nicholpi

• Amundsen, Monika; Pike, Nicholas; Løvvik, Ole Martin; Carvalho, Patricia Almeida & Gunnæs, Anette Eleonora (2022). New structure and insight on the phase transition within the Cu-Pd-Sn system with 25 at. % Sn. Materialia. 24, s. 1–6. doi: 10.1016/j.mtla.2022.101461. Fulltekst i vitenarkiv Vis sammendrag

  The Cu-Pd-Sn material system has a (Cu, Pd) 3 Sn phase region that shows an interesting and unexplored marten- sitic phase transformation. Literature on this system is limited and the description of the phases present is incom- plete and partly contradictory. With focus on a specific Cu-rich composition in this phase region (Cu 33 Pd 42 Sn 25 ) and having employed a combination of differential scanning calorimetry, X-ray diffraction, optical microscopy and density functional theory (DFT) methods we analyze the phases involved and the phase transformation tem- peratures. The martensitic transformation was found to occur at approximately 378 K upon heating and at 328 K under cooling conditions. The crystal structure of the high temperature (HT) austenite phase was identified as face centered cubic with lattice constant 𝑎 = 6 . 2261(1) Åat 473 K. DFT simulations of plausible atomic arrange- ments indicated that the HT phase was an L2 1 ( 𝐹 𝑚 ̄3 𝑚 ) type structure. The martensitic LT phase was found to be orthorhombic with a D0 11 ( 𝑃 𝑛𝑚𝑎 ) type structure with lattice parameters a = 8.9860(3) Å, 𝑏 = 5 . 9347(2) Åand 𝑐 = 4 . 5328(2) Åat 298 K.

• Pike, Nicholas; Dewandre, Antoine; Troeye, Benoit Van; Gonze, Xavier & Verstraete, Matthieu (2019). Vibrational and dielectric properties of monolayer transition metal dichalcogenides. PHYSICAL REVIEW MATERIALS. 3(7). doi: 10.1103/PhysRevMaterials.3.074009. Fulltekst i vitenarkiv
• Pike, Nicholas; Matt, Amina & Løvvik, Ole Martin (2019). Determining the Optimal Phase-Change Material via High-Throughput Calculations. MRS Advances. doi: 10.1557/adv.2019.235. Fulltekst i vitenarkiv
• Pike, Nicholas & Løvvik, Ole Martin (2019). Calculation of the anisotropic coefficients of thermal expansion: A first-principles approach. Computational Materials Science. ISSN 0927-0256. 167, s. 257–263. doi: 10.1016/j.commatsci.2019.05.045. Fulltekst i vitenarkiv Vis sammendrag

  Predictions of the anisotropic coefficients of thermal expansion are needed to not only compare to experimental measurement, but also as input for macroscopic modeling of devices which operate over a large temperature range. While most current methods are limited to isotropic systems within the quasiharmonic approximation, our method uses first-principles calculations and includes anharmonic effects to determine the temperature-dependent properties of materials. These include the lattice parameters, anisotropic coefficients of thermal expansion, isotherm bulk modulus, and specific heat at constant pressure. Our method has been tested on two compounds (Cu and AlN) and predicts thermal properties which compare favorably to experimental measurement over a wide temperature range.

• Pike, Nicholas; Dewandre, Antoine; Troeye, Benoit Van; Gonze, Xavier & Verstraete, Matthieu (2018). Vibrational and dielectric properties of the bulk transition metal dichalcogenides. PHYSICAL REVIEW MATERIALS. 2(6). doi: 10.1103/PhysRevMaterials.2.063608. Fulltekst i vitenarkiv Vis sammendrag

  Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range e−−e− correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased, and data trends on the full set of 15 existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications.

• Tripathi, Mukesh; Mittelberger, Andreas; Pike, Nicholas; Mangler, Clemens; Meyer, Jannik & Verstraete, Matthieu [Vis alle 8 forfattere av denne artikkelen] (2018). Electron-Beam Manipulation of Silicon Dopants in Graphene. Nano Letters. ISSN 1530-6984. 18(8), s. 5319–5323. doi: 10.1021/acs.nanolett.8b02406. Fulltekst i vitenarkiv Vis sammendrag

  The direct manipulation of individual atoms in materials using scanning probe microscopy has been a seminal achievement of nanotechnology. Recent advances in imaging resolution and sample stability have made scanning transmission electron microscopy a promising alternative for single-atom manipulation of covalently bound materials. Pioneering experiments using an atomically focused electron beam have demonstrated the directed movement of silicon atoms over a handful of sites within the graphene lattice. Here, we achieve a much greater degree of control, allowing us to precisely move silicon impurities along an extended path, circulating a single hexagon, or back and forth between the two graphene sublattices. Even with manual operation, our manipulation rate is already comparable to the state-of-the-art in any atomically precise technique. We further explore the influence of electron energy on the manipulation rate, supported by improved theoretical modeling taking into account the vibrations of atoms near the impurities, and implement feedback to detect manipulation events in real time. In addition to atomic-level engineering of its structure and properties, graphene also provides an excellent platform for refining the accuracy of quantitative models and for the development of automated manipulation.

