Step-like Increase Research Articles

In Situ Optical Quantification of Adsorbates and Surface Charges on Copper Crystals and Their Impact on the Hydrogen Evolution Reaction in Hydrochloric Electrolytes

In situ spectroscopic ellipsometry is combined with cyclic voltammetry to discover and quantify potential-dependent surface adsorbates and the electronic charge on copper single crystals in HCl solution. In comparison with electrochemical scanning tunneling microscopy, it is demonstrated that ellipsometry provides not only an extremely high finger print sensitivity to sub-monolayer surface modifications but that it is furthermore possible to determine qualified values with an appropriate optical model. As a critical bench mark we use the amount of adsorbed Cl– at the Cu(111) surface. In this context, we found clear optical evidence for a densified water layer at the Cu(111) surface. Particular attention is drawn to the potential range of the hydrogen evolution reaction and the catalytic efficiency of the relatively stable Cu(111) and the more open corrugated (110) surface. With the introduced ellipsometric method, we disclose a steplike increase of the surface electron excess and a decreasing lateral surf...

Humidity induced inhibition and enhancement of spontaneous emission of dye molecules in a single PEG nanofiber

Intrinsic properties of a polyethylene glycol nanofiber are utilized for the first time to dynamically modify the spontaneous emission rate of encapsulated boradiazaindacene dye molecules with fluorescence lifetime imaging. Nanofibers, fabricated by the electrospinning technique, are exposed to relative humidity up to 80%. The spontaneous emission rate of the confined boradiazaindacene is observed to inhibit and enhance the nanofibers’ swelling. The Purcell factor is determined to demonstrate an oscillatory behavior resulting from the distinct characteristics of the mode volume, and the step-like increase of the quality factor of the nanofiber cavity due to the changes in the refractive index and fiber diameter.

Pollution distribution in floodplain structure visualised by electrical resistivity imaging in the floodplain of the Litavka River, the Czech Republic

Shape and position of modern polluted sediment bodies in the floodplain internal structure can provide insight into past and future erosional/deposition behaviour of fluvial systems. Extensive field works needed for such studies can be mitigated by the use of geophysical imaging, originally developed for engineering practice but still rather sparsely used in floodplain research. We demonstrate the performance of electrical resistivity tomography (ERT), dipole electromagnetic profiling (DEMP), and X-ray fluorescence spectroscopy for identification of subsurface sediment bodies of contrasting lithology, age, and pollution status; we found minor remnants of unpolluted (ancient) floodplain segments that would likely have been overlooked in arbitrary “blind” sampling. Pollution chemostratigraphy of floodplains based on the concentrations of certain risk elements can be affected by post-depositional migration. We thus identified signatures related to mining and smelting that are carried by coarser particles (gangue, fly ash) not expected to undergo post-depositional translocations in alluvium: the local enrichment factor of Zr (LEF Zr) and mass-specific magnetic susceptibility (χ). The LEF Zr differentiated pre-mining floodplain sediments which originated from erosion of Zr-rich catchment loess soils from Zr-poor, gangue-derived material. χ exhibits a step-like increase above the threshold value of approximately 25·10−8 m3/kg in severely polluted sediments; we assign that χ step to the replacement of charcoal and wood by coke in smelting technology during the late 19th century. The shape of the unpolluted sedimentary bodies, as well as bodies of historically changing pollution extent, indicate considerable recent lateral river instability. A mild aggradation flattened the floodplain surface and prevented the identification of buried sedimentary bodies by surface topography, but did not restrict the power of the geophysical imaging.

