge of the quantum state of the universe. Suppose the universal quantum state is a typical unit vector in a high-dimensional subspace $\mathscr{H}_0$ of Hilbert space $\mathscr{H}$, such as the low-entropy subspace defined by the Past Hypothesis. We show that: (1) Any particular observation is incapable of identifying the universal state vector in $\mathscr{H}_0$ or substantially reducing the set of possibilities. In other words, the overwhelming majority of possible state vectors are observationally indistinguishable from each other. (2) For any reasonably probable measurement outcome and for most pairs of vectors in $\mathscr{H}_0$, that outcome will not appreciably favor one vector over the other. (3) Bayesian updating on any measurement result, unless it is extraordinarily improbable, has a negligible effect on the initial uniform probability distribution over the states in $\mathscr{H}_0$. These findings represent the most stringent epistemic constraints known for a quantum universe and are derived from a typicality theorem in quantum statistical mechanics. We close by considering how theoretical considerations beyond empirical evidence might inform our understanding of this fact and our knowledge of the universal quantum state. △ Less

Submitted 18 January, 2026; v1 submitted 22 October, 2024; originally announced October 2024.

Comments: Accepted version in The British Journal for the Philosophy of Science. 23 pages LaTeX, no figures; v3 minor revision

High Pressure Structural Behavior of Silicon Telluride (Si2Te3) Nanoplates

Authors: Bohan Li, Frank Cerasoli, Ethan Chen, Martin Kunz, Davide Donadio, Kristie J. Koski

Abstract: The high-pressure behavior of silicon telluride (Si2Te3), a two-dimensional (2D) layered material, was investigated using synchrotron X-ray powder diffraction in a diamond anvil cell to 11.5 GPa coupled with first-principles theory. Si2Te3 undergoes a phase transition at < 1 GPa from a trigonal to a hexagonal crystal structure. At higher pressures (> 8.5 GPa), X-ray diffraction showed the appearan… ▽ More The high-pressure behavior of silicon telluride (Si2Te3), a two-dimensional (2D) layered material, was investigated using synchrotron X-ray powder diffraction in a diamond anvil cell to 11.5 GPa coupled with first-principles theory. Si2Te3 undergoes a phase transition at < 1 GPa from a trigonal to a hexagonal crystal structure. At higher pressures (> 8.5 GPa), X-ray diffraction showed the appearance of new peaks possibly coincident with a new phase transition, though we suspect Si2Te3 retains a hexagonal structure. Density functional theory calculations of the band structure reveal metallization above 9.1 GPa consistent with previous measurements of the Raman spectra and disappearance of color and transparency at pressure. The theoretical Raman spectra reproduce the prominent features of the experiment, though a deeper analysis suggests that the orientation of Si dimers dramatically influences the vibrational response. Given the complex structure of Si2Te3, simulation of the resulting high-pressure phase is complicated by disordered vacancies and the initial orientations of Si-Si dimers in the crushed layered phase. △ Less

Submitted 8 October, 2024; originally announced October 2024.

Comments: 11 pages, 7 figures

Grand canonical generative diffusion model for crystalline phases and grain boundaries

Authors: Bo Lei, Enze Chen, Hyuna Kwon, Tim Hsu, Babak Sadigh, Vincenzo Lordi, Timofey Frolov, Fei Zhou

Abstract: The diffusion model has emerged as a powerful tool for generating atomic structures for materials science. This work calls attention to the deficiency of current particle-based diffusion models, which represent atoms as a point cloud, in generating even the simplest ordered crystalline structures. The problem is attributed to particles being trapped in local minima during the score-driven simulate… ▽ More The diffusion model has emerged as a powerful tool for generating atomic structures for materials science. This work calls attention to the deficiency of current particle-based diffusion models, which represent atoms as a point cloud, in generating even the simplest ordered crystalline structures. The problem is attributed to particles being trapped in local minima during the score-driven simulated annealing of the diffusion process, similar to the physical process of force-driven simulated annealing. We develop a solution, the grand canonical diffusion model, which adopts an alternative voxel-based representation with continuous rather than fixed number of particles. The method is applied towards generation of several common crystalline phases as well as the technologically important and challenging problem of grain boundary structures. △ Less

Submitted 28 August, 2024; originally announced August 2024.

A framework for extracting the rates of photophysical processes from biexponentially decaying photon emission data

Authors: Jill M. Cleveland, Tory A. Welsch, Eric Y. Chen, D. Bruce Chase, Matthew F. Doty, Hanz Y. Ramírez-Gómez

Abstract: There is strong interest in designing and realizing optically-active semiconductor nanostructures of greater complexity for applications in fields ranging from biomedical engineering to quantum computing. While these increasingly complex nanostructures can implement progressively sophisticated optical functions, the presence of more material constituents and interfaces also leads to increasingly c… ▽ More There is strong interest in designing and realizing optically-active semiconductor nanostructures of greater complexity for applications in fields ranging from biomedical engineering to quantum computing. While these increasingly complex nanostructures can implement progressively sophisticated optical functions, the presence of more material constituents and interfaces also leads to increasingly complex exciton dynamics. In particular, the rates of carrier trapping and detrapping in complex heterostructures are critically important for advanced optical functionality, but they can rarely be directly measured. In this work, we develop a model that includes trapping and release of carriers by optically inactive states. The model explains the widely observed biexponential decay of the photoluminescence signal from neutral excitons in low dimensional semiconductor emitters. The model also allows determination of likelihood intervals for all the transition rates involved in the emission dynamics, without the use of approximations. Furthermore, in cases for which the high temperature limit is suitable, the model leads to specific values of such rates, outperforming reduced models previously used to estimate those quantities. We demonstrate the value of this model by applying it to time resolved photoluminescence measurements of CdSeTe/CdS heterostructures. We obtain values not only for the radiative and nonradiative lifetimes, but also for the delayed photoluminescence originating in trapping and release. △ Less

Submitted 21 May, 2026; v1 submitted 22 August, 2024; originally announced August 2024.

Harnessing an elastic flow instability to improve the kinetic performance of chromatographic columns

Authors: Fabrice Gritti, Emily Y. Chen, Sujit S. Datta

Abstract: Despite decades of research and development, the optimal efficiency of slurry-packed HPLC columns is still hindered by inherent long-range flow heterogeneity from the wall to the central bulk region of these columns. Here, we show an example of how this issue can be addressed through the straightforward addition of a semidilute amount (500~ppm) of a large, flexible, synthetic polymer (18~MDa parti… ▽ More Despite decades of research and development, the optimal efficiency of slurry-packed HPLC columns is still hindered by inherent long-range flow heterogeneity from the wall to the central bulk region of these columns. Here, we show an example of how this issue can be addressed through the straightforward addition of a semidilute amount (500~ppm) of a large, flexible, synthetic polymer (18~MDa partially hydrolyzed polyacrylamide, HPAM) to the mobile phase (1\% NaCl aqueous solution) during operation of a 4.6 mm $\times$ 300 mm column packed with 10~$μ$m BEH$^{\mathrm{TM}}$ 125~Å\ Particles. Addition of the polymer imparts elasticity to the mobile phase, causing the flow in the interparticle pore space to become unstable above a threshold flow rate. We verify the development of this elastic flow instability using pressure drop measurements of the friction factor versus Reynolds number. In prior work, we showed that this flow instability is characterized by large spatiotemporal fluctuations in the pore-scale flow velocities that may promote analyte dispersion across the column. Axial dispersion measurements of the quasi non-retained tracer thiourea confirm this possibility: they unequivocally reveal that operating above the onset of the instability improves column efficiency by significantly reducing peak asymmetry. These experiments thereby provide a proof-of-concept demonstration that elastic flow instabilities can be harnessed to mitigate the negative impact of trans-column flow heterogeneities on the efficiency of slurry-packed HPLC columns. While this approach has its own inherent limitations and constraints, our work lays the groundwork for future targeted development of polymers that can impart elasticity when dissolved in commonly used liquid chromatography mobile phases, and can thereby generate elastic flow instabilities to help improve the resolution of HPLC columns. △ Less

Submitted 15 July, 2024; originally announced July 2024.

