Researchers Use Modern Techniques to Probe Ancient Chinese Pigment Han Purple

University of California scientists from the National High Magnetic Field Laboratory, Pulsed Field Facility at Los Alamos National Laboratory, working with colleagues from Tokyo Metropolitan University, the University of Buenos Aires in Argentina, the National Institute of Chemical Physics and Biophysics in Estonia, the National High Magnetic Field Laboratory in Tallahassee, Florida and the University of Tokyo, have discovered an ideal candidate for Bose-Einstein condensation in the ancient Chinese pigment, Han Purple.

In research featured recently on the cover of Physical Review Letters, the team describes how the application of a strong magnetic field to Han Purple (BaCuSi2O6) creates a gas of bosonic spin triplet excitations. The field acts as a chemical potential causing the weakly interacting bosonic gas to undergo Bose-Einstein condensation (BEC) when the temperature is reduced to minus 453 degrees Fahrenheit, six orders of magnitude higher than the temperature normally required for BEC in atomic gases.

According to Los Alamos physicist Marcelo Jaime, the work had both social and scientific rewards. "This research was rewarding from several perspectives. First, it was a pleasure collaborating with a truly international team to make the discovery and the work wouldn't have been possible without a strong international collaboration in the environment of a national facility like the National High Magnetic Field Laboratory. Second, it was enjoyable working with the Han Purple pigment, which has such a long and intriguing history of use. Finally, the discovery of BEC at such higher temperatures was a surprising scientific advance that will no doubt foster more research in this area."

Chinese chemists synthesized Han Purple pigment from barium copper silicates for the first time roughly 2,000 years ago and used the pigments for pottery and trade, in addition to large imperial projects such as the Qin Terracotta Warriors of Xi'an in Shaanxi Province. Preceding the invention of both paper and the compass, the ancient creation of Han Purple possibly makes it the first man-made compound containing a metallic bond.

A Bose-Einstein condensate is an unusual state of matter that is unlike both liquids and solids. BEC was first predicted in 1925, but it took roughly 70 years of intense theoretical and experimental effort before it was discovered in very dilute atomic gases in 1995. In essence, a BEC is a strange consequence of quantum theory regarding the wave nature of matter: atoms can act like the waves in water or sound in certain ways. At very low temperatures certain types of atoms spontaneously occupy the same quantum state. As a result, the atomic "waves" overlap, forming a giant "matter wave": each atom loses its identity, the assembly of atoms, in effect, acting as one giant atom or condensate.

The study of the BEC is indispensable to our fundamental understanding of quantum mechanics and could also be utilized in advanced "quantum computers", but very few examples have been found. Besides dilute atomic gases, the BEC is thought to occur in "superfluid" liquid helium and some forms of superconductivity. The possibility that a magnetic BEC exists in Han Purple represents a significant contribution to quantum physics. The original discovery of the BEC effect has since been confirmed by the same team using new samples prepared simultaneously by Tsuyoshi Kimura at Los Alamos and by Ian Fisher at Stanford University.

According to Alex Lacerda, NHMFL Pulsed Facility Director, this is the second NHMFL -LANL experimental result to be featured on the cover of Physical Review Letters. The first cover was on March 7, 2003 and this latest one on August 23.

The research team included Jaime, as well as Victor Correa, Neil Harrison, Cristian Batista and Peter Sharma, all of Los Alamos, Guillemo Jorge of the University of Buenos Aires and Los Alamos, Naoki Kawashima and Yoichi Kazuma from Tokyo Metropolitan University, Raivo Stern and Ivo Heinmaa from the National Institute of Chemical Physics and Biophysics, Sergei Zvyagin from the National High Magnetic Field Laboratory in Tallahassee and Yoshitaka Sasago and Kunimitsu Uchinokura from the University of Tokyo.

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