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We study transport in twisted bilayer graphene and show that electrostatic barriers can act as valley splitters, where electrons from the $K$ ($K'$) valley are transmitted only to e.g.\ the top (bottom) layer, leading to valley-layer locked currents. We show that such a valley splitter is obtained when the barrier varies slowly on the moir\'e scale and induces a Lifshitz transition across the junction, i.e.\ a change in the Fermi surface topology. Furthermore, we show that for a given valley the reflected and transmitted current are transversely deflected, as time-reversal symmetry is effectively broken in each valley separately, resulting in valley-selective transverse focusing at zero magnetic field., Comment: 13 pages, 13 figures

4-dodecyloxy-phenyl-4′,4″-diaminotriphenylamine(DPDTA) containing triphenylamine were successfully synthesized.Polyimides(PIs) containing triphenylamine units in the main chain were synthesized with the co-condensation of DMMDA,DPDTA and aromatic dianhydride 4,4′-oxydiphthalic anhydride(ODPA) by thermal imidization.The structures of the PIs were confirmed by FT-IR.The alignment behavior of PIs as liquid crystal alignment layers was also examined with polarized optical micrograph(POM).The properties of the PIs were characterized by TGA,UV-Vis and DSC etc.It was found that side chain density of PIs had important influence on properties of liquid crystal alignment film.Before rubbing,PI containing 25% DPDTA could induce uniform vertical alignment and a pretilt angle of 89.7°.The liquid crystal was not aligned on the surface of PI film which contained 10% DPDTA without the rubbing treatment.After the rubbing process,the pretilt angle of liquid crystal was 2.4,when PI film contained 25% DPDTA and the pretilt angle of liquid crystal was 1.8°,when PI contained 10% DPDTA.The transmittances of the PIs in 500～800 nm wavelengths are more than 80%,and their glass transition temperatures are above 230 ℃.

In dark-pulse Brillouin optical time domain analyzer(DP-BOTDA) DP-BOTDA the effective sensing length is greatly limited by the severe interaction between the Quasi-CW pump light and pump light.A simple DP-BOTDA was proposed to prolong the total sensing length without using multi-section fibers.To mitigate the impact of the high power Quasi-CW pump light,both Brillouin gain and loss effect along fiber were measured simultaneously in this scheme,yielding the Brillouin signal by subtracting the two side bands of the optical carrier suppressed(OCS) Stokes light.Based on this proposal,the total sensing length in DP-BOTDA could be increased.Simulations and discussions were conducted to validate the proposal,and the results are useful for designing high spatial resolution distributed Brillouin fiber sensor.

Composite magnetic fluids were obtained by doping silicon dioxide and polystyrene nonmagnetic microspheres into the pure ferrite magnetic fluids.The relationship between the magnetic-field-induced birefringence of composite magnetic fluids and the species,concentrations of nonmagnetic microspheres and pure ferrite magnetic fluids were investigated qualitatively.The results indicate that pure magnetic fluids with different concentrations doped with equal quantity of polystyrene microspheres have different variation trends for the birefringence with respect to the externally magnetic field.Different kinds of nonmagnetic microspheres have different influences on the change of birefringence after doping into the same pure magnetic fluids.When doping with two-kind-mixed nonmagnetic microspheres(equal quantity but different proportion between the two kinds of microspheres),one kind of the nonmagnetic microspheres will play a leading role in the variation of birefringence with externally magnetic field.Then,the variation trend of the birefringence with externally magnetic field for the as-prepared composite magnetic fluids is similar to that only doped with the leading nonmagnetic microspheres.

The relationship between the birefringence of Fe3O4 magnetic fluids and the ambient temperature(in the range of 20~80 ℃)at several different magnetic field strengths(lies in 0~200 mT)are investigated.The results show that similar dependence of birefringence on magnetic field strength and temperature happens for magnetic fluids with different concentrations.Moreover,the birefringence of the magnetic fluid is inversely proportional to the ambient temperature at fixed magnetic strength and proportional to the magnetic field strength at fixed ambient temperature.In a given temperature and magnetic field strength,the birefringence of the magnetic fluid with high concentration is larger than that with low concentration.The phenomena and results are analyzed carefully and the physical mechanism is clarified in detail.

In order to solve the low external quantum efficiency of the GaN-based LED caused by the total reflection,which occurs at the semiconductor-air interface,the Monte Carlo and wave theory are applied to simulate based on the basic structure of the GaN-based LED with double gratings,and the impact of the main loss of photon on the extraction efficiency is analyzed.Through the numerical simulation,different grating groove depth,period,and the absorption coefficient are applied to analyze the dynamic effects of the LED light extraction efficiency.The results show that light extraction change as similar periodic cosin with the increase of grating groove depth,which is in accordance with the relationship between grating diffraction efficiency and grating depth;the perfect grating period is similar to the wavelength in GaN or ITO.The light extraction efficiency of traditional slab LED decrease slower than grating structure LED with the increase of GaN material absorption coefficient.The depth and period both in transmitted and reflected gratings are 350 nm,300 nm,and 230 nm,250 nm,respectively.When the light absorption coefficient of GaN value is 0,the maximum light extraction efficiency of 67% can be obtained,while the light extraction efficiency of traditional slab LED is 18.5%.The light extraction efficiency of GaN-based LED can be improved more than 3 times after being applied with double gratings.Light extraction efficiency can be enhanced more effectively by improving crystal quality and decreasing light absorption coefficient.