Influence of fluid rheology on multistability in the unstable flow of polymer solutions through pore constriction arrays

Authors: Emily Y. Chen, Sujit S. Datta

Abstract: Diverse chemical, energy, environmental, and industrial processes involve the flow of polymer solutions in porous media. The accumulation and dissipation of elastic stresses as the polymers are transported through the tortuous, confined pore space can lead to the development of an elastic flow instability above a threshold flow rate. This flow instability can generate complex flows with strong spa… ▽ More Diverse chemical, energy, environmental, and industrial processes involve the flow of polymer solutions in porous media. The accumulation and dissipation of elastic stresses as the polymers are transported through the tortuous, confined pore space can lead to the development of an elastic flow instability above a threshold flow rate. This flow instability can generate complex flows with strong spatiotemporal fluctuations, despite the low Reynolds number ($\mathrm{Re} \ll 1$); for example, in 1D ordered arrays of pore constrictions, this unstable flow can be multistable, with distinct pores exhibiting distinct unstable flow states. Here, we examine how this multistability is influenced by fluid rheology. Through experiments using diverse polymer solutions having systematic variations in fluid shear-thinning or elasticity, in pore constriction arrays of varying geometries, we show that the onset of multistability can be described using a single dimensionless parameter. This parameter, the streamwise Deborah number, compares the stress relaxation time of the polymer solution to the time required for the fluid to be advected between pore constrictions. Our work thus helps to deepen understanding of the influence of fluid rheology on elastic instabilities, helping to establish guidelines for the rational design of polymeric fluids with desirable flow behaviors. △ Less

Submitted 30 June, 2024; originally announced July 2024.

Topological grain boundary segregation transitions

Authors: Vivek Devulapalli, Enze Chen, Tobias Brink, Timofey Frolov, Christian H. Liebscher

Abstract: Engineering structure of grain boundaries (GBs) by solute segregation is a promising strategy to tailor the properties of polycrystalline materials. Theoretically it has been suggested that solute segregation can trigger phase transitions at GBs offering novel pathways to design interfaces. However, an understanding of their intrinsic atomistic nature is missing. Here, we combine atomic resolution… ▽ More Engineering structure of grain boundaries (GBs) by solute segregation is a promising strategy to tailor the properties of polycrystalline materials. Theoretically it has been suggested that solute segregation can trigger phase transitions at GBs offering novel pathways to design interfaces. However, an understanding of their intrinsic atomistic nature is missing. Here, we combine atomic resolution electron microscopy atomistic simulations to discover that iron segregation to GBs in titanium stabilizes icosahedral units (cages) that form robust building blocks of distinct GB phases. Due to their five-fold symmetry, the Fe cages cluster and assemble into hierarchical GB phases characterised by a different number and arrangement of the constituent icosahedral units. Our advanced GB structure prediction algorithms and atomistic simulations validate the stability of these observed phases and the high excess of Fe at the GB that is accommodated by the phase transitions. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 23 pages, 13 figures

Journal ref: Science vol. 386, 2024, pg. 420-424

Density Matrix Realism

Authors: Eddy Keming Chen

Abstract: Realism about quantum theory naturally leads to realism about the quantum state of the universe. It leaves open whether it is a pure state represented by a wave function, or an impure (mixed) one represented by a density matrix. I characterize and elaborate on Density Matrix Realism, the thesis that the universal quantum state is objective but can be impure. To clarify the thesis, I compare it wit… ▽ More Realism about quantum theory naturally leads to realism about the quantum state of the universe. It leaves open whether it is a pure state represented by a wave function, or an impure (mixed) one represented by a density matrix. I characterize and elaborate on Density Matrix Realism, the thesis that the universal quantum state is objective but can be impure. To clarify the thesis, I compare it with Wave Function Realism, explain the conditions under which they are empirically equivalent, consider two generalizations of Density Matrix Realism, and answer some frequently asked questions. I end by highlighting an implication for scientific realism. △ Less

Submitted 1 November, 2024; v1 submitted 2 May, 2024; originally announced May 2024.

Comments: 23 pages; accepted version for Michael E. Cuffaro and Stephan Hartmann (eds.), Open Systems: Physics, Metaphysics, and Methodology (2025: Oxford University Press)

Grand canonically optimized grain boundary phases in hexagonal close-packed titanium

Authors: Enze Chen, Tae Wook Heo, Brandon C. Wood, Mark Asta, Timofey Frolov

Abstract: Grain boundaries (GBs) profoundly influence the properties and performance of materials, emphasizing the importance of understanding the GB structure and phase behavior. As recent computational studies have demonstrated the existence of multiple GB phases associated with varying the atomic density at the interface, we introduce a validated, open-source GRand canonical Interface Predictor (GRIP) to… ▽ More Grain boundaries (GBs) profoundly influence the properties and performance of materials, emphasizing the importance of understanding the GB structure and phase behavior. As recent computational studies have demonstrated the existence of multiple GB phases associated with varying the atomic density at the interface, we introduce a validated, open-source GRand canonical Interface Predictor (GRIP) tool that automates high-throughput, grand canonical optimization of GB structures. While previous studies of GB phases have almost exclusively focused on cubic systems, we demonstrate the utility of GRIP in an application to hexagonal close-packed titanium. We perform a systematic high-throughput exploration of tilt GBs in titanium and discover previously unreported structures and phase transitions. In low-angle boundaries, we demonstrate a coupling between point defect absorption and the change in the GB dislocation network topology due to GB phase transformations, which has important implications for the accommodation of radiation-induced defects. △ Less

Submitted 5 April, 2024; originally announced April 2024.

Journal ref: Nature Communications vol. 15, 2024, pg. 7049

Lattice dynamics of quasi-2D perovskites from first-principles

Authors: Emily Y. Chen, Bartomeu Monserrat

Abstract: We present the vibrational properties and phonon dispersion for quasi-2D hybrid organic-inorganic perovskites (BA)$_2$CsPb$_2$I$_7$, (HA)$_2$CsPb$_2$I$_7$, (BA)$_2$(MA)Pb$_2$I$_7$, and (HA)$_2$(MA)Pb$_2$I$_7$ calculated from first principles. Given the highly complex nature of these compounds, we first perform careful benchmarking and convergence testing to identify suitable parameters to describe… ▽ More We present the vibrational properties and phonon dispersion for quasi-2D hybrid organic-inorganic perovskites (BA)$_2$CsPb$_2$I$_7$, (HA)$_2$CsPb$_2$I$_7$, (BA)$_2$(MA)Pb$_2$I$_7$, and (HA)$_2$(MA)Pb$_2$I$_7$ calculated from first principles. Given the highly complex nature of these compounds, we first perform careful benchmarking and convergence testing to identify suitable parameters to describe their structural features and vibrational properties. We find that the inclusion of van der Waals corrections on top of generalized gradient approximation (GGA) exchange-correlation functionals provides the best agreement for the equilibrium structure relative to experimental data. We then compute vibrational properties under the harmonic approximation. We find that stringent energy cut-offs are required to obtain well-converged phonon properties, and once converged, the harmonic approximation can capture key physics for such a large, hybrid inorganic-organic system with vastly different atom types, masses, and interatomic interactions. We discuss the obtained phonon modes and dispersion behavior in the context of known properties for bulk 3D perovskites and ligand molecular crystals. While many vibrational properties are inherited from the parent systems, we also observe unique coupled vibrations that cannot be associated with vibrations of the pure constituent perovskite and ligand subphases. Dispersive low energy phonon branches primarily occur in the in-plane direction and within the perovskite subphase, and arise from bending and breathing modes of the equatorial Pb-I network within the perovskite octahedral plane. △ Less

Submitted 19 January, 2024; originally announced January 2024.

