laboratoire de physique statistique
 
 
laboratoire de physique statistique

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JOURNAL OF LOW TEMPERATURE PHYSICS 


16
P U B L I C A T I O N S



 
2015
Search for Dislocation Free Helium 4 Crystals - Souris, F. and Fefferman, A. D. and Haziot, A. and Garroum, N. and Beamish, J. R. and Balibar, S.
JOURNAL OF LOW TEMPERATURE PHYSICS 178149-161 (2015)

Abstract : The giant plasticity of He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996-1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm, sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10-10 per cm, that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.
On the Destabilization of Metastable Solid He-4 - Souris, F. and Qu, A. and Dupont-Roc, J. and Grucker, J. and Jacquier, Ph
JOURNAL OF LOW TEMPERATURE PHYSICS 179390-395 (2015)

Abstract : Recently, a metastable state of solid He has been produced by locally lowering the density of the solid below the melting density using a focused acoustic wave pulse. An unexpected instability of the solid has been found about 4 bar below the melting line. This paper reports on experiments precisely localizing in time the instability birth within the acoustic pulse. It is found that, as expected, the instability always appears during a depression swing of the wave. However a metastability pressure limit does not emerge clearly. Total stress duration seems also to play a role in the instability triggering, suggesting a fatigue effect.
Remarks on Solid Helium - Balibar, S. and Beamish, J. and Hallock, R. B.
JOURNAL OF LOW TEMPERATURE PHYSICS 1803-5 (2015)
 
2014
Low Frequency Elastic Measurements on Solid He in Vycor Using a Torsional Oscillator - Fefferman, A. D. and Beamish, J. R. and Haziot, A. and Balibar, S.
JOURNAL OF LOW TEMPERATURE PHYSICS 1761-15 (2014)

Abstract : Torsional oscillator (TO) experiments involving solid He confined in the nanoscale pores of Vycor glass showed anomalous frequency changes at temperatures below 200 mK. These were initially attributed to decoupling of some of the helium's mass from the oscillator, the expected signature of a supersolid. However, these and similar anomalous effects seen with bulk He now appear to be artifacts arising from large shear modulus changes when mobile dislocations are pinned by He impurities. We have used a TO technique to directly measure the shear modulus of the solid He/Vycor system at a frequency (1.2 kHz) comparable to that used in previous TO experiments. The shear modulus increases gradually as the TO is cooled from 1 K to 20 mK. We attribute the gradual modulus change to the freezing out of thermally activated relaxation processes in the solid helium. The absence of rapid changes below 200 mK is expected since mobile dislocations could not exist in pores as small as those of Vycor. Our results support the interpretation of a recent TO experiment that showed no anomaly when elastic effects in bulk helium were eliminated by ensuring that there were no gaps around the Vycor sample.
 
2012
Disorder, Supersolidity, and Quantum Plasticity in Solid Helium 4 - Balibar, S. and Fefferman, A. D. and Haziot, A. and Rojas, X.
JOURNAL OF LOW TEMPERATURE PHYSICS 168221-234 (2012)

Abstract : Several years after Kim and Chan's discovery of an anomaly in the rotation properties of solid helium (Kim and Chan in Nature 427:225, 2004; Science 305:1941, 2004), the interpretation of the observed phenomena as a manifestation of supersolidity remains controversial. J. Beamish and his collaborators have shown that the rotation anomaly is accompanied by an elastic anomaly (Day and Beamish in Nature 450:853, 2007; Day et al. in Phys. Rev. Lett. 104:075302, 2010; Syshchenko et al. in Phys. Rev. Lett. 104:195301, 2010): when the rotational inertia apparently increases, the shear modulus decreases. This softening is due to the appearance, in the solid, of a large reversible plasticity that is a consequence of the evaporation of He-3 impurities from dislocations that become mobile. This plasticity is called ``quantum plasticity'' because the dislocations move by quantum tunneling in the low temperature limit. Since the main evidence for supersolidity comes from torsional oscillator (TO) experiments, and since the TO period depends on both the inertia and the stiffness of solid He-4, it is not totally clear if supersolidity really induces a change in inertia or if it is the disappearance of quantum plasticity that mimics supersolidity in TO experiments. In order to distinguish between supersolidity and quantum plasticity, we have studied the rotational and the acoustic properties of solid He-4 samples with a variable amount of disorder and of He-3 impurities. Of particular interest is the comparison of single crystals to polycrystals but the whole problem is not yet solved. This short review article is an opportunity to discuss several questions regarding the exact role of disorder in supersolidity and in quantum plasticity.
 
