a square transparent cell:a square transparent cell:a square hole (11 x 11 mma square hole (11 x 11 mm22) in a gold-) in a gold-plated copper plate (here 10 mm thick)plated copper plate (here 10 mm thick)2 glass windows sealed with indium 2 glass windows sealed with indium O-rings.O-rings.this cell stands pressures up to at least 64 this cell stands pressures up to at least 64 bar and can be cooled down to 35 mK.bar and can be cooled down to 35 mK.

see Sasaki, Caupin and Balibar, submitted see Sasaki, Caupin and Balibar, submitted to J. Low Temp. Phys., July 2008.to J. Low Temp. Phys., July 2008.

helium dendriteshelium dendrites

are are 44He crystals also facetted at high T ?He crystals also facetted at high T ?S. Balibar, F. Caupin and S. Sasaki S. Balibar, F. Caupin and S. Sasaki aa

Laboratoire de Physique Statistique de l'ENS (Paris, France)Laboratoire de Physique Statistique de l'ENS (Paris, France)a - now at North Western Univ. (USA)a - now at North Western Univ. (USA)

conclusions and perspectives: conclusions and perspectives: It would be interesting to confirm this observation by a systematic study as a function of temperature and It would be interesting to confirm this observation by a systematic study as a function of temperature and growth velocity. If confirmed, one possible interpretation of this re-entrant facetting could be that the liquid-solid interfacial energy strongly growth velocity. If confirmed, one possible interpretation of this re-entrant facetting could be that the liquid-solid interfacial energy strongly increases along the melting curve because the pressure Pincreases along the melting curve because the pressure Pmm(T) increases.(T) increases.

AbstractAbstract:: We have observed helium 4 crystals growing from normal liquid helium 4 at 2.58K. Their growth shape looks facetted, at least in We have observed helium 4 crystals growing from normal liquid helium 4 at 2.58K. Their growth shape looks facetted, at least in the c , i.e. [0001] directionthe c , i.e. [0001] direction..This is aThis is a surprising observation since, up to now, the c facets were believed to disappear above the roughening transition surprising observation since, up to now, the c facets were believed to disappear above the roughening transition temperature Ttemperature TR1R1 = 1.30 K. Do facets re-appear above 2K ? = 1.30 K. Do facets re-appear above 2K ?

crystallization from the normal liquid crystallization from the normal liquid mixing chambermixing chamber

pressure gaugepressure gauge

fill line (0.6 mm ID)fill line (0.6 mm ID)

fast crystallization from the normal liquid at 1.87K produces a tangle of dendrites. Here fast crystallization from the normal liquid at 1.87K produces a tangle of dendrites. Here this is obtained by opening a valve which connects the cell to a reservoir at 50 bar. The this is obtained by opening a valve which connects the cell to a reservoir at 50 bar. The tangle scatters lights so much that the solid appears dark. Similar observations were done by tangle scatters lights so much that the solid appears dark. Similar observations were done by Maekawa et al. (Phys. Rev. B 65, 144525, 2002) and by Ford et al. (J. Low Temp. Phys. 148, Maekawa et al. (Phys. Rev. B 65, 144525, 2002) and by Ford et al. (J. Low Temp. Phys. 148, 653, 2007).653, 2007).In quench-frozen samples, Rittner and Reppy (PRL 2006, 2007, 2008) found «supersolid In quench-frozen samples, Rittner and Reppy (PRL 2006, 2007, 2008) found «supersolid fractions» of order 20%. This large value might be due to such a large disorderfractions» of order 20%. This large value might be due to such a large disorder

At 2.58K, we were suprised to observe that helium snow flakes look facetted.At 2.58K, we were suprised to observe that helium snow flakes look facetted.Crystallization is obtained by injecting mass through the fill line at a Crystallization is obtained by injecting mass through the fill line at a moderate rate.moderate rate.Snow flakes are produced in the upper left corner where the fill line is Snow flakes are produced in the upper left corner where the fill line is connected to the cell.connected to the cell.Their size is about 0.5 mm, homogeneous, close to the diameter of the fill Their size is about 0.5 mm, homogeneous, close to the diameter of the fill line orifice (0.6 mm).line orifice (0.6 mm).They fall down in the liquid and often stick to the glass windows.They fall down in the liquid and often stick to the glass windows.They exhibit the 6-fold symmetry of the hcp structure.They exhibit the 6-fold symmetry of the hcp structure.Those landing on the cell floor show that Those landing on the cell floor show that they are thin and flat.they are thin and flat.It seems that, at least in the « c » direction (ie the [0001] direction) the hcp It seems that, at least in the « c » direction (ie the [0001] direction) the hcp crystals are facetted.crystals are facetted.This is surprising because the roughening temperature TThis is surprising because the roughening temperature TR1R1 of hcp helium 4 of hcp helium 4 crystals is at 1.30K (for a review on the surface of helium crystals, especially crystals is at 1.30K (for a review on the surface of helium crystals, especially the roughening transition, see S. Balibar, H. Alles and A. Ya. Parshin, Rev. the roughening transition, see S. Balibar, H. Alles and A. Ya. Parshin, Rev. Mod. Phys. 77, 317, 2005.)Mod. Phys. 77, 317, 2005.)If confirmed, and since the Kosterlitz-Thouless universal relation imposes If confirmed, and since the Kosterlitz-Thouless universal relation imposes

kkBBTTRR = = (T(TRR) d) d22 where where (T(TRR) is the surface stiffness of the liquid-solid interface at the ) is the surface stiffness of the liquid-solid interface at the roughening temperature Troughening temperature TRR , it would mean that the roughening transition is , it would mean that the roughening transition is re-entrant along the melting curve because the surface stiffness increases as re-entrant along the melting curve because the surface stiffness increases as a function of pressure, as predicted by H.J. Maris and F. Caupin (JLTP 131, a function of pressure, as predicted by H.J. Maris and F. Caupin (JLTP 131, 145, 2003). V.M. Giordano and F. Datchi (PRL 99, 165701, 2007) have 145, 2003). V.M. Giordano and F. Datchi (PRL 99, 165701, 2007) have found a re-entrant roughening in COfound a re-entrant roughening in CO22. . It is also possible that replacing the It is also possible that replacing the superfluid by the normal liquid increases the interfacial energy, superfluid by the normal liquid increases the interfacial energy, consequently the stiffness consequently the stiffness ..

facetted helium snowflakesfacetted helium snowflakes

T = 2.58 KT = 2.58 KP = 62 barP = 62 bar

flat flakes on the cell floorflat flakes on the cell floor

helium snow flake helium snow flake falling downfalling down

orifice of the fill lineorifice of the fill line

glass plateglass plate

2 crosses carved 2 crosses carved on the window on the window to help focusing to help focusing

the camerathe camera

helium snow flakes helium snow flakes sticking to sticking to

the front windowthe front window

T = 1.87 KT = 1.87 K

11 mm11 mm