Temperature signal in suspended sediment export from an Alpine catchment

Abstract. Suspended sediment export from large Alpine catchments (> 1000 km2) over decadal timescales is sensitive to a number of factors, including long-term variations in climate, the activation–deactivation of different sediment sources (proglacial areas, hillslopes, etc.), transport through the fluvial system, and potential anthropogenic impacts on the sediment flux (e.g. through impoundments and flow regulation). Here, we report on a marked increase in suspended sediment concentrations observed near the outlet of the upper Rhône River Basin in the mid-1980s. This increase coincides with a statistically significant step-like increase in basin-wide mean air temperature. We explore the possible explanations of the suspended sediment rise in terms of changes in water discharge (transport capacity), and the activation of different potential sources of fine sediment (sediment supply) in the catchment by hydroclimatic forcing. Time series of precipitation and temperature-driven snowmelt, snow cover, and ice melt simulated with a spatially distributed degree-day model, together with erosive rainfall on snow-free surfaces, are tested to explore possible reasons for the rise in suspended sediment concentration. We show that the abrupt change in air temperature reduced snow cover and the contribution of snowmelt, and enhanced ice melt. The results of statistical tests show that the onset of increased ice melt was likely to play a dominant role in the suspended sediment concentration rise in the mid-1980s. Temperature-driven enhanced melting of glaciers, which cover about 10 % of the catchment surface, can increase suspended sediment yields through an increased contribution of sediment-rich glacial meltwater, increased sediment availability due to glacier recession, and increased runoff from sediment-rich proglacial areas. The reduced extent and duration of snow cover in the catchment are also potential contributors to the rise in suspended sediment concentration through hillslope erosion by rainfall on snow-free surfaces, and increased meltwater production on snow-free glacier surfaces. Despite the rise in air temperature, changes in mean discharge in the mid-1980s were not statistically significant, and their interpretation is complicated by hydropower reservoir management and the flushing operations at intakes. Overall, the results show that to explain changes in suspended sediment transport from large Alpine catchments it is necessary to include an understanding of the multitude of sediment sources involved together with the hydroclimatic conditioning of their activation (e.g. changes in precipitation, runoff, air temperature). In addition, this study points out that climate signals in suspended sediment dynamics may be visible even in highly regulated and human-impacted systems. This is particularly relevant for quantifying climate change and hydropower impacts on streamflow and sediment budgets in Alpine catchments.

Standing wave-mediated molecular reorientation and spontaneous formation of tunable, concentric defect arrays in liquid crystal cells

Standing waves are involved in various fundamental, natural phenomena over a wide range of scales, that is, from electron orbitals to planetary oscillations. Interactions between standing waves and matter are of interest. Here we demonstrate spontaneous, nematodynamic standing waves in a nonlinear liquid crystal medium and their interactions with director fields. The standing waves hold unique features such as diagonal nodal lines, self-adaption to broad resonance frequencies and a step-like increase in the wavelength with the increasing frequency. Furthermore, the standing wave causes unexpected phenomena that can be used in various applications; for example, it can manipulate the molecular orientation to form tunable, periodic defect arrays with concentric director profiles, which can serve as an optical vortex array inducer or tunable micro-liquid crystal lens array. The standing wave also exerts asymmetric mechanical pressure that can relocate small particles. The results lead to new approaches in director manipulation, colloidal assembly and singular optics.

Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization

We present a detailed analysis of the energetic landscape of phthalocyanine (H2Pc) tautomerization on Ag(111), i.e., the switching of protons between different sites in the molecular frame, which is induced and measured by a low-temperature scanning tunneling microscope (STM). We demonstrate that tautomerization of this molecule is preferentially triggered by the excitation of the N–H stretching mode. Interestingly, a step-like increase of the tautomerization rate is observed at a bias voltage that corresponds to the second harmonic of this vibrational mode, which we attribute to the crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. This hypothesis is supported by the analysis of four modified versions of H2Pc, produced by single silver atom manipulation and/or deprotonation of the pristine H2Pc molecule. Depending on the particular modification, the step-like increase varies strongly, spanning the entire range from further enhancement to almost comple...

Magnetization, specific heat, and thermal conductivity of hexagonal ErMnO3 single crystals

We report a study of magnetism and magnetic transitions of hexagonal ErMnO$_3$ single crystals by magnetization, specific heat and heat transport measurements. Magnetization data show that the $c$-axis magnetic field induces three magnetic transitions at 0.8, 12 and 28 T. The specific heat shows a peak at 2.2 K, which is due to a magnetic transition of Er$^{3+}$ moments. For low-$T$ thermal conductivity ($\kappa$), a clear dip-like feature appears in $\kappa(H)$ isotherm at 1--1.25 T for $H \parallel ab$; while in the case of $H \parallel c$, a step-like increase is observed at 0.5--0.8 T. The transition fields in $\kappa(H)$ are in good agreement with those obtained from magnetization, and the anomaly of $\kappa$ can be understood by a spin-phonon scattering scenario. The natures of magnetic structures and corresponding field-induced transitions at low temperatures are discussed.

