Sharp Minimum Research Articles

Wetting transition for carbon nanotube arrays under metal contacts.

Structural arrays with nanoscale spacing arise in many device concepts. Carbon nanotube transistors are an extreme example, where a practical technology will require arrays of parallel nanotubes with spacing of order 10nm or less. We show that with decreasing pitch there is a first-order transition, from a robust structure in which the metal wets the substrate between tubes, to a poorly wetting structure in which the metal rides atop the nanotube array without touching the substrate. The latter is analogous to the superhydrophobic "lotus leaf effect." There is a sharp minimum in the delamination energy of metal contacts at the transition pitch. We discuss implications for contact resistance and possible mitigation strategies.

Structural, magnetic and electrical properties of calcium modified bismuth manganite thin films

The present investigation involves the growth of Bi1−xCaxMnO3 (BCMO) thin films with x = 0, 0.1, 0.2, 0.3 and 0.4 on platinized silicon substrates at two different substrate temperatures (400 °C and 800 °C) using RF magnetron sputtering. It also deals with their characterization by employing various techniques: X-ray diffraction analysis (XRD), Atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), Vibrating sample magnetometer (VSM), Leakage, dielectric and ferroelectric measurements. The XRD reveals that the single phasic monoclinic structure of pure BiMnO3 (BMO) is distorted to a small degree with the variation in growth temperatures and the addition of calcium at different concentrations. The composition and valence state of the elements analyzed with XPS confirms the formation of BiMnO3 phase and substitution of Ca ions on Bi site. The room temperature hysteresis loops of BCMO (x = 0.2) films grown at 400 and 800 °C show better magnetic moments of 754 and 858 emu/cc respectively than the values of the other prepared films. The leakage measurements indicate that the leakage current density decreases gradually with the increasing Ca concentration. For both the temperatures, a sharp minimum leakage current density of 5.7 × 10−3 A/cm2 has been observed for BCMO (x = 0.2) films. Moreover, the study evaluates the effects of calcium substitution and growth temperatures on morphology, elemental, dielectric and ferroelectric properties of BMO films.

Vortex dynamics in Co-Fe-B magnetic tunnel junctions in presence of defects

We investigate the frequency of thermally excited vortex oscillations in Co-Fe-B magnetic tunnel junction (MTJ) pillars in the presence of defects. Under a variable in-plane magnetic field, a characteristic behavior is observed: the frequency oscillates from a maximum at certain field values to a steep minimum, which tends towards zero frequency. These frequency variations are described qualitatively well by an analytical model based on the Thiele equation taking into account a single Gaussian pinning potential. It is thus possible to calculate the in-plane depinning field for certain pinning potential parameters. For steep potentials, the depinning is hysteretic and jumps between the pinned and unpinned regime occur due to the presence of an energy barrier. A sharp frequency minimum occurs at an applied field, where a large flat region in the energy landscape is present. From the experiments, the pinning potentials are estimated to be between −0.2eV and −0.4eV. We also perform micromagnetic simulations of the vortex oscillations in the presence of a distribution of pinning centers. The simulations confirm the validity of the Thiele-approach showing that the vortex remains sufficiently rigid.

The effect of annealing on induction like properties of (FeCoZr)x(CaF2)(100−x) nanocomposite films produced by ion-beam sputtering in the vacuum environment

