Research
My main research theme is "study of quantum many-body dynamics based on Time-Dependent Density Functional Theory (TDDFT)." I'm interested in various phenomena ranging from collisions of atomic nuclei, superfluid dynamics inside neutron stars, to ultracold atomic gasses, and so forth.
Solitonic excitations in collisions of superfluid nuclei
At the heart of an atom, a tiny massive entity—an atomic nucleus—resides, which is composed of neutrons and protons (nucleons). In nuclear physics community, it is well-known that neutrons behave like superfluid and protons behave like superconducting in majority of atomic nuclei. Actually, superfluidity or superconductivity is actively studied in a variety of fields of science. You may have heard about related things such as persistent current, Meissner effect, Maglev train, and so on. Among those fascinating phenomena, one of the most peculiar is "topological excitations" of superfluid, in a form of a quantized vortex or a soliton, which are associated with a "winding" or a "discontinuity" of the phase of complex-valued spatially-modulating function that characterizes superfluid properties. Rcently, we have shown [1,2,3], that a soliton-like excitation (an abrupt distortion of the pairing phase) hinders energy dissipation as well as neck formation, leading to dramatic changes of the reaction dynamics: most intriguingly, it can even prevent fusion reaction that might be detectable with current experimental technology. From the link below, you can watch a typical movie that shows how the solitonic excitation alters the reaction dynamics. Theoretically-predicted novel role of superfluidity is now awaiting for experimental discovery.
Left column: the total density; Right column: absolute value of the pairing field of neutrons; Each row displays reaction dynamics with different pairing phase difference. Keywords:
Nuclear fusion, pairing, superfluidity, soliton, TDDFT/TDSLDA/TDHFB
References:
[1] P. Magierski, K. Sekizawa, and G. Wlazłowski,
Phys. Rev. Lett. 119, 042501 (2017);
arXiv:1611.10261.
[2] K. Sekizawa, P. Magierski, and G. Wlazłowski,
PoS(INPC2016)214 (2017);
arXiv:1702.00069.
[3] K. Sekizawa, G. Wlazłowski, and P. Magierski,
EPJ Web of Conf. 163, 00051 (2017);
arXiv:1705.04902.
Collaborators: P. Magierski, G. Wlazłowski
Topological excitations in spin-imbalanced superfluid Fermi gas
In fermionic systems, superfluidity arises due to the Cooper-pairing mechanism, where two fermions with opposite spins form a pair. Now, imagine that you are participating in a rigorous folk-dance party, where only man and woman are allowed to form a pair. If the numbers of male/female participants are equal, everyone can enjoy. But, what if those numbers are imbalanced? There will be a bunch of poor participants who cannot find a partner. Question is: How would they behave? Would they get angry and disturb the dance party? — A similar situation is realized in a spin-imbalanced superfluid fermionic system, where the Cooper-pairing mechanism is frustrated. Actually, the unpaired particles are expelled from a superfluid cloud of paired particles — so paired participants occupy the dance hall :/ But, that's not the end of the story. Very recently, we have found [1], that when some "defect" is created in the superfluid cloud (topological defects, like domain wall, vortex ring, or vortex line), the unpaired particles are sucked into the defects, changing its stability. Moreover, we have found that vortex crossings/reconnections, fundamental processes at the basis of quantum turbulence [2], are hindered by the sucked unpaired particles — at first, single (unpaired) participants just spectate the dance party, but when they find oppotunity they enter the dance hall and affect the other (paired) dancers ;) Exploring a new degree of freedom of atomic gases, spin-polarization, brought us into a terra incognita — a field where qualitatively new effects may emerge.
Keywords:
Topological excitations, superfluidity, unitary Fermi gas, spin imbalance/polarization, TDDFT/TDSLDA/TDHFB/TDBdG
References:
[1] G. Wlazłowski, K. Sekizawa, M. Marchwiany, and P. Magierski , Phys. Rev. Lett. 120, 253002 (2018);
arXiv:1711.05803.
