2016
- Evolution of an isolated monopole in a spin-1 Bose–Einstein condensate,
K. Tiurev, P. Kuopanportti, A. M. Gunyhó, M. Ueda, and M. Möttönen.
Physical Review A 94, 53616 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]
We simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number κ. It is shown to decay into a coreless Anderson-Toulouse vortex if κ=1 and into a singular vortex with an empty core if κ=2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.
@article{tiurev_evolution_2016, title = {Evolution of an isolated monopole in a spin-1 {Bose--Einstein} condensate}, volume = {94}, url = {http://link.aps.org/doi/10.1103/PhysRevA.94.053616}, doi = {10.1103/PhysRevA.94.053616}, abstract = {We simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number κ. It is shown to decay into a coreless Anderson-Toulouse vortex if κ=1 and into a singular vortex with an empty core if κ=2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.}, number = {5}, urldate = {2016-11-18TZ}, journal = {Physical Review A}, author = {Tiurev, Konstantin and Kuopanportti, Pekko and Gunyhó, András Márton and Ueda, Masahito and Möttönen, Mikko}, month = nov, year = {2016}, eprint = {1609.02506}, pages = {053616} }
- Skyrmionic vortex lattices in coherently coupled three-component Bose–Einstein condensates,
N. V. Orlova, P. Kuopanportti, and M. Milošević.
Physical Review A 94, 23617 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]
We show numerically that a harmonically trapped and coherently Rabi-coupled three-component Bose-Einstein condensate can host unconventional vortex lattices in its rotating ground state. The discovered lattices incorporate square and zig-zag patterns, vortex dimers and chains, and doubly quantized vortices, and they can be quantitatively classified in terms of a skyrmionic topological index, which takes into account the multicomponent nature of the system. The exotic ground-state lattices arise due to the intricate interplay of the repulsive density-density interactions and the Rabi couplings as well as the ubiquitous phase frustration between the components. In the frustrated state, domain walls in the relative phases can persist between some components even at strong Rabi coupling, while vanishing between others. Consequently, in this limit the three-component condensate effectively approaches a two-component condensate with only density-density interactions. At intermediate Rabi coupling strengths, however, we face unique vortex physics that occurs neither in the two-component counterpart nor in the purely density-density-coupled three-component system.
@article{orlova_skyrmionic_2016, title = {Skyrmionic vortex lattices in coherently coupled three-component {Bose--Einstein} condensates}, volume = {94}, copyright = {© 2016 American Physical Society}, url = {http://link.aps.org/doi/10.1103/PhysRevA.94.023617}, doi = {10.1103/PhysRevA.94.023617}, abstract = {We show numerically that a harmonically trapped and coherently Rabi-coupled three-component Bose-Einstein condensate can host unconventional vortex lattices in its rotating ground state. The discovered lattices incorporate square and zig-zag patterns, vortex dimers and chains, and doubly quantized vortices, and they can be quantitatively classified in terms of a skyrmionic topological index, which takes into account the multicomponent nature of the system. The exotic ground-state lattices arise due to the intricate interplay of the repulsive density-density interactions and the Rabi couplings as well as the ubiquitous phase frustration between the components. In the frustrated state, domain walls in the relative phases can persist between some components even at strong Rabi coupling, while vanishing between others. Consequently, in this limit the three-component condensate effectively approaches a two-component condensate with only density-density interactions. At intermediate Rabi coupling strengths, however, we face unique vortex physics that occurs neither in the two-component counterpart nor in the purely density-density-coupled three-component system.}, language = {EN}, number = {2}, journal = {Physical Review A}, author = {Orlova, Natalia V. and Kuopanportti, Pekko and Milošević, Milorad}, month = aug, year = {2016}, eprint = {1603.05813}, keywords = {Condensed Matter - Quantum Gases, Superfluid, Vortex}, pages = {023617} }
- Onsager vortex formation in Bose–Einstein condensates in two-dimensional power-law traps,
A. J. Groszek, T. P. Simula, D. M. Paganin, and K. Helmerson.
Physical Review A 93, 43614 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]
We study computationally dynamics of quantized vortices in two-dimensional superfluid Bose-Einstein condensates confined in highly oblate power-law traps. We have found that the formation of large-scale Onsager vortex clusters prevalent in steep-walled traps is suppressed in condensates confined by harmonic potentials. However, the shape of the trapping potential does not appear to adversely affect the evaporative heating efficiency of the vortex gas. Instead, the suppression of Onsager vortex formation in harmonic traps can be understood in terms of the energy of the vortex configurations. Furthermore, we find that the vortex-antivortex pair annihilation that underpins the vortex evaporative heating mechanism requires the interaction of at least three vortices. We conclude that experimental observation of Onsager vortices should be the most apparent in flat or inverted-bottom traps.
@article{groszek_onsager_2016, title = {Onsager vortex formation in {Bose--Einstein} condensates in two-dimensional power-law traps}, volume = {93}, url = {http://link.aps.org/doi/10.1103/PhysRevA.93.043614}, doi = {10.1103/PhysRevA.93.043614}, abstract = {We study computationally dynamics of quantized vortices in two-dimensional superfluid Bose-Einstein condensates confined in highly oblate power-law traps. We have found that the formation of large-scale Onsager vortex clusters prevalent in steep-walled traps is suppressed in condensates confined by harmonic potentials. However, the shape of the trapping potential does not appear to adversely affect the evaporative heating efficiency of the vortex gas. Instead, the suppression of Onsager vortex formation in harmonic traps can be understood in terms of the energy of the vortex configurations. Furthermore, we find that the vortex-antivortex pair annihilation that underpins the vortex evaporative heating mechanism requires the interaction of at least three vortices. We conclude that experimental observation of Onsager vortices should be the most apparent in flat or inverted-bottom traps.}, number = {4}, urldate = {2016-04-18TZ}, journal = {Physical Review A}, author = {Groszek, Andrew J. and Simula, Tapio P. and Paganin, David M. and Helmerson, Kristian}, month = apr, year = {2016}, eprint = {1511.06552}, pages = {043614} }
