Author

# L. A. Toikka

Other affiliations: Helsinki Institute of Physics, University of Turku, Massey University

Bio: L. A. Toikka is an academic researcher from University of Innsbruck. The author has contributed to research in topics: Soliton & Bose–Einstein condensate. The author has an hindex of 7, co-authored 23 publications receiving 203 citations. Previous affiliations of L. A. Toikka include Helsinki Institute of Physics & University of Turku.

##### Papers

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TL;DR: In this article, it was shown that the onset of the snake instability of ring dark solitons requires a broken symmetry, which can be used to predict the number of vortex-antivortex pairs produced.

Abstract: We show by numerically solving the Bogoliubov--de Gennes equations and numerically integrating the Gross-Pitaevskii equation that the onset of the snake instability of ring dark solitons requires a broken symmetry. We elucidate explicitly the connection between imaginary Bogoliubov modes and the snake instability, predicting the number of vortex-antivortex pairs produced. In addition, we propose a simple model to give a physical motivation as to why the snake instability takes place and needs a broken symmetry. Finally, we show that tight confinement in a toroidal potential actually enhances soliton decay due to inhibition of soliton motion, but can lead to a periodic revival of the ring dark soliton after the first snake instability.

34 citations

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TL;DR: By combining analytical molecular-dynamics with density-functional theory simulations, this article studied the radiation hardness of mechanically strained low-dimensional nanosystems such as carbon nanotubes, graphene, and Si nanowires.

Abstract: By combining analytical molecular-dynamics with density-functional theory simulations, we study the radiation hardness of mechanically strained low-dimensional nanosystems such as carbon nanotubes, graphene, and Si nanowires. We show that the radiation hardness of all these structures decreases with strain but the effect is most pronounced in nanowire due to the bulk structure of its core in contrast with the planar structure of nanotubes and graphene. Our results not only elucidate the microscopic mechanism of irradiation-induced defect production in strained nanomaterials but also provide quantitative information required for assessing the stability of nanocomponents in composite materials subjected to mechanical strain and irradiation, e.g., in space applications.

28 citations

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TL;DR: In this paper, the hydrodynamic equations of vortex motion in a compressible superfluid can be solved exactly in a model "slab" geometry, starting from an exact solution for an incompressible fluid, the hydroynamical equations are solved with a series expansion in a small tunable parameter provided by the ratio of the healing length, characterizing the vortex cores, to the slab width.

Abstract: Vortex motion is a complex problem due to the interplay between the short-range physics at the vortex core level and the long-range hydrodynamical effects. Here we show that the hydrodynamic equations of vortex motion in a compressible superfluid can be solved exactly in a model "slab" geometry. Starting from an exact solution for an incompressible fluid, the hydrodynamic equations are solved with a series expansion in a small tunable parameter provided by the ratio of the healing length, characterizing the vortex cores, to the slab width. The key dynamical properties of the vortex, the inertial and physical masses, are well defined and renormalizable. They are calculated at leading order beyond the logarithmic accuracy that has limited previous approaches. Our results provide a solid framework for further detailed study of the vortex mass and vortex forces in strongly-correlated and exotic superfluids. The proposed geometry can be realised in quantum-gas experiments where high-precision measurements of vortex mass parameters are feasible.

24 citations

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TL;DR: In this article, exact ring soliton-like solutions of the cylindrically symmetric (i.e., radial) Gross-Pitaevskii equation with a potential, using the similarity transformation method, were constructed.

Abstract: We construct exact ring soliton-like solutions of the cylindrically symmetric (i.e. radial) Gross–Pitaevskii equation with a potential, using the similarity transformation method. Depending on the choice of the allowed free functions, the solutions can take the form of stationary dark or bright rings whose time dependence is in the phase dynamics only, or oscillating and bouncing solutions, related to the second Painleve transcendent. In each case the potential can be chosen to be time independent.

20 citations

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TL;DR: In this article, exact ring soliton-like solutions of the cylindrically symmetric (i.e., radial) Gross- Pitaevskii equation with a potential, using the similarity transformation method, were constructed.

