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“Unusual structural rearrangement and superconductivity in infinite layer cuprate superlattices”. Samal D, Gauquelin N, Takamura Y, Lobato I, Arenholz E, Van Aert S, Huijben M, Zhong Z, Verbeeck J, Van Tendeloo G, Koster G, Physical review materials 7, 054803 (2023). http://doi.org/10.1103/PhysRevMaterials.7.054803
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.4
DOI: 10.1103/PhysRevMaterials.7.054803
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“Charge order, frustration relief, and spin-orbit coupling in U3O8”. Saniz R, Baldinozzi G, Arts I, Lamoen D, Leinders G, Verwerft M, Physical review materials 7, 054410 (2023). http://doi.org/10.1103/PhysRevMaterials.7.054410
Abstract: Research efforts on the description of the low-temperature magnetic order and electronic properties of U3O8 have been inconclusive so far. Reinterpreting neutron scattering results, we use group representation theory to show that the ground state presents collinear out-of-plane magnetic moments, with antiferromagnetic coupling both in-layer and between layers. Charge order relieves the initial geometric frustration, generating a slightly distorted honeycomb sublattice with Néel-type order. The precise knowledge of the characteristics of this magnetic ground state is then used to explain the fine features of the band gap. In this system, spin-orbit coupling (SOC) is of critical importance, as it strongly affects the electronic structure, narrowing the gap by ∼38%, compared to calculations neglecting SOC. The predicted electronic structure actually explains the salient features of recent optical absorption measurements, further demonstrating the excellent agreement between the calculated ground state properties and experiment.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.4
DOI: 10.1103/PhysRevMaterials.7.054410
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“Imaging the suppression of ferromagnetism in LaMnO₃, by metallic overlayers”. Folkers B, Jansen T, Roskamp TJ, Reith P, Timmermans A, Jannis D, Gauquelin N, Verbeeck J, Hilgenkamp H, Rosario CMM, Physical review materials 8, 054408 (2024). http://doi.org/10.1103/PHYSREVMATERIALS.8.054408
Abstract: LaMnO 3 (LMO) thin films epitaxially grown on SrTiO 3 (STO) usually exhibit ferromagnetism above a critical layer thickness. We report the use of scanning SQUID microscopy (SSM) to study the suppression of the ferromagnetism in STO / LMO / metal structures. By partially covering the LMO surface with a metallic layer, both covered and uncovered LMO regions can be studied simultaneously. While Au does not significantly influence the ferromagnetic order of the underlying LMO film, a thin Ti layer induces a strong suppression of the ferromagnetism, over tens of nanometers, which increases with time on a timescale of days. Detailed electron energy loss spectroscopy analysis of the Ti-LaMnO 3 interface reveals the presence of Mn 2 + and an evolution of the Ti valence state from Ti 0 to Ti 4 + over approximately 5 nm. Furthermore, we demonstrate that by patterning Ti / Au overlayers, we can locally suppress the ferromagnetism and define ferromagnetic structures down to sub -micrometer scales.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.4
DOI: 10.1103/PHYSREVMATERIALS.8.054408
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“Silver Ions Direct Twin-Plane Formation during the Overgrowth of Single-Crystal Gold Nanoparticles”. Grzelczak M, Sanchez-Iglesias A, Heidari H, Bals S, Pastoriza-Santos I, Perez-Juste J, Liz-Marzan LM, ACS Omega 1, 177 (2016). http://doi.org/10.1021/ACSOMEGA.6B00066
Abstract: It is commonly agreed that the crystalline structure of seeds dictates the crystallinity of final nanoparticles in a seeded-growth process. Although the formation of monocrystalline particles does require the use of single-crystal seeds, twin planes may stem from either single-or polycrystalline seeds. However, experimental control over twin-plane formation remains difficult to achieve synthetically. Here, we show that a careful interplay between kinetics and selective surface passivation offers a unique handle over the emergence of twin planes (in decahedra and triangles) during the growth over single-crystalline gold nanoparticles of quasi-spherical shape. Twinning can be suppressed under conditions of slow kinetics in the presence of silver ions, yielding single-crystalline particles with high-index facets.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Times cited: 18
DOI: 10.1021/ACSOMEGA.6B00066
