19 research outputs found
Anti-site defect-induced disorder in compensated topological magnet MnBiSbTe
The gapped Dirac-like surface states of compensated magnetic topological
insulator MnBiSbTe (MBST) are a promising host for exotic
quantum phenomena such as the quantum anomalous Hall effect and axion
insulating states. However, it has become clear that atomic defects undermine
the stabilization of such quantum phases as they lead to spatial variations in
the surface state gap and doping levels. The large number of possible defect
configurations in MBST make studying the influence of individual defects
virtually impossible. Here, we present a statistical analysis of the nanoscale
effect of defects in MBST with , by scanning tunneling
microscopy/spectroscopy (STM/S). We identify (Bi,Sb) anti-site
defects to be the main source of the observed doping fluctuations, leading
towards the formation of nanoscale charge puddles and effectively closing the
transport gap. Our findings will guide further optimization of this material
system via defect engineering, to enable exploitation of its promising
properties
Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping
Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Its formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform
Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO 1 − x F x and Sr(Fe 1 − x Co x ) 2 As 2
A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3 core level photoemission spectra in the parent and optimally doped CeFeAsO{}_{1\ensuremath{-}x}F ( 0, 0.11) and Sr(Fe{}_{1\ensuremath{-}x}Co)As ( 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10{}^{\ensuremath{-}15} s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3{\ensuremath{\mu}}_{B} in CeFeAsO and 2.1{\ensuremath{\mu}}_{B} in SrFeAs. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9{\ensuremath{\mu}}_{B} in CeFeAsOF and 1.3{\ensuremath{\mu}}_{B} in Sr(FeCo)As. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity
Controllable Strain-driven Topological Phase Transition and Dominant Surface State Transport in High-Quality HfTe5 Samples
Controlling materials to create and tune topological phases of matter could
potentially be used to explore new phases of topological quantum matter and to
create novel devices where the carriers are topologically protected. It has
been demonstrated that a trivial insulator can be converted into a topological
state by modulating the spin-orbit interaction or the crystal lattice. However,
there are limited methods to controllably and efficiently tune the crystal
lattice and at the same time perform electronic measurements at cryogenic
temperatures. Here, we use large controllable strain to demonstrate the
topological phase transition from a weak topological insulator phase to a
strong topological insulator phase in high-quality HfTe5 samples. After
applying high strain to HfTe5 and converting it into a strong topological
insulator, we found that the sample's resistivity increased by more than two
orders of magnitude (24,000%) and that the electronic transport is dominated by
the topological surface states at cryogenic temperatures. Our findings show
that HfTe5 is an ideal material for engineering topological properties, and it
could be generalized to study topological phase transitions in van der Waals
materials and heterostructures. These results can pave the way to create novel
devices with applications ranging from spintronics to fault-tolerant
topologically protected quantum computers
Dinamica del trasferimento di carica ultra-veloce in film sottili e ultra-sottili composti di molecole organiche studiata con luce di sincrotrone
2006/2007The increasing energy crisis has induced science and technology world to drive a lot of efforts in
the study of new materials suitable to develop renewable and with a low environmental impact
energy sources as an alternative to petroleum. In this context photo-voltaic cells are a good solution,
nevertheless the high costs and the low light-to-current efficiency still inhibits a large production
and a common usage. The employment of organic based materials, i.e. the materials inspired by
biological processes, finds a place in this research field. The wide availability of these materials in
nature, the ease in material processing and the intriguing chemical and physical properties places
the organics as good candidates in the production of photovoltaic devices.
Moreover, their electronic properties allow a usage as charge injector to enhance the light-to-current
efficiency in inorganic-based photovoltaic devices (Gratzel-cells).
The aim of this thesis is to study the growth, the electronic properties, and the chargetransfer
dynamic in thin and ultra-thin film(single molecular layer) composed by zinc-tetraphenylporphyrin
and C70 and thicker melanin films. We choose these molecules both because of their high
visible light sensitivity and because porphyrins are electron donor and fullerenes are electron
acceptors. In fact, it is well know in biology that the chlorophyll (Mg-poprhyrin) when illuminated
with visible light, act as an electron injector in the biochemical environment supplying the amount
of energy needed to activate the production of glucose starting from water and carbon dioxide
(chlorophyll synthesis). The fullerene C70 consist in an arrangement of 70 carbon atoms in a closed
cage structure and is a good electron acceptor. Then, the our purpose is to use the different
electronic properties of these molecules to generate donor/acceptor junctions at the molecular scale.