Laws of Physics

Authors: Eddy Keming Chen

Abstract: Despite its apparent complexity, our world seems to be governed by simple laws of physics. This volume provides a philosophical introduction to such laws. I explain how they are connected to some of the central issues in philosophy, such as ontology, possibility, explanation, induction, counterfactuals, time, determinism, and fundamentality. I suggest that laws are fundamental facts that govern th… ▽ More Despite its apparent complexity, our world seems to be governed by simple laws of physics. This volume provides a philosophical introduction to such laws. I explain how they are connected to some of the central issues in philosophy, such as ontology, possibility, explanation, induction, counterfactuals, time, determinism, and fundamentality. I suggest that laws are fundamental facts that govern the world by constraining its physical possibilities. I examine three hallmarks of laws--simplicity, exactness, and objectivity--and discuss whether and how they may be associated with laws of physics. △ Less

Submitted 7 September, 2023; originally announced September 2023.

Comments: 76 pages, 10 figures, monograph manuscript prepared for Cambridge Elements in Philosophy of Physics. arXiv admin note: substantial text overlap with arXiv:2109.09226

Realizing the Nishimori transition across the error threshold for constant-depth quantum circuits

Authors: Edward H. Chen, Guo-Yi Zhu, Ruben Verresen, Alireza Seif, Elisa Bäumer, David Layden, Nathanan Tantivasadakarn, Guanyu Zhu, Sarah Sheldon, Ashvin Vishwanath, Simon Trebst, Abhinav Kandala

Abstract: Preparing quantum states across many qubits is necessary to unlock the full potential of quantum computers. However, a key challenge is to realize efficient preparation protocols which are stable to noise and gate imperfections. Here, using a measurement-based protocol on a 127 superconducting qubit device, we study the generation of the simplest long-range order -- Ising order, familiar from Gree… ▽ More Preparing quantum states across many qubits is necessary to unlock the full potential of quantum computers. However, a key challenge is to realize efficient preparation protocols which are stable to noise and gate imperfections. Here, using a measurement-based protocol on a 127 superconducting qubit device, we study the generation of the simplest long-range order -- Ising order, familiar from Greenberger-Horne-Zeilinger (GHZ) states and the repetition code -- on 54 system qubits. Our efficient implementation of the constant-depth protocol and classical decoder shows higher fidelities for GHZ states compared to size-dependent, unitary protocols. By experimentally tuning coherent and incoherent error rates, we demonstrate stability of this decoded long-range order in two spatial dimensions, up to a critical point which corresponds to a transition belonging to the unusual Nishimori universality class. Although in classical systems Nishimori physics requires fine-tuning multiple parameters, here it arises as a direct result of the Born rule for measurement probabilities -- locking the effective temperature and disorder driving this transition. Our study exemplifies how measurement-based state preparation can be meaningfully explored on quantum processors beyond a hundred qubits. △ Less

Submitted 8 December, 2023; v1 submitted 6 September, 2023; originally announced September 2023.

Comments: 16 pages, 18 figures

Journal ref: Nature Physics 21, 161 (2025)

Why accumulation mode organic electrochemical transistors turn off much faster than they turn on

Authors: Jiajie Guo, Shinya E. Chen, Rajiv Giridharagopa, Connor G. Bischak, Jonathan W. Onorato, Kangrong Yan, Ziqiu Shen, Chang-Zhi Li, Christine K. Luscombe, David S. Ginger

Abstract: Understanding the factors underpinning device switching times is crucial for the implementation of organic electrochemical transistors (OECTs) in neuromorphic computing and real-time sensing applications. Existing models of device operation cannot explain the experimental observations that turn-off times are generally much faster than turn-on times in accumulation mode OECTs. Through operando opti… ▽ More Understanding the factors underpinning device switching times is crucial for the implementation of organic electrochemical transistors (OECTs) in neuromorphic computing and real-time sensing applications. Existing models of device operation cannot explain the experimental observations that turn-off times are generally much faster than turn-on times in accumulation mode OECTs. Through operando optical microscopy, we image the local doping level of the transistor channel and show that device turn-on occurs in two stages, while turn-off occurs in one stage. We attribute the faster turn-off to a combination of engineering as well as physical and chemical factors including channel geometry, differences in doping and dedoping kinetics, and the physical phenomena of carrier density-dependent mobility. We show that ion transport is limiting the device operation speed in our model devices. Our study provides insights into the kinetics of OECTs and guidelines for engineering faster OECTs. △ Less

Submitted 1 May, 2023; originally announced May 2023.

Studies of two-dimensional material resistive random-access memory by kinetic Monte Carlo simulations

Authors: Ying-Chuan Chen, Yu-Ting Chao, Edward Chen, Chao-Hsin Wu, Yuh-Renn Wu

Abstract: Resistive memory based on 2D WS2, MoS2, and h-BN materials has been studied, including experiments and simulations. The influences with different active layer thicknesses have been discussed, including experiments and simulations. The thickness with the best On/Off ratio is also found for the 2D RRAM. This work reveals fundamental differences between a 2D RRAM and a conventional oxide RRAM. Furthe… ▽ More Resistive memory based on 2D WS2, MoS2, and h-BN materials has been studied, including experiments and simulations. The influences with different active layer thicknesses have been discussed, including experiments and simulations. The thickness with the best On/Off ratio is also found for the 2D RRAM. This work reveals fundamental differences between a 2D RRAM and a conventional oxide RRAM. Furthermore, from the physical parameters extracted with the KMC model, the 2D materials have a lower diffusion activation energy from the vertical direction, where a smaller bias voltage and a shorter switching time can be achieved. It was also found the diffusion activation energy from the CVD-grown sample is much lower than the mechanical exfoliated sample. The result shows MoS2 has the fastest switching speed among three 2D materials. △ Less

Submitted 30 September, 2023; v1 submitted 22 April, 2023; originally announced April 2023.

Journal ref: Phys. Rev. Materials 7, 094001, 2023

A data-driven framework for structure-property correlation in ordered and disordered cellular metamaterials

Authors: Shengzhi Luan, Enze Chen, Joel John, Stavros Gaitanaros

Abstract: Cellular solids and micro-lattices are a class of lightweight architected materials that have been established for their unique mechanical, thermal, and acoustic properties. It has been shown that by tuning material architecture, a combination of topology and solid(s) distribution, one can design new material systems, also known as metamaterials, with superior performance compared to conventional… ▽ More Cellular solids and micro-lattices are a class of lightweight architected materials that have been established for their unique mechanical, thermal, and acoustic properties. It has been shown that by tuning material architecture, a combination of topology and solid(s) distribution, one can design new material systems, also known as metamaterials, with superior performance compared to conventional monolithic solids. Despite the continuously growing complexity of synthesized microstructures, mainly enabled by developments in additive manufacturing, correlating their morphological characteristics to the resulting material properties has not advanced equally. This work aims to develop a systematic data-driven framework that is capable of identifying all key microstructural characteristics and evaluating their effect on a target material property. The framework relies on integrating virtual structure generation and quantification algorithms with interpretable surrogate models. The effectiveness of the proposed approach is demonstrated by analyzing the effective stiffness of a broad class of two-dimensional (2D) cellular metamaterials with varying topological disorder. The results reveal the complex manner in which well-known stiffness contributors, including nodal connectivity, cooperate with often-overlooked microstructural features such as strut orientation, to determine macroscopic material behavior. We further re-examine Maxwell's criteria regarding the rigidity of frame structures, as they pertain to the effective stiffness of cellular solids and showcase microstructures that violate them. This framework can be used for structure-property correlation in different classes of metamaterials as well as the discovery of novel architectures with tailored combinations of material properties. △ Less

Submitted 10 April, 2023; originally announced April 2023.