[2] C.F. Barenghi, L. Skrbek, and K.R. Sreenivasan, Proc. Natl. Acad. Sci. USA 111, 4647 (2014).
Collaborators: P. Magierski, G. Wlazłowski, M. Marchwiany
Dynamics of superfluid vortices in the neutron star crust
As a remnant of supernova explosion of a massive star, a rotating neutron star can be created. From continuous observations, sudden changes of the rotational frequency — so-called "glitches" [1] — have been found, whose origin is still a debatable problem in nuclear astrophysics. It has been suggested [2,3] that the glitch is caused by a catastrophic unpinning of a huge number of vortices from pinning sites, a lattice of neutron-rich nuclei, in the inner crust of a neutron star. Although the idea was proposed more than 40 years ago [2,3], we have still quite poor knowledge about the vortex-nucleus interaction, which is undoubtedly a key quantity to understand the mechanism of glitches. To establish microscopic understanding of the vortex-nucleus interaction and to unveil the glitch mechanism, we have performed fully-microscopic simulations based on TDDFT including superfluidity, TDSLDA [4]. From the results, we have found that the vortex-nucleus interaction is actually "repulsive", at least for two background neutron densities examined, 0.014 fm-3 and 0.031 fm-3. Moreover, we have found significance of internal degrees of freedom, "vortex bending" and "nuclear-shape deformation". From the link below, you can watch a typical movie which shows one of the results of our simulations for the vortex-nucleus dynamics [5]. To our knowledge, it is the world-first dynamic 3D simulation for the vortex-nucleus system, with an explicit treatment of nucleonic degrees of freedom.
Blue line: center of the vortex line; Red dot: position of the nucleus; Arrow: the extracted vortex-nucleus force.
(Neutron density: 0.014 fm-3, Number of protons: Z=50) Keywords:
Glitch, quantum vortex, pinning, superfluid, pairing, inner crust, neutron star, TDDFT/TDSLDA/TDHFB
References:
[1] N. Andersson et al., Phys. Rev. Lett. 109, 241103 (2012).
[2] Richard E. Packard, Phys. Rev. Lett. 28, 1080 (1972).
[3] P.W. Anderson and N. Itoh, Nature 256, 25 (1975).
[4] A. Bulgac et al., Science 332, 1288 (2011).
[5] G. Wlazłowski, K. Sekizawa, P. Magierski, A. Bulgac, M.M. Forbes, Phys. Rev. Lett. 117, 232701 (2016);
arXiv:1606.04847.
Collaborators: P. Magierski, G. Wlazłowski, A. Bulgac, M.M. Forbes
Toward production of neutron-rich unstable nuclei
Thanks to continuous developments of experimental techniques, it is now feasible to study properties of unstable nuclei which do not exist naturally on Earth. Unstable nuclei can be produced by nucleus-nucleus collision experiments in accelerator laboratories. Because how to produce unstable nuclei is not obvious, it is crucially important to develop microscopic understanding of underlying reaction mechanisms and to provide reliable theoretical predictions. Recently, multi-nucleon transfer (MNT) reactions have been considered to be useful to produce neutron-rich unstable nuclei [1, 2]. Since models which have been used to describe MNT reactions are to some extent empirical, containing adjustable parameters to reproduce measurements, it has been desired to develop a fully microscopic framework with predictive power. To reveal applicability of TDDFT in describing MNT reactions, we performed detailed analyses [3]. From our analyses, we have found that MNT cross sections can be reasonably described by TDDFT. To find optimal conditions for producing objective unstable nuclei, we are now conducting systematic TDDFT simulations of heavy-ion reactions for various initial conditions, using supercomputers. From the link below, you can watch a typical movie showing a result of our TDDFT simulations for 58Ni+208Pb reaction, reported in Ref. [3]. Dynamics of a thick neck formation and its breaking causes transfer of many (more than 10) nucleons from the heavier nucleus to the lighter one.
