Structural Changes of a Doubly Spin‐Labeled Chemically Driven Molecular Shuttle Probed by PELDOR Spectroscopy

Gaining detailed information on the structural rearrangements associated with stimuli‐induced molecular movements is of utmost importance for understanding the operation of molecular machines. Pulsed electron–electron double resonance (PELDOR) was employed to monitor the geometrical changes arising upon chemical switching of a [2]rotaxane that behaves as an acid–base‐controlled molecular shuttle. To this aim, the rotaxane was endowed with stable nitroxide radical units in both the ring and axle components. The combination of PELDOR data and molecular dynamic calculations indicates that in the investigated rotaxane, the ring displacement along the axle, caused by the addition of a base, does not alter significantly the distance between the nitroxide labels, but it is accompanied by a profound change in the geometry adopted by the macrocycle.     

Relativistic Diffusion in Gödel’s Universe

K. Gödel [G1] published his exact solution to Einstein’s field equations in 1949. On the other hand, a general Lorentz invariant operator, associated to the so-called “relativistic diffusion”, and making sense in any Lorentz manifold, was introduced recently by Franchi-Le Jan in [F-LJ]. Here is proposed a study of the relativistic diffusion in the framework of Gödel’s universe, which contains matter. Such study is related to the determination of a boundary for this non-causal universe.     

Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.     

Kirchhoff plates with viscous boundary dissipation

In this paper, we confine our attention to Kirchhoff thin plates in presence of boundary viscoelastic dissipative mechanisms, in order to investigate the well-posedness and the asymptotic behavior within the minimal state approach, following the guidelines proposed in Deseri et al. (Arch Rational Mech Anal 181:43–96, 2006) [see also Fabrizio et al. (Arch Rational Mech Anal 198:189–232, 2010)].     

Homogenization for strongly anisotropic nonlinear elliptic equations

In this paper we present homogenization results for elliptic degenerate differential equations describing strongly anisotropic media. More precisely, we study the limit as e? 0 \epsilon \to 0 of the following Dirichlet problems with rapidly oscillating periodic coefficients:¶¶ . \cases {{ -div(\alpha(\frac{x}{\epsilon}}, \nabla u) A(\frac{x}{\epsilon}) \nabla u) = f(x) \in L^{\infty}(\Omega) \atop u = 0 su \eth\Omega\ } ¶¶where, p > 1,     a: \Bbb Rⁿ ×\Bbb Rⁿ ? \Bbb R,     a(y,x) ? áA(y)x,x?^p/2-1, A ? M^{n ×n}(\Bbb R) p>1, \quad \alpha : \Bbb R^n \times \Bbb R^n \to \Bbb R, \quad \alpha(y,\xi) \approx \langle A(y)\xi,\xi \rangle ^{p/2-1}, A \in M^{n \times n}(\Bbb R) , A being a measurable periodic matrix such that A^t(x) = A(x) 3 0A^t(x) = A(x) \ge 0 almost everywhere.¶¶The anisotropy of the medium is described by the following structure hypothesis on the matrix A:¶¶l^2/p(x) |x|² ￡ áA(x)x,x? ￡ L ^2/p(x) |x|², \lambda^{2/p}(x) |\xi|^2 \leq \langle A(x)\xi,\xi \rangle \leq \Lambda ^{2/p}(x) |\xi|^2, ¶¶where the weight functions l \lambda and L \Lambda (satisfying suitable summability assumptions) can vanish or blow up, and can also be "moderately" different. The convergence to the homogenized problem is obtained by a classical compensated compactness argument, that had to be extended to two-weight Sobolev spaces.     

Nitroxide Radical Spin Probes for Exploring Halogen‐Bonding Interactions in Solution

The synthesis of 2,3,5,6‐tetrafluoro‐4‐iodobenzyl tert‐butyl nitroxide ( 2‐I ) and its use as spin probe for the detection of halogen‐bond (XB) complexes by EPR is reported. Formation of a XB complex between 2‐I and several XB acceptors was evidenced by a significant change in the value of the benzylic hyperfine splitting upon complexation. Thermodynamic parameters for the formation of XB complex with quinuclidine were obtained by recording EPR spectra in the temperature range 203–294 K. In addition, competitive experiments allowed for the measurement of the equilibrium constant of the XB complex with a chloride anion. The proposed procedure constitutes the first direct EPR methodology providing a reliable determination of the strength of the XB bond in solution.     

Quantum dot nanostructures and molecular beam epitaxy

In order to fulfil the requirements of the information society there is a growing demand for nanoelectronic devices with new or largely improved performances; these devices are based on low-dimensional carrier systems, and in particular on zero-dimensional ones, that have peculiar properties as compared to the three- and two-dimensional counterparts.

In this paper we review and discuss the basic features of the Molecular Beam Epitaxy growth of quantum dots that are very interesting archetypes of zero-dimensional nanostructures; quantum dots can be obtained by the three-dimensional growth of self-assembled nanoislands that takes place during the preparation of structures based on highly lattice-mismatched materials. Aspects of the morphological, electronic and optical properties of quantum dots will be reviewed and it will be shown how the energy of confined levels for carriers is determined by design and growth parameters of nanostructures and how quantum dot emission wavelengths can be tuned in the windows of optoelectronic and photonic interest, such as that at 0.98, 1.31 and 1.55 μm. An overview of quantum dot devices will be given, with particular attention paid to the quantum dot laser, unarguably the most important application of quantum dots so far.     

On the behaviour of one-dimensional waves in thermo-viscoelastic fluids

Summary We investigate the behaviour of one-dimensional acceleration waves propagating into a thermo-viscoelastic fluid through a model characterized by constitutive equations with both thermal and viscous relaxation times. The differential equation governing the amplitude of thermomechanical longitudinal waves is shown to be a Bernoulli equation. Waves entering a region at rest in thermal equilibrium are precisely discussed: our results confirm that longitudinal waves are not exceptional. Finally, attention is confined to purely mechanical transverse waves: it is proved that the amplitude of such waves satisfies a linear equation, hence transverse waves propagating into a region at equilibrium are exceptional.

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Sommario Si analizza il comportamento delle onde di accelerazione unidimensionali che si propagano in un fluido termo-viscoelastico caratterizzato da equazioni costitutive con tempi di rilassamento sia termico che viscoso. Si dimostra che l'ampiezza delle onde longitudinali termomeccaniche soddisfa un'equazione differenziale del tipo di Bernoulli. Si esaminano più in dettaglio le onde che entrano in regioni in equilibrio, termico e meccanico: i risultati ottenuti confermano che le onde longitudinali ammesse dalla teoria non sono eccezionali. Infine, si concentra l'attenzione sulle onde trasversali puramente meccaniche: si prova che l'ampiezza di tali onde soddisfa un'equazione lineare, quindi le onde trasversali propagantisi in regioni in equilibrio sono eccezionali.