Two-dimensional ferroelectric tunnel junction: the case of SnSe Xin-Wei Shen , Yue-Wen Fang 1,† , Bo-Bo Tian , Chun-Gang Duan 1,2,* State Key Laboratory of Precision Spectroscopy and Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai, 200241, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China * Correspondence and requests for materials should be addressed to C.G.D<EMAIL_ADDRESS>(Date: 6 July 2018) n -type semiconductor homostructure, coupling the electron tunneling with the robust in- % is obtained. Ferroelectrics exhibiting spontaneous electric polarization ( P s ) play a crucial role in the modern information era. The opposite polarization states + P s and - P s in ferroelectric materials can be switched upon application of an electric field. This binary operation under uniform polarization reversal makes ferroelectrics a promising material for use in non-volatile memory applications, and opens up a wide range of practical applications in data storage devices 1-5 . Among the prototypical model devices, ferroelectric tunnel junctions (FTJs), which are typically composed of a thin ferroelectric layer sandwiched by two metallic electrodes, have attracted significantly increasing research interest 6-11 owing to the availability of giant tunneling electroresistance (TER) effect 12,13 . The mechanism can be affected by changing different electrodes or adopting
asymmetric electrode/ferroelectric interfaces. The electron tunneling mechanism requires that the ferroelectricity of insulating barrier should be sustained at nanometer scale, which is decisive for the occurrence of direct tunneling . Over the past few decades, fortunately, the emergence of high-quality ultrathin ferroelectric films 15-19 makes it possible to achieve the FTJ devices. However, the preparation of stable ferroelectricity in a few unit cells is still challenging. For example, as the pioneer of ferroelectrics, perovskite oxides thin films 20-24 have been widely adopted as the ferroelectric tunneling barrier, whereas sophisticated experimental approaches such as strain engineering , interface engineering , and careful control of epitaxial growth are generally required to maintain ferroelectricity in nanometre-thick oxide films. On account of the critical thickness in traditional ferroelectrics 28-30 , another approach is to find inherent atomic-thick materials with robust nanoscale ferroelectricity. Recently, a great deal of new members of ferroic family have been substantially demonstrated at layered van der Waals materials 31-37 . In particular, the discovery of intrinsic ferroelectricity in two-dimensional (2D) materials is attractive for breaking through the restriction of the critical thickness in traditional ferroelectrics, and hence they are showing great potential for the applications of next-generation nanoelectronic devices. A pristine monolayer MXs semiconductor with in-plane ferroelectricity serves as a tunneling barrier. The left and right electrodes are obtained by p -type and n -type doping of the same MXs, resulting in a homostructural tunneling device. The inset illustrates the wave nature of the quantum tunneling. As the ferroelectric polarization ( P s ) is rightward polarized, the left energy barrier ( Φ ) is raised up beyond the energy of electrons. Through the quantum-mechanical phenomenon, in this case, the electrons tunnel across the barrier material in the nature of evanescent state since its amplitude decays exponentially through the barrier.
image.2.1[405*12] image.2.2[405*12] image.2.3[405*12] image.2.4[405*12] image.2.5[405*12] image.2.6[405*12] image.2.7[405*12] image.2.8[405*12] image.2.9[405*12] image.2.10[405*12] image.2.11[405*12] image.2.12[405*12] image.2.13[405*12] image.2.14[405*12] image.2.15[405*12] image.2.16[405*12] image.2.17[405*12] image.2.18[405*12] image.2.19[405*12] image.2.20[405*12] image.2.21[405*12] image.2.22[405*9]In this Letter, by combining robust in-plane ferroelectricity with electron tunneling, we propose a distinctive FTJ consisted of homostructural 2D ferroelectric materials, as illustrated in Fig. 1. Different from a conventional vertical tunneling junctions, this lateral 2D-FTJ in our study features a barrier region of group-IV monochalcogenides (MXs) monolayer in ferroelectric state, and two electrodes are formed by doped p -type and n -type MXs semiconductors, constructing a p -type semiconductor/ferroelectric/ n -type semiconductor ( p -SC/FE/ n -SC) homostructure. Owing to the different screening lengths exhibited in the two electrodes, the potential energy profile at the semiconductor/ferroelectric interface is asymmetric. Therefore, the resistance switching in 2D p -SC/FE/ n -SC FTJ is associated with the inequivalent tunneling barrier height on the reversal of polarization in the ferroelectric barrier. In addition, the electron tunneling transmission also relies exponentially on the barrier width. Very recently, an enhancement of TER effect depending on ferroelectric modulated barrier width by using a doped semiconductor as the electrode has been reported in experimental works 38-40 . Combining the phenomenological model and first-principles calculations, we demonstrate this 2D p -SC/FE/ n - SC FTJ also provides the possibility to realize this barrier width altering mechanism. Owing to the polarization field effect , the majority carriers can be accumulated or depleted near two doped semiconductor surfaces in response