由N-E-V分布及赝势法研究弱磁场中弱相互作用费米子气体的热力学性质

基于低温下量子系统的相关实验多是在体积、能量和粒子数都可变的外场束缚下进行的事实, 由体积、能量和粒子数可变的完全开放系统的统计分布(N-E-V分布)研究了弱磁场中弱相互作用费米系统的热力学性质. 首先求出了一般情况下由费米积分表示的内能和热容的解析表达式. 在此基础上, 又给出了在低温极限条件下内能与热容的解析表达式和数值计算结果, 并将N-E-V分布(粒子数密度变化)的结果与赝势法(粒子数密度不变)的结果进行了比较. 结果表明: N-E-V分布方法的计算结果总是补偿赝势法计算结果的过度偏差. 由N-E-V 分布方法所得结果最特异之处在于: 在低温条件下, 弱磁场中弱相互作用费米系统存在一相变温度t_c, 其正处于费米系统发生玻色-爱因斯坦凝聚(BEC)和费米原子形成库珀对的超流状态(BCS)相变及BEC-BCS跨越的温度范围内, 且不随反映弱相互作用大小和特征的散射长度a (a<0引力, a>0斥力)变化, 但随弱磁场的加强而降低, 即弱磁场可调节该相变温度. 磁场为零时, 相变温度最高, 为费米温度的0.184倍.     

高温超导体混合态磁通涡旋结构

利用紧束缚模型,对s-d混合波的存在性作了讨论.在Ginzburg-Landan理论框架下,严格求解了高温超导体混合配对态磁通涡旋结构.结果表明,其磁通涡旋的分布与单个涡旋的结构密切相关.在低温下,由于序参量s波成分的存在及它与d波分量的耦合,引起局域磁场和d波分量的各向异性,使磁通涡旋呈斜格子分布,而在温度近于转变温度时,各向异性消失,涡旋态转化为三角格子.这与YBa₂Cu₃O₇样品的小角度中子散射实验的结果相符合. 关键词：     

基于I-V-T和C-V-T的GaN上Ni/Au肖特基接触特性研究

基于对制作在n-GaN上的肖特基二极管的变温I-V测试和C-V测试，采用表面势垒减薄模型对肖特基二极管的电流输运特性进行了研究.试验结果表明，肖特基接触的电流输运机理非常复杂，在不同的温度条件和偏压条件下有着不同的电流输运机理.在此基础上对肖特基接触I-V特性方程进行了修正，得到了很好的拟合曲线.试验表明，高温I-V法提取的势垒高度与常温C-V法提取的势垒高度接近于根据金属功函数得出的理论势垒高度值. 关键词： 氮化镓 肖特基二极管 表面势垒减薄模型 热电子场发射     

InGaN/GaN多量子阱蓝色发光二极管的实验与模拟分析

分别采用量子阱模型和量子点模型对蓝色InGaN/GaN多量子阱发光二极管电学和光学特性进行模拟,并和实验测量结果进行了比对,结果发现,量子点模型的引入,很好地解决了I-V和电致发光二方面的实验与理论模型间符合程度不好的问题.同时,在I-V曲线特性模拟中发现,在量子点理论模型的基础上,只有考虑到载流子的非平衡量子传输效应,才能得到和实验相接近的I-V曲线,揭示着在InGaN/GaN 多量子阱发光二极管电输运特性中,载流子的非 关键词： InGaN/GaN 发光二极管 数值模拟 量子点模型     

ITO/有机半导体/金属结构OLED器件的数值模拟

基于稳态的小信号漂移扩散方程,建立了有电极的单层有机电致发光(OLED)器件的数值模型,编制的MATLAB程序,首先模拟了文献中的OLED器件电极附近正电荷层(面电荷)对器件J-V的影响,得到了和文献中一致的结果. 模拟了ITO/PPV/Ca结构的OLED器件,模拟时,考虑了OLED阳极附近存在正体电荷,得到的J-V曲线和文献中的实验结果一致,体电荷产生了势垒,影响了电流曲线. 关键词： 有机电致发光 数值模拟 漂移扩散模型     

