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Protons in concert
《自然•物理》发文:江颖、王恩哥等在水科学领域再次获得重要研究进展

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.

 

继去年获得界面水/冰氢键构型的高分辨图像后,北京大学量子材料中心、量子物质科学协同创新中心的江颖课题组和王恩哥课题组通过合作再进一步,实现了对单个水团簇的氢键构型动态变化的实时监测,在实空间直接观察到了质子在氢键网络内的协同量子隧穿过程。相关研究成果于216日在线发表在物理领域最具影响力的杂志《自然·物理》(Nature Physics doi:10.1038/nphys3225)。同期的Nature Physics在“News and Views”栏目中专门刊发了德国著名学者Dominik Marx教授以“Tunnelling in chiral water clusters: Protons in concert”为题对该工作进行的评述。这也是在短短一年时间内,两个课题组通过实验和理论的密切合作,在Nature系列期刊发表的第三篇文章。

水的氢键构型往往不是静态的,而是不断的在变换和重组,信息和能量也常常在氢键构型中来回传递。因此,氢键动力学过程的研究对于人们理解真实条件下水的物理化学特性具有非常重要的意义。质子(氢核)沿氢键的转移是一种非常重要的氢键动力学过程,由于质子的质量很小,在转移过程中往往会表现出显著的核量子效应,比如:量子隧穿、零点运动等。更为有趣的是,氢键网络中的质子并不是相互独立的,通常具有很强的关联性。因此,氢键体系中的质子转移实际上会涉及到多体量子行为,目前人们对它的理解还非常的有限。

近年来,江颖课题组基于超高真空低温扫描隧道显微镜(STM)发展了一套独特的亚分子级轨道成像技术[Nature Materials 13, 184 (2014), Nature Communications 5, 4056 (2014)],可以在实空间直接识别氢键的方向性,并对质子进行精确定位,为原子尺度上质子动力学研究提供了必要条件。最近,江颖等进一步将亚分子级轨道成像技术和实时探测技术相结合,实现了对NaCl(001)表面上单个水团簇内质子转移的实时跟踪,直接观察到了质子在水团簇内的量子隧穿动力学过程。王恩哥等基于第一性原理计算方法并特别考虑了氢核的全量子化,发现这种隧穿过程由多个质子协同完成,是一种全新的相干量子过程。这种多体量子行为对实验探测技术的要求非常苛刻,任何微小的外界环境干扰都有可能破坏质子之间的相干性,从而抑制这种协同隧穿过程。此外,他们还利用功能化的STM针尖控制质子隧穿势垒和质子之间的相干性,实现了对这种多体量子态的原子尺度操控。

该工作表明,多体关联量子行为在水的氢键动力学过程扮演着不可忽视的角色。实验中观察到的质子协同隧穿,远比单粒子隧穿容易发生,很有可能广泛存在于氢键体系,这对于理解冰和有机铁电材料的相变过程以及生物体系的信号传递过程有非常重要的意义。另外,该工作发展的技术不仅为氢键动力学的研究开辟了新的途径,而且为实验上精确、定量描述氢键体系的核量子效应提供了可能。

直博生孟祥志、郭静和彭金波是文章的共同第一作者,物理学院的李新征研究员和量子材料中心的施均仁教授在理论方面提供了重要的支持和帮助。这项工作得到了国家自然科学基金委、科技部国家重点基础研究发展计划、“万人计划”青年拔尖人才支持计划等项目的资助。

 


图:四个质子的协同量子隧穿导致水四聚体的氢键方向性来回转换,这种氢键动力学过程可以通过监测STM隧道电流随时间的变化来探测。该图由真实的实验数据通过颜色渲染而成。