- Hamiltonian dynamics of two same-sign point vortices,
A. V. Murray, A. J. Groszek, P. Kuopanportti, and T. Simula.
Physical Review A 93, 33649 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]
We have studied numerically the Hamiltonian dynamics of two same-sign point vortices in an effectively two-dimensional, harmonically trapped Bose-Einstein condensate. We have found in the phase space of the system an impenetrable wall that divides the dynamics into two distinct and exhaustive types. In the two-dimensional position-coordinate space, the first type corresponds to intersecting single-vortex orbits and the second type to orbits that have no points in common. The two types are also easily distinguished in the two-dimensional space spanned by the radial and angular velocities of the vortices: In the first type, both single-vortex orbits are the same simple loop in this two-dimensional space, whereas in the second type the two orbits constitute two nonintersecting loops. The phase-space-dividing wall is distinct from the bifurcation curve of rigidly rotating states found by Navarro et al. [Phys. Rev. Lett. 110, 225301 (2013)].
@article{murray_hamiltonian_2016, title = {Hamiltonian dynamics of two same-sign point vortices}, volume = {93}, url = {http://link.aps.org/doi/10.1103/PhysRevA.93.033649}, doi = {10.1103/PhysRevA.93.033649}, abstract = {We have studied numerically the Hamiltonian dynamics of two same-sign point vortices in an effectively two-dimensional, harmonically trapped Bose-Einstein condensate. We have found in the phase space of the system an impenetrable wall that divides the dynamics into two distinct and exhaustive types. In the two-dimensional position-coordinate space, the first type corresponds to intersecting single-vortex orbits and the second type to orbits that have no points in common. The two types are also easily distinguished in the two-dimensional space spanned by the radial and angular velocities of the vortices: In the first type, both single-vortex orbits are the same simple loop in this two-dimensional space, whereas in the second type the two orbits constitute two nonintersecting loops. The phase-space-dividing wall is distinct from the bifurcation curve of rigidly rotating states found by Navarro et al. [Phys. Rev. Lett. 110, 225301 (2013)].}, number = {3}, urldate = {2016-04-18TZ}, journal = {Physical Review A}, author = {Murray, Anderson V. and Groszek, Andrew J. and Kuopanportti, Pekko and Simula, Tapio}, month = mar, year = {2016}, eprint = {1512.02809}, pages = {033649} }
- Interatomic Fe–H potential for irradiation and embrittlement simulations,
P. Kuopanportti, E. Hayward, C. Fu, A. Kuronen, and K. Nordlund.
Computational Materials Science 111, 525-531 (2016).
[arXiv] [DOI] [Bibtex] [Abstract]
The behavior of hydrogen in iron and iron alloys is of interest in many fields of physics and materials science. To enable large-scale molecular dynamics simulations of systems with Fe–H interactions, we develop, based on density-functional theory calculations, an interatomic Fe–H potential in the Tersoff–Brenner formalism. The obtained analytical potential is suitable for simulations of H in bulk Fe as well as for modeling small FeH molecules, and it can be directly combined with our previously constructed potential for the stainless steel Fe–Cr–C system. This will allow simulations of, e.g., hydrocarbon molecule chemistry on steel surfaces. In the current work, we apply the potential to simulating hydrogen-induced embrittlement in monocrystalline bulk Fe and in an Fe bicrystal with a grain boundary. In both cases, hydrogen is found to soften the material.
@article{kuopanportti_interatomic_2016, title = {Interatomic Fe–H potential for irradiation and embrittlement simulations}, volume = {111}, copyright = {© 2016 Elsevier B.V.}, issn = {0927-0256}, url = {http://www.sciencedirect.com/science/article/pii/S092702561500590X}, doi = {10.1016/j.commatsci.2015.09.021}, abstract = {The behavior of hydrogen in iron and iron alloys is of interest in many fields of physics and materials science. To enable large-scale molecular dynamics simulations of systems with Fe–H interactions, we develop, based on density-functional theory calculations, an interatomic Fe–H potential in the Tersoff–Brenner formalism. The obtained analytical potential is suitable for simulations of H in bulk Fe as well as for modeling small FeH molecules, and it can be directly combined with our previously constructed potential for the stainless steel Fe–Cr–C system. This will allow simulations of, e.g., hydrocarbon molecule chemistry on steel surfaces. In the current work, we apply the potential to simulating hydrogen-induced embrittlement in monocrystalline bulk Fe and in an Fe bicrystal with a grain boundary. In both cases, hydrogen is found to soften the material.}, language = {EN}, journal = {Computational Materials Science}, author = {Kuopanportti, Pekko and Hayward, Erin and Fu, Chu-Chun and Kuronen, Antti and Nordlund, Kai}, month = jan, year = {2016}, eprint = {1604.02829}, keywords = {Iron, hydrogen embrittlement, interatomic potential, materials science, molecular dynamics, tensile testing}, pages = {525--531} }
2015
- Electron Singularities, Matter Wave Catastrophes, and Vortex Lattice Singularimetry,
T. C. Petersen, M. Weyland, D. M. Paganin, T. P. Simula, S. A. Eastwood, A. I. Bishop, and M. J. Morgan.
Microscopy and Microanalysis 21, 673-674 (2015).
[DOI] [Bibtex]@article{petersen_electron_2015, title = {Electron Singularities, Matter Wave Catastrophes, and Vortex Lattice Singularimetry}, volume = {21}, issn = {1435-8115}, url = {http://journals.cambridge.org/article_S143192761500416X}, doi = {10.1017/S143192761500416X}, number = {Supplement S3}, urldate = {2016-01-04TZ}, journal = {Microscopy and Microanalysis}, author = {Petersen, T. C. and Weyland, M. and Paganin, D. M. and Simula, T. P. and Eastwood, S. A. and Bishop, A. I. and Morgan, M. J.}, month = aug, year = {2015}, pages = {673--674} }
- Route to non-Abelian quantum turbulence in spinor Bose–Einstein condensates,
T. Mawson, G. Ruben, and T. Simula.
Physical Review A 91, 63630 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]
We have studied computationally the collision dynamics of spin-2 Bose-Einstein condensates initially confined in a triple-well trap. Depending on the phase structure of the initial-state spinor wave function, the collision of the three condensate fragments produces one of many possible vortex-antivortex lattices, after which the system transitions to quantum turbulence. We find that the emerging vortex lattice structures can be described in terms of multiwave interference. We show that the three-fragment collisions can be used to systematically produce staggered vortex-antivortex honeycomb lattices of fractional-charge vortices, whose collision dynamics are known to be non-Abelian. Such condensate collider experiments could potentially be used as a controllable pathway to generating non-Abelian superfluid turbulence with networks of vortex rungs.
