The Experts below are selected from a list of 49518 Experts worldwide ranked by ideXlab platform
David N Hendrickson - One of the best experts on this subject based on the ideXlab platform.
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magnetic quantum tunneling insights from Simple Molecule based magnets
arXiv: Mesoscale and Nanoscale Physics, 2010Co-Authors: Stephen Hill, Euan K. Brechin, Saiti Datta, Ross Inglis, Patrick L. Feng, Enrique Del Barco, Constantinos J. Milios, Junjie Liu, John J. Henderson, David N HendricksonAbstract:This article takes a broad view of the understanding of magnetic bistability and magnetic quantum tunneling in single-Molecule magnets (SMMs), focusing on three families of relatively Simple, low-nuclearity transition metal clusters: spin S = 4 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin Molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the Molecules highlighted in this study proves to be of crucial importance.
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Magnetic quantum tunneling: Insights from Simple Molecule-based magnets
Dalton Transactions, 2010Co-Authors: Stephen Hill, Euan K. Brechin, Saiti Datta, Ross Inglis, Patrick L. Feng, Enrique Del Barco, Constantinos J. Milios, Junjie Liu, John J. Henderson, David N HendricksonAbstract:This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-Molecule magnets (SMMs), focusing on three families of relatively Simple, low-nuclearity transition metal clusters: spin S = 4 Ni(II)(4), Mn(III)(3) (S = 2 and 6) and Mn(III)(6) (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn(III)(3) units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin Molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of Simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn(6)) has barely increased in the 15 years since the first studies of Mn(12)-acetate, and why the tiny Mn(3) Molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn(3) and Ni(4)). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the Molecules highlighted in this study proves to be of crucial importance. Not only that, these Simple Molecules may be considered among the best SMMs: Mn(6) possesses the record anisotropy barrier, and Mn(3) is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the Molecule.
Johannes Kastner - One of the best experts on this subject based on the ideXlab platform.
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the role of atom tunneling in gas phase reactions in planet forming disks
Astronomy and Astrophysics, 2019Co-Authors: Jan Meisner, I Kamp, W F Thi, Johannes KastnerAbstract:Context. Chemical Gas-phase reactions of Simple Molecules have been recently revised to include atom tunneling at very low temperatures. This paper investigates the impact of the increased reaction rate constant due to tunneling effects on planet-forming disks. Aims: Our aim is to quantify the astrophysical implications of atom tunneling for Simple Molecules that are frequently used to infer disk structure information or to define the initial conditions for planet (atmosphere) formation. Methods: We quantify the tunneling effect on reaction rate constants by using H2 + OH → H2O + H as a scholarly example in comparison to previous UMIST2012 rate constants. In a chemical network with 1299 reactions, we identify all chemical reactions that could show tunneling effects. We devise a Simple formulation of reaction rate constants that overestimates tunneling and screen a standard T Tauri disk model for changes in species abundances. For those reactions found to be relevant, we find values of the most recent literature for the rate constants including tunneling and compare the resulting disk chemistry to the standard disk model(s), a T Tauri and a Herbig disk. Results: The rate constants in the UMIST2012 database in many cases already capture tunneling effects implicitly, as seen in the curvature of the Arrhenius plots of some reactions at low temperature. A rigorous screening procedure identified three neutral- neutral reactions where atom tunneling could change Simple Molecule abundances. However, by adopting recent values of the rate constants of these reactions and due to the layered structure of planet-forming disks, the effects are limited to a small region between the ion- Molecule dominated regime and the ice reservoirs where cold ( 500 K) water line fluxes, decrease by 60% at most when tunneling effects are explicitly excluded. On the other hand, disk midplane quantities relevant for planet formation such as the C-to-O ratio and also the ice-to-rock ratio are clearly affected by these gas-phase tunneling effects.
