Publications

2021

• First-principles study of excitons in the optical spectra of silver chloride
  
  • Lorin Arnaud
  • Gatti Matteo
  • Reining Lucia
  • Sottile Francesco
  Physical Review B, American Physical Society, 2021, 104, pp.235149. Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong excitonic effects, which are correctly captured only by solving the Bethe-Salpeter equation starting from quasiparticle self-consistent GW results. Numerical simulations are made feasible by using a model screening for the electron-hole interaction in a way that avoids the calculation of the static dielectric constant. A thorough analysis permits us to discuss localization in bright and dark excitons of silver chloride. (10.1103/PhysRevB.104.235149)
  DOI : 10.1103/PhysRevB.104.235149
• Theoretical description of non-linear process in magnetic materials
  
  • Rani Shalu
  , 2021. Second harmonic generation (SHG) is a process in which two photons of energy ω are absorbed by a material and a photon of energy 2ω is emitted. This process is theoretically described by the second order macroscopic susceptibility χ((2). This spectroscopy is used to study the optical properties of materials and it reveals additional information which cannot be accessed with linear optical spectroscopies. Indeed, as the dipolar selection rules prohibit SHG in centro-symmetric materials, it is possible to obtain a structural and electronic characterization of complex systems. In particular, the absence of time inversion symmetry, due to a magnetic order, reveals new contributions in second harmonic generation. For the specific case of antiferromagnetic materials, magnetic symmetry determines the polarization of the material and SHG then reveals the arrangement of spins in the solid. It was shown that it can be used to study ultrafast processes in magnetic materials, such as demagnetization.There are few satisfactory ab initio theoretical descriptions for nonlinear processes in magnetic materials. These theoretical approaches must be able to treat the electron-electron interactions, the effects of local fields (reflecting the microscopic inhomogeneities in the material) and the spin distribution of the electrons on the same footing.The aim of my thesis was to numerically calculate the optical, linear and second order responses for antiferromagnetic materials. I calculated these two answers for a chromium oxide (Cr2O3) as part of an ab-initio formalism, based on TDDFT (Time-Dependent Density Functional). In this approach, the spin distribution was taken into account explicitly and this extension was implemented in the 2light code.The electron-electron interaction is described mathematically by the exchange -correlation kernel fxc which must be approximated. Finding a good approximation for fxc is extremely important because the modeling and the interpretation of experiments is fundamentally based on these approximations. In particular, fxc must be able to describe the excitonic effects (electron-hole interactions) in the optical response. I studied the influence of these approximate kernel on the optical properties of Cr2O3. In the case of absorption spectra, I compared my results to the spectra calculated from the Bethe-Salpeter equation which explicitly takes into account the excitonic effects and I was thus able to demonstrate the presence of a strongly bound exciton.I was finally interested in different possible structures for Cr2O3, differing from each other only in the spin distribution, and I showed that my results can discriminate between these structures.
• Ultrafast dynamics with time-resolved ARPES: photoexcited electrons in monochalcogenide semiconductors
  
  • Chen Zhesheng
  • Caillaux Jonathan
  • Zhang Jiuxiang
  • Papalazarou Evangelos
  • Dong Jingwei
  • Rueff Jean-Pascal
  • Taleb-Ibrahimi Amina
  • Perfetti Luca
  • Marsi Marino
  Comptes Rendus. Physique, Académie des sciences (Paris), 2021, 22 (S2), pp.103-110. (10.5802/crphys.57)
  DOI : 10.5802/crphys.57
• Subpicosecond metamagnetic phase transition in FeRh driven by non-equilibrium electron dynamics
  
  • Pressacco Federico
  • Sangalli Davide
  • Uhlíř Vojtěch
  • Kutnyakhov Dmytro
  • Arregi Jon Ander
  • Agustsson Steinn Ymir
  • Brenner Günter
  • Redlin Harald
  • Heber Michael
  • Vasilyev Dmitry
  • Demsar Jure
  • Schönhense Gerd
  • Gatti Matteo
  • Marini Andrea
  • Wurth Wilfried
  • Sirotti Fausto
  Nature Communications, Nature Publishing Group, 2021, 12 (1). Abstract Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350 ± 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer. (10.1038/s41467-021-25347-3)
  DOI : 10.1038/s41467-021-25347-3
• Tuning the parity mixing of singlet-septet pairing in a half-Heusler superconductor
  
