Browsing by Department "Department of Applied Physics"

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(1+1)-Dimensional Scalar Field Theory on q-Deformed Space
(2023) Tadros, Poula
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study scalar field theory in a one-space and one-time dimension on a q-deformed space with a static background.We write the Lagrangian and the equation of motion and solve it to first order in q -1, where q is the deformation parameter of the space.
1/f noise and avalanche scaling in plastic deformation
(2006) Laurson, L.; Alava, Mikko J.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the intermittency and noise of dislocation systems undergoing shear deformation. Simulations of a simple two-dimensional discrete dislocation dynamics model indicate that the deformation rate exhibits a power spectrum scaling of the type 1/fα. The noise exponent is far away from a Lorentzian, with α≈1.5. This result is directly related to the way the durations of avalanches of plastic deformation activity scale with their size.
14th International Workshop on New Approaches to High-Tech: Nano-Design, Technology, Computer Simulations (NDTCS-2011) : abstract book
(2011) Susi, Toma (graphic design)
School of Science | D4 Julkaistu kehittämis- tai tutkimusraportti tai -selvitys
172 fs, 24.3 kW peak power pulse generation from a Ho-doped fiber laser system
(2018-10-01) Wang, Mengmeng; Zhang, Hui; Wei, Rongling; Zhu, Zexiu; Ruan, Shuangchen; Yan, Peiguang; Wang, Jinzhang; Hasan, Tawfique; Sun, Zhipei
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We demonstrate a high-peak-power femtosecond fiber laser system based on single-mode holmium (Ho)-doped fibers. 833 fs, 27.7 MHz pulses at 2083.4 nm generated in a passively mode-locked Ho fiber laser are amplified and compressed to near transform-limited 172 fs, 7.2 nJ pulses with 24.3 kW peak power. We achieve this performance level by using the soliton effect and high-order soliton compression. To the best of our knowledge, this is the first demonstration of sub-200 fs pulses, with peak power exceeding 10 kW from a Ho-doped single-mode fiber laser system without using bulk optics compressors.
1D kinetic modelling of the JET SOL with tungsten divertor
(2013-07) Tskhakaya, D.; Groth, M.; Contributors, JET EFDA
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this work a fully kinetic model of the JET SOL with tungsten divertor plates has been developed. It includes the dynamics of main-ions (D+) and electrons, the neutrals (D, C, W) and the impurity particles (C+m, W+n). Our simulations show extremely low concentration of W impurity. We identify two reasons which are responsible for this effect: (1) for low temperature divertor plasma the energy of most of the main-ions and the impurities in a low-ionization state impinging the divertor plates is below the W-sputtering threshold energy; (2) with increasing temperature the W-sputtering increases, but the potential drop across the divertor plasma increases too, so that most of the W ions are reabsorbed at the divertors.
2-Methylresorcinarene: a very high packing coefficient in a mono-anion based dimeric capsule and the X-ray crystal structure of the tetra-anion
(2016-05) Puttreddy, Rakesh; Beyeh, Ngong Kodiah; Kalenius, Elina; Ras, Robin H A; Rissanen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Mono- and tetra-deprotonated 2-methylresorcinarene anions (1 and 2) as their trans-1,4-diammoniumcyclohexane (TDAC)2+ inclusion complexes are reported. The mono-anion forms a fully closed dimeric capsule [1·H2O·MeOH]2 2- with a cavity volume of 165 3 and (TDAC)2+ as the guest with an extremely high packing coefficient, PC = 84.2%, while the tetra-anion forms a close-packed structure with two structurally isomeric tetra-anions 2a and 2b with a 50:50 ratio in the crystal lattice.
The 2021 Magnonics Roadmap
(2021-10-13) Barman, Anjan; Gubbiotti, Gianluca; Ladak, S.; Adeyeye, A. O.; Krawczyk, M.; Grafe, J.; Adelmann, C.; Cotofana, S.; Naeemi, A.; Vasyuchka, V. I.; Hillebrands, B.; Nikitov, S. A.; Yu, H.; Grundler, D.; Sadovnikov, A. V.; Grachev, A. A.; Sheshukova, S. E.; Duquesne, J. Y.; Marangolo, M.; Csaba, G.; Porod, W.; Demidov, V. E.; Urazhdin, S.; Demokritov, S. O.; Albisetti, E.; Petti, D.; Bertacco, R.; Schultheiss, H.; Kruglyak, V. V.; Poimanov, V. D.; Sahoo, S.; Sinha, J.; Yang, H.; Münzenberg, M.; Moriyama, T.; Mizukami, S.; Landeros, P.; Gallardo, R. A.; Carlotti, G.; Kim, J. V.; Stamps, R. L.; Camley, R. E.; Rana, B.; Otani, Y.; Yu, W.; Yu, T.; Bauer, G. E.W.; Back, C.; Uhrig, G. S.; Dobrovolskiy, O. V.; Budinska, B.; Qin, H.; Van Dijken, S.; Chumak, A. V.; Khitun, A.; Nikonov, D. E.; Young, I. A.; Zingsem, B. W.; Winklhofer, M.
