| 1. |
- Negi, Devendra Singh, et al.
(författare)
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Spin-entropy induced thermopower and spin-blockade effect in CoO
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:14
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Tidskriftsartikel (refereegranskat)abstract
- We report spin-entropy-induced thermopower and the occurrence of a spin-blockade effect in stoichiometric disordered CoO. Cation defect-driven distortion in the octahedral ligand field of CoO leads to a charge transfer process and favors the stabilization of Co+3 charge states at defect adjacent atomic sites. Moreover, a higher extent of local stoichiometric disruption triggers the spin crossover and magnetic collapse into a Co+3 state. Degenerated spin-orbital states on vacancy neighbored atomic sites render the spin-orbital degeneracy to enhance the thermopower in CoO. Furthermore, we unravel an operating spin-blockade effect in CoO. The localized combination of active magnetic states-high-spin Co+2 and neutral magnetic states-low-spin Co+3 on alternate atomic sites suppress the charge carrier hopping due to a spin blockade. In the pursuit of efficient thermoelectric material, the present investigation explores the potential of the recipe of spin entropy and defect-engineered CoO.
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| 2. |
- Ali, Hasan, 1985-, et al.
(författare)
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Atomic resolution energy-loss magnetic chiral dichroism measurements enabled by patterned apertures
- 2020
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Ingår i: Physical Review Research. - College Park USA. - 2643-1564. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- Electron energy-loss magnetic chiral dichroism (EMCD) has the potential to measure magnetic properties of the materials at atomic resolution but the complex distribution of magnetic signals in the zone axis and the overlapping diffraction discs at higher beam convergence angles make the EMCD signal acquisition challenging. Recently, the use of ventilator apertures to acquire the EMCD signals with atomic resolution was proposed. Here we give the experimental demonstration of several types of ventilator apertures and obtain a clear EMCD signal at beam semiconvergence angles of 5 mrad. To simplify the experimental procedures, we propose a modified ventilator aperture which not only simplifies the complex scattering conditions but reduces the influence of lens aberrations on the EMCD signal as compared to the originally proposed ventilator apertures. In addition, this modified aperture can be used to analyze magnetic crystals with various symmetries and we demonstrate this feature by acquiring EMCD signals on different zone axis orientations of an Fe crystal. With the same aperture we obtain EMCD signals with convergence angles corresponding to atomic resolution electron probes. After the theoretical demonstration of the EMCD signal on a zone axis orientation at high beam convergence angles, this work thus overcomes the experimental and methodological hurdles and enables atomic resolution EMCD on the zone axis by using apertures.
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| 3. |
- Negi, Devendra, et al.
(författare)
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Coexisting commensurate and incommensurate charge ordered phases in CoO
- 2021
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- The subtle interplay of strong electronic correlations in a distorted crystal lattice often leads to the evolution of novel emergent functionalities in the strongly correlated materials (SCM). Here, we unravel such unprecedented commensurate (COM) and incommensurate (ICOM) charge ordered (CO) phases at room temperature in a simple transition-metal mono-oxide, namely CoO. The electron diffraction pattern unveils a COM (q(1) = 1/2 (1, 1, (1) over bar) and ICOM (q(2) = 0.213(1, 1, (1) over bar)) periodic lattice distortion. Transmission electron microscopy (TEM) captures unidirectional and bidirectional stripe patterns of charge density modulations. The widespread phase singularities in the phase-field of the order parameter (OP) affirms the abundant topological disorder. Using, density functional theory (DFT) calculations, we demystify the underlying electronic mechanism. The DFT study shows that a cation disordering (Co1-x O, with x = 4.17%) stabilizes Jahn-Teller (JT) distortion and localized aliovalent Co3+ states in CoO. Therefore, the lattice distortion accompanied with mixed valence states (Co3+, Co2+) states introduces CO in CoO. Our findings offer an electronic paradigm to engineer CO to exploit the associated electronic functionalities in widely available transition-metal mono-oxides.
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| 4. |
- Negi, Devendra Singh, et al.
(författare)
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Defect driven spin state transition and the existence of half-metallicity in CoO
- 2019
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 31:11
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Tidskriftsartikel (refereegranskat)abstract
- We unveil the native defect induced high spin to low spin state transition in Co+3 and half- metallicity in CoO. First principles calculations unravel that, defect density holds a key role in dictating the spin-state transition in Co+3 ion in CoO, and introducing the half-metallicity. Charge transfer in the vicinity of vacancy plane favors the stabilization and coexistence of bivalent Co+2 and trivalent Co+3 ion in CoO. We propose that defect engineering could serve as a route to design the half metallicity in transition metal mono-oxides.
