A field-cycling NMR investigation of resonant spin-lattice relaxation features arising from tunnelling methyl groups

(1)H nuclear spin-lattice relaxation has been investigated in sodium acetate trihydrate and sorbic acid using field-cycling NMR in the solid state. The relaxation is dominated by the reorientation of the methyl groups. Resonant features arising from coherent tunnelling are observed in both the magnetic field dependence of the spin lattice relaxation rate, T(1)(-1)(B(z)) and in the inverse temperature dependence, T(1)(-1)(1/T). The two systems have different barrier heights and tunnelling frequencies, providing different perspectives on the tunnel resonance phenomena. The magnetic field dependence enables different spectral density components to be separately investigated and in the carboxylic acid, sorbic acid, concerted proton transfer in the hydrogen bonds is also identified at low field and low temperature. The methyl hindering barriers and the correlation times characterising the reorientational dynamics has been accurately determined in both materials.     

Theoretical study of rearrangements in water dimer and trimer

The global minimum and transition states for the acceptor-tunnelling, donor-acceptor interchange and bifurcation tunnelling rearrangements of the water dimer, and the single-flip, bifurcation and concerted proton transfer processes in the water trimer have been reinvestigated. Our analysis of the tunnelling splittings and spectroscopy is based on ab initio calculations at the computational level of second-order M?ller-Plesset (MP2) theory with basis sets of aug-cc-pVXZ quality (X = D, T, Q for the dimer; X = D, T for the trimer). In both water dimer and trimer, the binding energy, barrier heights, intermonomer distances, and harmonic frequencies converge smoothly as the size of the basis set increases. In the water dimer, the binding energy was evaluated as 5.09kcal mol^?1, while the activation energies are 0.52 (acceptor-tunnelling) 0.79 (donor-acceptor interchange), and 1.94 kcal mol^?1 (bifurcation tunnelling) at the MP2/aug-cc-pVQZ level. In the water trimer, the binding energy was evaluated as 16.29 kcal mol^?1, while the activation energies are 0.28 (single-flip), 2.34 (bifurcation), and 26.36 (proton transfer) kcal mol^?1 at the MP2/aug-cc-pVTZ level.     

Optimised polychromatic field-mediated suppression of H-atom tunnelling in a coupled symmetric double well: two-dimensional malonaldehyde model

The cis–cis isomerisation motion of malonaldehyde can be modelled as a symmetric double well coupled with an asymmetric double well, which includes the effect of the cis–trans out-of-plane motion on the cis–cis motion. We have presented an effective method for having control over the tunnelling dynamics of the symmetric double well which is coupled with the asymmetric double well by monitoring tunnelling splitting. When a suitable external field is allowed to interact with the system, the tunnelling splitting gets modified. As the external time perturbation is periodic in nature, the Floquet theory can be applied to calculate the quasi-energies of the perturbed system and hence the tunnelling splitting. The Floquet analysis is coupled with a stochastic optimiser in order to minimise the tunnelling splitting, which is related to slowering of the tunnelling process. The minimisation has been done by one of the stochastic optimisers, simulated annealing. Optimisation has been performed on the parameters which define the external polychromatic field, such as intensities and frequencies of the components of the polychromatic field. With the optimised sets of parameters, we have followed the dynamics of the system and have found suppression of tunnelling which is manifested by a much higher tunnelling time.     

Particle tunnelling in the 7-azaindole dimer

A model has been developed that attempts to calculate the particle (proton and deuteron) tunnelling probability in the molecular dimer of 7-azaindole.     

