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DTSTART;TZID=Europe/Paris:20260622T140000
DTEND;TZID=Europe/Paris:20260622T150000
DTSTAMP:20260522T124835Z
CREATED:20260522T124820Z
LAST-MODIFIED:20260522T124835Z
UID:10000160-1782136800-1782140400@sfp-alpes.fr
SUMMARY:Maurizio SACCHI (CNRS - Institut des NanoSciences de Paris and Synchrotron SOLEIL)
DESCRIPTION:X-rays with Orbital Angular Momentum for spectroscopy and imaging\nRésumé : \nIn addition to the spin angular momentum (SAM) associated to the light polarization\, Laguerre-Gaussian light beams carry also an orbital angular momentum (OAM) of ℓ /photon [1]ℏ associated to an azimuthal dependence exp(iℓϕ) of the electric field phase. Over the last thirty years\, OAM beams at vis-IR wavelengths found applications in fields as different as biology\, telecommunication and imaging [2]. The azimuthal phase dependence\, with a singularity on the propagation axis\, is accompanied by a radial modulation of the intensity (ring-shaped beams)\, properties that have been used to modify local magnetic ordering\, to improve the spatial resolution in microscopy\, and to enhance the edge sharpness in phase-contrast imaging. \nOver the last decade\, several approaches to the generation of OAM beams at shorter wavelengths\, from XUV to hard x-rays\, were proposed. Potential applications are often based on the extrapolation of previous work carried out in the vis-IR range. For instance\, as for the SAM\, the handedness imposed by the OAM has been exploited to study magnetic materials [3] and chiral molecules [4]. The interest of extending the use of OAM beams from the vis-IR to the x-ray range has been growing steadily over the last few years. Nonetheless\, the offer of user accessible beamlines and endstations remains limited\, especially when one aims at independently varying both SAM and OAM in a controlled way. At the SEXTANTS beamline of the SOLEIL synchrotron\, we have implemented and commissioned a new setup for soft x-ray spectroscopy (absorption and resonant scattering experiments) with OAM beams [5]. \n1. L. Allen at al.\, Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes\, Phys. Rev. A 45\, 8185 (1992).\n2. Y. Shen et al.\, Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities\, Light: Science & Applications 8\, 90 (2019).\n3. M. Fanciulli et al.\, Electromagnetic theory of Helicoidal Dichroism in reflection from magnetic structures\, Phys. Rev. A 103\, 013501 (2021); Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices\, Phys. Rev. Lett. 128\, 077401 (2022); Magnetic vortex dynamics probed by time-resolved magnetic helicoidal\ndichroism\, Phys. Rev.Lett. (2025).\n4. J. R. Rouxel et al.\, Hard X-ray helical dichroism of disordered molecular media\, Nature Phot. 16\, 570 (2022).\n5. P. Carrara et al.\, Soft x-rays with Orbital Angular Momentum for resonant scattering experiments at the SOLEIL synchrotron\, J. Synchr. Rad. 33\, 858 (2026). \nContact : matteo.dastuto@neel.cnrs.fr
URL:https://sfp-alpes.fr/event/maurizio-sacchi-cnrs-institut-des-nanosciences-de-paris-and-synchrotron-soleil/
LOCATION:CNRS – Salle Louis Weil (E424)\, CNRS - Institut Néel 25 avenue des Martyrs\, Grenoble\, 38042\, France
CATEGORIES:Séminaire
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DTSTART;TZID=Europe/Paris:20260629T140000
DTEND;TZID=Europe/Paris:20260629T150000
DTSTAMP:20260604T134030Z
CREATED:20260604T134030Z
LAST-MODIFIED:20260604T134030Z
UID:10000185-1782741600-1782745200@sfp-alpes.fr
SUMMARY:Dario DAGHERO (Department of Applied Science and Technology\, Politecnico di Torino)
DESCRIPTION:Point Contact Andreev-Reflection Spectroscopy : mechanisms\, models and examples\nRésumé : \nPoint-contact spectroscopy [1] in superconductors\, also known as point-contact Andreev-Reflection spectroscopy (PCARS) is a simple but powerful and versatile technique that allows a direct determination of the number\, the amplitude and the symmetry of the energy gap(s) in superconducting materials [2\,3]. The technique is rather simple in principle\, i.e. it just consists in creating a small (point-like) contact between a normal metal and a superconductor\, and to measure its differential conductance as a function of the bias voltage across the junction. However\, there are several complications that make this simple recipe fairly difficult to realize in practice. First of all\, the contact must be in the spectroscopic regime [1\,2\,3]\, i.e. electrons from the normal metal must be injected in the superconductor with an excess energy that coincides with eV\, V being the bias voltage. Hence\, they must not lose energy in the banks and in the contact itself. The ideal condition is that of ballistic conduction through the N/S interface\, which ensures no Joule effect and requires in turns that the contact size is smaller than both the coherence length and the electronic mean free path in the superconductor.\nWhen these conditions are met\, the conduction through the contact is dominated by Andreev reflection\, a quantum phenomenon that is responsible for the conversion of the normal current into supercurrent\, and occurs in a specific range of voltages (electron energies) set by the amplitude of the superconducting gap. Several models have been proposed to describe the phenomenon and are currently used to extract information on the amplitude and symmetry of the order parameter from the spectra. The simplest one [4] was only suited for superconductors with an isotropic (s-wave) gap\, but has been successfully generalized to the case of layered materials with anisotropic gaps\, like cuprates [5\,6] or strontium ruthenate [7]) and finally to the 3D case\, while taking into account the shape of the actual Fermi surface [3]. The latter generalization allows calculating the point-contact spectrum for any symmetry of the order parameter\, including exotic ones with horizontal node lines.\nAfter discussing these general aspects\, I will briefly describe the application of the technique to some example materials\, from the conventional multiband superconductors MgB2 [8] to unconventional ones like Pu-based heavy fermion compounds [9] or Fe-based compounds [3]\, to transition-metal dichalcogenides [10]. \nReferences\n1. Y. G. Naidyuk and I. K. Yanson\, Point-Contact Spectroscopy\, Springer Series in Solid-State Sciences\, Vol. 145 (Springer\, 2004).\n2. D. Daghero and R.S. Gonnelli\, Supercond. Sci. Technol. 23\, 043001 (2010).\n3. D. Daghero et al.\, Rep. Prog. Phys. 74\, 124509 (2011).\n4. G. E. Blonder\, M. Tinkham and T. M. Klapwijk\, Phys. Rev. B 25\, 4515 (1982)\n5. Y. Tanaka and S. Kashiwaya\, Phys. Rev. Lett. 74\, 3451 (1995)\n6. S. Kashiwaya and Y. Tanaka\, Rep. Prog. Phys. 63\, 1641 (2000).\n7. M. Yamashiro\, Y. Tanaka\, and S. Kashiwaya\, Phys. Rev. B 56\, 7847 (1997)\n8. R. S. Gonnelli et al.\, Phys Rev. Lett. 89\, 247004 (2002)\n9. D. Daghero et al.\, Nature Communications 3\, 786 (2012)\n10. E. Piatti et al.\, Materials Today Physics 59 (2025) 101883 \n_ \nContact : matteo.dastuto@neel.cnrs.fr
URL:https://sfp-alpes.fr/event/dario-daghero-department-of-applied-science-and-technology-politecnico-di-torino/
LOCATION:CNRS – Salle Louis Weil (E424)\, CNRS - Institut Néel 25 avenue des Martyrs\, Grenoble\, 38042\, France
CATEGORIES:Séminaire
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