La0.67Sr0.08Na0.25MnO3的奇特输运性质及CMR效应

用固相反应法制备了La_0.67Sr_0.08Na_0.25MnO₃样品.通过磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线以及ρ-T拟合曲线研究了样品的输运性质及庞磁电阻(colossal magnetoresistance，CMR)效应.结果表明，ρ-T曲线和磁电阻-温度(MR-T)曲线均出现双峰现象；高温峰是伴随顺磁-铁磁(PM-FM)相变出现绝缘体-金属(I-M)相变，低温峰是颗粒界面效应；两个绝缘相输运机理不同：较低温度下(248K<T<274K)，ρ(T)符合极化子的可变程跃迁模型，而在更高温区(330K<T<374K)，ρ(T)符合极化子近邻跃迁模型；两个类金属相输运机理也不同：在低温区(67K<T<186K)，满足ρ-T^2.5关系，输运机理是自旋波散射和电-磁子散射作用，而在高温区(292K<T<304K)，满足ρ-T²关系，输运机理是单磁子散射作用. 关键词： 庞磁电阻 金属-绝缘体转变 晶界效应 输运行为     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

伽玛暴180620A外激波的能量注入研究

通过GRB 180620A研究伽玛射线暴余辉阶段的能量注入行为。通过数据拟合,我们发现GRB 180620A的X射线余辉存在一个平台,同时Swift的UVOT望远镜的观测也显示GRB 180620A的光学余辉存在一个平台结构。这个平台可能起源于中心引擎的能量注入。我们在这样的图像下,拟合了余辉的观测数据,并估计了瞬时辐射的辐射效率。     

Sr2CoO4-δ薄膜中的自旋玻璃态的磁性质研究

用脉冲激光沉积方法(PLD)在铝酸镧衬底上制备了c取向的高氧空位含量的锶钴氧薄膜.X射线衍射分析表明薄膜单一取向且没有明显杂相.原位的高气压反射式高能电子衍射仪(RHEED)监测显示,薄膜为层状生长.通过对薄膜磁化强度随温度、磁场及时间的变化曲线进行测量,发现零场冷曲线上可能存在两个特征温度:T_f和T_a.T_f为对应玻璃态的冻结温度而T_a对应少量的不缺 关键词： 自旋玻璃 超交换相互作用 双交换相互作用 脉冲激光沉积     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

VOn±(n=0,1,2)的势能函数与光谱常数研究

用密度泛函B3LYP/6-311++G(d,p)方法和相对论有效实势(Lanl2dz基组)对VO^n±(n=0,1,2)分子离子的势能函数及光谱常数进行了分析. 结果表明它们都能稳定存在, 其基态电子状态分别是:⁴Σ(VO^2-), ³Σ(VO^-), ⁴Σ(VO), ³Σ(VO⁺)和²Σ(VO²⁺). 其中VO^2-和VO²⁺的势能函数曲线呈“火山口”型, 属于亚稳态分子离子. 用七参数Murell-Sorbie势拟合VO^2-和VO²⁺分子亚稳态双原子分子离子势能函数, 发现其拟合曲线与势能函数曲线符合得很好. 同时,讨论了电荷对势能函数和能级的影响. 关键词： 分子离子 密度泛函理论 势能函数 能级     

非线性耗散对亚稳态系统量子衰变速率的影响

发展了一种快速傅里叶变换路径积分方法，研究非线性耗散系统的量子衰变速率，得到了Bounce轨道的作用量S_B，即衰变速率的指数因子.在系统与环境存在非线性耦合f(x)=tanh［λ(x-x_b)］的情形下，发现其对衰变速率具有抑制作用.指数因子随温度T的关系不再满足S_B=a［1-b(T/T_c)²］法则；与通常的线性耗散情形相比，跨越温度T_c回升，即系统更早地进入穿透区域. 关键词： 量子衰变 非线性耦合 路径积分 快速傅里叶变换     

