Sébastien BALIBAR

Born: Tours (France) 14 october 1947.
Citizenship: French

Scientific activity

Summary

For my PhD (1976), I obtained the first experimental evidence of quantum evaporation with superfluid helium 4, a phenomenon which had been predicted as an equivalent of the photoelectric effect by P.W. Anderson in 1969. More precisely I discovered that atoms evaporated by ballistic rotons are ejected from the superfluid with a minimum kinetic energy of 1.5K. I continued on this subject as a postdoc in  A.F.G.Wyatt’s laboratory (Exeter, 1977). A.F.G.Wyatt and his group later performed a precise and extensive study of this effect which is now used for the detection of particles by G. Seidel and H.J.Maris (Brown University).

Together with my successive students and visitors, we then started a long study of the surface of helium crystals (1978 - 1994). This includes
- the first direct measurement of the surface tension of helium crystals
- the discovery of roughening transitions at the surface of helium 4 and helium 3 crystals
- a quantitative analysis of crystal shapes, growth rates and step energies , which brought the first experimental verification of several theories (Chui and Weeks, van Beijeren, Nozières) predicting that it is a Kosterlitz Thouless transition
- a complete study of vicinal and stepped surfaces which led to the first measurement of the step energy, step width, step fluctuations, step-step interactions (elastic and entropic) and step mobilities on a crystal (helium in the low T limit). This difficult experiment used the generation and detection of crystallization waves on rotating  helium crystals in the temperature range 40 mK - 500 mK.
- a study of the dendritic instability of helium 3 crystals
- a study of wetting and contact angles of helium crystals on various substrates
- a complete study of the growth kinetics and Kapitza resistance of rough helium crystal surfaces, more precisely the temperature behaviour of the three dissipative coefficients of helium crystal surfaces, first in helium 4 then in helium 3.
- a study of the mechanical instability of helium crystals under stress with direct experimental evidence and quantitative comparison with theoretical predictions on the threshold of the instablity which leads to the formation of periodic corrugations. 
- a study of the coalescence of crystalline drops, whose dynamics is original because of the absence of viscous dissipation in the bulk phases.
On these subjects I keep interacting with several groups (Helsinki, Konstanz, Kyoto, Leiden, Harvard). Together with P.Nozières and F.Pistolesi, I have recently calculated the shape of soft crystals such as those studied by Pieranski et al.. When the elastic modulus is small compared to the surface tension,  steps are burried as edge dislocations below the crystal surface and we calculated all their surface properties, including the large number of facets they show at room temperature.

In 1992, we decided to study nucleation. We started with cavitation, that is the nucleation of bubbles in liquid helium at negative pressure. In order to produce such negative pressures for a short time, we focussed high intensity ultrasonic waves and obtained evidence for homogeneous nucleation at large negative pressure in helium 4 (at- 9 ± 1 bar). We then discovered a crossover from a  classical nucleation where the nucleation is activated by thermal fluctuations to a quantum nucleation by quantum tunneling of a sizeable region of the liquid (about 1000 atoms). In 1998, we extended our measurements to liquid helium 3 down to 40 mK, where we found a less negative cavitation threshold (-2.7 ± 0.3 bar) . At such temperatures helium 3 is a Fermi liquid, not a superfluid. As a consequence, even very close to the spinodal limit where the low frequency compressibility diverges, the velodity  of high frequency sound named "zero sound" does not vanish. This is how we now understand that , in liquid helium 3 below about 100 mK, cavitation has to take place much closer to the spinodal limit than previously thought. A study of the temperature dependence of cavitation in helium 3 has led us to the discovery of an anomaly in its phase diagram, a minimum in its liquid-gas spinodal line.
More recently, we have extended our studies to nucleation near a clean glass wall. It allowed us to measure the instantaneous density  when nucleation occurs. We have discovered two different mechanisms for the cavitation which we need to understand. One looks heterogeneous on the wall (although liquid helium wets it perfectly) and the other one looks homogeneous near the wall at a pressure which is less negative than in the absence of wall.
In the same experiment, we have discovered the nucleation of solid helium at a pressure 4.3 ± 0.2 bar higher than the equilibrium solidification pressure (25.3 bar in helium 4). This rather high nucleation threshold confirms ideas which we have proposed on heterogeneous nucleation during a visit at Kyoto university. We explained that in many cases the energy barrier is more likely to represent a pinning energy for an already existing interface  than the energy necessary to create this interface. A systematic study of the statistics and of the temperature dependence of our "acoustic crystallization" has shown that we observe heterogeneous nucleation on one single defect of the glass wall.  In the absence of any wall, and by using even higher acoustic power, we have later discovered that homogneous nucleation of solid helium from pressurized liquid heliumoccurs at a much higher pressure, possibly close to + 200 bars where a liquid-solid spinodal limit may exist. This limit could be due to the vanishing of  the roton gap energy vanishes so that the local order in the liquid should spontaneously extend to infinity.  Thanks to light scattering measurements, we plan to address the question of whether liquid helium remains superfluid at very high density and how the roton gap decreases as a function of pressure.

