Tunneling Gap Research Articles

Plasmonics Yields Efficient Electron Transport via Assembly of Shell-Insulated Au Nanoparticles

SummaryJunctions built from metallic nanoparticles (NPs) can circumvent the diffraction limit and combine molecular/nanoelectronics with plasmonics. However, experimental advances in plasmon-assisted electron transport at the nanoscale have been limited. We construct junctions of a robust, molecule-free, suspended film, built solely from AuNPs, capped by SiO2 shells (Au@SiO2), which give insulating tunneling gaps up to 3.6 nm between the NPs. Current measured across monolayers of such AuNPs shows ultra-long-range, plasmon-enabled electron transport (P-transport), beyond the range of normal electron tunneling across insulators. This finding challenges the present understanding of electron transport in such systems and opens possibilities for future combinations of plasmonics and nanoelectronics.

Field- and temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule magnet

Understanding quantum tunnelling of the magnetisation (QTM) in single-molecule magnets (SMMs) is crucial for improving performance and achieving molecule-based information storage above liquid nitrogen temperatures. Here, through a field- and temperature-dependent study of the magnetisation dynamics of [Dy(tBuO)Cl(THF)5][BPh4]·2THF, we elucidate the different relaxation processes: field-independent Orbach and Raman mechanisms dominate at high temperatures, a single-phonon direct process dominates at low temperatures and fields >1 kOe, and a field- and temperature-dependent QTM process operates near zero field. Accounting for the exponential temperature dependence of the phonon collision rate in the QTM process, we model the magnetisation dynamics over 11 orders of magnitude and find a QTM tunnelling gap on the order of 10−4 to 10−5 cm−1. We show that removal of Dy nuclear spins does not suppress QTM, and argue that while internal dipolar fields and hyperfine coupling support QTM, it is the dynamic crystal field that drives efficient QTM.

Nonlinear Plasmonic Metasurfaces

AbstractThe nonlinear optical response of materials allows optical functionality not seen in linear devices, such as switching, wavelength conversion, and adaptive optics. Unfortunately, the nonlinear optical response is weak in naturally occurring materials, making many ultrafast information processing applications impractical from an efficiency point of view. Nonlinear plasmonic metasurfaces, as a subset of metamaterials, aim to provide a more efficient and functional nonlinear optical response by tailoring the configuration of nanostructures. Metasurfaces are compact, cascadable, and easy to fabricate with established planar technologies, and therefore deserve particular attention. In this review, advances in nonlinear plasmonic metasurfaces are presented, including theoretical approaches, design methodologies, and key demonstrations of functionality. The theoretical approach first considers the linear response of the plasmonic metal and then uses this to calculate the nonlinear scattering. Design methodologies are considered including limits on gap size enhancements, tunneling and charging effects, and thermal management. Key demonstrations such as efficiency in wavelength conversion, functional wavelength conversion, and switching are also reviewed. Finally, an outlook on the future development in this field of research is offered, aiming at efficient and ultrafast optical information processing.

Unrelated BCS-like pairing pseudogap and critical superconducting transition temperature in various cuprate compounds

The smooth evolution of the energy gap observed in the tunneling and angle-resolved photoemission spectra (ARPES) of high-[Formula: see text] cuprates with lowering the temperature from a pseudogap state above the critical temperature [Formula: see text] to a superconducting state below [Formula: see text], has been poorly interpreted as the evidence that the pseudogap must have the same origin as the superconducting order parameter, and therefore, must be related to [Formula: see text]. We argue that such an explanation of the tunneling gap and ARPES data is misleading. We show that the BCS-like energy gap (or pseudogap) opening in the electronic excitation spectrum of underdoped-to-overdoped cuprates at a characteristic temperature [Formula: see text] and the true superconducting order parameter appearing only at [Formula: see text] are unrelated. The superconducting phenomenon in unconventional cuprate superconductors is fundamentally different from the BCS-like pairing of fermionic quasiparticles, and the superconducting transition temperature [Formula: see text] is not determined by the BCS-like gap formation. The unusual superconducting order parameter in these high-[Formula: see text] materials appears at [Formula: see text] and coexists with the BCS-like gap (or pseudogap) below [Formula: see text].