2011
Effect of Helium Elasticity on Torsional Oscillator Measurements - Maris, Humphrey J. and Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 16212-22 (2011)

Abstract : In 2004 Kim and Chan performed a torsional oscillator measurement of the rotational inertia of solid helium-4. They found frequency changes which were interpreted in terms of a non-classical rotational inertia, that is a partial superfluidity or ``supersolidity'' of solid helium-4. Since then there have been many further studies using various versions of this technique. One important question that arises in these experiments is the possible effect on the oscillator frequency of changes in the elasticity of the solid helium; this can produce a change in frequency that adds to any effect due to superfluidity. In this paper we give a general discussion of the effect of changes in elasticity on the oscillator frequency and consider how the magnitude of the effect is influenced by the oscillator design. Our results should help make it possible to discriminate between frequency changes due to changes in elasticity and changes due to supersolidity.
 
2010
Acoustic Properties of Solid He-4 in the Limit of Zero Impurity - Rojas, X. and Pantalei, C. and Maris, H. J. and Balibar, S.
JOURNAL OF LOW TEMPERATURE PHYSICS 158478-484 (2010)

Abstract : We have studied the elasticity of solid He-4 in relation with its possible supersolidity. For this we have measured acoustic resonance frequencies in a 1 cm(3) cell filled either with polycrystals or with single crystals of He-4. We have observed a large stiffening at low temperature as first observed by Day and Beamish in polycrystals. The He-3 impurity content has been varied from 300 ppb to 0.4 ppb. When kept in equilibrium with liquid helium, single crystals should be impurity free. In these crystals, a large stiffening is observed, which should not be the result of the pinning of dislocations by impurities.
How to Prepare an Ideal Helium 4 Crystal - Pantalei, Claudia and Rojas, Xavier and Edwards, David O. and Maris, Humphrey J. and Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 159452-461 (2010)

Abstract : Given the running controversies on the interpretation of supersolidity, it appears important to measure more accurately what are the properties of helium 4 crystals in the absence of disorder. We recall how to prepare single crystals with no or very few dislocations. We then show that these crystals are extremely fragile and how to avoid the formation of defects. The main purpose of this article is to show how one can eliminate all helium 3 impurities from the solid: if the crystals are in equilibrium with liquid helium 4 below 50 mK, all helium 3 impurities are bound to the liquid phase with a large binding energy which we have calculated from the description of helium mixtures by Edwards and Balibar (Phys. Rev. B 39:4083, 1989).
Elastic Properties of Polycrystalline Solid Helium - Maris, Humphrey J. and Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 1605-11 (2010)

Abstract : Recent experiments have shown that the shear modulus of solid helium undergoes a large temperature variation in the range 20 to 200 mK, possibly due to changes in the pinning of dislocations. In this note we report on computer simulations of the elastic properties of polycrystalline solid helium. We calculate how the elastic coefficients of a sample made up of a large number of randomly oriented grains are affected by the changes in the shear modulus c (44) of the individual grains.
 
2008
DOI
10
Supersolidity and disorder in solid helium 4 - Caupin, Frederic and Sasaki, Satoshi and Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 150267-275 (2008)

Abstract : Supersolidity is a rather controversial issue which has been revived recently by a number of torsional oscillator (ac-rotation) experiments with solid helium 4. One possibility would be that the ground state of helium 4 crystals contains a Bose-Einstein condensate of mobile vacancies. However, the supersolid signal was shown to depend on sample history. In addition, dc-flow experiments show that superfluid transport of mass does not occur in solid helium, except if it contains appropriate grain boundaries. As a consequence, we believe that supersolidity is not an intrinsic property of helium single crystals, but that it is due to quenched disorder. We report experiments on pressure relaxation in and light scattering from solid samples, that give additional evidence for the existence of disorder in helium crystals. We also describe our recent study of the wetting properties of grain boundaries.
DOI
11
Static structure factor and static response function of superfluid helium 4: A comparative analysis - Caupin, Frederic and Boronat, Jordi and Andersen, Ken H.
JOURNAL OF LOW TEMPERATURE PHYSICS 152108-121 (2008)