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“Evidence of tetragonal distortion as the origin of the ferromagnetic ground state in gamma-Fe nanoparticles”. Augustyns V, van Stiphout K, Joly V, Lima TAL, Lippertz G, Trekels M, Menendez E, Kremer F, Wahl U, Costa ARG, Correia JG, Banerjee D, Gunnlaugsson HP, von Bardeleben J, Vickridge I, Van Bael MJ, Hadermann J, Araujo JP, Temst K, Vantomme A, Pereira LMC, Physical review B 96, 174410 (2017). http://doi.org/10.1103/PHYSREVB.96.174410
Abstract: <script type='text/javascript'>document.write(unpmarked('gamma-Fe and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of gamma-Fe nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature Mossbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters a = 3.76(2) angstrom and c = 3.50(2) angstrom, and a magnetic moment of 2.45(5) mu(B) per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured gamma-Fe is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of gamma-Fe taking tetragonal distortion into account.'));
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 1
DOI: 10.1103/PHYSREVB.96.174410
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“Interplay of morphology, composition, and optical properties of InP-based quantum dots emitting at the 1.55 \mum telecom wavelength”. Carmesin C, Schowalter M, Lorke M, Mourad D, Grieb T, Müller-Caspary K, Yacob M, Reithmaier JP, Benyoucef M, Rosenauer A, Jahnke F, Physical review B 96, 235309 (2017). http://doi.org/10.1103/PHYSREVB.96.235309
Abstract: <script type='text/javascript'>document.write(unpmarked('Results for the development and detailed analysis of self-organized InAs/InAlGaAs/InP quantum dots suitable for single-photon emission at the 1.55 mu m telecom wavelength are reported. The structural and compositional properties of the system are obtained from high-resolution scanning transmission electron microscopy of individual quantum dots. The system is composed of almost pure InAs quantum dots embedded in quaternary InAlGaAs barrier material, which is lattice matched to the InP substrate. When using the measured results for a representative quantum-dot geometry as well as experimentally reconstructed alloy concentrations, a combination of strain-field and electronic-state calculations is able to reproduce the quantum-dot emission wavelength in agreement with the experimentally determined photoluminescence spectrum. The inhomogeneous broadening of the latter can be related to calculated variations of the emission wavelength for the experimentally deduced In-concentration fluctuations and size variations.'));
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 3
DOI: 10.1103/PHYSREVB.96.235309
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“Atomic-scale quantification of charge densities in two-dimensional materials”. Müller-Caspary K, Duchamp M, Roesner M, Migunov V, Winkler F, Yang H, Huth M, Ritz R, Simson M, Ihle S, Soltau H, Wehling T, Dunin-Borkowski RE, Van Aert S, Rosenauer A, Physical review B 98, 121408 (2018). http://doi.org/10.1103/PHYSREVB.98.121408
Abstract: The charge density is among the most fundamental solid state properties determining bonding, electrical characteristics, and adsorption or catalysis at surfaces. While atomic-scale charge densities have as yet been retrieved by solid state theory, we demonstrate both charge density and electric field mapping across a mono-/bilayer boundary in 2D MoS2 by momentum-resolved scanning transmission electron microscopy. Based on consistency of the four-dimensional experimental data, statistical parameter estimation and dynamical electron scattering simulations using strain-relaxed supercells, we are able to identify an AA-type bilayer stacking and charge depletion at the Mo-terminated layer edge.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 10
DOI: 10.1103/PHYSREVB.98.121408
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“Evidence for exchange bias coupling at the perovskite/brownmillerite interface in spontaneously stabilized SrCoO3-\delta/SrCoO2.5 bilayers”. Behera BC, Jana S, Bhat SG, Gauquelin N, Tripathy G, Kumar PSA, Samal D, Physical review B 99, 024425 (2019). http://doi.org/10.1103/PHYSREVB.99.024425
Abstract: Interface effect in complex oxide thin-film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study of perovskite/brownmillerite interfaces remains in its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet (SrCoO3-delta)/brownmillerite-antiferromagnet (SrCoO2.5) bilayer with T-N > T-C and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spin-glass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of SrCoO3-delta,whereas the exchange bias field vanishes at T-C . We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagnetic layer for the ferromagnetic spins at the interface. Our results not only unveil a paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems, paving a big step forward in thin-film magnetism.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 2