Journal ref: Sci. Adv. 9, eadi1453 (2023)

ANTN: Bridging Autoregressive Neural Networks and Tensor Networks for Quantum Many-Body Simulation

Authors: Zhuo Chen, Laker Newhouse, Eddie Chen, Di Luo, Marin Soljačić

Abstract: Quantum many-body physics simulation has important impacts on understanding fundamental science and has applications to quantum materials design and quantum technology. However, due to the exponentially growing size of the Hilbert space with respect to the particle number, a direct simulation is intractable. While representing quantum states with tensor networks and neural networks are the two sta… ▽ More Quantum many-body physics simulation has important impacts on understanding fundamental science and has applications to quantum materials design and quantum technology. However, due to the exponentially growing size of the Hilbert space with respect to the particle number, a direct simulation is intractable. While representing quantum states with tensor networks and neural networks are the two state-of-the-art methods for approximate simulations, each has its own limitations in terms of expressivity and inductive bias. To address these challenges, we develop a novel architecture, Autoregressive Neural TensorNet (ANTN), which bridges tensor networks and autoregressive neural networks. We show that Autoregressive Neural TensorNet parameterizes normalized wavefunctions, allows for exact sampling, generalizes the expressivity of tensor networks and autoregressive neural networks, and inherits a variety of symmetries from autoregressive neural networks. We demonstrate our approach on quantum state learning as well as finding the ground state of the challenging 2D $J_1$-$J_2$ Heisenberg model with different systems sizes and coupling parameters, outperforming both tensor networks and autoregressive neural networks. Our work opens up new opportunities for quantum many-body physics simulation, quantum technology design, and generative modeling in artificial intelligence. △ Less

Submitted 16 April, 2024; v1 submitted 4 April, 2023; originally announced April 2023.

Report number: MIT-CTP/5549

Image-Force Barrier Lowering of Schottky Barriers in Two-Dimensional Materials as a Function of Metal Contact Angle

Authors: Sarah R. Evans, Emeric Deylgat, Edward Chen, William G. Vandenberghe

Abstract: Two-dimensional (2D) semiconductors are a promising solution for the miniaturization of electronic devices and for the exploration of novel physics. However, practical applications and demonstrations of physical phenomena are hindered by high Schottky barriers at the contacts to 2D semiconductors. While the process of image-force barrier lowering (IFBL) can considerably decrease the Schottky barri… ▽ More Two-dimensional (2D) semiconductors are a promising solution for the miniaturization of electronic devices and for the exploration of novel physics. However, practical applications and demonstrations of physical phenomena are hindered by high Schottky barriers at the contacts to 2D semiconductors. While the process of image-force barrier lowering (IFBL) can considerably decrease the Schottky barrier, IFBL is not fully understood for the majority of prevalent contact geometries. We introduce a novel technique to determine the IFBL potential energy with application spanning far beyond that of any existing method. We do so by solving Poisson's equation with the boundary conditions of two metal surfaces separated by an angle Omega. We then prove that our result can also be obtained with the method of images provided a non-Euclidean, cone-manifold space is used. The resulting IFBL is used to calculate the expected contact resistance of the most prevalent geometric contacts. Finally, we investigate contact resistance and show how the stronger IFBL counteracts the effect of larger depletion width with increasing contact angle. We find that top contacts experience lower contact resistance than edge contacts. Remarkably, our results identify tunable parameters for reducing Schottky barriers and likewise contact resistance to edge-contacted 2D materials, enhancing potential applications. △ Less

Submitted 8 September, 2023; v1 submitted 12 January, 2023; originally announced January 2023.

Comments: 22 pages, 6 figures

The Wentaculus: Density Matrix Realism Meets the Arrow of Time

Authors: Eddy Keming Chen

Abstract: Two of the most difficult problems in the foundations of physics are (1) what gives rise to the arrow of time and (2) what the ontology of quantum mechanics is. They are difficult because the fundamental dynamical laws of physics do not privilege an arrow of time, and the quantum-mechanical wave function describes a high-dimensional reality that is radically different from our ordinary experiences… ▽ More Two of the most difficult problems in the foundations of physics are (1) what gives rise to the arrow of time and (2) what the ontology of quantum mechanics is. They are difficult because the fundamental dynamical laws of physics do not privilege an arrow of time, and the quantum-mechanical wave function describes a high-dimensional reality that is radically different from our ordinary experiences. In this paper, I characterize and elaborate on the ''Wentaculus'' theory, a new approach to time's arrow in a quantum universe that offers a unified solution to both problems. Central to the Wentaculus are (i) Density Matrix Realism, the idea that the quantum state of the universe is objective but can be impure, and (ii) the Initial Projection Hypothesis, a new law of nature that selects a unique initial quantum state. On the Wentaculus, the quantum state of the universe is sufficiently simple to be a law, and the arrow of time can be traced back to an exact boundary condition. It removes the intrinsic vagueness of the Past Hypothesis, eliminates the Statistical Postulate, provides a higher degree of theoretical unity, and contains a natural realization of ''strong determinism.'' I end by responding to four recent objections. In a companion paper, I elaborate on Density Matrix Realism. △ Less

Submitted 7 November, 2022; originally announced November 2022.

Comments: 14 pages. Written for the book Bassi, A., Goldstein, S., Tumulka, R., Zanghì, N. (eds.) Physics and the Nature of Reality: Essays in Memory of Detlef Dürr

Hydration of a side-chain-free n-type semiconducting ladder polymer driven by electrochemical doping

Authors: Jiajie Guo, Lucas Q. Flagg, Duyen K. Tran, Shinya E. Chen, Ruipeng Li, Nagesh B. Kolhe, Rajiv Giridharagopal, Samson A. Jenekhe, Lee J. Richter, David S. Ginger

Abstract: We study the organic electrochemical transistors (OECTs) performance of the ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Both BBLs show… ▽ More We study the organic electrochemical transistors (OECTs) performance of the ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance (eQCM) gravimetry, in-operando atomic force microscopy (AFM), and both ex-situ and in-operando grazing incidence wide-angle x-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized, and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability. △ Less

Submitted 2 November, 2022; originally announced November 2022.

Comments: 24 pages, 5 figures

X-ray Free Electron Laser Studies of Electron and Phonon Dynamics of Graphene Adsorbed on Copper

Authors: Hirohito Ogasawara, Han Wang, Jörgen Gladh, Alessandro Gallo, Ralph Page, Johannes Voss, Alan Luntz, Elias Diesen, Frank Abild-Pedersen, Anders Nilsson, Markus Soldemo, Marc Zajac, Andrew Attar, Michelle E. Chen, Sang Wan Cho, Abhishek Katoch, Ki-Jeong Kim, Kyung Hwan Kim, Minseok Kim, Soonnam Kwon, Sang Han Park, Henrique Ribeiro, Sami Sainio, Hsin-Yi Wang, Cheolhee Yang , et al. (1 additional authors not shown)

Abstract: We report optical pumping and X-ray absorption spectroscopy experiments at the PAL free electron laser that directly probe the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab-initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the opt… ▽ More We report optical pumping and X-ray absorption spectroscopy experiments at the PAL free electron laser that directly probe the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab-initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the optical laser pulse. However, once the excitation is created in graphene, its decay follows a similar path as in many previous studies of graphene adsorbed on semiconductors, i e. rapid excitation of SCOPS (Strongly Coupled Optical Phonons) and eventual thermalization. It is likely that the lifetime of the hot electron-hole pairs in copper governs the lifetime of the electronic excitation of the graphene. △ Less

Submitted 1 November, 2022; originally announced November 2022.