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.
3D Printing of Superhydrophobic Objects with Bulk Nanostructure
(2021-11-11) Dong, Zheqin; Vuckovac, Maja; Cui, Wenjuan; Zhou, Quan; Ras, Robin H.A.; Levkin, Pavel A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The rapid development of 3D printing (or additive manufacturing) technologies demands new materials with novel properties and functionalities. Superhydrophobic materials, owing to their ultralow water adhesion, self-cleaning, anti-biofouling, or superoleophilic properties are useful for myriad applications involving liquids. However, the majority of the methods for making superhydrophobic surfaces have been based on surface functionalization and coatings, which are challenging to apply to 3D objects. Additionally, these coatings are vulnerable to abrasion due to low mechanical stability and limited thickness. Here, a new materials concept and methodology for 3D printing of superhydrophobic macroscopic objects with bulk nanostructure and almost unlimited geometrical freedom is presented. The method is based on a specific ink composed of hydrophobic (meth)acrylate monomers and porogen solvents, which undergoes phase separation upon photopolymerization to generate inherently nanoporous and superhydrophobic structures. Using a desktop Digital Light Processing printer, superhydrophobic 3D objects with complex shapes are demonstrated, with ultralow and uniform water adhesion measured with scanning droplet adhesion microscopy. It is shown that the 3D-printed objects, owing to their nanoporous structure throughout the entire volume, preserve their superhydrophobicity upon wear damage. Finally, a superhydrophobic 3D-printed gas-permeable and water-repellent microfluidic device and a hierarchically structured 3D-printed super-oil-absorbent are demonstrated.
3He Universe 2020
(2021-01) Volovik, G. E.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The latest news from 3He Universe are presented together with the extended map of the Universe.
4D and 5D phase-space tomography using slowing-down physics regularization
(2023-07) Schmidt, B. S.; Salewski, M.; Moseev, D.; Baquero-Ruiz, M.; Hansen, P. C.; Eriksson, J.; Ford, O.; Gorini, G.; Järleblad, H.; Kazakov, Ye O.; Kulla, D.; Lazerson, S.; Mencke, J. E.; Mykytchuk, D.; Nocente, M.; Poloskei, P.; Rud, M.; Snicker, A.; Stagner, L.; Äkäslompolo, S.; , W7-X Team
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We compute reconstructions of 4D and 5D fast-ion phase-space distribution functions in fusion plasmas from synthetic projections of these functions. The fast-ion phase-space distribution functions originating from neutral beam injection (NBI) at TCV and Wendelstein 7-X (W7-X) at full, half, and one-third injection energies can be distinguished and particle densities of each component inferred based on 20 synthetic spectra of projected velocities at TCV and 680 at W7-X. Further, we demonstrate that an expansion into a basis of slowing-down distribution functions is equivalent to regularization using slowing-down physics as prior information. Using this technique in a Tikhonov formulation, we infer the particle density fractions for each NBI energy for each NBI beam from synthetic measurements, resulting in six unknowns at TCV and 24 unknowns at W7-X. Additionally, we show that installing 40 LOS in each of 17 ports at W7-X, providing full beam coverage and almost full angle coverage, produces the highest quality reconstructions.
A-site deficient semiconductor electrolyte Sr1−xCoxFeO3−δ for low-temperature (450-550 °C) solid oxide fuel cells
(2022-08-30) Lu, Yuzheng; Yousaf Shah, M. A.K.; Mushtaq, Naveed; Yousaf, Muhammad; Lund, Peter D.; Zhu, Bin; Asghar, Muhammad Imran
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Fast ionic conduction at low operating temperatures is a key factor for the high electrochemical performance of solid oxide fuel cells (SOFCs). Here an A-site deficient semiconductor electrolyte Sr1−xCoxFeO3−δ is proposed for low-temperature solid oxide fuel cells (LT-SOFCs). A fuel cell with a structure of Ni/NCAL-Sr0.7Co0.3FeO3−δ-NCAL/Ni reached a promising performance of 771 mW cm−2 at 550 °C. Moreover, appropriate doping of cobalt at the A-site resulted in enhanced charge carrier transportation yielding an ionic conductivity of >0.1 S cm−1 at 550 °C. A high OCV of 1.05 V confirmed that neither short-circuiting nor power loss occurred during the operation of the prepared SOFC device. A modified composition of Sr0.5Co0.5FeO3−δ and Sr0.3Co0.7FeO3−δ also reached good fuel cell performance of 542 and 345 mW cm−2, respectively. The energy bandgap analysis confirmed optimal cobalt doping into the A-site of the prepared perovskite structure improved the charge transportation effect. Moreover, XPS spectra showed how the Co-doping into the A-site enhanced O-vacancies, which improve the transport of oxide ions. The present work shows that Sr0.7Co0.3FeO3−δ is a promising electrolyte for LT-SOFCs. Its performance can be boosted with Co-doping to tune the energy band structure.
