Quantum Tunneling Research Articles

Floating-gate memristor based on a MoS2/h-BN/AuNPs mixed-dimensional heterostructure

Memristors have recently received substantial attention because of their promising and unique emerging applications in neuromorphic computing, which can achieve gains in computation speed by mimicking the topology of the brain in electronic circuits. Traditional memristors made of bulk MoO3 and HfO2, for example, suffer from a low switching ratio and poor durability and stability. In this work, a floating-gate memristor is developed based on a mixed-dimensional heterostructure comprising two-dimensional (2D) molybdenum disulfide (MoS2) and zero-dimensional (0D) Au nanoparticles (AuNPs) separated by an insulating hexagonal boron nitride (h-BN) layer (MoS2/h-BN/AuNPs). We find that under the modulation of back-gate voltages, the MoS2/h-BN/AuNPs device operates reliably between a high-resistance state (HRS) and a low-resistance state (LRS) and shows multiple stable LRS states, demonstrating the excellent potential of our memristor in multibit storage applications. The modulation effect can be attributed to electron quantum tunneling between the AuNP charge-trapping layer and the MoS2 channel. Our memristor exhibits excellent durability and stability: the HRS and LRS are retained for more than 104 s without obvious degradation and the on/off ratio is >104 after more than 3000 switching cycles. We also demonstrate frequency-dependent memory properties upon stimulation with electrical and optical pulses.

Investigation of the toxicity of Cr (VI) against cyanobacteria and the mechanism of tolerance of the cyanobacterial consortia: a quantum mechanical approach.

Hexavalent chromium (Cr (VI)) is a heavy metal that is distributed globally and poses a significant threat to the environment through various mechanisms. It can react with soil and water, leading to severe environmental damage. In this study, the toxicity of Cr (VI) was investigated by analyzing two major cyanobacteria species, Nostoc commune and Anabaena variabilis, commonly found in soil along with their consortia. The findings revealed that the toxicity mechanisms of Cr (VI) differed in individual monocultures, with Cr (VI) competing with various components. However, when the cyanobacteria species were combined, i.e., in consortia, they demonstrated an impressive retention of their functioning even in Cr (VI) concentration at 10ppm. The study also concluded that non-photochemical quenching played a critical role in minimizing Cr (VI) toxicity. Furthermore, the research examined the role of the S-cycle in the process. The quantum yield of electron flux revealed that the Cr (VI) was competing with Qa in A. variabilis and with Qb in N. commune, albeit the photosystem dysfunction is only visible in the latter. The mechanism seemed to be quantum tunneling alteration because of the Cr (VI) having different energized quantum wells. The consortia proved to be behaving in a better manner as compared to the control. Overall, this study reveals the mode of toxicity of Cr (VI) in these two important cyanobacterial strains as well as it also discusses the mechanism of tolerance of consortia against Cr (VI) toxicity.

Reactions of Hydroperoxyl Radicals (HO2) with Oxygenated Aromatic Bio-oil Model Compounds

During thermal utilization of biomass, a complex array of reactions ensues in the oxidative environment between fragments of the biomass and reactive species. In the low-temperature window, hydroperoxyl (HO[Formula: see text] radicals assume a central role prior to the establishment of the O/H radical pool. Anisole, cresol, guaiacol and vanillin are often utilized as representative model compounds to comprehend the combustion chemistry of the phenolic entities in biomass and oxygen-containing derived bio-oil. This paper reports thermo-kinetic parameters for the reactions of HO2 radicals with anisole, cresol, guaiacol and vanillin. Computed activation enthalpies entail facile values for H abstraction from the side-chain functional groups in the investigated compounds. H abstraction from the hydroxyl groups incurs lower activation energies in reference to methyl sites in cresol and guaiacol. Hence, H abstraction from the OH sites in these two molecules proceeds at a faster reaction rate constant when compared to abstraction from the methyl site. In the vanillin molecule, abstraction from the aldehydic and hydroxyl sites portrays comparable importance. The effect of scaling vibrational frequencies and the type of applied quantum tunneling on the computed reaction rate constants were thoroughly explored. Analysis of entropies of activation indicates that the transition states exhibit a product-like character. While ignition delay times of fuels often exhibit a profound sensitivity towards initial H abstraction reactions by HO2, we found that ignition delay times of the four title molecules remain almost unchanged when updating literature kinetics models with new reaction rate constants calculated herein. It is anticipated that reported outcomes from this study aid in formulating a better understanding of the complex chemical reactions that dictate oxidative decomposition of oxygenated bio-oil model compounds.