Percolation theory based statistical resistance model for resistive random access memory

A comprehensive understanding of the disorder-induced transport characteristics in resistive random-access memory (RRAM) is critical for its thermal stability analysis and analog switching for the coming neuromorphic computing application. Superior to the previous transport mechanisms which are only valid within their respective ranges of temperatures, we propose a unified physics-based model that can accurately predict the transport dependence on all temperature ranges up to 300 K. By utilizing percolation theory and the Fermi Golden Rule, the probability distributions for both the tunnel junction energy barrier and gap distance based statistical resistance model are described. It is found that different programming cycles and resistance states contribute to transition behavior between various low-temperature transport mechanisms. Moreover, the model can also investigate the dependence of electrical characteristics on defect generation like radiation damage. Therefore, it quantitatively relates the thermal stability and percolation effects to the structural disorders in RRAM. The good agreement between the simulation and experimental results indicates that our physics-based model can provide an accurate prediction of temperature and disorder dependent effects in RRAMs.

Understanding quantum tunneling using diffusion Monte Carlo simulations

In simple ferromagnetic quantum Ising models characterized by an effective double-well energy landscape the characteristic tunneling time of path-integral Monte Carlo (PIMC) simulations has been shown to scale as the incoherent quantum-tunneling time, i.e., as $1/\Delta^2$, where $\Delta$ is the tunneling gap. Since incoherent quantum tunneling is employed by quantum annealers (QAs) to solve optimization problems, this result suggests there is no quantum advantage in using QAs w.r.t. quantum Monte Carlo (QMC) simulations. A counterexample is the recently introduced shamrock model, where topological obstructions cause an exponential slowdown of the PIMC tunneling dynamics with respect to incoherent quantum tunneling, leaving the door open for potential quantum speedup, even for stoquastic models. In this work, we investigate the tunneling time of projective QMC simulations based on the diffusion Monte Carlo (DMC) algorithm without guiding functions, showing that it scales as $1/\Delta$, i.e., even more favorably than the incoherent quantum-tunneling time, both in a simple ferromagnetic system and in the more challenging shamrock model. However a careful comparison between the DMC ground-state energies and the exact solution available for the transverse-field Ising chain points at an exponential scaling of the computational cost required to keep a fixed relative error as the system size increases.

Observation of Tunneling Gap in Epitaxial Ultrathin Films of Pyrite-Type Copper Disulfide**Supported by the National Natural Science Foundation of China under Grant Nos 11574174, 11774193 and 11790311, and the National Basic Research Program of China under Grant No 2015CB921000.

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide. Layer-by-layer growth of CuS2 films with a preferential orientation of (111) on SrTiO3(001) and Bi2Sr2CaCu2O8+δ substrates is achieved by molecular beam epitaxy growth. For ultrathin films on both kinds of substrates, we observe symmetric tunneling gap around the Fermi level that persists up to ∼15 K. The tunneling gap degrades with either increasing temperature or increasing thickness, suggesting new matter states at the extreme twodimensional limit.

Quaterrylene molecules on Ag(111): self-assembly behavior and voltage pulse induced trimer formation.

The self-assembly behavior of quaterrylene (QR) molecules on Ag(111) surfaces has been investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that the QR molecules are highly mobile on the Ag(111) surface at 78 K. No ordered assembled structure is formed on the surface with a sub-monolayer coverage up to 0.8 monolayer due to the intermolecular repulsive interactions, whereas ordered molecular structures are observed at one monolayer coverage. According to our DFT calculations, charge transfer occurs between the substrate and the adsorbed QR molecule. As a result, out-of-plane dipoles appear at the interface, which are ascribed to the repulsive dipole-dipole interactions between the QR molecules. Furthermore, due to the planar geometry, the QR molecules exhibit relatively low diffusion barriers on Ag(111). By applying a voltage pulse between the tunneling gap, immobilization and aggregation of QR molecules take place, resulting in the formation of a triangle-shaped trimer. Our work demonstrates the ability of manipulating intermolecular repulsive and attractive interactions at the single molecular level.

Plasmonics Yields Surprisingly Efficient Electron Transport Via Assembly of Shell-Insulated Au Nanoparticles

Junctions built from metallic nanoparticles can circumvent the diffraction limit and marry molecular-/nano-electronics with plasmonics. However, experimental advances in plasmon-assisted electron transport at the nanoscale have been limited. We construct junctions of a robust, molecule-free, suspended film, built solely from AuNPs, capped by SiO2 shells (Au@SiO2), which give insulating tunneling gaps up to 3.6 nm between the NPs. Current measured across monolayers of such AuNPs shows ultra-long-range, plasmon-enabled electron transport (P-transport), beyond the range of normal electron tunneling across insulators. The finding challenges present understanding of electron transport in such systems and opens possibilities for future combinations of plasmonics and nanoelectronics.

