Quantum Mechanical Tunneling Effect Research Articles

Critical Examination of Gamow's Theory of Alpha Particle Decay

Gamow’s Theory of Alpha Particle Decay was initially formulated for a limited set of nuclei. In this study, the researchers extend the scope and assessed the applicability of the theory to a broader range of nuclides especially those with different proton and neutron compositions. Three objectives were formulated to undertake the research. The researcher utilized one-dimensional WKB approximation to calculate the probability of tunneling through the potential barrier, which is a simplification compared to other formulas. The Geiger-Nuttall law, which describes a dependence of the disintegration constant on the range of α-particles, was deduced using the Gamow theory describing the passage of the α-particles through the Coulomb barrier by the quantum mechanical tunneling effect. Ground-to-ground state α-transitions for α-active nuclides were analyzed based on their half-lives and their dependence on various factors. The study revealed that all α-active nuclides whose Z ranges between 70 to100 undergoes similar alpha decay processes.

Silver induced chirality controlled spin filtration observed in ss-DNA functionalized with MoS2.

Chiral molecules can exhibit strong spin-orbit coupling, which can result in a large spin polarization. This is due to the fact that the energy levels of the electrons in a chiral molecule are strongly influenced by the chiral structure of the molecule, which can result in the separation of the energy levels for electrons with different spin orientations. We report a controlled spin-selective transmission of electrons through 20 base-paired poly-cytosine molecules functionalized with MoS2 flakes on ITO glass via the quantum mechanical tunneling effect. A reversion in spin polarization was observed after the silver ions interact with poly-cytosine due to the strong coordination of Ag(I) with cytosine-cytosine (C-C) mismatches, indicating the formation of duplex structural motifs, as confirmed by the circular dichroism spectroscopy at room temperature. Manipulating the spin of an electron through such a small molecule merely controlled by special cations could pave the way for major advances in spin-independent charge transport, advanced bioanalytical system design, and related applications.

From a microscopic inertial active matter model to the Schrödinger equation

Active field theories, such as the paradigmatic model known as ‘active model B+’, are simple yet very powerful tools for describing phenomena such as motility-induced phase separation. No comparable theory has been derived yet for the underdamped case. In this work, we introduce active model I+, an extension of active model B+ to particles with inertia. The governing equations of active model I+ are systematically derived from the microscopic Langevin equations. We show that, for underdamped active particles, thermodynamic and mechanical definitions of the velocity field no longer coincide and that the density-dependent swimming speed plays the role of an effective viscosity. Moreover, active model I+ contains an analog of the Schrödinger equation in Madelung form as a limiting case, allowing one to find analoga of the quantum-mechanical tunnel effect and of fuzzy dark matter in active fluids. We investigate the active tunnel effect analytically and via numerical continuation.

Manipulating electron-spin polarization using cysteine-DNA chiral conjugates.

The chiral molecules are potential generators of high spin-filters due to their inherent inversion asymmetric helical electric field. We report a controlled spin-selective transmission of electrons through self-assembled monolayers of 15 base-paired double-stranded deoxyribonucleic acid functionalized with two enantiomeric cysteine molecules on gold explored through the quantum mechanical tunneling effect. We observed a controlled spin polarization of 33% with dextro-cysteine, whereas a mere 8% was observed with levo-cysteine molecules using these functionalizations at room temperature. The manipulation of electron's spin merely through such a small molecule could lead to significant advancement in the spin-dependent charge transport phenomena and related applications.

Systematics of alpha-active nuclides and on the validity of the Geiger-Nuttall law

Geiger-Nuttall law, which describes a dependence of the disintegration constant on the range of α-particles, was deduced using the Gamow theory describing the passage of the α-particles through the Coulomb barrier by the quantum mechanical tunneling effect. Ground-to-ground state α-transitions for natural and artificial α-active nuclides were analyzed utilizing the Geiger-Nuttall rule. From rough analysis five group-like branches on the dependence of α-decay half-lives on α-particle energy was observed. Detailed analysis shows that precise linear dependence of the logarithm of α-decay half-lives on the reciprocal of square root ofthe α-particle energy for even-even isotopes of the U, Pu and Cm there are. However, for some even-even isotopes of the Po, Ra and Th regular behaviour of mass numbers was broken. This non-regularity of the mass numbers on the Geiger-Nuttall line is explained by the nuclear shell model.

