3D Neutron Microscopy

This week we highlight a paper in Nature Physics by Nikolay Kardjilov and co-authors “Three-dimensional imaging of magnetic fields with polarized neutrons”.

3D tomography and microscopy with x-rays is nothing new. Neutrons, however, provide the advantage of strong scattering from magnetic spins – but microscopy with neutrons is limited due to lack of focusing optics, low brightness and monochromacity of neutron sources.

Kardjilov and co-authors present a new technique based on observing the rotation of spin polarization of neutrons as they travel through magnetic material. The result is a 3D view of local magnetization with 100-micron spatial resolution. This technique requires highly polarized and monochromatic neutron beams.

Quasi-Forbidden Bragg peaks from soft matter

This week’s item is a rather technical Nature Materials paper by Forster et al., “Order causes secondary Bragg peaks in soft materials”[Nature Materials 6, 888 - 893 (2007)].

Atomic crystals can often be well-ordered, meaning that the correlation length on which “perfect” atomic order exist can extend over many thousands (or even millions) unit cells. Grain boundaries, dislocations and other defects are a common cause of breaking the perfectly ordered chain of atoms.

Soft materials – liquid crystals, colloids, mesoporous materials etc. – typically consist of fairly large unit cells and it is more difficult to get these materials as well-ordered as atomic crystals. All atoms are identical, but colloidal solutions, for example, are often fairly polydisperse, and therefore crystallize with some difficulty – if at all. It is no surprise that the correlation lengths – especially when expressed in unit cells – is far shorter in soft matter, compared to atomic crystals, such as Si or Pb.

Correlation lengthscales can be determined via Debye-Scherrer formalism that relates width of the x-ray or neutron scattering peak to the typical coherent domain size within the sample.

Forster et al. address the issue of finite correlation lengths by analysis of secondary “forbidden” (or quasi-forbidden) Bragg reflections. For example, for a perfect body-centered cubic lattice 001 reflection does not exist – only (002), (011) and other indices that add up to an even number. But once you introduce some disorder, these forbidden peaks become “alive”, since destructive interference responsible for precisely canceling out contributions to these forbidden reflections becomes somewhat faulty.

Surprisingly enough, people haven’t dealt much with ordered, but only over short-range distances materials, at least not to the extent of coming up with sophisticated treatment of intensities of these secondary Bragg peaks that can answer questions like: is the material truly homogeneous but has a lot of disorder, or is it “patchy”? Forster’s paper represents a key step in dealing with these important issues.

Observation of the density minimum in deeply supercooled confined water

This week’s item is the recent PNAS paper Liu et al., “Observation of the density minimum in deeply supercooled confined water” PNAS 104, 9547 (2007), by a collaboration lead by S. H. Chen at MIT.

By using Small Angle Neutron Scatterig (SANS) they are able to study water confined in mesoporous silica channels and find a density minimum at -63 deg C.

The density *maximum* of water at 4 deg C is a well known anomaly – one of many, many anomalies of water. The 4 deg maximum has some interesting consequences for living organisms – the denser 4 C water “sinks” to the bottom and makes it possible for fish and other water organisms to survive the winters, since bottoms of lakes and ponds don’t freeze out even in the harshest winters and remain at “balmy” 4 C.

Now SANS measurements find the opposite phenomena – a density *minimum*, at -63 C. But since bulk water in equilibrium freezes at 0 C, the researchers had to play some tricks to produce metastable, “supercooled” water. In absence of nucleation centers one could supercool bulk water by some 20-30 degrees – in fact this is the state in which water can exist in atmospheric clouds, in a form of supercooled droplets. But to go further than that one has to confine water to nanoscopic cylindrical channels, as was done in this work.

Several other previous studies on this topic by Chen group at MIT:

Chen et al., The violation of the Stokes–Einstein relation in supercooled water, Proc. Nat. Acad. Sci. (2006)

Liu et al., Pressure Dependence of Fragile-to-Strong Transition and a Possible Second Critical Point in Supercooled Confined Water Phys. Rev. Lett. 95, 117802 (2005).

And the same June 5 issue of PNAS contains a paper by Eugene Stanley’s group at BU on Relation between the Widom line and the breakdown of the Stokes–Einstein relation in supercooled water.