Melanin is a natural pigment present in living beings responsible, in human body, of the colour of
skin and of its variation due to the exposition to the sun light; it is a semiconductor with electron
donor properties. The combined usage of the properties of these molecules opens the way to the
production of complexes to realize high-efficiency and low cost photovoltaic devices.
In this context, and at the present state of the art in the production of organic-based
photovoltaic devices, investigations about the basic mechanism of molecular interaction and
electronic properties are needed to clarify the problems that are still open. In fact the light-tocurrent
conversion is just one of the possible processes successive to the absorption of a visible
photon in a material. In fact the large number of dissipative processes dissipates the charges excited
by the light and inhibits the light-to-current conversion efficiency. In this context, two aspect are
fundamental: the presence of empty states in the conduction band that are not allowed for dipole
transitions from the valence band but energetically favourable with respect to the first allowed ones,
in order to brake the excitonic bond and a good charge mobility in order to transport the excited
charges up to the collecting electrodes of the device. because the mobility is higher in ordered
systems instead of non ordered ones, the molecular interaction and the growth condition have a
fundamental role because they determine the molecular packing in the film. In this sense we used
soft X-rays and UV-rays photoemission to study the interaction between ZnTPP and C70 and
between these molecules and the Si(111)7x7 surface, one of the most common substrate used to
produce electronic devices. We studied the order in the various films in the sense of “orientational
order” using Near Edge Absorption Fine structure Spectroscopy at SuperESCA and ALOISA
beamlines at ELETTRA synchrotron radiation facility in Trieste. Because the NEXAFS spectra,
obtained with linearly polarized radiation, are sensitive to the direction of the chemical bonds, the
dependence of the absorption structures intensity on the angle between the electrical field of the
incoming radiation and the direction of the empty states yields informations about the geometrical
(orientational) arrangements of the molecules in the film. The films were produced by sublimation
in ultra-high-vacuum in order to obtain a film as pure as possible.
We produced a melanin film via “drop casting”, by in air deposition of a suspension of
synthetic melanin powder in mineral free water on a polycristal copper surface and drying the
water. We obtained the first photoemission data available in literature about this system.
A particular attention was dedicated to the ultra-fast delocalization processes of the excited
charges. We used Resonant Photoemission technique (SuperESCA beamline at ELETTRA) to study
the excitation de-excitation processes: a core electron is pumped to an empty state in the conduction
band, the following decay of the core hole (scale of fs) reveals time scale of the excited charge
delocalization with a chemical sensitivity typical of core spectroscopiesLa crescente crisi energetica ha indotto la scienza e la tecnologia ad indirizzarsi verso lo studio di
nuovi materiali da utilizzarsi per sviluppare fonti di energia alternative al petrolio che siano
rinnovabili e a basso impatto ambientale. In questo ambito le celle foto-voltaiche sono una buona
risposta, tuttavia i costi elevati e la bassa efficienza nella conversione luce-corrente fanno sì che
esse non siano ancora di uso comune. Lo studio dei materiali organici, ovvero di quelli ispirati da
processi biologici, trova spazio in questo ambito di ricerca. La larga diffusione in natura dei
costituenti, la facilità nel processare il materiale, e le interessanti proprietà chimico-fisiche fanno
dei materiali organici una delle possibili scelte nella realizzazione di dispositivi fotovoltaici.
Inoltre, la versatilità di questi materiali li rende utilizzabili anche come iniettori di cariche per
aumentare l’efficienza di conversione luce-corrente se accoppiati con semiconduttori inorganici
(Gratzel-cells).
Oggetto di questa tesi è lo studio della crescita e delle proprietà elettroniche di trasferimento
di carica di film sottili e monostrati molecolari composti di zinco-tetrafenil-porfirina e C70, e film di
melanina. La scelta di queste molecole origina sia dalle loro proprietà di sensibilità alla luce visibile
che dalle loro proprietà elettroniche di essere donori ed accettori di elettroni. Infatti, è ben noto in
natura che la clorofilla (magnesio-porfirina) svolge la funzione di iniettore di carica nell’ambiente
biochimico per fornire l’energia necessaria all’attivazione della produzione di glucosio a partire da
acqua e anidride carbonica, quando esposta a luce solare. Il fullerene C70 è una molecola costituita
da settanta atomi di carbonio disposti in una struttura chiusa a gabbia ed ha la proprietà di essere un
accettare di elettroni. Uno degli obiettivi è, quindi sfruttare le diverse proprietà elettroniche di
queste molecole per realizzare delle giunzioni donore/accettore su scala molecolare. La melanina è
il pigmento naturale presente negli esseri viventi responsabile, nel corpo umano, del colore della
pelle e del suo cambiamento in seguito all’esposizione alla luce ed è anch’essa un semiconduttore
con proprietà di donore di elettroni. L’uso combinato di queste caratteristiche apre la strada alla
realizzazione di materiali complessi che possano essere utilizzati nella realizzazione di dispositivi
fotovoltaici.
