A04 TSUBOTA, Makoto |Proposed Research Projects (2016-2017)

Paper | Original Paper


Jim Gao, *Wei Guo, Satoshi Yui, Makoto Tsubota, and W. F. Vinen,
Dissipation in quantum turbulence in superfluid 4He above 1 K,
Physical Review B 97, 184518/1-10 (2018).

[Summary] There are two commonly discussed forms of quantum turbulence in superfluid 4He above 1 K: in one there isa random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the secondthere is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scaleslarger than the separation between the quantized vortex lines in the superfluid component. We study experimentally and numerically the decay of vortex tangle in thermal counterflow.

*Satoshi Yui, Makoto Tsubota, and Hiromichi Kobayashi,
Three-Dimensional Coupled Dynamics of the two-fluid model in superfluid 4He: Deformed velocity profile of normal fluid in thermal counterflow,
Physical Review Letters 120, 155301/1-4 (2018).

[Summary] The coupled dynamics of the two-fluid model of superfluid 4He is numerically studied for quantum turbulence of the thermal counterfow in a square channel. We combine the vortex filamentmodel of the superfluid and the Navier{Stokes equations of normal fluid. Simulations of the coupled dynamics show that the velocity profile of the normal fluid is deformed significantly by superfluid turbulence as the vortices become dense. This result is consistent with recently performed visualization experiments. We introduce a dimensionless parameter that characterizes the deformation of the velocity profile.

Toshiaki Kanai, *Wei Guo, Makoto Tsubota,
Flows with fractional quantum circulation in Bose-Einstein condensates induced by nontopological phase defects,
Physical Review A 97, 013612/1-5 (2018).

Toshiaki Kanai, *Wei Guo, Makoto Tsubota, Flows with fractional quantum circulation in Bose-Einstein condensates induced by nontopological phase defects, Physical Review A 97, 013612/1-5 (2018).


*Che-hsiu Hsueh, Wen-Chin Wu, and Makoto Tsubota,
Quantum crystallography of Rydberg-dressed Bose gases on a square lattice,
Physical Review A 95, 013631/1-5 (2017).

[Summary] We numerically investigate the quantum crystallographic phases of a Rydberg-dressed Bose gas loaded on a square lattice by using the mean-field Gross–Pitaevskii model. For a relatively weak lattice confinement, the phases of ground state undergo amorphism, polycrystal, and polymorphism following the increase of the blockade radius, and if the confinement is stronger, a single crystal with a specific fillingfactor will be formed. In order to distinctively characterize these phases, the structure function is also studied. In such an anisotropic system, we report that the first diagonal element of the superfluid-fraction tensor should be a measurable quantity, and an anisotropy parameter can be defined.


*Shinichi Ikawa, and Makoto Tsubota,
Coflow turbulence of superfluid 4He in a square channel: Vortices trapped on a cylindrical attractor,
Physical Review B 93, 184508/1-8 (2016).

[Summary] We perform a numerical simulation of the dynamics of quantized vortices produced by coflow in a squarechannel using the vortex filament model. Unlike the situation in thermal counterflow, where the superfluidvelocity vs and normal-fluid velocity vn flow in opposite directions, in coflow, vs and vn flow in the samedirection. Quantum turbulence in thermal counterflow has been long studied theoretically and experimentally,and its various features have been revealed. In recent years, an experiment on quantum turbulence in coflow hasbeen performed to observe different features of thermal counterflow. By supposing that vs is uniform and vntakes the Hagen-Poiseuille profile, which is different from the experiment where vn is thought to be turbulent,we calculate the coflow turbulence. Vortices preferentially accumulate on the surface of a cylinder for vs  vn bymutual friction; namely, the coflow turbulence has an attractor. How strongly the vortices are attracted depends onthe temperature and velocity. The length of the vortices increases as the vortices protruding from the cylindricalattractor continue to wrap around it. As the vortices become dense on the attractor, they spread toward its interiorby their repulsive interaction. Then, the superfluid velocity profile induced by the vortices gradually mimics thenormal-fluid velocity profile. This is an indication of velocity matching, which is an important feature of coflowturbulence.

*Kazuya Fujimoto, Makoto Tsubota,
Direct and inverse cascades of spin-wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates,
Physical Review A 93, 033620/1-10 (2016).

[Summary] We theoretically and numerically study spin wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates, finding direct and inverse cascades with power-law behavior. To derive these power exponents analytically, the conventional weak wave turbulence theory is applied to the spin-1 spinor Gross-Pitaevskiiequation. Thus we obtain the −7/3 and −5/3 power laws in the transverse spin correlation function for the directand inverse cascades, respectively.