Sharp Minimum Research Articles

Flow regime transitions in a bubble column

In a 0.102m ID bubble column, various techniques for flow regime identification based on pressure fluctuation measurement were applied: standard deviation, fractal analysis, Kolmogorov entropy KE and power spectral density. The superficial gas velocity reached up to 0.2ms−1 and the pressure was varied up to 2MPa. Up to four flow regimes could be identified: intermittent, homogeneous, transition and heterogeneous flow. KE shows a sharp minimum at the end of the homogeneous regime and start of the transition or heterogeneous regime, respectively. Overall, KE is the most accurate method to determine this main transition. The fractal analysis shows a minimum at a clearly lower gas velocity. The standard deviation and the power spectral density are less accurate than KE but more convenient since the value obtained in a single measurement could be compared to a threshold.

Tamm-plasmon resonance based temperature sensor in a Ta2O5/SiO2 based distributed Bragg reflector

We demonstrate a Tamm-plasmon-polariton (TPP) based sensor for accurate determination of temperature. The sensing architecture is comprised of thin silver (Ag) film on a distributed-Bragg-reflector (DBR) structure made of periodically stratified multilayers of Ta2O5 and SiO2. TPP mode is excited using direct incidence of white-light-source at normal incidence as well as oblique incidence. The mode excitation is characterized by a sharp reflectivity minimum within the photonic bandgap (PBG) of DBR. It was observed that the depth of TPP resonance reduces uniformly as a function of temperature without affecting its sharpness. Defining temperature sensitivity in terms of change in reflectivity from the composite structure, a sensitivity of 7.8×10−4/°C is estimated for the sensing geometry. Also, the metallo-dielectric geometry supporting TPP modes is modeled using transfer-matrix-method for ascertaining the salient features of TPP modes which have a direct impact on sensing properties.

Influence of local strain heterogeneity on high piezoelectricity in 0.5Ba(Zr0.2Ti0.8)O3−0.5(Ba0.7Ca0.3)TiO3 ceramics

Dielectric and mechanical spectroscopies have been used to investigate ferroelectric transitions and twin wall dynamics in the lead-free ceramic $0.5\mathrm{Ba}(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}){\mathrm{O}}_{3}\ensuremath{-}0.5(\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3})\mathrm{Ti}{\mathrm{O}}_{3}$ (abbreviated as BZT-50BCT), which is known to have a high piezoelectric coefficient (${d}_{33}g545\phantom{\rule{0.16em}{0ex}}\mathrm{pC}/\mathrm{N}$). Results from dynamical mechanical analysis in the frequency range 0.2--20 Hz and resonant ultrasound spectroscopy in the frequency range $\ensuremath{\sim}0.1--1.2\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ confirm the existence of three phase transitions with falling temperature, at $\ensuremath{\sim}360\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (cubic-tetragonal), $\ensuremath{\sim}304\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (tetragonal-orthorhombic), and $\ensuremath{\sim}273\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (orthorhombic-rhombohedral). In comparison with $\mathrm{BaTi}{\mathrm{O}}_{3}$, however, the transitions are marked by rounded rather than sharp minima in the shear modulus. The pattern of acoustic loss is also quite different from that shown by $\mathrm{BaTi}{\mathrm{O}}_{3}$ in having a broad interval of high loss at low temperatures, consistent with a spectrum of relaxation times for interactions of ferroelastic twin walls. Differences in the dielectric properties also suggest more relaxor like characteristics for BZT-50BCT. It is proposed that the overall pattern of behavior is significantly influenced by strain heterogeneity at a local length scale in the perovskite structure due to the substitution of cations with different ionic radii. The existence of this strain heterogeneity and its influence on the elastic behavior near the transition points could be contributory factors to the development of adaptive nanoscale microstructures and enhanced piezoelectric properties.