Is It Possible to Build a Simple Telescope for Solar Neutrinos Registration?

The question of neutrino registration is closely related to the problem of beta decay. The question of whether the phenomenon of radioactive decay is purely accidental was actively discussed at the beginning of the last century, in particular, at Solvay congresses. N. Bohr and other creators of quantum mechanics proved that radioactive decay is a consequence of the quantum mechanical tunneling effect. Einstein and his supporters categorically objected to the fact that this phenomenon could be accidental.

Kinetics and thermodynamics of enzymatic decarboxylation of α,β-unsaturated acid: a theoretical study.

Enzymatic decarboxylation of α,β-unsaturated acid through ferulic acid decarboxylase (FDC1) has been of interest because this reaction has been anticipated to be a promising, environmentally friendly industrial process for producing styrene and its derivatives from natural resources. Because the local dielectric constant at the active site is not exactly known, enzymatic decarboxylation to generate β-methylstyrene (β-MeSt) was studied under two extreme conditions (ε = 1 and 78 in the gas phase and aqueous solution, respectively) using the B3LYP/DZP method and transition state theory (TST). The model molecular clusters consisted of an α-methylcinnamate (Cin) substrate, a prenylated flavin mononucleotide (PrFMN) cofactor and all relevant residues of FDC1. Analysis of the equilibrium structures showed that the FDC1 backbone does not play the most important role in the decarboxylation process. The potential energy profiles confirmed that the increase in the polarity of the solvent could lead to significant changes in the energy barriers, especially for the transition states that involve proton transfer. Analysis of the rate constants confirmed the low/no quantum mechanical tunneling effect in the studied temperature range and that inclusion of the fluctuation of the local dielectric environment in the mechanistic model was essential. Because the computed rate constants are not compatible with the time resolution of the stopped-flow spectrophotometric experiment, the direct route for generating β-MeSt after CO2 elimination (acid catalyst (2)) is unlikely to be utilized, thereby confirming that indirect cycloelimination in a low local dielectric environment is the rate determining step. The thermodynamic results showed that the elementary reactions that involve charge (proton) transfer are affected by solvent polarity, thereby leading to the conclusion that overall, the enzymatic decarboxylation of α,β-unsaturated acid is thermodynamically controlled at high ε. The entropy changes due to the generation of molecules in the active site appeared more pronounced than that due to only covalent bond breaking/formation or structural reorientation. This work examined in detail for the first time the scenarios in each elementary reaction and provided insight into the effect of the fluctuations in the local dielectric environment on the enzymatic decarboxylation of α,β-unsaturated acids. These results could be used as guidelines for further theoretical and experimental studies on the same and similar systems.

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures.

We reported a time-resolved study of quantum-mechanical tunneling of holes between two MoSe2 monolayers that are separated by a monolayer WS2 energy barrier. Four-layer heterostructures of MoSe2/WS2/MoSe2/graphene, as well as control samples, were fabricated by mechanical exfoliation and dry transfer techniques. To time-resolve the hole tunneling process, an ultrashort laser pulse was used to excite electrons and holes in both MoSe2 layers. By utilization of the graphene layer to eliminate carriers in the third MoSe2 layer, the first MoSe2 layer is selectively populated with the holes, which then tunnel to the third MoSe2 layer. By monitoring decay of the hole population with an ultrashort probe pulse, we measure a hole tunneling time of about 20 ps, which is found to slightly increase with the injected carrier density. Besides the fundamental interests of real-time observation of the quantum-mechanical tunneling effect across a nanometer barrier, these results provide quantitative understanding on tunneling mechanisms of charge transfer in van der Waals heterostructures, which is useful for designing sophisticated van der Waals multilayer heterostructures.