DOI: 10.1103/PHYSREVB.99.024425
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“Low-field switching of noncollinear spin texture at La0.7Sr0.3MnO3-SrRuO3interfaces”. Das S, Rata AD, Maznichenko I V, Agrestini IS, Pippel E, Gauquelin N, Verbeeck J, Chen K, Valvidares SM, Vasili HB, Herrero-Martin J, Pellegrin E, Nenkov K, Herklotz A, Ernst A, Mertig I, Hu Z, Doerr K, Physical review B 99, 024416 (2019). http://doi.org/10.1103/PHYSREVB.99.024416
Abstract: Interfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as BiFeO3. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the MnO2/SrO termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 19
DOI: 10.1103/PHYSREVB.99.024416
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“Depth-resolved resonant inelastic x-ray scattering at a superconductor/half-metallic-ferromagnet interface through standing wave excitation”. Kuo C-T, Lin S-C, Ghiringhelli G, Peng Y, De Luca GM, Di Castro D, Betto D, Gehlmann M, Wijnands T, Huijben M, Meyer-Ilse J, Gullikson E, Kortright JB, Vailionis A, Gauquelin N, Verbeeck J, Gerber T, Balestrino G, Brookes NB, Braicovich L, Fadley CS, Physical review B 98, 235146 (2018). http://doi.org/10.1103/PHYSREVB.98.235146
Abstract: We demonstrate that combining standing wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La1.85Sr0.15CuO4 (LSCO) and a half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 5
DOI: 10.1103/PHYSREVB.98.235146
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“Band flattening in buckled monolayer graphene”. Milovanović, SP, Andelkovic M, Covaci L, Peeters FM, Physical Review B 102, 245427 (2020). http://doi.org/10.1103/PHYSREVB.102.245427
Abstract: The strain fields of periodically buckled graphene induce a periodic pseudomagnetic field (PMF) that modifies the electronic band structure. From the geometry, amplitude, and period of the periodic pseudomagnetic field, we determine the necessary conditions to access the regime of correlated phases by examining the band flattening. As compared to twisted bilayer graphene the proposed system has the advantages that (1) only a single layer of graphene is needed, (2) one is not limited to hexagonal superlattices, and (3) narrower flat bandwidth and larger separation between flat bands can be induced. We, therefore, propose that periodically strained graphene single layers can become a platform for the exploration of exotic many-body phases.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Impact Factor: 3.7
Times cited: 27
DOI: 10.1103/PHYSREVB.102.245427
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“Anomalous behavior of the electronic structure of (Bi1-xInx)2Se3across the quantum phase transition from topological to trivial insulator”. Sanchez-Barriga J, Aguilera I, Yashina L V, Tsukanova DY, Freyse F, Chaika AN, Callaert C, Abakumov AM, Hadermann J, Varykhalov A, Rienks EDL, Bihlmayer G, Blugel S, Rader O, Physical review B 98, 235110 (2018). http://doi.org/10.1103/PHYSREVB.98.235110
Abstract: Using spin- and angle-resolved photoemission spectroscopy and relativistic many-body calculations, we investigate the evolution of the electronic structure of (Bi1-xInx)(2)Se-3)(2)Se-3 bulk single crystals around the critical point of the trivial to topological insulator quantum-phase transition. By increasing x, we observe how a surface gap opens at the Dirac point of the initially gapless topological surface state of Bi2Se3, leading to the existence of massive fermions. The surface gap monotonically increases for a wide range of x values across the topological and trivial sides of the quantum-phase transition. By means of photon-energy-dependent measurements, we demonstrate that the gapped surface state survives the inversion of the bulk bands which occurs at a critical point near x = 0.055. The surface state exhibits a nonzero in-plane spin polarization which decays exponentially with increasing x, and which persists in both the topological and trivial insulator phases. Our calculations reveal qualitative agreement with the experimental results all across the quantum-phase transition upon the systematic variation of the spin-orbit coupling strength. A non-time-reversal symmetry-breaking mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as explanation.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1103/PHYSREVB.98.235110
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“Cubic lead perovskite PbMoO3 with anomalous metallic behavior”. Takatsu H, Hernandez O, Yoshimune W, Prestipino C, Yamamoto T, Tassel C, Kobayashi Y, Batuk D, Shibata Y, Abakumov AM, Brown CM, Kageyama H, Physical review B 95, 155105 (2017). http://doi.org/10.1103/PHYSREVB.95.155105
