Spectroscopic detection of biological NO with a quantum cascade laser

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near λ=5.2 μm. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO₂ concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed. Received: 1 November 2000 / Revised version: 19 January 2001 / Published online: 20 April 2001     

Magnetic anisotropy and domain patterns in electrodeposited cobalt nanowires

The magnetic anisotropy and domain structure of electrodeposited cylindrical Co nanowires with length of 10 or 20 μm and diameters ranging from 30 to 450 nm are studied by means of magnetization and magnetic torque measurements, as well as magnetic force microscopy. Experimental results reveal that crystal anisotropy either concurs with shape anisotropy in maintaining the Co magnetization aligned along the wire or favours an orientation of the magnetization perpendicular to the wire, hence competing with shape anisotropy, depending on whether the diameter of the wires is smaller or larger than a critical diameter of 50 nm. This change of crystal anisotropy, originating in changes in the crystallographic structure of Co, is naturally found to strongly modify the zero (or small) field magnetic domain structure in the nanowires. Except for nanowires with parallel-to-wire crystal anisotropy (very small diameters) where single-domain behaviour may occur, the formation of magnetic domains is required to explain the experimental observations. The geometrical restriction imposed on the magnetization by the small lateral size of the wires proves to play an important role in the domain structures formed. Received 14 September 2000     

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Single crystal RFe₂(110) films were grown by molecular beam epitaxy to a total thickness of 1000 ? at different substrate temperatures ranging from 450 ^° C to 660 ^° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe₂(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound. Received 29 June 2000     

Infrared transmission study of Pr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>CuO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript> crystal-field excitations

We present an infrared crystal-field study of Pr₂CuO₄ single crystals and thin films. Excitations from the ground state multiplet ³H₄ to the ³H₅, ³H₆, ³F₂ and ³F₃ excited multiplets are observed in both single crystals and thin films. A precise set of crystal-field parameters, that reproduces the energy and the symmetry of the levels, is determined. Received 25 April 2001     

The origin of polarized blackbody radiation from resistively heated multiwalled carbon nanotubes

We observed very pronounced polarization of light emitted by highly aligned free-standing multiwall carbon nanotube (MWNT) sheet in axial direction which is turned to the perpendicular polarization when a number of layers are increased. The radiation spectrum of resistively heated MWNT sheet closely follows to the Plank's blackbody radiation distribution. The obtained polarization features can be described by a classical dielectric cylindrical shell model, taking into consideration the contribution of delocalized π-electrons (π surface plasmons). In absorption (emission) the optical transverse polarizability, which is much smaller than longitudinal one, is substantially suppressed by depolarization effect due to screening by induced charges. This phenomenon suggests very simple and precise method to estimate the alignment of nanotubes in bundles or large assemblies.     

Rényi and Tsallis entropies for incomplete or overcomplete systems of events

In this article, Shannon, Rényi and Tsallis entropies are considered for a system of events characterized by an arbitrary probability distribution P that can be incomplete, complete or overcomplete. After a suitable transformation that leads to the escort probabilities of P, these can be written as the canonical probability distribution for a set of pseudo-energies (Hartley information, E_n=−lnP_n) and a dimensionless parameter q that plays the role of thermodynamics β. Several relations between the entropies are presented, including the analysis of compound systems. The method is illustrated with an example.     

A comparison study of scratch and wear properties using atomic force microscopy

The patterning technique that uses an AFM (atomic force microscopy) tip as a scratch tool, also known as AFM scratching, has been a vital technique for nanofabrication because of its low cost and potential to reach a resolution into the sub-nanometer domain. The AFM scratching technique was first used to study the scratch characteristics of silicon, with an emphasis on establishing its scratchability or the nanoscale machinability. The effects of the scratch parameters, including the applied tip force and number of scratches, on the size of the scratched geometry were specifically evaluated. The primary property that measures the scratchability was identified and assessed. To illustrate its suitability and reliability, the value of the scratchability, based on the present Si scratching experiments, was compared with the values based on the data available in the literatures for different scratching conditions or for materials other than Si. Since AFM scratching is in some aspects similar to the nanoscale wear test, the scratchability property identified is also compared with two major wear resistance indicators, wear coefficient and hardness. All comparison results indicate that the scratchability property identified, the scratch ratio, is an appropriate manufacturability indicator for measuring the degree of the ease or difficulty of a material scratched by an AFM tip and more suitable than the wear coefficient and hardness to gauge the nanoscale AFM scratchability.     

Effect of allied and alien ions on the EPR spectrum of Mn-containing lithium-manganese spinel oxides

Electron paramagnetic resonance spectroscopy was used for studying the effect of allied and alien ions on the EPR spectrum of Mn⁴⁺-containing lithium-manganese spinel oxides. Manganese spinel oxides with paramagnetic Mn⁴⁺ and diamagnetic substituents in the 16d spinel sites were studied: Li[Mg_0.5Mn_1.5]O₄, Li[Mg_0.5−xCo_2xMn_1.5−x]O₄, 0<x≤0.5, and Li[Li_1/3Mn_5/3]O₄. Ni²⁺-ions with integer-spin-ground state (S=1) were selected as alien ions: Li[Mg_0.5−xNi_xMn_1.5]O₄ (0≤x≤0.5), Li[Li_(1−2x)/3Ni_xMn_(5−x)/3]O₄ (0≤x≤0.5), and Li[Ni_0.5Mn_1.5−yTi_y]O₄ (0≤y≤1.0). It was shown that in Ni-substituted oxides the low temperature EPR response comes from magnetically correlated Ni-Mn spins, while at high registration temperature Mn⁴⁺ ions give rise to the EPR profile. Analysis of the EPR line width allows differentiating between the contributions of the density of paramagnetic species and the strength of the exchange interactions in magnetically concentrated systems. The density of allied and alien paramagnetic species has no effect on the EPR line width in cases when the strengths of antiferro- and ferromagnetic interactions on an atomic site are close. On the contrary, when antiferro- or ferromagnetic interactions on an atomic site are dominant, the EPR line width increases with the density of paramagnetic species.     

Thermodynamics of the Casimir effect

A complete thermodynamic treatment of the Casimir effect is presented. Explicit expressions for the free and the internal energy, the entropy and the pressure are discussed. As an example we consider the Casimir effect with different temperatures between the plates (T) resp. outside of them (T'). For T'<T the pressure of heat radiation can eventually compensate the Casimir force and the total pressure can vanish. We consider both an isothermal and an adiabatic treatment of the interior region. The equilibrium point (vanishing pressure) turns out instable in the isothermal case. In the adiabatic situation we have both an instable and a stable equilibrium point, if T'/T is sufficiently small. Quantitative aspects are briefly discussed. Received 24 February 1999 and Received in final form 26 April 1999     

Magnetic properties of ErxY1−xF3 solid solutions

The Er_xY_1−xF₃ (x=0.1, 0.2, 0.7, 0.9, 1) solid solutions were synthesized and characterized by X-ray powder diffraction and magnetic measurements. The crystal structure refinements done by the Rietveld profile method show that no significant change of the structure parameters with the erbium concentration occurs. On the basis of DC susceptibility measurements in the 2-300 K range the lowest four crystal field levels have been determined, giving the ground level magnetic moment value of 6.7 μ_B. Results of M(H) studies point to the presence of complex exchange interactions between erbium ions.