to the polarization reversal, leading to the tunability of effective tunneling barrier width. Furthermore, the interfacial ferroelectric insulator becomes conducting as a result of reversible metallization of the barrier according to the band alignment . Taking the two distinct contributions into account, the effective barrier width can be dynamically manipulated via the electrical control of ferroelectric polarization reversal. Consequently, through ferroelectric modulation of barrier height together with the barrier width in our novel 2D-FTJ, the electronic resistance through the junction can be switched between two states allowing either a large (ON) or small (OFF) flow of the current from one side to the other, is therefore expected to achieve an enhanced TER effect, offering a significant potential in the next-generation memory applications. The schematic diagram of the 2D p -SC/FE/ n -SC FTJ with two opposite ferroelectric states is depicted in Fig. 2a. Both electrodes can be realized by heavy impurity doping. Below a critical thin thickness, the behavior of electrons transporting through the barrier can be described by quantum mechanics, i.e., dominated by direct tunneling. According to the basic Wentzel-Kramers-Brillouin (WKB) approximation 43,44 , the TER ratio in terms of ferroelectric polarization reversal can be approximated as follows 2Δ TER = =exp RL L JJm d J Φ Φ − ℏ (1)
where J R ( J L ) is the current density in the high (low) conductance state, d is the width of ferroelectric barrier, Φ is the average potential barrier height, and Δ= RL ΦΦΦ − is the height change with reversal of polarization. It is noteworthy that the barrier width and height are the decisive factors for TER effect, indicating an effective mechanism to realize the TER enhancement as d and Δ Φ increase. p -SC/FE/ n -SC FTJ between “ON” (left panel) and “OFF” (right panel) states. (a) The respective schematics of 2D-FTJ as ferroelectric polarization is in P + x and P - x states. The black plus and minus symbols in the ferroelectric barrier region represent positive and negative ferroelectric bound charges, respectively. The red plus and blue minus in the semiconductor electrodes represent hole and electrons, respectively. The circled plus and minus symbols represent ionized donors and acceptors respectively. The distributions of (b) charge densities and (c) electrostatic potential energy profiles. (d) The overall potential energy profiles with corresponding band diagrams. . The 2D monolayer ferroelectric with barrier width d is sandwiched between p -type doped semiconductor (left electrode) and n -type doped semiconductor (right electrode) with corresponding screening widths of δ and δ , respectively. Here, we assume δ ＜ δ to illustrate the formation of asymmetric potential
energy. When the ferroelectric polarization is taken into account, the ferroelectric bound charge densities ± P bc are induced at the semiconductor/ferroelectric interfaces, as shown in Fig. 2b. In order to quantitatively understand the interplay between the resistive switching and tunneling barrier variation as the ferroelectric polarization reversed, we adopt a Thomas-Fermi screening model 46-48 , and the electrostatic potential within left and right electrodes can be expressed as: s 0FE , 1or2 i i i Pd i d δ ϕ δδ εεε εε = ± = ++ (2) where ε is the permittivity of free space, ε FE is the relative permittivity of the ferroelectric layer, and ε i is the dielectric permittivity in electrode i ( i =1 or 2). The sign of “±” depends on polarization orientation ( P s ) where positive and negative signs correspond to polarization pointing to and away from this electrode respectively. The Thomas-Fermi screening length in the left or right electrodes can be obtained by i i i e ε δ ρ = , where ρ i is the density of states at the Fermi energy. Based on the equation (2), the electrostatic potential energy at semiconductor/ferroelectric interfaces can be calculated as ()() eeeel ϕϕϕϕ≡<≡ due to the assumption of δ ＜ δ , resulting in asymmetric potential energy profiles, as shown in Fig. 2c. The overall barrier height Φ experienced by the tunneling electrons needs to include the potential energy barrier of the ferroelectric insulator. For simplification, a potential barrier height with forbidden gap E g seen by tunneling electrons is assumed for the barrier region in paraelectric state. When the ferroelectric polarization points to the right n -type doped electrode (labeled as P + x state), the average potential barrier height can be obtained as +(+)2 Rg Eeφφ Φ= , as seen in Fig. 2d. In addition to the barrier height, owing to the specific p -SC/FE/ n -SC homostructure, the band bending in response to ferroelectric polarization will influence the effective width of the tunneling barrier. Compared with the width of pristine ferroelectric barrier, the regions near left and right semiconductor/ferroelectric interfaces become conducting, reducing the effective barrier width d ＜ d . Therefore, the device under P + x state realizes a low resistance “ON” state. When polarization is pointing to the left p -type doped electrode (labeled as P - x state), the average potential barrier height +()2 Lg Eeφφ Φ=+ can be calculated analogous to that in the case of P + x state. With δ ＜ δ , the barrier height for right polarization Φ R is smaller than the Φ L for the polarization pointing to left, resulting in different height of tunneling barrier seen by transport electrons. More interestingly, the left and right semiconductor surfaces are depleted of holes and
