Abstracts of Posters:

 


Complete photoemission experiment for probing spin texture at surfaces: commissioning of VLEED detectors for 3D spin-resolved ARPES and first experiments

Chiara Bigi 1, Ivana Vobornik 2, Jun Fujii 2, Pasquale Orgiani 3, Pranab Kumar Das 2, Giancarlo Panaccione 2, Giorgio Rossi 1,2,3

1 Department of Physics, University of Milano, I-20133 Milano, Italy

2 CNR-IOM Laboratorio TASC, I-34149 Trieste, Italy

3 CNR-SPIN, UOS Salerno, I-84084 Fisciano (SA), Italy

Measuring spin-resolved density of states of solid surfaces is the key to access the magnetic properties of low dimensional systems as well as to investigate the band structure spin texture of complex materials where the electronic properties arise from spin-orbit coupling effects, such as the topological insulators.

We designed and developed the 3D vectorial spin polarimeter based on very low-energy electron diffraction (VLEED) within the NFFA-Trieste demonstrator project[1]. The spin polarimeters are combined with a state of art ARPES analyzer and the whole system is installed on the Low-Energy branch of the Advance Photoelectric Effect beamline (APE-LE), working with variably polarized synchrotron radiation tunable in 10-100 eV range from the Elettra storage ring at Trieste. The detector performances were tested on the small Rashba splitting of the Au(111) surface state and the results show the system is well-performing and competitive at international level (120 ± 5 meV resolution, Seff = 0.5), making up an advanced system for spin and angular-resolved photoemission (Spin-ARPES).

Topological properties of in-situ grown Bi2Se3 thin films were then investigated with the newly built Spin-ARPES apparatus. Epitaxial bismuth-selenide thin films were grown by means of Pulsed Laser Deposition system, implanted on the operational infrastructure of the APE-NFFA laboratory[1]. Such an infrastructure consists of a suite of interconnected UHV chambers that provide surface science environment and methods to support the manipulation and characterization of the as-grown PLD samples directly connected to spectroscopic probing technique using Elettra synchrotron facility. All the surface-related problems were therefore circumvented by allowing the synthesis and the characterization within the same UHV manifold.

X-ray diffraction technique was used to investigate the bulk structural properties and demonstrated the Bi2Se3 films have (00l) orientation along the perpendicular axis, while the surface structure and quality was investigated with low-energy electron diffraction (LEED). The electronic and topological properties of the surface state of the films were investigated by Spin-ARPES experiments, performed on untreated in-situ transferred as-grown Bi2Se3 thin films. ARPES band dispersion of the Bi2Se3 thin films show a single Dirac cone, fingerprint of the topological surface states, centered at the Γ point. The spin-resolved spectra with the respective polarization are ascribed to the corresponding points of the k-space. Opposite spin polarization for opposite momenta have been found, confirming the fingerprint of the topological surface states, whose spin helical texture is strictly linked to the crystal momentum in the topological surface state[2].

[1] http://www.trieste.nffa.eu/
[2] Chiara Bigi, master thesis, Università degli Studi di Milano, 2016

 


Transmission Electron Microscopy studies of radiation sensitive single nanostructured caffeine-based cocrystals

Valeria Cotta 1,2, Elvio Carlino 2, Dritan Hasa 3, Giorgio Rossi 1,2

1 University of Milano, Via G. Celoria 16, 20133 Milano

2 IOM CNR Laboratorio TASC, Area Science Park – Basovizza, Bld MM SS 14, 34149 Trieste, Italy

3 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom

Transmission Electron Microscopy (TEM) is a widely established tool for atomic resolution studies of Nano-structured materials. TEM experiments on single particle of radiation sensitive materials, like protein or polymers, would require the use of low dose of irradiation coupled to cryo-techniques and extensive software assisted image reconstruction techniques [1]. The case of nano-drug materials is of particular interest as the structure of the nanoparticles determines the way it interacts with the living cells and hence its potential effectiveness. A class of nano-drugs of recent interest is based on cocrystals. Cocrystals are stoichiometric multicomponent systems in which each component is solid under ambient conditions. In pharmaceutical cocrystals, one solid component is an Active Pharmaceutical Ingredient (API) and the others components are called coformers. An ideal API model for cocrystal formation is caffeine, which has been synthetized with different coformers [2]. The resulting crystalline nanoparticles are strongly sensitive to the irradiation with high-energy electrons. Here we show how single crystalline nanoparticles of caffeine-based cocrystals can be studied, at room temperature and atomic resolution, by low dose phase contrast High Resolution TEM (HRTEM) determining straightforwardly the morphology and the polytype of individual particles.