Two liquid states of distinguishable helium-4: The existence of another non-superfluid frozen by heating.

We show that two liquid states can exist in distinguishable helium-4 (4He) obeying Boltzmann statistics by path integral centroid molecular dynamics (CMD) simulations. This is an indication of quantum liquid polyamorphism induced by the nuclear quantum effect. For 0.08-3.3K and 1-500bar, we extensively conducted the isothermal-isobaric CMD simulations to explore not only possible states and state diagram but also the state characteristics. The distinguishable 4He below 25bar does not freeze down to 0.1K even though it includes no Bosonic exchange effect and, therefore, no Bose condensation. One liquid state, low quantum-dispersion liquid (LQDL), is nearly identical to normal liquid He-I of real 4He. The other is high quantum-dispersion liquid (HQDL) consisting of atoms with longer quantum wavelength. This is another non-superfluid existing below 0.5K or the temperatures of LQDL. The HQDL is also a low-entropy and fragile liquid to exhibit, unlike conventional liquids, rather gas-like relaxation of velocity autocorrelation function, while there the atoms diffuse without noticeable contribution from quantum tunneling. The LQDL-HQDL transition is not a thermodynamic phase transition but a continuous crossover accompanied by the change in the expansion factor of quantum wavelength. Freezing of HQDL into the low quantum-dispersion amorphous solid occurs by heating from 0.2 to 0.3K at 40-50bar, while this P-T condition coincides with the Kim-Chan normal-supersolid phase boundary of real 4He. The obtained state diagram was compared to that of the confined subnano-scale 4He systems, where Bosonic correlation is considerably suppressed.

A semi-classical Floquet-NEGF approach to model photon-assisted tunneling in quantum well devices

A semi-classical Floquet-NEGF approach to model photon-assisted tunneling in quantum well devices

DNA-Based Conductors: From Materials Design to Ultra-Scaled Electronics.

Photolithography has been the foundational fabrication paradigm in current high-performance electronics. However, due to the limitation in fabrication resolution, scaling beyond a 20-nm critical dimension for metal conductors presents a significant challenge for photolithography. Structural DNA nanotechnology emerges as a promising alternative to photolithography, allowing for the site-specific assembly of nano-materials at single-molecule resolution. Substantial progresses have been achieved in the ultra-scaled DNA-based conductors, exhibiting novel transport characteristics and small critical dimensions. This review highlights the structure-transport property relationship for various DNA-based conductors and their potential applications in quantum/semiconductor electronics, going beyond the conventional scope focusing mainly on the shape diversity of DNA-templated metals. Different material synthesis methods and their morphological impacts on the conductivities are discussed in detail, with particular emphasis on the conducting mechanisms, such as insulating, metallic conducting, quantum tunneling, and superconducting. Furthermore, the ionic gating effect of self-assembled DNA structures in electrolyte solutions is examined. This review also suggests potential solutions to address current challenges in DNA-based conductors, encouraging multi-disciplinary collaborations for the future development of this exciting area.

Nonadiabatic Tunneling in Chemical Reactions.

Quantum tunneling can have a dramatic effect on chemical reaction rates. In nonadiabatic reactions such as electron transfers or spin crossovers, nuclear tunneling effects can be even stronger than for adiabatic proton transfers. Ring-polymer instanton theory enables molecular simulations of tunneling in full dimensionality and has been shown to be far more reliable than commonly used separable approximations. First-principles instanton calculations predict significant nonadiabatic tunneling of heavy atoms even at room temperature and give excellent agreement with experimental measurements for the intersystem crossing of two nitrenes in cryogenic matrix isolation, the spin-forbidden relaxation of photoexcited thiophosgene in the gas phase, and singlet oxygen deactivation in water at ambient conditions. Finally, an outlook of further theoretical developments is discussed.

