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Ultrafast Formation and Annihilation of Strongly Bound, Anisotropic Excitons
Authors:
Lawson T. Lloyd,
Tommaso Pincelli,
Mohamed Amine Wahada,
Alessandro De Vita,
Ferdinand Menzel,
Kseniia Mosina,
Túlio H. L. G. Castro,
Alexander Neef,
Andreas V. Stier,
Nathan P. Wilson,
Zdeněk Sofer,
Jonathan J. Finley,
Martin Wolf,
Laurenz Rettig,
Ralph Ernstorfer
Abstract:
Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the qu…
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Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large exciton binding energy (~800 meV) and a highly anisotropic momentum space distribution of the exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.
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Submitted 25 August, 2026; v1 submitted 27 March, 2026;
originally announced March 2026.
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Valence Modifications in Hygroscopic VI3 Degraded Crystals via Soft X-Ray Synchrotron Radiation
Authors:
A. De Vita,
V. Polewczyk,
G. Panaccione,
G. Vinai
Abstract:
Among van der Waals crystals, transition metal trihalide VI3 has driven attention for its magnetic and orbital properties. However, its chemical instability under ambient conditions make its exploitation challenging for technological implementation. In this context, here we show how synchrotron radiation soft X-rays partially restore stoichiometric chemical and electronic properties of VI3 crystal…
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Among van der Waals crystals, transition metal trihalide VI3 has driven attention for its magnetic and orbital properties. However, its chemical instability under ambient conditions make its exploitation challenging for technological implementation. In this context, here we show how synchrotron radiation soft X-rays partially restore stoichiometric chemical and electronic properties of VI3 crystals. By combining X-ray absorption and X-ray photoemission spectroscopies, we show as-cleaved and aged (in ultra-high vacuum conditions) chemical degradation of VI3 crystal surface, with the formation of vanadates, and its, at least partial, recovery under high-flux soft X-ray beam exposure, revealing that superficial hygroscopic contamination couples relatively weakly to the crystal surface.
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Submitted 3 July, 2026; v1 submitted 5 December, 2025;
originally announced December 2025.
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Uncovering surface states of the Dirac semimetal BaMg2Bi2
Authors:
A. De Vita,
J. Bakkelund,
H. Świątek,
M. J. Winiarski,
S. Malick,
C. V. B. Nielsen,
F. Bertran,
A. J. H. Jones,
P. Majchrzak,
F. Miletto Granozio,
J. A. Miwa,
R. Ernstorfer,
T. Pincelli,
T. Klimczuk,
C. Bigi,
F. Mazzola
Abstract:
BaMg2Bi2 is a Dirac semimetal characterized by a simple Dirac cone crossing the Fermi level at the center of the Brillouin zone, protected by C3 rotational symmetry. Together with its Sr-based analogue SrMg2Bi2, it has been proposed as a promising candidate for a chemically driven topological switch: while SrMg2Bi2 is an insulator, BaMg2Bi2 exhibits non-trivial topological features. A detailed und…
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BaMg2Bi2 is a Dirac semimetal characterized by a simple Dirac cone crossing the Fermi level at the center of the Brillouin zone, protected by C3 rotational symmetry. Together with its Sr-based analogue SrMg2Bi2, it has been proposed as a promising candidate for a chemically driven topological switch: while SrMg2Bi2 is an insulator, BaMg2Bi2 exhibits non-trivial topological features. A detailed understanding of its electronic structure is essential to elucidate its electronic and transport properties. Previous photoemission studies confirmed the Dirac nature of BaMg2Bi2, but were limited to high photon energies, which hindered direct comparison with density functional theory calculations (DFT), due to reduced resolution and higher-frequency matrix-element modulation in that regime. In this work, we combine high-resolution angle-resolved photoemission spectroscopy (ARPES) and DFT calculations to get full insight on the valence band states, providing a comprehensive picture of the low-energy electronic structure. Our measurements reveal the presence of previously unobserved surface states. We found that they are topologically trivial, but they unlock a more comprehensive understanding of the material's behavior, reconciling previous discrepancies between experiment and theory.