@article{mawson_route_2015, title = {Route to non-Abelian quantum turbulence in spinor {Bose--Einstein} condensates}, volume = {91}, url = {http://link.aps.org/doi/10.1103/PhysRevA.91.063630}, doi = {10.1103/PhysRevA.91.063630}, abstract = {We have studied computationally the collision dynamics of spin-2 Bose-Einstein condensates initially confined in a triple-well trap. Depending on the phase structure of the initial-state spinor wave function, the collision of the three condensate fragments produces one of many possible vortex-antivortex lattices, after which the system transitions to quantum turbulence. We find that the emerging vortex lattice structures can be described in terms of multiwave interference. We show that the three-fragment collisions can be used to systematically produce staggered vortex-antivortex honeycomb lattices of fractional-charge vortices, whose collision dynamics are known to be non-Abelian. Such condensate collider experiments could potentially be used as a controllable pathway to generating non-Abelian superfluid turbulence with networks of vortex rungs.}, number = {6}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Mawson, Thomas and Ruben, Gary and Simula, Tapio}, month = jun, year = {2015}, eprint = {1412.6186}, pages = {063630} }
- Ground-state multiquantum vortices in rotating two-species superfluids,
P. Kuopanportti, N. V. Orlova, and M. V. Milošević.
Physical Review A 91, 43605 (2015).
[arXiv] [DOI] [Bibtex] [Abstract]
We show numerically that a rotating, harmonically trapped mixture of two Bose-Einstein-condensed superfluids can—contrary to its single-species counterpart—contain a multiply quantized vortex in the ground state of the system. This giant vortex can occur without any accompanying single-quantum vortices, may either be coreless or have an empty core, and can be realized in a Rb87−K41 Bose-Einstein condensate. Our results not only provide a rare example of a stable, solitary multiquantum vortex but also reveal exotic physics stemming from the coexistence of multiple, compositionally distinct condensates in one system.
@article{kuopanportti_ground-state_2015, title = {Ground-state multiquantum vortices in rotating two-species superfluids}, volume = {91}, url = {http://link.aps.org/doi/10.1103/PhysRevA.91.043605}, doi = {10.1103/PhysRevA.91.043605}, abstract = {We show numerically that a rotating, harmonically trapped mixture of two Bose-Einstein-condensed superfluids can—contrary to its single-species counterpart—contain a multiply quantized vortex in the ground state of the system. This giant vortex can occur without any accompanying single-quantum vortices, may either be coreless or have an empty core, and can be realized in a Rb87−K41 Bose-Einstein condensate. Our results not only provide a rare example of a stable, solitary multiquantum vortex but also reveal exotic physics stemming from the coexistence of multiple, compositionally distinct condensates in one system.}, number = {4}, urldate = {2016-01-06TZ}, journal = {Physical Review A}, author = {Kuopanportti, Pekko and Orlova, Natalia V. and Milošević, Milorad V.}, month = apr, year = {2015}, eprint = {1504.02178}, pages = {043605} }
- Persistent Gravitational Radiation from Glitching Pulsars,
A. Melatos, J. A. Douglass, and T. P. Simula.
The Astrophysical Journal 807, 132 (2015).
[DOI] [Bibtex] [Abstract]
Quantum mechanical simulations of neutron star rotational glitches, triggered by vortex avalanches in the superfluid stellar interior, reveal that vortices pin nonaxisymmetrically to the crust during the intervals between glitches. Hence a glitching neutron star emits a persistent current quadrupole gravitational wave signal at the star’s rotation frequency, whose interglitch amplitude is constant and determined by the avalanche history since birth. The signal can be detected in principle by coherent searches planned for the Laser Interferometer Gravitational Wave Observatory (LIGO), whether or not a glitch occurs during the observation, if the power-law distribution of glitch sizes extends up to \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn1.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax/{\textbackslash}rmØmega {\textbackslash}gtrsim 10{\textasciicircum}-6{\textbackslash}eta {\textasciicircum}-1({\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi ){\textasciicircum}-1({\textbackslash}rmØmega /10{\textasciicircum}3{\textbackslash};{\textbackslash}mathrmrad{\textbackslash};{\textbackslash}rms{\textasciicircum}-1){\textasciicircum}-3(D/1{\textbackslash};{\textbackslash}mathrmkpc)\$ in the targeted object, where \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn2.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax\$ and \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn3.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi \$ are the largest angular velocity jump and avalanche opening angle, respectively, to have occurred in a glitch since birth, Ω is the angular velocity at present, η is the crustal fraction of the moment of inertia, and D is the distance from the Earth. A major caveat concerning detectability is whether the nonaxisymmetries observed in existing simulations with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn4.gif] \${\textbackslash}lesssim 10{\textasciicircum}3\$ vortices extrapolate to realistic neutron stars with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn5.gif] \${\textbackslash}gtrsim 10{\textasciicircum}15\$ vortices. The arguments for and against extrapolation are discussed critically in the context of avalanche dynamics in self-organized critical systems, but the issue cannot be resolved without larger simulations and tighter observational limits on \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn6.gif] \${\textbackslash}eta {\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi {\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega _{\textbackslash}mathrmmax\$ from future LIGO (non)detections and radio timing campaigns.
