Tunneling Effect Research Articles

Is Quantum Above-Barrier Reflection Important for Molecular Barrier Crossing Rates?

Understanding quantum tunneling and above-barrier reflection effects on unimolecular and bimolecular reaction rate constants remains challenging to this very day. In many applications, especially when considering moderate-to-high temperatures, the "standard" procedure is to use the parabolic barrier approximation. Recent work has shown though that this may be insufficient, and one cannot ignore anharmonicity. In this work, we study the analytic theory, including anharmonicity obtained when expanding the thermal rate up to order ℏ4. Such theories need high-order derivatives of the potential at the barrier top. We show that such derivatives are computed straightforwardly for six different reactions. We suggest a straightforward methodology for assessing whether the parabolic barrier approximation is valid and show that when the reaction asymmetry is large, this may lead to significant quantum above-barrier reflection and transmission coefficients, which are less than unity.

Nonadiabatic Hydrogen Tunneling Dynamics for Multiple Proton Transfer Processes with Generalized Nuclear-Electronic Orbital Multistate Density Functional Theory.

Proton transfer and hydrogen tunneling play key roles in many processes of chemical and biological importance. The generalized nuclear-electronic orbital multistate density functional theory (NEO-MSDFT) method was developed in order to capture hydrogen tunneling effects in systems involving the transfer and tunneling of one or more protons. The generalized NEO-MSDFT method treats the transferring protons quantum mechanically on the same level as the electrons and obtains the delocalized vibronic states associated with hydrogen tunneling by mixing localized NEO-DFT states in a nonorthogonal configuration interaction scheme. Herein, we present the derivation and implementation of analytical gradients for the generalized NEO-MSDFT vibronic state energies and the nonadiabatic coupling vectors between these vibronic states. We use this methodology to perform adiabatic and nonadiabatic dynamics simulations of the double proton transfer reactions in the formic acid dimer and the heterodimer of formamidine and formic acid. The generalized NEO-MSDFT method is shown to capture the strongly coupled synchronous or asynchronous tunneling of the two protons in these processes. Inclusion of vibronically nonadiabatic effects is found to significantly impact the double proton transfer dynamics. This work lays the foundation for a variety of nonadiabatic dynamics simulations of multiple proton transfer systems, such as proton relays and hydrogen-bonding networks.

Ferroelectric tunnel junctions based on a HfO2/dielectric composite barrier

The ferroelectric tunnel junction (FTJ), which possesses a simple structure, low power consumption, high operation speed, and nondestructive reading, has attracted great attention for the application of next-generation nonvolatile memory. The complementary metal–oxide–semiconductor-compatible hafnium oxide (HfO2) ferroelectric thin film found in the recent decade is promising for the scalability and industrialization of FTJs. However, the electric performance, such as the tunneling electroresistance (TER) effect, of the current HfO2-based FTJs is not very satisfactory. In this work, we propose a type of high-performance HfO2-based FTJ by utilizing a ferroelectric/dielectric composite barrier strategy. Using density functional theory calculations, we study the electronic and transport properties of the designed Ni/HfO2/MgS/Ni (001) FTJ and demonstrate that the introduction of an ultra-thin non-polar MgS layer facilitates the ferroelectric control of effective potential barrier thickness and leads to a significant TER effect. The OFF/ON resistance ratio of the designed FTJ is found to exceed 4 × 103 based on the transmission calculation. Such an enhanced performance is driven by the resonant tunneling effect of the ON state, which significantly increases transmission across the FTJ when the ferroelectric polarization of HfO2 is pointing to the non-polar layer due to the aroused electron accumulation at the HfO2/MgS interface. Our results provide significant insight for the understanding and development of the FTJs based on the HfO2 ferroelectric/non-polar composite barrier.

Reassessment of the High-Temperature Oxidation of Di Ethyl Ether through Ab Initio Calculations.