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Submitted 5 December, 2025;
originally announced December 2025.
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Orbital mixing and strong Hund's coupling stabilize spin order in van der Waals ferromagnet CrI3
Authors:
Alessandro De Vita,
Srdjan Stavrić,
Roberto Sant,
Nicholas B. Brookes,
Ivana Vobornik,
Giancarlo Panaccione,
Silvia Picozzi,
Martin Wolf,
Laurenz Rettig,
Ralph Ernstorfer,
Tommaso Pincelli
Abstract:
Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3d-transition metal elements, combined with chalcogenides or halogens. Despite the high technological promise in the field of spintronics, the connection between the d-orbital configuration and the occurrence of low-dimensional magnetic order is curr…
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Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3d-transition metal elements, combined with chalcogenides or halogens. Despite the high technological promise in the field of spintronics, the connection between the d-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI3, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI3 in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I p and Cr eg states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.
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Submitted 9 July, 2026; v1 submitted 5 July, 2025;
originally announced July 2025.
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Discovery of magnetic-field-tunable density waves in a layered altermagnet
Authors:
Christopher Candelora,
Muxian Xu,
Siyu Cheng,
Alessandro De Vita,
Davide Romanin,
Chiara Bigi,
My Bang Petersen,
Alexander LaFleur,
Matteo Calandra,
Jill Miwa,
Younghun Hwang,
Ziqiang Wang,
Federico Mazzola,
Ilija Zeljkovic
Abstract:
Altermagnets recently came into the spotlight as a new class of magnetic materials, arising as a consequence of specific crystal symmetries. They are characterized by a spin-polarized electronic band structure similar to ferromagnets, but with net zero magnetization, and touted as a promising platform to host a slew of exotic properties, many of which are yet to be explored. Here we study a new la…
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Altermagnets recently came into the spotlight as a new class of magnetic materials, arising as a consequence of specific crystal symmetries. They are characterized by a spin-polarized electronic band structure similar to ferromagnets, but with net zero magnetization, and touted as a promising platform to host a slew of exotic properties, many of which are yet to be explored. Here we study a new layered triangular lattice altermagnet, Co-intercalated NbSe$_2$ using scanning tunneling microscopy and spectroscopy (STM/S). Differential conductance dI/dV spectra at low temperature reveal a surprising partial gap opening centered at the Fermi level, which is not captured by density functional theory calculations of the system in the pure altermagnetic state. Spatial mapping using spectroscopic-imaging STM and spin-polarized STM further reveals emergent tri-directional charge and spin density modulations with a 2a$_0$ wave length. Interestingly, we discover that out-of-plane magnetic field can serve as a knob to tune the amplitudes of the modulations as well as alter the overall electronic density-of-states in a manner that is strongly dependent on the field direction and strength. This can be attributed to the tilting of spins by the external magnetic field, which can have profound implications on the electronic properties of the altermagnet. By providing elusive atomic-scale insights, our work uncovers a tunable density wave accompanied by concomitant changes in the electronic band structure, and sets the foundation for studies of correlated electronic phenomena in altermagnets.
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Submitted 6 June, 2025; v1 submitted 5 March, 2025;
originally announced March 2025.