electrons, respectively, as a result of ferroelectric polarization field effect. The majority carriers disappear near the semiconductor/ferroelectric interfaces, only leaving the immobile ionized acceptors and donors, as shown in the right panel of Fig. 2a. Under this condition, two separated insulating space charge regions will be created near the left and right electrode surfaces. Consequently, the tunneling electrons have to overcome an additional barrier. We further illustrate the mechanism by showing the band diagram in Fig. 2d. The valence band of p -type semiconductor bends downwards below the Fermi energy ( E F ), while the conduction band of n -type semiconductor bends beyond the E F , thus, increasing the effective barrier width, i.e., d ＞ d . Compared with the P + x state, a much wider and higher tunneling barrier is introduced in P - x state, driving the device enter a low tunneling transmission state, i.e., a high resistance “OFF” state. Therefore, the 2D p -SC/FE/ n -SC FTJs would provide a fascinating platform to realize enhanced TER effect through engineering the barrier width and barrier height electrically. To realize the model of p- SC/FE/ n -SC FTJ based on all-2D materials, we construct a homostructure implemented in the monolayer GIVMs 49-51 which display intrinsic in-plane ferroelectric polarization with high Curie temperature 52-54 . Taking monolayer SnSe as an example, the atomic structures of such 2D-FTJ at two polarization states have been illustrated in Fig. 3. The in-plane polarization P s of SnSe reaches up to 1.89×10 C/m. More excitingly, its estimated Curie temperature T c is around 460 K, implying the possibility of devices working at room temperature. Compared with the widely studied ferroelectrics such as BaTiO and PbTiO with out-of-plane ferroelectric polarization , the atomic-thick pristine SnSe displays in-plane ferroelectric polarization along x axis on the 2D plane, namely its P + x and P - x states. Note that the in-plane polarization along y axis is exactly equivalent to that along x axis owing to the symmetry of monolayer SnSe. The left electrode is hole doped by replacing a Sn cation with an In atom (In:SnSe), while the right electrode is electron doped with Sn substituted by Sb atom (Sb:SnSe). The doping concentration of each semiconductor electrode reaches up to 6.2×10 elections/cm , resulting in a heavily doped condition. Then, the homostructure is constructed by stacking unit cells along the [100] direction, where the pristine ferroelectric SnSe with 18 unit cells are in the middle of p -type and n -type semiconductor electrodes. In contrast to the conventional Metal- 1/ferroelectric/Metal-2 FTJ or all-oxide FTJ in which lattice mismatch is naturally generated by the electrodes and ferroelectric due to their different lattice constants, our In:SnSe/SnSe/Sb:SnSe homostructure completely eliminates the lattice constant mismatch and will not introduce abrupt structure distortions at the ferroelectric/electrode interfaces. More importantly, the ferroelectric polarization of monolayer SnSe in this 2D-FTJ is spontaneous, which avoids sophisticated external methods such as chemically functionalized approach to maintain ferroelectricity, making it more applicable for experimental investigations and use in further practical devices.
p -type semiconductor/ferroelectric/ n -type semiconductor homostructure. The green region is the ferroelectric SnSe monolayer. The red and blue arrows correspond to two opposite orientations of polarization along x axis. The grey regions represent the left and right electrodes composed of hole and electron doped SnSe semiconductors, respectively. Only a few unit cells are presented for illustrating the ferroelectric barrier region and electrode regions here because of the page width limitations. The stability and robustness of the in-plane ferroelectricity in 2D ferroelectric materials sandwiched between two doped electrodes lie at the heart of such 2D-FTJ device. To examine the stability of this in-plane ferroelectricity, we show the total energy evolution during the of ferroelectric switching in Fig. 4 by calculating the double-well potential profile for the In:SnSe/SnSe/Sb:SnSe homostructure. On account of the two types of electrodes, the energy potential profile shows an asymmetric feature associated with two inequivalent energy minima, which has also been observed in our previous study on tricolor-superlattice-based FTJ with asymmetric ferroelectric/electrode interfaces . The normalized atomic displacement λ is used to describe the ferroelectric distortion amplitude. Different from a free- standing ferroelectric material with symmetric potential profile, the paraelectric state with the energy maximum is approximately located around λ = 0.2, which slightly deviates from the coordinate of zero. The P - x state with the lowest energy occurs at λ = -1, while the P + x state is stabilized at λ = +1. The energy difference between these two opposite ferroelectric states is estimated to be 161.25 meV for the whole structure. These results confirm the stability of in-plane ferroelectricity in monolayer SnSe-based FTJ, and also reveal the characteristics of asymmetric ferroelectricity which will be important to achieve the enhanced TER effect in the following sections.
