Protons in concert

Proton transfer through the hydrogen bond plays an essential role across an incredibly broad spectrum of physics, chemistry and biology. The mystery of proton dynamics mainly arises from the nuclear quantum effect such as quantum tunneling due to the small mass of proton. Moreover, the tunneling of protons within the H-bonded network 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 Prof. Ying Jiang and Prof. Enge Wang of International Center for Quantum Materials (ICQM) of Peking University report the use of a cryogenic scanning tunneling microscope (STM) to directly visualize the concerted proton tunneling within a hydrogen-bonded cyclic water tetramer adsorbed on NaCl(001) surface.

“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 [see Nat. Mater. 13, 184 (2014)],” says 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,” adds Jiang.

Detailed control experiments combined with state-of-the-arts 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 as a delocalized quasiparticle.

“The concerted tunneling of protons is extremely sensitive to the coupling with atomic-scale environment due to the demanding phase coherence between the protons,” emphasizes Jiang. The researchers found that the Cl-terminated tip can either enhance or suppress the concerted tunneling process depending on the details of coupling symmetry between the Cl 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.

Research was published online in Nature Physics on Feb. 16, 2015 (Nat. Phys.doi:10.1038/nphys3225) and also featured in “News and Views” (Nat. Phys. doi:10.1038/nphys3269). This work received supports from Ministry of Science and Technology of China, National Natural Science Foundation of China, Ministry of Education of China, and National Program for Support of Eminent Professionals.

Figure: Chirality switching of a water tetramer. The two different chiral states (blue and red) arise from the concerted quantum tunneling of four protons. The switching dynamics can be monitered by recording the time-dependent current trace using a scanning tunneling microscope.