Recent investigations of diethyl ether (DEE) high-temperature pyrolysis and fuel-rich oxidation have highlighted the failure of existing kinetic models to describe experimental CO production. The DEE high-temperature pyrolysis and oxidation chemistry is thus investigated through ab initio calculations. Geometries, frequencies, and hindered-rotor potentials of reactants, products, and transition states of key reactions (fuel decomposition radical decomposition and H-abstraction reactions) are calculated with the B2PLYP-D3/def2-TZVPD method, whereas final energies are refined using CCSD(T)/aug-cc-pV(D,T)Z. Temperature- and pressure-dependent rate constants are then derived from either canonical transition state theory (CTST) or ME/RRKM analysis with the inclusion of tunneling effect and hindered-rotor corrections and compared to experimental measurements when available as well as to previously suggested values. This new information is then merged with a C0-C3 core chemistry model and a low-temperature chemistry DEE subset from the literature to propose a new kinetic model for the combustion of DEE. This model is tested successfully against a large database related to the high-temperature oxidation and pyrolysis chemistry of DEE, including ignition delay times, shock tube speciation data, time-resolved CO profiles, laminar flame speeds, flame structures, and jet-stirred reactor data.

Investigating Entropic Dynamics of Multiqubit Cavity QED System

AbstractEntropic dynamics of a multiqubit cavity quantum electrodynamics system is simulated and various aspects of entropy are explored. In the modified version of the Tavis–Cummings–Hubbard model, atoms are held in optical cavities through optical tweezers and can jump between different cavities through the tunneling effect. The interaction of atom with the cavity results in different electronic transitions and the creation and annihilation of corresponding types of photon. Electron spin and the Pauli exclusion principle are considered. Formation and break of covalent bond and creation and annihilation of phonon are also introduced into the model. The system is bipartite. The effect of all kinds of interactions on entropy is studied. And the von Neumann entropy of different subsystems is compared. The results show that the entropic dynamics can be controlled by selectively choosing system parameters, and the entropy values of different subsystems satisfy certain inequality relationships.

Study on the coupling mechanism and reinforcement effect of a shield tunnel reinforced by channel steel under side pit excavation

To investigate the internal force and deformation response law and reinforcement effect of a shield tunnel reinforced by channel steel in a pit excavation, a self-developed “hydraulic loading system” is used to conduct a full-size test and numerical simulation analysis on a three-ring staggered-seam assembled tube sheet. The results show that the internal force response of the tube sheet on the excavation side of the pit is complex and sensitive, the internal force is transformed in the direction under different load levels, and the bearing capacity can be effectively increased by 62.5 % through the channel steel reinforcement; the channel steel fails due to weld cracking, and the nonstructural bonding surface is damaged by sliding; and the maximum reinforcing efficiency of the combination of the channel steel and steel plate reaches 45.8 %, making it an economical and reliable reinforcing program.

Behavior of defects in GaN avalanche photodiodes grown on GaN substrates

GaN avalanche photodiodes grown on GaN substrates were successfully fabricated. These devices displayed a low dark current, measuring <80 pA at a reverse bias of 82.0 V. Notably, the response spectrum of the devices showed new out-of-band response peaks with increasing reverse bias. Moreover, at high reverse bias, the devices emitted visible light. These phenomena were attributed to inherent defects within the materials. The defect level fitted from the tunneling currents closely matched the experimental value, indicating that the defect-assisted tunneling effect, with a defect level at 0.127 eV relative to the conduction band, contributed to the out-of-band response peak in the response spectrum. The Franz–Keldysh effect led to a redshift in the response spectrum. Additionally, the Mg-related deep energy level situated approximately 0.498 eV above the valence band, facilitated radiative recombination at high reverse bias. Meanwhile, the device’s luminescent image displayed a consistently square shape, suggesting uniform avalanche breakdown throughout the device.