[1] E. Callaway, Nature 125 (2015) 172
[2] D. Hasa, E. Carlino, W. Jones, ACS Cryst. Growth Des. 16 (2016) 1772

 


Spectroscopy of strongly correlated materials: many-body approaches

Evgeny Gorelov

European XFEL GmbH, Albert-Einstein-Ring 19, 22671 Hamburg, Germany

Materials with strong electronic correlations are one of the most fascinating problems in modern solid state physics. Strongly correlated compounds exhibit a variety of intriguing properties and phenomena, that are very sensitive to a change of control parameters (e.g. magnetization, temperature, pump-probe time delay, pressure). A modern experimental techniques, as well as recently developed theoretical methods help us to understand this striking behavior.  In the present talk a brief introduction to novel theoretical approaches for description of spectroscopic properties of correlated solids will be given. We discuss details of metal-to-insulator transition in Ca2RuO4 [1] and LaCoO3 [2], and spin-state transition in the latter material. We calculate such spectroscopic data, as shape of Fermi surface [3], density of electronic states, and resonant X-ray absorption spectra.

References:
[1] E. Gorelov, M. Karolak, T. O. Wehling, F. Lechermann, A. I. Lichtenstein, E. Pavarini, PRL  104 226401 (2010).
[2] G. Zhang, E. Gorelov, E. Koch, E. Pavarini, PRB 86, 184413 (2012).
[3] G. Zhang, E. Gorelov, E. Sarvestani, E. Pavarini, PRL 116, 106402 (2016).

 


Hidden Structural Features of Organic Materials at Nanoscale

Souren Grigorian

Soft Matter Physics Group, University of Siegen, Germany

Over the last decades the surface sensitive X-ray techniques proven to be a versatile and frequently used tool for the analysis of thin films and patterned nanostructures. A combination of X-ray reflectivity, grazing incidence X-ray diffraction and grazing incidence small-angle X-ray scattering allows us to probe the structural properties and morphology of the organic materials.

Starting from the conventional X-ray approaches for investigations of the conjugated organic systems, in the talk will be presented results of the recent spatially-resolved experiments performed at synchrotron radiation sources. A particular advantage of nanoscale studies of the organic materials is to reveal a local orientation and, moreover, to probe an interconnectivity of the conjugated network by X-ray cross-correlation analysis [1-2].

References

[1] C. Gutt, L. Grodd, E. Mikayelyan, U. Pietsch, R. J. Kline, S. Grigorian, J. Phys. Chem. Lett., 5, 2335, 2014

[2] R. P. Kurta, L. Grodd, E. Mikayelyan, O. Y. Gorobtsov, I. Fratoddi, I. Venditti, M.V.Russo, M. Sprung, I. A. Vartanyants, S. Grigorian, , Phys. Chem. Chem. Phys., 17, 7404, 2015

 


Phase retrieval in X-ray imaging of biological objects using Bragg Magnifier Microscope

Stanislav Hrivňak (a), Jozef Uličný (a), Patrik Vagovic (b)

(a) P.J. Safarik University, Kosice, Slovakia

(b) European XFEL, Hamburg, Germany

We present the improved, single-distance phase retrieval algorithm for holographic imaging of biological objects using in-line germanium Bragg Magnifier Microscope (BMM).
The proposed algorithm takes advantage from the combination of the modified shrink-wrap algorithm for phase objects, robust unwrapping algorithm as well as other reasonable constraints applied to the wavefield at the object and the detector plane.
The robustness of the algorithm is tested on different samples and the results are shown.
First it is applied to the experimentally measured diffractograms of well-defined samples of polystyrene spheres and siemens star, where we determine the resolution to be ~ 300 nm.
Subsequently we present the successful reconstruction of more complex biological object Tardigrade. Results confirm the potential of in-vivo single-shot imaging of biological objects using BMM.