  • Ishihara K.
  • Takenaka T.
  • Miao Y.
  • Mizukami Y.
  • Hashimoto K.
  • Yamashita M.
  • Konczykowski M.
  • Masuki R.
  • Hirayama M.
  • Nomoto T.
  • Arita R.
  • Pavlosiuk O.
  • Wiśniewski P.
  • Kaczorowski D.
  • Shibauchi T.
  Physical Review X, American Physical Society, 2021, 11 (4), pp.041048. In superconductors, electrons with spin s ¼ 1=2 form Cooper pairs whose spin structure is usually singlet (S ¼ 0) or triplet (S ¼ 1). When the electronic structure near the Fermi level is characterized by fermions with angular momentum j ¼ 3=2 due to strong spin-orbit interactions, novel pairing states such as even-parity quintet (J ¼ 2) and odd-parity septet (J ¼ 3) states are allowed. Prime candidates for such exotic states are half-Heusler superconductors, which exhibit unconventional superconducting properties, but their pairing nature remains unsettled. Here, we show that the superconductivity in the noncentrosymmetric half-Heusler LuPdBi can be consistently described by the admixture of isotropic even-parity singlet and anisotropic odd-parity septet pairing, whose ratio can be tuned by electron irradiation. From magnetotransport and penetration depth measurements, we find that carrier concentrations and impurity scattering both increase with irradiation, resulting in a nonmonotonic change of the superconducting gap structure. Our findings shed new light on our fundamental understanding of unconventional superconducting states in topological materials. (10.1103/PhysRevX.11.041048)
  DOI : 10.1103/PhysRevX.11.041048
• Probing spin chirality of photoexcited topological insulators with circular dichroism: multi-dimensional time-resolved ARPES on Bi 2 Te 2 Se and Bi 2 Se 3
  
  • Zhang J.
  • Caillaux J.
  • Chen Z.
  • Konczykowski M.
  • Hruban A.
  • Wołoś A.
  • Materna A.
  • Perfetti L.
  • Papalazarou E.
  • Marsi M.
  Journal of Electron Spectroscopy and Related Phenomena, Elsevier, 2021, 253, pp.147125. Using time-resolved multi-dimensional angle-resolved photoelectron spectroscopy (ARPES) we explore the angular momentum transfer of low energy polarized photons to two prototype topological insulators, Bi 2 Te 2 Se and Bi 2 Se 3. Our comparative study is based on the analysis of circular dichroism in the photoemission yield of photoexcited Dirac states, and reveals that the spin vector of in-gap Dirac electrons in Bi 2 Te 2 Se presents a more pronounced out-of-plane component compared to that of Bi 2 Se 3. We show that the multi-dimensional ARPES approach can be effectively used to observe the spin texture of photoexcited topological insulators, and to unambiguously disentangle experimental geometry and matrix element effects. (10.1016/j.elspec.2021.147125)
  DOI : 10.1016/j.elspec.2021.147125
• Optoelectronic mixing with high-frequency graphene transistors
  
  • Montanaro A.
  • Wei W.
  • de Fazio D.
  • Sassi U.
  • Soavi G.
  • Aversa P.
  • Ferrari A.
  • Happy H.
  • Legagneux P.
  • Pallecchi Emiliano
  Nature Communications, Nature Publishing Group, 2021, 12 (1), pp.2728. Abstract Graphene is ideally suited for optoelectronics. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We show how high speed optoelectronic mixing can be achieved with high frequency (~20 GHz bandwidth) graphene field effect transistors (GFETs). These devices mix an electrical signal injected into the GFET gate and a modulated optical signal onto a single layer graphene (SLG) channel. The photodetection mechanism and the resulting photocurrent sign depend on the SLG Fermi level ( E F ). At low E F (<130 meV), a positive photocurrent is generated, while at large E F (>130 meV), a negative photobolometric current appears. This allows our devices to operate up to at least 67 GHz. Our results pave the way for GFETs optoelectronic mixers for mm-wave applications, such as telecommunications and radio/light detection and ranging (RADAR/LIDARs.) (10.1038/s41467-021-22943-1)
  DOI : 10.1038/s41467-021-22943-1
• Bridge in micron-sized Bi$_2$ Sr$_2$ CaCu$_2$ O$_{8 + y}$ sample act as converging lens for vortices
  