image.7.1[421*43] image.7.2[421*43] image.7.3[421*43]p- SC/FE/ n - The ion displacements along [100] directions are characterized by λ, where λ = +1 and -1 correspond to the ‘ P + x ’ and ‘ P - x ’ states, respectively. The energy is summed up to all the atoms in the p -SC/FE/ n -SC supercell. The ‘ P - x ’ state with minimum energy is set as the reference. Near the semiconductor/ferroelectric interfaces, the reversible metallization of insulating barrier may be brought about by polarization reversal . To study this underlying property and further understand the change of barrier width, we calculate the layer-resolved local density of states (LDOS) for this 2D p- SC/FE/ n -SC FTJ. As shown in Fig. 5a, for the hole doped electrode on the left, the valence band maximum (VBM) lies beyond the E F , whereas the conduction band minimum (CBM) dips below E F in the electron doped electrode on the right. According to the difference in work function step between two electrodes , a notable built-in electric field is generated here, pointing from right to left across the ferroelectric SnSe barrier. When the ferroelectric polarization is pointing to the right, the built-in electric field is parallel to the depolarizing field, leading to an obvious band bending existing in the band alignment, where the valence band or conduction band of ferroelectric SnSe layers near the interface lies in the Fermi level. As a result, the effective tunnel barrier for the electrons tunneling is reduced in terms of these additional conducting regions added on the left and right interfaces. If the polarization orientation is reversed to left, we can find a relatively slight band bending appearing in Fig. 5b as the built-in electric field is antiparallel to the depolarizing field. In addition, fewer SnSe unit cells become metallic
image.8.1[364*39] image.8.2[364*39] image.8.3[364*39] image.8.4[364*39] image.8.5[364*39] image.8.6[364*39] image.8.7[364*39]compared to Fig. 5a. Consequently, the effective tunneling barrier width in Fig. 5b is wider than that in Fig. 5a. Note that the same direction of band bending for two opposite ferroelectric states indicates the built-in electric field is stronger than the depolarizing field. The red and blue circles indicate the position of the CBM and VBM, respectively. The zero in energy axis is E F marked by the black dashed line. The red shaded areas denote the electrode regions, the grey shaded ones represent the reversible metallization areas near the interfaces and the green ones correspond to the effective ferroelectric barrier regions. Here each LDOS of SnSe regions is summed up in two unit cells of SnSe monolayer in sequence. Based on the above first-principles calculations, not only the height but also the width of the barrier can be tuned as a result of ferroelectric polarization reversal in 2D p- SC/FE/ n -SC FTJ, which is consistent with the phenomenological model analysis. In order to evaluate the performance of SnSe-based 2D p -SC/FE/ n -SC FTJ, non-equilibrium Green’s function formalism calculations implemented in Atomistix ToolKit (ATK) is used to study the electrical conductance and TER effect. The central scattering region of this tunneling device is constructed by pristine SnSe monolayer, with semi-infinite p- type and n -type SnSe electrodes attached on the left and right sides, respectively. Transmission coefficients and reflection matrices are determined by matching the wave functions of the scattering region with linear combinations of propagating Bloch states in the electrodes. Since the electronic states at the E F dominate the transport properties, the zero-bias electrical conductance within the Landauer-Büttiker formula can be evaluated as:
image.9.1[364*46] image.9.2[364*46] image.9.3[364*46] image.9.4[364*46] image.9.5[364*46] image.9.6[364*46]() 0F k =,k GGTE ? ? . (3) Here G = 2e / h is the conductance quantum, e is the electron charge and h is the Planck’s constant. The T ( E F , k || ) is transmission coefficient at the Fermi energy for a given Bloch wave vector k || = ( k x , k y ) in the two-dimensional Brillouin zone, which corresponds to the periodicity in the plane of the junction. In terms of integrating the transmission probability for states at the Fermi energy over the 2D Brillouin zone, we find that the total conductance G R = 1.295×10 G for the polarization state pointing to the right electrode. However, G L is decreased to be 8.306×10 G when the ferroelectric orientation is reversed to the left. Following the conventional definition , the TER ratio here is defined as: TER= RL L GG G − . (4) As a result, the reversal of ferroelectric polarization in such 2D p- SC/FE/ n -SC FTJ will lead to a significantly enhanced TER effect, which is approximately about 1460 %. Note that the migrations of electrons or holes near the semiconductor surfaces are nearly ignored during the ATK calculations, where only considering the electron cloud diffusion, making the change of barrier width underestimated by ferroelectric polarization reversal. Hence, a giant TER effect can be expected in the practical applications, which is in accord with the enhanced results reported in Wu’s work 38,40 . In order to intuitively understand the large changes in the conductance ratio during polarization reversal, the k || - resolved transmission at E F has been plotted in Fig. 6. For the in-plane ferroelectric polarization along x axis, the main contribution to the transmission comes from the region around the edges of 2D Brillouin zone. However, we notice that the transmission peak in the P - x state is significantly reduced compared to that of the P + x state, which verifies the characteristics of a sizable TER effect existing in this proposed In:SnSe/SnSe/Sb:SnSe homostructure. || -resolved transmission in 2D Brillouin zone at the Fermi energy through the tunnel junction. (a) Under P + x and (b) P - x states. Polarization directions are shown by arrows. The two panels use the same color bar.