@article{melatos_persistent_2015, title = {Persistent Gravitational Radiation from Glitching Pulsars}, volume = {807}, issn = {0004-637X}, url = {http://stacks.iop.org/0004-637X/807/i=2/a=132}, doi = {10.1088/0004-637X/807/2/132}, abstract = {Quantum mechanical simulations of neutron star rotational glitches, triggered by vortex avalanches in the superfluid stellar interior, reveal that vortices pin nonaxisymmetrically to the crust during the intervals between glitches. Hence a glitching neutron star emits a persistent current quadrupole gravitational wave signal at the star’s rotation frequency, whose interglitch amplitude is constant and determined by the avalanche history since birth. The signal can be detected in principle by coherent searches planned for the Laser Interferometer Gravitational Wave Observatory (LIGO), whether or not a glitch occurs during the observation, if the power-law distribution of glitch sizes extends up to \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn1.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax/{\textbackslash}rmØmega {\textbackslash}gtrsim 10{\textasciicircum}-6{\textbackslash}eta {\textasciicircum}-1({\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi ){\textasciicircum}-1({\textbackslash}rmØmega /10{\textasciicircum}3{\textbackslash};{\textbackslash}mathrmrad{\textbackslash};{\textbackslash}rms{\textasciicircum}-1){\textasciicircum}-3(D/1{\textbackslash};{\textbackslash}mathrmkpc)\$ in the targeted object, where \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn2.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax\$ and \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn3.gif] \${\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi \$ are the largest angular velocity jump and avalanche opening angle, respectively, to have occurred in a glitch since birth, Ω is the angular velocity at present, η is the crustal fraction of the moment of inertia, and D is the distance from the Earth. A major caveat concerning detectability is whether the nonaxisymmetries observed in existing simulations with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn4.gif] \${\textbackslash}lesssim 10{\textasciicircum}3\$ vortices extrapolate to realistic neutron stars with \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn5.gif] \${\textbackslash}gtrsim 10{\textasciicircum}15\$ vortices. The arguments for and against extrapolation are discussed critically in the context of avalanche dynamics in self-organized critical systems, but the issue cannot be resolved without larger simulations and tighter observational limits on \#\#IMG\#\# [http://ej.iop.org/images/0004-637X/807/2/132/apj515723ieqn6.gif] \${\textbackslash}eta {\textbackslash}rm{\textbackslash}Delta {\textbackslash}phi {\textbackslash}rm{\textbackslash}Delta {\textbackslash}rmØmega \_{\textbackslash}mathrmmax\$ from future LIGO (non)detections and radio timing campaigns.}, number = {2}, urldate = {2016-01-04TZ}, journal = {The Astrophysical Journal}, author = {Melatos, A. and Douglass, J. A. and Simula, T. P.}, year = {2015}, pages = {132} }
2014
- Emergence of Order from Turbulence in an Isolated Planar Superfluid,
T. Simula, M. J. Davis, and K. Helmerson.
Physical Review Letters 113, 165302 (2014).
[arXiv] [DOI] [Bibtex] [Abstract]
We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent “Onsager vortex” clusters that are stable indefinitely—despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism—the evaporative heating of the vortices—that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.
@article{simula_emergence_2014, title = {Emergence of Order from Turbulence in an Isolated Planar Superfluid}, volume = {113}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.165302}, doi = {10.1103/PhysRevLett.113.165302}, abstract = {We study the relaxation dynamics of an isolated zero temperature quasi-two-dimensional superfluid Bose-Einstein condensate that is imprinted with a spatially random distribution of quantum vortices. Following a period of vortex annihilation the remaining vortices self-organize into two macroscopic coherent “Onsager vortex” clusters that are stable indefinitely—despite the absence of driving or external dissipation in the dynamics. We demonstrate that this occurs due to a novel physical mechanism—the evaporative heating of the vortices—that results in a negative-temperature phase transition in the vortex degrees of freedom. At the end of our simulations the system is trapped in a nonthermal state. Our computational results provide a pathway to observing Onsager vortex states in a superfluid Bose gas.}, number = {16}, urldate = {2016-01-04TZ}, journal = {Physical Review Letters}, author = {Simula, Tapio and Davis, Matthew J. and Helmerson, Kristian}, month = oct, year = {2014}, eprint = {1405.3399}, pages = {165302} }
- Vortex Gyroscope Imaging of Planar Superfluids,
A. T. Powis, S. J. Sammut, and T. P. Simula.
Physical Review Letters 113, 165303 (2014).
[arXiv] [DOI] [Bibtex] [Abstract]
We propose a robust imaging technique that makes it possible to distinguish vortices from antivortices in quasi-two-dimensional Bose-Einstein condensates from a single image of the density of the atoms. Tilting the planar condensate prior to standard absorption imaging excites a generalized gyroscopic mode of the condensate, revealing the sign and location of each vortex. This technique is anticipated to enable experimental measurement of the incompressible kinetic energy spectrum of the condensate and the observation of a negative-temperature phase transition of the vortex gas, driven by two-dimensional superfluid turbulence.
@article{powis_vortex_2014, title = {Vortex Gyroscope Imaging of Planar Superfluids}, volume = {113}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.165303}, doi = {10.1103/PhysRevLett.113.165303}, abstract = {We propose a robust imaging technique that makes it possible to distinguish vortices from antivortices in quasi-two-dimensional Bose-Einstein condensates from a single image of the density of the atoms. Tilting the planar condensate prior to standard absorption imaging excites a generalized gyroscopic mode of the condensate, revealing the sign and location of each vortex. This technique is anticipated to enable experimental measurement of the incompressible kinetic energy spectrum of the condensate and the observation of a negative-temperature phase transition of the vortex gas, driven by two-dimensional superfluid turbulence.}, number = {16}, urldate = {2016-01-04TZ}, journal = {Physical Review Letters}, author = {Powis, A. T. and Sammut, S. J. and Simula, T. P.}, month = oct, year = {2014}, eprint = {1405.4352}, pages = {165303} }
- Unifying interpretations of the Gouy phase anomaly for electron waves,
T. C. Petersen, D. M. Paganin, M. Weyland, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review A 89, 63801 (2014).