Thermoluminescence as a Research Tool to Investigate Luminescence Mechanisms.

Thermally stimulated luminescence (TSL) is known as a technique used in radiation dosimetry and dating. However, since the luminescence is very sensitive to the defects in a solid, it can also be used in material research. In this review, it is shown how TSL can be used as a research tool to investigate luminescent characteristics and underlying luminescent mechanisms. First, some basic characteristics and a theoretical background of the phenomenon are given. Next, methods and difficulties in extracting trapping parameters are addressed. Then, the instrumentation needed to measure the luminescence, both as a function of temperature and wavelength, is described. Finally, a series of very diverse examples is given to illustrate how TSL has been used in the determination of energy levels of defects, in the research of persistent luminescence phosphors, and in phenomena like band gap engineering, tunnelling, photosynthesis, and thermal quenching. It is concluded that in the field of luminescence spectroscopy, thermally stimulated luminescence has proven to be an experimental technique with unique properties to study defects in solids.

Bipolar resistive switching with coexistence of mem-elements in the spray deposited CoFe2O4 thin film

In the present investigation, we have experimentally demonstrated the bipolar resistive switching with the coexistence of three fundamental memelements in the Ag/CoFe2O4/FTO thin film metal-insulator-metal (MIM) device. The device shows the analog resistive switching behavior and charge transport follows the Ohmic and space charge limited conduction (SCLC) mechanisms. The device transforms from asymmetric to symmetric resistive switching when the SCLC conduction mechanism change to the Ohmic conduction mechanism at higher voltage sweep rates. It was observed that the I–V crossing location of MIM device shifted towards the higher voltage range with increasing voltage sweep rates for both bias regions due to the nanobattery effect. The significant tunneling gap between immature conductive filament(s) and percolation channels was responsible for the coexistence of memelements and nanobattery effect in the Ag/CoFe2O4/FTO thin film MIM device.

Complementary Switching in 3D Resistive Memory Array

AbstractHigh‐density application of 3D vertical crossbar resistive random access memory (3D‐VRRAM) is challenging due to sneak leakage paths, which can be combated by using nonlinear low resistance state or complementary resistive switching (CRS). This work presents high‐resolution transmission electron microscopy (HRTEM) observation of nanofilaments during resistive switching (RS) in a TiOx/Al2O3‐based bilayer submicrometer 3D‐VRRAM hybrid device and the reset‐failure‐induced transformation from RS to CRS. The complete concept is divided into two parts. The first part is RS, where the devices show reliable switching, stable resistance levels, good retention, and moderately higher nonlinear behavior at the low resistance state. The RS mechanism is confirmed by an HRTEM image of a clear formation of conductive Ti5O9 nanofilament followed by a tunnel gap. The second part presents the realization of CRS operation after the reset failure in common electrodeless TiOx/Al2O3‐based 3D‐VRRAM hybrid devices. The formation of a series resistor along with the filament is the reason behind the evolution of the CRS mode. Higher nonlinearity is achieved under CRS as compared to RS. Further to this, for broad applications, it is essential to examine similar phenomena in other 3D‐VRRAM systems.

Two Series of Homodinuclear Lanthanide Complexes: Greatly Enhancing Energy Barriers through Tuning Terminal Solvent Ligands in Dy2 Single-Molecule Magnets.

The utilization of 2-ethoxy-6-{[(2-hydroxy-3-methoxybenzyl)imino]methyl}phenol (H2 L) as a chelating ligand, in combination with the employment of alcohols (EtOH and MeOH) as auxiliary ligands, in 4 f-metal chemistry afforded two series of dinuclear lanthanide complexes of compositions [Ln2 L2 (NO3 )2 (EtOH)2 ] (Ln=Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7)) and [Ln2 L2 (NO3 )2 (MeOH)2 ] (Ln=Sm (8), Eu (9), Gd (10), Tb (11), Dy (12), Ho (13), Er (14)). The structures of 1-14 were determined by single-crystal X-ray crystallography. Complexes 1-7 are isomorphous. The two lanthanide(III) ions in 1-7 are doubly bridged by two deprotonated aminophenoxide oxygen atoms of two μ2 :η0 :η1 :η2 :η1 :η1 :η0 -L2- ligands. One nitrogen atom, two oxygen atoms of the NO3- anion, two methoxide oxygen atoms of two ligand sets, and one oxygen atom of the terminally coordinated EtOH molecule complete the distorted dodecahedron geometry of each lanthanide(III) ion. Compounds 8-14 are isomorphous and their structures are similar to those of 1-7. The slight difference between 1-7 and 8-14 stems from purposefully replacing the EtOH ligands in 1-7 with MeOH in 8-14. Direct-current magnetic susceptibility studies in the 2-300 K range reveal weak antiferromagnetic interactions for 3, 4, 7, 10, 11, and 14, and ferromagnetic interactions at low temperature for 5, 6, 12, and 13. Complexes 5 and 12 exhibit single-molecule magnet (SMM) behavior with energy barriers of 131.3 K for 5 and 198.8 K for 12. The energy barrier is significantly enhanced by dexterously regulating the terminal ligands. To rationalize the observed difference in the magnetic behavior, complete-active-space self-consistent field (CASSCF) calculations were performed on two Dy2 complexes. Subtle variation in the angle between the magnetic axes and the vector connecting two dysprosium(III) ions results in a weaker influence on the tunneling gap of individual dysprosium(III) ions by the dipolar field in 12. This work proposes an efficient strategy for synthesizing Dy2 SMMs with high energy barriers.