image.10.1[433*28] image.10.2[433*28] image.10.3[433*28] image.10.4[433*28] image.10.5[433*28] image.10.6[433*28]Finally, we further study the electronic structure across the 2D p- SC/FE/ n -SC FTJ device by carrying out the analysis of real-space projected LDOS (PLDOS). The corresponding results for the two polarization states are plotted in Fig. 7, where the band bending at the left interface ( L interface ) and right interface ( R interface ) are clearly displayed, revealing the p - type and n -type doping features in the left and right electrodes, respectively. The green regions with high electronic density of states demonstrate the doped semiconductor electrodes. Moreover, the tunneling barrier width can be defined by the black regions without electronic states between the left and right electrodes. As expected, the effective tunneling regions will stay away from the initial semiconductor/ferroelectric interfaces due to the interfacial metallization of ferroelectric barrier. Note that the electrons tunnel across the junction under P - x state will experience a much wider barrier compared with that of the P + x state, which is completely consistent with our model analysis and calculations. p- SC/FE/ n -SC FTJ device with (a) P + x and (b) P - x states. The abscissa is the Cartesian coordinate of central region along the x -direction. The color bar on the right indicates the DOS amplitude. E ci and E vi are the c onduction band minimum and valance band maximum of the electrode with i -type ( i = p , n ) doping, respectively. The Fermi level (i.e., E Fp and E Fn ) is set to zero. The interface of semiconductor/ferroelectric is initially set to be located around 35 Å and 114 Å. To conclude, we have proposed an all-2D materials based ferroelectric tunnel junction with the structure configuration of p -type semiconductor/ferroelectric/ n -type semiconductor. We find that not only the tunneling barrier height but also the barrier width can be continuously tunable as a result of the polarization reversal. In particular, the mechanism of barrier width variation is not only ascribed to depletion/accumulation of majority carriers near the semiconductor surface in response to the reversal of ferroelectricity, but also influenced by the reversible metallization of barrier near the ferroelectric surface due to the band alignment. This well-designed 2D p -SC/FE/ n -SC FTJ brings up a new idea approaching the giant
image.11.1[488*26] image.11.2[488*26] image.11.3[488*26] image.11.4[488*26] image.11.5[488*26] image.11.6[488*26] image.11.7[488*26] image.11.8[488*26]TER effect for the non-volatile resistive switching memories, which is of great importance in paving the way for the blueprint of the future nanodevice applications. calculations. The geometry optimizations and electronic structure calculations of slab models are performed within density-functional theory (DFT) using the projector augmented wave (PAW) method , as implemented in the Vienna ab initio Simulation Package (VASP) 61,62 . The exchange correlation potential is described by generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functions . The kinetic-energy cutoff of 500 eV is applied to the plane wave expansion and a Γ-centered 1×12×1 k -points grid is adopted for Brillouin zone sampling. All the structures are optimized until the Hellmann-Feynman forces tolerance below 1 meV/Å and self-consistent convergence for electronic energy is 10 eV. A vacuum space of 15 Å is used to avoid interactions between adjacent layers. The device properties of the 2D-FTJs are calculated using density functional theory plus non-equilibrium Green’s function formalism 64,65 (DFT+NEGF approach) as implemented in Atomistix ToolKit-Virtual NanoLab (ATK-VNL) software package . Double-ζ plus polarization basis set is employed, and a real-space mesh cut-off energy of 80 Hartree is used to guarantee the good convergence of the device configuration. The electron temperature is set at 300K. The 1×21×101 k -points are used for the self-consistent calculations to eliminate the mismatch of Fermi level between electrodes and central region. An increased number 201 of k -pints along y axis is adopted during the calculations of transmission spectra and spatially resolved projected local density of states. The data that support the findings of this study are available from the corresponding author upon reasonable request. 1. Scott, J. F. Applications of modern ferroelectrics . Science , 954-959 (2007). 2. Hoffman, J. et al. Ferroelectric field effect transistors for memory applications . Adv. Mater. , 2957-2961 (2010). 3. Garcia, V. & Bibes, M. Inside story of ferroelectric memories . Nature , 279-281 (2012). 4. Chanthbouala, A. et al. A ferroelectric memristor . Nat. Mater. , 860-864 (2012). 5. Kim, D. J. et al. Ferroelectric tunnel memristor . Nano Lett. , 5697-5702 (2012). 6. Garcia, V. et al. Giant tunnel electroresistance for non-destructive readout of ferroelectric states . Nature , 81-84 (2009).
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to the reversible metallization of the barrier . Phys. Rev. Lett. , 197602 (2016). 43. Pantel, D. & Alexe, M. Electroresistance effects in ferroelectric tunnel barriers . Phys. Rev. B , 134105 (2010). 44. Velev, J. P., Burton, J. D., Zhuravlev, M. Y. & Tsymbal, E. Y. Predictive modelling of ferroelectric tunnel junctions . npj Comput. Mater. , 16009 (2016). 45. Gruverman, A. et al. Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale . Nano Lett. , 3539-3543 (2009). 46. Zhuravlev, M. Y., Sabirianov, R. F., Jaswal, S. S. & Tsymbal, E. Y. Giant electroresistance in ferroelectric tunnel junctions . Phys. Rev. Lett. , 246802 (2005). 47. Soni, R. et al. Giant electrode effect on tunnelling electroresistance in ferroelectric tunnel junctions . Nat. Commun. , 5414 (2014). 48. Tian, B. B. et al. Space-charge effect on electroresistance in metal-ferroelectric-metal capacitors . Sci. Rep. , 18297 (2015). 49. Gomes, L. C. & Carvalho, A. Phosphorene analogues: Isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure . Phys. Rev. B , 085406 (2015). 50. Xu, L., Yang, M., Wang, S. J. & Feng, Y. P. Electronic and optical properties of the monolayer group-IV monochalcogenides MX (M= Ge, Sn; X= S, Se, Te) . Phys. Rev. B , 235434 (2017). 51. Shen, X. W., Tong, W. Y., Gong, S. J. & Duan, C. G. Electrically tunable polarizer based on 2D orthorhombic ferrovalley materials . 2D Mater. , 011001 (2018). 52. Fei, R. X., Kang, W. & Yang, L. Ferroelectricity and phase transitions in monolayer group-IV monochalcogenides . Phys. Rev. Lett. , 097601 (2016). 53. Chang, K. et al. Discovery of robust in-plane ferroelectricity in atomic-thick SnTe . Science , 274-278 (2016). 54. Wu, M. H. & Zeng, X. C. Intrinsic ferroelasticity and/or multiferroicity in two-dimensional phosphorene and phosphorene analogues . Nano Lett. , 3236-3241 (2016). 55. Fang, Y. W. et al. First-principles studies of multiferroic and magnetoelectric materials . Sci. Bull. , 156-181 (2015). 56. Yang, Q., Xiong, W., Zhu, L., Gao, G. Y. & Wu, M. H. Chemically functionalized phosphorene: Two-dimensional multiferroics with vertical polarization and mobile magnetism . J. Am. Chem. Soc. , 11506-11512 (2017). 57. Gao, Y. C. et al. A first-principles study on the intrinsic asymmetric ferroelectricity of the SrTiO –BaTiO –CaTiO tricolor superlattice at the nanoscale . J. Phys. Condens. Matter , 165901 (2013). 58. Gerra, G., Tagantsev, A. K. & Setter, N. Ferroelectricity in asymmetric metal-ferroelectric-metal heterostructures: A combined first-principles-phenomenological approach . Phys. Rev. Lett. , 207601 (2007).