Systematical Analysis of Alpha-active Nuclides

The systematical analysis of alpha-active nuclides is useful for both the nuclear technology applications and understanding of nuclear structure study. Geiger-Nuttall law, which describes a dependence of the disintegration constant on the range of α-particles, was deduced from the Gamow theory explaining the passage of the α-particles through the Coulomb barrier by the quantum mechanical tunneling effect. Ground-to-ground state α-transitions for natural and artificial α-active nuclides were analyzed utilizing the Geiger-Nuttall rule. From rough analysis five group-like branches on the dependence of α-decay half- lives on α-particle energy were observed. Detailed analysis shows that precise linear dependences of the logarithm of α-decay half-lives on the reciprocal of square root of the α- particle energy for even-even isotopes of the U, Pu and Cm there are. However, for some even-even isotopes of the Po, Ra and Th regular behaviour of mass numbers was broken. This non-regularity of the mass numbers on the Geiger-Nuttall line is explained by the nuclear shell model.

The Hydride Transfer Process in NADP-dependent Methylene-tetrahydromethanopterin Dehydrogenase

NADP-dependent methylene-tetrahydromethanopterin (methylene-H4MPT) dehydrogenase (MtdA) catalyzes the reversible dehydrogenation of methylene-H4MPT to form methenyl-H4MPT+ by using NADP+ as a hydride acceptor. This hydride transfer reaction is involved in the oxidative metabolism from formaldehyde to CO2 in methylotrophic and methanotrophic bacteria. Here, we report on the crystal structures of the ternary MtdA-substrate complexes from Methylorubrum extorquens AM1 obtained in open and closed forms. Their conversion is accomplished by opening/closing the active site cleft via a 15° rotation of the NADP, relative to the pterin domain. The 1.08 Å structure of the closed and active enzyme-NADP−methylene-H4MPT complex allows a detailed geometric analysis of the bulky substrates and a precise prediction of the hydride trajectory. Upon domain closure, the bulky substrate rings become compressed resulting in a tilt of the imidazolidine group of methylene-H4MPT that optimizes the geometry for hydride transfer. An additional 1.5 Å structure of MtdA in complex with the nonreactive NADP+ and methenyl-H4MPT+ revealed an extremely short distance between nicotinamide-C4 and imidazoline-C14a of 2.5 Å, which demonstrates the strong pressure imposed. The pterin-imidazolidine-phenyl butterfly angle of methylene-H4MPT bound to MtdA is smaller than that in the enzyme-free state but is similar to that in H2- and F420-dependent methylene-H4MPT dehydrogenases. The concept of compression-driven hydride transfer including quantum mechanical hydrogen tunneling effects, which are established for flavin- and NADP-dependent enzymes, can be expanded to hydride-transferring H4MPT-dependent enzymes.

CALCULATION AND VISUALIZATION OF QUANTUM-MECHANICAL TUNNEL EFFECT

The article presents calculations and visualization of wave packet tunneling through the potentialbarrier of various widths done by using the MATLAB software. The graph of distribution of the probability densityalong barrier width is obtained in two and three dimensions. The article contains materials about quantummechanical tunnel effect such as the reflection of the part of the wave packet from the potential barrier and passingthrough the barrier of the other part. Calculations and visualization of the tunneling process are performed forvarious widths of the barrier. Figures given in the article show the probability of finding the particle at a certain time,i.e. the square of the absolute value of the state vector 2 . The part of the total number of particles tunnels throughbarrier, the other part reflects from it. The probability of tunneling increases with decrease of the barrier width. Thedetailed analysis of the graphs reveals that the wave function exponentially decreases inside the potential barriertherefore the observation of this effect requires a rather small width of the barrier. Students are offered to developprograms for various widths of the potential barrier. Results of experiments are used during the study of the quantummechanics.

High-level theoretical study of the reaction between hydroxyl and ammonia: Accurate rate constants from 200 to 2500 K.