[DOI] [Bibtex] [Abstract]
We provide a unified description of the Gouy phase anomaly, highlighting the pivotal role of local fluctuations in transverse momentum and quantal phase changes associated with electron trajectories that touch caustics. Using arguments based on wave optics, at the interface between classical and quantum mechanics, we derive the magnitude of the quantized Gouy phase changes for scalar waves. Our analysis unifies disparate descriptions of the Gouy anomaly within a single theoretical framework. In particular, we find that the phase anomaly connects Maslov indices, quantal Berry phases, Heisenberg momentum fluctuations, and confinement-induced Lévy-Leblond phase shifts.
@article{petersen_unifying_2014, title = {Unifying interpretations of the Gouy phase anomaly for electron waves}, volume = {89}, url = {http://link.aps.org/doi/10.1103/PhysRevA.89.063801}, doi = {10.1103/PhysRevA.89.063801}, abstract = {We provide a unified description of the Gouy phase anomaly, highlighting the pivotal role of local fluctuations in transverse momentum and quantal phase changes associated with electron trajectories that touch caustics. Using arguments based on wave optics, at the interface between classical and quantum mechanics, we derive the magnitude of the quantized Gouy phase changes for scalar waves. Our analysis unifies disparate descriptions of the Gouy anomaly within a single theoretical framework. In particular, we find that the phase anomaly connects Maslov indices, quantal Berry phases, Heisenberg momentum fluctuations, and confinement-induced Lévy-Leblond phase shifts.}, number = {6}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Petersen, T. C. and Paganin, D. M. and Weyland, M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.}, month = jun, year = {2014}, pages = {063801} }
2013
- Diffraction catastrophes threaded by quantized vortex skeletons caused by atom-optical aberrations induced in trapped Bose–Einstein condensates,
T. P. Simula, T. C. Petersen, and D. M. Paganin.
Physical Review A 88, 43626 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]
We propose a nonlinear atom-optics experiment to create diffraction catastrophes threaded by quantized vortex skeletons in Bose-Einstein-condensed matter waves. We show how atom-optical aberrations induced in trapped Bose-Einstein condensates evolve into specific caustic structures due to imperfect focusing. Vortex skeletons, whose cross sections are staggered vortex lattices, are observed to nucleate inside the universal diffraction catastrophes. Our observations shed further light on the structure and dynamics of Bose novae and suggest applications of matter wave diffraction catastrophes, including the detection of the order parameter pairing symmetry in cold gas experiments.
@article{simula_diffraction_2013, title = {Diffraction catastrophes threaded by quantized vortex skeletons caused by atom-optical aberrations induced in trapped {Bose--Einstein} condensates}, volume = {88}, url = {http://link.aps.org/doi/10.1103/PhysRevA.88.043626}, doi = {10.1103/PhysRevA.88.043626}, abstract = {We propose a nonlinear atom-optics experiment to create diffraction catastrophes threaded by quantized vortex skeletons in Bose-Einstein-condensed matter waves. We show how atom-optical aberrations induced in trapped Bose-Einstein condensates evolve into specific caustic structures due to imperfect focusing. Vortex skeletons, whose cross sections are staggered vortex lattices, are observed to nucleate inside the universal diffraction catastrophes. Our observations shed further light on the structure and dynamics of Bose novae and suggest applications of matter wave diffraction catastrophes, including the detection of the order parameter pairing symmetry in cold gas experiments.}, number = {4}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Simula, T. P. and Petersen, T. C. and Paganin, D. M.}, month = oct, year = {2013}, eprint = {1305.6413}, pages = {043626} }
- Measurement of the Gouy phase anomaly for electron waves,
T. C. Petersen, D. M. Paganin, M. Weyland, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review A 88, 43803 (2013).
[DOI] [Bibtex] [Abstract]
We measure the Gouy phase anomaly for matter waves using in-line holography to retrieve the full complex field of an astigmatic electron wave function. Sequential phase shifts of π/2 rad are observed for electron trajectories along the optic axis that pass through each line-focus caustic of subnanometer transverse width. Our observations demonstrate that anomalous phase shifts of matter waves in the vicinity of caustics can be robustly measured using phase retrieval, extending the current scope of singular electron optics.
@article{petersen_measurement_2013, title = {Measurement of the Gouy phase anomaly for electron waves}, volume = {88}, url = {http://link.aps.org/doi/10.1103/PhysRevA.88.043803}, doi = {10.1103/PhysRevA.88.043803}, abstract = {We measure the Gouy phase anomaly for matter waves using in-line holography to retrieve the full complex field of an astigmatic electron wave function. Sequential phase shifts of π/2 rad are observed for electron trajectories along the optic axis that pass through each line-focus caustic of subnanometer transverse width. Our observations demonstrate that anomalous phase shifts of matter waves in the vicinity of caustics can be robustly measured using phase retrieval, extending the current scope of singular electron optics.}, number = {4}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Petersen, T. C. and Paganin, D. M. and Weyland, M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.}, month = oct, year = {2013}, pages = {043803} }
- Vibrations of a columnar vortex in a trapped Bose–Einstein condensate,
L. Koens, T. P. Simula, and A. M. Martin.
Physical Review A 87, 63614 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]
We derive a governing equation for a Kelvin wave supported on a vortex line in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic trap, where it is assumed that the shape of the vortex line is dominated by the properties of the condensate at the center of the trap. From this solution the Kelvin wave dispersion relation is determined. In the limit of an oblate trap and in the absence of longitudinal trapping our results are consistent with previous work. We show that the derived Kelvin wave dispersion in the general case is in quantitative agreement with numerical calculations of the Bogoliubov spectrum.
@article{koens_vibrations_2013, title = {Vibrations of a columnar vortex in a trapped {Bose--Einstein} condensate}, volume = {87}, url = {http://link.aps.org/doi/10.1103/PhysRevA.87.063614}, doi = {10.1103/PhysRevA.87.063614}, abstract = {We derive a governing equation for a Kelvin wave supported on a vortex line in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic trap, where it is assumed that the shape of the vortex line is dominated by the properties of the condensate at the center of the trap. From this solution the Kelvin wave dispersion relation is determined. In the limit of an oblate trap and in the absence of longitudinal trapping our results are consistent with previous work. We show that the derived Kelvin wave dispersion in the general case is in quantitative agreement with numerical calculations of the Bogoliubov spectrum.}, number = {6}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Koens, Lyndon and Simula, Tapio P. and Martin, Andrew M.}, month = jun, year = {2013}, eprint = {1208.1313}, pages = {063614} }
- PHI: A powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes,
N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini, and K. S. Murray.
