MI weekly selection #2
Five hundred phases
Condensed matter physics – the branch of physics responsible for discovering and describing most of these phases – has traditionally classified phases by the way their fundamental building blocks – usually atoms – are arranged. The key is something called symmetry. Classifying the phases of matter by describing their symmetries and where and how those symmetries break is known as the Landau paradigm.
Topological order is a quantum mechanical phenomenon: it is not related to the symmetry of the ground state, but instead to the global properties of the ground state’s wave function. Therefore, it transcends the Landau paradigm, which is based on classical physics concepts.
Using modern mathematics – specifically group cohomology theory and group super-cohomology theory – researchers have constructed and classified the symmetry-protected phases in any number of dimensions and for any symmetries. Their new classification system will provide insight about these quantum phases of matter, which may in turn increase our ability to design states of matter for use in superconductors or quantum computers.
X. Chen, Z.-C. Gu, Z.-X. Liu, X.-G. Wen. Symmetry-Protected Topological Orders in Interacting Bosonic Systems. Science, 2012; 338 (6114): 1604 DOI:10.1126/science.1227224
Quantum spin liquid experimentally discovered
A spin liquid state exists in Herbertsmithite—a mineral whose atoms form a kagome lattice, named for a simple weaving pattern of repeating triangles well-known in Japan. Kagome lattices are one of the simplest structures believed to possess a spin liquid state, and the new findings, revealed by neutron scattering, indeed show striking evidence for a fundamental prediction of spin liquid physics. This prediction was made by X.G. Wen (see above).
T.-H. Han, J.S. Helton, S. Chu, D.G. Nocera, J.A. Rodriguez-Rivera, C. Broholm and Y.S. Lee. Fractionalized excitations in the spin liquid state of a kagome lattice antiferromagnet. Nature, 492, Dec. 20, 2012, DOI: 10.1038/nature11659
A question of connectivity
A study comparing human and chimpanzee brains shows that humans experience an exponential period of white matter growth, or networks between brain cells, before they reach two years of age. Scientists previously knew human brains experience rapid connectivity growth, but the study showed that such growth rapidly outpaced connectivity growth in chimpanzee brains.
The findings, while not unexpected, are unique because the researchers followed the same individual chimpanzees over time; past studies have instead pieced together brain development from scans on several apes of different ages.
Tokomo Sakai et al. Developmental patterns of chimpanzee cerebral tissues provide important clues for understanding the remarkable enlargement of the human brain Proc. R. Soc. B 22 February 2013 vol. 280 no. 1753 DOI: 10.1098/rspb.2012.2398
Neolithic wooden water wells made without metal tools
Prehistoric farming communities in Europe constructed water wells out of oak timbers, revealing that these first farmers were skilled carpenters long before metal was discovered or used for tools. The research contradicts the common belief that metal tools were required to make complex wooden structures.
Tegel W, Elburg R, Hakelberg D, Stauble H, Buntgen U (2012) Early Neolithic Water Wells Reveal the World’s Oldest Wood Architecture. PLOS ONE 7(12): e51374. doi:10.1371/journal.pone.0051374
A memory device using a loose and moving part that actually enhances performance.
The untethered shuttle takes up less area than other designs and is not expected to suffer from mechanical fatigue because it avoids the use of components that need to bend or flex—such as the cantilevers used in competing mechanical memory approaches. In a simulation, the researchers found that the shuttle memory should be able to switch at speeds in excess of 1 megahertz.
Pott, V., Li, C. G., Vaddi, R., Tsai, J. M.-L. & Kim, T. T. The shuttle nanoelectromechanical nonvolatile memory. IEEE Transactions on Electron Devices 59, 1137–1143 (2012). ieeexplore.ieee.org/xpl/login.jsp?reload=true&tp=&arnumber=6135495
Delaying the aging process in mice
Feeding healthy mice resveratrol, a substance in the skin of red grapes and other fruits, delayed their aging process. The scientists believe that resveratrol increases the binding efficiency between Lamin A, a protein found in human cells, and the gene SIRT1, which is linked to longevity.
Boahua et al., Resveratrol Rescues SIRT1-Dependent Adult Stem Cell Decline and Alleviates Progeroid Features in Laminopathy-Based Progeria Cell Metabolism Volume 16, Issue 6, 5 December 2012, Pages 738–750 DOI: 10.1016/j.cmet.2012.11.007