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| 5. |
- Negi, Devendra Singh, et al.
(författare)
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Probing the localization of magnetic dichroism by atomic-size astigmatic and vortex electron beams
- 2018
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- We report localization of a magnetic dichroic signal on atomic columns in electron magnetic circular dichroism ( EMCD), probed by beam distorted by four-fold astigmatism and electron vortex beam. With astigmatic probe, magnetic signal to noise ratio can be enhanced by blocking the intensity from the central part of probe. However, the simulations show that for atomic resolution magnetic measurements, vortex beam is a more effective probe, with much higher magnetic signal to noise ratio. For all considered beam shapes, the optimal SNR constrains the signal detection at low collection angles of approximately 6-8 mrad. Irrespective of the material thickness, the magnetic signal remains strongly localized within the probed atomic column with vortex beam, whereas for astigmatic probes, the magnetic signal originates mostly from the nearest neighbor atomic columns. Due to excellent signal localization at probing individual atomic columns, vortex beams are predicted to be a strong candidate for studying the crystal site specific magnetic properties, magnetic properties at interfaces, or magnetism arising from individual atomic impurities.
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| 6. |
- Negi, Devendra Singh, et al.
(författare)
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Proposal for a three-dimensional magnetic measurement method with nanometer-scale depth resolution
- 2018
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:17
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Tidskriftsartikel (refereegranskat)abstract
- We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2 nm.
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| 7. |
- Negi, Devendra Singh, et al.
(författare)
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Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 122:3
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Tidskriftsartikel (refereegranskat)abstract
- We propose a magnetic measurement method utilizing a patterned post-sample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes -- from wide parallel beams down to atomic resolution magnetic spectrum imaging.
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| 8. |
- Negi, Devendra Singh, et al.
(författare)
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Prospect for detecting magnetism of a single impurity atom using electron magnetic chiral dichroism
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:10
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Tidskriftsartikel (refereegranskat)abstract
- Dopants, even single atoms, can influence the electrical and magnetic properties of materials. Here we demonstrate the opportunity for detecting the magnetic response of an embedded magnetic impurity in a nonmagnetic host material. We combine a depth sectioning approach with electron magnetic circular dichroism in scanning transmission electron microscopy to compute the depth-resolved magnetic inelastic-scattering cross section of single Co impurity buried in the host crystal of GaAs. Our calculations suggest that the magnetic dichroic signal intensity is sensitive to the depth and lateral position of the electron probe relative to the magnetic impurity. Additionally, a more precise dichroic signal localization can be achieved via choosing higher-collection-angle (beta) apertures. Quantitative evaluation of the inelastic-scattering cross section and signal-to-noise ratio indicates that the magnetic signal from a single Co atom is on the verge of being detectable with today's state-of-the-art instrumentation.
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| 9. |
- Schneider, Sebastian, et al.
(författare)
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Simple method for optimization of classical electron magnetic circular dichroism measurements : The role of structure factor and extinction distances
- 2018
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:11
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Tidskriftsartikel (refereegranskat)abstract
- Electron magnetic circular dichroism (EMCD), the electron wave analog of x-ray magnetic circular dichroism (XMCD), allows for the element specific measurement of the spin and orbital magnetic moments with up to nanometer resolution. However, due to dynamical diffraction effects, the signal-to-noise ratios of EMCD spectra are often very low. We describe a simple set of rules, how to set up a geometry for a classical EMCD experiment on an arbitrary crystal structure to get a maximum dichroic signal. The procedure is based on an evaluation of the structure factor and extinction distances. Proof-of-concept simulations and experiments on a FeGe crystal present a successful test of these guidelines.
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| 10. |
- Vats, Nilesh, et al.
(författare)
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Catalyzing Bond-Dissociation in Graphene via Alkali-Iodide Molecules
- 2021
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Ingår i: Small. - : John Wiley & Sons. - 1613-6810 .- 1613-6829. ; 17:42
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Tidskriftsartikel (refereegranskat)abstract
- Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons. Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to charge-transfer phenomena the C-C bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.
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