Abstract: We construct exact ring soliton-like solutions of the cylindrically symmetric (i.e., radial) Gross- Pitaevskii equation with a potential, using the similarity transformation method. Depending on the choice of the allowed free functions, the solutions can take the form of stationary dark or bright rings whose time dependence is in the phase dynamics only, or oscillating and bouncing solutions, related to the second Painlev\'e transcendent. In each case the potential can be chosen to be time-independent.

18 citations

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28 Apr 2017TL;DR: In this paper, the authors study the production of defects in free-standing metal dichalcogenides (TMDs) under noble gas ion irradiation for a wide range of ion energies when nuclear stopping dominates, and assess the probabilities for different defects to appear.

Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDs), like MoS2, have unique electronic and optical properties, which can further be tuned using ion bombardment and post-synthesis ion-beam mediated methods combined with exposure of the irradiated sample to precursor gases. The optimization of these techniques requires a complete understanding of the response of 2D TMDs to ion irradiation, which is affected by the reduced dimensionality of the system. By combining analytical potential molecular dynamics with first-principles calculations, we study the production of defects in free-standing MoS2 sheets under noble gas ion irradiation for a wide range of ion energies when nuclear stopping dominates, and assess the probabilities for different defects to appear. We show that depending on the incident angle, ion type and energy, sulfur atoms can be sputtered away predominantly from the top or bottom layers, creating unique opportunities for engineering mixed MoSX compounds where X are chemical elements from group V or VII. We study the electronic structure of such systems, demonstrate that they can be metals, and finally discuss how metal/semiconductor/metal junctions, which exhibit negative differential resistance, can be designed using focused ion beams combined with the exposure of the system to fluorine.

144 citations

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TL;DR: A critical evaluation of the characterization techniques and methodologies for the characterization and understanding of the mechanical properties of nanowires, including elasticity, plasticity, anelasticity and strength were presented.

Abstract: Applications of nanowires into future generation nanodevices require a complete understanding of the mechanical properties of the nanowires. A great research effort has been made in the past two decades to understand the deformation physics and mechanical behaviors of nanowires, and to interpret the discrepancies between experimental measurements and theoretical predictions. This review focused on the characterization and understanding of the mechanical properties of nanowires, including elasticity, plasticity, anelasticity and strength. As the results from the previous literature in this area appear inconsistent, a critical evaluation of the characterization techniques and methodologies were presented. In particular, the size effects of nanowires on the mechanical properties and their deformation mechanisms were discussed.

140 citations

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TL;DR: In this paper, the dynamics of a ring-shaped Bose-Einstein condensate were studied both experimentally and theoretically, and it was shown that the expansion redshifts long-wavelength excitations, as in an expanding universe.

Abstract: We study the dynamics of a supersonically expanding ring-shaped Bose-Einstein condensate both experimentally and theoretically. The expansion redshifts long-wavelength excitations, as in an expanding universe. After expansion, energy in the radial mode leads to the production of bulk topological excitations - solitons and vortices - driving the production of a large number of azimuthal phonons and, at late times, causing stochastic persistent currents. These complex nonlinear dynamics, fueled by the energy stored coherently in one mode, are reminiscent of a type of "preheating" that may have taken place at the end of inflation.

139 citations

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TL;DR: An unexpectedly high stability of armchair edges is reported, comparable to that of pristine graphene, and it is demonstrated that the electron energy should be below ~50 keV to minimize the knock-on damage.

Abstract: Electron beam of a transmission electron microscope can be used to alter the morphology of graphene nanoribbons and create atomically sharp edges required for applications of graphene in nanoelectronics. Using density-functional-theory-based simulations, we study the radiation hardness of graphene edges and show that the response of the ribbons to irradiation is not determined by the equilibrium energetics as assumed in previous experiments, but by kinetic effects associated with the dynamics of the edge atoms after impacts of energetic electrons. We report an unexpectedly high stability of armchair edges, comparable to that of pristine graphene, and demonstrate that the electron energy should be below ∼50 keV to minimize the knock-on damage.

137 citations