# Presentation: 696032

## Controlled Anisotropy in Superfluid HeliumWilliam Halperin, Northwestern University DMR 0703656

- We have discovered[1,2] that anisotropic superfluids can be stabilized by globally anisotropic porous media, (image of silica aerogel at right).  We synthesize these highly porous silica aerogels and impose uniform anisotropy within the structure by applying uniaxial stress and/or controlling the aerogel growth conditions.  This 98% porous sample is viewed with polarized light to display growth inhomogeneity on a cm scale which suppresses and controls the direction of the order parameter. Naturally occurring porous media with such anisotropy include river beds, bone tissue, and cement.
- In this project we develop, and transfer, the basic science of superfluid 3He[1-3] to give a better understanding of porous materials.

- A 98% porous silica aerogel under polarized light shows long length scale inhomogeneity (1 cm).  These optical birefringence  patterns are responsible for the texture of the order parameter of  the superfluid imbibed in the pore space measured at low temperatures.

**Notes:**
 Compressed Silica Aerogels for the Study of Superfluid 3He in Aerogel,  J. Pollanen, H. Choi,  J.P. Davis, S. Blinstein, T.M. Lippman, L.B. Lurio, N. Mulders, W.P. Halperin, AIP Proceedings, AIP Proceedings 850, 237 (2006).
2. Anisotropic Aerogels for Studying Superfluid 3He, J. Pollanen, S. Blinstein, H. Choi, J.P. Davis, T.M. Lippman, L.B. Lurio, W.P. Halperin, J. Low Temp. Phys. 148, 579 (2007).
3. Specific heat of Disordered Superfluid 3He, H. Choi, K. Yawata, T.M. Haard, J.P. Davis, G. Gervais, N. Mulders, P. Sharma, J.A. Sauls, and W.P. Halperin, Phys. Rev. Lett. 93, 145301 (2004).

## Controlled Anisotropy in Superfluid HeliumWilliam Halperin, Northwestern University DMR 0703656

- Northwestern University undergraduates, Sam Blinstein (at right preparing an aerogel sample), Tom Lippman, and Kent Shirer together with, Northwestern graduate students, Johannes Pollanen, Hyongsoon Choi, and John Davis have found new ways to control global anisotropy in highly dilute silica aerogels, a key to exploring anisotropic superfluidity. Our projects and publications involving undergraduate students[1,2] include, small angle Xray scattering, optical birefringence, stress strain analysis, and properties of confined fluids.

**Outreach:**
- We have collaborations with four other laboratories in France, Japan, England, and in the USA supplying them with samples of silica aerogels made by our  group.  Outgrowths of this collaborative work include a novel scheme for nanoparticle suspensions for catalysis and for non-linear optics applications.

- Controlled Anisotropy in Superfluid HeliumWilliam Halperin, Northwestern University DMR 0703656

**Notes:**
Highly porous silica aerogel glass (98 to 99.5 % porous) has found important applications in industry, basic science, and in every day life.  These fascinating materials are solid, although they are a hundred times lighter (less dense) than regular glass.  Like ordinary glass however they are almost transparent as can be seen in the picture.  The fact that they have a bluish cast is for the same reason that the sky is blue.  Blue light is scattered preferentially since it has a short wavelength compared to red light which has a long wavelength.  Just as light is scattered by aerogel so is helium when imbibed into the pore space.  Helium at low temperatures condenses into a liquid which then becomes a superfluid below 0.002 mK.  The properties of this unusual superfluid are the main focus of our basic scientific program.  In a recent publication we show that an energy barrier called the energy gap, that exists in pure superfluids and superconductors disappears for the case of superfluid in aerogel, Choi et al. [3].  This gapless behavior is directly demonstrated by our measurements of the heat capacity at very low temperatures.  Furthermore, this basic result gives important information on what is called the density-density correlation function that describes how the porous material is organized on distances that correspond to the wavelength of visible light.  Unlike most nano-porous materials, aerogel can be significantly strained by squeezing in one direction.  The anisotropy induced, according to the theory, should give rise to a preferential direction for scattering of either visible light (see figure), or for superfluid helium.  Work is in progress to understand these effects, including contributions from graduate students, undergraduates and high school students.