Graphene induced effects on epitaxial Cr2O3 thin films grown on a Ni(111) substrate

LAUREA MAGISTRALEFilm ultra sottili di Cr2O3 sono stati cresciuti su superfici di Ni(111) e Ni(111) ricoperto da grafene. Le proprietà chimiche dei campioni sono state studiate mediante spettroscopia elettronica Auger e spettroscopia fotoelettronica a raggi x. La struttura reticolare, la topografia della superficie e le proprietà elettroniche sono state investigate rispettivamente tramite diffrazione elettronica a bassa energia, microscopia e spettroscopia a effetto tunnel. Il grafene rimane sepolto all'interfaccia Cr2O3/Ni(111), proteggendo il substrato dall'ossidazione. Quando invece il Cr2O3 viene cresciuto sul Ni pulito, uno strato di ossido (NiO) si forma all'interfaccia. L'ossidazione del substrato nel campione privo di grafene provoca lo sviluppo di una superficie rugosa, mentre nel caso di substrato ricoperto da grafene il film di Cr2O3 mantiene una morfologia piatta. Inoltre, siccome il parametro reticolare del grafene è più piccolo rispetto a quello di NiO, il film di Cr2O3 è soggetto a una diversa deformazione. Un semplice modello fenomenologico viene presentato per relazionare la deformazione del Cr2O3 alle proprietà elettroniche di Cr2O3/Ni(111) e Cr2O3/Gr/Ni(111), che sono rispettivamente isolanti e metalliche.Cr2O3 ultra-thin films have been epitaxially grown on a Ni(111) and on a graphene covered Ni(111) surface. The chemical properties of the sam- ples have been studied by means of Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy. The lattice structure, surface topography and electronic properties of the Cr2O3 films have been elucidated by means of Low Energy Electron Diffraction, Scanning Tunneling Microscopy and Spec- troscopy, respectively. The graphene layer remains buried at the Cr2O3/Ni(111) interface, pro- tecting the substrate from oxidation. On the other hand, when Cr2O3 is grown on the bare Ni substrate, a nickel oxide (NiO) film develops at the interface. The oxidation of the substrate in the graphene-free sample in- duces the development of a rough surface, while the Cr2O3 film grown on the graphene covered substrate retains a flat morphology. Furthermore, because of the smaller lattice parameter of graphene with respect to that of NiO, the Cr2O3 film experiences a different state of strain. A simple phenomenological model is proposed to relate the strain of Cr2O3 to the electronic properties of Cr2O3/Ni(111) and Cr2O3/Gr/Ni(111), which are insulating and metallic, respectively

Studio multidisciplinare finalizzato alla riqualificazione ambientale della valle del Rio della Rocca (Comune di Castellarano, Provincia di Reggio Emilia)

La valle del Rio della Rocca nel Comune di Castellarano (Reggio Emilia) è stata interessata a partire dagli anni ‘50 del secolo scorso da attività estrattive che in parte ne hanno profondamente modificato le caratteristiche ambientali, in particolare quelle geomorfologiche. In considerazione dell’interesse paesaggistico della valle e ai fini di una riqualificazione ambientale del territorio, è stato realizzato uno studio multidisciplinare volto a delineare i principali aspetti geologici, paleontologici, floristici e faunistici. Specifica attenzione è stata posta alla valutazione dell’instabilità dei versanti e alla individuazione dei geositi presenti nell’area. Sulla scorta dei risultati delle ricerche effettuate sono state elaborate proposte di riqualificazione del territorio che prevedono una valorizzazione della valle a fini geoturistici e ricreativi

Self-assembly of C60 on a ZnTPP/Fe(001)–p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

Fullerene (C(60)) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)–p(1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C(60)/ZnTPP/Fe(001)–p(1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C(60) nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C(60) and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C(60)/ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C(60) is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C(60) interfaced with a metallic electrode is required

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

The macroscopic coherence in superconductors supports dissipationless supercurrents which could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called superconducting (SC) diode effect. We demonstrate strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish non-volatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge/surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens door for novel technologies. Meanwhile, we reveal the ubiquity of Meissner screening effect induced SC diode effect in superconducting films, which should be eliminated with great care in the search of exotic superconducting states harboring finite-momentum Cooper pairing.Comment: 27 pages, 16 figure

Effects of the introduction of a chromium oxide monolayer at the C60/Fe(001) interface

The introduction of a two-dimensional oxide layer at the interface between an organic semiconductor and a ferromagnetic metal (spinterface) can help in tailoring the formation of spin-polarized hybridized interface states. Here, we consider the case of a Cr4O5 monolayer at the C-60/Fe(001) interface, which is already known to feature the occurrence of spin-polarized states in the fullerene molecules. In this work, we employ scanning tunneling microscopy/spectroscopy and photoemission spectroscopy to show that the C-60/Cr4O5/Fe(001) spinterface is characterized by the formation of a well-ordered fullerene monolayer and of strongly hybridized interface states. These experimental results are discussed in terms of state-of-the-art ab lingo calculations of the structural, electronic, and magnetic properties at the interface

Supplemental material for “Ubiquitous Superconducting Diode Effect in Superconductor Thin Films”

Supplemental material: -Materials and Methods. -Note 1: Calculation of experimentally measured critical current with edge asymmetry. -Note 2: False “in-plane” magnetic field induced diode effect. -Note 3: Suppression of critical current by out-of-plane magnetic field. -Note 4: Precise removal of the out-of-plane magnetic field. -Note 5: Estimation of the screening current in FM/SC bilayers. .Peer reviewe

