Field-only integral equation method for time domain scattering of electromagnetic pulses

The scattering of electromagnetic pulses is described using a non-singular boundary integral method to solve directly for the field components in the frequency domain, and Fourier transform is then used to obtain the complete space-time behavior. This approach is stable for wavelengths both small and large relative to characteristic length scales. Amplitudes and phases of field values can be obtained accurately on or near material boundaries. Local field enhancement effects due to multiple scattering of interest to applications in microphotonics are demonstrated.Comment: 7 pages, 9 figure

Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures

Hydrogen-rich compounds are important for understanding the dissociation of dense molecular hydrogen, as well as searching for room temperature Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure experiment reported the successful synthesis of novel insulating lithium polyhydrides when above 130 GPa. However, the results are in sharp contrast to previous theoretical prediction by PBE functional that around this pressure range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. In order to address this discrepancy, we perform unbiased structure search with first principles calculation by including the van der Waals interaction that was ignored in previous prediction to predict the high pressure stable structures of LiHn (n = 2-11, 13) up to 200 GPa. We reproduce the previously predicted structures, and further find novel compositions that adopt more stable structures. The van der Waals functional (vdW-DF) significantly alters the relative stability of lithium polyhydrides, and predicts that the stable stoichiometries for the ground-state should be LiH2 and LiH9 at 130-170 GPa, and LiH2, LiH8 and LiH10 at 180-200 GPa. Accurate electronic structure calculation with GW approximation indicates that LiH, LiH2, LiH7, and LiH9 are insulative up to at least 208 GPa, and all other lithium polyhydrides are metallic. The calculated vibron frequencies of these insulating phases are also in accordance with the experimental infrared (IR) data. This reconciliation with the experimental observation suggests that LiH2, LiH7, and LiH9 are the possible candidates for lithium polyhydrides synthesized in that experiment. Our results reinstate the credibility of density functional theory in description H-rich compounds, and demonstrate the importance of considering van der Waals interaction in this class of materials.Comment: 34 pages, 15 figure

Magnetic properties of transition-metal-doped Zn1−xTxO (T=Cr, Mn, Fe, Co, and Ni) thin films with and without intrinsic defects: A density functional study

Theoretical calculations based on density-functional theory and generalized gradient approximation have been carried out in studying the electronic structure and magnetic properties of transition-metal-doped Zn1−xTxO (T=Cr, Mn, Fe, Co, and Ni) (112¯0) thin films systematically with and without intrinsic point defects (e.g., vacancies and interstitials), and as function of concentration and distribution of dopants and vacancies. Using large supercells and geometry optimization without symmetry constraint, we are able to determine the sites that metal atoms prefer to occupy, their tendency to cluster, the preferred magnetic coupling between magnetic moments at transition-metal sites, and the effect of intrinsic point defects on the nature of their coupling. Except for Mn atom, which distributes uniformly in ZnO thin films in dilute condition, transition-metal atoms occupying Zn sites prefer to reside on the surface and couple antiferromagnetically. The presence of native point defects has a large effect on the ground-state magnetic structure. In particular, p-type defects such as Zn vacancies play a crucial role in tuning and stabilizing ferromagnetism in Zn1−xTxO thin films (T=Cr, Mn, Fe, and Ni), while n-type defects such as O vacancies or Zn interstitials greatly enhance the ferromagnetic coupling in Zn1−xCoxO thin films. The present study provides a clear insight into the numerous conflicting experimental results on the magnetic properties of T-doped ZnO systems