Abstract: A previously unreported Pb-based perovskite PbMoO3 is obtained by high-pressure and high-temperature synthesis. This material crystallizes in the Pm3m cubic structure at room temperature, making it distinct from typical Pb-based perovskite oxides with a structural distortion. PbMoO3 exhibits a metallic behavior down to 0.1 K with an unusual T-sublinear dependence of the electrical resistivity. Moreover, a large specific heat is observed at low temperatures accompanied by a peak in C-P/T-3 around 10 K, in marked contrast to the isostructural metallic system SrMoO3. These transport and thermal properties for PbMoO3, taking into account anomalously large Pb atomic displacements detected through diffraction experiments, are attributed to a low-energy vibrational mode, associated with incoherent off-centering of lone-pair Pb2+ cations. We discuss the unusual behavior of the electrical resistivity in terms of a polaronlike conduction, mediated by the strong coupling between conduction electrons and optical phonons of the local low-energy vibrational mode.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1103/PHYSREVB.95.155105
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“Wave-packet scattering at a normal-superconductor interface in two-dimensional materials : a generalized theoretical approach”. Linard FJA, Moura VN, Covaci L, Milošević, MV, Chaves A, Physical review B 107, 165306 (2023). http://doi.org/10.1103/PHYSREVB.107.165306
Abstract: A wave-packet time evolution method, based on the split-operator technique, is developed to investigate the scattering of quasiparticles at a normal-superconductor interface of arbitrary profile and shape. As a practical application, we consider a system where low-energy electrons can be described as Dirac particles, which is the case for most two-dimensional materials, such as graphene and transition-metal dichalcogenides. However, the method is easily adapted for other cases such as electrons in few-layer black phosphorus or any Schrodinger quasiparticles within the effective mass approximation in semiconductors. We employ the method to revisit Andreev reflection in mono-, bi-, and trilayer graphene, where specular-and retro-reflection cases are observed for electrons scattered by a steplike superconducting region. The effect of opening a zero-gap channel across the superconducting region on the electron and hole scattering is also addressed, as an example of the versatility of the technique proposed here.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Impact Factor: 3.7
DOI: 10.1103/PHYSREVB.107.165306
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“Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles”. Kleibert A, Balan A, Yanes R, Derlet PM, Vaz CAF, Timm M, Fraile Rodríguez A, Béché, A, Verbeeck J, Dhaka RS, Radovic M, Nowak U, Nolting F, Physical review B 95, 195404 (2017). http://doi.org/10.1103/PhysRevB.95.195404
Abstract: Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size- and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 21
DOI: 10.1103/PhysRevB.95.195404
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“Spin-reorientation transitions in the Cairo pentagonal magnet Bi4Fe5O13F”. Tsirlin AA, Rousochatzakis I, Filimonov D, Batuk D, Frontzek M, Abakumov AM, Physical review B 96, 094420 (2017). http://doi.org/10.1103/PhysRevB.96.094420
Abstract: We show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi4Fe5O13F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mossbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi4Fe5O13F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 7
DOI: 10.1103/PhysRevB.96.094420
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“Quantitative modeling of secondary electron emission from slow-ion bombardment on semiconductors”. Bercx M, Partoens B, Lamoen D, Physical review B 99, 085413 (2019). http://doi.org/10.1103/PhysRevB.99.085413
Abstract: When slow ions incident on a surface are neutralized, the excess potential energy is passed on to an electron inside the surface, leading to emission of secondary electrons. The microscopic description of this process, as
well as the calculation of the secondary electron yield, is a challenging problem due to its complexity as well
as its sensitivity to surface properties. One of the first quantitative descriptions was articulated in the 1950s by
Hagstrum, who based his calculation on a parametrization of the density of states of the material. In this paper, we
present a model for calculating the secondary electron yield, derived from Hagstrum’s initial approach. We use
first-principles density functional theory calculations to acquire the necessary input and introduce the concept of
electron cascades to Hagstrum’s model in order to improve the calculated spectra, as well as remove its reliance
