Anisotropic charge transport properties of chrysene derivatives as organic semiconductor: A computational study

We used density functional theory to calculate the angular resolution anisotropic charge mobility of the substituted chrysene molecules, viz, 4,10‐diphenoxychrysene (DPC), 4,10‐bis(phenylsulfanyl)chrysene (BPSC), and ethyl 8,9,12‐trimethoxychrysene‐6‐carboxylate (ETCC). The highest occupied molecular orbital–lowest unoccupied molecular orbital gap for DPC, BPSC, and ETCC was calculated to be 3.92, 3.83, and 3.81 eV, respectively, which inferred the compounds to be wide‐band‐gap semiconductors indicating that the compounds should have high stability in atmospheric conditions. The fact is also supported by electronic band‐structure calculation. In addition, higher electron affinity of studied compounds as compared with the bare chrysene molecule imparts enhancement of n‐type character in the compounds. The maximum hole ( ) and electron mobilities ( ) for DPC compound were found to be 0.739 cm²V^?1s^?1 and 0.319 cm²V^?1s^?1, respectively, at Φ = 0°. On the other hand, in the case of BPSC crystal, comparatively larger anisotropic electron mobility (0.709 cm²V^?1s^?1 at Φ = 0° and Φ = 179.90°) than the hole mobility (0.208 cm²V^?1s^?1 at Φ = 127.19° and Φ = 307.10°) was noted. Similarly, in ETCC, the parallel dimers were found to contribute maximum and of 0.052 and 0.102 cm²V^?1s^?1, respectively, at Φ = 0°. The substitution of ‐SPh in BPSC and ‐OCH₃ and ‐CO₂CH₂CH₃ in ETCC have relatively more impact on band reduction than ‐OPh in DPC, thus facilitating electron transport in BPSC and ETCC.     

Ultra‐Fast Control of Magnetic Relaxation in a Periodically Driven Hubbard Model

Motivated by cold atom and ultra‐fast pump‐probe experiments we study the melting of long‐range antiferromagnetic order of a perfect Néel state in a periodically driven repulsive Hubbard model. The dynamics is calculated for a Bethe lattice in infinite dimensions with non‐equilibrium dynamical mean‐field theory. In the absence of driving melting proceeds differently depending on the quench of the interactions to hopping ratio from the atomic limit. For decay occurs due to mobile charge‐excitations transferring energy to the spin sector, while for it is governed by the dynamics of residual quasi‐particles. Here we explore the rich effects that strong periodic driving has on this relaxation process spanning three frequency ω regimes: (i) high‐frequency , (ii) resonant with integer l, and (iii) in‐gap away from resonance. In case (i) we can quickly switch the decay from quasi‐particle to charge‐excitation mechanism through the suppression of ν₀. For (ii) the interaction can be engineered, even allowing an effective regime to be reached, giving the reverse switch from a charge‐excitation to quasi‐particle decay mechanism. For (iii) the exchange interaction can be controlled with little effect on the decay. By combining these regimes we show how periodic driving could be a potential pathway for controlling magnetism in antiferromagnetic materials. Finally, our numerical results demonstrate the accuracy and applicability of matrix product state techniques to the Hamiltonian DMFT impurity problem subjected to strong periodic driving.     

The gravitational energy‐momentum pseudo‐tensor of higher‐order theories of gravity

We derive the gravitational energy momentum tensor for a general Lagrangian of any order and in particular for a Lagrangian such as . We prove that this tensor, in general, is not covariant but only affine, then it is a pseudo‐tensor. Furthermore, the pseudo‐tensor is calculated in the weak field limit up to a first non‐vanishing term of order h² where h is the metric perturbation. The average value of the pseudo‐tensor over a suitable spacetime domain is obtained. Finally we calculate the power per unit solid angle Ω carried by a gravitational wave in a direction for a fixed wave number under a suitable gauge. These results are useful in view of searching for further modes of gravitational radiation beyond the standard two modes of General Relativity and to deal with nonlocal theories of gravity where terms involving are present. The general aim of the approach is to deal with theories of any order under the same standard of Landau pseudo‐tensor.     

