Diagrammatic approach to excitonic effects on nonlinear optical response

Optical responses of atomically thin 2D materials are greatly influenced by electron-hole interactions. It is by far established that exciton signatures can be well-identified in the optical absorption spectrum of quasi-2D materials. However, the same level of understanding of excitonic effects on nonlinear optical responses and the ability to compute them accurately is still much desired. Based on the functional integral formalisms and working in the velocity gauge, we introduce a convenient Feynman diagram approach for calculating nonlinear responses including excitonic effects. By dressing electron-photon interactions with electron-hole ladder diagrams, we derive an expression for second-order optical responses and provide a comprehensive description of excitonic effects. We apply our approach to a monolayer h-BN model and show qualitative changes in the second harmonic generation spectrum when comparing with results assuming independent particles. Our approach can be readily extended to higher order optical responses and is feasible for first-principles calculations

Tensor network simulation of phase diagram of frustrated J1-J2 Heisenberg model on a checkerboard lattice

We use the recently developed tensor network algorithm based on infinite projected entangled pair states (iPEPS) to study the phase diagram of frustrated antiferromagnetic J1-J2 Heisenberg model on a checkerboard lattice. The simulation indicates a Neel ordered phase when J2 < 0.88J1, a plaquette valence bond solid state when 0.88 1.11J1, with two first-order transitions across the phase boundaries. The calculation shows the cross-dimer state proposed before is unlikely to be the ground state of the model, although such a state indeed arises as a metastable state in some parameter region.Comment: 4 pages, 5 figure

Large shift current via in-gap and charge-neutral exciton excitations in BN nanotubes and single BN layer

We perform {\it ab initio} many-body calculations to investigate the exciton shift current in small diameter zigzag BN nanotubes and also single BN sheet, using the GW plus Bethe-Salpeter equation (GW-BSE) method with the newly developed efficient algorithms. Our GW-BSE calculations reveal a giant in-gap peak in the shift current spectrum in all the studied BN systems due to the excitation of the A exciton. The peak value of the excitonic shift current is more than three times larger than that of the quasiparticle shift current, and is attributed to the gigantic enhancement of the optical dipole matrix element by the A exciton resonance. The effective exciton shift current conductivity is nearly ten times larger than the largest shift conductivity observed in ferroelectric semiconductors. Importantly, the direction of the shift current in the BN nanotubes is found to be independent of the tube chirality ($n,0$) (or diameter), contrary to the simple rule of $sgn(J_\text{shift})=\text{mod}(n,3)$ predicted by previous model Hamiltonian studies. Finally, our {\it ab initio} calculations also show that the exciton excitation energies decrease significantly with the decreasing diameter due to the curvature-induced orbital rehybridization in small diameter zigzag BN nanotubes.Comment: 12 pages, 8 figures and 2 table

Excitonic interactions and mechanism for ultrafast interlayer photoexcited response in van der Waals heterostructures

Optical dynamics in van der Waals heterobilayers is of fundamental scientific and practical interest. Based on a time-dependent adiabatic GW approach, we discover a new many-electron (excitonic) channel for converting photoexcited intralayer to interlayer excitations and the associated ultrafast optical responses in heterobilayers, which is conceptually different from the conventional single-particle picture. We find strong electron-hole interactions drive the dynamics and enhance the pump-probe optical responses by an order of magnitude with a rise time of ~300 fs in MoSe$_2$/WSe$_2$ heterobilayers, in agreement with experiment

In-situ strain tuning of the Dirac surface states in Bi2Se3 films

Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudo-magnetic-field effects, helical flat bands and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here, we show that the Dirac surface states of the topological insulator Bi2Se3 can be reversibly tuned by an externally applied elastic strain. Performing in-situ x-ray diffraction and in-situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi2Se3 films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting reversible changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of Bi2Se3 and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes

Mutation of SLC35D3 causes metabolic syndrome by impairing dopamine signaling in striatal D1 neurons

We thank Dr. Ya-Qin Feng from Shanxi Medical University, Dr. Tian-Yun Gao from Nanjing University and Dr. Yan-Hong Xue from Institute of Biophysics (CAS) for technical assistance in this study. We are very thankful to Drs. Richard T. Swank and Xiao-Jiang Li for their critical reading of this manuscript and invaluable advice. Funding: This work was partially supported by grants from National Basic Research Program of China (2013CB530605; 2014CB942803), from National Natural Science Foundation of China 1230046; 31071252; 81101182) and from Chinese Academy of Sciences (KSCX2-EW-R-05, KJZD-EW-L08). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD

Endometrial transcriptome in recurrent miscarriage and recurrent implantation failure

The endometrium becomes receptive to the embryo only in the mid-luteal phase, but not other stages of the menstrual cycle. Endometrial factors play an important role in implantation. Women with recurrent miscarriage and recurrent implantation failure have both been reported to have altered expression of receptivity markers during the window of implantation. We aimed to compare the gene expression profiles of the endometrium in the window of implantation among women with unexplained recurrent implantation failures (RIF) and unexplained recurrent miscarriages (RM) by RNA sequencing (RNA-Seq). In total 20 patients (9 RIF and 11 RM) were recruited. In addition 4 fertile subjects were included as reference. Endometrium samples were precisely timed on the 7th day after luteal hormone surge (LH+7). All the 24 samples were extracted for total RNA. The transcriptome was determined by RNA-Seq in first 14 RNA samples (5 RIF, 6 RM, and 3 fertile). Differentially expressed genes between RM and RIF were validated by quantitative real time PCR (qPCR) in all 24 RNA samples (9 RIF, 11 RM and 4 fertile). Complementary and coagulation cascades pathway was the significantly up-regulated in RIF while down-regulated in RM. Differentially expressed genes C3, C4, C4BP, DAF, DF and SERPING1 in complement and coagulation cascade pathway between RM and RIF were further validated by qPCR. This study identified differential molecular pathways in endometrium between RIF and RM, which potentially affect the implantation process