  • Puig Joaquín
  • Cejas Bolecek Néstor René
  • Aragón Sánchez Jazmín
  • Dolz Moira Inés
  • Konczykowski Marcin
  • Fasano Yanina
  Physica C: Superconductivity and its Applications, Elsevier, 2021, 590, pp.1353948. We report on direct imaging of vortex matter nucleated in micron-sized Bi$_2$ Sr$_2$ CaCu$_2$ O$_{8 + y}$ superconducting samples that incidentally present a bridge structure. We find that when nucleating vortices in a field-cooling condition the deck of the bridge acts as a converging lens for vortices. By means of Bitter decoration images allowing us to quantify the enhancement of vortex-vortex interaction energy per unit length in the deck of the bridge, we are able to estimate that the deck is thinner than ∼ 0.6 μm. We show that the structural properties of vortex matter nucleated in micron-sized thin samples are not significantly affected by sample-thickness variations of the order of half a micron, an important information for type-II superconductors-based mesoscopic technological devices. (10.1016/j.physc.2021.1353948)
  DOI : 10.1016/j.physc.2021.1353948
• Programming the microstructure of magnetic nanocomposites in DLP 3D printing
  
  • Lantean Simone
  • Roppolo Ignazio
  • Sangermano Marco
  • Hayoun Marc
  • Dammak Hichem
  • Rizza Giancarlo
  Additive Manufacturing, Elsevier, 2021, 47, pp.102343. The ability to program the behavior of magneto-reactive polymers requires the fine control of their magnetic microstructure during each step of the printing process. Here, a systematic study of magnetically driven self-assembly of Fe3O4 nanoparticles into chain-like structures is presented and used in a 3D printable formulation. The kinetics of chains formation, as well as their rotation, are studied by varying several experimental parameters: i.e. the viscosity of the formulation, the content of nanoparticles, the intensity of the applied magnetic field, and its application time. Experimental results are coupled to numerical simulations based on the dipolar approximation model, and the collected data are used to produce a dataset to precisely program the microstructure during the printing step. Thus, a desired microstructure in a 3D printed piece can be obtained by controlling the orientation and the length of the magnetic chains in each printed layer. This is achieved by modifying a commercial Digital Light Processing (DLP) 3D printer to apply magnetic fields of tunable intensity and direction. Finally, as a proof of concept, a pyramid-like structure was 3D printed, where each layer contains a specific and spatially oriented microstructure. (10.1016/j.addma.2021.102343)
  DOI : 10.1016/j.addma.2021.102343
• L'utilisation de la matière active dans les pratiques de recherche-création : utilisation d'un vocabulaire artistique pour l'impression 4D de polymères magnéto-actifs déployés dans des dispositifs expérimentaux et d'observation
  
  • Desjardins Antoine
  • Rizza Giancarlo
  , 2021. En sciences sociales et en philosophie l'agentivité est définie comme la capacité d'un objet d'agir, sa faculté d'action dans et sur l'environnement qui l'entoure. Dans le cadre de nos pratiques de recherchecréation, nous utilisons l'impression 4D de polymères magnéto-actifs pour questionner la notion de comportement, une notion de plus en plus fondamentale dans la création artistique contemporaine. Le répertoire de formes est le résultat d'une hybridation entre différents registres appartenant au monde naturel, plus précisément le floral et l'animal. L'objectif de notre projet n'est pas celui d'imiter la nature en imprimant des objets programmables avec des mouvements déterministes, mais de combiner des modèles structurels et des éléments naturels pour concevoir et fabriquer des objets à comportement qui manifestent un ensemble d'actions qui ne sont pas prévisibles et qui ne peuvent être déduites directement de leur environnement magnétique afin de créer et explorer des mondes artificiels, allusifs et dystopiques. (10.5281/zenodo.6059525)
  DOI : 10.5281/zenodo.6059525
• A proposal for the solution of the paradox of the double-slit experiment
  