Comments: 22 pages, 8 Figures including Supplementary Material

Strong Determinism

Authors: Eddy Keming Chen

Abstract: A strongly deterministic theory of physics is one that permits exactly one possible history of the universe. In the words of Penrose (1989), ''it is not just a matter of the future being determined by the past; the entire history of the universe is fixed, according to some precise mathematical scheme, for all time.'' Such an extraordinary feature may appear unattainable in a world like ours. In th… ▽ More A strongly deterministic theory of physics is one that permits exactly one possible history of the universe. In the words of Penrose (1989), ''it is not just a matter of the future being determined by the past; the entire history of the universe is fixed, according to some precise mathematical scheme, for all time.'' Such an extraordinary feature may appear unattainable in a world like ours. In this paper, I show that it can be achieved in a simple way and discuss its implications for metaphysics and philosophy of science, including natural properties, free will, explanation, and modality. First, I propose a precise definition of strong determinism. Next, I discuss its philosophical ramifications and a toy example. Finally, I provide a realistic example of a strongly deterministic (and simple) physical theory -- the Everettian Wentaculus. A surprising consequence is that whether or not our world is strongly deterministic may be empirically underdetermined. △ Less

Submitted 29 April, 2024; v1 submitted 6 March, 2022; originally announced March 2022.

Comments: Accepted version, Philosophers' Imprint

Governing Without A Fundamental Direction of Time: Minimal Primitivism about Laws of Nature

Authors: Eddy Keming Chen, Sheldon Goldstein

Abstract: The Great Divide in metaphysical debates about laws of nature is between Humeans, who think that laws merely describe the distribution of matter, and non-Humeans, who think that laws govern it. The metaphysics can place demands on the proper formulations of physical theories. It is sometimes assumed that the governing view requires a fundamental / intrinsic direction of time: to govern, laws must… ▽ More The Great Divide in metaphysical debates about laws of nature is between Humeans, who think that laws merely describe the distribution of matter, and non-Humeans, who think that laws govern it. The metaphysics can place demands on the proper formulations of physical theories. It is sometimes assumed that the governing view requires a fundamental / intrinsic direction of time: to govern, laws must be dynamical, producing later states of the world from earlier ones, in accord with the fundamental direction of time in the universe. In this paper, we propose a minimal primitivism about laws of nature (MinP) according to which there is no such requirement. On our view, laws govern by constraining the physical possibilities. Our view captures the essence of the governing view without taking on extraneous commitments about the direction of time or dynamic production. Moreover, as a version of primitivism, our view requires no reduction / analysis of laws in terms of universals, powers, or dispositions. Our view accommodates several potential candidates for fundamental laws, including the principle of least action, the Past Hypothesis, the Einstein equation of general relativity, and even controversial examples found in the Wheeler-Feynman theory of electrodynamics and retrocausal theories of quantum mechanics. By understanding governing as constraining, non-Humeans who accept MinP have the same freedom to contemplate a wide variety of candidate fundamental laws as Humeans do. △ Less

Submitted 17 October, 2021; v1 submitted 19 September, 2021; originally announced September 2021.

Comments: Forthcoming in Yemima Ben-Menahem (ed.), Rethinking Laws of Nature, Springer. The main change from v1 is the addition of "A Sharper Contrast" on p.36

Journal ref: In: Ben-Menahem, Y. (eds.) 2022. Rethinking the Concept of Law of Nature. pp 21-64. Jerusalem Studies in Philosophy and History of Science. Springer, Cham

Top-Gated Carbon Nanotube FETs from Quantum Simulations: Comparison with Experiments

Authors: Alfonso Sanchez-Soares, Thomas Kelly, Giorgos Fagas, James C. Greer, Edward Chen

Abstract: We present quantum simulations of carbon nanotube field-effect transistors (CNT-FETs) based on top-gated architectures and compare to electrical characterization on devices with 15 nm channel lengths. A non-equilibrium Green's function (NEGF) quantum transport method coupled with a $\vec{k} \cdot \vec{p}$ description of the electronic structure is demonstrated to achieve excellent agreement with t… ▽ More We present quantum simulations of carbon nanotube field-effect transistors (CNT-FETs) based on top-gated architectures and compare to electrical characterization on devices with 15 nm channel lengths. A non-equilibrium Green's function (NEGF) quantum transport method coupled with a $\vec{k} \cdot \vec{p}$ description of the electronic structure is demonstrated to achieve excellent agreement with the reported experimental data. Factors influencing the electrostatic control of the channel are investigated and reveal that detailed modeling of the electrostatics and the electronic band structure of the CNT is required to achieve quantitative agreement with experiment. △ Less

Submitted 26 October, 2021; v1 submitted 16 August, 2021; originally announced August 2021.

Journal ref: 2021 International Symposium on VLSI Technology, Systems and Applications (VLSI-TSA), 2021, pp. 1-2

Twin-Boundary Structural Phase Transitions in Elemental Titanium

Authors: Mohammad S. Hooshmand, Ruopeng Zhang, Yan Chong, Enze Chen, Timofey Frolov, David L. Olmsted, Andrew M. Minor, Mark Asta

Abstract: Twinning in crystalline materials plays an important role in many transformation and deformation processes, where underlying mechanisms can be strongly influenced by the structural, energetic and kinetic properties of associated twin boundaries (TBs). While these properties are well characterized in common cases, the possibility that TBs can display multiple complexions with distinct properties, a… ▽ More Twinning in crystalline materials plays an important role in many transformation and deformation processes, where underlying mechanisms can be strongly influenced by the structural, energetic and kinetic properties of associated twin boundaries (TBs). While these properties are well characterized in common cases, the possibility that TBs can display multiple complexions with distinct properties, and phase transitions between them, has not been widely explored, even though such phenomena are established in a few more general grain boundaries. We report experimental findings that {11-24} TBs in titanium display a thick interfacial region with crystalline structure distinct from the bulk. First-principles calculations establish that this complexion is linked to a metastable polymorph of titanium, and exhibits behavior consistent with a solid-state wetting transition with compressive strain, and a first-order structural transition under tension. The findings document rich TB complexion behavior in an elemental metal, with important implications for mechanical behavior and phase-transformation pathways. △ Less

Submitted 20 July, 2021; v1 submitted 10 March, 2021; originally announced March 2021.

Comments: 62 Pages, 17 Figures

Graphene-Based Electromechanical Thermal Switches

Authors: Michelle E. Chen, Miguel Muñoz Rojo, Feifei Lian, Justin Koeln, Aditya Sood, Stephanie M. Bohaichuk, Christopher M. Neumann, Sarah G. Garrow, Andrew G. Alleyne, Kenneth E. Goodson, Eric Pop

Abstract: Thermal management is an important challenge in modern electronics, avionics, automotive, and energy storage systems. While passive thermal solutions (like heat sinks or heat spreaders) are often used, actively modulating heat flow (e.g. via thermal switches or diodes) would offer additional degrees of control over the management of thermal transients and system reliability. Here we report the fir… ▽ More Thermal management is an important challenge in modern electronics, avionics, automotive, and energy storage systems. While passive thermal solutions (like heat sinks or heat spreaders) are often used, actively modulating heat flow (e.g. via thermal switches or diodes) would offer additional degrees of control over the management of thermal transients and system reliability. Here we report the first thermal switch based on a flexible, collapsible graphene membrane, with low operating voltage, < 2 V. We also employ active-mode scanning thermal microscopy (SThM) to measure the device behavior and switching in real time. A compact analytical thermal model is developed for the general case of a thermal switch based on a double-clamped suspended membrane, highlighting the thermal and electrical design challenges. System-level modeling demonstrates the thermal trade-offs between modulating temperature swing and average temperature as a function of switching ratio. These graphene-based thermal switches present new opportunities for active control of fast (even nanosecond) thermal transients in densely integrated systems. △ Less

Submitted 21 November, 2020; originally announced November 2020.