Loss of adiabaticity with increasing tunneling gap in nonintegrable multistate Landau-Zener models

We consider the simplest non-integrable model of multistate Landau-Zener transition. In this model two pairs of levels in two tunnel coupled quantum dots are swept passed each other by the gate voltage. Although this 2 * 2 model is non-integrable, it can be solved analytically in the limit when the inter-level energy distance is much smaller than their tunnel splitting. The result is contrasted to the similar 2 * 1 model, in which one of the dots contains only one level. The latter model does not allow interference of the virtual transition amplitudes, and it is exactly solvable. In 2 * 1 model, the probability for a particle, residing at time t -> -\infty in one dot, to remain in the same dot at t -> \infty falls off exponentially with tunnel coupling. By contrast, in 2 * 2 model, this probability grows exponentially with tunnel coupling. The physical origin of this growth is the formation of the tunneling-induced collective states in the system of two dots. This can be viewed as manifestation of the Dicke effect.

Effect and Mechanism of High-Pressure Processing: A Case Study of Flue-Cured Tobacco

Summary After a high-pressure processing (HPP) treatment sensory evaluation of flue-cured tobacco showed modifications. There was no significant difference (P > 0.05) between the routine chemical components (total sugar, reducing sugar, nicotine, and total nitrogen) of flue-cured tobacco after high-pressure processing treatment (HPP sample) and that of an untreated control group (CG). An overall judgement, which can be made from the observations of scanning electron microscopy (SEM), X-ray computed microtomography (micro-CT) and transmission electron microscopy (TEM), is that HPP could compress the inner tunnel and tissue gap in a flue-cured tobacco leaf. However, the ultrastructure, such as the cellular cytoskeleton, would not be changed. Compared with CG, the apparent density of the HPP sample rose by 19.3%, while the true density only rose by 1.4%. This also explained that the main effect of high-pressure processing on flue-cured tobacco was microstructure compression rather than compression on the ultrastructure level. The differences between the lamina (leaf-shaped) sample, which were caused by high-pressure processing, were reflected in terahertz time-domain spectroscopy (THz-TDS), simultaneous thermal analysis (STA), and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). When the same tests were carried out using a sample that was milled to a powder, however, these differences were nearly removed. The milling process destroyed most of the microstructure of the flue-cured tobacco lamina; therefore, the results of THz-TDS, STA, and Py-GC/MS confirmed the hypothesis: That 400 MPa high-pressure processing treatment minimally changes the ultrastructure of flue-cured tobacco and only changes its relatively larger microstructure.

Probing Gap Plasmons Down to Subnanometer Scales Using Collapsible Nanofingers.

Gap plasmonic nanostructures are of great interest due to their ability to concentrate light into small volumes. Theoretical studies, considering quantum mechanical effects, have predicted the optimal spatial gap between adjacent nanoparticles to be in the subnanometer regime in order to achieve the strongest possible field enhancement. Here, we present a technology to fabricate gap plasmonic structures with subnanometer resolution, high reliability, and high throughput using collapsible nanofingers. This approach enables us to systematically investigate the effects of gap size and tunneling barrier height. The experimental results are consistent with previous findings as well as with a straightforward theoretical model that is presented here.