Convection in the east Pacific Intertropical Convergence Zone

AbstractThe eastern tropical Pacific exhibits a strong, cross‐equatorial sea surface temperature (SST) gradient, which drives a southerly flow in the atmospheric boundary layer. Convergence in this flow is generally considered to drive deep convection in the east Pacific Intertropical Convergence Zone. However, results from cloud modeling and recent field programs provide an alternative thermodynamic mechanism for controlling this convection. While shallow convection responds to boundary layer convergence, deep convection appears to be controlled by a combination of convective inhibition, surface moist entropy fluxes, tropospheric relative humidity, and moist convective instability. These factors explain the sharp minimum in infrared brightness temperature near 8°N while boundary layer convergence occurs over a much broader range of latitudes.

De Gennes Narrowing and Hard-Sphere Approach

The energy width Δω of the quasielastic coherent dynamic structure factor S(Q, ω) for a simple liquid exhibits the oscillating dependence on wavenumber Q with the sharp minimum at Qmax corresponding to the maximum of the structure factor S(Q). The only known expression for Δω(Q) was derived for a dense hard-sphere (HS) fluid (Cohen et al., Phys. Rev. Lett. 1987, 59, 2872). Though this expression has been frequently used for the analysis of the experimental data obtained for liquid metals, until now, it has never been tested against a true HS fluid. A test performed by means of HS molecular dynamic simulations reveals a considerable discrepancy between the simulations results and the examined model. The main output of the analysis is the finding that the ΔωHS(Q) behavior is defined in terms of the average cage size, ⟨Lc⟩, rather than of the HS diameter, σHS. The simulated ΔωHS(Q) has been compared with the results for the soft-spherical potential. The microscopic dynamics of the soft-sphere fluid shows significant difference in comparison to the HS system. Nevertheless, the diffusive mobility of soft spheres can be characterized within the HS approximation using an effective diameter, σeff, and this parameter can be found from Δω(Q) at Q ≈ Qmax. A similar result has been obtained for the neutron scattering data measured for liquid Rb.

Detecting depinning of polar nanoregions under dc external electric field in canonical relaxor ferroelectric Pb(Mg1/3Ta2/3)O3 via acoustic emission

Canonical relaxor ferroelectric Pb(Mg1/3Ta2/3)O3 single crystals were studied via acoustic emission in temperature range of 180–250 K covering their dielectric constant temperature maximum at Tm ≈ 202 K, and under dc external electric fields up to 1.2 kV cm−1. The dependence of Tm on dc external electric field exhibits a nonlinear behavior, characteristic of relaxors: it initially decreases, attains a sharp minimum at the threshold field of 0.25 kV cm−1 and then starts to increase, as the field enhances. The count rate of the accompanying acoustic emission exhibits two maxima: at 0.1 and 0.6 kV cm−1. The latter obviously corresponds to the end critical point, also characteristic of relaxors, while the former is attributed to a polar nanoregions depinning field at which the polar nanoregions start to be switched off from the intrinsic random electric fields to the dc external electric field. These data are discussed from the viewpoint of competition between random electric fields and an external electric field when affecting the polar nanoregions.

Ultra-low magnetic damping of a metallic ferromagnet.

Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials1-3. This presents a challenge for a broad range of applications in spintronics4 and spin-orbitronics that depend on materials and structures with ultra-low damping5,6. It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10-4, which is comparable to values reported only for ferrimagnetic insulators7,8. We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues3.

Double minimum creep of single crystal Ni-base superalloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.

Variational Analysis on Local Sharp Minima via Exact Penalization

In this paper we study local sharp minima of the nonlinear programming problem via exact penalization. Utilizing generalized differentiation tools in variational analysis such as subderivatives and regular subdifferentials, we obtain some primal and dual characterizations for a penalty function associated with the nonlinear programming problem to have a local sharp minimum. These general results are then applied to the l p penalty function with 0 ≤ p ≤ 1. In particular, we present primal and dual equivalent conditions in terms of the original data of the nonlinear programming problem, which guarantee that the l p penalty function has a local sharp minimum with a finite penalty parameter in the case of $p\in (\frac {1}{2}, 1]$ and $p=\frac {1}{2}$ respectively. By assuming the Guignard constraint qualification (resp. the generalized Guignard constraint qualification), we also show that a local sharp minimum of the nonlinear programming problem can be an exact local sharp minimum of the l p penalty function with p ∈ [0, 1] (resp. $p\in [0, \frac {1}{2}]$ ). Finally, we give some formulas for calculating the smallest penalty parameter for a penalty function to have a local sharp minimum.

