1,721,059 research outputs found
Grotta Bella (Avigliano Umbro - TR). La fauna
No full textSi presenta all'interno della Guida del Museo Archeologico Nazionale di Perugia una breve scheda relativa agli studi faunistici effettuati nel sito di Grotta Bella
Grotta del Lago di Triponzo (Cerreto di Spoleto - PG). La fauna
No full textSi presenta all'interno della Guida del Museo Archeologico Nazionale di Perugia una breve scheda relativa agli studi faunistici effettuati nel sito di Grotta del Lago
Mesoscopic disorder and intrinsic charge instability in oxide heterostructures
Get PDFAfter Ohtomo and Hwang detected a high-mobility two-dimensional electron gas (2DEG) at the interface between the two insulating perovskite oxides Strontium Titanate
(SrTiO) and Lanthanum Aluminate (LaAlO) \cite{Ohtomo2004}, an increasingly intense theoretical and experimental investigation has been devoted to
these systems.
Oxide heterostructures are of great interest for both fundamental and applicative reasons.
In particular, the two-dimensional electron gas at the LaAlO/SrTiO (LAO/STO) interfaces displays
many different properties and functionalities.
It can be made superconducting when its carrier density is tuned by means of a gate voltage \cite{Reyren2007,Caviglia2008}, thus opening the way to voltage-driven
superconducting devices.
Also, it exhibits magnetic properties \cite{Ariando2011,Li2011,Bert2011,Dikin2011,Mehta2012,Bert2012};
displays a strong and tunable \cite{Caviglia2010,Caprara2012} Rashba spin-orbit coupling; it is extremely two-dimensional,
having a lateral extension \approx \SI{5}{\nano\meter}, thereby enhancing the effect of disorder due to extrinsic and/or intrinsic sources.
Similar results hold true also for other oxides such as
LaTiO/SrTiO \cite{Biscaras2010,Biscaras2012}, where the role of Lanthanum Aluminate is taken by
Lanthanum Titanate LaTiO (LTO).
In these kind of systems, superconductivity is possibly related to the presence of high-mobility carriers (HMC) and low-mobility carriers (LMC),
which presence is revealed by
magneto-transport experiments, and seems to develop as soon as high-mobility carriers
appear \cite{Biscaras2012}, when the carrier density is tuned above a threshold value by means of gate voltage, V.
Along with these intriguing physical properties, there are clear experimental indications that the
interface electronic state is strongly inhomogeneous,
as revealed in various magnetic experiments \cite{Ariando2011,Li2011,Bert2011,Bert2012}, in tunneling spectra \cite{Ristic2011},
and possibly in piezoforce microscopy \cite{Feng2013}.
It seems that inhomogeneities at nanometric scale coexist with larger (e.g., micrometric) scale inhomogeneities, revealed by the occurrence of striped
textures in the current distribution \cite{Kalisky2013} and in the surface potential \cite{Ilani2013}.
The inhomogeneous character of the 2DEG may be responsible of the anomalously large width of the superconducting transition, which is not compatible
with any reasonable superconducting fluctuations (e.g., Aslamazov-Larkin contribution to the paraconductivity) \cite{Caprara2011}, but instead
it is well accounted for assuming that the 2DEG consists of superconducting ``puddles '' embedded in weakly localizing metallic background.
This scenario could also help to explain the measurements carried out at low carrier density, showing a saturation plateau with finite resistance,
a clear signature of the percolating character of the metal-to-superconductor transition.
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The aim of the present thesis is to assemble all these experimental evidences into a coherent overall theoretical framework.
The outline is as follows.
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In chapter \ref{ch:overview} we review some of the experimental facts about LAO/STO and LTO/STO interface, stressing our attention
on the peculiar transport properties, both in the normal and in the superconducting state.
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In chapter \ref{ch:sust} we first provide new,
compelling evidences of the inhomogeneous character of the 2DEG.
We consider the phenomenological assumption that the electron gas consists of a metallic sea, where only LMC are present, hosting metallic islands, where LMC and HMC
coexist and become superconducting below a randomly distributed
critical temperature.
