In the hydrogen-bond network of ice, orientation of water molecules is strongly correlated and geometrically frustrated under the Bernal-Fowler-Pauling ice rules. In general, materials with a strongly correlated and highly frustrated degree of freedom have potential for exhibiting dramatic and unusual responses to external stimuli. Nevertheless, little has been understood about cooperative dielectric and thermodynamic responses of ice to external perturbations. To open up a new route to unveil hidden exotic ferroelectric properties of protons with many-body interactions in ice beyond the current phase diagram, we have investigated a possibility of interface-induced ferroelectric proton ordering by focusing on heteroepitaxially grown crystalline-ice films on metal substrates as model systems [1-4]. We have used developed phase-resolved sum-frequency generation (SFG) spectroscopy with heterodyne detection under UHV conditions [5]. Although the SFG spectra obtained with the conventional homodyne detection show positive sign regardless of the orientation of water molecules, Imχ(2) SFG spectra obtained by the heterodyne detection exhibits positive or negative sign for net orientation of OH with hydrogen pointing away (H-up) or toward substrate (H-down), respectively.
Recently, we have succeeded in directly demonstrating that the adsorbed first-layer water molecules prefer an H-down configuration on model platinum substrate: Pt(111) [4]. The coverage dependence of the Imχ(2) SFG spectra in the hydrogen-bonded OH stretching regions clearly reveals that the H-down proton ordering in the first layer is significantly pinned by the Pt(111) substrate and is subsequently propagated to the overlayer during the film growth. Temperature dependence of the SFG spectra revealed that such an exotic proton ordering is thermodynamically stable and has an extremely high critical temperature of ~175 K [4], which is more than twice as large as that of ferroelectric bulk ice XI (Tc~72 K). It was clarified that anisotropy and protolysis driven by the electrostatistics at the heterointerface are key factors in stimulating the novel exotic ordering in the many-body correlated proton system [4].
[1] Phys. Rev. B 97, 075410 (2018).
[2] Phys. Rev. B 96, 115405 (2017).
[3] Phys. Chem. Chem. Phys. 19, 17677-17684 (2017).
[4] Nature Physics 12, 1063–1068 (2016).
[5] Phys. Rev. Lett. 117, 186101 (2016).