We also studied the wetting of walls by phase separated mixtures of helium 3 and helium 4,  and proposed that an exception may exist to "critical point wetting" , the very general phenomenon which had been proposed  by J. Cahn (1980). This would be due to the "critical Casimir effect ", that is the confinement of critical fluctuations in films of finite thickness. However, our most recent measurements  show that, in fact, the Casimir force is smaller than the van der Waals force in most cases and that the observation of an exception to critical wetting would require a very weekly attracting substrate.

We are now extending our cavitation studies to liquid water. Indeed,  Speedy's model of water predicts the existence of a minimum in its liquid-gas spinodal. Just like in liquid helium 3, although for very different reasons, liquid water may exhibit such a minimum due to the negative sign of its thermal expansion coefficient in the low T part of its phase diagram. However, Sastry, Stanley et al. have predicted that the liquid-gas spinodal line in water is an increasing function of temperature. Our measurements of the cavitation threshold in pure water shows a monotonic increase with temeprature but the threshold is 5 times less negative than predicted. Our results do not depend on the water preparation and we confirmed them with a fiber optic hydrophone. We are now trying to measure the equation of state of water at negative pressure in order to see if this equation has any anomaly which could explaiin our measurements.

In 2006, we have discovered that solid helium 4 samples allow superfluid mass transport if they are polycrystalline. This was one of the results which showed that , if it exist, supersolidity is related to disorder, that is to the presence of grain boundaries , dislocations and perhaps also glassy regions in the solid samples. We have measured the energy of the grain boundaries and showed that their thickness remians microscopic even at the liquid-solid equilibrium. This result confirms a predictioin for materials with long range interactions (here, van der Waals). We have also shown that liquid channels exist where grain boudnaires intersect each other and wehre they meet walls. We are now trying to distinguish mass transport along the liquid channels from transport along the inside of grain boundarieswhich arre new original 2D quantum systems.

Throughout this whole work, I have tried to find model situations in helium for very general properties of matter (phase transitions, dynamics of interfaces, crystal growth and crystal shapes, fluctuations, nucleation, wetting, instabilities, non-equilibrium thermodynamics...). I already used some of the knowledge acquired in helium to study properties of matter at room teperature (liquid crystals, water...).
 
 

Invited talks in International meetings

 1985 5th general conf. of the Europ. Phys. Soc.(Cond. Matt.), Berlin (RFA)
 1985 "Brookhaven Seminar on Wetting", Brookhaven (USA).
 1986 Quantum Fluids and Solids, Banff (Canada), présentée par F. Gallet.
 1987 "The Physics of Amphiphilic Layers", Les Houches (France).
 1989 "Low Temp. Physics: present and future", Aspen, Colorado (USA).
 1989 9th Gen. Conf. of the EPS (Cond. Matt.), Nice (France).
 1989 "Interface between Quant.Field Theory and Cond.Mat.Phys.", Trieste (Italy).
 1989 Stat. Phys. 17, Rio de Janeiro (Brazil).
 1989 4th Medyfinol Conf. on "Non-equilibrium stat. mech.", Iguazu Falls (Argentina).
 1989 Internat. Conf. on Cryst. Growth (ICCG-9), Sendai (Japan).