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Robust spin splitting and fermiology in a layered altermagnet
Authors:
Alessandro De Vita,
Chiara Bigi,
Davide Romanin,
Matthew D. Watson,
Vincent Polewczyk,
Marta Zonno,
François Bertran,
My Bang Petersen,
Federico Motti,
Giovanni Vinai,
Manuel Tuniz,
Federico Cilento,
Mario Cuoco,
Brian M. Andersen,
Andreas Kreisel,
Luciano Jacopo D'Onofrio,
Oliver J. Clark,
Mark T. Edmonds,
Christopher Candelora,
Muxian Xu,
Siyu Cheng,
Alexander LaFleur,
Tommaso Antonelli,
Giorgio Sangiovanni,
Lorenzo Del Re
, et al. (20 additional authors not shown)
Abstract:
Altermagnetism defies conventional classifications of collinear magnetic phases, standing apart from ferromagnetism and antiferromagnetism with its unique combination of spin-dependent symmetries, net-zero magnetization, and anomalous Hall transport. Although altermagnetic states have been realized experimentally, their integration into functional devices has been hindered by the structural rigidi…
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Altermagnetism defies conventional classifications of collinear magnetic phases, standing apart from ferromagnetism and antiferromagnetism with its unique combination of spin-dependent symmetries, net-zero magnetization, and anomalous Hall transport. Although altermagnetic states have been realized experimentally, their integration into functional devices has been hindered by the structural rigidity and poor tunability of existing materials. First, through cobalt intercalation of the superconducting 2H-NbSe$_2$ polymorph, we induce and stabilize a robust altermagnetic phase and using both theory and experiment, we directly observe the lifting of Kramers degeneracy. Additionally, we present spectroscopic insight into a previously hinted low-temperature phase, and provide evidence of its electronic origin. While shedding light on overlooked aspects of altermagnetism, these findings open pathways to spin-based technologies and lay a foundation for advancing the emerging field of altertronics.
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Submitted 20 January, 2026; v1 submitted 27 February, 2025;
originally announced February 2025.
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Anomalous spin-optical helical effect in Ti-based kagome metal
Authors:
Federico Mazzola,
Wojciech Brzezicki,
Chiara Bigi,
Armando Consiglio,
Luciano Jacopo D' Onofrio,
Maria Teresa Mercaldo,
Adam Kłosiński,
François Bertran,
Patrick Le Fèvre,
Oliver J. Clark,
Mark T. Edmonds,
Manuel Tuniz,
Alessandro De Vita,
Vincent Polewczyk,
Jeppe B. Jacobsen,
Henrik Jacobsen,
Jill A. Miwa,
Justin W. Wells,
Anupam Jana,
Ivana Vobornik,
Jun Fujii,
Niccolò Mignani,
Narges Samani Tarakameh,
Alberto Crepaldi,
Giorgio Sangiovanni
, et al. (10 additional authors not shown)
Abstract:
The kagome lattice stands as a rich platform for hosting a wide array of correlated quantum phenomena, ranging from charge density waves and superconductivity to electron nematicity and loop current states. Direct detection of loop currents in kagome systems has remained a formidable challenge due to their intricate spatial arrangements and the weak magnetic field signatures they produce. This has…
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The kagome lattice stands as a rich platform for hosting a wide array of correlated quantum phenomena, ranging from charge density waves and superconductivity to electron nematicity and loop current states. Direct detection of loop currents in kagome systems has remained a formidable challenge due to their intricate spatial arrangements and the weak magnetic field signatures they produce. This has left their existence and underlying mechanisms a topic of intense debate. In this work, we uncover a hallmark reconcilable with loop currents: spin handedness-selective signals that surpass conventional dichroic, spin, and spin-dichroic responses. We observe this phenomenon in the kagome metal CsTi$_3$Bi$_5$ and we call it the anomalous spin-optical helical effect. This effect arises from the coupling of light' s helicity with spin-orbital electron correlations, providing a groundbreaking method to visualize loop currents in quantum materials. Our discovery not only enriches the debate surrounding loop currents but also paves the way for new strategies to exploit the electronic phases of quantum materials via light-matter interaction.
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Submitted 26 February, 2025;
originally announced February 2025.
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Bilayer orthogonal ferromagnetism in CrTe$_2$-based van der Waals system
Authors:
Chiara Bigi,
Cyriack Jego,
Vincent Polewczyk,
Alessandro De Vita,
Thomas Jaouen,
Hulerich C. Tchouekem,
François Bertran,
Patrick Le Fèvre,
Pascal Turban,
Jean-François Jacquot,
Jill A. Miwa,
Oliver J. Clark,
Anupam Jana,
Sandeep Kumar Chaluvadi,
Pasquale Orgiani,
Mario Cuoco,
Mats Leandersson,
Thiagarajan Balasubramanian,
Thomas Olsen,
Younghun Hwang,
Matthieu Jamet,
Federico Mazzola
Abstract:
Systems with pronounced spin anisotropy play a pivotal role in advancing magnetization switching and spin-wave generation mechanisms, which are fundamental for spintronic technologies. Quasi-van der Waals ferromagnets, particularly Cr$_{1+δ}$Te$_2$ compounds, represent seminal materials in this field, renowned for their delicate balance between frustrated layered geometries and magnetism. Despite…
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Systems with pronounced spin anisotropy play a pivotal role in advancing magnetization switching and spin-wave generation mechanisms, which are fundamental for spintronic technologies. Quasi-van der Waals ferromagnets, particularly Cr$_{1+δ}$Te$_2$ compounds, represent seminal materials in this field, renowned for their delicate balance between frustrated layered geometries and magnetism. Despite extensive investigation, the precise nature of their magnetic ground state, typically described as a canted ferromagnet, remains contested, as does the mechanism governing spin reorientation under external magnetic fields and varying temperatures. In this work, we leverage a multimodal approach, integrating complementary techniques, to reveal that Cr$_{1+δ}$Te$_2$ ($δ= 0.25 - 0.50$) hosts a previously overlooked magnetic phase, which we term orthogonal-ferromagnetism. This single phase consists of alternating atomically sharp single layers of in-plane and out-of-plane ferromagnetic blocks, coupled via exchange interactions and as such, it differs significantly from crossed magnetism, which can be achieved exclusively by stacking multiple heterostructural elements together. Contrary to earlier reports suggesting a gradual spin reorientation in CrTe$_2$-based systems, we present definitive evidence of abrupt spin-flop-like transitions. This discovery, likely due to the improved crystallinity and lower defect density in our samples, repositions Cr$_{1+δ}$Te$_2$ compounds as promising candidates for spintronic and orbitronic applications, opening new pathways for device engineering.
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Submitted 13 December, 2024;
originally announced December 2024.
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Pomeranchuk instability from electronic correlations in CsTi$_3$Bi$_5$ kagome metal
Authors:
Chiara Bigi,
Matteo Dürrnagel,
Lennart Klebl,
Armando Consiglio,
Ganesh Pokharel,
Francois Bertran,
Patrick Le Févre,
Thomas Jaouen,
Hulerich C. Tchouekem,
Pascal Turban,
Alessandro De Vita,
Jill A. Miwa,
Justin W. Wells,
Dongjin Oh,
Riccardo Comin,
Ronny Thomale,
Ilija Zeljkovic,
Brenden R. Ortiz,
Stephen D. Wilson,
Giorgio Sangiovanni,
Federico Mazzola,
Domenico Di Sante
Abstract:
Among many-body instabilities in correlated quantum systems, electronic nematicity, defined by the spontaneous breaking of rotational symmetry, has emerged as a critical phenomenon, particularly within high-temperature superconductors. Recently, this behavior has been identified in CsTi$_3$Bi$_5$, a member of the AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome family, recognized for its intricate and unconven…
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Among many-body instabilities in correlated quantum systems, electronic nematicity, defined by the spontaneous breaking of rotational symmetry, has emerged as a critical phenomenon, particularly within high-temperature superconductors. Recently, this behavior has been identified in CsTi$_3$Bi$_5$, a member of the AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome family, recognized for its intricate and unconventional quantum phases. Despite accumulating indirect evidence, the fundamental mechanisms driving nematicity in CsTi$_3$Bi$_5$ remain inadequately understood, sparking ongoing debates. In this study, we employ polarization-dependent angle-resolved photoemission spectroscopy to reveal definitive signatures of an orbital-selective nematic deformation in the electronic structure of CsTi$_3$Bi$_5$. This direct experimental evidence underscores the pivotal role of orbital degrees of freedom in symmetry breaking, providing new insights into the complex electronic environment. By applying the functional renormalization group technique to a fully interacting ab initio model, we demonstrate the emergence of a finite angular momentum ($d$-wave) Pomeranchuk instability in CsTi$_3$Bi$_5$, driven by the concomitant action of electronic correlations within specific orbital channels and chemical potential detuning away from Van Hove singularities. By elucidating the connection between orbital correlations and symmetry-breaking instabilities, this work lays a crucial foundation for future investigations into the broader role of orbital selectivity in quantum materials, with far-reaching implications for the design and manipulation of novel electronic phases.