Quantum Tunneling Enhanced Hydrogen Desorption from Graphene Surface: Atomic versus Molecular Mechanism

Abstract We study the desorption mechanism of hydrogen isotopes from graphene surface using first-principles calculations, with focus on the effects of quantum tunneling. At low temperatures, quantum tunneling plays a dominant role in the desorption process of both hydrogen monomers and dimers. In the case of dimer desorption, two types of mechanisms, namely the traditional one-step desorption in the form of molecules (molecular mechanism), and the two-step desorption in the form of individual atoms (atomic mechanism) are studied and compared. For the ortho-dimers, the dominant desorption mechanism is found to switch from the molecular mechanism to the atomic mechanism above a critical temperature, which is respectively ~300 K and 200 K for H and D.

Klein tunneling and Fabry-Perot resonances in the α - T 3 bilayer with aligned stacking

This paper investigates the quantum tunneling effect on the α − T 3 bilayer with aligned stacking. An effective model is constructed to describe the properties around the triple band crossings for stacking with a vertical alignment of sites in the bilayer system. Focusing on these band crossings, it is found that while the energy spectrum remains gapless throughout, it is characterized by flat energy bands. Subsequently, the transmission coefficient, T, for Dirac quasi-electrons across a rectangular potential barrier is calculated, alongside the relationship between the transmission rate and the coupling parameter α. It is observed that super-tunnel phenomena occur at certain values of the quasiparticle energy, where the transmission is perfect regardless of the angle of incidence on the barrier, with α = 1. Furthermore, it is found that for a wide range of parameter values, the transmittance evolves monotonically and exponentially with increasing alpha. The paper also highlights the occurrence of the Klein paradox in the system, where quasiparticles approaching the barrier with zero-angle incidence exhibit ideal quantum transparency.

Impact of Nuclear Quantum Effects on the Structural Properties of Protonated Water Clusters.

Nuclear quantum effects (NQEs) play a crucial role in hydrogen-bonded systems due to quantum tunneling and proton fluctuation. Our understanding of how NQEs affect microstructures mainly focuses on bulk phases of liquids and solids but remains deficient for water clusters, including their hydrogen nuclei, hydrogen-bonded configurations, and temperature dependence. Here, we conducted ab initio molecular dynamics (MD) and path integral MD simulations to investigate the influence of NQEs on the structural properties of protonated water clusters H+(H2O)n (n = 3, 6, 9, 12). The results reveal that the NQEs become less evident as the cluster size increases due to the competition between NQEs and electrostatic interactions. Simulations of several H+(H2O)6 isomers at different temperatures indicate that the effect of elevated temperature on proton transfer is related to the initial structure. Interestingly, the process of proton transfer also involves the interconversion between Zundel-type and Eigen-type isomers. These findings significantly deepen our understanding of ion-water and water-water interactions, opening new avenues for the study of hydrated ion clusters and related systems.

Direct Observation of HOON Intermediate in the Photochemistry of HONO.

The photochemistry of nitrous acid (HONO), encompassing dissociation into OH and NO as well as the reverse association reaction, plays a pivotal role in atmospheric chemistry. Here, we report the direct observation of nitrosyl-O-hydroxide (HOON) in the photochemistry of HONO, employing matrix-isolation IR and UV-vis spectroscopy. Despite a barrier of approximately 30 kJ/mol, HOON undergoes spontaneous rearrangement to the more stable HONO isomer through quantum mechanical tunneling, with a half-life of 28 min at 4 K. Kinetic isotope effects and instanton theory calculations reveal that the tunneling process involves the concerted motion of the NO moiety (65.2%) and the hydrogen atom (32.3%). Our findings underscore the significance of HOON as a key intermediate in the photolytic dissociation-association cycle of HONO at low temperatures.