Detuning Axis Pulsed Spectroscopy of Valley-Orbital States in Si/SiGe Quantum Dots

Authors: Edward H. Chen, Kate Raach, Andrew Pan, Andrey A. Kiselev, Edwin Acuna, Jacob Z. Blumoff, Teresa Brecht, Maxwell Choi, Wonill Ha, Daniel Hulbert, Michael P. Jura, Tyler Keating, Ramsey Noah, Bo Sun, Bryan J. Thomas, Matthew Borselli, C. A. C. Jackson, Matthew T. Rakher, Richard S. Ross

Abstract: Silicon quantum dot qubits must contend with low-lying valley excited states which are sensitive functions of the quantum well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated Si/SiGe double quantum dots using standard baseband pulsing techniq… ▽ More Silicon quantum dot qubits must contend with low-lying valley excited states which are sensitive functions of the quantum well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated Si/SiGe double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited state energies spanning multiple wafers, quantum dots, and orbital states, crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for improving valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley splitting measurements for guiding the development of Si qubits. △ Less

Submitted 26 February, 2021; v1 submitted 9 October, 2020; originally announced October 2020.

Comments: 13 pages, 12 figures. accepted for publication by Physical Review Applied

Journal ref: Phys. Rev. Applied 15, 044033 (2021)

The Past Hypothesis and the Nature of Physical Laws

Authors: Eddy Keming Chen

Abstract: If the Past Hypothesis underlies the arrows of time, what is the status of the Past Hypothesis? In this paper, I examine the role of the Past Hypothesis in the Boltzmannian account and defend the view that the Past Hypothesis is a candidate fundamental law of nature. Such a view is known to be compatible with Humeanism about laws, but as I argue it is also supported by a minimal non-Humean "govern… ▽ More If the Past Hypothesis underlies the arrows of time, what is the status of the Past Hypothesis? In this paper, I examine the role of the Past Hypothesis in the Boltzmannian account and defend the view that the Past Hypothesis is a candidate fundamental law of nature. Such a view is known to be compatible with Humeanism about laws, but as I argue it is also supported by a minimal non-Humean "governing" view. Some worries arise from the non-dynamical and time-dependent character of the Past Hypothesis as a boundary condition, the intrinsic vagueness in its specification, and the nature of the initial probability distribution. I show that these worries do not have much force, and in any case they become less relevant in a new quantum framework for analyzing time's arrows -- the Wentaculus. Hence, the view that the Past Hypothesis is a candidate fundamental law should be more widely accepted than it is now. △ Less

Submitted 17 September, 2020; v1 submitted 2 August, 2020; originally announced August 2020.

Comments: Forthcoming in Barry Loewer, Eric Winsberg, and Brad Weslake (eds.), Time's Arrows and the Probability Structure of the World, Harvard University Press

Monte Carlo analysis of phosphorene nanotransistors

Authors: Gautam Gaddemane, Maarten L. Van de Put, William G. Vandenberghe, Edward Chen, Massimo V. Fischetti

Abstract: Experimental studies on two-dimensional (2D) materials are still in the early stages, and most of the theoretical studies performed to screen these materials are limited to the room-temperature carrier-mobility in the free standing 2D layers. With the dimensions of devices moving towards nanometer-scale lengths, the room-temperature carrier-mobility -- an equilibrium concept -- may not be the main… ▽ More Experimental studies on two-dimensional (2D) materials are still in the early stages, and most of the theoretical studies performed to screen these materials are limited to the room-temperature carrier-mobility in the free standing 2D layers. With the dimensions of devices moving towards nanometer-scale lengths, the room-temperature carrier-mobility -- an equilibrium concept -- may not be the main quantity that controls the performance of devices based on these 2D materials, since electronic transport occurs under strong off--equilibrium conditions. Here we account for these non-equilibrium conditions and, for the case of monolayer phosphorene (monolayer black phosphorus), show the results of device simulations for a short channel n-MOSFET, using the Monte Carlo method coupled with the Poisson equation, including full bands and full electron-phonon matrix elements obtained from density functional theory. Our simulations reveal significant intrinsic limitations to the performance of phosphorene as a channel material in nanotransistors. △ Less

Submitted 29 July, 2020; originally announced July 2020.

Spin-Blockade Spectroscopy of Si/SiGe Quantum Dots

Authors: A. M. Jones, E. J. Pritchett, E. H. Chen, T. E. Keating, R. W. Andrews, J. Z. Blumoff, L. A. De Lorenzo, K. Eng, S. D. Ha, A. A. Kiselev, S. M. Meenehan, S. T. Merkel, J. A. Wright, L. F. Edge, R. S. Ross, M. T. Rakher, M. G. Borselli, A. Hunter

Abstract: We implement a technique for measuring the singlet-triplet energy splitting responsible for spin-to-charge conversion in semiconductor quantum dots. This method, which requires fast, single-shot charge measurement, reliably extracts an energy in the limits of both large and small splittings. We perform this technique on an undoped, accumulation-mode Si/SiGe triple-quantum dot and find that the mea… ▽ More We implement a technique for measuring the singlet-triplet energy splitting responsible for spin-to-charge conversion in semiconductor quantum dots. This method, which requires fast, single-shot charge measurement, reliably extracts an energy in the limits of both large and small splittings. We perform this technique on an undoped, accumulation-mode Si/SiGe triple-quantum dot and find that the measured splitting varies smoothly as a function of confinement gate biases. Not only does this demonstration prove the value of having an $in~situ$ excited-state measurement technique as part of a standard tune-up procedure, it also suggests that in typical Si/SiGe quantum dot devices, spin-blockade can be limited by lateral orbital excitation energy rather than valley splitting. △ Less

Submitted 21 September, 2018; originally announced September 2018.

Comments: 18 pages, 9 figures

Journal ref: Phys. Rev. Applied 12, 014026 (2019)

Theoretical studies of electronic transport in mono- and bi-layer phosphorene: A critical overview

Authors: Gautam Gaddemane, William G. Vandenberghe, Maarten L. Van de Put, Shanmeng Chen, Sabyasachi Tiwari, Edward Chen, Massimo V. Fischetti

Abstract: Recent $\textit{ab initio}$ theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26,000 to 35 cm$^{2}$ V$^{-1}$ s$^{-1}$, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critic… ▽ More Recent $\textit{ab initio}$ theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26,000 to 35 cm$^{2}$ V$^{-1}$ s$^{-1}$, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group V material, as a specific example. We argue that the use of the most accurate models results in a calculated performance that is at the disappointing lower-end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in mono- and bi-layer phosphorene. For thin multi-layer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed $\sim$ 30 cm$^{2}$ V$^{-1}$ s$^{-1}$ at 300 K for electrons along the armchair direction. △ Less

Submitted 25 January, 2018; originally announced January 2018.

Journal ref: Phys. Rev. B 98, 115416 (2018)

Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State

Authors: Eddy Keming Chen

Abstract: In a quantum universe with a strong arrow of time, we postulate a low-entropy boundary condition (the Past Hypothesis) to account for the temporal asymmetry. In this paper, I show that the Past Hypothesis also contains enough information to simplify the quantum ontology and define a natural initial condition. First, I introduce Density Matrix Realism, the thesis that the quantum state of the unive… ▽ More In a quantum universe with a strong arrow of time, we postulate a low-entropy boundary condition (the Past Hypothesis) to account for the temporal asymmetry. In this paper, I show that the Past Hypothesis also contains enough information to simplify the quantum ontology and define a natural initial condition. First, I introduce Density Matrix Realism, the thesis that the quantum state of the universe is objective and impure. This stands in sharp contrast to Wave Function Realism, the thesis that the quantum state of the universe is objective and pure. Second, I suggest that the Past Hypothesis is sufficient to determine a natural density matrix, which is simple and unique. This is achieved by what I call the Initial Projection Hypothesis: the initial density matrix of the universe is the (normalized) projection onto the Past Hypothesis subspace (in the Hilbert space). Third, because the initial quantum state is unique and simple, we have a strong case for the Nomological Thesis: the initial quantum state of the universe is on a par with laws of nature. This new package of ideas has several interesting implications, including on the harmony between statistical mechanics and quantum mechanics, theoretical unity of the universe and the subsystems, and the alleged conflict between Humean supervenience and quantum entanglement. △ Less

Submitted 5 March, 2022; v1 submitted 4 December, 2017; originally announced December 2017.