A 13C field-cycling NMR relaxometry investigation of proton tunnelling in the hydrogen bond: dynamic isotope effects, the influence of heteronuclear interactions and coupled relaxation

Concerted double proton transfer in the hydrogen bonds of a carboxylic acid dimer has been studied using 13C field-cycling NMR relaxometry. Heteronuclear 13C-1H dipolar interactions dominate the 13C spin-lattice relaxation which is significantly influenced by the polarisation state of the 1H Zeeman reservoir. The methodology of field-cycling experiments for such heteronuclear spin-coupled systems is studied experimentally and theoretically, including an investigation of various saturation-recovery and polarisation-recovery pulse sequence schemes. A theoretical model of the spin-lattice relaxation of this coupled system is presented which is corroborated by experiment. Spectral density components with frequencies omega(C), omega(C) + omega(H), and omega(C) - omega(H) are mapped out experimentally from the magnetic field dependence of the 13C and 1H spin-lattice relaxation and the proton transfer rate at low temperature is determined from their widths. Any dynamic isotope effect on the proton tunnelling in the hydrogen bond arising from 13C enrichment in the skeletal framework of the dimer is found to be smaller than experimental uncertainties (approximately 5%).     

Ac response of a coupled double quantum dot

The effect of phase-breaking process on the ac response of a coupled double quantum dot is studied in this paper based on the nonequilibrium Green function formalism. A general expression is derived for the ac current in the presence of electron--phonon interaction. The ac conductance is numerically computed and the results are compared with those in [Anatram M P and Datta S 1995 Phys. Rev. B 51 7632]. Our results reveal that the inter-dot electron tunnelling interplays with that between dots and electron reservoirs, and contributes prominently to the ac current when inter-dot tunnelling coupling is much larger than the tunnelling coupling between dots and electron reservoirs. In addition, the phase-breaking process is found to have a significant effect on the ac transport through the coupled double dot.     

Effect of Rashba spin–orbit coupling on the spin polarized transport in diluted magnetic semiconductor barrier structures

We investigate theoretically the effects of Rashba spin–orbit coupling on the spin dependent transport through diluted magnetic semiconductor single and double barrier structures in the presence of a magnetic field. We find that the Rashba spin–orbit coupling gives rise to an enhancement of the negative tunnelling magnetoresistance of the diluted magnetic semiconductor single barrier structure and a pronounced beating pattern in the tunnelling magnetoresistance and spin polarization of the diluted magnetic semiconductor double barrier structure.     

Thermally induced nuclear dipolar polarization in powders

Photon spin symmetry conversion of methyl groups undergoing tunnelling rotation in powder samples at low temperature is accompanied by induced proton dipolar polarization.     

Confinement in a double barrier structure in the presence of an electric field

The effect of an electric field on the electron resonant tunnelling into a double barrier structure is studied. We show for particular field strengths an increase of the tunnelling time which helps us to explain the Stark-ladder localization and to discuss Bloch oscillations and the quenching of luminescence in multiple quantum wells.     

Quantum phase transition and dynamics induced by atom-pair tunnelling of Bose-Einstein condensates in a double-well potential

We investigate the quantum phase transition (QPT) and dynamics induced by atom-pair tunnelling of Bose-Einstein condensates in a symmetric double well under the mean-field approximation. We find the system undergoes a new QPT towards phase-locking state when atom-pair tunnelling is strong enough, and the critical point of self-trapping QPT is shifted by atom-pair tunnelling. As for the dynamics, the system displays localized dynamical behaviour: phase-locking motion and self-trapping motion. We further study the correlation between this localized dynamics and QPT, and find that the area of the localized trajectories in the phase space can serve as an order parameter for both QPTs. The critical exponent of this order parameter is also discussed.     

Effects of C5-substituent group on the hydrogen peroxide-mediated tautomerisation of protonated cytosine: a theoretical perspective