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the role of atom tunneling in gas phase reactions in planet forming disks
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Jan Meisner, I Kamp, W F Thi, Johannes KastnerAbstract:This paper investigates the impact of the increased reaction rate constant due to tunneling effects on planet-forming disks. Our aim is to quantify the astrophysical implications of atom tunneling for Simple Molecules that are frequently used to infer disk structure information or to define the initial conditions for planet (atmosphere) formation. We explain the tunneling effect on reaction rates by using a scholarly example in comparison to previous UMIST2012 rate constants. In a chemical network with 1299 reactions, we identify all reactions that could show atom tunneling. We devise a Simple formulation of reaction rate constants that overestimates tunneling and screen a standard T Tauri disk model for changes in species abundances. For those reactions found to be relevant, we find values of the most recent literature for the rate constants including tunneling and compare the resulting disk chemistry to the standard disk models. The rate constants in the UMIST2012 database in many cases already capture tunneling effects implicitly. A rigorous screening procedure identified three neutral-neutral reactions where atom tunneling could change Simple Molecule abundances. However, by adopting recent values of the rate constants of these reactions and due to the layered structure of planet-forming disks, the effects are limited to a small region where cold neutral-neutral chemistry dominates. Abundances of water close to the midplane snowline can increase by a factor of two at most compared to previous results with UMIST2012 rates. Observables from the disk surface, such as high excitation (> 500 K) water line fluxes, decrease by 60% at most when tunneling effects are explicitly excluded. On the other hand, disk midplane quantities relevant for planet formation such as the C-to-O ratio and also the ice-to-rock ratio are clearly affected by these gas-phase tunneling effects.
Lidan Xing - One of the best experts on this subject based on the ideXlab platform.
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theoretical investigations on oxidative stability of solvents and oxidative decomposition mechanism of ethylene carbonate for lithium ion battery use
Journal of Physical Chemistry B, 2009Co-Authors: Lidan Xing, Fenglong Gu, Mengqing Xu, Chaoyang Wang, Weishan Li, Jin YiAbstract:The electrochemical oxidative stability of solvent Molecules used for lithium ion battery, ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in the forms of Simple Molecule and coordination with anion PF6−, is compared by using density functional theory at the level of B3LYP/6-311++G (d, p) in gas phase. EC is found to be the most stable against oxidation in its Simple Molecule. However, due to its highest dielectric constant among all the solvent Molecules, EC coordinates with PF6− most strongly and reaches cathode most easily, resulting in its preferential oxidation on cathode. Detailed oxidative decomposition mechanism of EC is investigated using the same level. Radical cation EC•+ is generated after one electron oxidation reaction of EC and there are five possible pathways for the decomposition of EC•+ forming CO2, CO, and various radical cations. The formation of CO is more difficult than CO2 during the initial decomposition of EC•+ due to...
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theoretical investigations on oxidative stability of solvents and oxidative decomposition mechanism of ethylene carbonate for lithium ion battery use
Journal of Physical Chemistry B, 2009Co-Authors: Lidan Xing, Chaoyang Wang, Chunlin TanAbstract:The electrochemical oxidative stability of solvent Molecules used for lithium ion battery, ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in the forms of Simple Molecule and coordination with anion PF(6)(-), is compared by using density functional theory at the level of B3LYP/6-311++G (d, p) in gas phase. EC is found to be the most stable against oxidation in its Simple Molecule. However, due to its highest dielectric constant among all the solvent Molecules, EC coordinates with PF(6)(-) most strongly and reaches cathode most easily, resulting in its preferential oxidation on cathode. Detailed oxidative decomposition mechanism of EC is investigated using the same level. Radical cation EC(*+) is generated after one electron oxidation reaction of EC and there are five possible pathways for the decomposition of EC(*+) forming CO(2), CO, and various radical cations. The formation of CO is more difficult than CO(2) during the initial decomposition of EC(*+) due to the high activation energy. The radical cations are reduced and terminated by gaining one electron from anode or solvent Molecules, forming aldehyde and oligomers of alkyl carbonates including 2-methyl-1,3-dioxolane, 1,3,6-trioxocan-2-one, 1,4,6,9-tetraoxaspiro[4.4]nonane, and 1,4,6,8,11-pentaoxaspiro[4.6]undecan-7-one. The calculation in this paper gives a detailed explanation on the experimental findings that have been reported in literatures and clarifies the mechanism on the oxidative decomposition of EC.
Stephen Hill - One of the best experts on this subject based on the ideXlab platform.
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magnetic quantum tunneling insights from Simple Molecule based magnets
arXiv: Mesoscale and Nanoscale Physics, 2010Co-Authors: Stephen Hill, Euan K. Brechin, Saiti Datta, Ross Inglis, Patrick L. Feng, Enrique Del Barco, Constantinos J. Milios, Junjie Liu, John J. Henderson, David N HendricksonAbstract:This article takes a broad view of the understanding of magnetic bistability and magnetic quantum tunneling in single-Molecule magnets (SMMs), focusing on three families of relatively Simple, low-nuclearity transition metal clusters: spin S = 4 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin Molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the Molecules highlighted in this study proves to be of crucial importance.
