Pairing of few Fermi atoms in one dimension

We study a few Fermi atoms interacting through attractive contact forces in a one-dimensional trap by means of numerical exact diagonalization. From the combined analysis of energies and wave functions of correlated ground and excited states we find evidence of BCS-like pairing even for very few atoms. For moderate interaction strength, we reproduce the even-odd oscillation of the separation energy observed in [G. Zuern, A. N. Wenz, S. Murmann, A. Bergschneider, T. Lompe, and S. Jochim, Phys. Rev. Lett. 111, 175302 (2013)]. For strong interatomic attraction the arrangement of dimers in the trap differs from the homogeneous case as a consequence of Pauli blockade in real space.Comment: Major revision to appear in Physical Review

Three interacting atoms in a one-dimensional trap: A benchmark system for computational approaches

We provide an accurate calculation of the energy spectrum of three atoms interacting through a contact force in a one-dimensional harmonic trap, considering both spinful fermions and spinless bosons. We use fermionic energies as a benchmark for exact-diagonalization technique (also known as full configuration interaction), which is found to slowly converge in the case of strong interatomic attraction.Comment: To appear in Journal of Physics B: Atomic, Molecular and Optical Physic

Engineering interband tunneling in nanowires with diamond cubic or zincblende crystalline structure based on atomistic modeling

We present an investigation in the device parameter space of band-to-band tunneling in nanowires with a diamond cubic or zincblende crystalline structure. Results are obtained from quantum transport simulations based on Non-Equilibrium Green's functions with a tight-binding atomistic Hamiltonian. Interband tunneling is extremely sensitive to the longitudinal electric field, to the nanowire cross section, through the gap, and to the material. We have derived an approximate analytical expression for the transmission probability based on WKB theory and on a proper choice of the effective interband tunneling mass, which shows good agreement with results from atomistic quantum simulation.Comment: 4 pages, 3 figures. Final version, published in IEEE Trans. Nanotechnol. It differs from the previous arXiv version for the title and for some changes in the text and in the reference

Trasporto termoelettrico spin-selettivo in strutture ibride SINIS

La nanoelettronica studia le proprietà dei gas elettronici presenti all'interno di materiali di dimensioni tipiche che vanno da qualche decina a qualche centinaia di nm. A queste scale di lunghezza è possibile investigare proprietà inusuali dei gas elettronici e possono essere realizzate situazioni del tutto particolari, molto diverse da quelle comunemente osservate in sistemi di dimensioni macroscopiche, in quanto gli elettroni manifestano le proprie caratteristiche quantomeccaniche. Tipicamente nella nanoelettronica le temperature di interesse sono criogeniche (dell'ordine o minore di 1K). Assemblando materiali con diverse proprietà, quali superconduttori, semiconduttori, metalli normali, isolanti e ferromagneti, è possibile costruire nanostrutture in cui le proprietà dei singoli costituenti vengono adoperate per realizzare situazioni fisiche esotiche. Un'effetto particolarmente interessante ottenibile nelle nanostrutture è il non-equilibrio termodinamico elettronico. Infatti le temperature criogeniche e le dimensioni ridotte dei costituenti, permettono di poter sopprimere i vari tipi di interazione a cui sono soggetti gli elettroni e quindi impediscono il raggiungimento dell'equilibrio termodinamico. Altri effetti interessanti possono manifestarsi in presenza di campi magnetici, in quanto questi possono modificare le proprietà dello stato superconduttivo. In questa tesi analizziamo una struttura Superconduttore/Isolante/Metallo normale/Isolante/Superconduttore (SINIS) in presenza di campi magnetici localizzati sui superconduttori. In particolare siamo interessati alle proprietà del gas elettronico all'interno della regione N. Le dimensioni mesoscopiche della regione N permettono di portare il gas elettronico fuori dall'equilibrio termodinamico e la presenza dei campi magnetici localizzati sui superconduttori consente di manipolare spin selettivamente la funzione di distribuzione degli elettroni. I risultati originali di questo lavoro di tesi risiedono nell'aver mostrato che le due popolazioni di spin si comportano distintamente all'interno della regione N della struttura SINIS. Nella configurazione parallela dei campi (theta = 0) le temperature efficaci delle due popolazioni sono uguali mentre i potenziali chimici hanno comportamento diverso. Nella situazione opposta (theta = pi greco), le due popolazioni hanno stesso potenziale chimico ma temperature diverse. Nelle situazioni intermedie le due popolazioni mostrano appunto situazioni intermedie, cioè ci sono differenze sia nella temperatura che nel potenziale chimico efficaci. Del tutto originale è anche il dispositivo proposto per la generazione di correnti spin-polarizzate. Infatti sfruttando la configurazione (theta = pi) siamo riusciti ad ottenere correnti di carica spin-polarizzate tra la regione N della struttura SINIS ed un ulteriore elettrodo superconduttivo. Il trasporto di carica avviene in maniera termoelettrica, in quanto sfrutta le diverse temperature delle popolazioni di spin up e down. La tesi è così organizzata: Nel capitolo 1 introduciamo la teoria BCS della superconduttività, che utilizziamo per descrivere la densità degli stati dei superconduttori immersi in campo magnetico; analizziamo le proprietà della giunzione NIS (la struttura SINIS è composta da due giunzioni NIS con la regione N in comune) studiandone le proprietà di trasporto. Nel capitolo 2 descriviamo lo stato di non equilibrio termodinamico di un gas elettronico, partendo dallo stato di equilibrio e generalizzando i concetti di temperatura e potenziale chimico per il caso del non equilibrio. Ricaviamo le equazioni che permettono di ottenere la temperatura ed il potenziale chimico efficaci, utilizzate per descrivere la regione N della struttura SINIS. Nel capitolo 3 analizziamo la struttura SINIS con campi magnetici localizzati sui superconduttori, ricavando le diverse funzioni di distribuzione per le due popolazioni di spin all'interno della regione N, studiandone il comportamento al variare dell'angolo relativo (theta) tra i due campi. Dalle funzioni di distribuzione ricaviamo le temperature ed i potenziali chimici efficaci relativamente alle due popolazioni di spin. Utilizziamo la configurazione antiparallela dei campi (theta = pi), per generare correnti spin-polarizzate tra la regione N della struttura e un ulteriore elettrodo superconduttivo. In appendice mostriamo il formalismo utilizzato comunemente per la descrizione dettagliata dei processi di interazione elettronica presenti all'interno di fili mesoscopici di metallo normale