  • Coddens Gerrit
  , 2021. We propose a solution for the apparent paradox of the double-slit experiment within the framework of our reconstruction of quantum mechanics (QM), based on the geometrical meaning of spinors. We argue that the double-slit experiment can be understood much better by considering it as an experiment whereby the particles yield information about the set-up rather than an experiment whereby the set-up yields information about the behaviour of the particles. The probabilities of QM are conditional, whereby the conditions are defined by the macroscopic measuring device. Consequently, they are not uniquely defined by the local interaction probabilities in the point of the interaction. They have to be further fine-tuned in order to fit in seamlessly within the macroscopic probability distribution, by complying to its boundary conditions. When a particle interacts incoherently with the set-up the answer to the question through which slit it has moved is experimentally decidable. When it interacts coherently the answer to that question is experimentally undecidable. We provide a rigorous mathematical proof of the expression $\psi_{3}= \psi_{1} + \psi_{2}$ for the wave function $\psi_{3}$ of the double-slit experiment, whereby $\psi_{1}$ and $\psi_{2}$ are the wave functions of the two related single-slit experiments. This proof is algebraically perfectly logical and exact, but geometrically flawed and meaningless for wave functions. The reason for this weird-sounding distinction is that the wave functions are representations of symmetry groups and that these groups are curved manifolds instead of vector spaces. The identity $\psi_{3}= \psi_{1} + \psi_{2}$ must therefore be replaced in the interference region by the expression $\psi'_{1} + \psi'_{2}$, for which a geometrically correct meaning can be constructed in terms of sets (while this is not possible for $\psi_{1} + \psi_{2}$). This expression has the same numerical value as $\psi_{1} + \psi_{2}$ , such that $\psi'_{1} + \psi'_{2} = \psi_{1} + \psi_{2}$, but with $\psi'_{1} = e^{\imath \pi/4 } (\psi_{1} +\psi_{2})/\sqrt{2} \neq \psi_{1}$ and $\psi'_{2} = e^{-\imath \pi/4 } (\psi_{1} +\psi_{2})/\sqrt{2} \,\neq \psi_{2}$. Here $\psi'_{1}$ and $\psi'_{2}$ are the correct (but experimentally unknowable) contributions from the slits to the total wave function $\psi_{3} = \psi'_{1} + \psi'_{2}$. We have then $p = \vert \psi'_{1} + \psi'_{2}\vert^{2} = \vert\psi'_{1}\vert^{2} + \vert\psi'_{2}\vert^{2} = p'_{1}+p'_{2} $ such that the apparent paradox that quantum mechanics would not follow the traditional rules of probability calculus for mutually exclusive events disappears.
• Crucial Role of Conjugation in Monolayer-Protected Metal Clusters with Aromatic Ligands: Insights from the Archetypal Au 144 L 60 Cluster Compounds
  
  • Sinha-Roy Rajarshi
  • López-Lozano Xóchitl
  • Whetten Robert
  • Weissker Hans-Christian
  Journal of Physical Chemistry Letters, American Chemical Society, 2021, 12 (38), pp.9262-9268. (10.1021/acs.jpclett.1c02597)
  DOI : 10.1021/acs.jpclett.1c02597
• Mixing nanostructured Ni/piezoPVDF composite thin films with e-beam irradiation: A beneficial synergy to piezoelectric response
  
  • Potrzebowska Natalia
  • Cavani Olivier
  • Oral Ozlem
  • Doaré Olivier
  • Melilli Giuseppe
  • Wegrowe Jean-Eric
  • Clochard Marie-Claude.
  Materials Today Communications, Elsevier, 2021, 28, pp.102528. (10.1016/j.mtcomm.2021.102528)
  DOI : 10.1016/j.mtcomm.2021.102528
• Flat bands and giant light-matter interaction in hexagonal boron nitride
  