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Submitted 30 October, 2024;
originally announced October 2024.
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Evidence of temperature-dependent interplay between spin and orbital moment in van der Waals ferromagnet VI3
Authors:
A. De Vita,
R. Sant,
V. Polewczyk,
G. van der Laan,
N. B. Brookes,
T. Kong,
R. J. Cava,
G. Rossi,
G. Vinai,
G. Panaccione
Abstract:
Van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and the fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent x-ray absorption spectroscop…
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Van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and the fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent x-ray absorption spectroscopy and x-ray magnetic circular and linear dichroism. We detect localized electronic states and reduced magnetic dimensionality, due to electronic correlations. We furthermore provide experimental evidence of (a) an unquenched orbital magnetic moment (up to 0.66(7)) in the ferromagnetic state, and (b) an instability of the orbital moment in proximity of the spin reorientation transition. Our results support a coherent picture where electronic correlations give rise to a strong magnetic anisotropy and a large orbital moment, and establish VI3 as a prime candidate for the study of orbital quantum effects.
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Submitted 6 February, 2024;
originally announced February 2024.
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Observation of termination-dependent topological connectivity in a magnetic Weyl kagome-lattice
Authors:
Federico Mazzola,
Stefan Enzner,
Philipp Eck,
Chiara Bigi,
Matteo Jugovac,
Iulia Cojocariu,
Vitaliy Feyer,
Zhixue Shu,
Gian Marco Pierantozzi,
Alessandro De Vita,
Pietro Carrara,
Jun Fujii,
Phil D. C. King,
Giovanni Vinai,
Pasquale Orgiani,
Cephise Cacho,
Matthew D. Watson,
Giorgio Rossi,
Ivana Vobornik,
Tai Kong,
Domenico Di Sante,
Giorgio Sangiovanni,
Giancarlo Panaccione
Abstract:
Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designer, with the opportunity of driving new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co$_3$Sn$_2$S$_2$ and show how for different sample's terminations the Weyl-points connect also differently, still preserv…
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Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designer, with the opportunity of driving new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co$_3$Sn$_2$S$_2$ and show how for different sample's terminations the Weyl-points connect also differently, still preserving the bulk-boundary correspondence. Scanning-tunnelling microscopy has suggested such a scenario indirectly. Here, we demonstrate this directly for the fermiology of Co$_3$Sn$_2$S$_2$, by linking it to the system real space surfaces distribution. By a combination of micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co$_3$Sn$_2$S$_2$ for different surface terminations and show the existence of topological features directly depending on the top-layer electronic environment. Our work helps to define a route to control bulk-derived topological properties by means of surface electrostatic potentials, creating a realistic and reliable methodology to use Weyl kagome metals in responsive magnetic spintronics.
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Submitted 18 August, 2023;
originally announced August 2023.
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Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet
Authors:
R. Sant,
A. De Vita,
V. Polewczyk,
G. Pierantozzi,
F. Mazzola,
G. Vinai,
G. van der Laan,
G. Panaccione,
N. B. Brookes
Abstract:
Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI$_3$ van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within…
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Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI$_3$ van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI$_6$ unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V$^{3+}$ ion due to the unbalanced hybridization between the Vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the $t_{2g}^2$ ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn-Teller mechanism. Our results clarify the role of the distortion in VI$_3$ and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.
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Submitted 9 May, 2023;
originally announced May 2023.
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Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles
Authors:
Maria Sygletou,
Stefania Benedetti,
Marzia Ferrera,
Gian Marco Pierantozzi,
Riccardo Cucini,
Giuseppe Della Valle,
Pietro Carrara,
Alessandro De Vita,
Alessandro di Bona,
Piero Torelli,
Daniele Catone,
Giancarlo Panaccione,
Maurizio Canepa,
Francesco Bisio
Abstract:
The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambigu…
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The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.