on fitting parameters. We apply our model to He+ and Ne+ ions incident on Ge(111) and Si(111) and obtain
yield spectra that match closely to the experimental results of Hagstrum.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 5
DOI: 10.1103/PhysRevB.99.085413
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“Single femtosecond laser pulse excitation of individual cobalt nanoparticles”. Savchenko TM, Buzzi M, Howald L, Ruta S, Vijayakumar J, Timm M, Bracher D, Saha S, Derlet PM, Béché, A, Verbeeck J, Chantrell RW, Vaz CAF, Nolting F, Kleibert A, Physical Review B 102, 205418 (2020). http://doi.org/10.1103/PhysRevB.102.205418
Abstract: Laser-induced manipulation of magnetism at the nanoscale is a rapidly growing research topic with potential for applications in spintronics. In this work, we address the role of the scattering cross section, thermal effects, and laser fluence on the magnetic, structural, and chemical stability of individual magnetic nanoparticles excited by single femtosecond laser pulses. We find that the energy transfer from the fs laser pulse to the nanoparticles is limited by the Rayleigh scattering cross section, which in combination with the light absorption of the supporting substrate and protective layers determines the increase in the nanoparticle temperature. We investigate individual Co nanoparticles (8 to 20 nm in size) as a prototypical model system, using x-ray photoemission electron microscopy and scanning electron microscopy upon excitation with single femtosecond laser pulses of varying intensity and polarization. In agreement with calculations, we find no deterministic or stochastic reversal of the magnetization in the nanoparticles up to intensities where ultrafast demagnetization or all-optical switching is typically reported in thin films. Instead, at higher fluences, the laser pulse excitation leads to photo-chemical reactions of the nanoparticles with the protective layer, which results in an irreversible change in the magnetic properties. Based on our findings, we discuss the conditions required for achieving laser-induced switching in isolated nanomagnets.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.7
Times cited: 1
DOI: 10.1103/PhysRevB.102.205418
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“Symmetry-constrained electron vortex propagation”. Clark L, Guzzinati G, Béché, A, Lubk A, Verbeeck J, Physical review A 93, 063840 (2016). http://doi.org/10.1103/PhysRevA.93.063840
Abstract: Electron vortex beams hold great promise for development in transmission electron microscopy but have yet to be widely adopted. This is partly due to the complex set of interactions that occur between a beam carrying orbital angular momentum (OAM) and a sample. Herein, the system is simplified to focus on the interaction between geometrical symmetries, OAM, and topology. We present multiple simulations alongside experimental data to study the behavior of a variety of electron vortex beams after interacting with apertures of different symmetries and investigate the effect on their OAM and vortex structure, both in the far field and under free-space propagation.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.925
Times cited: 7
DOI: 10.1103/PhysRevA.93.063840
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“Extension of Friedel's law to vortex-beam diffraction”. Juchtmans R, Guzzinati G, Verbeeck J, Physical Review A 94, 033858 (2016). http://doi.org/10.1103/PhysRevA.94.033858
Abstract: Friedel's law states that the modulus of the Fourier transform of real functions is centrosymmetric, while the phase is antisymmetric. As a consequence of this, elastic scattering of plane-wave photons or electrons within the first-order Born-approximation, as well as Fraunhofer diffraction on any aperture, is bound to result in centrosymmetric diffraction patterns. Friedel's law, however, does not apply for vortex beams, and centrosymmetry in general is not present in their diffraction patterns. In this work we extend Friedel's law for vortex beams by showing that the diffraction patterns of vortex beams with opposite topological charge, scattered on the same two-dimensional potential, always are centrosymmetric to one another, regardless of the symmetry of the scattering object. We verify our statement by means of numerical simulations and experimental data. Our research provides deeper understanding in vortex-beam diffraction and can be used to design new experiments to measure the topological charge of vortex beams with diffraction gratings or to study general vortex-beam diffraction.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.925
Times cited: 13
DOI: 10.1103/PhysRevA.94.033858