Comments: Minor changes to conform to the published version

Journal ref: The British Journal for the Philosophy of Science, Volume 72, Number 4, December 2021

Random sequential adsorption of discs on constant-curvature surfaces: plane, sphere, hyperboloid, and projective plane

Authors: Elizabeth R. Chen, Miranda Holmes-Cerfon

Abstract: We present an algorithm to simulate random sequential adsorption (random "parking") of discs on constant-curvature surfaces: the plane, sphere, hyperboloid, and projective plane, all embedded in three-dimensional space. We simulate complete parkings by explicitly calculating the boundary of the available area in which discs can park and concentrating new points in this area. This makes our algorit… ▽ More We present an algorithm to simulate random sequential adsorption (random "parking") of discs on constant-curvature surfaces: the plane, sphere, hyperboloid, and projective plane, all embedded in three-dimensional space. We simulate complete parkings by explicitly calculating the boundary of the available area in which discs can park and concentrating new points in this area. This makes our algorithm efficient and also provides a diagnostic to determine when each parking is complete, so there is no need to extrapolate data from incomplete parkings to study questions of physical interest. We use our algorithm to study the number distribution and density of discs parked in each space, where for the plane and hyperboloid we consider two different periodic tilings each. We make several notable observations: (i) On the sphere, there is a critical disc radius such the number of discs parked is always exactly four: the random parking is actually deterministic. We prove this statement rigorously, and also show that random parking on the surface of a $d$-dimensional sphere would have deterministic behaviour at the same critical radius. (ii) The average number of parked discs does not always monotonically increase as the disc radius decreases: on the plane (square with periodic boundary conditions), there is an interval of decreasing radius over which the average *decreases*. We give a heuristic explanation for this counterintuitive finding. (iii) As the disc radius shrinks to zero, the density (average fraction of area covered by parked discs) appears to converge to the same constant for all spaces, though it is always slightly larger for a sphere and slightly smaller for a hyperboloid. Therefore, for parkings on a general curved surface we would expect higher local densities in regions of positive curvature and lower local densities in regions of negative curvature. △ Less

Submitted 14 September, 2017; originally announced September 2017.

Journal ref: Journal of Nonlinear Science, May 2017

High-sensitivity, spin-based electrometry with an ensemble of nitrogen-vacancy centers in diamond

Authors: Edward H. Chen, Hannah A. Clevenson, Kerry A. Johnson, Linh M. Pham, Dirk R. Englund, Philip R. Hemmer, Danielle A. Braje

Abstract: We demonstrate a spin-based, all-dielectric electrometer based on an ensemble of nitrogen-vacancy (NV$^-$) defects in diamond. An applied electric field causes energy level shifts symmetrically away from the NV$^-$'s degenerate triplet states via the Stark effect; this symmetry provides immunity to temperature fluctuations allowing for shot-noise-limited detection. Using an ensemble of NV$^-$s, we… ▽ More We demonstrate a spin-based, all-dielectric electrometer based on an ensemble of nitrogen-vacancy (NV$^-$) defects in diamond. An applied electric field causes energy level shifts symmetrically away from the NV$^-$'s degenerate triplet states via the Stark effect; this symmetry provides immunity to temperature fluctuations allowing for shot-noise-limited detection. Using an ensemble of NV$^-$s, we demonstrate shot-noise limited sensitivities approaching 1 V/cm/$\sqrt{\text{Hz}}$ under ambient conditions, at low frequencies ($<$10 Hz), and over a large dynamic range (20 dB). A theoretical model for the ensemble of NV$^-$s fits well with measurements of the ground-state electric susceptibility parameter, $\langle k_\perp\rangle$. Implications of spin-based, dielectric sensors for micron-scale electric-field sensing are discussed. △ Less

Submitted 22 March, 2017; originally announced March 2017.

Comments: 8 pages, 7 figures

Journal ref: Phys. Rev. A 95, 053417 (2017)

Review Article: Quantum Nanophotonics in Diamond

Authors: Tim Schröder, Sara Mouradian, Jiabao Zheng, Matthew E. Trusheim, Michael Walsh, Edward H. Chen, Luozhou Li, Igal Bayn, Dirk Englund

Abstract: The past decade has seen great advances in developing color centers in diamond for sensing, quantum information processing, and tests of quantum foundations. Increasingly, the success of these applications as well as fundamental investigations of light-matter interaction depend on improved control of optical interactions with color centers -- from better fluorescence collection to efficient and pr… ▽ More The past decade has seen great advances in developing color centers in diamond for sensing, quantum information processing, and tests of quantum foundations. Increasingly, the success of these applications as well as fundamental investigations of light-matter interaction depend on improved control of optical interactions with color centers -- from better fluorescence collection to efficient and precise coupling with confined single optical modes. Wide ranging research efforts have been undertaken to address these demands through advanced nanofabrication of diamond. This review will cover recent advances in diamond nano- and microphotonic structures for efficient light collection, color center to nanocavity coupling, hybrid integration of diamond devices with other material systems, and the wide range of fabrication methods that have enabled these complex photonic diamond systems. △ Less

Submitted 16 March, 2016; originally announced March 2016.

Crossover Behavior in the Packing and Assembly of Multivalent Lock-and-Key Colloids

Authors: N. Khalid Ahmed, Greg van Anders, Elizabeth R. Chen, Sharon C. Glotzer

Abstract: Emergent behaviors occur in a vast array of systems across many scales, and are of fundamental physical importance because of the intrinsic difficulty in linking microscopic system properties to macroscopic behaviors. Here we study the emergent self-assembly behavior of model systems of recently synthesized families of concave dimpled hard spheres, or lock-and-key colloids. We find that as dimple… ▽ More Emergent behaviors occur in a vast array of systems across many scales, and are of fundamental physical importance because of the intrinsic difficulty in linking microscopic system properties to macroscopic behaviors. Here we study the emergent self-assembly behavior of model systems of recently synthesized families of concave dimpled hard spheres, or lock-and-key colloids. We find that as dimple size increases each family exhibits a crossover from a structure that does not reflect the particle symmetry to one that does and, surprisingly, the point at which this crossover occurs is approximately independent of the particle symmetry. Using a combination of numerical and analytic techniques we study systems at infinite and finite pressure, and find different common control parameters in each limit. Our results suggest there exists a set of experimentally realizable colloidal systems that exhibit complex emergent behaviors that can be traced to a common underlying microscopic control parameter. △ Less

Submitted 4 October, 2016; v1 submitted 13 January, 2015; originally announced January 2015.