Gold Nanoparticles on Functionalized Silicon Substrate under Coulomb Blockade Regime: An Experimental and Theoretical Investigation.

Single charge electronics offer a way for disruptive technology in nanoelectronics. Coulomb blockade is a realistic way for controlling the electric current through a device with the accuracy of one electron. In such devices the current exhibits a step-like increase upon bias which reflects the discrete nature of the fundamental charge. We have assembled a double tunnel junction on an oxide-free silicon substrate that exhibits Coulomb staircase characteristics using gold nanoparticles (AuNPs) as Coulomb islands. The first tunnel junction is an insulating layer made of a grafted organic monolayer (GOM) developed for this purpose. The GOM also serves for attaching AuNPs covalently. The second tunnel junction is made by the tip of an STM. We show that this device exhibits reproducible Coulomb blockade I-V curves at 40 K in vacuum. We also show that depending on the doping of the silicon substrate, the whole Coulomb staircase can be adjusted. We have developed a simulation approach based on the orthodox theory that was completed by calculating the bias dependent tunnel barriers and by including an accurate calculation of the band bending. This model accounts for the experimental data and the doping dependence of Coulomb oscillations. This study opens new perspectives toward designing new kind of single electron transistors (SET) based on this dependence of the Coulomb staircase with the charge carrier concentration.

Rapid enhancement of nodal quasiparticle mass with heavily underdoping in Bi2212

We report substantial advance of our low-energy angle-resolved photoemission study of nodal quasiparticles in Bi2Sr2CaCu2O8+δ. The new data cover the samples from underdoped down to heavily underdoped levels. We also present the nodal Fermi velocities that determined by using an excitation-photon energy of hν=7.0 eV over a wide doping range. The consistency between the results with hν=8.1 and 7.0eV allows us to rule out the effect of photoemission matrix elements. In comparison with the data previously reported, the nodal effective mass increases by a factor of ∼1.5 in going from optimally doped to heavily underdoped levels. We find a rapid enhancement of the nodal quasiparticle mass at low doping levels near the superconductor-to-insulator transition. The effective coupling spectrum, λ(ω), is extracted directly from the energy derivatives of the quasiparticle dispersion and scattering rate, as a causal function of the mass enhancement factor. A steplike increase in Reλ(ω) around ∼ 65meV is demonstrated clearly by the Kramers-Kronig transform of Imλ(ω). To extract the low-energy renormalization effect, we calculated a simple model for the electron-boson interaction. This model reveals that the contribution of the renormalization at |ω|≤15meV to the quasiparticle mass is larger than that around 65meV in underdoped samples.

Anomalous elastic properties across the \u03b3 to \u03b1 volume collapse in cerium

The behavior of the f-electrons in the lanthanides and actinides governs important macroscopic properties but their pressure and temperature dependence is not fully explored. Cerium with nominally just one 4f electron offers a case study with its iso-structural volume collapse from the γ-phase to the α-phase ending in a critical point (pC, VC, TC), unique among the elements, whose mechanism remains controversial. Here, we present longitudinal (cL) and transverse sound speeds (cT) versus pressure from higher than room temperature to TC for the first time. While cL experiences a non-linear dip at the volume collapse, cT shows a step-like change. This produces very peculiar macroscopic properties: the minimum in the bulk modulus becomes more pronounced, the step-like increase of the shear modulus diminishes and the Poisson’s ratio becomes negative—meaning that cerium becomes auxetic. At the critical point itself cerium lacks any compressive strength but offers resistance to shear.

Hall field-induced resistance oscillations in a tunable-density GaAs quantum well

We report on Hall field-induced resistance oscillations (HIRO) in a 60 nm-wide GaAs/AlGaAs quantum well with an \emph{in situ} grown back gate, which allows tuning the carrier density $n$. At low $n$, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with $n^{-1/2}$, as expected. Remarkably, population of the second subband (SB2) significantly enhances HIRO, while their frequency now scales as $n^{-1}$. We demonstrate that in this two-subband regime HIRO still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, while that of SB1 remains essentially unchanged. The enhancement of HIRO manifests an unexpected, step-like increase of the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime.