The influence of magnetic and structural heterogeneity on thermopower, magnetothermopower, electrical resistivity, and magnetoresistivity of Nd0.5Sr0.5MnO3 manganite

The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd0.5Sr0.5MnO3 single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature TNCE ~ 145 K) and an A-type with TNA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature TC = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to TNCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to TC. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to TNCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.

Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI - the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Delta = 2.5 and 3.5 meV were identified.

Electric field dependences of Curie and Néel phase transition temperatures in magnetoelectric relaxor multiferroic Pb(Fe0.5 Ta0.5 )O3 crystals seen via acoustic emission

Pb(Fe0.5Ta0.5)O3 magnetic and ferroelectric relaxor multiferroic crystals have been studied under both temperature and dc external electric field conditions by means of acoustic emission technique. All the characteristic relaxor points as Burns Td ≈ 610 K and intermediate T* ≈ 500 K temperatures, as well as both Curie ferroelectric cubic-tetragonal Tc1 ≈ 259 K and tetragonal–monoclinic Tc2 ≈ 201 K structural and Neel antiferromagnetic–paramagnetic TN ≈ 187 K phase transitions have been successfully detected. It is shown that, when an external dc electric field is applied, both Tc1 and Tc2 exhibit a nonlinear or V-shape behavior and attain the sharp minima at threshold temperatures Tth1 ≈ 247 K and Tth2 ≈ 200 K with the threshold field Eth ≈ 0.1 kV cm−1, as the field enhances, while their acoustic emission count rate monotonically increases. It is shown, too, that the TN behaves linearly in dependence on dc electric field with a negative slope of about −20 K cm kV−1, while its acoustic emission count rate monotonically decreases. All these data are discussed from a viewpoint of presence of polar nanoregions and magnetoelectric coupling effect in Pb(Fe0.5Ta0.5)O3 crystals.

Physicochemical and functional properties of quinoa protein isolate

This study was focused on the optimum conditions for preparing the protein isolate of quinoa seeds and investigates the physicochemical and functional properties of the isolated protein to assess the potential use of quinoa protein isolate in food applications and manufacturing. The protein isolate of quinoa was obtained by protein solubility at alkaline pH value (10), followed by precipitation at an acidic pH value (4.5). SDS–PAGE showed protein bands with 55KDa corresponding to globulin and 31–33KDa corresponding to chenoprotein in all extraction pHs. Quinoa protein had reasonable concentrations of essential amino acids (except tryptophan) with a high level of lysine (17.13%). A sharp minimum solubility was observed at the pH value (4.5), and the maximum value was observed at the alkaline pH value (10) (P>0.05). Quinoa protein showed a high In Vitro digestibility (78.37±1.08%). The quinoa protein showed water absorption (3.94±0.06ml/g) and (1.88±0.02ml/g) oil absorption. The foaming capacity of quinoa protein isolate was (69.28±9.39% in average) and the foaming capacity was increased with the increase in the protein concentration. Quinoa protein isolate registered 54.54±15.31% foam stability after 60min. Emulsion ability index was ranged from 1.24±0.05m2/g for 0.1% protein suspension to 3.38±0.31m2/g for 3% protein suspension with average 2.10±0.99m2/g. The average of emulsion stability index was (38.43±7.22min). Quinoa protein isolate is a promising and impressive nutritive source, which is leading to candidate it as a food supplement and functional food but still needs more advanced research to improve and proof its functional properties to be convenient for using in food processing and additives.

Effect of Cationic and Zwitterionic Surfactants on Contact Angle of Quartz–Water–Crude Oil System

The influences of four cationic surfactants hexadecyl glycidyl ether ammonium chloride and four zwitterionic surfactants hexadecyl glycidyl ether glycine Betaine solutions on contact angle of crude oil on a quartz surface were investigated using a captive drop method. The effects of surfactant type, structure, and concentration on contact angle were expounded. From obtained results it appears that the adsorbed surfactant at oil–water interface reduces the interfacial tension and the adsorption at quartz–water interface improves interfacial free energy, which results in reducing the stable value of contact angle and weakening dynamic behavior. At high concentration, the zwitterionic surfactant with branched-chain may form semi-micelle at quartz surface. As a result, the stable value of contact angle passes through a sharp minimum with the increasing concentration.

