Do Two Symmetry-Breaking Transitions in Photosynthetic Light Harvesting Complexes (PLHC) Form One, Two or More Kibble–Zurek (KZ) Topological Defect(s)?

Abstract

Kibble (J Phys A: Math Gen, 9(8):1387, 1976) [1] and Zurek (Nature, 317(6037):505, 1985) [2] (KZ) proposed that rapid symmetry-breaking transitions in the hot, early universe could result in causally disconnected topological defects such as cosmic strings. This type of first order transition has analogues in certain second order transitions present in condensed matter such as liquid crystals, superfluids and charge density waves in terms of flux tubes or vortices. Recently, we discovered that Rhodopseudomonas acidophila’s Photosynthetic Light Harvesting Complex might have different types of coherent ground and excited states, suggesting that there are two different symmetry-breaking transitions. The B850 ground states comprise eight identical rings each containing 18 bacteriochlorophyll components, and each ring has undergone a Bose–Einstein phase transition to a charge density wave that lowers the energy. The excited state coherence results from polariton formation from the non-crossing of bosons, here excitons and photons, an extension of exciton theory (Knox, Theory of excitons. Academic Press, New York, 1963) [3]. The result is short-lived quasiparticles with very low mass that can form an unusual Bose–Einstein condensate (BEC). We suggest that the oriented, circular B850’s and enclosed single B875 create new cavity structure with some attributes of toroidal nanopillars (Fan et al., Nano Lett, 10(10):3823, 2010) [4], (Pelton and Bryant, Introduction to metal-nanoparticle plasmonics. Wiley, Hoboken, 2013) [5], (Vahala, Nature, 424(6950):839, 2003) [6]. Since both the ground and excited states should contain solitons, we envisage trARPES (time and angle resolved photoemission spectroscopy) in conjunction with a three-pulse probe with various appropriate time delays should be able to map both the KZ phase transitions and energy transfers as a function of light intensity and time in this complex at room temperature (Mihailovic et al., J Phys: Condens Matt, 25(40):404206, 2013) [7], (Rettig et al., Nat Comms, 7:10459, 2016) [8].