Harmful impact on presynaptic glutamate and GABA transport by carbon dots synthesized from sulfur-containing carbohydrate precursor.

Carbon nanoparticles that may be potent air pollutants with adverse effects on human health often contain heteroatoms including sulfur. In order to study in detail their effects on different physiological and biochemical processes, artificially produced carbon dots (CDs) with well-controlled composition that allows fluorescence detection may be of great use. Having been prepared from different types of organic precursors, CDs expose different atoms at their surface suggesting a broad variation of functional groups. Recently, we demonstrated neurotoxic properties of CDs synthesized from the amino acid β-alanine, and it is of importance to analyze whether CDs obtained from different precursors and particularly those exposing sulfur atoms induce similar neurotoxic effects. This study focused on synthesis of CDs from the sulfur-containing precursor thiourea-CDs (TU-CDs) with a size less than 10nm, their characterization, and neuroactivity assessment. Neuroactive properties of TU-CDs were analyzed based on their effects on the key characteristics of glutamatergic and γ-aminobutyric acid (GABA) neurotransmission in isolated rat brain nerve terminals. It was observed that TU-CDs (0.5-1.0mg/ml) attenuated the initial velocity of Na+-dependent transporter-mediated uptake and accumulation of L-[14C]glutamate and [3H]GABA by nerve terminals in a dose-dependent manner and increased the ambient level of the neurotransmitters. Starting from the concentration of 0.2mg/ml, TU-CDs evoked a gradual dose-dependent depolarization of the plasma membrane of nerve terminals measured with the cationic potentiometric dye rhodamine 6G. Within the concentration range of 0.1-0.5mg/ml, TU-CDs caused an "unphysiological" step-like increase in fluorescence intensity of the рН-sensitive fluorescent dye acridine orange accumulated by synaptic vesicles. Therefore, despite different surface properties and fluorescent features of CDs prepared from different starting materials (thiourea and β-alanine), their principal neurotoxic effects are analogous but displayed at a different level of efficiency. Sulfur-containing TU-CDs exhibit lower effects (by ~30%) on glutamate and GABA transport in the nerve terminals in comparison with sulfur-free β-alanine CDs. Our results suggest considering that an uncontrolled presence of carbon-containing particulate matter in the human environment may pose a toxicity risk for the central nervous system.

Films fabricated from partially fluorinated graphene suspension: structural, electronic properties and negative differential resistance

The band structure and electric properties of films created from a partially fluorinated graphene suspension are analyzed in this paper. As may be inferred from the structural study, graphene islands (quantum dots) are formed in these films. Various types of negative differential resistance (NDR) and a step-like increase in the current are found for films created from the fluorinated graphene suspension. NDR resulting from the formation of the potential barrier system in the film and corresponding to the theoretical prediction is observed for a relatively low fluorination degree. The origin of the NDR varies with an increase in the fluorination degree of the suspension. The observation of NDR in the fluorinated films widens the range of application of such films, including as active device layers fabricated using 2D printed technologies on rigid and flexible substrates.

First principles calculation of the elasticity of ice VIII and X

The elastic constants of ice VIII and ice X phases under pressure have been determined at static 0 K conditions using first principles calculation. A step-like increase of the elastic constants of ice VIII phase occurred at 100-110 GPa due to hydrogen bond symmetrization. The elastic constants, and the pressure dependencies of these constants, of ice X and VIII are completely distinct. Due to these differences, these two phases can be distinguished on the basis of the elastic behavior. Conversely, the experimental elastic constant of C11 of ice VII gradually changes from an ice VIII like asymmetric hydrogen bond to a symmetric bond character within a wide pressure range. This finding suggests that the transition from ice VII to ice X starts around 30 GPa and completes at 110 GPa.