Study of the influence of spatial effects on the ground surface and buildings in a two-lane tunnel boring

There are a series of engineering risks, such as ground subsidence, building tilt and cracking, in the process of shield tunnel driving through buildings, which have many adverse effects on urban residents and engineers. In particular, the differences in the effects of the interaction of two-lane tunnels on the building structure and ground deformation field are less often considered under different spatial effects such as construction sequences and tunnel spacings. As well as the problem of analyzing the building as a whole out of reality. To solve these problems, the spatial effect of the shield tunnel underpassing the shallow foundation building is simulated by Plaxis3D software to study the sensitivity analysis of surface settlement and the internal forces and deformation law of the building above in the process of tunneling underpassing with different depths of burial H and horizontal distance D of the double line tunnel. The engineering impact zoning method can investigate the safety of tunnels and buildings under different spatial effects when tunnels pass through buildings. The splitting of the building into plates and columns can reveal the forces and deformation laws of different structural parts. The results show that during the construction process of the double-line tunnel, the tunnel constructed first has a “blocking effect” on the tunnel constructed later, which affects the distribution of the disturbance area to a certain extent and changes the curve shape of the settlement trough. When the “blocking effect” occurs, the surface settlement and building deformation will be significantly reduced. In the internal forces of the building, the plate structure is mainly subjected to changes in axial forces, while the column structure is mainly affected by shear forces and bending moments. The factor of safety of tunnels decreases as the tunnel spacing decreases and as the building loads above increase.

An epsilon‐near‐zero circuit based on half‐mode SIW with enhanced integrability for applications of narrowband large group delay

AbstractThe tunneling effect of an epsilon‐near‐zero (ENZ) is exploited to fulfill a narrowband large group delay by using a compact half‐mode substrate integrated waveguide structure built on thin substrate. The ENZ channel possesses a finite input impedance at the tunneling frequency point, and the finite input impedance can be matched to the final I/O port simply by a quarter wavelength impedance transformer. Thus, the enhanced integrability is realized in a planar form in the proposed circuit comparing to the conventional waveguide ENZ channel. Meanwhile, in the real ENZ channel, the effective impedance is observed to be close to 50 Ω, so that the ENZ can be connected with 50 Ω microstrip directly, which has been experimentally validated. The final measured net group delay is 1.36 ns at 3.08 GHz with 3.6 dB insertion loss, and the proposed method provides an alternative for the group delay engineering.

Transient characteristics of carrier transport in an isotopically enriched 28Si/SiGe undoped heterostructure

An isotopically enriched 28Si/SiGe undoped heterostructure is a promising platform for Si-based qubits due to the long coherence time by reducing 29Si isotopes with non-zero nuclear spins. Carriers in the buried Si quantum well (QW) of the Si/SiGe heterostructures could tunnel to the oxide/Si interface, increasing charge noise and leading to charge instability. In this work, we investigate the tunneling effects on carrier distribution and transport properties in an isotopically enriched 28Si/SiGe undoped heterostructure and its transient characteristics by controlling the hold time of gate biasing under different drain biases. By holding the gate bias for a longer time, the drain is reduced since more carriers in the buried QW tunnel to the oxide interface. Furthermore, under a larger drain bias, more carriers can move across the Si/SiGe heterojunction to the oxide interface and are trapped, resulting in a stronger current reduction, which is explained by a lucky electron model.

Neuron-astrocyte interaction-inspired percolative networks with metal microdendrites and nanostars for ultrasensitive and transparent electronic skins

Biological systems provide innovative designs for electronic devices, optimizing network configurations for high-performance signal transmission with minimal energy consumption. The brain, as one of the most complex biological structures, demonstrates efficient network design through the multiscale radial networks of neurons and astrocytes. Emulating these brain networks offers a blueprint for the development of ultrasensitive pressure sensors for electronic skin, aiming to provide a more intuitive and sensitive mode of interaction between humans and machines. Herein, we propose a neuromorphic percolative network inspired by neuron-astrocyte interactions for ultrasensitive pressure sensors employing metal microdendrites and nanostars. Electromechanical investigation through representative volume elements simulation reveals that the optimized arrangement of microdendrites and nanostars in the neuromorphic percolative system enhances the percolation threshold and probability. Following these simulation results, we developed a neuromorphic percolative polyurethane (NP-PU) matrix utilizing the metal microdendrite-nanostar networks. The augmented quantum tunneling effect in the NP-PU matrix was investigated through electrochemical impedance spectroscopy and capacitance analysis. The fabricated piezoresistive pressure sensor with the NP-PU matrix shows ultrahigh sensitivity (160.3 kPa−1) at a low pressure range and a low limit of detection resolution (4 Pa), enabled by multi-channel quantum tunneling in the metal particle networks. Furthermore, the sensor maintains excellent mechanical flexibility and high optical transparency (75.4 %), improving its efficacy in applications like electronic skin and force touch panel. Our study highlights the potential of leveraging biological system-inspired network designs for crafting advanced electronic devices.