 1989 Internat Summer School on Cryst. Growth (ISSCG-7), Zao (Japan).
 1989 Soviet French Conference, Grenoble (France), présenté par E.Rolley.
 1990 Nato ASI Conf. "Growth and Form, Non-linear aspects", Cargese (France), présenté par E.Rolley.
 1990 "Thermal Excitations in 2D and 3D Quantum Fluids", Exeter (UK).
 1990 Low Temp. Conf. LT-19, Brighton, Sussex (UK).
 1991 CMMP 91, Institute of Physics, Birmingham (UK).
 1992 Symposium on Surface Science 3S* 92, La Plagne (France).
 1992 "Quantum Fluids and Solids", Penn. State University (USA), présenté par R. Torii.
 1992 "The Growth of Disordered Surfaces, Roughening and Phase Transitions",Les Houches (France).
 1992 3rd Int. Symp. "Dynamical Processes at Surfaces", Tokyo (Japan).

 1992 Conférence SFP - Matière Condensée, Lille (France).
 1993 ESF conf. "Fundamental aspects of Surface Science", Davos (Switzerland).
 1993 "Role of the condensate and vorticity in dense Bose systems", Minneapolis (USA), présenté par H.J. Maris.
 1994 "Vortices, Interfaces and Mesoscopic Phenomena in Quantum Systems", Jyväskylä (Finland), présenté par E.Rolley.
 1994 "The vicinal surfaces of helium crystals", Konstanz (Germany), présenté par E.Rolley.
 1995 Summer School "Quantum Fluctuations", Les Houches (France)
 1995 Gordon Conference "Chemistry and Physics of Liquids", Plymouth (USA).
 1995 Cryocrystals 95, Almaty (Kazakhstan).
 1995 "Quantum evaporation, Condensation and Sticking phenomena", Pisa (Italie)
 1996 "Liquid/Solid Interfaces in Helium", Helsinki (Finland)

 1996 "Solid State Lasers for application to inertial confinement fusion", Paris (France)
 1998 "Quantum Physics at low Temperature" (Lammi, Finland)
 1998 "Quantum Fluids and Solids (QFS98)", Amherst (USA)
 1998 "Applications of Field Theory to Stat. Phys. ..." Bonn (Germany)
 1999 "Hydrodynamic and magnetohydrodynamic interface instabilities", Paris (France)
 1999 3rd International Conference on Low Temperature Chemistry, Nagoya (Japan)
 1999 European Research Conference "Solid-Fluid Interfaces", Castelvecchio Pascoli (Italy)
 2000 NEDO workshop, Hawai (USA)
 2000 Quantum Fluids and Solids (QFS-2000), Minneapolis (USA)
 2001 "Ultralow energy physics: methods and methodology", Helsinki (Finland)

 2001 "Horizons in complex systems", Messina (Italy)
 2002 "Liquids under negative pressure", Budapest (Hungary)
 2002 "Liquid Matter Conference", Konstanz (Allemagne)
 2002 "Frontiers in condensed matter and cosmology", Kevo (Finland)
 2002 "Ultralow Temperature Physics",  Kanazawa (Japon)
 2002 "88th Statistical Mechanics Conference", Rutgers University (USA)
 2003  Séminaire Poincaré "Condensation de Bose-Einstein", IHP, Paris (France)
 2004 Quantum Phenomena at Low Temp. ULTI III, Lammi (Finland)
 2004 APS march meeting, Montreal (Canada)
 2004 "Soft Matter, Biology and Statistical Physics", Les Houches (France)