For this subject, we are conducting a collaborative work with the experimental group in Bhabha Atomic Research Centre (BARC), Mumbai, India [4,5].
Keywords:Unstable nuclei, multi-nucleon transfer, low-energy heavy-ion reaction, TDDFT/TDHF
References:
[1] C.H. Dasso et al., Phys. Rev. Lett. 73, 1907 (1994).
[2] V.I. Zagrebaev and W. Greiner, Phys. Rev. C 87, 034608 (2013).
[3] K. Sekizawa and K. Yabana, Phys. Rev. C 88, 014614 (2013).
[4] Sonica et al., Phys. Rev. C 92, 024603 (2015).
[5] B.J. Roy et al., Phys. Rev. C 97, 034603 (2018).
Collaborators: K. Yabana, B.J. Roy
Toward synthesis of the heaviest element
Does it surprise you that the periodic table is not only incomplete, but is actually growing from time to time? For example, new elements with the atomic numbers, 113, 115, 117, and 118, have been added to the table, filling its seventh row in June 2016! Actually, the heaviest stable element which exists naturally on Earth is Uranium (Symbol "U", Z=92), and heavier elements (Z>92) have been produced in accelerator laboratories. Among them, those with the atomic number Z>103 are called "superheavy elements" (SHEs). It is one of the most challenging subjects in nuclear physics to reveal the largest possible atomic number Z [1]. A typical example of SHE researches in Japan is the syntheses of the element 113 (named Nihonium; Symbol "Nh") by K. Morita et al. [2], an experimental group in RIKEN (at that time). The SHE synthesis is extremely difficult, since the fusion reaction is substantially hindered by the so-called quasi-fission (QF) process. In order to synthesize an objective SHE, it is important to understand the main competing process, QF. We have thus performed detailed TDDFT calculations for 64Ni+238U reaction which has been considered as a candidate for synthesizing an unknown element with Z=120. Our results indicate that QF dynamics are sensitive to effects of quantum shells and orientation of deformed 238U. From the links below, you can watch typical movies showing results of our TDDFT simulations for the 64Ni+238U reaction, reported in Ref. [3]. Although the incident energy is the same for both cases, you can see quite different reaction dynamics associated with different orientations of 238U: a compact composite system is formed, which might lead to synthesis of the element 120, only when 238U collides from its side with 64Ni.
1) "tip" collision -> quasi-fission
2) "side" collision -> capture
Superheavy element synthesis, nuclear fusion, quasi-fission, low-energy heavy-ion reaction, TDDFT/TDHF
References:
[1] Y. Oganessian, J. Phys. G 34, R165 (2007).
[2] Special page of the element 113 by RIKEN.
[3] K. Sekizawa and K. Yabana, Phys. Rev. C 93, 054616 (2016).
Collaborators: K. Hagino, K. Yabana
Doctoral Program in Physics (University of Tsukuba)
- Theme
-
Multinucleon Transfer Reactions and Quasifission Processes in Time-Dependent Hartree-Fock Theory
- Keywords
- Time-dependent Hartree-Fock theory, multi-nucleon transfer reactions, quasi-fission processes, low-energy heavy ion reactions, particle-number projection
Abstract (PDF, 35 KB); Download: Tsukuba repository
Master's Program in Physics (University of Tsukuba)
- Thesis Title
-
Time-Dependent Mean Field Theory for Multi-Nucleon Transfer Reaction
- Keywords
- Time-dependent mean-field theory, multi-nucleon transfer reactions, time-dependent Hartree-Fock (TDHF) theory, Numerical simulation of nuclear reactions, particle-number projection
Abstract (PDF, 26 KB)
Undergraduate Course (Tokyo University of Science)
- Theme
-
Study of high-density hadronic phase by random phase approximation
- Keywords
- Quark matter, two-flavor color-superconducting phase, random phase approximation (RPA)