59. Landauer, R. Electrical resistance of disordered one-dimensional lattices . Phil. Mag. , 863-867 (1970). 60. Blöchl, P. E. Projector augmented-wave method . Phys. Rev. B , 17953-17979 (1994). 61. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set . Phys. Rev. B , 11169-11186 (1996). 62. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method . Phys. Rev. B , 1758-1775 (1999). 63. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple . Phys. Rev. Lett. , 3865- 3868 (1996). 64. Taylor, J., Guo, H. & Wang, J. Ab initio modeling of quantum transport properties of molecular electronic devices . Phys. Rev. B , 245407 (2001). 65. Brandbyge, M., Mozos, J.-L., Ordejón, P., Taylor, J. & Stokbro, K. Density-functional method for nonequilibrium electron transport . Phys. Rev. B , 165401 (2002). 66. Atomistix ToolKit version 2014.3-Virtual NanoLab version 2017.2, QuantumWise A / S (www.quantumwise.com) . This work was supported by the National Key Project for Basic Research of China (Grant No. 2014CB921104), National Key R&D Program of China (2017YFA0303403), the National Natural Science Foundation of China (Grant No. 11774092, 51572085) and Shanghai Science and Technology Innovation Action Plan (No. 17JC1402500). Computations were performed at the ECNU computing center. C.G.D conceived the idea and supervised the work. X.W.S, Y.W.F and B.B.T did the model analysis. X.W.S carried out the first-principles calculations and non-equilibrium Green’s function transport calculations and did the data analysis with the assistance of Y.W.F. B.B.T and Y.W.F contributed to the interpretation of the results. X.W.S, Y.W.F and C.G.D co-wrote the paper. All the authors reviewed and modified the manuscript. † Present address: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan


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| struct node_for * | rule_for |
| struct node_while * | rule_while |
| struct node_until * | rule_until |
| struct node_case * | rule_case |
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Sorry, you are on a smaller screen than intended. We wanted to make this fully responsive, but have been busy with other things.
An increase of 36% compared to 2013
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Generators: The Final FrontierCopyright (C) 2014David M. Beazley http://www.dabeaz.com Presented at PyCon'14, April 10, 2014, Montreal. IntroductionThis tutorial discusses advanced uses of using generators to alter program control flow, explode brains, and exponentially increase your job security. Topics include context managers, inlined futures, concurrency, asyncio, actors, compilers, and more.
Support Data FilesThe following file contains some supporting data files that are used by the various code samples. Download this to your machine to work the examples that follow. This download includes the presentation slides and all of the code sample below.Background MaterialThis tutorial is part 3 of a trilogy of tutorials involving generators and coroutines. Although it stands on its own, you may want to review the first two parts to get the bigger picture:
Code SamplesHere are various code samples that are used in the course. You can cut and paste these to your own machine to try them out. The order in which these are listed follow the course outline. You'll need Python 3.4.Part 2 : And Now For Something Completely Different
Part 4: Yield From Yield From Yield From Future
Part 5: GIL
Part 6 : Fake it Until You Make It (Actors)
Part 7: A Terrifying Visitor
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The primary type of documents on the internet are HTML. HTML (Hypertext Markup Language) documents are a tree-like structure made up of elements. These elements have a beginning and ending tag; of which, can be text, spans, paragraphs, etc. So you can have elements within other elements.
Tags make up elements and allow you to affect the output of the code that is written. You can turn words into links to other websites or even images. There are two types of tags inline and block. Inline allows the code to run continuously without structure around it. Whereas block contains the content within a virtual box with adjustable height and width.
CSS compliments HTML by providing style to the elements. These can include text & background color, alignment, height and width.
Avoiding repetition is important to avoid rework and errors. Applying the same style to multiple elements can mean having to reapply it over and over. Attention to detail when making the changes is crucial. If the overall design requires a consistent format, the programmer can assign the same div class to each element and make the changes simultaneously.
