Magnetic proximity effect between topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of coupling constant. Such FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of FMI/TI interface, but also provides insights in designing the magnetic storage devices with ultrafast response.

Using neutron diffraction technique, we measure the average density of the heavy water confined in a nanoporous silica matrix, MCM-41, over the pressure-temperature plane. The result suggests the existence of a line of liquid-liquid phase transition with its end point at 1.29 ± 0.34 kbar and 213 ± 3 K in a fully hydrated sample. This point would be the liquid-liquid critical point (LLCP) according to the "liquid-liquid critical point" scenario. The phase diagram of the deeply cooled confined heavy water is then discussed. Moreover, in a partially hydrated sample, the phase transition completely disappears. This result shows that it is the free water part, rather than the bound water part, of the confined water that undergoes a liquid-liquid transition.

High-resolution inelastic X-ray scattering was used to investigate the collective vibrational excitations in hydrated lysozyme powders as a function of hydration level and temperature. It is found that the samples with strong enzymatic function are "soft", in the sense that they exhibit low frequency and large amplitude intraprotein collective vibrational motions on certain length scales. This is not the case for samples with weak or no enzymatic activity. Thus, we identify a possible correlation between the short-time intraprotein collective vibrational motions and the establishment of enzymatic function in hydrated lysozyme powders, and bring new insight to notions of protein "conformational flexibility" and "softness" in terms of these motions.

With quasi-elastic neutron scattering, we study the single-particle dynamics of the water confined in a hydrophilic silica material, MCM-41, at 4 kbar. A dynamic crossover phenomenon is observed at 219 K. We compare this dynamic crossover with the one observed at ambient pressure and find that (a) above the crossover temperature, the temperature dependence of the characteristic relaxation time at ambient pressure exhibits a more evident super-Arrhenius behavior than that at 4 kbar. Especially, at temperatures below about 230 K, the relaxation time at 4 kbar is even smaller than that at ambient pressure. This feature is different from many other liquids. (b) Below the crossover temperature, the Arrhenius behavior found at ambient pressure has a larger activation energy compared to the one found at 4 kbar. We ascribe the former to the difference between the local structure of the low-density liquid (LDL) phase and that of the high-density liquid (HDL) phase, and the latter to the difference between the strength of the hydrogen bond of the LDL and that of the HDL. Therefore, we conclude that the phenomena observed in this paper are consistent with the LDL-to-HDL liquid-liquid transition hypothesis.

The boson peak in deeply cooled water confined in nanopores is studied with inelastic neutron scattering. We show that in the (P, T) plane, the locus of the emergence of the boson peak is nearly parallel to the Widom line below ∼1600 bar. Above 1600 bar, the situation is different and from this difference the end pressure of the Widom line is estimated. The frequency and width of the boson peak correlate with the density of water, which suggests a method to distinguish the hypothetical "low-density liquid" and "highdensity liquid" phases in deeply cooled water. DOI: 10.1103/PhysRevLett.112.237802 PACS numbers: 61.20.Lc, 63.50.−x The boson peak is a broad peak observed at low frequencies (∼2-10 meV) in the inelastic neutron, x-ray, and Raman scattering spectra of many amorphous materials and supercooled liquids [1][2][3]. Preliminary measurements on the boson peak in deeply cooled water confined in nanopores [4] and protein surfaces [5] under ambient pressure have been reported. The measured spectra include a boson peak at about 6 meV, which only emerges below ∼230 K. A similar result was also observed in a computer simulation study [6]. Since this temperature is close to the ambient-pressure Widom line temperature T W ∼224 K (the Widom line is the locus of specific heat maxima [7]), the authors tentatively explained this phenomenon as due to transformation of the local structure of water from a predominantly high-density liquid (HDL) to a predominantly low-density liquid (LDL) form as T W is crossed from above. However, this experimental result is insufficient. First, it is clear that this emergence of the boson peak in the raw spectra depends on the energy resolution of the experimental facility. Second, and more importantly, the situation at high pressures has, heretofore, remained unknown. Therefore, without quantitative analysis of the entire spectra, including quasielastic and inelastic components at high pressure, definite conclusions cannot be drawn.In this Letter, we have measured the boson peak in deeply cooled water with a series of inelastic neutron scattering (INS) experiments in the temperature (T) range from 120 to 230 K and the pressure (P) range from 400 to 2400 bar employing the Disk Chopper Spectrometer (DCS) [8] at the National Institute of Standards and Technology (NIST) Center for Neutron Research and the Cold Neutron Chopper Spectrometer (CNCS) [9] at the Spallation Neutron Source of the Oak Ridge National Laboratory (ORNL). The DAVE software was used for the data reduction [10]. We find that below ∼1600 bar, the emergence of the boson peak is correlated (but does not overlap) with the Widom line and with the fragile-to-strong crossover (FSC) in deeply cooled water [11]. Above ∼1600 bar, the locus of the emergence of the boson peak in the (P, T) plane has a different slope as compared with its behavior below ∼1600 bar, and the end pressure of the Widom line is estimated by determining where the slope begins to change. Moreover, the (P, T) dependences of the shape of the boson peak are f...