Theoretical study of tip-enhanced photoluminescence of single molecule in nanocavity formed by apex protrusion and substrate

Tip-enhanced photoluminescence techniques, characterized by ultra-high detection sensitivity and spatial resolution, have exhibited unprecedented capabilities in exploring optoelectronic phenomena and unraveling intrinsic physical mechanism at the single-molecule level. To suppress the quenching of molecular luminescence and achieve high-resolution optical imaging, a rational design of the tip-substrate configuration is required. In this paper, we systematically investigate the tip-enhanced photoluminescence characteristics of single molecules located within the plasmonic nanocavity formed by the tip protrusion and the substrate from a theoretical perspective. The impacts of electron tunneling and electron-hole pair creation on localized electric field and quantum yield are discussed using the quantum correction model and nonlocal dielectric theory, respectively. The crucial roles of atomic-scale protrusion at the tip apex and the decoupling layer on the substrate in enhancing vertical and horizontal electric fields as well as quantum yields have been emphasized. The results indicate that in smaller sub-nanometer cavities, electron tunneling and nonlocal dielectric effects greatly weaken the electric field intensity and fluorescence quantum yield. Meanwhile, due to the sharp lightning rod effect, the presence of atomic-level protrusion at the tip apex can provide a strong and highly localized plasmonic field. We find that the added NaCl layer not only blocks charge transfer between molecules and the substrate but also effectively prevents fluorescence quenching. The maximum fluorescence and Raman enhancement factors near the tip apex approach up to 5 and 12 orders of magnitude, respectively, achieving sub-nanometer spatial resolution. Finally, compared to vertical electric field and dipole, sharp protrusion can excite a stronger horizontal electric field and couple with horizontal dipoles to yield a higher quantum yield, resulting in a three orders of magnitude higher in the fluorescence enhancement of horizontal dipole compared to the tip without protrusion. Our theoretical research not only confirms the experimental results of spectral enhancement with the proximity of the tip to the molecule in tip-enhanced photoluminescence, but also provides effective guidance for a deeper understanding of the mechanism of tip-enhanced fluorescence and Raman spectroscopy, as well as optimizing the experimental system.

Evaluating the significance of rare earth and iron doping on structure-property of lead titanate

The samples (Pb0.8Ln0.2 Fe0.2 Ti0.8O3, (Ln= La, Nd, Sm, Gd)) were synthesized successfully by mixed oxide method. A significant reduction in tetragonality is observed with the modification of structure. The grain size from FESEM study was observed to be decreasing with increase in ionic radii of rare earth ions. Specifically, the dielectric constant is found to be maximum for the Sm-doped sample. Conductivity studies at varying temperature and frequencies reveal the NTCR behaviour of these samples. In Sm, Nd and Gd doped samples, tunnelling of small polarons is responsible for conduction process whereas for La doped sample, the conduction process is mediated through the combined effect of small polaron tunnelling and bipolaron hopping. The Sm-doped sample exhibits highest remanent polarization. All the results of this comprehensive study provide valuable insights into the effect of dopant ions on various properties of the studied materials, paving the way for device applications.