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Dynamic SEM imaging at the μs scale

Gabriele Irde¹², Silvia M. Pietralunga²³, Vittorio Sala¹², Maurizio Zani¹, James Ball², Alex Barker², Annamaria Petrozza², Guglielmo Lanzani¹² and Alberto Tagliaferri¹²

¹ Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano
² IIT – CNST @ Politecnico di Milano, Via Giovanni Pascoli, 70/3, 20133 Milano
³ Consiglio Nazionale delle Ricerche, Istituto di Fotonica e Nanotecnologie, Dipartimento di Fisica, Piazza Leonardo da Vinci, 32, 20133 Milano


Since decades Scanning Electron Microscopy has been a well established technique for characterizing samples’ morphology and composition. While being a very versatile and accurate experimental technique though, it lacks typically any temporal resolution and is unable to capture and describe time – evolving phenomena. Recent advances in modern electron microscopy have showed that dynamical electron Microscopy can be conceived and implemented, either  by exploiting  time-resolved direct detection methods[1], or by developing an optically assisted pump and probe technique, in which a pulsed primary electron beam is obtained by pulsed photo-electronic effect from the tip of the microscope, and a synchronized optical pump beam excites the physical phenomena under study, in a repetitive way. Dynamics is retrieved by temporally delaying the two pulses respectively, without the need for fast electronics in detection[2]. Our work, here presented, relies on a pump-and-probe approach where electron and photon pulses are obtained respectively by gating the continuous electron beam with the aid of  a Beam Blanker and by a modulated laser beam. Specifically, the laser beam is externally modulated by using an Acousto Optic gating technique, synchronized to the Beam Blanker.  In this way, temporal resolution at the μs scale is obtained.
We aim to study the evolution of time dependent phenomena such as photo-induced charge transport and diffusion, and morphological modifications, up to the Megaherz range.
We show some preliminary results obtained on samples of interest as hybrid organometal-halide Perovskites, which are a novel and breaking-through materials in organic photovoltaics [3], and present an interesting behavior at the μs scale.

[1] T. Sekiguchi, K. Sumino, Quantitative electron-beam tester for defects in semiconductors (CL/EBIIC/SDLTS system), Review of Scientific instruments (1995) 66, 8.

[2] O. F. Mohammed, A. H. Zewail et al., 4D Scanning Ultrafast Electron Microscopy: Visualization of Materials Surface Dynamics JACS (2011) 133, 20.

[3] T. Chen et al., 17.6% stabilized efficiency in low-temperature processed planar perovskite solar cells, Energy & Environmental Science (2015) 8, 2365-2370.

 


Electronic and optical properties of uracil-like nucleobases on Silicon(001)

Elena Molteni (a,b), Giovanni Onida (a,b), Guido Fratesi (a,b), Giancarlo Cappellini (c)

(a) Department of Physics, Universita’ degli Studi di Milano, via Celoria 16, 20133 Milano, Italy

(b) European Theoretical Spectroscopy Facility (ETSF)

(c) Department of Physics and CNR-IOM SLACS Cagliari, Universita’ degli Studi di Cagliari, Cittadella Universitaria di Monserrato, S.P. Monserrato – Sestu, Km. 0.700, 09042 Monserrato (CA), Italy

The adsorption of nucleobases on silicon surfaces is relevant both for the potential applications related to the incorporation of molecular functionalities within the semiconductor technology[1], and for the role of prebiotic molecules in models of the origin of life[2].

In this work we study the electronic properties of uracil-like nucleobases, thymine (THY), uracil (URA) and 5-fluorouracil (5-FU), adsorbed on the Si(001) surface[3], with first principles methods, based on density functional theory (DFT) with pseudopotentials and plane wave basis set. We analyze both the effects of chemical substitutions and of molecule orientation, finding important changes in band dispersion and energy gaps as a function of molecular tilting with respect to the surface normal.

We then calculate optical absorption and reflectance anisotropy spectra (RAS) of the clean Si(001) surface and of the Si(001):X system (X=THY,URA,5-FU), in order to probe the effects of molecule adsorption, chemical substitutions and geometric details of the adsorbate on the optical properties of the functionalized silicon surface. RAS is in fact a surface sensitive technique[4], which could be successfully used for in-situ, non-destructive surface monitoring. We obtain that the optical response of the Si(001) surface is strongly modified by the presence of the adsorbed molecules. We discuss the possible importance of these results for applications in hybrid silicon/biomolecule-based nanodevices.