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“Spiral phase plate contrast in optical and electron microscopy”. Juchtmans R, Clark L, Lubk A, Verbeeck J, Physical review A 94, 023838 (2016). http://doi.org/10.1103/PhysRevA.94.023838
Abstract: The use of phase plates in the back focal plane of a microscope is a well-established technique in optical microscopy to increase the contrast of weakly interacting samples and is gaining interest in electron microscopy as well. In this paper we study the spiral phase plate (SPP), also called helical, vortex, or two-dimensional Hilbert phase plate, which adds an angularly dependent phase of the form exp(iℓϕk) to the exit wave in Fourier space. In the limit of large collection angles, we analytically calculate that the average of a pair of l=+-1
SPP filtered images is directly proportional to the gradient squared of the exit wave, explaining the edge contrast previously seen in optical SPP work. We discuss the difference between a clockwise-anticlockwise pair of SPP filtered images and derive conditions under which the modulus of the wave's gradient can be seen directly from one SPP filtered image. This work provides the theoretical background to interpret images obtained with a SPP, thereby opening new perspectives for new experiments to study, for example, magnetic materials in an electron microscope.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.925
Times cited: 10
DOI: 10.1103/PhysRevA.94.023838
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“Low-cost electron detector for scanning electron microscope”. Vlasov E, Denisov N, Verbeeck J, HardwareX 14, e00413 (2023). http://doi.org/10.1016/j.ohx.2023.e00413
Abstract: Electron microscopy is an indispensable tool for the characterization of (nano) materials. Electron microscopes are typically very expensive and their internal operation is often shielded from the user. This situation can provide fast and high quality results for researchers focusing on e.g. materials science if they have access to the relevant instruments. For researchers focusing on technique development, wishing to test novel setups, however, the high entry price can lead to risk aversion and deter researchers from innovating electron microscopy technology further. The closed attitude of commercial entities about how exactly the different parts of electron microscopes work, makes it even harder for newcomers in this field. Here we propose an affordable, easy-to-build electron detector for use in a scanning electron microscope (SEM). The aim of this project is to shed light on the functioning of such detectors as well as show that even a very modest design can lead to acceptable performance while providing high flexibility for experimentation and customization.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Times cited: 1
DOI: 10.1016/j.ohx.2023.e00413
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“Occupancy of lattice positions probed by X-ray photoelectron diffraction : a case study of tetradymite topological insulators”. Vladimirova NV, Frolov AS, Sanchez-Barriga J, Clark OJ, Matsui F, Usachov DY, Muntwiler M, Callaert C, Hadermann J, Neudachina VS, Tamm ME, Yashina LV, Surfaces and interfaces 36, 102516 (2023). http://doi.org/10.1016/J.SURFIN.2022.102516
Abstract: Occupancy of different structural positions in a crystal lattice often seems to play a key role in material prop-erties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 6.2
DOI: 10.1016/J.SURFIN.2022.102516
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“Study of rechargeable batteries using advanced spectroscopic and computational techniques”. Barbiellini B, Kuriplach J, Saniz R, Condensed Matter 6, 26 (2021). http://doi.org/10.3390/CONDMAT6030026
Abstract: Improving the efficiency and longevity of energy storage systems based on Li- and Na-ion rechargeable batteries presents a major challenge. The main problems are essentially capacity loss and limited cyclability. These effects are due to a hierarchy of factors spanning various length and time scales, interconnected in a complex manner. As a consequence, and in spite of several decades of research, a proper understanding of the ageing process has remained somewhat elusive. In recent years, however, combinations of advanced spectroscopy techniques and first-principles simulations have been applied with success to tackle this problem. In this Special Issue, we are pleased to present a selection of articles that, by precisely applying these methods, unravel key aspects of the reduction-oxidation reaction and intercalation processes. Furthermore, the approaches presented provide improvements to standard diagnostic and characterisation techniques, enabling the detection of possible Li-ion flow bottlenecks causing the degradation of capacity and cyclability.