Aq++H碰撞中的电荷转移

利用CTMC方法计算C^q+,N^q+,O^q+与H原子的碰撞过程的截面.入射粒子的能量范围为10~250keV/amu,离子为部分电离和完全电离的离子.给出了十二个耦合的Hamilton运动方程.用六个伪随机数确定粒子的轨道. A^q+离子和作用电子的势为模型势,其他两个势为纯库仑势。在计算中,需要2000个以上的轨道.引入约化变量σ_tr=σ_tr/q,E=E/q^1/2,得到了q标度的俘获截面,这些截面数据都集中在一条曲线附近.计算结果与其他理论计算结果作了比较,它们符合得很好.     

硼掺杂对四面体非晶碳膜电导性能的影响

以单质硼和高纯石墨的混合粉末压制成型的靶材作为靶源,采用过滤阴极真空电弧技术制备不同硼含量的掺硼四面体非晶碳膜.分别采用四探针法、阻抗分析仪和电化学界面对薄膜的变温电导率、I-V特性和C-V特性进行了测试和研究.实验结果表明,当B含量由0增加至6.04 at%时,薄膜的室温电导率先逐渐增大而后逐渐减小,相应薄膜的电导激活能先逐渐减小而后逐渐增大,并在2.13 at%时分别出现最大和最小值1.42×10^-7S/cm和0.1eV.此外,掺硼四面体非晶碳/n型硅异质结的I-V曲线表现出典型的整流特性,表明p-n结二极管已经形成,且结两端的掺杂能级在空间上连续统一. 关键词： 四面体非晶碳 电导率 I-V曲线')" href="#">I-V曲线 C-V曲线')" href="#">C-V曲线     

荷电流体中静电关联效应的有效势模型

基于Nordholm关于等离子体的库仑关联孔穴的概念,提出了描述荷电流体的有效库仑势模型.采用经典密度泛函理论,计算了受限于纳米微腔中荷电流体的平衡结构.通过对比计算结果,研究了粒子间的关联效应对体系平衡密度分布和剩余吸附量的影响.此外,计算并研究了较大尺寸受限空间中荷电粒子的关联效应对体系结构的影响.研究结果表明:本文提出的有效两体势模型可有效地预测荷电粒子间的库仑关联对体系结构和物理化学性质的影响.研究结果可为研究和理解其他复杂的势模型体系中的关联效应提供可能的线索.     

一维扩展t-J模型中密度-自旋相互作用诱导的相分离

t-J模型是研究高温超导电性的重要理论模型之一. 最近的冷分子实验表明可用极性分子模拟t-J模型. 实验模拟的t-J模型除了引进长程的偶极相互作用外, 还引进了密度-自旋相互作用. 本文使用密度矩阵重整化群方法研究了密度-自旋相互作用对一维t-J 模型基态性质的影响. 选取了t-J模型基态相图中不同相区的三个点, 计算了不同密度-自旋相互作用强度下的粒子数和自旋的实空间分布,以及密度-密度关联函数和自旋-自旋关联函数与相应的结构因子. 计算结果表明, 密度-自旋相互作用强度较弱时, 对系统的性质不会产生定性影响;当其强度足够大时, 系统会进入相分离, 该相分离与传统t-J模型的相分离有很大区别.     

应变SiGe SOI量子阱沟道PMOSFET阈值电压模型研究

在绝缘层附着硅(SOI)结构的Si膜上生长SiGe合金制作具有SiGe量子阱沟道的SOI p型金属氧化物半导体场效应晶体管(PMOSFET)，该器件不仅具有SOI结构的优点，而且因量子阱中载流子迁移率高，所以进一步提高了器件的性能.在分析常规的Si SOI MOSFET基础上，建立了应变SiGe SOI 量子阱沟道PMOSFET的阈值电压模型和电流-电压(I-V)特性模型，利用Matlab对该结构器件的I-V特性、跨导及漏导特性进行了模拟分析，且与常规结构的器件作了对比.模拟结果表明，应变SiGe SOI量子阱沟道PMOSFET的性能均比常规结构的器件有大幅度提高. 关键词： 应变SiGe SOI MOSFET 阈值电压 模型     