The paper presents frequency f and temperature T dependences of the phase angle θ, conductivity σ, capacity Cp and tgδ for (FeCoZr)x(CaF2)(100−x) nanocomposite films deposited with the application of ion-beam sputtering of a complex target in the gas atmosphere of mixed argon and oxygen. The examined films are composed of metallic FeCoZr “cores” covered with FeCo-based oxide “shells” embedded in an oxygen-free dielectric matrix (fluorite).It has been found that in nanocomposites (FeCoZr)62.7(CaF2)37.3 produced with ion-beam sputtering using combined argon–oxygen ions, the phase angle values change from the negative, through zero, up to the positive ones. It means that the phase angle changes from the capacitive to the inductive type of conduction.Thermo-gravimetric analyses show that nanogranules have metallic phase cores covered by oxides of metallic-phase atoms. The inductive type of nanocomposites conductivity is related to the presence of the oxide cover. Annealing of the nanocomposites at the temperature of 398 K and higher causes an extension of the phase angle change range to an area exceeding +90°. This is due to the fact that during the sputtering of the dielectric CaF2 with argon and oxygen ion beam, some fluorine ions get released and participate in the chemical reaction with surface atoms of nanogranules during the annealing. Such reactions produce a coating composed of oxides and fluorides of the metallic phase on the surface of metallic nanogranules, which results in the extended range of the phase angle changes beyond the area of +90°. In the annealed nanocomposites, frequency dependences of capacity exhibit a sharp minimum which is related to the phase angle transition through zero, as with the voltage resonance phenomena in series RLC circuits. Frequency vs. conductivity characteristics show strong minima, which are related to the phase angle passing through the values of θ = +90°. They correspond to the current resonances in parallel RLC circuits.Relaxation times of the capacity τC and conductivity τσ correspond to the phase angle transition through zero 0° and through +90°. Relaxation time dependences of activation energy on annealing temperature have been determined with the use of the Arrhenius dependences for capacity and conductivity.It has been found that for annealing temperatures higher than 573 K an increase of activation energy occurs, which is related to the oxidation of metal atoms in the core of metallic-phase nanoparticles.

Formation and nature of InGaN quantum dots in GaN nanowires.

InGaN/GaN disk-in-nanowire heterostructures on silicon substrates have emerged as important gain media for the realization of visible light sources. The nature of quantum confinement in the disks is largely unknown. From the unique nature of the measured temperature dependence of the radiative lifetime and direct transmission electron microscopy, it is evident that such self-organized islands (disks) behave as quantum dots. This is confirmed by the observation of single photon emission from a single disk-in-nanowire and the presence of a sharp minimum in the line width enhancement factor of edge emitting lasers having the InGaN disks as the gain media.

Electrical Characterization of Pd‐Doped CMAS‐TiO2 Glass‐Ceramics

Electrical characterization was performed on Pd‐bearing and Pd‐free, electrically conductive glass‐ceramics in the CMAS‐TiO2 system. AC impedance plots clearly revealed both grain‐core and grain‐boundary semicircles. Grain boundaries had a lower magnitude of conductivity but a similar activation energy to grain‐core conduction except when Pd was present, for which much lower activation energies were indicated. The lack of any frequency (f) dependence of conductivity for f < 100 Hz, while using noble metal electrodes, suggests the conduction was dominantly electronic in origin in these glass‐ceramics. Time‐dependent polarization measurements showed no change in electrical current over several days, also consistent with this conclusion. Overall, the temperature dependence of the conductivity was relatively low over a very large temperature range, but quite complex. The appearance of a sharp minimum in conduction seen near 600°C suggests some type of transition occurred with our glass‐ceramics, possibly related to semiconductor‐to‐metal transitions seen with some transition‐metal crystals. The available data suggest that Pd did have some effect on grain‐boundary conduction, but that a percolating Pd network was not formed.

Influence of the magnetic and structural heterogeneities on the thermopower, magnetothermpower, and spontaneous generation of electric voltage in the manganite Sm0.55Sr0.45MnO3

The thermopower α and magnetothermopower Δα/α have been studied on single-crystal Sm0.55Sr0.45MnO3 samples consisting of three types of magnetic clusters (ferromagnetic clusters with the Curie temperature TC = 134 K, the A-type antiferromagnetic clusters with the Neel temperature TNA ≥ TC, and the CE-type antiferromagnetic clusters with TNCE = 240 K). The temperature dependences of α and Δα/α have extremes in the vicinity of TNCE, namely, a wide maximum in curve α(T) and a sharp minimum in curve{Δα/α}(T). The negative magnetothermopower in the minimum has a giant value of 50% in the magnetic field of 13.2 kOe. The thermopower is shown to be mainly due to the existence of the CE-type antiferromagnetic clusters, in which there is a charge-orbital ordering that displaces the oxygen atoms. The changed crystal lattice inside these clusters changes the value of the thermopower inside them. This thermopower influences the voltage drop across the sample during measuring the thermopower and, thus, the effective value of α of the whole sample. The application of a magnetic field near TNCE accelerating the destruction of the CE-type antiferromagnetic order causes the sharp decrease in the thermopower of the whole sample. This implies that the CE-type antiferromagnetic clusters with the charge-orbital ordering make main contribution to the thermopower of the whole sample.