Three-dimensional numerical analysis of twin tunnelling in two-layered soil strata

Tunnelling induces stress change and displacement in the ground. The excavation of a new tunnel in stratified soil can trigger different patterns of stress redistribution, which may adversely influence nearby tunnels. Research on multi-tunnel interaction has mainly been performed on the assumption of a uniform ground. The effects of different soil stratifications on tunnelling interaction remain poorly understood. In this paper, three-dimensional numerical parametric studies verified by previous centrifuge tests were carried out to analyse the twin tunnelling effects in two-layered soil. An advanced hypoplastic constitutive model that can capture stress-, path-, and strain-dependency of soil behaviour is adopted. Numerical cases investigated include perpendicular twin tunnelling in two sand layers with different relative densities and the location of the interface between the two sand layers. It is revealed that larger settlements and a wider surface settlement trough occur when tunnelling in two-layered soil strata than in a uniform ground. This is because of the wider and larger soil arch induced in two-layered soil strata. The structural response including tunnel deformation, induced bending moment, and induced hoop stress of the existing tunnel can be greater when tunnelling in layered soil strata than in a uniform ground owing to larger stress relief. Moreover, the combination of bending moment and hoop stress can exceed the M−N failure envelope of the structure in layered soil. A conventional simplified assumption of a uniform ground can underestimate of the influence of new tunnel excavation on existing tunnels, resulting in unsafe designs.

Strength characteristics of cement slurry in high geothermal tunnel environment

The early-age strength of grouting material is crucial for tunnel support under high geothermal temperatures. Grouting materials (such as the most commonly used cement slurry) hydrate and solidify under the condition of variable-temperature curing after entering the real stratum. However, the development process of the early strength of cement paste under the condition of variable-temperature curing is still unclear. This aim of this study was to elucidate the environmental effects of high-ground-temperature tunnels on the early strength development of cement stone. Cement slurry was subjected to variable-temperature curing under three different temperatures (T = 40°C, 60°C and 80°C) and two different relative humidities (H = 5% and 95%) through design experiments. Compressive strength tests were conducted, along with microscopic analysis using X-ray diffraction, scanning electron microscopy and nuclear magnetic resonance. The results indicate that the strength of cement stone under variable-temperature curing was lower than that under constant-temperature curing at the same temperature. The influence of variable-temperature curing conditions on the strength variation of samples with a high water/cement ratio at later age was greater for curing at 80°C. The research results are helpful to understand the influence of environmental effects of high-ground-temperature tunnels on the properties of grouting materials.

A novel method for establishing typical daily profile of PM concentrations in underground railway stations

The air quality in underground railway stations (URS) poses a significant public health concern due to extremely high concentrations of particulate matter: PM10 and PM2.5. Indeed, PM sources are strong and numerous, such as train braking and tunnel effect and URS are often confined spaces with low air change rates. Despite multiple PM measurements within URS, the variability of those concentrations from stations to stations is still poorly understood. We present here a methodology for establishing a daily profile of particle mass concentrations, based on a 5-year long measurement series in a Parisian URS. This approach incorporates an extensive data cleaning process based on the identification of URS operation periods and physically inconsistent or mathematically aberrant data, together with a linear regression model. This methodology delivers three usable outcomes: a typical profile for weekdays, a typical profile for weekends, and a PM concentration Daily Amplitude Coefficient (DAC) for the considered period. The DAC is a daily metric of the pollution levels, that enables the analysis of temporal trends and facilitates the comparison with other data with other acquisition frequency. The methodology developed here in a specific URS for PM10 measurements can be easily applied to different particle size fractions or to other measured parameters exhibiting a daily profile. Weekdays PM10 concentrations exhibit two distinct peaks corresponding to morning and evening rush hours, with an average daytime concentration of 193 µg/m³. These peaks are delayed by ∼1 hour compared to the train traffic. Weekends show consistently lower PM levels with no observable peaks, averaging 157 µg/m³ during the day. Our analysis reveals the long-term temporal evolution of PM concentration within the URS, highlighting seasonal patterns with higher PM10 concentrations observed in summer (up to 400 µg/m3) and lower values in winter (down to 250 µg/m3). This indoor seasonal evolution is not correlated with the outdoor temporal evolution, showing higher concentrations during the winter. Furthermore, our results show that the optimal period (DAC∼1) for conducting experiments to obtain reliable profiles is during the spring months (April, May, June).

Advanced In-Situ functionalized conductive hydrogels with high mechanical strength for hypersensitive soft strain sensing applications

Conductive hydrogels are used in wearable electronics, soft robotics and artificial skin due to their flexibility, biocompatibility and multifunctionality. However, combining high mechanical strength, conductivity and sensitivity in these hydrogels remains a challenge. This paper proposes a novel in-situ conductive functionalization technology for hydrogels with high mechanical strength, achieving high conductivity (3223.727 S·m−1) and low resistance (0.92 Ω) without sacrificing mechanical properties. Comprehensive experiments reveal the mechanisms and optimal parameters for conductive functionalization. AgNO3 and ascorbic acid (VC) concentrations influence the distribution, reaction rate and aggregation of Ag particles on hydrogel surfaces. Sodium carboxymethylcellulose (CMC) enhances mechanical strength and Ag+ adsorption capacity. Optimal parameters are 1.00 M AgNO3 and 0.08 M VC. An in-situ reduction reaction of silver particles establishes strong interfacial bonding between the silver layer and hydrogel matrix, forming the basis for strain sensing. The hypersensitized sensing mechanism, involving the appearance and recovery of microcracks on the silver layer via the tunneling effect, enables high sensitivity (gauge factor = 36.65). The conductive hydrogels can monitor human physiological signals, tiny strain signals from water droplets (0.01 g) and airflow, with an average response time of 12.7 ms. The developed conductive hydrogels meet the technical requirements for combining high mechanical strength and sensitivity, offering an efficient new approach for conductive hydrogel-based soft strain sensors.

Development of self-sensing cement composites by incorporating hybrid biochar and nano carbon black

The self-sensing cement composites (SSCC) containing traditional conductive carbon materials are of great significance but cause serious pollution. In this study, the green SSCC is innovatively developed utilizing the synergistic effects of nano carbon black and biochar from coconut shell. The SSCC mixtures are compared in terms of fluidity, mechanical properties, resistivity, microstructure and comprehensive properties. The results indicate that in SSCC containing nano carbon black, the addition of biochar can obtain comparable compressive strength for its internal curing, interlocking and bridging effects. Besides, 4.5 % carbon material prolongs the crack initiation stage during compression, and 9 % carbon material causes the multiple cracks. Moreover, the addition of 4.5 % biochar can reduce the 35-day resistivity of sample with 1.5 % biochar and 3 % nano carbon black by 35.6 % owing to the increasing tunneling effect and contact conduction. It is also found that in conductive SSCC, the slope Kc of the non-contact resistivity curve is dominated by the conductive additives rather than cement hydration. And the self-sensing cement composites exhibit excellent piezoresistivity. The rates of change in resistivity of samples after cycle loading are exponentially related to residual damages, with R2 values greater than 0.95. Finally, multi-indicator evaluation analyses show that the sample containing more biochar is more environmentally friendly. This study provides a new pathway for the sustainable development of SSCC.

Influence of Nb Doping on the Performance of Oxygen Reduction Reaction on TiO2 in Acidic Environmental