Journal of Computational Chemistry 34, 1164-1175 (2013).
[DOI] [Bibtex] [Abstract]
A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d-block and f-block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well-known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc.
@article{chilton_phi:_2013, title = {{PHI:} {A} powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes}, volume = {34}, copyright = {Copyright © 2013 Wiley Periodicals, Inc.}, issn = {1096-987X}, shorttitle = {{PHI}}, url = {http://onlinelibrary.wiley.com/doi/10.1002/jcc.23234/abstract}, doi = {10.1002/jcc.23234}, abstract = {A new program, PHI, with the ability to calculate the magnetic properties of large spin systems and complex orbitally degenerate systems, such as clusters of d-block and f-block ions, is presented. The program can intuitively fit experimental data from multiple sources, such as magnetic and spectroscopic data, simultaneously. PHI is extensively parallelized and can operate under the symmetric multiprocessing, single process multiple data, or GPU paradigms using a threaded, MPI or GPU model, respectively. For a given problem PHI is been shown to be almost 12 times faster than the well-known program MAGPACK, limited only by available hardware. © 2013 Wiley Periodicals, Inc.}, number = {13}, urldate = {2014-04-25TZ}, journal = {Journal of Computational Chemistry}, author = {Chilton, Nicholas F. and Anderson, Russell P. and Turner, Lincoln D. and Soncini, Alessandro and Murray, Keith S.}, month = may, year = {2013}, pages = {1164--1175} }
- Collective dynamics of vortices in trapped Bose–Einstein condensates,
T. Simula.
Physical Review A 87, 23630 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]
We have calculated collective-mode spectra for three-dimensional, rotating Bose-Einstein condensates in oblate harmonic traps using the microscopic Bogoliubov–de Gennes field theory. For condensates with Nv vortices, Nv Kelvin-Tkachenko-mode branches are obtained. The features of these modes are compared with those predicted by a classical point-vortex model. We have created movies to visualize the motion of the vortices corresponding to the Kelvin-Tkachenko waves.
@article{simula_collective_2013, title = {Collective dynamics of vortices in trapped {Bose--Einstein} condensates}, volume = {87}, url = {http://link.aps.org/doi/10.1103/PhysRevA.87.023630}, doi = {10.1103/PhysRevA.87.023630}, abstract = {We have calculated collective-mode spectra for three-dimensional, rotating Bose-Einstein condensates in oblate harmonic traps using the microscopic Bogoliubov–de Gennes field theory. For condensates with Nv vortices, Nv Kelvin-Tkachenko-mode branches are obtained. The features of these modes are compared with those predicted by a classical point-vortex model. We have created movies to visualize the motion of the vortices corresponding to the Kelvin-Tkachenko waves.}, number = {2}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Simula, Tapio}, month = feb, year = {2013}, eprint = {1303.0947}, pages = {023630} }
- Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes,
T. C. Petersen, M. Weyland, D. M. Paganin, T. P. Simula, S. A. Eastwood, and M. J. Morgan.
Physical Review Letters 110, 33901 (2013).
[DOI] [Bibtex] [Abstract]
An aberration corrected electron microscope is used to create electron diffraction catastrophes, containing arrays of intensity zeros threading vortex cores. Vortices are ascribed to these arrays using catastrophe theory, scalar diffraction integrals, and experimentally retrieved phase maps. From measured wave function phases, obtained using focal-series phase retrieval, the orbital angular momentum density is mapped for highly astigmatic electron probes. We observe vortex rings and topological reconnections of nodal lines by tracking the vortex cores using the retrieved phases.
@article{petersen_electron_2013, title = {Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes}, volume = {110}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.110.033901}, doi = {10.1103/PhysRevLett.110.033901}, abstract = {An aberration corrected electron microscope is used to create electron diffraction catastrophes, containing arrays of intensity zeros threading vortex cores. Vortices are ascribed to these arrays using catastrophe theory, scalar diffraction integrals, and experimentally retrieved phase maps. From measured wave function phases, obtained using focal-series phase retrieval, the orbital angular momentum density is mapped for highly astigmatic electron probes. We observe vortex rings and topological reconnections of nodal lines by tracking the vortex cores using the retrieved phases.}, number = {3}, urldate = {2016-01-04TZ}, journal = {Physical Review Letters}, author = {Petersen, T. C. and Weyland, M. and Paganin, D. M. and Simula, T. P. and Eastwood, S. A. and Morgan, M. J.}, month = jan, year = {2013}, pages = {033901} }
- Zero-energy states in rotating trapped Bose–Einstein condensates,
T. Simula.
Journal of Physics: Condensed Matter 25, 285602 (2013).
[arXiv] [DOI] [Bibtex] [Abstract]
We have calculated low-lying quasiparticle excitation spectra of rotating three-dimensional Bose–Einstein condensates. We find, as opposed to the prediction of hydrodynamic continuum theories, a minimum in the Tkachenko mode spectrum at intermediate rotation frequencies of the harmonic trap. Such a minimum can harbour a Tkachenko quasiparticle with zero excitation energy. We discuss the experimental signatures of such a zero mode.
@article{simula_zero-energy_2013, title = {Zero-energy states in rotating trapped {Bose}–{Einstein} condensates}, volume = {25}, issn = {0953-8984}, url = {http://stacks.iop.org/0953-8984/25/i=28/a=285602}, doi = {10.1088/0953-8984/25/28/285602}, abstract = {We have calculated low-lying quasiparticle excitation spectra of rotating three-dimensional Bose–Einstein condensates. We find, as opposed to the prediction of hydrodynamic continuum theories, a minimum in the Tkachenko mode spectrum at intermediate rotation frequencies of the harmonic trap. Such a minimum can harbour a Tkachenko quasiparticle with zero excitation energy. We discuss the experimental signatures of such a zero mode.}, number = {28}, urldate = {2016-01-04TZ}, journal = {Journal of Physics: Condensed Matter}, author = {Simula, Tapio}, year = {2013}, eprint = {1207.5892}, pages = {285602} }
2012
- Synthetic charge-flux quantum liquids,
T. P. Simula.
Physical Review B 85, 144521 (2012).