Do Molecules Tunnel through Nanoporous Graphene?

The molecular transport and quantum tunneling of H2 and H2O molecules through nanoporous graphene is studied using computational modeling and first-principles density functional theory. It is demonstrated that molecules with sufficiently high kinetic energies can tunnel through nanopores. It is also demonstrated that molecules can be trapped in front of a nanopore or behind it. These investigations help us learn the behavior of molecules in and around the nanopores of graphene. They also help us learn the fundamentals of molecular tunneling. We believe nanoporous graphene can play important roles for gas separation and nanofiltration.

Thin-wall monopoles in a false vacuum

We study numerically the existence in a false vacuum of magnetic monopoles that are “thin walled,” i.e., which correspond to a spherical region of radius R that is essentially trivial surrounded by a wall of thickness Δ≪R, hence the name thin wall, and finally an exterior region that essentially corresponds to a pure Abelian magnetic monopole. Such monopoles were dubbed false monopoles and can occur in non-Abelian gauge theories where the symmetry-broken vacuum is actually the false vacuum. This idea was first proposed in Kumar []; however, the proof of the existence of thin-wall, false monopoles given there was incorrect. Here, we fill this lacuna and demonstrate numerically, for an appropriately modified potential, the existence of thin-wall false monopoles. The decay via quantum tunneling of the false monopoles could be of importance to cosmological scenarios that entertain epochs in which the Universe is trapped in a symmetry-broken false vacuum. Published by the American Physical Society 2024

Record Quantum Tunneling Time in an Air-Stable Exchange-Bias Dysprosium Macrocycle.

In recent years, dysprosium macrocycle single-molecule magnets (SMMs) have received increasing attention due to their excellent air/thermal stability, strong magnetic anisotropy, and rigid molecular skeleton. However, they usually display fast zero-field quantum tunneling of the magnetization (QTM) rate, severely hindering their data storage applications. Herein, we report the design, synthesis, and characterization of an air-stable monodecker didysprosium macrocycle integrating strong single-ion anisotropy, near-perfect local crystal field (CF) symmetry, and efficient exchange bias. These indispensable features enable clear-cut elucidation of the crucial role of very weak antiferromagnetic coupling on magnetization dynamics, creating a prominent SMM with a large effective energy barrier (Ueff) of 670 cm-1, open hysteresis loops at zero field up to 14.9 K, and a record relaxation time of QTM (τQTM), 24281 s, for all known nonradical-bridged lanthanide SMMs.

Developing an efficient analytical model for predicting the electrical conductivity of polymeric nanocomposites containing hybrid carbon nanotube/carbon black nanofillers

Inter-cluster bridging of carbon nanotubes (CNTs) and carbon black (CB) nanoparticles conjoins inactive branches of carbonaceous nanofillers within the matrix and reduces the electron tunneling distance. This mechanism moderately overcomes quantum tunneling and provides percolative polymer networks exhibiting favorable electrical responses. This study focuses on devising an analytical procedure to scrutinize the electrical conductivity and percolation threshold of CNT/CB/polymer nanocomposites. To involve the physics of electrical processes, the modeling approach considers cylindrical CNTs and spherical CB nanoparticles surrounded by a continuum interphase, which serves as an electron hopping duct. This model is extended in a bottom-up micromechanics generalization to a level where it is capable of predicting the effects of a wide range of microstructural properties. The comparison of predictions with those obtained via experimental examinations, while affirming the infrastructure for investigating the electrical behavior of binary systems, convincingly captures the electrical conductivity/percolation threshold of ternary nanocomposites containing CNT/CB nanofillers.

Auxiliary Rather Than Dominant. The Role of Direct Dy-S Coordination in Single-Molecule Magnet Unveiled via ab initio Study.