  • Elias Christine
  • Fugallo Giorgia
  • Valvin Pierre
  • Lhenoret Christian
  • Li Jiahan
  • Edgar James H.
  • Sottile Francesco
  • Lazzeri Michele
  • Ouerghi Abdelkarim
  • Gil Bernard
  • Cassabois Guillaume
  Physical Review Letters, American Physical Society, 2021, 127 (13), pp.137401. Dispersionless energy bands in k space are a peculiar property gathering increasing attention for the emergence of novel electronic, magnetic, and photonic properties. Here, we explore the impact of electronic flat bands on the light-matter interaction. The van der Waals interaction between the atomic layers of hexagonal boron nitride induces flat bands along specific lines of the Brillouin zone. The macroscopic degeneracy along these lines leads to van Hove singularities with divergent joint density of states, resulting in outstanding optical properties of the excitonic states. For the direct exciton, we report a giant oscillator strength with a longitudinal-transverse splitting of 420 meV, a record value, confirmed by our ab initio calculations. For the fundamental indirect exciton, flat bands result in phonon-assisted processes of exceptional efficiency, that compete with direct absorption in reflectivity, and that make the internal quantum efficiency close to values typical of direct band gap semiconductors. (10.1103/PhysRevLett.127.137401)
  DOI : 10.1103/PhysRevLett.127.137401
• Moving Dirac nodes by chemical substitution
  
  • Nilforoushan Niloufar
  • Casula Michele
  • Amaricci Adriano
  • Caputo Marco
  • Caillaux Jonathan
  • Khalil Lama
  • Papalazarou Evangelos
  • Simon Pascal
  • Perfetti Luca
  • Vobornik Ivana
  • Das Pranab Kumar
  • Fujii Jun
  • Barinov Alexei
  • Santos-Cottin David
  • Klein Yannick
  • Fabrizio Michele
  • Gauzzi Andrea
  • Marsi Marino
  Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2021, 118 (33), pp.e2108617118. Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step toward the realization of novel concepts of electronic devices and quantum computation. By means of Angle-Resolved Photo-Emission Spectroscopy (ARPES) experiments and ab initio simulations, here, we show that Dirac states can be effectively tuned by doping a transition metal sulfide, B a N i S 2 , through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge-transfer gap of B a C o 1 − x N i x S 2 across its phase diagram, lead to the formation of Dirac lines, whose position in k-space can be displaced along the Γ − M symmetry direction and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making B a C o 1 − x N i x S 2 a model system to functionalize Dirac materials by varying the strength of electron correlations. (10.1073/pnas.2108617118)
  DOI : 10.1073/pnas.2108617118
• Anatomy of inertial magnons in ferromagnetic nanostructures
  
  • Lomonosov Alexey
  • Temnov Vasily
  • Wegrowe Jean-Eric
  Physical Review B, American Physical Society, 2021, 104 (5), pp.054425. (10.1103/PhysRevB.104.054425)
  DOI : 10.1103/PhysRevB.104.054425
• Spectroscopy for Materials Characterization
  
  • Agnello Simonpietro
  • Alessi Antonino
  • Gelardi Franco Mario
  , 2021 (1). (10.1002/9781119698029.ch9)
  DOI : 10.1002/9781119698029.ch9
• Why spinors do not form a vector space
  
  • Coddens Gerrit
  , 2021. Spinors ψj of SU(2) represent group elements, i.e. three-dimensional rotations Rj , because they are shorthands of SU(2) representation matrices Rj obtained by taking their first columns. We explain that making linear combinations of spinors of SU(2) is feasible algebraically but geometrically meaningless. E.g. ψ1 + ψ2 ∈ C 2 would have to correspond to R1 + R2. But in SO(3), R1 + R2 would be a function that transforms r ∈ R 3 into R1(r) + R2(r), while in SU(2) it would correspond to a different function that is up to a multiplication constant equal to S : r → S(r) = R1(r) + R2(r) + 2 [ cos(ϕ1/2) cos(ϕ2/2) − s1•s2 ] r + 2 sin(ϕ1/2) sin(ϕ2/2)[ (s2•r) s1 + (s1•r) s2 ] + 2 sin(ϕ1/2) cos(ϕ2/2)(s1 ∧ r) + 2 cos(ϕ1/2) sin(ϕ2/2)(s2 ∧ r). The multiplication constant must be chosen such that |r| is preserved. Such extensions by linear combinations for spinors from a manifold to an embedding vector space are therefore mindless algebra with a spurious, conceptually impenetrable geometrical counterpart. This should not surprise anybody because the group axioms only define products of group elements, not linear combinations of them. Redefining spinors as vectors of a Hilbert space is therefore a purely formal, would-be scholar generalization.
• Key parameters for surface plasma wave excitation in the ultra-high intensity regime
  