[arXiv] [DOI] [Bibtex] [Abstract]
We apply rotating optical flux lattices to spinor Bose-Einstein condensates. Distinct quantum states emerge for fractional ratios of vortex charge density to optical flux density. We exhibit the calculated charge-flux states and discuss their topological structure and experimental signatures.
@article{simula_synthetic_2012, title = {Synthetic charge-flux quantum liquids}, volume = {85}, url = {http://link.aps.org/doi/10.1103/PhysRevB.85.144521}, doi = {10.1103/PhysRevB.85.144521}, abstract = {We apply rotating optical flux lattices to spinor Bose-Einstein condensates. Distinct quantum states emerge for fractional ratios of vortex charge density to optical flux density. We exhibit the calculated charge-flux states and discuss their topological structure and experimental signatures.}, number = {14}, urldate = {2016-01-04TZ}, journal = {Physical Review B}, author = {Simula, Tapio P.}, month = apr, year = {2012}, eprint = {1201.4636}, pages = {144521} }
- Coherence simplices,
T. P. Simula and D. M. Paganin.
New Journal of Physics 14, 113015 (2012).
[arXiv] [DOI] [Bibtex] [Abstract]
Coherence simplices are generic topological correlation-function defects supported by a hierarchy of coherence functions. We classify coherence simplices based on their topology and discuss their structure and dynamics, together with their relevance to several physical systems.
@article{simula_coherence_2012, title = {Coherence simplices}, volume = {14}, issn = {1367-2630}, url = {http://stacks.iop.org/1367-2630/14/i=11/a=113015}, doi = {10.1088/1367-2630/14/11/113015}, abstract = {Coherence simplices are generic topological correlation-function defects supported by a hierarchy of coherence functions. We classify coherence simplices based on their topology and discuss their structure and dynamics, together with their relevance to several physical systems.}, number = {11}, urldate = {2016-01-04TZ}, journal = {New Journal of Physics}, author = {Simula, Tapio P. and Paganin, David M.}, year = {2012}, eprint = {1111.5114}, pages = {113015} }
2011
- Coherence vortices in one spatial dimension,
T. P. Simula and D. M. Paganin.
Physical Review A 84, 52104 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]
Coherence vortices are screw-type topological defects in the phase of Glauber’s two-point degree of quantum coherence, associated with pairs of spatial points at which an ensemble-averaged stochastic quantum field is uncorrelated. Coherence vortices may be present in systems whose dimensionality is too low to support spatial vortices. We exhibit lattices of such quantum-coherence phase defects for a one-dimensional model quantum system. We discuss the physical meaning of coherence vortices and propose how they may be realized experimentally.
@article{simula_coherence_2011, title = {Coherence vortices in one spatial dimension}, volume = {84}, url = {http://link.aps.org/doi/10.1103/PhysRevA.84.052104}, doi = {10.1103/PhysRevA.84.052104}, abstract = {Coherence vortices are screw-type topological defects in the phase of Glauber's two-point degree of quantum coherence, associated with pairs of spatial points at which an ensemble-averaged stochastic quantum field is uncorrelated. Coherence vortices may be present in systems whose dimensionality is too low to support spatial vortices. We exhibit lattices of such quantum-coherence phase defects for a one-dimensional model quantum system. We discuss the physical meaning of coherence vortices and propose how they may be realized experimentally.}, number = {5}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Simula, Tapio P. and Paganin, David M.}, month = nov, year = {2011}, eprint = {1111.5096}, pages = {052104} }
- Crow instability in trapped Bose–Einstein condensates,
T. P. Simula.
Physical Review A 84, 21603 (2011).
[arXiv] [DOI] [Bibtex] [Abstract]
We show theoretically that elongated vortex-antivortex dipoles can be created controllably in trapped Bose-Einstein condensates, using known experimental techniques. Vortex dipoles of sufficient length are unstable and cascade into slow vortex rings which ultimately decay via sound emission. This instability of antiparallel vortex line elements, which self-generates Kelvin waves on vortex loops in trapped atomic gases, may play a role in bridging the Kelvin-wave and Kolmogorov-Richardson cascades of quantum turbulence.
@article{simula_crow_2011, title = {Crow instability in trapped {Bose--Einstein} condensates}, volume = {84}, url = {http://link.aps.org/doi/10.1103/PhysRevA.84.021603}, doi = {10.1103/PhysRevA.84.021603}, abstract = {We show theoretically that elongated vortex-antivortex dipoles can be created controllably in trapped Bose-Einstein condensates, using known experimental techniques. Vortex dipoles of sufficient length are unstable and cascade into slow vortex rings which ultimately decay via sound emission. This instability of antiparallel vortex line elements, which self-generates Kelvin waves on vortex loops in trapped atomic gases, may play a role in bridging the Kelvin-wave and Kolmogorov-Richardson cascades of quantum turbulence.}, number = {2}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Simula, Tapio P.}, month = aug, year = {2011}, eprint = {1107.3379}, pages = {021603} }
2010
- Rotating Dipolar Spin-1 Bose–Einstein Condensates,
T. P. Simula, J. A. M. Huhtamäki, M. Takahashi, T. Mizushima, and K. Machida.
Journal of the Physical Society of Japan 80, 13001 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]
We have computed phase diagrams for rotating spin-1 Bose–Einstein condensates with long-range magnetic dipole–dipole interactions. Spin textures including vortex sheets, staggered half-quantum- and skyrmion vortex lattices and higher order topological defects have been found. These systems exhibit both superfluidity and magnetic crystalline ordering and they could be realized experimentally by imparting angular momentum in the spinor condensate.
