196,305 research outputs found
A density-functional study of heterometallic Cr-based molecular rings
No full textWe present a density-functional theoretical investigation of the electronic and magnetic properties of octametallic Cr-based molecular antiferromagnetic rings. The presence of the divalent magnetic ion M unbalances the charge and the spin of the parent Cr8 ring, leading to a finite total spin in the molecules. Results are presented for Cr8, i.e., [Cr8F 8(O2CH)16] (1), and for Cr7M rings belonging to two different derivatives, i.e., [Me2NH 2][Cr7MF8(O2CH)16], with M = Ni, Mn, Fe, and Cu, and Me=CH3 (2, "green" derivative), and [Cr7 NiF3(C6H10NO 5)(O2CH)15 (H2O)] (3, "purple" derivative). Exchange interaction parameters have been extracted from broken-symmetry calculations and compared with the available experiments; in agreement with them, we find that exchange parameters are rather similar in the two derivatives, although somewhat larger in the "purple" derivative. The analysis of the electronic properties shows some differences depending on M, in particular in the size of the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps. For all the "green" rings we observe that the HOMOs are localized on the divalent ion site, while the HOMOs for the "purple" Cr7Ni have a more delocalized nature; LUMOs, instead, are, except for "green" Cr7Cu, localized on the Cr atoms opposite to the M site. We discuss how these findings may show up in terms of an asymmetric I-V curve in molecular junctions working in the sequential tunneling regime, or help in discerning the orientation of the molecules with respect to a surface, in scanning tunneling experiments. © 2010 American Chemical Society
RELIABILITY OF THE SKELETOCHRONOLOGICAL METHOD IN CARETTA CARETTA FROM THE ADRIATIC SEA.
No full textEffects of Al doping on the normal and superconducting properties of MgB2: A specific heat study
No full textEffects of Al doping on the normal and superconducting properties of MgB2: a specific heat study.The effects of Mg substitution by Al on the specific heat of MgB2 have been studied in the normal and in the superconducting state. The Sommerfeld coefficient, evaluated by measurements in magnetic field up to 7 T, progressively decreases from gamma=3.0+/-0.2 mJ/mol K-2 for undoped MgB2 down to gamma=1.9+/-0.2 mJ/mol K-2 for Mg0.6Al0.4B2 as a result of the change of the density of states and of the electron-phonon coupling constants. The superconducting contribution to the specific heat c(sc) clearly shows an excess at low temperature compared with conventional single-gap BCS behavior also for doped samples with x up to 0.3. The two-band model has been used to fit the temperature dependence of c(sc) and the amplitude of the two gaps has been evaluated for different Al concentrations. The changes of the energy gaps are in a rather poor agreement with those predicted by taking into account changes in the electronic and phononic structure only. We suggest that disorder also plays an important role when Al substitutes Mg
Microwave photon detectors based on semiconducting double quantum dots
No full textDetectors of microwave photons find applications in different fields ranging from security to cosmology. Due to the intrinsic difficulties related to the detection of vanishingly small energy quanta ̄hω, significant portions of the microwave electromagnetic spectrum are still uncovered by suitable techniques. No prevailing technology has clearly emerged yet, although different solutions have been tested in different contexts. Here, we focus on semiconductor quantum dots, which feature wide tunability by external gate voltages and scalability for large architectures. We discuss possible pathways for the development of microwave photon detectors based on photon-assisted tunneling in semiconducting double quantum dot circuits. In particular, we consider implementations based on either broadband transmission lines or resonant cavities, and we discuss how developments in charge sensing techniques and hybrid architectures may be beneficial for the development of efficient photon detectors in the microwave range
Microwave dual-mode resonators for coherent spin-photon coupling
No full textWe implement superconducting Yttrium barium copper oxide planar resonators with two fundamental modes for circuit quantum electrodynamics experiments. We first demonstrate good tunability in the resonant microwave frequencies and in their interplay, as emerges from the dependence of the transmission spectra on the device geometry. We then investigate the magnetic coupling of the resonant modes with bulk samples of 2,2-diphenyl-1-picrylhydrazyl organic radical spins. The transmission spectroscopy performed at low temperature shows that the coherent spin-photon coupling regime with the spin ensembles can be achieved by each of the resonator modes. The analysis of the results within the framework of the input-output formalism and by means of entropic measures demonstrates coherent mixing of the degrees of freedom corresponding to two remote spin ensembles and, with a suitable choice of the geometry, the approaching of a regime with spin-induced mixing of the two photon modes
Magnetothermal properties of molecule-based materials
No full text16 pages, 14 figures.-- El pdf del artículo es la versión post-print.We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecule-based materials. After introducing the experimental techniques and basic theoretical framework needed for heat capacity determination and understanding, we report on the magnetothermal properties of molecular antiferromagnetic wheels. For selected molecular high-spin clusters, particular emphasis is devoted to magnetic quantum tunnelling and coherence as well as collective phenomena as probed by heat capacity experiments. We discuss also the
possibilities of application of molecule-based materials for magneto-cooling at low temperatures and the limitations in other temperature ranges. Perspectives for future developments are mentioned as well.Special thanks go to J. C. Lasjaunias, P. Gandit, J. Chaussy and I. Sheikin (CNRS, Grenoble) and M. A. Novak who contributed much to the development of this field. This work was carried out within the framework of the EC-Network of Excellence ‘‘MAGMANet’’ (No. 515767), WorkPackages 2, 9 and 10.Peer reviewe
Lattice effects in the ferromagnetic insulating phase of manganites
No full textThe origin of the ferromagnetic insulating (FMI) phase observed at low temperature in a large variety of manganites is addressed by using x-ray absorption spectroscopy. The variations in the Mn L2,3 and O K-edge x-ray absorption and circular dichroism spectra as a function of temperature are consistent with a carrier localization and localized lattice distortions associated with this phase
Estrazione , accelerazione, condizionamento di correnti di elettroni da plasma generato da catodo cavo di opportuna costruzione e loro impiego in apparecchiature e sistemi industriali di irraggiamento di elettroni
No full textEstrazione , accelerazione, condizionamento di correnti di elettroni da plasma generato da catodo cavo di opportuna costruzione e loro impiego in apparecchiature e sistemi industriali di irraggiamento di elettron
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