Ubiquitous superconducting diode effect in superconductor thin films

The macroscopic coherence in superconductors supports dissipationless supercurrents that could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called the superconducting (SC) diode effect. We demonstrate the strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish the nonvolatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge and surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens the door for novel technologies while revealing the ubiquity of the Meissner screening effect induced SC diode effect in superconducting films, and it should be eliminated with great care in the search for exotic superconducting states harboring finite-momentum Cooper pairing.This work was supported by Air Force Office of Sponsored Research (FA9550-23-1-0004 DEF), Office of Naval Research (N00014-20-1-2306), National Science Foundation (NSF-DMR 1700137, 2218550 and 1231319); Army Research Office (W911NF-20-2-0061, DURIP W911NF-20-1-0074). F. N., M. F. R., and D. Z. H. acknowledge support from the European Research Council (Grant No. 804273). H. C. is sponsored by the Army Research Laboratory under Cooperative Agreement Number W911NF-19-2-0015. S. I. and F. S. B. are supported by European Union’s Horizon 2020 Research and Innovation Framework Programme under Grant No. 800923 (SUPERTED), and the Spanish Ministerio de Ciencia e Innovacion (MICINN) through Project PID2020–114252 GBI00 (SPIRIT). F. S. B. acknowledges financial support by the A.v. Humboldt Foundation. A. K. acknowledges the support by the Spanish Ministry for Science and Innovation—AEI Grant CEX2018-000805-M (through the “Maria de Maeztu” Programme for Units of Excellence in R&D, and Grant RYC2021-031063-I.). P. A. L. acknowledges the support by DOE office of Basic Sciences Grant No. DE-FG0203ER46076.With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2018-000805-M).Peer reviewe

Functionalization of oxide-metal and molecules-metal interfaces through monoatomic buffer layers

DOTTORATOQuesto concetto è qui utilizzato per ottimizzare le eterostrutture per applicazioni nel campo della spintronica. Studiamo gli effetti che un buffer layer di grafene su Ni(111) o di FeO su Fe(001) ha sull’interfaccia studiando gli stadi iniziali di crescita del film superiore. Questo viene fatto tramite un confronto schematico tra i campioni cresciuti con o senza il buffer layer. I substrati ferromagnetici Ni(111) e Fe(001) sono usati, rispettivamente, come supporto per a crescita di ossidi di metalli di transizione e metallo tetra-fenil porfirine M-TPP (M = Co, Ni, Zn). Tutto questo è fatto tramite un approccio sperimentale a più tecniche, parzialmente supportato da simulazioni DFT. In particolare, dimostriamo che, inserendo grafene ad un’interfaccia metallo-ossido, si può indurre sia una crescita tridimensionale che la formazione di un layer piatto e altamente stressato, a seconda della scelta dei materiali e delle condizioni di crescita. Inoltre, dimostriamo che è possibile indurre il self-assembly a temperatura ambiente di porfirine su un substrato di Fe(001) tramite l’inserimento di un singolo layer di ossido, a differenza del layer disordinato che si forma sul substrato metallico pulito. Questi risultati forniscono un metodo affidabile e riproducibile per realizzare interfacce nette, con la possibilità di modificarne le proprietà.In this work we propose to use monoatomic buffer layers with the aim of controlling the morphology and the properties of both organic and inorganic ultra-thin films stabilized on metal surfaces. Here, this concept is applied to optimize the heterostructures for applications in the field of spintronics. We investigate the effects that a buffer layer of graphene on Ni(111) or FeO on Fe(001) has on the interface by studying the early stages of growth of the upper layer, following a systematic comparison between the sample synthesized with or without the buffer layer. The ferromagnetic Ni(111) and Fe(001) substrates are used as supports for the growth of transition metal oxides and metal tetra phenyl porphyrins M-TPP (M=Co, Ni, Zn), respectively. This is done with a multi-technique experimental approach, partly supported by density functional theory calculations. In particular, we demonstrate that inserting graphene at a metal-oxide interface can induce both a three-dimensional growth and the formation of an atomically flat highly strained layer, depending on growth conditions and materials choice. Moreover, we prove that it is possible to induce the self-assembly at room temperature of porphyrin molecules on a Fe(001) substrate through surface passivation with a single layer of oxygen, whereas, on the pristine surface, the molecules form a disordered overlayer. These results provide a reliable and reproducible method to realize atomically sharp interfaces with the possibility of tailoring their properties.DIPARTIMENTO DI FISICA33BRAMBILLA, ALBERTO LUIGIFINAZZI, MARC

3-dimensional nucleation of Fe oxide induced by a graphene buffer layer

Shaping the morphology of oxide nanolayers is of paramount importance in tailoring their physical and chemical properties. Here, the influence of a two dimensional graphene buffer layer on the growth of Fe oxide has been investigated by comparing the oxide deposition on a Ni(111) and a graphene/Ni(111) substrate. Scanning tunneling microscopy images acquired at a mesoscopic scale indicate that Fe oxide grows layer-by-layer on the bare Ni(111) surface, while the nucleation of three-dimensional clusters is induced by graphene. Atomically resolved images reveal that Fe oxide adopts an in-plane lattice constant similar to that of the FeO(111) surface when deposited on Ni(111) and graphene/Ni(111), indicating in both cases, a weak interaction between the overlayer and the substrate. Accordingly, it is suggested that the different growth mode is mainly driven by the graphene-induced lowering of the substrate surface free energy