Synthesis and thermochemical study of quinoxaline‐N‐oxides: enthalpies of dissociation of the N–O bond

The synthesis of three new quinoxaline mono‐N‐oxides derivatives, namely, 2‐tert‐butoxycarbonyl‐3‐methylquinoxaline‐N‐oxide, 2‐phenylcarbamoyl‐3‐ethylquinoxaline‐N‐oxide, and 2‐carbamoyl‐3‐methylquinoxaline‐N‐oxide, from their corresponding 1,4‐di‐N‐oxides is reported. Samples of these compounds were used for a thermochemical study, which allowed derivation of their gaseous standard molar enthalpies of formation, , from their enthalpies of formation in the condensed phase, , determined by static bomb combustion calorimetry, and from their enthalpies of sublimation, , determined by Calvet microcalorimetry. Finally, combining the for the quinoxaline‐N‐oxides derived in this work with literature values for the corresponding 1,4‐di‐N‐oxides and atomic oxygen, the bond dissociation enthalpies for cleavage of the first N?O bond in the di‐N‐oxides, DH₁(N–O), were obtained and compared with existing data. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermodynamic Properties of the 1D Robin Quantum Well

The thermodynamic properties of the Robin quantum well with extrapolation length Λ are analyzed theoretically both for the canonical and two grand canonical ensembles, with special attention being paid to the situation when the energies of one or two lowest‐lying states are split off from the rest of the spectrum by the large gap that is controlled by the varying Λ. For the single split‐off level, which exists for the geometry with the equal magnitudes but opposite signs of the Robin distances on the confining interfaces, the heat capacity of the canonical averaging is a nonmonotonic function of the temperature T with its salient maximum growing to infinity as for the decreasing to zero extrapolation length and its position being proportional to . The specific heat per particle of the Fermi–Dirac ensemble depends nonmonotonically on the temperature too, with its pronounced extremum being foregone on the T axis by the plateau, whose value at the dying Λ is , with N being the number of fermions. The maximum of , similar to the canonical averaging, unrestrictedly increases as Λ goes to zero and is largest for one particle. The most essential property of the Bose–Einstein ensemble is the formation, for a growing number of bosons, of the sharp asymmetric shape on the characteristics, which is more protrusive at the smaller Robin distances. This cusp‐like structure is a manifestation of the phase transition to the condensate state. For two split‐off orbitals, one additional maximum emerges whose position is shifted to colder temperatures with the increase of the energy gap between these two states and their higher‐lying counterparts and whose magnitude approaches a Λ‐independent value. All these physical phenomena are qualitatively and quantitatively explained by the variation of the energy spectrum by the Robin distance. Parallels with other structures are drawn and similarities and differences between them are highlighted. Generalization to higher dimensions is also provided.     

TeV‐ and MeV‐Physics Out of an Model

The standard electroweak interaction is here re‐assessed to accommodate two different situations in Particle Physics. The first one is a ‐model at the TeV‐scale physics. The second one tackles the recent discussion of a possible fifth force mediated by a 17‐MeV X‐boson associated with an electron‐positron emission in the transition of an excited 8‐Beryllium to its ground state. The anomaly‐free model that provides these two scenarios is based on an ‐symmetry. It yields a new massive neutral boson, an exotic massive neutral fermion, right‐neutrinos and an additional neutral Higgs particle, which stems from a supplementary Higgs field, introduced along with the usual Higgs doublet responsible for the electroweak breaking and the masses of and Z⁰. Yukawa interactions of the two scalars generate the masses of the Standard Model leptons, neutrinos and a new exotic fermion of the model. The vacuum expectation values of the Higgses fix up two independent energy scales. One of them is the well‐confirmed electroweak scale, 246 GeV, whereas the other one is set up by adopting an experimental estimate for the ‐mass.     

London penetration depth and thermal fluctuations in the sulphur hydride 203 K superconductor

Recently, compressed H₂S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground‐state London penetration depth, λ₀=189 nm, and the superconducting energy gap, Δ₀=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, K, which shows that, unlike the cuprates, of the hydride is not limited by fluctuations. This is due to its three dimensionality and suggests the search for better superconductors should refocus on three‐dimensional systems where the inevitable thermal fluctuations are less likely to reduce the observed .

