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Multiferroic tunnel junctions
Yue-Wei Yin, Muralikrishna Raju, Wei-Jin Hu, Xiao-Jun Weng, Ke Zou, Jun Zhu, Xiao-Guang Li, Zhi-Dong Zhang, Qi Li
Frontiers of Physics. 2012, 7 (4 ): 380-385.
https://doi.org/10.1007/s11467-012-0266-8
Magnetic tunnel junctions with ferroelectric barriers, often referred to as multiferroic tunnel junctions, have been proposed recently to display new functionalities and new device concepts. One of the notable predictions is that the combination of two charge polarizing states and the parallel and antiparallel magnetic states could make it a four resistance state device. We have recently studied the ferroelectric tunneling using a scanning probe technique and multiferroic tunnel junctions using ferromagnetic La0.7 Ca0.3 MnO3 and La0.7 Sr0.3 MnO3 as the electrodes and ferroelectric (Ba, Sr)TiO3 as the barrier in trilayer planner junctions. We show that very thin (Ba, Sr)TiO3 films can sustain ferroelectricity up till room temperature. The multiferroic tunnel junctions show four resistance states as predicted and can operate at room temperatures.
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Synthesis, microstructures, and magnetoelectric couplings of electrospun multiferroic nanofibers
Shu-Hong Xie (谢淑红), Yun-Ya Liu(刘运牙), Jiang-Yu Li(李江宇)
Frontiers of Physics. 2012, 7 (4 ): 399-407.
https://doi.org/10.1007/s11467-011-0210-3
Multiferroic materials with two or more types of ferroic orders have attracted a great deal of attention in the last decade for their magnetoelectric coupling, and new ideas and concepts have been explored recently to develop multiferroic materials at nano-scale. Motivated by theoretical analysis, we synthesized single-phase BiFeO3 (BFO) nanofibers, Pb(Zr0.52 Ti0.48 )O3 -CoFe2 O4 (PZT-CFO) and Pb(Zr0.52 Ti0.48 )O3 -NiFe2 O4 (PZT-NFO) composite nanofibers, and CoFe2 O4 -Pb(Zr0.52 Ti0.48 )O3 (CFO-PZT) core-shell nanofibers using sol-gel based electrospinning. These nanofibers typically have diameters in the range of a few hundred nanometers and grain size in the range of 10s nanometers, and exhibits both ferroelectric and ferromagnetic properties. Piezoresponse force microscopy (PFM) based techniques have also been developed to examine the magnetoelectric coupling of the nanofibers, which is estimated to be two orders of magnitude higher than that of thin films, consistent with our theoretical analysis. These nanofibers are promising for a variety of multiferroic applications.
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Electronic structure of YMn2 O5 studied by EELS and first-principles calculations
Zhen Chen, Rui-Juan Xiao, Chao Ma, Yuan-Bin Qin, Hong-Long Shi, Zhi-Wei Wang, Yuan-Jun Song, Zhen Wang, Huan-Fang Tian, Huai-Xin Yang, Jian-Qi Li
Frontiers of Physics. 2012, 7 (4 ): 429-434.
https://doi.org/10.1007/s11467-011-0201-4
The electronic structure of multiferroic YMn2 O5 material has been studied by use of the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U , we performed the GGA+U calculations for 0<U ≤8 eV, and it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn–O hybridization. The experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O–K edge and Mn–L edges. A theoretical and experimental analysis on the O–K edge suggests that the on-site Coulomb interaction (U ) in YMn2 O5 could be less than 4 eV. Certain electronic structural features of LaMn2 O5 have been discussed in comparison with those of YMn2 O5 .
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Adsorption and separation of methane/hydrogen in octaphenylsilsesquioxane based covalently-linked organic-inorganic hybrid framework
Xiao-Dong Li, Hong Zhang, Yong-Jian Tang, Chao-Yang Wang
Frontiers of Physics. 2012, 7 (4 ): 453-460.
https://doi.org/10.1007/s11467-011-0243-7
The adsorption and separation of CH4 /H2 in two covalently-linked organic-inorganic hybrid frameworks polyoctaphenylsilsesquioxane (JUC-Z1) were computationally studied using the Grand Canonical Monte Carlo (GCMC) simulations. The results show that JUC-Z1 with Linde type A (LTA) and polycubane (zeolite code ACO) net topologies can adsorb up to 20.32, 18.57 mmol/g of CH4 and 19.04, 17.89 mmol/g of H2 at 298 K and 10 MPa, respectively. For the adsorption of binary mixture, the selectivity of CH4 over H2 in LTA-JUC-Z1 decrease gradually with the increase of the pressure or the CH4 mole fraction of the mixture. As to ACO-JUC-Z1, the selectivity first increases at low pressure or CH4 mole fraction, and then begins to decrease with the further increase of the corresponding amount. Anyhow, the two materials both exhibit excellent adsorption and separation capacities of CH4 /H2 .