The role of Dy-S coordination in a single-molecule magnet (SMM) is investigated via an ab initio study in a group of mononuclear structures. The SMM performance of this group is well interpreted via a concise criterion consisting of long quantum tunneling of magnetization (QTM) time τQTM and high effective barrier for magnetic reversal Ueff. The best SMMs in the selected group, i.e., 1Dy (CCDC refcode: PUKFAF) and 2Dy (CCDC refcode: NIKSEJ), are just those holding the longest τQTM and the highest Ueff simultaneously. Further analysis based on the crystal field model and ab initio magneto-structural exploration indicates that the influence of Dy-S coordination on the SMM performance of 1Dy is weaker than that of axial Dy-O coordination. Thus, Dy-S coordination is more likely to play an auxiliary role rather than a dominant one. However, if placed at the suitable equatorial position, Dy-S coordination could provide important support for good SMM performance. Consequently, starting from 1Dy, we built two new structures where Dy-S coordination only exists at the equatorial position and two axial positions are occupied by strong Dy-O/Dy-F coordination. Compared to 1Dy and 2Dy, these new ones are predicted to have significantly longer τQTM and higher Ueff, as well as a nearly doubled blocking temperature TB. Thus, they are probable candidates of SMM having clearly improved performance.

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.

Kinetic isotope effects on hydrogen/deuterium disordering and ordering in ice crystals: A Raman and dielectric study of ice VI, XV, and XIX.

Ice XIX and ice XV are both partly hydrogen-ordered counterparts to disordered ice VI. The ice XIX → XV transition represents the only order-to-order transition in ice physics. Using Raman and dielectric spectroscopies, we investigate the ambient-pressure kinetics of the two individual steps in this transition in real time (of hours), that is, ice XIX → transient ice VI (the latter called VI‡) and ice VI‡ → ice XV. Hydrogen-disordered ice VI‡ appears intermittent between 101 and 120K, as inferred from the appearance and subsequent disappearance of the ice VI Raman marker bands. A comparison of the rate constants for the H2O ices reported here with those from D2O samples [Thoeny et al., J. Chem. Phys. 156, 154507 (2022)] reveals a large kinetic isotope effect for the ice XIX decay, but a much smaller one for the ice XV buildup. An enhancement of the classical overbarrier rate through quantum tunneling for the former can provide a possible explanation for this finding. The activation barriers for both transitions are in the 18-24kJ/mol range, which corresponds to the energy required to break a single hydrogen bond. These barriers do not show an H/D isotope effect and are the same, no matter whether they are derived from Raman scattering or from dielectric spectroscopy. These findings favor the notion that a dipolar reorientation, involving the breakage of a hydrogen bond, is the rate determining step at the order-to-order transition.

Proton Collective Quantum Tunneling Induces Anomalous Thermal Conductivity of Ice under Pressure.

Proton tunneling is believed to be nonlocal in ice, but its range has been shown to be limited to only a few molecules. Here, we measured the thermal conductivity of ice under pressure up to 50GPa and found it increases with pressure until 20GPa but decreases at higher pressures. We attribute this nonmonotonic thermal conductivity to the collective tunneling of protons at high pressures, supported by large-scale quantum molecular dynamics simulations. The collective tunneling loops span several picoseconds in time and are as large as nanometers in space, which match the phonon periods and wavelengths, leading to strong phonon scattering at high pressures. Our results show direct evidence of global quantum motion existing in high-pressure ice and provide a new perspective to understanding the coupling between phonon propagation and atomic tunneling.

Superoscillations in high energy physics and gravity

We explore superoscillations within the context of classical and quantum field theories, presenting novel solutions to Klein–Gordon’s, Dirac’s, Maxwell’s and Einstein’s equations. In particular, we illustrate a procedure of second quantization of fields and how to construct a Fock space which encompasses Superoscillating states. Furthermore, we extend the application of superoscillations to quantum tunnelings, scatterings and mixings of particles, squeezed states and potential advancements in laser interferometry, which could open new avenues for experimental tests of quantum gravity effects. By delving into the relationship among superoscillations and phenomena such as Hawking radiation, the black hole (BH) information and the Firewall paradox, we propose an alternative mechanism for information transfer across the BH event horizon.