Basing on this idea, we extend previous multicarrier analyses of magneto-transport measurements to deal with inhomogeneous
systems and show how Hall resistivity, superfluid density and tunneling spectra measurements can be explained within this scenario.
The connection between superconductivity and the appearing of the high-mobility carriers is also investigated.
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Chapter \ref{ch:QW} is devoted to formulate a possible justification of the inhomogeneous scenario and the appearing of two type of carriers whose distribution
is not uniform in space. The mechanism that we propose is based on the quantum confinement experienced
by the 2DEG because of the strong electric field perpendicular to the interface.
We will show that such an \emph{intrinsic} mechanism is due to the fact that the confining potential well depends on the electron density,
therefore giving rise to a non-rigid band structure which
evolves as a function of the electron density too. In a wide range of reasonable parameters, compatible with the experiments,
the non-rigidity determines a negative compressibility in the uniform system, which it avoids by phase separation.
The densities of the phase separated regions are thus found by Maxwell construction and depend on the doping level.
The phase separation scenario also naturally explains the inhomogeneous distribution of low- and high-mobility carriers, and their behavior as a function
of the doping level.
In chapter \ref{ch:dds} we explore the fluctuations of the superconducting order parameter in the vicinity of the metal-to-superconductor phase transition,
that is reached as the gate voltage is changed.
In the case of superconductivity, the order parameter has an amplitude and a phase, which can both fluctuate according to well identified scenarios. We address the
possibility that a new type of fluctuations occurs in the superconducting 2DEG at LAO/STO and LTO/STO interfaces, with an anomalous dynamics. In particular,
we show that the superconducting-to-metal quantum phase transition displays anomalous scaling properties, which can be explained by density driven
superconducting critical fluctuations.
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Finally, the concluding remarks will be given in chapter \ref{ch:conclusion}. We will critically discuss the merits
and limitations of our approach and provide a brief outlook on future work
Tane del Diavolo (Parrano - TR). La fauna
No full textSi presenta all'interno della Guida del Museo Archeologico Nazionale di Perugia una breve scheda relativa agli studi faunistici effettuati nel sito di Tane del Diavolo
Insediamenti francescani e produzione miniata tra XIII e XIV secolo: modelli francesi nel messale di San Francesco a Tagliacozzo
No full textRicostruzione delle vicende che interessarono il cantiere medievale del convento di San Francesco a Tagliacozzo
Superconductive proximity in a topological insulator slab and excitations bound to an axial vortex
No full textWe consider the proximity effect in a topological insulator (TI) sandwiched between two conventional superconductors by comparing s-wave spin singlet superconducting pairing correlations and odd-parity triplet pairing correlations with a zero spin component orthogonal to the slab (“polar” phase). A superconducting gap opens in the Dirac dispersion of the surface states existing at the interfaces. An axial vortex is included, piercing the slab along the normal to the interfaces with the superconductors. It is known that, when proximity is s-wave, quasiparticles in the gap are Majorana bound states, localized at opposite interfaces. We report the full expression for the quantum field associated to the midgap neutral fermions, as derived in the two-orbital band model for the TI. When proximity involves odd-parity pairing, midgap modes are charged surface Andreev bound states, and they originate from interfacial circular states of definite chirality, centered at the vortex singularity and decaying in the TI film with oscillations. When the chemical potential is moved away from midgap, extended states along the vortex axis are also allowed. Their orbital structure depends on the symmetry of the bulk band from where the quasiparticle level splits off
Topological rf SQUID with a frustrating π junction for probing the Majorana bound state
No full textMajorana bound states are predicted to appear as boundary states of the Kitaev model. Here we show that a π-Josephson junction, inserted in a topologically nontrivial model ring, sustains a Majorana bound state, which is robust with respect to local and nonlocal perturbations. The realistic structure could be based on a high-Tc superconductor tricrystal structure, similar to the one used to spot the d-wave order parameter. The presence of the Majorana bound state changes the ground state of the topologically nontrivial ring in a measurable way with respect to that of a conventional one
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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