Nucleation induced by “Short-Term Pressurization” of an undercooled isotactic polypropylene melt

Nucleation of semi-crystalline polymers is very sensitive to perturbations of the melt state. In contrast to the case of flow, the influence of pressure changes on nucleation has been almost neglected so far. In this work we explore the effect of the pressure history on isotactic polypropylene crystallization by applying a brief step-like increase of pressure to the undercooled melt. Using dilatometry and synchrotron X-ray diffraction, an enhancement of crystallization kinetics proportional to the magnitude of the pressure pulse is revealed. This acceleration is linked to an increase of the number of active nuclei after the short term pressurization, as confirmed by ex-situ optical microscopy observations. Up to an order of magnitude increase in nucleation density is found, for pressure pulses around 600–700bar. The pressure-induced nucleating effect is interpreted in the light of classical nucleation theory; although a non-classical “barrier-less” nucleation mechanism is also envisaged.

Quantum state-resolved molecular scattering of NO (2Π1/2) at the gas-[Cnmim][Tf2N] room temperature ionic liquid interface: Dependence on alkyl chain length, collision energy, and temperature

Room temperature ionic liquids (RTILs) represent a promising class of chemically tunable, low vapor pressure solvents with myriad kinetic applications that depend sensitively on the nature of gas-molecule interactions at the liquid surface. This paper reports on rovibronically inelastic dynamics at the gas-RTIL interface, colliding supersonically cooled hyperthermal molecular beams of NO (Π1/22, N = 0) from 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (or [Cnmim][Tf2N]) and probing the scattered NO molecules via laser induced fluorescence (LIF) from the A(2Σ) state. Specifically, inelastic energy transfer into NO rovibrational and electronic degrees of freedom is explored as a function of RTIL alkyl chain length (n), incident collision energy (Einc) and surface temperature (Ts). At low collision energies (Einc = 2.7(9) kcal/mol), the scattered NO molecules exhibit a rotational temperature (Trot) systematically colder than Ts for all chain lengths, which signals the presence of non-equilibrium dynamics in the desorption channel. At high collision energies (Einc = 20(2) kcal/mol), microscopic branching into trapping/desorption (TD) and impulsive scattering (IS) pathways is clearly evident, with the TD fraction (α) exhibiting a step-like increase between short (n = 2, 4) and long (n = 8, 12, 16) alkyl chains consistent with theoretical predictions. For all hydrocarbon chain lengths and RTIL temperature conditions, NO rotational excitation in the IS channel yields hyperthermal albeit Boltzmann-like distributions well described by a “temperature” (TIS = 900 -1200 K) that decreases systematically with increasing n. Non-adiabatic, collision induced hopping between ground and excited spin-orbit states is found to be independent of RTIL alkyl chain length and yet increase with collision energy. The scattering data confirm previous experimental reports of an enhanced presence of the alkyl tail at the gas-RTIL interface with increasing n, as well as provide support for theoretical predictions of an alkyl length dependent shift between chains oriented parallel vs. perpendicular to the surface normal.

Postnatal changes in glutamatergic inputs of jaw-closing motoneuron dendrites.

Dendrites of masseter (jaw-closing) motoneurons (MMNs) are well developed and ramify extensively throughout the trigeminal motor nucleus and often extend into the adjacent reticular formation. It is possible that the dendrites have active properties, which are altered with the development of the orofacial musculoskeletal system. Thus, we examined the changes in somatic voltage responses evoked by photostimulation of the MMN dendrites by laser photolysis of caged glutamate from postnatal day (P) 2-5 and 9-12 rats. We photostimulated 39 spots that were arranged around each recorded neuron in a concave shape and found that the dendritic stimulation induced somatic depolarization in the presence of tetrodotoxin in all MMNs. With increasing photostimulation intensity, the responses grew in amplitude up to a certain threshold, where a step-like increase in amplitude occurred. In 75% of P2-5 MMNs, the step-like increase in amplitude, which was blocked by 20μM D(-)-2-amino-5-phosphonovaleric acid application, corresponded to the NMDA spikes/plateau potentials. In contrast, at P9-12 the responses became significantly smaller in amplitude and shorter in duration and only one neuron out of 12 MMNs showed NMDA spikes/plateau potentials. These results suggest that the glutamatergic responses evoked by photostimulation of the MMN dendrites change during the first two postnatal weeks, and these changes may be involved in the transition from suckling to chewing during postnatal development.