I really appreciate the feedback; however, it seems that I am getting conflicting messages. My last submittal suggested that I had too many duplicate tags because I used the "div class=header" tag for all my headers. I did that using the logic that I could style them all at the same time. I then adjusted to unique "div class=" and am now being asked to revert. Just trying to understand when is it appropriate. Thanks for all you do!


"Air Jordan"
In his first MVP season, Jordan created one of the best All-Star moments ever by taking off from the free-throw line in the Slam Dunk contest against Dominique Wilkins. It was a revelation that pushed the boundaries by atesting that dunking from the free-throw line was even possible.
"The Move"
In Game 2 of the 1991 Finals against the Lakers, Jordan pulled off what has become one of his most famous highlights -- the up-and-under, swooping layup in the lane. Jordan ended the night with 33 points on 15-of-18 shooting (making 13 in a row at one point), along with 13 assists, seven rebounds, two steals and a block. Chicago would win Game 2 and win the series in five games.
"Be Like Mike" Campaign
The Gatorade campaign began after Jordans' 1991 MVP season concluded, which debuted in August during the year of '91. But Jordan’s second MVP season – and his first title with the Chicago Bulls – spawned the "Be Like Mike" commercial, which has become iconic to modern pop culture.
Jordan taunts Mutombo
Jordan was a superb trash talker and loved to play mind games on the court, which was evident in November ’91, when he pulled a funny stunt on the great Dikembe Mutombo. Near the end of a Bulls victory, Mutombo was bantering with Jordan while MJ was at the free-throw line. Jordan laughed and said, 'This one’s for you,' before closing his eyes and draining the free throw. It’s one of MJ’s most memorable moments.
'92 Playoffs: Game 3 (vs. Heats)
In Game 3 of the first round of the '92 playoffs, Chicago had a chance to close out Miami on the road. Was Jordan going to let that chance pass? Nope. He dropped 56 points on the Heat, along with five rebounds, five assists, four steals and two blocks. Watching performances like this, you can understand why Jordan began to think, 'The NBA is too easy for me. Let's go play professional baseball instead (Chicago White Sox).'
'92 Playoffs: Game 7 (vs. Knicks)
After taking out the Heat in ’92, Jordan faced the Knicks in the Eastern Conference semifinals and was pushed to the limit. In Game 7, Jordan countered Patrick Ewing’s 22 points and John Starks’ 18 with 42 of his own, including six rebounds and four assists. -- It was step two of a completely dominant postseason for MJ.
"The Shrug"
Step three -- not including defeating Cleveland in the Eastern Conference finals -- was the ’92 Finals against the Portland Trail Blazers, which included a top-three MJ moment: The Shrug. In Game 1 against the Blazers, Jordan dropped 35 in the first half (ended with 39).
Father's Day
Jordan won his fourth NBA title on Father’s Day 1996 in his first full season back from retirement. We remember it because of the backstory. Part of the reason Jordan retired was to cope with his father’s murder and pursue the baseball dream he had always shared with his dad. An emotional Jordan collapsed in tears after the game, providing one of the most poignant moments of his career.
"The Last Shot"
With a little more than five seconds left in Game 6 (NBA series are best of 7) of the '98 finals, Jordan crossed over Utah’s Bryon Russell and sank a jumper to give the Bulls the lead and clinch MJ’s sixth championship with Chicago. It’s widely regarded as the best shot Jordan ever made.
"The Flu Game"
It's been speculated that MJ actually played hungover rather than what game 5 of the '97 finals vs. Utah was known for, which was that MJ got out of bed at 5:50 p.m due to a flu but still played before tip-off at 7 o'clock. Regardless, he ended game 5 with 38 points and a clutch 3 pointer in the final 25 seconds of the 4th quarter!


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CREATE DATABASE
Section: SQL Commands (7)Updated: 2012-08-14
Index Return to Main Contents
NAME
CREATE DATABASE - create a new databaseSYNOPSIS
CREATE DATABASE name [ [ WITH ] [ OWNER [=] dbowner ] [ TEMPLATE [=] template ] [ ENCODING [=] encoding ] [ LC_COLLATE [=] lc_collate ] [ LC_CTYPE [=] lc_ctype ] [ TABLESPACE [=] tablespace ] [ CONNECTION LIMIT [=] connlimit ] ]
DESCRIPTION
CREATE DATABASE creates a new PostgreSQL database.
To create a database, you must be a superuser or have the special CREATEDB privilege. See CREATE USER [create_user(7)].
Normally, the creator becomes the owner of the new database. Superusers can create databases owned by other users, by using the OWNER clause. They can even create databases owned by users with no special privileges. Non-superusers with CREATEDB privilege can only create databases owned by themselves.
By default, the new database will be created by cloning the standard system database template1. A different template can be specified by writing TEMPLATE name. In particular, by writing TEMPLATE template0, you can create a virgin database containing only the standard objects predefined by your version of PostgreSQL. This is useful if you wish to avoid copying any installation-local objects that might have been added to template1.
PARAMETERS
- name
- The name of a database to create.
- dbowner
- The name of the database user who will own the new database, or DEFAULT to use the default (namely, the user executing the command).
- template
- The name of the template from which to create the new database, or DEFAULT to use the default template (template1).