Ferromagnetism in the Hubbard model: instability of the Nagaoka state on the square lattice

The instability of the fully polarized ferromagnetic ground state (Nagaoka state) of the Hubbard model on the square lattice is investigated. We use single spin flip variational wave functions including majority spin correlation effects and calculate spin flip energies in the thermodynamic limit. With very local wave functions and with moderate numbers of variational parameters we reproduce the best known estimate for the critical hole density δ_cr = 0.29 and we obtain an estimate of U_cr = 63 t for the critical coupling which is considerably better than the best estimate of U_cr = 42 t previously known. The simplicity of our wave functions makes the physical origin of the various aspects of the instability particularly transparent.     

Ferromagnetism in the Hubbard model on the square lattice: Improved instability criterion for the Nagaoka state

The instability of the fully polarized ferromagnetic state (Nagaoka state) with respect to single spin flips is re-examined for the Hubbard model on the square lattice with a large family of variational wave functions which include correlation effects of the majority spins in the vicinity of the flipped spin. We find a critical hole density of δ_cr = 0.251 for U = ∞ and a critical coupling of U_cr = 77.7t. Both values improve previous variational results considerably.     

Superconductivity with s and p symmetries in an extended Hubbard model with correlated hopping

We consider a generalized Hubbard model with on-site and nearest-neighbour repulsions U and V respectively, and nearest-neighbour hopping for spin up (down) which depends on the total occupation n_b of spin down (up) electrons on both sites involved. The hopping parameters are t _AA , t_AB and t_BB for n _b =0,1,2 respectively. We briefly summarize results which support that the model exhibits s-wave superconductivity for certain parameters and extend them by studying the Berry phases. Using a generalized Hartree-Fock(HF) BCS decoupling of the two and three-body terms, we obtain that at half filling, for t _AB <t _AA =t _BB and sufficiently small U and V the model leads to triplet p-wave superconductivity for a simple cubic lattice in any dimension. In one dimension, the resulting phase diagram is compared with that obtained numerically using two quantized Berry phases (topological numbers) as order parameters. While this novel method supports the previous results, there are quantitative differences. Received: 2 February 1998 / Accepted: 17 March 1998     

Magnetic properties of Mn-doped GaN with defects: ab-initio calculations

According to first-principles density functional calculations,we have investigated the magnetic properties of Mn-doped GaN with defects,Ga 1-x-y V Gx Mn y N 1-z-t V Nz O t with Mn substituted at Ga sites,nitrogen vacancies V N,gallium vacancies V G and oxygen substituted at nitrogen sites.The magnetic interaction in Mn-doped GaN favours the ferromagnetic coupling via the double exchange mechanism.The ground state is found to be well described by a model based on a Mn 3+-d 5 in a high spin state coupled via a double exchange to a partially delocalized hole accommodated in the 2p states of neighbouring nitrogen ions.The effect of defects on ferromagnetic coupling is investigated.It is found that in the presence of donor defects,such as oxygen substituted at nitrogen sites,nitrogen vacancy antiferromagnetic interactions appear,while in the case of Ga vacancies,the interactions remain ferromagnetic;in the case of acceptor defects like Mg and Zn codoping,ferromagnetism is stabilized.The formation energies of these defects are computed.Furthermore,the half-metallic behaviours appear in some studied compounds.     

The t‐U‐V‐J model in cuprates: Strengths of the interactions

The t‐U‐V‐J model fits together three major parts of the superconductivity puzzle of the cuprite compounds: (i) it describes the opening of a d‐wave pairing gap, (ii) it is consistent with the fact that the basic pairing mechanism arises from the antiferromagnetic exchange correlations, and (iii) it takes into account the charge fluctuations associated with double occupancy of a site which play an essential role in doped systems. The strengths of the interactions U, V and J in YBa₂Cu₃O_6.7 and La_2‐xSr_xCuO₄ (_x = 0.16) samples are obtained by requiring quantitative consistency between the angle‐resolved photoemission spectroscopy (ARPES) measurements, the sharp collective mode at the antiferromagnetic wave vector Q _AF=(π,π), and the observed inelastic neutron scattering resonance (INSR) positions of the incommensurate peaks at wave vectors Q _δ = ((1 ± δ)π,π) and Q _δ = (π(1 ± δ)π).     