References
[1] W. G. Schmidt et al., Appl. Phys. A 85 (2006) 387.
[2] G. Malloci et al., Astronomy & Astrophysics 432 (2005) 585.
[3] E. Molteni, G. Onida, G. Cappellini, Eur. Phys. J. B 89 (2016) 98.
[4] K. Seino, W. G. Schmidt, Surf. Sci. 548 (2004) 183.

 

Measuring coherence properties of X-ray and XUV radiation: the Heterodyne Near Field Scattering and Asymmetric Lateral Coherence approaches

M. Siano, B. Paroli, M. A.C. Potenza

Dipartimento di Fisica, Università degli Studi di Milano and INFN Sezione di Milano,

via G. Celoria, 16, 20133, Milano, Italy

In this work we present two techniques for measuring coherence properties of high brightness X-ray and XUV radiation emitted by third- and fourth-generation light sources: Heterodyne Near Field Scattering (HNFS) and Asymmetric Lateral Coherence.

HNFS is a well-established method for measuring the modulus of the 2-dimensional spatial coherence factor. It relies on the interference between the strong transmitted beam and the weak spherical waves scattered by each particle of a colloidal suspension. Despite the resulting intensity pattern appears as a stochastic speckle field, the complex degree of coherence is easily retrieved via power-spectra analysis. We report on experimental results obtained with the visible SASE FEL at SPARC_LAB, INFN-LNF [1], and with the soft X-ray undulator at beamline ID02, ESRF [2]. We also discuss the recent developments of the technique: extension to broadband sources and to single-shot measurements, as well as the modeling and reproducibility of X-ray experiments in the visible range with simple and cheap table-top setup [3].

Asymmetric Lateral Coherence is a novel technique based on a modified two-slit interferometer conceived for the characterization of transverse coherence of broad-spectrum radiation [4][5]. It exploits the limited temporal coherence and an asymmetric interferometric device to access the transverse properties of the radiation source by directly measuring the real part of the complex coherence factor. The advantages with respect to a classical interferometer are: i) higher sensitivity due to the absence of monochromators or narrow band-pass filters; ii) phase information is preserved, thus transverse inhomogeneities of the beam can be resolved.

[1] M. D. Alaimo, et al, “Mapping the transverse coherence of the self amplified spontaneous emission of a free electron laser with the heterodyne speckle method”, Opt. Express, 22 (24) (2014)

[2] M. D. Alaimo, M. A. C. Potenza, M. Manfredda, G. Geloni, M. Sztucki, T. Narayanan, and M. Giglio, “Probing the transverse coherence of an undulator X-ray beam using Brownian particles”, Phys. Rev. Lett., 103 (2009)

[3] M. Siano, B. Paroli, E. Chiadroni, M. Ferrario, and M. A. C. Potenza, “Measurement of power spectral density of broad-spectrum visible light with Heterodyne Near Field Scattering and its scalability to betatron radiation”, Opt. Express, 23 (26) (2015)

[4] B. Paroli, E. Chiadroni, M. Ferrario, V. Petrillo, M. A. C. Potenza, A. R. Rossi, L. Serafini, and V. Shpakov , “Asymmetric lateral coherence of betatron radiation emitted in laser-driven light sources”, Europhys. Lett., 111 (2015)

[5] B.Paroli, E. Chiadroni, M. Ferrario, and M. A. C. Potenza, “Analogical optical modeling of the asymmetric lateral coherence of betatron radiation”, Opt. Express, 23 (23) (2015)

 

 


Electronic excitations in small electron gas fragments and hot plasmons in gold nanoparticles. An experimental model for matter at extreme conditions

Pietro Tozzi

Università degli Studi di Perugia

In this poster we discuss the calculation of confined electron gases dielectric function for nanoparticles. We used the Random Phase Approximation for both cubic and spherical particles shape. We show some results obtained by this approach and numerical calculation and the comparison of these ones with characteristics for the infinite electron gas too. It will be stressed that there are some quantum mechanical effects caused by shape and dimensions of the nanoparticles specifically in cubic cases. We also consider the case of gold nanoparticles in the perspective of an experiment at the FERMI FEL to obtain plasmons in extreme conditions of pressure and temperature.