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Submitted 30 June, 2021;
originally announced July 2021.
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Gaussian Process States: A data-driven representation of quantum many-body physics
Authors:
Aldo Glielmo,
Yannic Rath,
Gabor Csanyi,
Alessandro De Vita,
George H. Booth
Abstract:
We present a novel, non-parametric form for compactly representing entangled many-body quantum states, which we call a `Gaussian Process State'. In contrast to other approaches, we define this state explicitly in terms of a configurational data set, with the probability amplitudes statistically inferred from this data according to Bayesian statistics. In this way the non-local physical correlated…
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We present a novel, non-parametric form for compactly representing entangled many-body quantum states, which we call a `Gaussian Process State'. In contrast to other approaches, we define this state explicitly in terms of a configurational data set, with the probability amplitudes statistically inferred from this data according to Bayesian statistics. In this way the non-local physical correlated features of the state can be analytically resummed, allowing for exponential complexity to underpin the ansatz, but efficiently represented in a small data set. The state is found to be highly compact, systematically improvable and efficient to sample, representing a large number of known variational states within its span. It is also proven to be a `universal approximator' for quantum states, able to capture any entangled many-body state with increasing data set size. We develop two numerical approaches which can learn this form directly: a fragmentation approach, and direct variational optimization, and apply these schemes to the Fermionic Hubbard model. We find competitive or superior descriptions of correlated quantum problems compared to existing state-of-the-art variational ansatzes, as well as other numerical methods.
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Submitted 17 September, 2020; v1 submitted 27 February, 2020;
originally announced February 2020.
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Space Charge Free Ultrafast Photoelectron Spectroscopy on Solids by a Narrowband Tunable Extreme Ultraviolet Light Source
Authors:
Riccardo Cucini,
Tommaso Pincelli,
Giancarlo Panaccione,
Damir Kopic,
Fabio Frassetto,
Paolo Miotti,
Gian Marco Pierantozzi,
Simone Peli,
Andrea Fondacaro,
Aleksander De Luisa,
Alessandro De Vita,
Pietro Carrara,
Damjan Krizmancic,
Daniel T. Payne,
Federico Salvador,
Andrea Sterzi,
Luca Poletto,
Fulvio Parmigiani,
Giorgio Rossi,
Federico Cilento
Abstract:
Here we report on a novel High Harmonic Generation (HHG) light source designed for space charge free ultrafast photoelectron spectroscopy (PES) on solids. The ultimate overall energy resolution achieved on a polycrystalline Au sample is ~22 meV at 40 K. These results have been obtained at a photon energy of 16.9 eV with a pulse bandwidth of ~19 meV, by varying, up to 200 kHz, the photon pulses rep…
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Here we report on a novel High Harmonic Generation (HHG) light source designed for space charge free ultrafast photoelectron spectroscopy (PES) on solids. The ultimate overall energy resolution achieved on a polycrystalline Au sample is ~22 meV at 40 K. These results have been obtained at a photon energy of 16.9 eV with a pulse bandwidth of ~19 meV, by varying, up to 200 kHz, the photon pulses repetition rate and the photon fluence on the sample. These features set a new benchmark for tunable narrowband HHG sources. By comparing the PES energy resolution and the photon pulse bandwidth with a pulse duration of ~105 fs, as retrieved from time-resolved (TR) angle resolved (AR) PES experiments on Bi$_2$Se$_3$, we validate a way for a space charge free photoelectric process close to Fourier transform limit conditions for ultrafast TR-PES experiments on solids.
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Submitted 11 October, 2019;
originally announced October 2019.