Hilbert space and ground-state structure of bilayer quantum Hall systems at ν=2/λ

We analyze the Hilbert space and ground state structure of bilayer quantum Hall (BLQH) systems at fractional filling factors $\nu=2/\lambda$ ($\lambda$ odd) and we also study the large $SU(4)$ isospin-$\lambda$ limit. The model Hamiltonian is an adaptation of the $\nu=2$ case [Z.F. Ezawa {\it et al.}, Phys. Rev. {B71} (2005) 125318] to the many-body situation (arbitrary $\lambda$ flux quanta per electron). The semiclassical regime and quantum phase diagram (in terms of layer distance, Zeeeman, tunneling, etc, control parameters) is obtained by using previously introduced Grassmannian $\mathbb{G}^4_{2}=U(4)/[U(2)\times U(2)]$ coherent states as variational states. The existence of three quantum phases (spin, canted and ppin) is common to any $\lambda$, but the phase transition points depend on $\lambda$, and the instance $\lambda=1$ is recovered as a particular case. We also analyze the quantum case through a numerical diagonalization of the Hamiltonian and compare with the mean-field results, which give a good approximation in the spin and ppin phases but not in the canted phase, where we detect exactly $\lambda$ energy level crossings between the ground and first excited state for given values of the tunneling gap. An energy band structure at low and high interlayer tunneling (spin and ppin phases, respectively) also appears depending on angular momentum and layer population imbalance quantum numbers.

Biased Nanoscale Contact as Active Element for Electrically Driven Plasmonic Nanoantenna

Electrically-driven optical antennas can serve as compact sources of electromagnetic radiation operating at optical frequencies. In the most widely explored configurations, the radiation is generated by electrons tunneling between metallic parts of the structure when a bias voltage is applied across the tunneling gap. Rather than relying on an inherently inefficient inelastic light emission in the gap, we suggest to use a ballistic nanoconstriction as the feed element of an optical antenna supporting plasmonic modes. We discuss the underlying mechanisms responsible for the optical emission, and show that with such a nanoscale contact, one can reach quantum efficiency orders of magnitude larger than with standard light-emitting tunneling structures.

Dynamic Stimulation of Superconductivity With Resonant Terahertz Ultrasonic Waves

An experiment is proposed to stimulate a superconducting thin film with terahertz (THz) acoustic waves, which is a regime not previously tested. For a thin film on a piezoelectric substrate, this can be achieved by coupling the substrate to a tunable coherent THz electromagnetic source. Suggested materials for initial tests are a niobium film on a quartz substrate, with a BSCCO intrinsic Josephson junction (IJJ) stack. This will create acoustic standing waves on the nm scale in the thin film. A properly tuned standing wave will enable electron diffraction across the Fermi surface, leading to electron localization perpendicular to the substrate. This is expected to reduce the effective dimensionality, and enhance the tendency for superconducting order parallel to the substrate, even well above the superconducting critical temperature. This enhancement can be observed by measuring the in-plane critical current and the perpendicular tunneling gap. A similar experiment may be carried out for a cuprate thin film, although the conduction electrons might be more responsive to spin waves than to acoustic waves. These experiments address a novel regime of large momentum transfer to the electrons, which should be quite distinct from the more traditional regime of large energy transfer obtained from direct electromagnetic stimulation. The experiments are also motivated in part by novel theories of the superconducting state involving dynamic charge-density waves and spin-density waves. Potential device applications are discussed.

Investigation of the channelling effect on pollutants dispersion between adjacent roadway tunnels

The health risks presented by noxious vehicle emissions inside tunnels has been amplified due to the increasing use of roadway tunnels. Particularly, for adjacent roadway tunnels, vehicular emissions from the upstream tunnel can further deteriorate the air quality within the following tunnels. A scale vehicle tunnel model was designed to experimentally modelled the airflow and pollutants dispersion in contiguous roadway tunnels. The channelling effect on pollutants dispersion between adjacent roadway tunnels was studied, and factors such as ventilation speed, open road section length, traffic condition (e.g. car free, car running and traffic congestion) were considered. Pollutants mass flow rate ratio between downstream and upstream tunnels was calculated to evaluate the variation of the entrained pollutants amount. For the car free condition, pollutant can be easily entrained into the downwind tunnel when the gap distance between roadway tunnels decreased. For the car running condition with fixed tunnel gap distance, the traffic speed variation barely changed the pollutants mass flow rate ratio. Furthermore, evident influences on pollutants concentration were observed from continuous congestion and partial congestion. Lastly, numerical simulation using computational fluid dynamics approach was conducted for the car free scenario, and reasonably good agreements were found for pollutants concentration ratio compared with the experimental data. The results yielded from this study further quantified the relationships among different influential factors on the pollutants dispersion between roadway tunnels, and can contribute to an improved tunnel ventilation system design, especially for the downstream tunnel.