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Register a new userLayer-dependence of dielectric response and water-enhanced ambient degradation of highly-anisotropic black As
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Black arsenic (BAs) is a van der Waals layered material with a puckered honeycomb structure and has received increased interest due to its anisotropic properties and promising performance in devices. Here, crystalline structure, thickness-dependent dielectric responses, and ambient stability of BAs nanosheets are investigated using STEM imaging and spectroscopy. Atomic-resolution HAADF-STEM images directly visualize the three-dimensional structure and evaluate the degree of anisotropy. STEM-EELS is used to measure the dielectric response of BAs as a function of the number of layers. Finally, BAs degradation under different ambient environments is studied highlighting high sensitivity to moisture in the air.
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The superconductivities of samples prepared by several procedures were found to degrade under ambient environment. The degradation mechanism was studied by measuring the change of surface chemical composition of dense MgB2 pellets (prepared by hot isostatic pressure, HIPed) under atmospheric exposure using X-ray Photoelectron Spectroscopy (XPS). Results showed that samples with poor connectivity between grains and with smaller grain sizes degrade with time when exposed to ambient conditions. In these samples, the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. In contrast, our well-sintered and the HIPed samples remained stable for several months under ambient condition. The degradation was found to be related to surface decomposition as observed by XPS. We observed the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the increase of C and O contents, and the reduction of B concentration in the surface layer of MgB2 samples.
We propose a fully ab initio theory to compute the electron density response under the perturbation in the local field. This method is based on our recently developed local dielectric response theory [Phys. Rev. B 92, 241107(R), 2015], which provides a rigorous theoretical framework to treat local electronic excitations in extended systems beyond the commonly employed dipole approximation. We have applied this method to study the electronic part of the molecular polarizability of water in ice Ih and liquid water. Our results reveal that the crystal field of the hydrogen-bond network has strong anisotropic effects, which significantly enhance the out-of-plane component and suppress the in-plane component perpendicular to the bisector direction. The contribution from the charge transfer is equally important, which increases the isotropic molecular polarizability by 5-6%. Our study provides new insights into the dielectric properties of water, which form the basis to understand the electronic excitations in water and to develop accurate polarizable force fields of water.
The nanostructural evolution of the strain-induced structural phase transition in BiFeO3 is examined. Using high-resolution X-ray diffraction and scanning-probe microscopy-based studies we have uniquely identified and examined the numerous phases present at these phase boundaries and have discovered an intermediate monoclinic phase in addition to the previously observed rhombohedral- and tetragonal-like phases. Further analysis has determined that the so-called mixed-phase regions of these films are not mixtures of rhombohedral- and tetragonal-like phases, but intimate mixtures of highly-distorted monoclinic phases with no evidence for the presence of the rhombohedral-like parent phase. Finally, we propose a mechanism for the enhanced electromechanical response in these films including how these phases interact at the nanoscale to produce large surface strains.
The microwave, near-millimetre and infrared (IR) dielectric response of Srn+1TinO3n+1 (n=1-4) Ruddlesden-Popper homologous series was studied in the temperature range 10 to 300 K. Remarkable softening of the polar optical mode was observed in Sr4Ti3O10 and Sr5Ti4O13 which explains the increase in microwave permittivity and dielectric loss upon cooling. However, both samples have a distinct content of SrTiO3 dispersed between SrO layers. It is proposed therefore that the observed soft mode originates from the SrTiO3 microscopic inclusions.
To date, the intrinsic thermal conductivity tensor of bulk black phosphorus (BP), an important 2D material, is still unknown, since recent studies focus on BP flakes not on bulk BP. Here we report the anisotropic thermal conductivity tensor of bulk BP, for temperature range 80 - 300 K. Our measurements are similar to prior measurements on submicron BP flakes along zigzag and armchair axes, but are >25% higher in the through-plane axis, suggesting that phonon mean-free-paths are substantially longer in the through-plane direction. We find that despite the anisotropy in thermal conductivity, phonons are predominantly scattered by the same Umklapp processes in all directions. We also find that the phonon relaxation time is rather isotropic in the basal planes, but is highly anisotropic in the through-plane direction. Our work advances fundamental knowledge of anisotropic scattering of phonons in BP and is an important benchmark for future studies on thermal properties of BP nanostructures.
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