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Radio jets and galaxies as cosmic string probes
Fa-Bo Feng
Frontiers of Physics. 2012, 7 (4 ): 461-470.
https://doi.org/10.1007/s11467-011-0188-x
The lensing effect of a cosmic string is studied, and some new methods are proposed to detect the cosmic string. The technique for using jets as extended gravitational lensing probes was first explored by Kronberg.We use the “alignment-breaking parameter” ηG as a sensitive indicator of gravitational distortion by a wiggly cosmic string. Then, we applied the non-constant deflection angle to jets, and ηG of a specific jet is just related to the projected slope of the jet. At least three jets in the sample of Square Kilometer Array (SKA) would have significant signals (ηG >10°) if the wiggly infinite cosmic string existed. The distortion of elliptical object is also studied and used to do a statistical research on the directions of axes and ellipticities of galaxies. In the direction of the string, we find that galaxies appear to be more elliptical for an observer and the distribution of apparent ellipticity changes correspondingly. The ellipticity distribution of current SDSS spiral sample has the signalto-noise ratio up to 8.48 which is large enough for astronomical observations. The future survey, such as Large Synoptic Survey Telescope (LSST) and Dark Energy Survey (DES) would weaken the requirement of special geometry in the data processing. As a result, all kinds of distributions, including ellipticity axis distribution, would serve as probes to detect wiggly strings in the near future. In brief, if a wiggly cosmic string existed, these signals would be convenient to be observed with the future weak lensing survey or other surveys in the deep space. If there was no lensing signal in these distributions, it would give the upper limit of the abundance of infinite strings.
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Theoretical prediction of cosmological constant Λ in Veneziano ghost theory of QCD
Wei-Xing Ma, Li-Juan Zhou, Tong-Quan Yuan, Jin-Song Peng, Xiao Lu, Ji-Huan Pan, Guang-Xiong Peng, Cheng-Ju Meng
Frontiers of Physics. 2012, 7 (4 ): 471-480.
https://doi.org/10.1007/s11467-012-0265-9
Based on the Veneziano ghost theory of QCD, we predict the cosmological constant Λ , which is related to energy density of cosmological vacuum by Λ = 8 π G 3 ρ Λ . In the Veneziano ghost theory, the vacuum energy density ρΛ is expressed by absolute value of the product of quark vacuum condensate and quark current mass: ρ Λ = 2 N f H m η ' c | m q ? 0 | ∶ q ˉ q ∶ | 0 ? | . We calculate the quark local vacuum condensates ? 0 | ∶ q ˉ q ∶ | 0 ? by solving Dyson–Schwinger Equations for a fully dressed confining quark propagator Sf (p ) with an effective gluon propagator G μ ν a b ( q ) . The quark current mass mq is predicted by use of chiral perturbation theory. Our theoretical result of Λ, with the resulting ? 0 | ∶ q ˉ q ∶ | 0 ? = - ( 235 MeV ) 3 and light quark current mass mq m q ? 3.29 - 6.15 M e V is in a good agreement with the observable of the Λ ≈ 10 - 52 m - 2 used widely in a great amount of literatures.
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Physical modeling of multiphase flow via lattice Boltzmann method: Numerical effects, equation of state and boundary conditions
Yan-Biao Gan, Ai-Guo Xu, Guang-Cai Zhang, Ying-Jun Li
Frontiers of Physics. 2012, 7 (4 ): 481-490.
https://doi.org/10.1007/s11467-012-0245-0
The aims of the present paper are threefold. First, we further study the fast Fourier transform thermal lattice Boltzmann (FFT–TLB) model for van der Waals (VDW) fluids proposed in Phys. Rev. E, 2011, 84(4): 046715. We analyze the merits of the FFT approach over the traditional finite difference scheme and investigate the effects of smoothing factors on accuracy and stability in detail. Second, we incorporate the VDW equation of state with flexible parameters into the FFT–TLB model. As a result, the revised model may be used to handle multiphase flows with various critical densities and temperatures. Third, we design appropriate boundary conditions for systems with solid walls. These improvements, from the views of numerics and physics, significantly extend the application scope of the model in science and engineering.
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