  • Marini S.
  • Kleij P. S.
  • Amiranoff F.
  • Grech M.
  • Riconda C.
  • Raynaud M.
  Physics of Plasmas, American Institute of Physics, 2021, 28 (7), pp.073104. Ultra-short high-power lasers can deliver extreme light intensities ($\ge 10^{20}$ W/cm$^2$ and $\leq 30 f$s) and drive large amplitude Surface Plasma Wave (SPW) at over-dense plasma surface. The resulting current of energetic electron has great interest for applications, potentially scaling with the laser amplitude, provided the laser-plasma transfer to the accelerated particles mediated by SPW is still efficient at ultra-high intensity. By mean of Particle-in-Cell simulations, we identify the best condition for SPW excitation and show a strong correlation between the optimum Surface Plasma Wave excitation angle and the laser's angle of incidence that optimize the electron acceleration along the plasma surface. We also discuss how plasma density and plasma surface shape can be adjusted in order to push to higher laser intensity the limit of Surface Plasma Wave excitation. Our results open the way to new experiments on forthcoming multi-petawatt laser systems. (10.1063/5.0052599)
  DOI : 10.1063/5.0052599
• Time resolved study of carrier relaxation dynamics in α-Al 2 O 3
  
  • Bildé A
  • Redeckas K
  • Melninkaitis A.
  • Vengris M
  • Guizard S
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2021, 33 (31), pp.315402. The relaxation of excited carriers in α-Al 2 O 3 is complex, depending for instance on the type of ionizing radiation. Using femtosecond time-resolved absorption spectroscopy, we can induce a controllable excitation density on a wide range, and follow the relaxation dynamics from 30 fs to 7 ns. We show that the excited carrier decay is non-exponential: it is dependent on the pump intensity, i.e. on the initial carrier concentration. We describe the relaxation as a two-steps process, involving the trapping of initially free electron-hole pairs, followed by recombination. A numerical model taking into account the initial electronic excitation by multiphoton absorption and the subsequent relaxation allows to quantitatively reproduce the amplitude of the measured absorption and its temporal evolution. (10.1088/1361-648X/ac0475)
  DOI : 10.1088/1361-648X/ac0475
• Spin injection efficiency at metallic interfaces probed by THz emission spectroscopy
  
  • Hawecker Jacques
  • Dang Thi‐huong H
  • Rongione Enzo
  • Boust James
  • Collin Sophie
  • George Jean-Marie
  • Drouhin Henri-Jean
  • Laplace Yannis
  • Grasset Romain
  • Dong Jingwei
  • Mangeney Juliette
  • Tignon Jerome
  • Jaffrès Henri
  • Perfetti Luca
  • Dhillon Sukhdeep
  Advanced Optical Materials, Wiley, 2021, pp.2100412. Terahertz (THz) spin-to-charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here we investigate nanometric trilayers of Co/X/Pt (X=Ti, Au or Au0:85W0:15) as a function of the 'X' layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the 'X' layer we show that the ultrafast spin current injected in the non-magnetic layer defines a direct spin conductance, whereas the Gilbert damping leads to an effective spin mixing-conductance of the trilayer. Importantly, we show that these two parameters are connected to each other and that spinmemory losses can be modeled via an effective Hamiltonian with Rashba fields. This work highlights that magneto-circuits concepts can be successfully extended to ultrafast spintronic devices, as well as enhancing the understanding of spin-to-charge conversion processes through the complementarity between ultrafast THz spectroscopy and steady state techniques. (10.1002/adom.202100412)
  DOI : 10.1002/adom.202100412
• Resonant inelastic x-ray scattering study of doping and temperature dependence of low-energy excitations in La 1 − x Sr x VO 3 thin films
  