Spin waves in the block checkerboard antiferromagnetic phase

Motivated by the discovery of a new family of 122 iron-based superconductors, we present the theoretical results on the ground state phase diagram, spin wave, and dynamic structure factor obtained from the extended J₁-J₂ Heisenberg model. In the reasonable physical parameter region of K₂Fe₄Se₅, we find that the block checkerboard antiferromagnetic order phase is stable. There are two acoustic spin wave branches and six optical spin wave branches in the block checkerboard antiferromagnetic phase, which have analytic expressions at the high-symmetry points. To further compare the experimental data on neutron scattering, we investigate the saddlepoint structure of the magnetic excitation spectrum and the inelastic neutron scattering pattern based on linear spin wave theory.     

Phase diagram of the extended hubbard model with pair hopping interaction

A one-dimensional model of interacting electrons with on-site U, nearest-neighbor V, and pair-hopping interaction W is studied at half-filling using the continuum limit field theory approach. The ground state phase diagram is obtained for a wide range of coupling constants. In addition to the insulating spin-density wave (SDW) and charge-density wave (CDW) phases for large U and V, respectively, we identify a bond-charge-density-wave (BCDW) phase W < 0, | U - 2V| < | 2W| and a bond-spin-density-wave (BSDW) for W > 0, | U - 2V| < W. The possibility of bond-located ordering results from the site-off-diagonal nature of the pair-hopping term and is a special feature of the half-filled band case. The BCDW phase corresponding to an enhanced Peierls instability in the system. The BdSDW is an unconventional insulating magnetic phase, characterized by a gapless spin excitation spectrum and a staggered magnetization located on bonds between sites. The general ground state phase diagram including insulating, metallic, and superconducting phases is discussed. A transition to the η-superconducting phase at | U - 2V| ≪ 2t?W is briefly discussed. Received 20 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002     

On the ground state of the half-filled Hubbard model

We calculate the ground state of the half-filled Hubbard model and its energy by starting from a spindensity wave approximation and improving it by incorporating transverse spin fluctuations. The calculations are done by employing a projection method. The quality of the proposed approximation is particularly high for intermediate and large Coulomb repulsionU, where it exceeds considerably e.g. that of the Gutzwiller projected spin-density wave state. To ordert ²/U (wheret is the hopping matrix element), our approximation is shown to be equivalent to a recent Coupled Cluster calculation for the Heisenberg antiferromagnet. Finally we show how to ordert ²/U the linear spin-wave approximation for the Heisenberg antiferromagnet may be obtained.     

ReSi1.75电子结构的第一性原理研究

基于第一性原理全势线性缀加平面波方法和局域密度近似(LDA)，对ReSi_1.75的基态晶格属性进行了研究. 结构优化的结果表明，ReSi_1.75的基态平衡晶格常数比实验值小约0.6%. 在LDA计算基础上，考虑局域的Re的d电子库仑作用，用LDA+U方法计算了ReSi_1.75的电子结构，发现当U_eff=U-J=4.4eV时，能带结构呈半导体性质. 具有0.12eV 关键词： 1.75')" href="#">ReSi_1.75 局域密度近似 自相互修正作用 电子结构     

The value of the cosmological constant

We make the cosmological constant, Λ, into a field and restrict the variations of the action with respect to it by causality. This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it requires that the cosmological constant measured today, t _U , be L ~ t_U^-2 ~ 10^-122{\Lambda \sim t_{U}^{-2} \sim 10^{-122}} , as observed. This is the classical value of Λ that dominates the wave function of the universe. Our new field equation determines Λ in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature parameter of the universe is W_k0 o -k/a₀²H²=-0.0055{\Omega _{\mathrm{k0}} \equiv -k/a_{0}^{2}H^{2}=-0.0055} , which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological history.