The Influence of Magnetic Inhomogeneous State on Thermopower and Magnetothermopower in Sm<sub>0.55</sub>Sr<sub>0.45</sub>MnO<sub>3</sub> Manganites

Thermopower α and magnetothermopower α/α were studied in the single-crystal Sm0.55Sr0.45MnO3 samples, containing clusters of three types: ferromagnetic clusters with the Curie temperature TC = 134 K, A-type antiferromagnetic clusters with the Neel temperature TNATC and CE-type antiferromagnetic clusters with the TNCE = 240 K. The curves of temperature dependence of α (T) and {α/α}(T) have extrema in the TNCE-region: large maximum on the first and sharp minimum on the second. Negative magnetothermopower in minimum achieves the giant value 50% in magnetic field 13,2 kOe. It is shown that thermopower is essentially caused by the presence of CE-type antiferromagnetic clusters, in which exists charge order, displacing oxygen ions

Thermoelectric properties of single- and polycrystalline RuGa3

The thermoelectric properties of the intermetallic semiconductor RuGa3 are investigated. Large single crystals were grown to study intrinsic properties. To investigate the influence of grain boundaries in this system, the single crystals were ground to powder and densified using spark plasma sintering treatment. The initial chemical composition is maintained with the introduction of grain boundaries. Electrical resistivity data show semiconducting behavior for single- and polycrystalline samples. The high thermal conductivity (>500 W K−1 m−1) obtained for single crystals at low temperatures is reduced by a factor of 10 in the polycrystalline specimen. The thermopower shows a change between n-type and p-type behavior with a sharp minimum of about −700 μV K−1 at 38 K for single crystals, which is completely suppressed by the introduction of grain boundaries. A comparison with RuIn3 shows the potential of RuGa3 as a thermoelectric material in its single- and polycrystalline form.

Morphology-dependent resonances of spherical droplets with numerous microscopic inclusions

We use the recently extended superposition T-matrix method to study the behavior of a sharp Lorenz-Mie resonance upon filling a spherical micrometer-sized droplet with tens and hundreds of randomly positioned microscopic inclusions. We show that as the number of inclusions increases, the extinction cross-section peak and the sharp asymmetry-parameter minimum become suppressed, widen, and move toward smaller droplet size parameters, while ratios of diagonal elements of the scattering matrix exhibit sharp angular features indicative of a distinctly nonspherical particle. Our results highlight the limitedness of the concept of an effective refractive index of an inhomogeneous spherical particle.

Generalized weak sharp minima in cone-constrained convex optimization on Hadamard manifolds

In this paper, a convex optimization problem with cone constraint (for short, CPC) is introduced and studied on Hadamard manifolds. Some criteria and characterizations for the solution set to be a set of generalized global weak sharp minima, generalized local weak sharp minima and generalized bounded weak sharp minima for (CPC) are derived on Hadamard manifolds.

The Influence of Magnetic Inhomogeneous State on Thermopower and Magnetothermopower in Sm0.55Sr0.45MnO3Manganites