The direct tautomerism (path A) and H₂O₂ as a catalyst (path B) have been studied in conversion of Cyt2t⁺ into CytN3⁺ isomer. The protonated 5-carboxycytosine (5-caCyt) is represented and has been further explored in the presence of H₂O₂ (path C). In going from a four-membered-ring transition state in the case of the direct tautomerism to the six-membered ring for H₂O₂, the H₂O₂ significantly contributes to decreasing the free energy barrier of tautomerisation. Although the carboxylic substituent of 5-carboxycytosine has certain affected on the electron distribution of the pyrimidine ring, the six-membered-ring transition state has not changed. This result illustrates that the C5-carboxylation has no significant effect on the H₂O₂-mediated isomerisation of Cyt2t⁺ to CytN3⁺ isomer. Meanwhile, these paths A–C have been further explored in the presence of two water molecules. Use of implicit solvent models (PCM) does not significantly alter the energetics of water-mediated paths A–C compared to those in gas phase. Furthermore, the rate constant with Wigner tunnelling correction of path A is obviously smaller than those of paths B and C. Finally, the lifetime τ_99.9% of paths B and C is 10^?5 s, which is implemented by the mechanism of the concerted synchronous double proton transfer.     

Performance assessment of nanoscale Schottky MOSFET as resonant tunnelling device: Non-equilibrium Green’s function formalism

A comprehensive study is performed on the electrical characteristics of Schottky barrier MOSFET (SBMOSFET) in nanoscale regime, by employing the non-equilibrium Green’s function (NEGF) approach. Quantum confinement results in the enhancement of effective Schottky barrier height (SBH). High enough Schottky barriers at the source/drain and the channel form a double barrier profile along the channel that results in the formation of resonance states. We have, for the first time, proposed a resonant tunnelling device based on SBMOSFET in which multiple resonance states are modulated by the gate voltage. Role of essential factors such as temperature, SBH, bias voltage and structural parameters on the feasibility of this device for silicon-based resonant tunnelling applications are extensively studied. Resonant tunnelling appears at low temperatures and low drain voltages and as a result negative differential resistance (NDR) is apparent in the transfer characteristic. Scaling down the gate length to 6 nm increases the peak-to-valley ratio (PVR) of the drain current. As the effective SBH reduces, the curvature of the double barrier profile is gradually diminished. Therefore, multiple resonant states are contributed to the current and consequently resonant tunnelling is smoothed out.     

Study on Non-Sequential Double Ionization of Aligned Diatomic Molecules in Strong Laser Fields

We develop a semiclassical model to describe the non-sequential double ionization of aligned diatomic molecules in an intense linearly polarized field. It is found that in the tunnelling regime, the oriented molecule shows geometric effects on double ionization process when aligned parallel and perpendicular to the external field. Our results are qualitatively consistent with the recent experimental observations.     

Resonant tunnelling in O-D systems

O-D energy spectroscopy by means of transport and magneto-transport measurements has been carried out in double barrier resonant tunnelling heterostructures. Two models for the fabrication imposed lateral confining potential have been considered to account for the resonant lines associated with tunnelling of electrons through the O-D states of the quantum well. Preliminary measurements in a magnetic field are consistent with the values of magnetic length and undepleted conducting core radius in our structure, and indicate that up to 6 T the energy spectrum is dominated by spatial quantization. Single-charging effects in our structures are discussed.     

Generation of Bell states in two-component Bose－Einstein condensates

We examine analytically the generation of Bell state in Bose condensates of two interacting species trapped in a double well configuration. The density of probability for finding the entangled Bell state is given. The effect of the tunnelling rate and the interspecies interaction strength on the generation of Bell state is discussed. We find that the oscillation amplitude of the density of probability for finding the entangled Bell state becomes greater as the tunnelling rate Ω increases, and the self-interaction strength of the component A(B) has no effect on it.     

Effect of silver doping on the current–voltage characteristic of PbS nanorods

We report on field emission property from a single nanorod measured by using scanning tunnelling spectroscopy. It has been shown that field emission from nanorods of small band gap semiconductor is significantly increasing by doping. The current transport mechanism is explained using double barrier tunnel junction formalism. It is observed experimentally that the Fowler–Nordheim tunnelling mechanism is dominant and governs the transport mechanism. The transport properties of PbS nanostructures in the form of nanorod are investigated in terms of various conduction mechanism. The minimum voltage necessary for triggering Fowler–Nordheim tunnelling under the revised biased for intrinsic sample ~0.95 V and decreases to ~0.67 V for increase doping concentration up to 1.76 wt%.     