In questo contesto, e all’attuale stato dell’arte della realizzazione di dispositivi fotovoltaici
basati su molecole organiche è necessario lo studio di base delle proprietà elettroniche dei film
composti di queste molecole per affrontare problematiche aperte. Infatti il processo di conversione
della luce in corrente è solo uno di quelli possibili in seguito all’assorbimento di un fotone visibile
da parte di un materiale. Infatti un gran numero di processi dissipativi rende le cariche eccitate in
gran parte inutilizzabili ai fini della conversione della luce in corrente. Due aspetti sono
fondamentali affinché il materiale possa essere efficiente nella conversione luce-corrente: la
presenza di stati di conduzione vuoti non accessibili tramite eccitazione con radiazione
elettromagnetica ma energeticamente favorevoli rispetto a quelli accessibili, e una buona mobilità
delle cariche eccitate in modo da essere trasportate senza dissipazione verso gli elettrodi di raccolta.
Dal momento che la mobilità delle cariche è maggiore in sistemi ordinati, diventano cruciali
sia le tecniche di crescita che le interazioni molecolari che determinano l’impacchettamento delle
molecole a formare il film. In questo senso ci siamo avvalsi della spettroscopia di fotoemissione nel
regime dei raggi X soffici e di raggi UV per studiare sia l’interazione tra le due specie molecolari e
substrato (superficie (111) del silicio) che tra porfirina e porfirina e porfirina e fullerene, crescendo
films a spessori via via crescenti. Per quanto riguarda la crescita, e quindi l’ordine con cui sono stati
cresciuti i films abbiamo utilizzato la spettroscopia di assorbimento vicino soglia (NEXAFS,
esperimenti eseguiti sulla beamline ALOISA ad ELETTRA). Poiché gli spettri di assorbimento
sono sensibili alla direzione di legami chimici qualora eccitati con radiazione polarizzata
linearmente, la dipendenza dell’intensità dei singoli picchi di assorbimento dall’angolo tra il vettore
campo elettrico della radiazione e la direzione del legame fornisce informazioni circa la geometria
del sistema. I films sono stati ottenuti per sublimazione di polveri in ultra alto vuoto al fine di
ottenere un sistema chimicamente puro. Per quanto riguarda la melanina, abbiamo realizzato un
film utilizzando la tecnica del “drop casting” depositando una sospensione di acqua e melanina su
una superficie di rame policristallino e lasciando evaporare l’acqua. Sono stati raccolti, quindi, i
primi dati di fotoemissione presenti in letteratura riguardo questo sistema.
Particolare attenzione è stata rivolta ai processi ultraveloci di delocalizzazione delle cariche
in stati eccitati. A tale scopo abbiamo utilizzato la tecnica di fotoemissione risonante (ResPES,
esperimenti eseguiti sulla beamline SuperESCA ad ELETTRA), in cui un elettrone di core viene
eccitato da radiazione di sincrotrone a riempire uno stato di conduzione, il successivo decadimento
della buca di core (scala temporale dei fs) permette di individuare l’avvenuta delocalizzazione
dell’elettrone eccitato ed ottenere una stima dell’efficienza di trasferimento di carica con specificità
chimicaXX Ciclo197
Electronic surface reconstruction and correlation in the fcc and dimer phases of RbC60
A combined photoemission and x-ray absorption study of a RbC60 crystalline film is presented. We find evidence for an electronic charge reconstruction of the film surface in both the fcc and the dimer phases of RbC60. We confirm the previous conclusion on less crystalline films that the dimer phase is insulating. Several observations, such as the presence of molecular features in the photoemission spectra, indicate that at least partial electron localization occurs in the high-temperature fcc phase. In the fcc phase, the surface consists in a half-charge C60(111) plane and appears weakly metallic, as found for the bulk. In the dimer phase, the charge reconstruction simply implies the presence of neutral C60 in the surface layer. The identification of neutral molecules in the surface layer drastically improves the agreement between calculations of the electronic density of states and photoelectron spectra in both phases.