Thermopower α and magnetothermopower ∆α/α were studied in the single-crystal Sm 0.55 Sr 0.45 MnO 3 samples, containing clusters of following three types: ferromagnetic clusters with the Curie temperature T C = 134 K, A-type antiferromagnetic clusters with the Neel temperature T NA ≤ T C and CE-type antiferromagnetic clusters with the T NCE = 240 K. Temperature dependence of α and ∆α/α have extrema in the T NCE -region: large maximum on α (T) and sharp minimum on {∆α/α} (T) . Negative magnetothermopower in minimum achieves the giant value 50% in magnetic field H = 13,2 kOe. As will be shown below thermopower is essentially caused by the presence of CE-type antiferromagnetic clusters, in which exists charge order (CO), displacing oxygen ions. Modified crystalline lattice inside clusters causes change of thermopower in them. This thermopower influences on the voltage drop on sample at measurement of thermopower and, consequently, on the effective value α of the whole sample. Enclosure of the magnetic field in the T NCE , which accelerates the destruction of the CE-type antiferromagnetic order, causes sharp decrease of total thermopower. This means that the CE-type antiferromagnetic clusters with CO order are the main contributors to the thermopower of the whole sample.

Error Bounds and Finite Termination for Constrained Optimization Problems

We present a global error bound for the projected gradient of nonconvex constrained optimization problems and a local error bound for the distance from a feasible solution to the optimal solution set of convex constrained optimization problems, by using the merit function involved in the sequential quadratic programming (SQP) method. For the solution sets (stationary points set andKKTpoints set) of nonconvex constrained optimization problems, we establish the definitions of generalized nondegeneration and generalized weak sharp minima. Based on the above, the necessary and sufficient conditions for a feasible solution of the nonconvex constrained optimization problems to terminate finitely at the two solutions are given, respectively. Accordingly, the results in this paper improve and popularize existing results known in the literature. Further, we utilize the global error bound for the projected gradient with the merit function being computed easily to describe these necessary and sufficient conditions.

Mollow triplet for cavity-mediated laser cooling

Here we analyse cavity-mediated laser cooling for an experimental setup with an external trap which strongly confines the motion of a particle in the direction of the cavity axis. It is shown that the stationary state phonon number exhibits three sharp minima as a function of the atom-cavity detuning due to a direct atom-phonon-photon coupling term in the system Hamiltonian. These resonances have the same origin as the Mollow triplet in the resonance fluorescence of a laser-driven atomic system. It is shown that a laser-Rabi frequency-dependent atom-cavity detuning yields the lowest stationary state phonon number for a wide range of experimental parameters.

Spin reorientation tuned by oxygen content in the double perovskite Sr2YRuO6

Sr2YRuO6 is a double perovskite with a complex magnetic state, including non-collinear arrangement of the ruthenium moments and spin re-orientation. One open issue is the possibility of magnetization reversal leading to an apparent diamagnetic response. In this work we show that when the oxygen content is reduced, the susceptibility curve develops a sharp minimum, in contrast to the broad maximum observed for a well-oxygenated sample. However, the magnetization remains positive regardless of the thermomagnetic history. We explain this effect in terms of the reversal of a net ferromagnetic moment in opposition to the applied field and by a spatial average of the contribution of the grains in the polycrystalline sample. The reversal is observed in deoxygenated samples because the delicate balance between competitive ferromagnetic and antiferromagnetic interactions is altered. The significant reduction of the antiferromagnetic correlations at high temperatures, as evidenced by the magnetic contribution to the specific heat, is consistent with this scenario. The role of the oxygen content is conclusively confirmed by reversibility after re-oxygenation. Our results suggest that Sr2YRuO6−x might host inhomogeneous superconductivity due to local enhancement of spin fluctuations in the neighborhood of the oxygen vacancies.

Simulation of Series Arrays of Superconducting Quantum Interference Devices

The voltage as a function of applied magnetic field (V-B) was calculated for arrays of superconducting quantum interference devices (SQUIDs) connected in series. Comparisons were made between arrays of equal area SQUIDs and superconducting quantum interference filters (SQIFs). The areas for the SQIFs were varied exponentially, so that the V-B had a sharp minimum at zero field. We used equations for the dc SQUID based on resistively shunted junctions, with typical parameters for YBa2Cu3O7 - δ ion damage Josephson junctions. The maximum transfer coefficient of the central minimum VB = (∂V/∂B)max of the SQIF decreases as the area range increases. We find that the equal area array is more robust to the effects of non-uniform junction critical currents than the SQIF, for the junction parameters and SQUID area distributions chosen. Furthermore, we find that slight variations (~5%) to the area due to fabrication irregularities have little effect on the central minimum of V-B for either device.

