AsianScientist (Mar. 6, 2015) – Scientists have developed a method to observe complex quantum tunneling effects in hydrogen bonded materials, paving the way for the possibility of controlling the quantum states of protons with atomic-scale precision. Their results have been published in Nature Physics.
Proton transfer through the hydrogen bond plays an essential role across an incredibly broad spectrum of physics, chemistry and biology. Due to its small mass, the proton exhibits unusual dynamics, controlled by nuclear quantum effects such as quantum tunneling.
Moreover, the tunneling of protons within H-bonded networks such as those present in water tends to involve many hydrogen bonds simultaneously, leading to correlated many-body tunneling. However, the direct evidence of such concerted proton tunneling still remains elusive, in spite of tremendous experimental and theoretical efforts for decades.
Recently, the teams led by Professors Jiang Ying and Wang Enge of the International Center for Quantum Materials of Peking University reported the use of a cryogenic scanning tunneling microscope (STM) to directly visualize concerted proton tunneling within a hydrogen-bonded cyclic water tetramer adsorbed on a surface of sodium chloride.
“This is made possible by monitoring in real time the reversible interconversion of the hydrogen-bonding chirality of the water tetramer based on a unique orbital imaging technique, which was newly developed by our groups last year,” said Jiang. “Another key step is using a chlorine-functionalized STM tip to tune the tunneling barrier through tip-proton coupling such that the tunneling events can be readily detectable”.
Detailed control experiments combined with state-of-the-art density functional theory calculations confirm the quantum nature of the proton transfer between the water molecules. Strikingly, it was revealed that the proton tunneling process involves a concerted motion of four protons, which are locked and move in a fully correlated manner.
“The concerted tunneling of protons is extremely sensitive to the coupling with atomic-scale environment due to the demanding phase coherence between the protons,” emphasized Jiang.
The researchers found that the chlorine-terminated tip can either enhance or suppress the concerted tunneling process depending on the details of coupling symmetry between the chlorine and the protons.
This work not only sheds new light on the understanding of phase transition in ices of high-pressure phases and hydrogen-bonded ferroelectric materials, but also opens a new route for controlling the quantum states of the protons with atomic scale precision.
The article can be found at: Meng et al. (2015) Direct Visualization of Concerted Proton Tunnelling in a Water Nanocluster.
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Source: Peking University.
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