 2004 StatPhys 22, plenary speaker, Bangalore (India)
 2004 "Pattern Formation in Nonequilibrium Systems", Calcutta (India)
 2004  "5th Conf. on cryocrystals and quantum crystals", Wroclaw (Poland)
 2004 "Todai international symposium on quantum condensed systems" (Tokyo)
 2004 "Collective Aspects of Stochastic Non-Equilibrium Phenomena at Surfaces and Interfaces" (Leiden)
 2005 "Quantique...mais macroscopique", Hommage à Fritz London, IHP Paris (France)
 2005 "LT24", plenary speaker, Orlando (Florida, USA)
 2006 "CMD21" (Eur. Phys. Soc.  and German Phys. Soc.), Dresden (Germany)
 2006 "Soft matter and interfaces" (in honor of R. Evans), Lisbon (Portugal)
 2006 "ULTI", Lammi (Finland).

Séminaires, colloques et séries de cours

Invited colloquia, series of lectures and seminars in Universities:

 1977 Nottingham, London
 1978 Columbus
 1981 Haifa, Amsterdam.
 1982 Bell Laboratories (Murray Hills), Brown University.
 1984 Moscou, Tbilissi, ETH (Zurich), Leiden (Ehrenfest colloquium).
 1985 Bell Laboratories (Murray Hills), Lausanne.
 1986 Brighton.
 1987 Mainz.
 1988 Leiden, Columbus, Chicago.
 1989 Seattle, Vancouver (UBC and SFU), Leiden (Ehrenfest colloquium), Kyoto, Cornell, Columbus.
 1990 Chicago, Aachen, Konstanz.
 1993 Amsterdam, Leiden (Ehrenfest colloquium).
 1994 Helsinki, Trento, Trieste.
 1995 Penn State University, Maryland University, Trieste, Cornell, Moscow
 1996 Leuven (Belgium)
 1997 Barcelona (Spain), Columbus, Brown Univ., Seattle, UCLA, Stanford (USA)
 1998 Boston University (USA)
 1999 Konstanz (20h), Kyoto (9h), Tokyo, Sendai, Harvard (Loeb lectures, 3h).
 2000 NEU Boston, Berkeley, U.Mass Amherst, Brown U. Providence, NWU Chicago (2h), Cornell, Buffalo.
 2001 CERN (Geneva)
 2002 Barcelona,  Kevo (Finland, 6 lectures), Leipzig, Calcutta, Bangalore, Pune, Bombay, MIT (Cambridge, USA), Brown University.
 2003 Stuttgart, Kyoto (2), Tokyo.
 2004 Rutgers
 2005 Tunis
 2006 Birmingham, Princeton, Columbus, Royal Holloway (UK), Geneva (2)
 2007 Oxford, Geneva, ISSP Tokyo, MIT (2), U. Mass Amherst (USA), Istanbul, Amsterdam.
 2008 Barcelone, Weizmann Inst. Israel (4 lectures), Haïfa (Israel), NWU Evanston, Rutgers Univ., U. Florida Gainesville
 2009  Regensburg and Duisburg (Germany), Fribourg (Switzerland)
 2010 Bangalore, Bristol, Stuttgart

and many others in France:

GPS-Universités Paris 6&7, ENS Paris, ENS Lyon, CRTBT Grenoble,  ILL Grenoble, Collège de France, CEA Saclay, LPMC Nice, CRMC2 Marseille, CRPP Bordeaux, LSP Grenoble, ESPCI Paris, LPS Orsay, Ecole Polytechnique, LPTMS Orsay, IRPHE and IUSTI Marseille, Inst. Louis Néel Grenoble, Poitiers, etc.

Publications (March 2010)

Theses:

T1. S. Balibar.
« Propagation d'ondes électromagnétiques de basse fréquence et excitation  d'ondes sonores dans l'Aluminum ». Thèse de 3ème cycle (Paris, 3 mars 71).
T2. S. Balibar.
« Contribution à l'étude de l'évaporation de l'Hélium superfluide » Thèse  de Doctorat ès Sciences (Paris, 6 novembre 1976).

Original articles in refereed scientific journals:

Conference proceedings:

Scientific information and other publications: