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Register a new userNovel Methods to Create Multielectron Bubbles in Superfluid Helium
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An equilibrium multielectron bubble in liquid helium is a fascinating object with a spherical two-dimensional electron gas on its surface. We describe two ways of creating them. MEBs have been observed in the dome of a cylindrical cell with an unexpectedly short lifetime; we show analytically why these MEBs can discharge by tunneling. Using a novel method, MEBs have been extracted from a vapor sheath around a hot filament in superfluid helium by applying electric fields up to 15 kV/cm, and photographed with high-speed video. Charges as high as 1.6x10-9 C (~1010 electrons) have been measured. The latter method provides a means of capture in an electromagnetic trap to allow the study of the extensive exciting properties of these elusive objects.
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We study two techniques to create electrons in a liquid helium environment. One is thermionic emission of tungsten filaments in a low temperature cell in the vapor phase with a superfluid helium film covering all surfaces; the other is operating a glowing filament immersed in bulk liquid helium. We present both the steady state and rapid sweep I-V curves and the electron current yield. These curves, having a negative dynamic resistance region, differ remarkably from those of a vacuum tube filament. A novel low temperature vapor-phase electron collector for which the insulating helium film on the collector surface can be removed is used to measure emission current. We also discuss our achievement of producing multi-electron bubbles (MEBs) in liquid helium by a new method.
The irrotational nature of superfluid helium was discovered through its decoupling from the container under rotation. Similarly, the resonant period drop of a torsional oscillator (TO) containing solid helium was first interpreted as the decoupling of solid from the TO and appearance of supersolid. However, the resonant period can be changed by mechanisms other than supersolid, such as the elastic stiffening of solid helium that is widely accepted as the reason for the TO response. To demonstrate the irrotational nature more directly, the previous experiments superimposed the dc rotation onto the TO and revealed strong suppression on the TO response without affecting the shear modulus. This result is inconsistent with the simple temperature-dependent elasticity model and supports the supersolid scenario. Here, we re-examine the rotational effect on solid helium with a two-frequency rigid TO to clarify the conflicting observations. Surprisingly, most of the result of previous rotation experiments were not reproduced. Instead, we found a very interesting superfluid-like irrotational response that cannot be explained by elastic models.
The $A$ phase and the $B$ phase of superfluid He-3 are well studied, both theoretically and experimentally. The decay time scale of the $A$ phase to the $B$ phase of a typical supercooled superfluid $^3$He-A sample is calculated to be $10^{20,000}$ years or longer, yet the actual first-order phase transition of supercooled $A$ phase happens very rapidly (in seconds to minutes) in the laboratory. We propose that this very fast phase transition puzzle can be explained by the resonant tunneling effect in field theory, which generically happens since the degeneracies of both the $A$ and the $B$ phases are lifted by many small interaction effects. This explanation predicts the existence of peaks in the $A to B$ transition rate for certain values of the temperature, pressure, and magnetic field. Away from these peaks, the transition simply will not happen.
We argue that classical transitions can be the key to explaining the long standing puzzle of the fast A-B phase transition observed in superfluid Helium 3 while standard theory expects it to be unobservably slow. Collisions between domain walls are shown to be capable of reaching phases inaccessible through homogenous nucleation on the measured timescales. We demonstrate qualitative agreements with prior observations and provide a definite, distinctive prediction that could be verified through future experiments or, perhaps, a specific analysis of existing data.
We study multielectron bubble phases in the $N=2$ and $N=3$ Landau levels in a high mobility GaAs/AlGaAs sample. We found that the longitudinal magnetoresistance versus temperature curves in the multielectron bubble region exhibit sharp peaks, irrespective of the Landau level index. We associate these peaks with an enhanced scattering caused by thermally fluctuating domains of a bubble phase and a uniform uncorrelated electron liquid at the onset of the bubble phases. Within the $N=3$ Landau level, onset temperatures of three-electron and two-electron bubbles exhibit linear trends with respect to the filling factor; the onset temperatures of three-electron bubbles are systematically higher than those of two-electron bubbles. Furthermore, onset temperatures of the two-electron bubble phases across $N=2$ and $N=3$ Landau levels are similar, but exhibit an offset. This offset and the dominant nature of the three-electron bubbles in the $N=3$ Landau level reveals the role of the short-range part of the electron-electron interaction in the formation of the bubbles.
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