Results 1 - 10 of 14941
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[en] We studied the temperature dependent magnetic phase evolution in spin frustrated TbMnO_3 affected by Fe doping via powder neutron diffraction. With the introduction of Fe (10% and 20%), the long range incommensurate magnetic orderings collapse. When the Fe content is increased to 30%, a long-range antiferromagnetic ordering develops, while a spin reorientation transition is found near 35 K from a canted G-type antiferromagnetic ordering to a collinear G-type antiferromagnetic ordering. This work demonstrates the complex magnetic interactions existing in transition metal oxides, which helps to understand the frustrated spin states in other similar systems and design magnetic materials as well.
[en] We have found experimentally that the rise time of voltage pulse in NbN superconducting single photon detectors increases nonlinearly with increasing the length of the detector L. The effect is connected with dependence of resistance of the detector R_n, which appears after photon absorption, on its kinetic inductance L_k and, hence, on the length of the detector. This conclusion is confirmed by our calculations in the framework of two temperature model.
[en] The intrinsic carrier transport dynamics in phosphorene is theoretically examined. Utilizing a density functional theory treatment, the low-field mobility and the saturation velocity are characterized for both electrons and holes in the monolayer and bilayer structures. The analysis clearly elucidates the crystal orientation dependence manifested through the anisotropic band structure and the carrier-phonon scattering rates. In the monolayer, the hole mobility in the armchair direction is estimated to be approximately five times larger than in the zigzag direction at room temperature (460 cm"2/V s vs. 90 cm"2/V s). The bilayer transport, on the other hand, exhibits a more modest anisotropy with substantially higher mobilities (1610 cm"2/V s and 760 cm"2/V s, respectively). The calculations on the conduction-band electrons indicate a comparable dependence while the characteristic values are generally smaller by about a factor of two. The variation in the saturation velocity is found to be less pronounced. With the anticipated superior performance and the diminished anisotropy, few-layer phosphorene offers a promising opportunity particularly in p-type applications.
[en] Full, reversible intercalation of two Li"+ has not yet been achieved in promising VOPO_4 electrodes. A pronounced Li"+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO_4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li"+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li"+ intercalation is a prerequisite for the formation of intermediate phases Li_1_._5_0VOPO_4 and Li_1_._7_5VOPO_4. The evolution from LiVOPO_4 to Li_2VOPO_4 via the intermediate phases is confirmed by direct comparison between O K–edge absorption spectroscopy and density functional theory.
[en] We present a comprehensive study of the In_2O_3 growth kinetics during plasma-assisted molecular beam epitaxy and compare it to that of the related oxide Ga_2O_3 [P. Vogt and O. Bierwagen, Appl. Phys. Lett. 108, 072101 (2016)]. The growth rate and desorbing fluxes were measured during growth in-situ by a laser reflectometry set-up and line-of-sight quadrupole mass spectrometer, respectively. We extracted the In incorporation as a function of the provided In flux, different growth temperatures T_G, and In-to-O flux ratios r. The data are discussed in terms of the competing formation of In_2O_3 and desorption of the suboxide In_2O and O. The same three growth regimes as in the case of Ga_2O_3 can be distinguished: (i) In-transport limited, O-rich (ii) In_2O-desorption limited, O-rich, and (iii) O-transport limited, In-rich. In regime (iii), In droplets are formed on the growth surface at low T_G. The growth kinetics follows qualitatively that of Ga_2O_3 in agreement with their common oxide and suboxide stoichiometry. The quantitative differences are mainly rationalized by the difference in In_2O and Ga_2O desorption rates and vapor pressures. For the In_2O, Ga_2O, and O desorption, we extracted the activation energies and frequency factors by means of Arrhenius-plots.
[en] The thermal conductivity of low-dimensional materials and graphene nanoribbons, in particular, is limited by the strength of line-edge-roughness scattering. One way to characterize the roughness strength is the dependency of the thermal conductivity on the channel's width in the form W"β. Although in the case of electronic transport, this dependency is very well studied, resulting in W"6 for nanowires and quantum wells and W"4 for nanoribbons, in the case of phonon transport it is not yet clear what this dependence is. In this work, using lattice dynamics and Non-Equilibrium Green's Function simulations, we examine the width dependence of the thermal conductivity of ultra-narrow graphene nanoribbons under the influence of line edge-roughness. We show that the exponent β is in fact not a single well-defined number, but it is different for different parts of the phonon spectrum depending on whether phonon transport is ballistic, diffusive, or localized. The exponent β takes values β < 1 for semi-ballistic phonon transport, values β ≫ 1 for sub-diffusive or localized phonons, and β = 1 only in the case where the transport is diffusive. The overall W"β dependence of the thermal conductivity is determined by the width-dependence of the dominant phonon modes (usually the acoustic ones). We show that due to the long phonon mean-free-paths, the width-dependence of thermal conductivity becomes a channel length dependent property, because the channel length determines whether transport is ballistic, diffusive, or localized.
[en] Lubricants play important roles in daily activities such as driving, walking, and cooking. The current understanding of mechanisms of lubrication, particularly in mechanical systems, has been limited by the lack of capability in direct observation. Here, we report an in situ approach to directly observe the motion of additive particles in grease under the influence of shear. Using the K-edge tomography technique, it is possible to detect particular additives in a grease and observe their distribution through 3D visualization. A commercial grease as a reference was studied with and without an inorganic additive of Fe_3O_4 microparticles. The results showed that it was possible to identify these particles and track their movement. Under a shear stress, Fe_3O_4 particles were found to adhere to the edge of calcium complex thickeners commonly used in grease. Due to sliding, the grease formed a film with increased density. This approach enables in-line monitoring of a lubricant and future investigation in mechanisms of lubrication.
[en] Silicon nanostructures with reduced dimensionality, such as nanowires, membranes, and thin films, are promising thermoelectric materials, as they exhibit considerably reduced thermal conductivity. Here, we utilize density functional theory and Boltzmann transport equation to compute the electronic properties of ultra-thin crystalline silicon membranes with thickness between 1 and 12 nm. We predict that an optimal thickness of ∼7 nm maximizes the thermoelectric figure of merit of membranes with native oxide surface layers. Further thinning of the membranes, although attainable in experiments, reduces the electrical conductivity and worsens the thermoelectric efficiency.
[en] Some group III elements such as Indium are known to produce the resonant impurity states in IV-VI compounds. The discovery of these impurity states has opened up new ways for engineering the thermoelectric properties of IV-VI compounds. In this work, resonant states in SnTe were studied by co-doping with both resonant (In) and extrinsic (Ag, I) dopants. A characteristic nonlinear relationship was observed between the Hall carrier concentration (n_H) and extrinsic dopant concentration (N_I, N_A_g) in the stabilization region, where a linear increase of dopant concentration does not lead to linear response in the measured n_H. Upon substituting extrinsic dopants beyond a certain amount, the n_H changed proportionally with additional dopants (Ag, I) (the doping region). The Seebeck coefficients are enhanced as the resonant impurity is introduced, whereas the use of extrinsic doping only induces minor changes. Modest zT enhancements are observed at lower temperatures, which lead to an increase in the average zT values over a broad range of temperatures (300–773 K). The improved average zT obtained through co-doping indicates the promise of fine carrier density control in maximizing the favorable effect of resonant levels for thermoelectric materials.
[en] We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors. To this purpose, we fabricated a 250 nm wide and 250 nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from λ = 900 nm to λ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for total absorbed energies >0.8 eV the current–energy relation is linear, similar to what was observed in NbN nanowires, whereas the current–energy relation deviates from linear behavior for total energies below 0.8 eV.