M. Krusius

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

We report the observation of vortex formation upon the absorption of a thermal neutron in a rotating container of superfluid

An intrinsic velocity-independent criterion for superfluid turbulence

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NMR experiments on the superfluid phases of3He in restricted geometries

He-B. The nuclear reaction n +

Shear Flow and Kelvin-Helmholtz Instability in Superfluids

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Quantum turbulence in a propagating superfluid vortex front

He = p +

Double-quantum vortex in superfluid 3He-A

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NMR and axial magnetic field textures in stationary and rotating superfluid3He-B

H + 0.76MeV heats a cigar shaped region of the superfluid into the normal phase. The subsequent cooling of this region back through the superfluid transition results in the nucleation of quantized vortices. Depending on the superflow velocity, sufficiently large vortex rings grow under the influence of the Magnus force and escape into the container volume where they are detected individually with nuclear magnetic resonance. The larger the superflow velocity the smaller the rings which can expand. Thus it is possible to obtain information about the morphology of the initial defect network. We suggest that the nucleation of vortices during the rapid cool-down into the superfluid phase is similar to the formation of defects during cosmological phase transitions in the early universe.

Structure of the surface vortex sheet between two rotating 3He superfluids

Hydrodynamic flow in classical and quantum fluids can be either laminar or turbulent. Vorticity in turbulent flow is often modelled with vortex filaments. While this represents an idealization in classical fluids, vortices are topologically stable quantized objects in superfluids. Superfluid turbulence is therefore thought to be important for the understanding of turbulence more generally. The fermionic 3He superfluids are attractive systems to study because their characteristics vary widely over the experimentally accessible temperature regime. Here we report nuclear magnetic resonance measurements and numerical simulations indicating the existence of sharp transition to turbulence in the B phase of superfluid 3He. Above 0.60Tc (where Tc is the transition temperature for superfluidity) the hydrodynamics are regular, while below this temperature we see turbulent behaviour. The transition is insensitive to the fluid velocity, in striking contrast to current textbook knowledge of turbulence. Rather, it is controlled by an intrinsic parameter of the superfluid: the mutual friction between the normal and superfluid components of the flow, which causes damping of the vortex motion.Hydrodynamic flow in both classical and quantum fluids can be either laminar or turbulent. To describe the latter, vortices in turbulent flow are modelled with stable vortex filaments. While this is an idealization in classical fluids, vortices are real topologically stable quantized objects in superfluids. Thus superfluid turbulence is thought to hold the key to new understanding on turbulence in general. The fermion superfluid 3He offers further possibilities owing to a large variation in its hydrodynamic characteristics over the experimentally accessible temperatures. While studying the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp transition at 0.60Tc between two regimes, with regular behaviour at high-temperatures and turbulence at low-temperatures. Unlike in classical fluids, this transition is insensitive to velocity and occurs at a temperature where the dissipative vortex damping drops below a critical limit. This discovery resolves the conflict between existing high- and low-temperature measurements in 3He-B: At high temperatures in rotating flow a vortex loop injected into superflow has been observed to expand monotonically to a single rectilinear vortex line, while at very low temperatures a tangled network of quantized vortex lines can be generated in a quiescent bath with a vibrating wire. The solution of this conflict reveals a new intrinsic criterion for the existence of superfluid turbulence.

Intrinsic and extrinsic mechanisms of vortex formation in superfluid3He-B

We report transverse cw NMR measurements on the superfluid phases of3He at temperatures between 3 and 0.7 mK. Nuclear demagnetization of copper was used for refrigeration. For thermometry we employed pulsed NMR on platinum powder immersed in the liquid. The measurements on3He were carried out in two NMR coil assemblies in which the liquid was confined between parallel Mylar foils with separations of 0.37 mm and 4 µm. The transition temperatureTcwas measured at pressures between 32 bars and the saturated vapor pressure; a pressure-independent increase of ∼11% was observed inTcwith respect to earlier data obtained with the same apparatus. We found that our temperature scale is not proportional to that used in La Jolla. In the 4-µm stack we observe a reduction in the B → A transition temperature. In our measurements on the orientational anisotropy of the B phase we found qualitative agreement with the theory of Brinkman et al. We also measured the longitudinal resonance frequencies of the A and B phases between 32 bars and the polycritical point. In the 4-µm stack we found a negative NMR shift in the A phase when the field was oriented perpendicular to the Mylar plates, in agreement with the prediction of Takagi. The static susceptibility XB of the B phase was measured as a function of temperature at 18.7 and 29 bars; its low-temperature limiting value was observed to be (0.33±0.02)XN, independent of pressure. We use our data to estimate the strong coupling corrections to the size of the energy gap. The initial slope of the reduced gap in the A phase, ΔA/Tc, was found to increase by ∼25% when the pressure increased from 21.1 bars to the melting curve, whereas in the low-temperature limit ΔB(0)/Tcwas found to be independent of pressure and close to its weak coupling value.

Dynamics of vortices and interfaces in superfluid 3he

The first realization of instabilities in the shear flow between two superfluids is examined. The interface separating the A and B phases of superfluid 3He is magnetically stabilized. With uniform rotation we create a state with discontinuous tangential velocities at the interface, supported by the difference in quantized vorticity in the two phases. This state remains stable and nondissipative to high relative velocities, but finally undergoes an instability when an interfacial mode is excited and some vortices cross the phase boundary. The measured properties of the instability are consistent with the classic Kelvin-Helmholtz theory when modified for two-fluid hydrodynamics.