Destructive impact of imperfect beam collimation in extraordinary optical transmission

We investigate the difference between analytic predictions, numerical simulations, and experiments measuring the transmission of energy through subwavelength, periodically arranged holes in a metal film. At normal incidence, theory predicts a sharp transmission minimum when the wavelength is equal to the periodicity, and sharp transmission maxima at one or more nearby wavelengths. In experiments, the sharpest maximum from the theory is not observed, while the others appear less sharp. In numerical simulations using commercial electromagnetic field solvers, we find that the sharpest maximum appears and approaches our predictions as the computational resources are increased. To determine possible origins of the destruction of the sharp maximum, we incorporate additional features in our model. Incorporating imperfect conductivity and imperfect periodicity in our model leaves the sharp maximum intact. Imperfect collimation, on the other hand, incorporated into the model causes the destruction of the sharp maximum as happens in experiments. We provide analytic support through an asymptotic calculation for both the existence of the sharp maximum and the destructive impact of imperfect collimation.

Spin Structures of the Hubbard Clusters and the Pseudogap

The spin and charge structures formed in the Hubbard model for a finite two-dimensional cluster have been studied in the mean field approximation. The self-consistent iterative procedure reduces an uncorrelated initial spin distribution into stable structures with spectral properties typical of stripes. It is shown that the density of states of the system for any doping has a sharp minimum at the Fermi level, the pseudogap. The pinning of the gap at the Fermi level is characteristic not only of a superconducting state, but also typical of a normal state of spin glasses. Our results support conception of the PG state as a state with frozen locally nematic spin structures of antiferromagnetic domains.

Setting the boundaries of prior influence on kinship relation testing: the case of many hypotheses

Kinship relation and, in particular, paternity probability estimation using a Bayesian approach require the input of a priori probabilities of different hypotheses. In practical case work, a priori probabilities or priors, for short, must often be estimated using only common sense and symmetry arguments because in most cases, there is no evidence-based information on which the priors may be determined. In contrast to the accuracy of the likelihood probabilities or the likelihood ratios, the precision of the priors is usually very poor. Thus, a quantitative estimation of the priors' influence on the paternity probability is desirable. This article presents exact formulae to define sharp minimum and maximum boundaries of posterior probabilities as a function of prior boundaries which may be applied in kinship cases with varying numbers of hypotheses and also presents two case examples.

Peculiarities of Magnetocaloric Effect in Manganites Connected with Magnetic Heterogeneous State

Magnetocaloric effect (MCE) or T-effect was studied in La1-xSrxMnO3, Sm0.55Sr0.45MnO3 and PrBaMn2O6 manganites. It has been found that the maximum value of MCE, measured by direct method, is far less than obtainable by computation from the change of the magnetic entropy in Curie temperature TC. This phenomenon is explained by presence in them of the magnetic two-phase ferromagnetic (F) – antiferromagnetic (AF) state. So in La1-xSrxMnO3 the negative contribution from AF portion of sample lowers MCE and displacesmaximum on T(T)-curve to the higher temperature than TC on 20-40 K. Maximum on T(T) curve of Sm0.55Sr0.45MnO3 is disposed near TC=134 K in the cooling in the air single-crystal and ceramic samples. The cooling inoxygen of single-crystal,restored the Mn-O-Mn broken connections, increases the volume of clusters with CE-type of AF order, results to that maximum on T(T) curve is disposed at Neel temperature TN of this phase (243 K). Magnetic field, applied to sample during T-measurement, transforms AF clusters into F state and theboth types of clusters decompose at TN. The PrBaMn2O6 manganite has two phase transactions: paramagnetic–F at TC=295 K and F–AF at TN=231 K. Phase with spontaneous magnetization obtains the understated magnetic moment. Curve T(T) has the wide maximum near TC and the sharp minimum near TN with the small T-values in both extremes: 0,13 K and - 0,2 K respectively. This is connected with presence of AF interactions in F phase and F interactions in AF phase.