Ab initio Kinetic Monte Carlo simulations of dissolution at the NaCl-water interface
(2014) Chen, Jian-Cheng; Reischl, Bernhard; Spijker, Peter; Holmberg, Nico; Laasonen, Kari; Foster, Adam S.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Ab initio studies of stepped Pd surfaces with and without S
(2003) Makkonen, Ilja; Salo, Petri; Alatalo, Matti; Rahman, Talat S.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Ab initio study of Cu diffusion in alpha-cristobalite
(2012) Zelený, M.; Hegedüs, J.; Foster, Adam S.; Drabold, D.A.; Elliott, S.R.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We have studied the geometries, formation energies, migration barriers and diffusion of a copper interstitial with different charge states with and without an external electric field in the α-cristobalite crystalline form of SiO2 using ab initio computer simulation. The most stable state almost throughout the band gap is charge q = + 1. The height of the migration barrier depends slightly on the charge state and varies between 0.11 and 0.18 eV. However, the charge has a strong influence on the shape of the barrier, as metastable states exist in the middle of the diffusion path for Cu with q = + 1. The heights and shapes of barriers also depend on the density of SiO2, because volume expansion has a similar effect to increase the positive charge on Cu. Furthermore, diffusion coefficients have been deduced from our calculations according to transition-state theory and these calculations confirm the experimental result that oxidation of Cu is a necessary condition for diffusion. Our molecular dynamics simulations show a similar ion diffusion, and dependence on charge state. These simulations also confirm the fact that diffusion of ions can be directly simulated using ab initio molecular dynamics.
Ab initio study of Cu diffusion in α-cristobalite
(2012-11) Zelený, M.; Hegedüs, J.; Foster, A. S.; Drabold, D. A.; Elliott, S. R.; Nieminen, R. M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We have studied the geometries, formation energies, migration barriers and diffusion of a copper interstitial with different charge states with and without an external electric field in the α-cristobalite crystalline form of SiO2 using ab initio computer simulation. The most stable state almost throughout the band gap is charge q = + 1. The height of the migration barrier depends slightly on the charge state and varies between 0.11 and 0.18 eV. However, the charge has a strong influence on the shape of the barrier, as metastable states exist in the middle of the diffusion path for Cu with q = + 1. The heights and shapes of barriers also depend on the density of SiO 2, because volume expansion has a similar effect to increase the positive charge on Cu. Furthermore, diffusion coefficients have been deduced from our calculations according to transition-state theory and these calculations confirm the experimental result that oxidation of Cu is a necessary condition for diffusion. Our molecular dynamics simulations show a similar ion diffusion, and dependence on charge state. These simulations also confirm the fact that diffusion of ions can be directly simulated using ab initio molecular dynamics.
Ab initio study of fully relaxed divacancies in GaAs
(1996) Pöykkö, S.; Puska, Martti J.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We report calculations of the electronic and atomic structures of neutral and charged divacancies in GaAs using the first-principles Car-Parrinello method. It is found that the divacancy relaxes inwards in all charge states (2-,1-,0,1+) studied. The defect-induced electron levels lie in the lower half of the fundamental band gap. The doubly negative divacancy is the most stable one for nearly all values of the electron chemical potential within the band gap. The deep-level electron density is localized at the Ga-vacancy end of the divacancy and the ionic relaxation is stronger there than at the As-vacancy end. We have also calculated the thermodynamic concentrations for several different native defects in GaAs, and the implications for self-diffusion are discussed.
Ab initio study of gamma-Al2O3 surfaces
(2004) Pinto, Henry P.; Nieminen, Risto M.; Elliott, Simon D.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Starting from the theoretical prediction of the γ−Al2O3 structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) α−Al2O3 and available experimental results for γ-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.
Ab initio study of oxygen point defects in GaAs, and AIN
(1996) Mattila, T.; Nieminen, R.M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. OAs in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ON in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ON in AlN is shown to be a deep center due to the wide band gap, in contrast with ON in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in AlxGa1−xN alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented.
Ab initio study of oxygen point defects in GaAs, GaN, and AlN
(1996) Mattila, T.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. OAs in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ON in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ON in AlN is shown to be a deep center due to the wide band gap, in contrast with ON in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in AlxGa1−xN alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented. © 1996 The American Physical Society.
Ab initio study of point defects in CdF2
(1997) Mattila, T.; Pöykkö, S.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The plane-wave pseudopotential method is used to study point defects in CdF2. We present comprehensive results for the native defects as well as for dominant impurities. In addition to Fi, VCd and OF were found to be easily formed compensating acceptors. For In and Ga impurities the experimentally observed large Stokes shift could not be established, and the results rule out symmetric atomic relaxation as the mechanism leading to the bistable behavior. The limitations of the present approach utilizing density-functional theory and the local-density approximation in the case of ionic materials are addressed.