Scholarly Works, Physics
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Browsing Scholarly Works, Physics by Department "Chemistry"
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- An adaptive variational algorithm for exact molecular simulations on a quantum computerGrimsley, Harper R.; Economou, Sophia E.; Barnes, Edwin Fleming; Mayhall, Nicholas J. (Springer Nature, 2019-07-08)Quantum simulation of chemical systems is one of the most promising near-term applications of quantum computers. The variational quantum eigensolver, a leading algorithm for molecular simulations on quantum hardware, has a serious limitation in that it typically relies on a pre-selected wavefunction ansatz that results in approximate wavefunctions and energies. Here we present an arbitrarily accurate variational algorithm that, instead of fixing an ansatz upfront, grows it systematically one operator at a time in a way dictated by the molecule being simulated. This generates an ansatz with a small number of parameters, leading to shallow-depth circuits. We present numerical simulations, including for a prototypical strongly correlated molecule, which show that our algorithm performs much better than a unitary coupled cluster approach, in terms of both circuit depth and chemical accuracy. Our results highlight the potential of our adaptive algorithm for exact simulations with present-day and near-term quantum hardware.
- Efficient symmetry-preserving state preparation circuits for the variational quantum eigensolver algorithmGard, Bryan T.; Zhu, Linghua; Barron, George S.; Mayhall, Nicholas J.; Economou, Sophia E.; Barnes, Edwin Fleming (2020-01-28)The variational quantum eigensolver is one of the most promising approaches for performing chemistry simulations using noisy intermediate-scale quantum (NISQ) processors. The efficiency of this algorithm depends crucially on the ability to prepare multiqubit trial states on the quantum processor that either include, or at least closely approximate, the actual energy eigenstates of the problem being simulated while avoiding states that have little overlap with them. Symmetries play a central role in determining the best trial states. Here, we present efficient state preparation circuits that respect particle number, total spin, spin projection, and time-reversal symmetries. These circuits contain the minimal number of variational parameters needed to fully span the appropriate symmetry subspace dictated by the chemistry problem while avoiding all irrelevant sectors of Hilbert space. We show how to construct these circuits for arbitrary numbers of orbitals, electrons, and spin quantum numbers, and we provide explicit decompositions and gate counts in terms of standard gate sets in each case. We test our circuits in quantum simulations of the H2 and LiH molecules and find that they outperform standard state preparation methods in terms of both accuracy and circuit depth.
- Enhanced nonlinear optical response of an endohedral metallofullerene through metal-to-cage charge transferHeflin, James R.; Marciu, D.; Figura, C.; Wang, S.; Burbank, P.; Stevenson, Steven A.; Dom, H. C. (AIP Publishing, 1998-06)A new mechanism for increasing the third-order nonlinear optical susceptibility, X-(3), is described for endohedral metallofullerenes. A two to three orders of magnitude increase in the nonlinear response is reported for degenerate four-wave mixing experiments conducted with solutions of Er-2@C-82 (isomer III) relative to empty-cage fullerenes. A value of - 8.7x 10(-32) esu is found for the molecular susceptibility, gamma(xyyx), of Er-2@C-82 compared to previously reported values of gamma(xxxx) = 3 x 10(-34) esu and gamma(xyyx) = 4 x 10(-35) esu for C-60. The results confirm the importance of the metal-to-cage charge-transfer mechanism for enhancing the nonlinear optical response in endohedral metallofullerenes. (C) 1998 American Institute of Physics.
- Imidazole-containing triblock copolymers with a synergy of ether and imidazolium sitesJangu, Chainika; Wang, Jing-Han Helen; Wang, Dong; Fahs, Gregory B.; Heflin, James R.; Moore, Robert Bowen; Colby, Ralph H.; Long, Timothy E. (The Royal Society of Chemistry, 2015-03-06)Reversible addition-fragmentation chain transfer (RAFT) polymerization enabled the synthesis of well-defined A-BC-A triblock copolymers containing a synergy of pendant ether and imidazolium sites. The soft central BC block comprises low Tg di(ethylene glycol) methyl ether methacrylate (DEGMEMA) and 1-(4-vinylbenzyl) methyl imidazolium units. External polystyrene blocks provide mechanical reinforcement within a nanoscale morphology. Dynamic mechanical analysis (DMA) of the A-BC-A triblock copolymers exhibited a plateau region, which suggested the formation of a microphase-separated morphology. Atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) collectively probed the morphology of the A-BC-A triblock copolymers, revealing long-range order at the nanoscale dimensions. Dielectric relaxation spectroscopy (DRS) examined the ion-transport properties of ionomeric A-BC-A triblock copolymers and random copolymers with different compositions. The role of morphology was demonstrated with block copolymer nanoscale structures providing superior ionic conductivity and mechanical performance compared to random copolymers. Under a 4 V direct current (DC) applied voltage, electromechanical transducers derived from these triblock copolymer membranes with added ionic liquid showed superior actuation performance compared to a benchmark Nafion[registered sign] membrane, suggesting potential for ionic polymer device applications. This was attributed to optimum modulus, improved ionic conductivity, and microphase-separated morphology of triblock copolymers.