Hydrogen abstraction from NH3 by OH to produce H2O and NH2-an important reaction in combustion of NH3 fuel-was studied with a theoretical approach that combines high level quantum chemistry and advanced chemical kinetics methods. Thermal rate constants calculated from first principles agree well (within 5%-20%) with available experimental data over a temperature range that extends from 200 to 2500 K. Quantum mechanical tunneling effects were found to be important; they lead to a decided curvature and non-Arrhenius behavior for the rate constant.

Addition and abstraction kinetics of H atom with propylene and isobutylene between 200 and 2500 K: A DFT study

The rate coefficients for the reactions of hydrogen (H) atom with propylene and isobutylene were studied at M06-2X/6-31+G(d,p) and MPWB1K/6-31+G(d,p) level of theories between 200 and 2500K. The possible mechanism for the reactions of propylene and isobutylene with H atom were examined. The rate coefficients for each reaction channels were calculated over a wide range of temperature using Conventional Transition State Theory (CTST). The quantum mechanical tunneling effect was computed using parabolic model and were included in the rate coefficient calculations. The Arrhenius expressions for the reactions, propylene+H and isobutylene+H were estimated to be kpropylene+H=(9.68±0.17)×10−18 T2.16exp[−(131±10)/T]cm3 molecule−1s−1 and kisobutylene+H=(1.40±0.22)×10−16 T1.89exp[−(215±12)/T]cm3molecule−1s−1, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the available experimentally measured rate coefficients.

Cascade photoconverter of concentrated radiations based on homogenous tunnel structures

A new photoelectric system with a cascade photoconverter (PhC) based on tunnel structures of a homogenous semiconductor of the n +-p-p +(t)n +-p-p +(t)n +-p-p + type with a quantum-mechanical tunneling effect for charge carriers at the p +(t)n + junction and with a radiation concentrator is presented and investigated. The problem on determining the optimal parameters of the structure and limit efficiency of an idealized cascade photoconverter is solved by neglecting the recombination of charge carriers in the thin photoactive layers. The optimal width of cascade photoconverting elements under different radiation spectra can be written via the distribution of the integral flux of the radiation quanta over a distance from an illuminated surface of the semiconductor structure. It is revealed that under light concentration the cascade photoconverters are preferable with respect to ordinary planar photoconverters, since their internal power losses are lower. The limit theoretical and real efficiencies of the examined systems as a function of radiation intensity and of the number of photoconverters in the cascade are determined.

Integration of Advanced MOSFET Device with Dual Effective Band Edge Work Function Metals Using Both HK and MG Last Scheme

The aggressive downscaling of CMOS requires both lateral and vertical shrinkage of dimensions. SiO2 served as gate dielectrics for more than four decades now is with severe leakage problem with quantum mechanical tunneling effect due to SiO2has been scaled down to only a few atomic layers to boost device performance. Thus high dielectric constant materials such as Hafnium oxide and Hafnium silicate are of great interests in recent years [1]-[3]. Meanwhile HK layers are not perfectly compatible with silicon channel surface as traditional silicon dioxide due to interface trap, dipole and defect are formed during processing which would be difficult for threshold voltage tuning. Gate stacks engineering like interfacial layer, post dielectric anneal and metal gate are needed to be carefully handled to alleviate these side effects. Due to excellent EOT scaling and short channel effect controllability, HKMG has been implemented into modern bulk silicon device to replace traditional Poly/SiON process. Different from Poly/SiON process which poly gate can be doped with N type and P type dopants respectively for NFET and PFET threshold voltage adjustment, Metal gate last process integration needs two kinds of metal to tune MOSFET work function in order to achieve suitable device performance [4, 5]. In this work, we have successfully fabricated devices with dual effective work function metals with 4.15eV and 5.04eV for NFET and PFET respectively. With EWF close to band edge, threshold voltage can be effectively achieved with 0.35V and -0.31V for NFET and PFET respectively. Process conditions and electrical results are also discussed.TiN served as PMOS work function metal and TiAl chosen as NMOS work function metal are integrated into CMOS. The detail integration flow of HKMG and TEM of cross section of gate structure are shown in Figure.1.In Fig. 2, threshold voltages with different Al% concentration are plotted. It can be shown that threshold voltage is strongly dependent on Al atomic concentration. With more percentage of concentration of Al, Vt would be lower due to work function Fermi level close to conduction band edge. Table.1 shows the effective work function values for NFET and PFET respectively. The EWFs are both close to band edges which are suitable for Vt tuning for devices. Fig.3 shows the NFET and PFET Id-Vg curves with low DIBL and sub-threshold swing performance. DIBL can be as low as 78mV/V and 82mV/V for NFET and PFET respectively. Swing can be as low as 62mV/dec and 64mV/dec for NFET and PFET respectively. It effectively demonstrates that controllability for metal gate to channel is much better comparing with Poly/SiON gate stackIn conclusion, we have successfully fabricated NFET and PFET devices with both HK and MG last integration scheme. By optimizing process conditions, Both NFET and PFET effective work functions can be tuned to be close to band edges with TiAl as NFET work function metal and TiN as PFET work function metal. Al concentration impaction on Vt is analyzed. Vt are well controlled with different channel length. DIBL and Swing are as low as to 78mV/V and 82mV/V and 62mV/dec and 64mV/dec for NFET and PFET respectively.