Polymer electrolyte fuel cells (PEFCs) have attracted attention as one of the next-generation power sources that are environmentally friendly. However, the high manufacturer's costs, short service life, and insufficient cell performance are solved to realize widespread commercialization. A non-platinum group metal catalyst is relatively low in cost and highly durable. In particular, the oxides of group 4 and 5 elements were reported to show high oxygen reduction reaction (ORR) activity and durability. Their oxides are considered dominant candidates for the non-platinum group metal catalysts, although they have a fateful flaw that their ORR activity is clearly poorer than traditional platinum-supported carbon catalysts. It was reported that introducing oxygen vacancies and other transition metals into the crystal structure of 4- and 5-group metal oxides effectively enhanced the ORR activity. The active site of 4- and 5-group metal oxides is unclear despite it being well-known that the oxygen vacancies and dopant atoms are related to its ORR activity. It is now indicated that candidates of active site were the oxygen vacancies around the dopant atom and the crystal distortion. However, it is hard to separate their effects because both dopant atoms and oxygen vacancies distort the local crystal structure. On the other hand, identifying the ORR active site leads to obtaining the catalyst design with high ORR activity. We focused on the oxide nanosheet, the simplest structure formed by a six-coordinate octahedral, as the model electrode. The thickness of oxide nanosheets is about 1 nm, and forming a single layer on a substrate is relatively easy. Thus, it was expected that the electron conductivity was guaranteed enough by the tunnel effect. This study employed titanium oxide nanosheets (TiO2ns) as the model catalyst and niobium as a dopant to investigate the effect of dopant atoms on ORR activity. The ratio of Nb/Ti in the prepared model electrode was 0/1, 1/1, and 2/1. In addition, the oxygen vacancies were introduced into all prepared electrodes to study the impact of oxygen vacancies on ORR activity at each Nb/Ti ratio. The thickness of Nb-doped TiO2ns (Nb-TiO2ns) was about 1.2 nm for the 1/1 ratio and slightly thicker for the 0/1 ratio. In contrast, the thickness of the 2/1 ratio of Nb-TiO2ns was about two times thicker than the 1/1 ratio. The thicknesses of Nb-TiO2ns in each ratio were increased by the reduction treatment to introduce the oxygen vacancies. One reason for this thickness increase may be related to the phase transition into the anatase structure. ORR activity was evaluated from the on-set potential of ORR (E ORR) measured at the step scan with 3 minutes step width. E ORR was slightly elevated with Nb doping. The ion radii of Nb and Ti atoms are 3 pm different when it is assumed that both ions are six-coordinated and their valence is Nb5+ and Ti4+. These results showed that the crystal structure distortion probably has a slight but positive effect on ORR activity.After the reduction treatments, the E ORR of the 0/1 ratio of Nb-TiO2ns increased and was the same as that of the 2/1 ratio. The introduction of oxygen vacancies certainly enhanced the ORR activity of the 0/1 ratio of Nb-TiO2ns. The oxygen vacancies are thought to be necessary to enhance the ORR activity, although the reduction condition wasn’t optimized for the Nb-doped samples.From the above results, we consider that the active site is the oxygen vacancy site rather than the crystal distortion site at this time.

Observation and characterization of titanium-like nano-filament in TiO2 memristor using superconducting electrode(s) and Andreev spectroscopy

A thin TiO2 semiconductor film embedded between two metal electrodes works as a memristor after being formed by soft breakdown. The forming creates a nano-filament that penetrates through the poorly conducting TiO2 film and connects the electrodes conductively. While previous works characterized the nano-filament properties (shape, composition, and resistivity) by electron microscopy techniques, we present a characterization by electrical measurements. In a typical memristor, both electrodes are made of normal metals. We study the metal/TiO2/metal memristors with a bottom electrode made of a superconducting NbN layer and a top electrode made of a normal (Pt) or superconducting (Nb) metal. The nano-filament connecting the electrodes touches the bottom electrode as a point contact, thus allowing us to perform point-contact Andreev reflection spectroscopy of the NbN superconductor. The spectra, measured below the critical temperature (15 K) of NbN, are analyzed theoretically. The analysis reveals the presence of one nano-filament and determines the nano-filament resistance, Sharvin resistance of the point contact, and Maxwell resistance of the electrodes. Moreover, it shows that the nano-filament is a conical-shaped Ti-like metal point contact with a tip diameter of ∼3–5 nm, Fermi velocity of 2×106m/s, and low-temperature resistivity of ∼10−8–10−7Ωm. Thus, the nano-filament in our device is not the Ti4O7 phase observed in previous works. Remarkably, the point contact spectrum of the superconducting NbN layer shows the Andreev peak typical for ballistic transport. This is because the point contact probes the NbN layer through a thin Al layer that mimics superconductivity of NbN via the proximity effect and eliminates the effects of tunneling and disorder.