Capacitive properties of nanocomposite (FeCoZr) x (PZT)(100−x) produced by sputtering with the use of argon and oxygen ions beam

The paper established that, the hopping mechanism of the charge exchange for the (FeCoZr)64.4(PZT)35.6 nanocomposite and the additional polarization of tested sample occurs. As frequency increases two frequency areas where capacitance decreases can be observed. Correlation between the conductivity increase and the capacitance decrease has been observed for the both stages of their variability. Voltage resonance phenomenon for the studied material was observed. The zero crossing in the frequency dependence of phase difference transition is accompanied with this phenomenon, which is reflected by a sharp minimum in the capacitance versus frequency characteristics.

Low-temperature thermal conductivity ofDy2Ti2O7andYb2Ti2O7single crystals

We study the low-temperature thermal conductivity (\kappa) of Dy_2Ti_2O_7 and Yb_2Ti_2O_7 single crystals in magnetic fields up to 14 T along the [111], [100] and [110] directions. The main experimental findings for Dy_2Ti_2O_7 are: (i) the low-T \kappa(H) isotherms exhibit not only the step-like decreases at the low-field (< 2 T) magnetic transitions but also obvious field dependencies in high fields (> 7 T); (ii) at T \le 0.5 K, the \kappa(H) curves show anisotropic irreversibility in low fields, that is, the \kappa(H) hysteresis locates at the first-order transition with H \parallel [100] and [110], while it locates between two successive transitions with H \parallel [111]; (iii) the \kappa in the hysteresis loops for H \parallel [100] and [110] show an extremely slow relaxation with the time constant of \sim 1000 min. The main experimental findings for Yb_2Ti_2O_7 are: (i) the zero-field \kappa(T) show a kink-like decrease at the first-order transition (\sim 200 mK) with decreasing temperature; (ii) the low-T \kappa(H) isotherms show a decrease in low field and a large enhancement in high fields; (iii) the low-T \kappa(H) curves show a sharp minimum at 0.5 T for H \parallel [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations and magnetoelastic coupling are discussed.

Dielectric Anomalies in Crystalline Ice: Indirect Evidence of the Existence of a Liquid–Liquid Critical Point in H2O

The phase diagram of H2O is extremely complex, in particular, it is believed that a second critical point exists deep below the supercooled water (SCW) region where two liquids of different densities coexist. The problem however, is that SCW freezes at temperatures just above this hypothesized liquid-liquid critical point (LLCP) so direct experimental verification of its existence has yet to be realized. Here, we report two anomalies in the complex dielectric constant during warming in the form of a peak anomaly near Tp=203 K and a sharp minimum near Tm=212 K from ice samples prepared from SCW under hydrostatic pressures up to 760 MPa. The same features were observed about 4 K higher in heavy ice. Tp is believed to be associated to the nucleation process of metastable cubic ice Ic and Tm the transitioning of ice Ic to either ices Ih or II depending on pressure. Given that Tp and Tm are nearly isothermal and present up to at least 620 MPa and ending as a critical point near 33-50 MPa, it is deduced that two types of SCW with different density concentrations exists which affects the surface energy of ice Ic nuclei in the "no man's land" region of the phase diagram. Our results are consistent with the LLCP theory and suggest that a metastable critical point exists in the region of 33-50 MPa and Tc > 212 K.