Theoretical kinetic investigation of thermal decomposition of methylcyclohexane

The thermal decomposition of methylcyclohexane (MCH) has been investigated at the CBS-QB3 and CCSD level of theory. The pyrolysis of MCH follows a radical chain mechanism, which mainly includes the C–C bond scission, H-atom abstraction, secondary and biradical reactions. Thermodynamic data for selected species involved in this study are computed at the CBS-QB3 level. The rate constants for all elementary reactions are also evaluated with conventional transition state theory (TST) in the temperature range of 298–2000K, where Eckart method is adopted to correct the quantum mechanical tunneling effect. The rate constants are reasonable agreement with experimental measurements and previous theoretical reports. Furthermore, the final products of MCH thermal decomposition are methane (CH4), ethylene (C2H4), propylene (C3H6), 1,3-butadiene (1,3-C4H6), isoprene (C5H8) and 1,3-pentadiene (1,3-C5H8). The main goal of this work is to give an exhaustive description of the MCH thermal decomposition by means of high level quantum chemical methods and provide a reliable reference for thermodynamic and kinetic information.

Reaction Kinetics of Hydrogen Abstraction Reactions by Hydroperoxyl Radical from 2-Methyltetrahydrofuran and 2,5-Dimethyltetrahydrofuran

Highly accurate rate parameters for H-abstraction reactions by HO2 radicals are needed for development of predictive chemical kinetic models for ignition. In this article, we report the rate coefficients for reaction of hydroperoxyl radical (HO2) with 2-methyltetrahydrofuran (MTHF) and 2,5-dimethyltetrahydrofuran (DMTHF) computed employing CBS-QB3 and CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory in the temperature range of 500-2000 K. Conventional transition state theory (CTST) with hindered rotor approximation for low frequency torsional modes and RRHO (rigid-rotor harmonic oscillator) approximation for all other vibrational modes is employed to evaluate the high pressure rate constants as a function of temperature. Rate constant of each individual hydrogen abstraction channel is taken into account to calculate the overall rate constant. Three-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of all abstraction channels between 500 and 2000 K. Eight transition states have been identified for MTHF and four for slightly more stable trans-DMTHF. Intrinsic reaction coordinates (IRC) calculations were performed to verify the connectivity of all the transition states (TSs) with reactants and products. One dimensional Eckart's asymmetrical method has been used to calculate quantum mechanical tunneling effect. Results of the theoretically calculated rate coefficients indicate that the hydrogen abstraction by HO2 from the C2 carbon of both MTHF and DMTHF is the most dominant path among all reaction pathways attributed to its lowest barrier height. The total rate coefficients of the MTHF and DMTHF with HO2 at CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory are k(T) = 8.60T(3.54) exp(-8.92/RT) and k(T)= 3.17T(3.63) exp(-6.59/RT) cm(3) mol(-1) s(-1), respectively. At both the level of theories, the predicted total abstraction rate constant for DMTHF is found to be higher as compared to that of MTHF over an entire temperature range of investigation. The overall rate constant calculated at CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory is lower by 1.43 and 3.44 times at 2000 K than the CBS-QB3 level for MTHF and DMTHF, respectively.