Deuteron spin-lattice relaxation in ammonium hexachlorometallates

Deuteron spin-lattice relaxation via the motion-dependent part of the electric quadrupole interaction is discussed in partly and fully deuterated ammonium ions of ammonium hexachlorometallates. The dominant motion at temperatures T>50K is normally 120 degrees reorientations of the ammonium ions. In some hexachlorometallates the instantaneous equilibrium directions of the nitrogen-hydrogen vectors make a certain angle Delta with the metal-nitrogen vectors and they appear in groups of six near each metal-hydrogen vector. Each N-D vector jumps between the six directions of one group and this motion (called limited jumps) dominates the deuteron relaxation at lower temperatures. In some samples one direction of each group seems to become more populated than the others when the deuteration degree exceeds a certain value and the ammonium ions become ordered. A model is derived for the relaxation rate in the absence of tunnelling splittings, which includes the effects of reorientations and limited jumps also in the ordered structure, where the limited-jump rate of a N-D vector to the preferred direction, r(p), differs from that to the nonpreferred direction, r(n). The obtained relaxation rate depends, in addition to the angle Delta, also on the ratio d=r(n)/r(p). The effect of d is discussed and estimates for it are presented on the basis of earlier experiments. The recent model for the deuteron relaxation in NH(3)D(+) ions, including the effect of proton tunnelling, is shortly reviewed. At lowest temperatures the motional rates can be dominated by corresponding incoherent tunnelling and the rate of the incoherent tunnelling contributing to limited jumps is argued to be clearly larger than that of the incoherent tunnelling contributing to approximately 120 degrees rotations.     

The mechanism of the amine-catalysed isomerization of dialkyl maleate: a computational study

DFT at B3LYP/6-31G (d,p) level calculation results for the amine-catalysed isomerization of dimethyl maleate revealed that the mechanism proceeds via four steps: (1) a concerted proton transfer from one amine molecule to another which subsequently enhances the addition of the adduct thus formed to the C–C double bond to yield INT1. (2) Abstraction of a proton from the β-carbon of INT1 by a second amine molecule to give intermediate INT2. (3) Rotation about the C–C single bond followed by proton abstraction by an amine molecule to yield unstable INT3, and (4) an elimination of an amine molecule to yield the trans isomer, dimethyl fumarate. Furthermore, it was found that step 1 is the rate limiting step. However, the activation energy difference between steps 1 and 2 was significantly low and its value depends on the amine catalyst used. The activation energy was found to be lower in water when compared to that calculated in the gas phase. In addition, linear correlation was found between the amine-catalysed isomerization experimental rate and the pKa of the amine catalyst on one hand and the enthalpic and free activation energies on the other hand. The calculations also confirmed that the reaction is first order in dimethyl maleate, second order in the amine catalyst and overall third order. This study disproves three of the four different intermediates that were previously suggested to explain the amine catalysed isomerization of dialkyl maleates. The study verifies the intermediate suggested by Rappoport.     

Wigner function study of a double quantum barrier resonant tunnelling diode

Resonant tunnelling structures are receiving attention as a testbed for theoretical approaches to quantum transport. We present a Wigner function study of a double quantum barrier resonant tunnelling device formed by layers of AlGaAs in GaAs. Our study deals with the influence of the boundary conditions on the initial distribution as well as on the time-evolution of the system. We use a Gaussian wave packet to study the numerical effects of the boundaries. We attempt to solve the system in both the time-evolution and steady-state cases, including self-consistency in the potential.     

Pourbaix diagrams in weakly coupled systems: a case study involving acridinol and phenanthridinol pseudobases

The thermodynamics of proton‐coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7‐dimethyl‐9‐hydroxy‐9‐phenyl‐10‐tolyl‐9,10‐dihydroacridine (AcrOH) and 6‐phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pK_a values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active –N center closer to –OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pK_a of hydronium ions as ?1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.