@article{simula_rotating_2010, title = {Rotating Dipolar Spin-1 {Bose}–{Einstein} Condensates}, volume = {80}, issn = {0031-9015}, url = {http://journals.jps.jp/doi/abs/10.1143/JPSJ.80.013001}, doi = {10.1143/JPSJ.80.013001}, abstract = {We have computed phase diagrams for rotating spin-1 Bose–Einstein condensates with long-range magnetic dipole–dipole interactions. Spin textures including vortex sheets, staggered half-quantum- and skyrmion vortex lattices and higher order topological defects have been found. These systems exhibit both superfluidity and magnetic crystalline ordering and they could be realized experimentally by imparting angular momentum in the spinor condensate.}, number = {1}, urldate = {2016-01-04TZ}, journal = {Journal of the Physical Society of Japan}, author = {Simula, Tapio P. and Huhtamäki, Jukka A. M. and Takahashi, Masahiro and Mizushima, Takeshi and Machida, Kazushige}, month = dec, year = {2010}, eprint = {1007.3551}, pages = {013001} }
- Kelvin-Tkachenko waves of few-vortex arrays in trapped Bose–Einstein condensates,
T. P. Simula and K. Machida.
Physical Review A 82, 63627 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]
We have calculated the low-lying elementary excitations of three-dimensional few-vortex arrays in trapped Bose-Einstein condensates. The number of different Kelvin-Tkachenko vortex wave branches found matches the number of vortices in the condensate. The lowest odd-parity modes exhibit superfluid gyroscopic vortex motion. Experimentally, these modes could be excited and observed individually or in connection with the formation and decay of quantum turbulence.
@article{simula_kelvin-tkachenko_2010, title = {{Kelvin}-Tkachenko waves of few-vortex arrays in trapped {Bose--Einstein} condensates}, volume = {82}, url = {http://link.aps.org/doi/10.1103/PhysRevA.82.063627}, doi = {10.1103/PhysRevA.82.063627}, abstract = {We have calculated the low-lying elementary excitations of three-dimensional few-vortex arrays in trapped Bose-Einstein condensates. The number of different Kelvin-Tkachenko vortex wave branches found matches the number of vortices in the condensate. The lowest odd-parity modes exhibit superfluid gyroscopic vortex motion. Experimentally, these modes could be excited and observed individually or in connection with the formation and decay of quantum turbulence.}, number = {6}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Simula, T. P. and Machida, K.}, month = dec, year = {2010}, eprint = {1007.3548}, pages = {063627} }
- Multi-Gaussian modes of diffusion in a quenched random medium,
T. Simula and M. Stenlund.
Physical Review E 82, 41125 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]
We have studied a model of a random walk in a quenched random environment. In addition to featuring anomalous diffusion and localization, for special regimes of disorder parameters the particle density decomposes into multi-Gaussian structure while its cumulative distribution is normal. We explain the observed fine structure of the density and point out its significance to experiments.
@article{simula_multi-gaussian_2010, title = {Multi-Gaussian modes of diffusion in a quenched random medium}, volume = {82}, url = {http://link.aps.org/doi/10.1103/PhysRevE.82.041125}, doi = {10.1103/PhysRevE.82.041125}, abstract = {We have studied a model of a random walk in a quenched random environment. In addition to featuring anomalous diffusion and localization, for special regimes of disorder parameters the particle density decomposes into multi-Gaussian structure while its cumulative distribution is normal. We explain the observed fine structure of the density and point out its significance to experiments.}, number = {4}, urldate = {2016-01-04TZ}, journal = {Physical Review E}, author = {Simula, Tapio and Stenlund, Mikko}, month = oct, year = {2010}, eprint = {1011.1992}, pages = {041125} }
- Spin textures in condensates with large dipole moments,
J. A. M. Huhtamäki, M. Takahashi, T. P. Simula, T. Mizushima, and K. Machida.
Physical Review A 81, 63623 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]
We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as Cr52 or Dy condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic (optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic F=1 spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number F or the origin of the dipolar interactions.
@article{huhtamaki_spin_2010, title = {Spin textures in condensates with large dipole moments}, volume = {81}, url = {http://link.aps.org/doi/10.1103/PhysRevA.81.063623}, doi = {10.1103/PhysRevA.81.063623}, abstract = {We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as Cr52 or Dy condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic (optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic F=1 spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number F or the origin of the dipolar interactions.}, number = {6}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Huhtamäki, J. A. M. and Takahashi, M. and Simula, T. P. and Mizushima, T. and Machida, K.}, month = jun, year = {2010}, eprint = {1007.2027}, pages = {063623} }
- Finite-temperature phase transitions in quasi-two-dimensional spin-1 Bose gases,
V. Pietilä, T. P. Simula, and M. Möttönen.
Physical Review A 81, 33616 (2010).
[arXiv] [DOI] [Bibtex] [Abstract]
Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [S. Mukerjee, C. Xu, and J. E. Moore, Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D system and find a crossover temperature at which free HQVs proliferate at the center of the trap. Above the crossover temperature, we observe transitions corresponding to the onset of a coherent condensate and a quasicondensate and discuss the absence of a fragmented condensate.
@article{pietila_finite-temperature_2010, title = {Finite-temperature phase transitions in quasi-two-dimensional spin-1 {Bose} gases}, volume = {81}, url = {http://link.aps.org/doi/10.1103/PhysRevA.81.033616}, doi = {10.1103/PhysRevA.81.033616}, abstract = {Recently, the Berezinskii-Kosterlitz-Thouless transition was found to be mediated by half-quantum vortices (HQVs) in two-dimensional (2D) antiferromagnetic Bose gases [S. Mukerjee, C. Xu, and J. E. Moore, Phys. Rev. Lett. 97, 120406 (2006)]. We study the thermal activation of HQVs in the experimentally relevant trapped quasi-2D system and find a crossover temperature at which free HQVs proliferate at the center of the trap. Above the crossover temperature, we observe transitions corresponding to the onset of a coherent condensate and a quasicondensate and discuss the absence of a fragmented condensate.}, number = {3}, urldate = {2016-01-04TZ}, journal = {Physical Review A}, author = {Pietilä, Ville and Simula, Tapio P. and Möttönen, Mikko}, month = mar, year = {2010}, eprint = {0909.1898}, pages = {033616} }