Control of the Power of Strokes and Muscle Activities in Cyclic Rowing Movements (a Research using Rowing Simulators)

We investigated the relationship between the power of rowing movements strokes and the rate of the latter in various testing modes and under different conditions of rowing performance; 25 elite sportsmen specialized in rowing on racing shells were involved in the tests. Two series of tests were carried out on rowing simulators of two types; mechanographic parameters (in particular, joint angles) and EMG activity of broad sets of the muscles involved in this type of locomotor activity were simultaneously recorded. Separate tasks included (i) evaluation of the maximum power of rowing movements, (ii) a controlled step-like increase in the power of the latter, (iii) passing a test “distance” with the maximum speed, (iv) performance of the rowing movements with the presence of visual feedback (with visual presentation of the parameters of motor activity on a monitor), and (v) “rowing” with variations of the external loading. It was found that increases in the power of rowing motions rather rigidly correlated with a nearly proportional increase in the rate of rowing cycles (at all performance modes); a subjectively comfortable rate of such cyclic movements increased with increase in the external loading. Under conditions where rowing movements were initiated with the presence of visual feedback that provided the subject with information on the characteristics of these movements, tested subjects were capable of controlling the power and rate of rowing movements separately. The intensities of EMG discharges of the muscles involved in realization of separate rowing movements correlated mostly with the velocity of these movements and not with the power of the latter. Thus, a strong interrelation between the power and rate of the movements in rowing is, to a great extent, a universal phenomenon; it can be disturbed only at the additional involvement of some external conditions. The value of this interrelation significantly varies between individuals and can be used for characterization of the functional productivity of the athletes and of their functional state.

Inter-ribbon tunneling in graphene: An atomistic Bardeen approach

A weakly coupled system of two crossed graphene nanoribbons exhibits direct tunneling due to the overlap of the wavefunctions of both ribbons. We apply the Bardeen transfer Hamiltonian formalism, using atomistic band structure calculations to account for the effect of the atomic structure on the tunneling process. The strong quantum-size confinement of the nanoribbons is mirrored by the one-dimensional character of the electronic structure, resulting in properties that differ significantly from the case of inter-layer tunneling, where tunneling occurs between bulk two-dimensional graphene sheets. The current-voltage characteristics of the inter-ribbon tunneling structures exhibit resonance, as well as stepwise increases in current. Both features are caused by the energetic alignment of one-dimensional peaks in the density-of-states of the ribbons. Resonant tunneling occurs if the sign of the curvature of the coupled energy bands is equal, whereas a step-like increase in the current occurs if the signs are opposite. Changing the doping modulates the onset-voltage of the effects as well as their magnitude. Doping through electrostatic gating makes these structures promising for application towards steep slope switching devices. Using the atomistic empirical pseudopotentials based Bardeen transfer Hamiltonian method, inter-ribbon tunneling can be studied for the whole range of two-dimensional materials, such as transition metal dichalcogenides. The effects of resonance and of step-like increases in the current we observe in graphene ribbons are also expected in ribbons made from these alternative two-dimensional materials, because these effects are manifestations of the one-dimensional character of the density-of-states.

Elastic anomalies at the magnetic phase transitions of TbTe 3

Abstract We report sound velocity and ultrasonic attenuation measurements in the vicinity of the successive magnetic phase transitions Tmag1~6.5 K, Tmag2~5.8 K and Tmag3~5.3 K in the charge density wave TbTe3 compound. A detailed investigation of the critical contributions to the temperature dependences of the sound velocity and ultrasonic attenuation is presented. Anisotropic stress dependences ∂ T m a g 1 / ∂ σ found at the antiferromagnetic phase transition Tmag1 is associated with the layered structure of this compound. An abrupt step-like increase in the velocity and a sharp peak in the attenuation are observed with the longitudinal and shear modes at the lock-in magnetic phase transition Tmag3=5.3 K. The critical velocity and attenuation behaviors in the high temperature paramagnetic above Tmag1 are described in terms of a phenomenological dynamic scaling expression.