Comments: V2: 18 pages, 22 figures. Revised text with clarifications, corrections, and additional supporting data

Generation of Ensembles of Individually Resolvable Nitrogen Vacancies Using Nanometer-Scale Apertures in Ultrahigh-Aspect Ratio Planar Implantation Masks

Authors: Igal Bayn, Edward H. Chen, Matthew E. Trusheim, Luozhou Li, Tim Schröder, Ophir Gaathon, Ming Lu, Aaron Stein, Mingzhao Liu, Kim Kisslinger, Hannah Clevenson, Dirk Englund

Abstract: A central challenge in developing magnetically coupled quantum registers in diamond is the fabrication of nitrogen vacancy (NV) centers with localization below ~20 nm to enable fast dipolar interaction compared to the NV decoherence rate. Here, we demonstrate the targeted, high throughput formation of NV centers using masks with a thickness of 270 nm and feature sizes down to ~1 nm. Super-resoluti… ▽ More A central challenge in developing magnetically coupled quantum registers in diamond is the fabrication of nitrogen vacancy (NV) centers with localization below ~20 nm to enable fast dipolar interaction compared to the NV decoherence rate. Here, we demonstrate the targeted, high throughput formation of NV centers using masks with a thickness of 270 nm and feature sizes down to ~1 nm. Super-resolution imaging resolves NVs with a full-width maximum distribution of $26\pm7$ nm and a distribution of NV-NV separations of $16\pm5$ nm. △ Less

Submitted 19 December, 2014; originally announced December 2014.

Complexity in surfaces of densest packings for families of polyhedra

Authors: Elizabeth R. Chen, Daphne Klotsa, Michael Engel, Pablo F. Damasceno, Sharon C. Glotzer

Abstract: Packings of hard polyhedra have been studied for centuries due to their mathematical aesthetic and more recently for their applications in fields such as nanoscience, granular and colloidal matter, and biology. In all these fields, particle shape is important for structure and properties, especially upon crowding. Here, we explore packing as a function of shape. By combining simulations and analyt… ▽ More Packings of hard polyhedra have been studied for centuries due to their mathematical aesthetic and more recently for their applications in fields such as nanoscience, granular and colloidal matter, and biology. In all these fields, particle shape is important for structure and properties, especially upon crowding. Here, we explore packing as a function of shape. By combining simulations and analytic calculations, we study three 2-parameter families of hard polyhedra and report an extensive and systematic analysis of the densest packings of more than 55,000 convex shapes. The three families have the symmetries of triangle groups (icosahedral, octahedral, tetrahedral) and interpolate between various symmetric solids (Platonic, Archimedean, Catalan). We find that optimal (maximum) packing density surfaces that reveal unexpected richness and complexity, containing as many as 130 different structures within a single family. Our results demonstrate the utility of thinking of shape not as a static property of an object in the context of packings, but rather as but one point in a higher dimensional shape space whose neighbors in that space may have identical or markedly different packings. Finally, we present and interpret our packing results in a consistent and generally applicable way by proposing a method to distinguish regions of packings and classify types of transitions between them. △ Less

Submitted 10 September, 2013; originally announced September 2013.

Comments: 16 pages, 8 figures

Journal ref: Phys. Rev. X 4 (2014) 011024

Packing and Self-assembly of Truncated Triangular Bipyramids

Authors: Amir Haji-Akbari, Elizabeth R. Chen, Michael Engel, Sharon C Glotzer

Abstract: Motivated by breakthroughs in the synthesis of faceted nano- and colloidal particles, as well as theoretical and computational studies of their packings, we investigate a family of truncated triangular bipyramids. We report dense periodic packings with small unit cells that were obtained via numerical and analytical optimization. The maximal packing fraction $φ_{\max}$ changes continuously with th… ▽ More Motivated by breakthroughs in the synthesis of faceted nano- and colloidal particles, as well as theoretical and computational studies of their packings, we investigate a family of truncated triangular bipyramids. We report dense periodic packings with small unit cells that were obtained via numerical and analytical optimization. The maximal packing fraction $φ_{\max}$ changes continuously with the truncation parameter $t$. Eight distinct packings are identified based on discontinuities in the first and second derivatives of $φ_{\max}(t)$. These packings differ in the number of particles in the fundamental domain (unit cell) and the type of contacts between the particles. In particular, we report two packings with four particles in the unit cell for which both $φ_{\max}(t)$ and $φ'_{\max}(t)$ are continuous and the discontinuity occurs in the second derivative only. In the self-assembly simulations that we perform for larger boxes with 2048 particles, only one out of eight packings is found to assemble. In addition, the degenerate quasicrystal reported previously for triangular bipyramids without truncation [{http://prl.aps.org/abstract/PRL/v107/i21/e215702}{Haji-Akbari \emph{et al.}, \emph{Phys. Rev. Lett.} \textbf{107}: 215702 (2011)}] assembles for truncations as high as 0.45. The self-assembly propensities for the structures formed in the thermodynamic limit are explained using the isoperimetric quotient of the particles and the coordination number in the disordered fluid and in the assembled structure. △ Less

Submitted 14 August, 2013; v1 submitted 10 April, 2013; originally announced April 2013.

Comments: 14 pages, 12 figures

Journal ref: Phys. Rev. E 88, 012127 (2013)

Optimal Fillings - A new spatial subdivision problem related to packing and covering

Authors: Carolyn L. Phillips, Joshua A. Anderson, Elizabeth R. Chen, Sharon C. Glotzer

Abstract: We present filling as a new type of spatial subdivision problem that is related to covering and packing. Filling addresses the optimal placement of overlapping objects lying entirely inside an arbitrary shape so as to cover the most interior volume. In n-dimensional space, if the objects are polydisperse n-balls, we show that solutions correspond to sets of maximal n-balls and the solution space c… ▽ More We present filling as a new type of spatial subdivision problem that is related to covering and packing. Filling addresses the optimal placement of overlapping objects lying entirely inside an arbitrary shape so as to cover the most interior volume. In n-dimensional space, if the objects are polydisperse n-balls, we show that solutions correspond to sets of maximal n-balls and the solution space can reduced to the medial axis of a shape. We examine the structure of the solution space in two dimensions. For the filling of polygons, we provide detailed descriptions of a heuristic and a genetic algorithm for finding solutions of maximal discs. We also consider the properties of ideal distributions of N discs in polygons as N approaches infinity. △ Less

Submitted 28 August, 2012; originally announced August 2012.

Comments: 38 pages, 21 figures

Tetrahedral colloidal clusters from random parking of bidisperse spheres

Authors: Nicholas B. Schade, Miranda C. Holmes-Cerfon, Elizabeth R. Chen, Dina Aronzon, Jesse W. Collins, Jonathan A. Fan, Federico Capasso, Vinothan N. Manoharan

Abstract: Using experiments and simulations, we investigate the clusters that form when colloidal spheres stick irreversibly to -- or "park" on -- smaller spheres. We use either oppositely charged particles or particles labeled with complementary DNA sequences, and we vary the ratio $α$ of large to small sphere radii. Once bound, the large spheres cannot rearrange, and thus the clusters do not form dense or… ▽ More Using experiments and simulations, we investigate the clusters that form when colloidal spheres stick irreversibly to -- or "park" on -- smaller spheres. We use either oppositely charged particles or particles labeled with complementary DNA sequences, and we vary the ratio $α$ of large to small sphere radii. Once bound, the large spheres cannot rearrange, and thus the clusters do not form dense or symmetric packings. Nevertheless, this stochastic aggregation process yields a remarkably narrow distribution of clusters with nearly 90% tetrahedra at $α=2.45$. The high yield of tetrahedra, which reaches 100% in simulations at $α=2.41$, arises not simply because of packing constraints, but also because of the existence of a long-time lower bound that we call the "minimum parking" number. We derive this lower bound from solutions to the classic mathematical problem of spherical covering, and we show that there is a critical size ratio $α_c=(1+\sqrt{2})\approx 2.41$, close to the observed point of maximum yield, where the lower bound equals the upper bound set by packing constraints. The emergence of a critical value in a random aggregation process offers a robust method to assemble uniform clusters for a variety of applications, including metamaterials. △ Less

Submitted 26 December, 2012; v1 submitted 18 January, 2012; originally announced January 2012.

Comments: 24 pages, 6 figures

Journal ref: Physical Review Letters, 110 (14), 148303 (2013)