- encoding
- Character set encoding to use in the new database. Specify a string constant (e.g., 'SQL_ASCII'), or an integer encoding number, or DEFAULT to use the default encoding (namely, the encoding of the template database). The character sets supported by the PostgreSQL server are described in in the documentation. See below for additional restrictions.
- lc_collate
- Collation order (LC_COLLATE) to use in the new database. This affects the sort order applied to strings, e.g. in queries with ORDER BY, as well as the order used in indexes on text columns. The default is to use the collation order of the template database. See below for additional restrictions.
- lc_ctype
- Character classification (LC_CTYPE) to use in the new database. This affects the categorization of characters, e.g. lower, upper and digit. The default is to use the character classification of the template database. See below for additional restrictions.
- tablespace
- The name of the tablespace that will be associated with the new database, or DEFAULT to use the template database's tablespace. This tablespace will be the default tablespace used for objects created in this database. See CREATE TABLESPACE [create_tablespace(7)] for more information.
- connlimit
- How many concurrent connections can be made to this database. -1 (the default) means no limit.
Optional parameters can be written in any order, not only the order illustrated above.
NOTES
CREATE DATABASE cannot be executed inside a transaction block.
Errors along the line of ``could not initialize database directory'' are most likely related to insufficient permissions on the data directory, a full disk, or other file system problems.
Use DROP DATABASE [drop_database(7)] to remove a database.
The program createdb [createdb(1)] is a wrapper program around this command, provided for convenience.
Although it is possible to copy a database other than template1 by specifying its name as the template, this is not (yet) intended as a general-purpose ``COPY DATABASE'' facility. The principal limitation is that no other sessions can be connected to the template database while it is being copied. CREATE DATABASE will fail if any other connection exists when it starts; otherwise, new connections to the template database are locked out until CREATE DATABASE completes. See in the documentation for more information.
The character set encoding specified for the new database must be compatible with the chosen locale settings (LC_COLLATE and LC_CTYPE). If the locale is C (or equivalently POSIX), then all encodings are allowed, but for other locale settings there is only one encoding that will work properly. (On Windows, however, UTF-8 encoding can be used with any locale.) CREATE DATABASE will allow superusers to specify SQL_ASCII encoding regardless of the locale settings, but this choice is deprecated and may result in misbehavior of character-string functions if data that is not encoding-compatible with the locale is stored in the database.
The encoding and locale settings must match those of the template database, except when template0 is used as template. This is because other databases might contain data that does not match the specified encoding, or might contain indexes whose sort ordering is affected by LC_COLLATE and LC_CTYPE. Copying such data would result in a database that is corrupt according to the new settings. template0, however, is known to not contain any data or indexes that would be affected.
The CONNECTION LIMIT option is only enforced approximately; if two new sessions start at about the same time when just one connection ``slot'' remains for the database, it is possible that both will fail. Also, the limit is not enforced against superusers.
EXAMPLES
To create a new database:
CREATE DATABASE lusiadas;
To create a database sales owned by user salesapp with a default tablespace of salesspace:
CREATE DATABASE sales OWNER salesapp TABLESPACE salesspace;
To create a database music which supports the ISO-8859-1 character set:
CREATE DATABASE music ENCODING 'LATIN1' TEMPLATE template0;In this example, the TEMPLATE template0 clause would only be required if template1's encoding is not ISO-8859-1. Note that changing encoding might require selecting new LC_COLLATE and LC_CTYPE settings as well.
COMPATIBILITY
There is no CREATE DATABASE statement in the SQL standard. Databases are equivalent to catalogs, whose creation is implementation-defined.
SEE ALSO
ALTER DATABASE [alter_database(7)], DROP DATABASE [drop_database(7)]
Index
This document was created by man2html, using the manual pages.
Time: 05:34:14 GMT, December 24, 2015


Use your arrow keys or swipe to combine similar Eric Andres and score points!
Unlock all 11 Eric Andres to win!
EricAndre-ified by Dubchuck
2048 Created by Gabriele Cirulli.
Fork at https://github.com/dubchuck/2048ericandre
Based on 1024 by Veewo Studio and conceptually similar to Threes by Asher Vollmer.
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BATTLE FURY
4350
Passive: Cleave - Deals a percent of attack damage in a 225 AoE around the target. Does not work on ranged heroes.
Cleave damage on non-primary targets is not reduced by armor values.
+ 65 Damage
+ 6 HP Regeneration
+ 150% Mana Regeneration
CLEAVE DAMAGE: 35%
The bearer of this mighty axe gains the ability to cut down swaths of enemies at once.


Welcome to Greater Noida Developer Group
We are the group of students and professional who help other people to get involved with some technology area. So, we conduct meetups for that. Its a volunteer driven community. If you want to help us out please feel free to contact us. And want to conduct meetup at your college or place please let us know.


"PHICOL "great, the chief captain of the army of Abimelech, the Philistine king of Gerar. He entered into an alliance with Abraham with reference to a certain well which, from this circumstance, was called Beersheba (q.v.),“the well of the oath” (Genesis 21:22, 32; 26:26).
"PHICOL "great, the chief captain of the army of Abimelech, the Philistine king of Gerar. He entered into an alliance with Abraham with reference to a certain well which, from this circumstance, was called Beersheba (q.v.),“the well of the oath” (Genesis 21:22, 32; 26:26).