LKB - Polarised Helium, Quantum Fluids and Solids

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# QUANTUM SOLID AND LIQUID HELIUM

The group does research on the quantum properties of solid and liquid helium, in particular in metastable phases (either solid, when it should be a liquid, or liquid, when it should be a gas). Another theme is the spectroscopy of atoms, specially cesium or rare earths, implanted by laser sputtering in a single-crystal of He4.

## Bubbles in liquid He-4

##### Publications

An Qu, Azer Trimeche, Jacques Dupont-roc, Jules Grucker and Philippe Jacquier Cavitation Density of Superfluid Helium-4 around 1 K Physical Review B
DOI: 10.1103/PhysRev. B 91, 214115 (2015)

An Qu, A. Trimeche, Ph. Jacquier, and J. Grucker Dramatic effect of superfluidity on the collapse of 4 He vapor bubbles Physical Review B DOI: 10.1103/PhysRevB.93.174521

##### Bubbles in liquid He-4

By using a hemispherical transducer immersed in liquid helium, we can induce a high amplitude pressure swing. Starting a few millibar above the boiling pressure, we thus bring the liquid in a metastable sate, a state where it remains liquid although at thermodynamic equilibrium, it should be a gas. Liquid helium being extremely pure, no heterogeneous nucleation can occur. The so called spinodal limit, the pressure under which the liquid is instable and cannot exist is predicted around -10 bar. In fact bubbles are created as soon as the pressure is of the order of -4 bar, no satisfying explication is available.

Lifetime of the bubbles has been studied, we find a dramatic change in the order of magnitude of this lifetime when crossing the superfluid transition : lifetime is almost constant at 1.5 ms under $T_\lambda$ and larger than 40 to 100 ms above it. This seems due to the difference in heat transport (from the condensation of gas into liquid).

## Toward supersolidity : depressurized single-crystal of He-4 in a metastable state

##### Publications

F. Souris, A. Qu, J. Dupont-Roc, J. Grucker, Ph. Jacquier On the destabilization of metastable solid 4He

##### Toward supersolidity : depressurized single-crystal of He-4 in a metastable state

With the help of a tailored-fitted piezoelectric transducer totally immersed in a single-crystal of Helium, we apply a high amplitude pressure swing to the solid, thus bringing it locally in space and time from just above the melting pressure to well below. Although theories predict a transition from solid to liquid at -10 bar, we observe the apparition of a defect of unknown nature at a pressure of +21.4 bar. This pressure is pretty
independent of anything.

No satisfactory explanation of the occurrence of this defect is available for now.

## Spectroscopy of atoms implanted in a single-crystal of He-4

##### Spectroscopy of atoms implanted in a single-crystal of He-4

The anisotropy of the hcp crystal leads to a modification of the energy levels of implanted atoms. We have studied these modifications in the case of a cesium atom.

Laser sputtering of solid cesium situated above a single crystal of He produces grains of various size , down to single atoms. A frequency doubled Nd:YAG pulsed laser is used (1mJ, 8ns). They fall onto the surface of the crystal and are incorporated into it by growing the crystal again after sputtering.

Figure at left sows the D2 resonant fluorescence of implanted atoms.

Hyperfine structure of the fundamental 6S1/2 state should be anisotropic. If this anisotropy is large enough, it would allow a measurement of the anapole moment of the cesium nucleus.

The lineshape of the optical absorption D2 line is modified as well: it is expected that this lineshape should depend on the relative orientation of the excitation polarization with respect to the optic axis of the crystal. It should also depend on the angle between excitation and detection directions.

Indeed, our measurements show a deformation of the lineshape with respect to a free atom. But this deformation does not seem to depend of the polarization, neither of the excitation, nor of the detection. We interpret this as evidence for our sputtering method (frequency-doubled Nd:YAG laser, 10 mJ, 15ns) to destroy the single-crystal and to make a multi-crystalline solid.