Keywords: Editorial; Electron microscopy for materials research (EMAT)
DOI: 10.3390/CONDMAT6030026
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“Enabling hydrate-based methane storage under mild operating conditions by periodic mesoporous organosilica nanotubes”. Beckwee EJ, Watson G, Houlleberghs M, Arenas Esteban D, Bals S, Van Der Voort P, Breynaert E, Martens J, Baron GV, Denayer JFM, Heliyon 9, e17662 (2023). http://doi.org/10.1016/J.HELIYON.2023.E17662
Abstract: Biomethane is a renewable natural gas substitute produced from biogas. Storage of this sustainable energy vector in confined clathrate hydrates, encapsulated in the pores of a host material, is a highly promising avenue to improve storage capacity and energy efficiency. Herein, a new type of periodic mesoporous organosilica (PMO) nanotubes, referred to as hollow ring PMO (HR-PMO), capable of promoting methane clathrate hydrate formation under mild working conditions (273 K, 3.5 MPa) and at high water loading (5.1 g water/g HR-PMO) is reported. Gravimetric uptake measurements reveal a steep single-stepped isotherm and a noticeably high methane storage capacity (0.55 g methane/g HR-PMO; 0.11 g methane/g water at 3.5 MPa). The large working capacity throughout consecutive pressure-induced clathrate hydrate formationdissociation cycles demonstrates the material's excellent recyclability (97% preservation of capacity). Supported by ex situ cryo-electron tomography and x-ray diffraction, HR-PMO nanotubes are hypothesized to promote clathrate hydrate nucleation and growth by distribution and confinement of water in the mesopores of their outer wall, along the central channels of the nanotubes and on the external nanotube surface. These findings showcase the potential for application of organosilica materials with hierarchical and interconnected pore systems for pressure-based storage of biomethane in confined clathrate hydrates.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Times cited: 4
DOI: 10.1016/J.HELIYON.2023.E17662
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“Optical versus electron diffraction imaging of Twist-angle in 2D transition metal dichalcogenide bilayers”. Psilodimitrakopoulos S, Orekhov A, Mouchliadis L, Jannis D, Maragkakis GM, Kourmoulakis G, Gauquelin N, Kioseoglou G, Verbeeck J, Stratakis E, npj 2D Materials and Applications 5, 77 (2021). http://doi.org/10.1038/S41699-021-00258-5
Abstract: Atomically thin two-dimensional (2D) materials can be vertically stacked with van der Waals bonds, which enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative direction between the two monolayers, coined as twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. Here, we demonstrate an all-optical method for pixel-by-pixel mapping of the twist-angle with a resolution of 0.55(degrees), via polarization-resolved second harmonic generation (P-SHG) microscopy and we compare it with four-dimensional scanning transmission electron microscopy (4D STEM). It is found that the twist-angle imaging of WS2 bilayers, using the P-SHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the optical approach are that the characterization is performed on the same substrate that the device is created on and that it is three orders of magnitude faster than the 4D STEM. We envisage that the optical P-SHG imaging could become the gold standard for the quality examination of TMD superlattice-based devices.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Times cited: 4
DOI: 10.1038/S41699-021-00258-5
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“Direct visualization of irreducible ferrielectricity in crystals”. Du K, Guo L, Peng J, Chen X, Zhou Z-N, Zhang Y, Zheng T, Liang Y-P, Lu J-P, Ni Z-H, Wang S-S, Van Tendeloo G, Zhang Z, Dong S, Tian H, npj Quantum Materials 5, 49 (2020). http://doi.org/10.1038/S41535-020-00252-Y
Abstract: In solids, charge polarity can one-to-one correspond to spin polarity phenomenologically, e.g., ferroelectricity/ferromagnetism, antiferroelectricity/antiferromagnetism, and even dipole-vortex/magnetic-vortex, but ferrielectricity/ferrimagnetism kept telling a disparate story in microscopic level. Since the definition of a charge dipole involves more than one ion, there may be multiple choices for a dipole unit, which makes most ferrielectric orders equivalent to ferroelectric ones, i.e., this ferrielectricity is not necessary to be a real independent branch of polarity. In this work, by using the spherical aberration-corrected scanning transmission electron microscope, we visualize a nontrivial ferrielectric structural evolution in BaFe2Se3, in which the development of two polar sub-lattices is out-of-sync, for which we term it as irreducible ferrielectricity. Such irreducible ferrielectricity leads to a non-monotonic behavior for the temperature-dependent polarization, and even a compensation point in the ordered state. Our finding unambiguously distinguishes ferrielectrics from ferroelectrics in solids.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
