401 research outputs found
Enrico Calandra. Ritratto di un architetto
No full textSu Enrico Calandra (1877-1946) ha pesato un lungo silenzio storiografico, interrotto solo sporadicamente da voci che provengono dal passato. Le memorie dei suoi allievi – tra questi Giuseppe Samonà, Lina Bo, Bruno Zevi – ci consegnano il ritratto di un maestro in anni che di maestri sono assai poveri. Attraverso lettere, scritti e disegni emersi dall’archivio dell’architetto, questo libro restituisce in un’unica immagine i molti volti di Enrico Calandra: quello del giovane architetto alla ricerca della modernità, quello del maestro amatissimo dagli allievi più diversi, quello dello storico dell’architettura impegnato nella definizione dei fondamenti di una disciplina che da allora, grazie anche ai suoi studi, appartiene agli architetti. Le tracce di una storia individuale e familiare che ha origine nella Sicilia di fine Ottocento ci conducono lontano verso una storia dell’architettura – e non solo – che racconta l’Italia negli anni difficili delle guerre mondiali e del regime fascista
Emberiza calandra
No full textRecopilación de observaciones de ecología general, anidamiento y cría del triguero (Miliaria calandra, nombrada por el autor como Emberiza calandra) realizadas durante varias salidas a diferentes enclaves de la provincia de Valladolid, Magaz de Pisuerga (Palencia), Toro (Zamora) y Burgos, entre el 7 de julio de 1947 y el 2 de julio de 1948.Compilation of nesting, breeding and general ecology observations of the Corn Bunting (Miliaria calandra, refered as Emberiza calandra by the author) carried out during some trips to different areas of the province of Valladolid, Magaz de Pisuerga (Palencia), Toro (Zamora) and Burgos, between the 7th of July of 1947 and the 2nd of July of 1948
Electron-phonon coupling and electron self-energy in electron-doped graphene: Calculation of angular-resolved photoemission spectra
No full textWe obtain analytical expressions for the electron self-energy and the electron-phonon coupling in electron-doped graphene using electron-phonon matrix elements extracted from density functional theory simulations. From the electron self-energies we calculate angle-resolved photoemission spectra (ARPES). We demonstrate that the measured kink at approximate to-0.2 eV from the Fermi level is actually composed of two features, one at approximate to-0.195 eV due to the twofold-degenerate E-2g mode, and a second one at approximate to-0.16 eV due to the A(1)(') mode. The electron-phonon coupling extracted from the kink observed in ARPES experiments is roughly a factor of 5.5 larger than the calculated one. This disagreement can be only partially reconciled by the inclusion of resolution effects. Indeed, we show that a finite resolution increases the apparent electron-phonon coupling by underestimating the renormalization of the electron velocity at energies larger than the kink positions. The discrepancy between theory and experiments is thus reduced to a factor of approximate to 2.5. From the linewidth of the calculated ARPES we obtain the electron relaxation time. A comparison with available experimental data in graphene shows that the electron relaxation time detected in ARPES is almost two orders of magnitudes smaller than that measured by other experimental techniques
Spin susceptibility and electron-phonon coupling of two-dimensional materials by range-separated hybrid density functionals: Case study of LixZrNCl
No full textWe investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility, chi(s), and the intervalley electron-phonon coupling in LixZrNCl. At low doping, LixZrNCl behaves as a two-dimensional two-valley electron gas, with parabolic bands. In such a system, chi(s) increases with decreasing doping because of the electron-electron interaction. We show that DFT with local functionals (LDA/GGA) is not capable of reproducing this behavior. The use of exact exchange in Hartree-Fock (HF) or in DFT hybrid functionals enhances chi(s). HF, B3LYP, and PBE0 approaches overestimate chi(s), whereas the range-separated HSE06 functional leads to results similar to those obtained in the random phase approximation (RPA) applied to a two-valley two-spin electron gas. Within HF, LixZrNCl is even unstable towards a ferromagnetic state for x < 0.16. The intervalley phonons induce an imbalance in the valley occupation that can be viewed as the effect of a pseudomagnetic field. Thus, similarly to what happens for chi(s), the electron-phonon coupling of intervalley phonons is enhanced by the electron-electron interaction. Only hybrid DFT functionals capture such an enhancement and the HSE06 functional reproduces the RPA results presented in M. Calandra et al. [Phys. Rev. Lett. 114, 077001 (2015)]. These results imply that the description of the susceptibility and electron-phonon coupling with a range-separated hybrid functional would be important also in other two-dimensional weakly doped semiconductors, such as transition-metal dichalcogenides and graphene
Phonon-mediated superconductivity in graphene by lithium deposition
No full textGraphene(1) is the physical realization of many fundamental concepts and
phenomena in solid-state physics(2). However, in the list of graphene's
many remarkable properties(3-6), superconductivity is notably absent. If
it were possible to find a way to induce superconductivity, it could
improve the performance and enable more efficient integration of a
variety of promising device concepts including nanoscale superconducting
quantum interference devices, single-electron superconductor-quantum dot
devices(7,8), nanometre-scale superconducting transistors(9) and
cryogenic solid-state coolers(10). To this end, we explore the
possibility of inducing superconductivity in a graphene sheet by doping
its surface with alkaline metal adatoms, in a manner analogous to which
superconductivity is induced in graphite intercalated compounds(11,12)
(GICs). As for GICs, we find that the electrical characteristics of
graphene are sensitive to the species of adatom used. However, contrary
to what happens in GICs, Li-covered graphene is superconducting at a
much higher temperature with respect to Ca-covered graphene
"El impresionante brillo de la felicidad de la gente norteamericana". Il pensiero Thomas Paine in America Latina
No full textIl saggio propone alcune riflessioni essenziali su come il pensiero rivoluzionario di Thomas Paine è stato accolto e applicato nel contesto delle lotte di indipendenza latinoamericane, a inizi del XIX secolo, e inserito in una riflessione piu ́complessiva sui modelli europei nei nascenti stati-nazione
Superconductivity in metal-coated graphene
No full textIn this work we explore, by first-principles density functional theory (DFT) calculations, the possibility of inducing electronphonon mediated superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms. We demonstrate that, contrary to what could be naively believed, simple exfoliation to one layer of superconducting graphite intercalated compounds (GICs) does not necessarily lead to superconducting graphene, as it is the case in CaC6. On the contrary, it is meaningful to look for superconductivity in monolayers obtained by exfoliating non-superconducting GICs. In particular, we demonstrate that Li coating and double-coating of graphene leads to superconductivity in graphene with T-c that could be as large as 18 K. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
First-Principles Theory of Anharmonicity and the Inverse Isotope Effect in Superconducting Palladium-Hydride Compounds
No full textPalladium hydrides display the largest isotope effect anomaly known in
the literature. Replacement of hydrogen with the heavier isotopes leads
to higher superconducting temperatures, a behavior inconsistent with
harmonic theory. Solving the self-consistent harmonic approximation by a
stochastic approach, we obtain the anharmonic free energy, the thermal
expansion, and the superconducting properties fully ab initio. We find
that the phonon spectra are strongly renormalized by anharmonicity far
beyond the perturbative regime. Superconductivity is phonon mediated,
but the harmonic approximation largely overestimates the superconducting
critical temperatures. We explain the inverse isotope effect, obtaining
a -0.38 value for the isotope coefficient in good agreement with
experiments, hydrogen anharmonicity being mainly responsible for the
isotope anomaly
Huge anharmonic effects in superconducting hydrides and transition metal dichalcogenides
No full textEven if the harmonic approximation describing the vibrations of atoms in solids suffices to interpret experimental measurements in many occasions, it can completely break down when the displacements of the atoms exceed the range in which the harmonic potential is valid. The stochastic self-consistent harmonic approximation method is precisely devised to calculate theoretically vibrational properties in strongly anharmonic solids in which the harmonic theory fails. We apply this method to palladium hydrides and 2H-NbSe2, two strongly anharmonic systems that exemplify the importance of anharmonic effects in metallic hydrides and transition metal dichalcogenides. First of all, we explain that the inversion of the isotope effect in palladium hydrides is a consequence of huge anharmonic effects. The temperature dependence of the phonon spectra in PdH, PdD, and PdT is also presented, where qualitative differences are predicted depending on the isotope. Secondly, we demonstrate that the high-temperature 2H-NbSe2 structure is fully stabilized dynamically by anharmonicity. The softening with temperature of the acoustic longitudinal mode in 2H-NbSe2 at the CDW momentum is predicted as well by our calculation. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Universal Increase in the Superconducting Critical Temperature of Two-Dimensional Semiconductors at Low Doping by the Electron-Electron Interaction
No full textIn two-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first-principles calculations beyond density functional theory, we prove that this effect accounts for the unconventional doping dependence of the superconducting transition temperature (T-c) and of the magnetic susceptibility measured in LixZrNCI. Finally, we discuss what are the conditions for a maximal T-c enhancement in weakly doped two-dimensional semiconductors
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