- Ion transport and storage of ionic liquids in ionic polymer conductor network compositesLiu, Yang; Liu, Sheng; Lin, Junhong; Wang, Dong; Jain, Vaibhav; Montazami, Reza; Heflin, James R.; Li, Jing; Madsen, Louis A.; Zhang, Q. M. (AIP Publishing, 2010-05-01)We investigate ion transport and storage of ionic liquids in ionic polymer conductor network composite electroactive devices. Specifically, we show that by combining the time domain electric and electromechanical responses, one can gain quantitative information on transport behavior of the two mobile ions in ionic liquids (i.e., cation and anion) in these electroactive devices. By employing a two carrier model, the total excess ions stored and strains generated by the cations and anions, and their transport times in the nanocomposites can be determined, which all depend critically on the morphologies of the conductor network nanocomposites. (C) 2010 American Institute of Physics. [doi:10.1063/1.3432664]
- Multi-photon patterning of photoactive o-nitrobenzyl ligands bound to gold surfacesMagill, Brenden A.; Guo, Xi; Peck, Cheryl L.; Reyes, Roberto L.; See, Erich M.; Santos, Webster L.; Robinson, Hans D. (Royal Society of Chemistry, 2019-01-01)We quantitatively investigate lithographic patterning of a thiol-anchored self-assembled monolayer (SAM) of photocleavable o-nitrobenzyl ligands on gold through a multi-photon absorption process at 1.7 eV (730 nm wavelength). The photocleaving rate increases faster than the square of the incident light intensity, indicating a process more complex than simple two-photon absorption. We tentatively ascribe this observation to two-photon absorption that triggers the formation of a long-lived intermediate aci-nitro species whose decomposition yield is partially determined either by absorption of additional photons or by a local temperature that is elevated by the incident light. At the highest light intensities, thermal processes compete with photoactivation and lead to damage of the SAM. The threshold is high enough that this destructive process can largely be avoided, even while power densities are kept sufficiently large that complete photoactivation takes place on time scales of tens of seconds to a few minutes. This means that this type of ligand can be activated at visible and near infrared wavelengths where plasmonic resonances can easily be engineered in metal nanostructures, even though their single-photon reactivity at these wavelengths is negligible. This will allow selective functionalization of plasmon hotspots, which in addition to high resolution lithographic applications would be of benefit to applications such as Surface Enhanced Raman Spectroscopy and plasmonic photocatalysis as well as directed bottom-up nanoassembly.
- Raman-scattering and optical studies of argon-etched GaAs surfacesFeng, G. F.; Zallen, Richard H.; Epp, June Miriam; Dillard, John G. (American Physical Society, 1991-04)We have studied the structual damage in low-energy argon-ion-bombarded (ion-etched) GaAs using Raman scattering and ultraviolet reflectivity. When combined with post-bombardment sequential chemical etching, the Raman results reveal a graded depth profile of the damage layer, with a nearly linear damage dropoff with depth. The total damage-layer thickness is about 600 angstrom for high-fluence bombardment with 3.89-keV Ar+ ions. The spectral effects produced by argon etching are very different from those produced by high-energy ion implantation. The longitudinal-optic Raman line seen for argon-etched GaAs is not shifted and broadened as in ion-implanted GaAs. More striking are the results of the reflectivity measurements. For argon-etched GaAs, the electronic interband peaks are both broadened and strongly red shifted relative to the crystal peaks; for ion-implanted GaAs, only the broadening occurs. Distinct nanocrystals, which account for the effects seen in ion-implanted GaAs, are evidently absent in argon-etched GaAs. Instead, the damage layer caused by argon etching appears to be characterized by a very high density of point defects, which previous work suggests may be arsenic vacancies.
- Upconverting nanocomposites dispersed in urea-containing acrylicsInglefield, David Lott, Jr.; Merritt, Travis R.; Magill, Brenden A.; Long, Timothy E.; Khodaparast, Giti A. (The Royal Society of Chemistry, 2015-05-08)Lanthanide-doped upconverting nanoparticles (UCNPs) have the ability to convert low energy photons into high energy photons, making this material appealing for a variety of scientific pursuits, from solar energy conversion to bioimaging. A combination of polymers and nanocomposites increases the utility of these upconverting nanoparticles allowing nanoparticles to be added to any device compatible with polymer coatings. Here, trifluoroacetate salt decomposition enables Er/Yb doped NaYF4 upconverting nanoparticle synthesis. The subsequent deposition of a silica nanoshell yields polar silica-coated upconverting nanoparticles, enabling composite formation with polar urea-containing methacrylic polymers. Hydrogen bonding between urea groups in the polymer and the silica-coated nanoparticles allowed for dispersion of the upconverting nanoparticles to form upconverting composite films. These films exhibit desirable upconversion comparable to the nanoparticles dispersed in methanol. Urea-containing polymers are promising candidates for matrices in nanocomposites formed with polar silica nanoparticles due to favorable polymer-nanoparticle interactions. This architecture is superior to urea-methacrylate homopolymers, since the central low glass transition temperature block will provide critical ductility to the film, thus rendering the film to be durable for optical applications.