• Pike, Nicholas; Troeye, Benoit Van; Dewandre, Antoine; Petretto, Guido; Gonze, Xavier & Rignanese, Gian-Marco [Vis alle 7 forfattere av denne artikkelen] (2017). Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides. Physical review B (PRB). ISSN 2469-9950. 95(20). doi: 10.1103/physrevb.95.201106. Fulltekst i vitenarkiv Vis sammendrag

  espite numerous studies of transition metal dichalcogenides, the diversity of their chemical bonding characteristics and charge transfer is not well understood. Based on density functional theory we investigate their static and dynamic charges. The dynamic charge of the transition metal dichalcogenides with trigonal symmetry are anomalously large, while in their hexagonally symmetric counterparts, we even observe a counterintuitive sign, i.e., the transition metal takes a negative charge, opposite to its static charge. This phenomenon, so far never remarked on or analyzed, is understood by investigating the perturbative response of the system and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, a link is established between the sign of the Born effective charge and the process of π backbonding from organic chemistry. Experiments are proposed to verify the calculated sign of the dynamical charge in these materials. Employing a high-throughput search we also identify other materials that present counterintuitive dynamic charges.

• Pike, Nicholas & Stroud, David (2017). Spin waves on chains of YIG particles: dispersion relations, Faraday rotation, and power transmission. European Physical Journal B : Condensed Matter Physics. ISSN 1434-6028. 90. doi: 10.1140/epjb/e2017-70609-7. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Pike, Nicholas; Matt, Amina & Løvvik, Ole Martin (2019). Determining the Optimal Phase-Change Material via High-Throughput Calculations. Fulltekst i vitenarkiv
• Løberg, Monika; Pike, Nicholas; Carvalho, Patricia Almeida; Karlsen, Ole Bjørn; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2019). Martensitic-like phase transformations within the Cu-Pd-Sn system. Fulltekst i vitenarkiv
• Sunding, Martin Fleissner; Løberg, Monika; Azar, Amin S.; Stange, Marit Synnøve Sæverud; Carvalho, Patricia Almeida & Karlsen, Ole Bjørn [Vis alle 10 forfattere av denne artikkelen] (2019). The Need for a Multi-Technique Approach in the Search for New Phase-Transforming Materials. Fulltekst i vitenarkiv Vis sammendrag

  Introduction/Purpose Reversible diffusionless first-order phase transformations are a fundamental part of phase change materials and shape memory alloys [1]. Some of these materials even possess useful physical characteristics that show abrupt changes e.g. in their electrical[2] and magnetic [3] polarizations. These phase transformations can be studied using multiple techniques, enabling access to different types of information including length scales and time periods, but how should this characterisation be approached? Methods TEM studies give highly relevant data in this field of research, from detailed crystallographic structures, to relative orientations between grains and phases and to the phase transformation temperature. However, TEM analyses are time consuming and requires small sample sizes and ad hoc preparation. High-throughput measurement techniques are thus required in the search for improved compositions and new materials in order to rapidly assess the properties of many samples or samples with graded composition [4]. Optical methods (for example unpolarised and polarised microscopy and thermal imaging) and electric conductivity measurements can for example be used to map phase transformation temperatures much faster and on much larger areas than traditional TEM techniques. XRD and SEM-EDS-EBSD can then be applied to obtain initial crystallographic and microstructure information on an intermediate size scale, before TEM is applied for the in-depth study of the most relevant compositions. Results We applied this multi-pronged approach in the search for Heusler and half-Heusler alloys with a low temperature martensitic phase transformation and with low thermal hysteresis, with potential applications in waste heat energy harvesting. Conclusions Drawing advantage of the strengths of multiple analysis techniques – with respect to the obtainable information, the sample size and/or the time frame of the analyses – renders the search for new phase change materials more effective. It enables faster identification of potential target compositions and of sample areas optimal for in depth studies. Selected references [1] David Dye, Nature Materials14 (8), 760 (2015). [2] M. M. Vopson, Critical Reviews in Solid State and Materials Sciences40 (4), 223 (2015). [3] L. Huang, D. Y. Cong, L. Ma, Z. H. Nie, Z. L. Wang, H. L. Suo, Y. Ren, and Y. D. Wang, Applied Physics Letters108 (3) (2016). [4] M. L. Green, C. L. Choi, J. R. Hattrick-Simpers, A. M. Joshi, I. Takeuchi, S. C. Barron, E. Campo, T. Chiang, S. Empedocles, J. M. Gregoire, A. G. Kusne, J. Martin, A. Mehta, K. Persson, Z. Trautt, J. Van Duren, and A. Zakutayev, Applied Physics Reviews 4 (1), 011105 (2017).

• Løberg, Monika; Pike, Nicholas; Karlsen, Ole Bjørn; Carvalho, Patricia Almeida; Løvvik, Ole Martin & Gunnæs, Anette Eleonora (2019). Characterization of phase transforming, Heusler based (Cu-Pd-Sn), alloys for energy harvesting. Fulltekst i vitenarkiv
• Pike, Nicholas & Løvvik, Ole Martin (2018). Determination of the pyroelectric coefficient in spontaneously polarized materials. Fulltekst i vitenarkiv
• Pike, Nicholas & Løvvik, Ole Martin (2018). High-throughput calculations of the pyroelectric coefficient in spontaneously polarized materials. Fulltekst i vitenarkiv

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