Abstract : Extensive neutron scattering data on liquid helium 4 are available, and have been analyzed to give a number of physical quantities, e.g. static structure factor S(q), excitation energy, and roton linewidth. X-rays also give access to S(q). However, a comprehensive comparison between experimental data and theoretical results, including their dependence on pressure, is still lacking. The static response function chi(q) has been particularly overlooked, despite its fundamental role in theories of inhomogeneous helium. We present here a critical review about the strength of the main peaks of S(q) and chi(q). We include in the comparison the analysis of unpublished neutron data and new Monte-Carlo calculations of chi(q). We find a significant discrepancy between experiments, and suggest corrections which account for some of the differences. We give recommendations for the best values to use for S(q) and chi(q).
DOI
12
Optical Observations of Disorder in Solid Helium 4 - Sasaki, Satoshi and Caupin, Frederic and Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 15343-76 (2008)

Abstract : It has been proposed that the supersolidity of helium 4 is associated with disorder in the solid samples. We report optical observations of this disorder in samples grown by different methods. We show that, when grown by the ``blocked capillary method'' as in most experiments showing anomalous phenomena, solid helium 4 can be polycrystalline with grain sizes down to the micrometer range, much smaller than the 0.1 mm found in some other experiments. We analyze the properties of grain boundaries, and in particular the existence of liquid channels at the contact of grain boundaries with walls. Our analysis includes measurements of the contact angle of the liquid-solid interface with these walls, which exhibits a large hysteresis. Furthermore, we predict that similar channels should exist at the junction between grain boundaries.
 
2007
DOI
13
The discovery of superfluidity - Balibar, Sebastien
JOURNAL OF LOW TEMPERATURE PHYSICS 146441-470 (2007)

Abstract : Superfluidity is a remarkable manifestation of quantum mechanics at the macroscopic level. This article describes the history of its discovery, which took place at a particularly difficult period of the twentieth century. A special emphasis is given to the role of J.F. Allen, D. Misener, P. Kapitza, F. London, L. Tisza and L.D. Landau. The nature and the importance of their respective contributions are analyzed and compared. Of particular interest is the controversy between Landau on one side, London and Tisza on the other, concerning the relevance of Bose-Einstein condensation to the whole issue, and also on the nature of thermal excitations in superfluid helium 4. In order to aid my understanding of this period, I have collected several testimonies which inform us about the work and attitude of these great scientists.
DOI
14
Freezing of helium-4: Comparison of different density functional approaches - Caupin, Frederic and Ancilotto, Francesco and Barranco, Manuel and Mayol, Ricardo and Pi, Marti
JOURNAL OF LOW TEMPERATURE PHYSICS 148731-736 (2007)

Abstract : Solidification of superfluid helium-4 has been addressed within the framework of density functional theory. Early studies used a variational approach, approximating the density distribution in the solid phase by a sum of Gaussians on each lattice site. Recently, we performed an unconstrained minimization of the functional describing the helium system as reported by Ancilotto et al. (Phys. Rev. B, 72, 214522, 2005). At sufficiently high density, we find stable solid like solutions, which exhibit an anisotropic density profile around each lattice site. We compare these results to the previous variational approach, and attempt to improve the family of trial functions by adding a variational parameter to account for anisotropy.
DOI
15
Supersolidity and superfluidity of grain boundaries - Sasaki, S. and Ishiguro, R. and Caupin, F. and Maris, H.-J. and Balibar, S.
JOURNAL OF LOW TEMPERATURE PHYSICS 148665-670 (2007)

Abstract : We have looked for dc mass transport through solid He-4 in a simple experiment with two communicating vessels filled with solid He-4 in equilibrium with liquid He-4. Through good quality crystals, we have observed no mass transport, in contradiction with the hypothesis of a Bose-Einstein condensation of vacancies. Through crystals containing grain boundaries, we have found superfluid flow along these grain boundaries. We discuss these results in the context of other experiments on supersolidity.
DOI
16
Homogeneous nucleation of He-4 crystals by acoustic waves - Ishiguro, R. and Caupin, F. and Balibar, S.
JOURNAL OF LOW TEMPERATURE PHYSICS 148645-652 (2007)

Abstract : We have recently found experimental evidence for the homogeneous nucleation of crystals in metastable liquid He-4 at high pressure. For this we combined the focusing of a high intensity spherical acoustic wave with a simple light scattering technique. We discuss the analysis used to distinguish between nucleation of bubbles in the negative pressure swings of the wave from nucleation of crystals in the positive swings. We also discuss the interest of our results and future developments of our experiment in the general context of the study of nucleation and instability limits in phase diagrams.