  • Ruotsalainen Kari
  • Gatti Matteo
  • Ablett James
  • Yakhou-Harris Flora
  • Rueff Jean-Pascal
  • David Adrian
  • Prellier Wilfrid
  • Nicolaou Alessandro
  Physical Review B, American Physical Society, 2021, 103 (23), pp.235158. We present a temperature- and doping-dependent resonant inelastic x-ray scattering experiment at the V L2,3 and O K edges in La1−xSrxVO3 thin films with x=0 and x=0.1. This material is a canonical example of a compound that exhibits a filling-control metal-insulator transition and undergoes orbital ordering and antiferromagnetic transitions at low temperature. Temperature-dependent measurements at the V L3 edge reveal an intra-t2g excitation that blueshifts by 40 meV from room temperature to 30 K at a rate that differs between the para- and antiferromagnetic phases. The line shape can be partially explained by a purely local model using crystal field theory calculations. For the low Sr concentration x=0.1, the doping is shown to affect the local electronic structure primarily on the O sites, beyond a simple Mott-Hubbard picture. Furthermore, the presence of phonon overtone features at the O K edge evidences that the low-energy part of the spectrum is dominated by phonon response. (10.1103/PhysRevB.103.235158)
  DOI : 10.1103/PhysRevB.103.235158
• Optical properties of Ag$_{29}$(BDT)$_{12}$(TPP)$_4$ in the VIS and UV and influence of ligand modeling based on real-time electron dynamics
  
  • Sinha-Roy Rajarshi
  • López-Lozano Xóchitl
  • Whetten Robert
  • Weissker H.C.
  Theoretical Chemistry Accounts: Theory, Computation, and Modeling, Springer Verlag, 2021, 140 (7). We study the optical properties of the Ag29(BDT)12(TPP)4 cluster, the geometry of which is available from experimental structure determination, by means of Fourier-transformed induced densities from real-time (time evolution) calculations of time-dependent density-functional theory. In particular, we demonstrate the influence of the ligands on the optical spectra in the visible region and, even more, in the UV. A strong peak in the UV reminiscent of the spectrum of isolated benzene is found to be caused by the phenyl rings of the TPP ligand molecules. Nonetheless, their absence in the modeling also impacts the absorption in the visible region substantially. By contrast, the aromatic rings of the BDT ligands are more strongly coupled to the silver core and loose the character of independent oscillators; they contribute a much less peaked UV absorption. Our results underline the importance of properly accounting for the full ligands for precise and reliable modeling. (10.1007/s00214-021-02783-4)
  DOI : 10.1007/s00214-021-02783-4
• Effect of controlled artificial disorder on the magnetic properties of EuFe$_2$(As$_{1−x}$P$_x)_2$ ferromagnetic superconductor
  
  • Ghimire Sunil
  • Kończykowski Marcin
  • Cho Kyuil
  • Tanatar Makariy A
  • Torsello Daniele
  • Veshchunov Ivan S
  • Tamegai Tsuyoshi
  • Ghigo Gianluca
  • Prozorov Ruslan
  Materials, MDPI, 2021, 14 (12), pp.3267. Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2(As1−xPx)2 (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, Tc(H), shows an extraordinarily large decrease. It starts at Tc(H=0)≈24K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only Tc moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in Tc to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe2(As1−xPx)2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu2+ lattice act as efficient pairbreakers leading to a significant Tc reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of Tm (less than 1 K) after the irradiation drives Tc to below Tm. Our results suggest that FM and SC phases coexist microscopically in the same volume. (10.3390/ma14123267)
  DOI : 10.3390/ma14123267
• Dynamical screening in SrVO$_3$: Inelastic x-ray scattering experiments and ab initio calculations
  
  • Sahle Christoph J.
  • Ablett James M.
  • Ruotsalainen Kari
  • Nicolaou Alessandro
  • Efimenko Anna
  • Ablett James
  • Rueff Jean-Pascal
  • Prabhakaran Dharmalingam
  • Gatti Matteo
  Physical Review B, American Physical Society, 2021, 103 (23). We characterize experimentally and theoretically the high-energy dielectric screening properties of the prototypical correlated metal SrVO$_3$. The dynamical structure factor measured by inelastic x-ray scattering spectroscopy as a function of momentum transfer is in very good agreement with first-principles calculations in the adiabatic local density approximation to time-dependent density-functional theory. Our results reveal the crucial importance of crystal local fields in the charge response function of correlated materials: They lead to depolarization effects for localised excitations and couple spectra from different Brillouin zones. (10.1103/PhysRevB.103.235136)
  DOI : 10.1103/PhysRevB.103.235136