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Imeall: A Computational Framework for the Calculation of the Atomistic Properties of Grain Boundaries
Authors:
Henry Lambert,
Adam Fekete,
James R Kermode,
A. De Vita
Abstract:
We describe the \texttt{Imeall} package for the calculation and indexing of atomistic properties of grain boundaries in materials. The package provides a structured database for the storage of atomistic structures and their associated properties, equipped with a programmable application interface to interatomic potential calculators. The database adopts a general indexing system that allows storin…
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We describe the \texttt{Imeall} package for the calculation and indexing of atomistic properties of grain boundaries in materials. The package provides a structured database for the storage of atomistic structures and their associated properties, equipped with a programmable application interface to interatomic potential calculators. The database adopts a general indexing system that allows storing arbitrary grain boundary structures for any crystalline material. The usefulness of the \texttt{Imeall} package is demonstrated by computing, storing, and analysing relaxed grain boundary structures for a dense range of low index orientation axis symmetric tilt and twist boundaries in $α$-iron for various interatomic potentials. The package's capabilities are further demonstrated by carrying out automated structure generation, dislocation analysis, interstitial site detection, and impurity segregation energies across the grain boundary range. All computed atomistic properties are exposed via a web framework, providing open access to the grain boundary repository and the analytic tools suite.
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Submitted 29 September, 2017;
originally announced September 2017.
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Accurate Interatomic Force Fields via Machine Learning with Covariant Kernels
Authors:
Aldo Glielmo,
Peter Sollich,
Alessandro De Vita
Abstract:
We present a novel scheme to accurately predict atomic forces as vector quantities, rather than sets of scalar components, by Gaussian Process (GP) Regression. This is based on matrix-valued kernel functions, on which we impose the requirements that the predicted force rotates with the target configuration and is independent of any rotations applied to the configuration database entries. We show t…
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We present a novel scheme to accurately predict atomic forces as vector quantities, rather than sets of scalar components, by Gaussian Process (GP) Regression. This is based on matrix-valued kernel functions, on which we impose the requirements that the predicted force rotates with the target configuration and is independent of any rotations applied to the configuration database entries. We show that such covariant GP kernels can be obtained by integration over the elements of the rotation group SO(d) for the relevant dimensionality d. Remarkably, in specific cases the integration can be carried out analytically and yields a conservative force field that can be recast into a pair interaction form. Finally, we show that restricting the integration to a summation over the elements of a finite point group relevant to the target system is sufficient to recover an accurate GP. The accuracy of our kernels in predicting quantum-mechanical forces in real materials is investigated by tests on pure and defective Ni, Fe and Si crystalline systems.
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Submitted 8 June, 2017; v1 submitted 10 November, 2016;
originally announced November 2016.
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Thermal Contraction and Disordering of the Al(110) Surface
Authors:
Nicola Marzari,
David Vanderbilt,
Alessandro De Vita,
M. C. Payne
Abstract:
Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface…
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Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface.
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Submitted 9 March, 1999;
originally announced March 1999.
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Amorphous indium phosphide from first principles
Authors:
Laurent J. Lewis,
Alessandro De Vita,
Roberto Car
Abstract:
We report detailed and extensive first-principles molecular-dynamics (MD) simulations of the structure and electronic properties of amorphous InP produced by rapid quenching from the liquid. The structure of the material is found to be strongly ordered chemically, even though there is a significant number of coordination defects and despite the presence of odd-membered rings. We find, as a conse…
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We report detailed and extensive first-principles molecular-dynamics (MD) simulations of the structure and electronic properties of amorphous InP produced by rapid quenching from the liquid. The structure of the material is found to be strongly ordered chemically, even though there is a significant number of coordination defects and despite the presence of odd-membered rings. We find, as a consequence, that there exists ``wrong bonds'' in the system, in an amount of about 8%; these result from the presence of coordination defects, not of local composition fluctuations, as has been conjectured. The system, in fact, is found to be over-coordinated, which might be the reason for the observed higher density of a-InP compared to c-InP. We have also investigated the possibility of pressure-amorphizing InP. Our calculations indicate that the cost of a transformation of the compressed zinc-blende crystal into an amorphous phase is so large that it is very unlikely that it would take place.
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Submitted 21 May, 1997;
originally announced May 1997.