Theoretical analysis on the kinetic isotope effects of bimolecular nucleophilic substitution (S(N)2) reactions and their temperature dependence.

Factors affecting the kinetic isotope effects (KIEs) of the gas-phase SN2 reactions and their temperature dependence have been analyzed using the ion-molecule collision theory and the transition state theory (TST). The quantum-mechanical tunneling effects were also considered using the canonical variational theory with small curvature tunneling (CVT/SCT). We have benchmarked a few ab initio and density functional theory (DFT) methods for their performance in predicting the deuterium KIEs against eleven experimental values. The results showed that the MP2/aug-cc-pVDZ method gave the most accurate prediction overall. The slight inverse deuterium KIEs usually observed for the gas-phase SN2 reactions at room temperature were due to the balance of the normal rotational contribution and the significant inverse vibrational contribution. Since the vibrational contribution is a sensitive function of temperature while the rotation contribution is temperature independent, the KIEs are thus also temperature dependent. For SN2 reactions with appreciable barrier heights, the tunneling effects were predicted to contribute significantly both to the rate constants and to the carbon-13, and carbon-14 KIEs, which suggested important carbon atom tunneling at and below room temperature.

Ab Initio Thermal Rate Calculations of HO + HO = O(3P) + H2O Reaction and Isotopologues

The forward and reverse reactions, HO + HO ⇌ O((3)P) + H2O, which play roles in both combustion and laboratory studies, were theoretically characterized with a master equation approach to compute thermal reaction rate constants at both the low and high pressure limits. Our ab initio k(T) results for the title reaction and two isotopic variants agree very well with experiments (within 15%) over a wide temperature range. The calculated reaction rate shows a distinctly non-Arrhenius behavior and a strong curvature consistent with the experiment. This characteristic behavior is due to effects of positive barrier height and quantum mechanical tunneling. Tunneling is very important and contributes more than 70% of total reaction rate at room temperature. A prereactive complex is also important in the overall reaction scheme.

Abstraction Kinetics of H-Atom by OH Radical from Pinonaldehyde (C10H16O2): Ab Initio and Transition-State Theory Calculations

The kinetics and abstraction rate coefficients of hydroxyl radical (OH) reaction with pinonaldehyde were computed using G3(MP2) theory and transition-state theory (TST) between 200 and 400 K. Structures of the reactants, reaction complexes (RCs), product complexes (PCs), transition states (TSs), and products were optimized at the MP2(FULL)/6-31G* level of theory. Fifteen transition states were identified for the title reaction and confirmed by intrinsic reaction coordinate (IRC) calculations. The contributions of all the individual hydrogens in the substrate molecule to the total reaction are computed. The quantum mechanical tunneling effect was computed using Wigner's and Eckart's methods (both symmetrical and unsymmetrical methods). The reaction exhibits a negative temperature dependent rate coefficient, k(T) = (1.97 ± 0.34) × 10(-13) exp[(1587 ± 48)/T] cm(3) molecule(-1) s(-1), k(T) = (3.02 ± 0.56) × 10(-13) exp[(1534 ± 52/T] cm(3) molecule(-1) s(-1), and k(T) = (4.71 ± 1.85) × 10(-14) exp[(2042 ± 110)/T] cm(3) molecule(-1) s(-1) with Wigner's, Eckart's symmetrical, and Eckart's unsymmetrical tunneling corrections, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the experimentally measured ones and other theoretical results. It is shown that hydrogen abstraction from -CHO position is the major channel, whereas H-abstraction from -COCH(3) is negligible. The atmospheric lifetime of pinonaldehyde is computed to be few hours and found to be in excellent agreement with the experimentally estimated ones.