     

Density functional theory study of methoxide promoted and Zn(II)‐complexed methoxide promoted cleavages of aryl‐ and alkyl acetates in methanol. Transition from concerted to stepwise processes as a function of leaving group ability

Density functional theory calculations were performed for the methanolysis reactions of a set of aryloxy and alkoxy acetates ( 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1h , 1i , 1j , 1k , 1l , 1m ) promoted by methoxide and a 1,5,9‐triazacyclododecane‐complexed Zn^(II)‐methoxide [2(OCH₃)]⁺ in order to give free energies and structural data for the various intermediates and transition states along the reaction pathway. The methoxide‐promoted reactions experience a transition of pathways from enforced‐concerted addition of CH₃O^? to the C = O unit for substrates having a good aryloxy leaving groups (LGs) with strong electron withdrawers ( 1a , 1b , 1c , 1d , 1e ) to a two step process with rate‐limiting CH₃O^‐ attack on aryloxy acetates having higher (the pK_a of the parent phenol of the LG in methanol) values. Only in the case of the substrates 1i‐m having alkoxy LGs is there an observed change in rate‐limiting step that occurs at the quasi‐symmetrical point where the . The methanolysis process for the 2,4‐dinitrophenoxy substrate ( 1a ) promoted by [2(OCH₃)]⁺ involves transient binding of the substrate to the metal complex followed by a rate‐limiting, enforced‐concerted attack of Zn^(II)‐coordinated ^–OCH₃, with fast breakdown of an addition intermediate that does not have a significant lifetime. For substrates 1b,c having slightly less electron withdrawing substituents, the reaction has two steps with rate‐limiting attack and an unassisted LG departure. As the increases, the reaction still has two steps with rate‐limiting attack, but departure of the LG is now assisted by its coordination to the metal ion. For alkoxy containing substrates, a change in rate‐limiting step occurs centered at methoxy acetate, 1j , (when ) for which the second step of metal ion assisted departure of methoxide becomes partially rate‐limiting. The Brønsted plots computed for the methoxide‐promoted and [2(OCH₃)]⁺‐promoted methanolyses are compared with the previously determined experimental data and are analyzed as arising not from a common line attributable to all substrates but rather in terms of separate, but intersecting, plots for aryl‐ and alkyl acetates. Copyright © 2014 John Wiley & Sons, Ltd.     

Evolution of electric‐field‐induced quasibound states and resonances in one‐dimensional open quantum systems

A comparative analysis of three different time‐independent approaches to studying open quantum structures in a uniform electric field was performed using the example of a one‐dimensional attractive or repulsive δ‐potential and the surface that supports the Robin boundary condition. The three considered methods exploit different properties of the scattering matrix as a function of energy E: its poles, real values, and zeros of the second derivative of its phase. The essential feature of the method of zeroing the resolvent, which produces complex energies, is the unlimited growth of the wave function at infinity, which is, however, eliminated by the time‐dependent interpretation. The real energies at which the unitary scattering matrix becomes real correspond to the largest possible distortion, , or its absence at which in either case leads to the formation of quasibound states. Depending on their response to the increasing electric intensity, two types of field‐induced positive energy quasibound levels are identified: electron‐ and hole‐like states. Their evolution and interaction in the enlarging field lead ultimately to the coalescence of pairs of opposite states, with concomitant divergence of the associated dipole moments in what is construed as an electric breakdown of the structure. The characteristic features of the coalescence fields and energies are calculated and the behavior of the levels in their vicinity is analyzed. Similarities between the different approaches and their peculiarities are highlighted; in particular, for the zero‐field bound state in the limit of the vanishing , all three methods produce the same results, with their outcomes deviating from each other according to growing electric intensity. The significance of the zero‐field spatial symmetry for the formation, number, and evolution of the electron‐ and hole‐like states, and the interaction between them, is underlined by comparing outcomes for the symmetric δ geometry and asymmetric Robin wall.