Efficient GW calculations via the interpolation of the screened interaction in momentum and frequency space: The case of graphene

The calculation of the GW self-energy may be a computational challenge due to the double convolution integrals over frequency and transferred momentum. In this work, we combine the recently developed multipole approximation (MPA) with the W-av method. MPA accurately approximates full-frequency response functions using a small number of poles, while W-av improves the convergence with respect to the Brillouin zone (BZ) sampling in 2D materials. The combination of these techniques is applied to obtain an accurate G0W0 QP band structure of graphene. The screened interaction of graphene shows a complex low-energy frequency dependence, that is poorly described with plasmon pole approximations (PPA), and a sharp q dependence of the dynamical dielectric function over momentum transfer, making standard BZ integration techniques inefficient. Within the present development, we compare the calculated QP band structure of graphene finding an excellent agreement with angle resolved photoemission spectroscopy (ARPES) measurements

Accurate ab initio tight-binding Hamiltonians: Effective tools for electronic transport and optical spectroscopy from first principles

The calculations of electronic transport coefficients and optical properties require a very dense interpolation of the electronic band structure in reciprocal space that is computationally expensive and may have issues with band crossing and degeneracies. Capitalizing on a recently developed pseudoatomic orbital projection technique, we exploit the exact tight-binding representation of the first-principles electronic structure for the purposes of (i) providing an efficient strategy to explore the full band structure E-n (k), (ii) computing the momentum operator differentiating directly the Hamiltonian, and (iii) calculating the imaginary part of the dielectric function. This enables us to determine the Boltzmann transport coefficients and the optical properties within the independent particle approximation. In addition, the local nature of the tight-binding representation facilitates the calculation of the ballistic transport within the Landauer theory for systems with hundreds of atoms. In order to validate our approach we study the multivalley band structure of CoSb3 and a large core-shell nanowire using the ACBN0 functional. In CoSb3 we point the many band minima contributing to the electronic transport that enhance the thermoelectric properties; for the core-shell nanowire we identify possible mechanisms for photo-current generation and justify the presence of protected transport channels in the wire

Charge-memory effect in a polaron model: equation-of-motion method for Green functions

We analyze a single-level quantum system placed between metallic leads and strongly coupled to a localized vibrational mode, which models a singlemolecule junction or an STM setup. We consider a polaron model describing the interaction between electronic and vibronic degrees of freedom and develop and examine different truncation schemes in the equation-of-motion method within the framework of non-equilibrium Green functions. We show that upon applying gate or bias voltage, it is possible to observe charge-bistability and hysteretic behavior which can be the basis of a charge-memory element. We further perform a systematic analysis of the bistability behaviour of the system for different internal parameters such as the electron-vibron and the lead-molecule coupling strength.Comment: 12 pages, 5 figure

SCIENZAPERTA: EARTH SCIENCE FOR EVERYONE... FINALLY IN MILAN!

ScienzAperta is an outreach science venue that the Istituto Nazionale di Geofisica e Vulcanologia started in 2011 as the spring of science: the doors of the headquarters of science were finally opened to public. A number of events, conferences, seminars, guided tours through the Institute and its laboratories are every year offered to general public. The venue is held in most of the cities where the Institute is located, priority to high seismic and/or volcanic risk regions. On May 2014 we held ScienzAperta for the first time in Milano and open up the doors to schools specifically dealing mostly with seismic hazard in a region where general public not necessarily think it might We offered students conferences, seminars and educational activities to highlight the fun of science and jet raise awareness on proper behaviours in case of earthquake shaking. We asked students and teachers, from elementary to high schools, to fill in a questionnaire that we use to evaluate the appreciation the venue had. One hundred years after Giuseppe Mercalli’s death we could not forget to celebrate his science the city where he was born.UnpublishedMilano3T. Pericolosità sismica e contributo alla definizione del rischiorestricte

Decoding the Folding of Burkholderia glumae Lipase: Folding Intermediates En Route to Kinetic Stability

The lipase produced by Burkholderia glumae folds spontaneously into an inactive near-native state and requires a periplasmic chaperone to reach its final active and secretion-competent fold. The B. glumae lipase-specific foldase (Lif) is classified as a member of the steric-chaperone family of which the propeptides of α-lytic protease and subtilisin are the best known representatives. Steric chaperones play a key role in conferring kinetic stability to proteins. However, until present there was no solid experimental evidence that Lif-dependent lipases are kinetically trapped enzymes. By combining thermal denaturation studies with proteolytic resistance experiments and the description of distinct folding intermediates, we demonstrate that the native lipase has a kinetically stable conformation. We show that a newly discovered molten globule-like conformation has distinct properties that clearly differ from those of the near-native intermediate state. The folding fingerprint of Lif-dependent lipases is put in the context of the protease-prodomain system and the comparison reveals clear differences that render the lipase-Lif systems unique. Limited proteolysis unveils structural differences between the near-native intermediate and the native conformation and sets the stage to shed light onto the nature of the kinetic barrier

Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb−1 of √ s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente