On the Magnetic Nature of Quantum Point Contacts

We present results for a model that describes a quantum point contact. We show how electron-electron correlations, within the unrestricted Hartree-Fock approximation, generate a magnetic moment in the point contact. Having characterized the magnetic structure of the contact, we map the problem onto a simple one-channel model and calculate the temperature dependence of the conductance for different gate voltages. Our results are in good agreement with experimental results obtained in GaAs devices and support the idea of Kondo effect in these systems.Comment: 7 pages, 4 figure

Theory of Core-Level Photoemission and the X-ray Edge Singularity Across the Mott Transition

The zero temperature core-level photoemission spectrum is studied across the metal to Mott insulator transition using dynamical mean-field theory and Wilson's numerical renormalization group. An asymmetric power-law divergence is obtained in the metallic phase with an exponent alpha(U,Q)-1 which depends on the strength of both the Hubbard interaction U and the core-hole potential Q. For Q <~ U_c/2 alpha decreases with increasing U and vanishes at the transition (U -> U_c) leading to a symmetric peak in the insulating phase. For Q >~ U_c/2, alpha remains finite close to the transition, but the integrated intensity of the power-law vanishes and there is no associated peak in the insulator. The weight and position of the remaining peaks in the spectra can be understood within a molecular orbital approach.Comment: 5 pages, 6 figure

The Residences of the House of Savoy, a Serial Site in the World Heritage List since 1997 in Piedmont

This contribution aims to place in the European framework a particular item of the World Heritage List, the serial site of the Royal Residences of the House of Savoy, trying to analyze its history as a planned system and its current situation in relation to the contemporary needs of valorization, focusing on the first results of the process. These residences constitute a heritage that documents most effectively the artistic and architectural culture of the Baroque period. When the Italian monarchy was abolished (1946), these palaces underwent a partial process of neglect. Works to remedy the damage began in the 1980s and culminated in the major restoration of Venaria Reale (1998-2007). The current challenge is to valorise this heritage, a cultural asset of utmost priority in the Piedmont Region, notwithstanding the multiplicity of institutions, public bodies and public-private foundations involved, through coordinated management and common policies. To this end, in 2017, the public-private consortium that had been in charge of the Reggia di Venaria Reale was recast as Consortium of the Royal Residences of the House of Savoy, with the specific aim of sharing skills, knowledge and policies in a network and achieve a more effective common management, visibility and tourism promotion

The Residences of the Royal House of Savoy, a serial site in the World Heritage List since 1997 in Piedmont (Italy)

Il 6 dicembre 1997 le Residenze Reali sabaude vengono inserite, come sito seriale, nella World Heritage List, sulla base dei criteri (i), (ii), (iv), (v). Solo le ville medicee costituiscono in Italia un sito seriale che riconosce il preciso carattere di sistema degli edifici. Le residenze delle corte piemontese, realizzate sostanzialmente tra il 1563, quando Torino diviene capitale degli Stati sabaudi, e il XVIII secolo, poi ampliate e integrate sino al XX secolo, costituiscono un patrimonio che documenta con chiarezza la cultura artistica e architettonica dell’epoca barocca. Il complesso di 22 edifici inseriti nella lista, realizzati per residenza della corte, per il loisir e per le cacce, nella capitale e intorno, hanno subito dopo il trasferimento della capitale a Firenze (1865) e a Roma (1870) e con la caduta della monarchia (1946) un parziale processo di abbandono a cui è stato posto rimedio a partire dagli anni ’80 del ‘900, concludendosi con il grande restauro del complesso di Venaria Reale (1998-2007). La sfida attuale consiste nel valorizzare questo patrimonio, bene culturale prioritario nella Regione Piemonte, nonostante il mosaico di istituzioni ed enti pubblici, fondazioni pubblico-private, coordinandone la gestione con politiche comuni. In questo senso il Consorzio pubblico-privato che dal 2008 ha in carico la Reggia di Venaria Reale nel 2017 è stato riformulato come Consorzio delle Residenze Reali Sabaude, con il preciso scopo di mettere in rete capacità, conoscenze, politiche per una migliore gestione comune, e per conseguire visibilità e promozione turistica. I primi risultati sono oggi visibili in un approccio comune alla cura dei parchi e dei giardini di queste residenze

Localized Spins on Graphene

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low energy electronic features of graphene make possible to inject spin polarized currents through the impurity using a scanning tunneling microscope (STM). Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.Comment: 4+ pages, 4 figures. Added reference

Magnetoconductance through a vibrating molecule in the Kondo regime

The effect of a magnetic field on the equilibrium spectral and transport properties of a single-molecule junction is studied using the numerical renormalization group method. The molecule is described by the Anderson-Holstein model in which a single vibrational mode is coupled to the electron density. The effect of an applied magnetic field on the conductance in the Kondo regime is qualitatively different in the weak and strong electron-phonon coupling regimes. In the former case, the Kondo resonance is split and the conductance is strongly suppressed by a magnetic field $g mu_B B \gtrsim k_BT_K$, with $T_K$ the Kondo temperature. In the strong electron-phonon coupling regime a charge analog of the Kondo effect develops. In this case the Kondo resonance is not split by the field and the conductance in the Kondo regime is enhanced in a broad range of values of $B$.Comment: 6 pages, 4 figure

Quantum Transport Through a Stretched Spin--1 Molecule

We analyze the electronic transport through a model spin-1 molecule as a function of temperature, magnetic field and bias voltage. We consider the effect of magnetic anisotropy, which can be generated experimentally by stretching the molecule. In the experimentally relevant regime the conductance of the unstretched molecule reaches the unitary limit of the underscreened spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an antiferromagnetic coupling between the remaining spin 1/2 and a singular density of quasiparticles, producing a second Kondo effect and a reduced conductance. The results explain recent measurements in spin-1 molecules [Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP

Electronic Transport through Magnetic Molecules with Soft Vibrating Modes

The low-temperature transport properties of a molecule are studied in the field-effect transitor geometry. The molecule has an internal mechanical mode that modulates its electronic levels and renormalizes both the interactions and the coupling to the electrodes. For a soft mechanical mode the spin fluctuations in the molecule are dominated by the bare couplings while the valence changes are determined by the dressed energies. In this case, the transport properties present an anomalous behavior and the Kondo temperature has a weak gate voltage dependence. These observations are in agreement with recent experimental data.Comment: 4 pages, 3 figures, accepted in PRB R

Transport through quantum dots in mesoscopic circuits

We study the transport through a quantum dot, in the Kondo Coulomb blockade valley, embedded in a mesoscopic device with finite wires. The quantization of states in the circuit that hosts the quantum dot gives rise to finite size effects. These effects make the conductance sensitive to the ratio of the Kondo screening length to the wires length and provide a way of measuring the Kondo cloud. We present results obtained with the numerical renormalization group for a wide range of physically accessible parameters.Comment: 4 pages, 5 figure