Connection of thermopower and giant magnetothermopower with magnetic and structural heterogeneity in Sm0.55Sr0.45MnO3 manganite

It was first shown that the thermoelectric power in a magnetic semiconductor Sm0.55Sr0.45MnO3 is caused by ferromagnetic (FM) ferron-type nanoclusters and antiferromagnetic (AFM) CE-type nanoclusters with charge-orbital (CO) ordering. The presence of these clusters is caused by heavily Sr doping of a SmMnO3 compound, which replaced Sm ions. Thermoelectric power S and magnetothermopower ΔS/S has been studied in three samples: ceramics and two single-crystal samples. One of single-crystals was cooled in the air, another one was cooled in an oxygen atmosphere. The annealing in oxygen closes oxygen vacancies and, thus, increases the fraction of the CE-type AFM phase with the CO ordering which displaces the oxygen ions. S(T) curves of single-crystal sample cooled in the air and ceramic sample have a sharp increase starting from Curie temperature (TC) region and a slow decline till 325K. At the same time {ΔS/S}(T) curves have a sharp minimum near TC=126K, reaching the giant absolute value of 87% in the magnetic field H=14.17kOe. This means that thermopower almost vanish with thermal destruction of FM clusters, i.e. thermopower is caused by these clusters in which crystal lattice is compressed. Oxygen cooled single-crystal's S(T) curves have a broad maximum near 270K, including Neel temperature of CE-type clusters TNCE=240K. {ΔS/S}(T) curve has a sharp minimum at the TNCE reaching the absolute value of 50% in H=13.2kOe. The decrease of S is caused by destruction of CO order displacing oxygen ions in CE-type AFM clusters. Thus changed crystal lattice in nanoclusters of ferron type or AFM of CE-type makes the main contribution in thermopower in these three samples.

Elastic anisotropy and Poisson's ratio of solid helium under pressure

The elastic moduli, elastic anisotropy coefficients, sound velocities and Poisson's ratio of hcp solid helium have been calculated using density functional theory in generalized gradient approximation (up to 30 TPa), and pair + triple semiempirical potentials (up to 100 GPa). Zero-point vibrations have been treated in the Debye approximation assuming $^{4}\mathrm{He}$ isotope (we exclude the quantum-crystal region at very low pressures from consideration). Both methods give a reasonable agreement with the available experimental data. Our calculations predict significant elastic anisotropy of helium ($▵P\ensuremath{\approx}1.14,\phantom{\rule{0.16em}{0ex}}▵{S}_{1}\ensuremath{\approx}1.7,\phantom{\rule{0.16em}{0ex}}▵{S}_{2}\ensuremath{\approx}0.93$ at low pressures). Under terapascal (TPa) pressures helium becomes more elastically isotropic. At the metallization point, there is a sharp feature in the elastic modulus ${C}_{S}$, which is the stiffness with respect to the isochoric change of the $c/a$ ratio. This is connected with the previously obtained sharp minimum of the $c/a$ ratio at the metallization point. Our calculations confirm the previously measured decrease of the Poisson's ratio with increasing pressure. This is not a quantum effect, as the same sign of the pressure effect was obtained when we disregarded zero-point vibrations. At TPa pressures, Poisson's ratio reaches the value of $0.31$ at the theoretical metallization point (${V}_{mol}=0.228\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}$/mol, $p=17.48$ TPa) and $0.29$ at 30 TPa. For $p=0$, we predict a Poisson's ratio of $0.38$, which is in excellent agreement with the low-$p$-low-$T$ experimental data.

Giant Magnetothermopower in Sm&lt;sub&gt;0.55&lt;/sub&gt;Sr&lt;sub&gt;0.45&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; Manganite

Thermopower α and magnetothermopower ∆α/α were studied in the Sm0.55Sr0.45MnO3 samples, containing clusters of three types: ferromagnetic clusters with the Curie temperature TC = 126 K, A-type antiferromagnetic clusters with the Neel temperature TNA ≥ TC and CE-type antiferromagnetic clusters with the TNCE = 240 K. The curves of temperature dependence of α (T) have a large maximum including TC and TNCE and the sharp minimum on the {∆α/α}(T) curves in the TC-region. Negative magnetothermopower in minimum achieves the giant value ~ 85% in magnetic field 14.17 kOe. It is shown that thermopower is largely caused by the presence of ferromagnetic nanoclusters of ferron-type and to a lesser degree of CE-type antiferromagnetic clusters, in which there is a charge ordering, displacing oxygen ions.