DOI: 10.1038/S41535-020-00252-Y
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“Revealing pH-Dependent Activities and Surface Instabilities for Ni-Based Electrocatalysts during the Oxygen Evolution Reaction”. Yang C, Batuk M, Jacquet Q, Rousse G, Yin W, Zhang L, Hadermann J, Abakumov AM, Cibin G, Chadwick A, Tarascon J-M, Grimaud A, ACS energy letters , 2884 (2018). http://doi.org/10.1021/acsenergylett.8b01818
Abstract: Multiple electrochemical processes are involved at the catalyst/ electrolyte interface during the oxygen evolution reaction (OER). With the purpose of elucidating the complexity of surface dynamics upon OER, we systematically studied two Ni-based crystalline oxides (LaNiO3−δ and La2Li0.5Ni0.5O4) and compared them with the state-of-the-art Ni−Fe (oxy)- hydroxide amorphous catalyst. Electrochemical measurements such as rotating ring disk electrode (RRDE) and electrochemical quartz microbalance microscopy (EQCM) coupled with a series of physical characterizations including transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) were conducted to unravel the exact pH effect on both the OER activity and the catalyst stability. We demonstrate that for Ni-based crystalline catalysts the rate for surface degradation depends on the pH and is greater than the rate for surface reconstruction. This behavior is unlike that for the amorphous Ni oxyhydroxide catalyst, which is found to be more stable and pH-independent.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
DOI: 10.1021/acsenergylett.8b01818
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“Low-temperature plasticity of olivine revisited with in situ TEM nanomechanical testing”. Idrissi H, Bollinger C, Boioli F, Schryvers D, Cordier P, Science Advances 2, e1501671 (2016). http://doi.org/10.1126/sciadv.1501671
Abstract: The rheology of the lithospheric mantle is fundamental to understanding how mantle convection couples with plate tectonics. However, olivine rheology at lithospheric conditions is still poorly understood because experiments are difficult in this temperature range where rocks and mineral become very brittle. We combine techniques of quantitative in situ tensile testing in a transmission electron microscope and numerical modeling of dislocation dynamics to constrain the low-temperature rheology of olivine. We find that the intrinsic ductility of olivine at low temperature is significantly lower than previously reported values, which were obtained under strain-hardened conditions. Using this method, we can anchor rheological laws determined at higher temperature and can provide a better constraint on intermediate temperatures relevant for the lithosphere. More generally, we demonstrate the possibility of characterizing the mechanical properties of specimens, which can be available in the form of submillimeter-sized particles only.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Times cited: 32
DOI: 10.1126/sciadv.1501671
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“Composite super-moiré, lattices in double-aligned graphene heterostructures = Composite super-moire lattices in double-aligned graphene heterostructures”. Wang Z, Wang YB, Yin J, Tovari E, Yang Y, Lin L, Holwill M, Birkbeck J, Perello DJ, Xu S, Zultak J, Gorbachev RV, Kretinin AV, Taniguchi T, Watanabe K, Morozov SV, Andelkovic M, Milovanović, SP, Covaci L, Peeters FM, Mishchenko A, Geim AK, Novoselov KS, Fal'ko VI, Knothe A, Woods CR, Science Advances 5, eaay8897 (2019). http://doi.org/10.1126/SCIADV.AAY8897
Abstract: When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moire pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moire patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moire pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moire super cell.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Times cited: 71
DOI: 10.1126/SCIADV.AAY8897
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