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Magnetic quantum tunneling: Insights from Simple Molecule-based magnets
Dalton Transactions, 2010Co-Authors: Stephen Hill, Euan K. Brechin, Saiti Datta, Ross Inglis, Patrick L. Feng, Enrique Del Barco, Constantinos J. Milios, Junjie Liu, John J. Henderson, David N HendricksonAbstract:This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-Molecule magnets (SMMs), focusing on three families of relatively Simple, low-nuclearity transition metal clusters: spin S = 4 Ni(II)(4), Mn(III)(3) (S = 2 and 6) and Mn(III)(6) (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn(III)(3) units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin Molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of Simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn(6)) has barely increased in the 15 years since the first studies of Mn(12)-acetate, and why the tiny Mn(3) Molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn(3) and Ni(4)). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the Molecules highlighted in this study proves to be of crucial importance. Not only that, these Simple Molecules may be considered among the best SMMs: Mn(6) possesses the record anisotropy barrier, and Mn(3) is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the Molecule.
J. Ignacio Cirac - One of the best experts on this subject based on the ideXlab platform.
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Analogue quantum chemistry simulation.
Nature, 2019Co-Authors: Javier Argüello-luengo, Alejandro Gonzalez-tudela, Tao Shi, Peter Zoller, J. Ignacio CiracAbstract:Computing the electronic structure of Molecules with high precision is a central challenge in the field of quantum chemistry. Despite the success of approximate methods, tackling this problem exactly with conventional computers remains a formidable task. Several theoretical1,2 and experimental3–5 attempts have been made to use quantum computers to solve chemistry problems, with early proof-of-principle realizations done digitally. An appealing alternative to the digital approach is analogue quantum simulation, which does not require a scalable quantum computer and has already been successfully applied to solve condensed matter physics problems6–8. However, not all available or planned setups can be used for quantum chemistry problems, because it is not known how to engineer the required Coulomb interactions between them. Here we present an analogue approach to the simulation of quantum chemistry problems that relies on the careful combination of two technologies: ultracold atoms in optical lattices and cavity quantum electrodynamics. In the proposed simulator, fermionic atoms hopping in an optical potential play the role of electrons, additional optical potentials provide the nuclear attraction, and a single-spin excitation in a Mott insulator mediates the electronic Coulomb repulsion with the help of a cavity mode. We determine the operational conditions of the simulator and test it using a Simple Molecule. Our work opens up the possibility of efficiently computing the electronic structures of Molecules with analogue quantum simulation. An analogue quantum simulator based on ultracold atoms in optical lattices and cavity quantum electrodynamics is proposed for the solution of quantum chemistry problems and tested numerically for a Simple Molecule.
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analog quantum chemistry simulation
arXiv: Quantum Physics, 2018Co-Authors: Javier Arguelloluengo, Tao Shi, Peter Zoller, Alejandro Gonzaleztudela, J. Ignacio CiracAbstract:Computing the electronic structure of Molecules with high precision is a central challenge in the field of quantum chemistry. Despite the enormous success of approximate methods, tackling this problem exactly with conventional computers is still a formidable task. This has triggered several theoretical and experimental efforts to use quantum computers to solve chemistry problems, with first proof-of-principle realizations done in a digital manner. An appealing alternative to the digital approach is analog quantum simulation, which does not require a scalable quantum computer, and has already been successfully applied in condensed matter physics problems. However, all available or planned setups cannot be used in quantum chemistry problems, since it is not known how to engineer the required Coulomb interactions with them. Here, we present a new approach to the simulation of quantum chemistry problems in an analog way. Our method relies on the careful combination of two technologies: ultra-cold atoms in optical lattices and cavity QED. In the proposed simulator, fermionic atoms hopping in an optical potential play the role of electrons, additional optical potentials provide the nuclear attraction, and a single spin excitation over a Mott insulator mediates the electronic Coulomb repulsion with the help of a cavity mode. We also provide the operational conditions of the simulator and benchmark it with a Simple Molecule. Our work opens up the possibility of efficiently computing electronic structures of Molecules with analog quantum simulation.
