Bobby G. Sumpter

Bobby G. Sumpter
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Bobby G. Sumpter

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Physics - Materials Science (8)
Physics - Soft Condensed Matter (6)
Physics - Mesoscopic Systems and Quantum Hall Effect (5)
Physics - Statistical Mechanics (3)
Physics - Chemical Physics (3)
Physics - Computational Physics (1)
Quantum Physics (1)

Publications Authored By Bobby G. Sumpter

Atomically thin circuits have recently been explored for applications in next-generation electronics and optoelectronics and have been demonstrated with two-dimensional lateral heterojunctions. In order to form true 2D circuitry from a single material, electronic properties must be spatially tunable. Here, we report tunable transport behavior which was introduced into single layer tungsten diselenide and tungsten disulfide by focused He$^+$ irradiation. Read More

Developing devices that can reliably and accurately demonstrate the principles of superposition and entanglement is an on-going challenge for the quantum computing community. Modeling and simulation offer attractive means of testing early device designs and establishing expectations for operational performance. However, the complex integrated material systems required by quantum device designs are not captured by any single existing computational modeling method. Read More

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no 'glassy' layer, but the alpha relaxation time near the nanoparticle grows with cooling faster than the alpha relaxation time in the bulk, and is ~ 20 times longer at low temperatures. The interfacial layer thickness increases from ~ 1. Read More

Bilayer graphene (BLG) with a tunable bandgap appears interesting as an alternative to graphene for practical applications, thus its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLGs in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLGs for a given field. Read More

We present a field theory approach to study changes in local temperature due to an applied electric field (the electrocaloric effect) in electrolyte solutions. Steric effects and a field-dependent dielectric function are found to be of paramount importance for accurate estimations of the electrocaloric effect. Interestingly, electrolyte solutions are found to exhibit negative electrocaloric effects. Read More

As a new two-dimensional layered material, black phosphorus (BP) is a promising material for nanoelectronics and nano-optoelectronics. We use Raman spectroscopy and first-principles theory to report our findings related to low-frequency (LF) interlayer breathing modes (<100 cm-1) in few-layer BP for the first time. The breathing modes are assigned to Ag symmetry by the laser polarization dependence study and group theory analysis. Read More

We present a generalized theory for studying phase separation in blends of polymers containing dipoles on their backbone. The theory is used to construct co-existence curves and to study the effects of dipolar interactions on interfacial tension for a planar interface between the coexisting phases. We show that a mismatch in monomeric dipole moments, or equivalently a mismatch in the dielectric constant of the pure components, leads to destabilization of the homogeneous phase. Read More

We present a first-principles theoretical study of electric field-and strain-controlled intrinsic half-metallic properties of zigzagged aluminium nitride (AlN) nanoribbons. We show that the half-metallic property of AlN ribbons can undergo a transition into fully-metallic or semiconducting behavior with application of an electric field or uniaxial strain. An external transverse electric field induces a full charge screening that renders the material semiconducting. Read More

Understanding the effect of inhomogeneity on the charge regulation and dielectric properties, and how it depends on the conformational characteristics of the macromolecules is a long-standing problem. In order to address this problem, we have developed a field-theory to study charge regulation and local dielectric function in planar polyelectrolyte brushes. The theory is used to study a polyacid brush, which is comprised of chains end-grafted at the solid-fluid interface, in equilibrium with a bulk solution containing monovalent salt ions, solvent molecules and pH controlling acid. Read More

Brownian Dynamics simulations are carried out to understand the effect of temperature and dielectric constant of the medium on microphase separation of charged-neutral diblock copolymer systems. For different dielectric media, we focus on the effect of temperature on the morphology and dynamics of model charged diblock copolymers. In this study we examine in detail a system of partially charged block copolymer consisting of 75% neutral blocks and 25% of charged blocks with 50% degree of ionization. Read More

We have carried out a quantitative analysis of the chain packing in polymeric melts using molecular dynamics simulations. The analysis involves constructing Voronoi tessellations in the equilibrated configurations of the polymeric melts. In this work, we focus on the effects of temperature and polymer backbone rigidity on the packing. Read More

Using detailed first principles calculations, we have found a new class of stable organic molecular magnets based on zwitterionic molecules possessing donor, $\pi$ bridge, and acceptor groups. These molecules are organic molecules containing only C, H and N. The quantum mechanical nature of the magnetic properties originates from the conjugated $\pi$ bridge (involving only p electrons) where the exchange interactions between electron spin are relatively strong and local and are independent of the length of the $\pi$ bridge, enabling the easy construction of a molecular magnetic device with specified length. Read More

Magnetism in nanographenes (also know as polycyclic aromatic hydrocarbons, or PAHs) are studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. Read More

The zigzag edge of a graphene nanoribbon possesses a unique electronic state that is near the Fermi level and localized at the edge carbon atoms. We investigate the chemical reactivity of these zigzag edge sites by examining their reaction energetics with common radicals from first principles. A "partial radical" concept for the edge carbon atoms is introduced to characterize their chemical reactivity, and the validity of this concept is verified by comparing the dissociation energies of edge-radical bonds with similar bonds in molecules. Read More