What makes parrots have colorful feathers?

Dr. Peter Mojzeš Institute of Physics of Charles University, Faculty of Mathematics and Physic Charles University and Dr. Jindřich Brejcha Department of Philosophy and History of Science, Faculty of Science, Charles University in Prague – conducted the research on a CIRI beamline, studying how parrots produce the colors of their feathers.

The multi-colored plumage of parrot feathers arouses admiration and delight, but where did all these colors actually come from?  How birds managed to develop such a range of colors and how has it evolved over the centuries? A group of researchers led by Dr. Miguel Carneiro from CIBIO (Research Centre in Biodiversity and Genetic Resources – InBIO Associate Laboratory) in Portugal decided to answer these questions. 

Dr. Jindřich Brejcha explained what their research is about – Specifically, we are interested in molecular differences of polyene pigment contained within parrots’ feathers. We use Raman spectroscopy combined with mass spectroscopy to look at the structure of molecules causing parrot color. However, due to the resonance Raman effect for the excitation throughout the visible region and high Raman cross-section of the C-C and C=C vibrations, only a few Raman bands related to the vibrational modes of the main polyene chain and disproportionately enhanced are visible in the Raman spectra. Raman bands associated with vibrations of functional end groups are hidden in stochastic noise. Hence, to overcome this shortcoming of Raman microscopy while preserving the same spatial resolution, O-PTIR microscopy seems to be a promising candidate method.

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Transition metal insulators: The origin of colour

In a theoretical study, researchers have explained the vibrant colours of two compounds whose electronic properties seemingly prohibit such colouring. The hues exhibited by the two insulators originate from transitions in the spins of the electrons, which modify the way the materials absorb and reflect light in such a way as to create the bright colours. The theoretical framework employed by the team promises new insights in fields such as optoelectronics or in the study of qubits, the quantum bits used in quantum computers. 

Although colour is a familiar phenomenon, it is sometimes challenging to explain how the hues of certain materials come about. This is the case with insulators that contain transition metals. In these compounds, the energy gap between the valence band, in which the electrons are tightly bound to the atoms, and the conduction band, in which the electrons can move freely, is larger than the highest energy of photons of visible light—meaning that these materials should not absorb visible light. As the colour of a compound is complementary to the wavelengths it absorbs, we should thus perceive these insulators as being transparent instead of coloured. 

A team of researchers including the head of the European XFEL Theory group, Alexander Lichtenstein, now used two complementary theoretical methods to study the origin of colour in two typical transition metal insulators: nickel(II) oxide (NiO)—a green compound used in the production of ceramics and nickel steel as well as in thin-film solar cells, nickel–iron batteries, and fuel cells—and manganese(II) fluoride (MnF2), a pink material employed in the manufacture of special kinds of glass and lasers.

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Image: Visualization of the orbital character of low-laying excitons in NiO, corresponding to a local ‘Frenkel’ exciton at an energy of 1.6 eV and a weakly bound, bright ‘Wannier-Mottâ’ exciton at an energy of 3.6 eV