Abstract
We present a combined numerical and experimental study of Andreev scattering from quantum turbulence in superfluid $^3$He-B at ultralow temperatures. We simulate the evolution of moderately dense, three-dimensional, quasiclassical vortex tangles and the Andreev reflection of thermal quasiparticle excitations by these tangles. This numerical simulation enables us to generate the two-dimensional map of local Andreev reflections for excitations incident on one of the faces of a cubic computational domain, and to calculate the total coefficient of Andreev reflection as a function of the vortex line density. Our numerical simulation is then compared with the experimental measurements probing quantum turbulence generated by a vibrating grid. We also address the question of whether the quasiclassical and ultraquantum regimes of quantum turbulence can be distinguished by their respective total Andreev reflectivities. We discuss the screening mechanisms which may strongly affect the total Andreev reflectivity of dense vortex tangles. Finally, we present combined numerical-experimental results for fluctuations of the Andreev reflection from a quasiclassical turbulent tangle and demonstrate that the spectral properties of the Andreev reflection reveal the nature and properties of quantum turbulence.
Highlights
A pure superfluid in the zero-temperature limit has no viscosity
We present a combined numerical and experimental study of Andreev scattering from quantum turbulence in superfluid 3He-B at ultralow temperatures
Thermal excitations no longer form the normal fluid, but a few remaining excitations, whose mean free paths greatly exceed the typical size of the experimental cell, form a ballistic gas, which has no influence on the vortex dynamics [4]
Summary
A pure superfluid in the zero-temperature limit has no viscosity. The superfluid is formed by atoms which have condensed into the ground state. Of quantized vortices result in the formation of a vortex tangle which displays complex dynamics, as each vortex moves according to the collective velocity field of all other vortices [2,3] This complex, disordered flow is known as quantum turbulence. We only consider the low temperature regime in 3He-B, T 0.17Tc, where Tc ≈ 0.9 mK, is the critical temperature of superfluid transition at zero pressure In this temperature regime, thermal excitations no longer form the normal fluid, but a few remaining excitations, whose mean free paths greatly exceed the typical size of the experimental cell, form a ballistic gas, which has no influence on the vortex dynamics [4]. DYNAMICS OF THE VORTEX TANGLE AND PROPAGATION OF THERMAL QUASIPARTICLE EXCITATIONS: GOVERNING EQUATIONS AND
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