Eu掺杂TbMnO3多晶材料的介电性质

采用固相反应法制备了Tb_0.8Eu_0.2MnO₃多晶材料.对样品的X射线衍射（XRD）分析表明Eu³⁺固溶于TbMnO₃中.测量了样品在低温(100 K ≤T≤ 300 K)和低频下(200 Hz≤f≤100 kHz)的复介电性质.在此温度区间内发现了两个介电弛豫峰.经分析认为低温峰（T≈170 K）起源于局域载流子漂移引起的偶极子极化效应,而高温峰（T≈290 K）则是由离子电导产生的边界和界面层的电容效应引起的.电阻率的测量显示在低温下（T≈230 K）存在明显的导电机制转变. 关键词： 多铁性材料 掺杂 介电性质     

光解苯半醌自由基的CIDEP谱

用高时间分辨ESR谱仪实验给出了光解对苯醌产生的苯半醌自由基的CIDEP发射谱.用三重态机理,同时考虑了自由基对机理的影响,从理论上分析了极化强度,计算出了增强因子V(Ⅰ)=-0.66,V(R)=0.2,一级反应速率k_1,A=0.05/μs,二级反应速率β=0.02/μs,横向和纵向弛豫时间T₁≈T₂=2.3μs.     

未掺杂反式聚乙炔(CH)x中核自旋晶格弛豫-孤子-维扩散模型

Nechtschein等人报道并分析了反式聚乙炔中质子自旋晶格弛豫时间对拉摩频率ω和温度T的依赖关系。观察到了质子自旋晶格弛豫速率T₁^-1和ω^-1/2的正比关系。但是在高频段,T₁^-1∝ω^-1/2关系发生偏离,且温度越低,发生偏离的频率也越低。 本文用另一种方法对这些实验结果作了分析。首先,论证了孤子一维扩散模型的合理性。排除了质子弛豫速率∝ω^-1/2的另一种解释,即仅仅是核自旋向着静止的顺磁中心扩散。孤子能处在运动状态或静止状态。当温度降低时,发生两个效应,即越来越少的孤子处于运动状态,且运动孤子的扩散系数减小。只有扩散的孤子对所观察到的质子弛豫有贡献,而固定孤子的贡献可以忽略。其次,描述了运动孤子的一维随机行走模型,计算了它的相关函数和谱密度函数。质子自旋晶格弛豫速率是: 其中C是运动孤子的浓度,τ是运动孤子沿链跳跃时,渡越相邻位置的跳跃时间,ω是质子的拉摩频率。 这个公式揭示了质子弛豫速率的频率和温度依赖关系的主要特征。它和Nechtschein的测量结果拟合得很好。从拟合中可以得到各个温度下运动孤子的跳跃时间和相对浓度。     

Cosmic γ-ray bursts: Observations and modeling

It is now commonly accepted that cosmic γ-ray bursts (GRBs) are of cosmological origin. This conclusion is based on the statistical analysis of GRBs and the measurements of line redshifts in GRB optical afterglows, i.e., in the so-called long GRBs. In this review, the models of radiation and models of GRB sources are considered. In most of these models, if not in all of them, the isotropic radiation cannot provide the energy release necessary for the appearance of a cosmological GRB. No correlation is noted between the redshift, the GRB-spectrum shape, and the total detected energy. The comparison between data obtained in the Soviet experiment KONUS and the American experiment BATSE shows that they substantially differ in statistical properties and the detection of hard x-ray lines. The investigation of hard gamma (0.1–10 GeV) afterglows, the measurement of prompt optical spectra during the GRB detection, and the further investigation of hard x-ray lines is of obvious importance for gaining insight into the GRB origin. Observations of two bright optical GRB afterglows point to the fact that an initially bright optical flare is directly related to the GRB itself, and the subsequent weak and much more continuous optical radiation is of a different nature. The results of observations of optical GRB afterglows are discussed. They point to the fact that the GRBs originate in distant galaxies with a high matter density, where intense star formation takes place. The interaction of the cosmological GRB radiation with a dense surrounding molecular cloud results in the appearance of long-duration (up to 10 years) weak optical afterglows associated with the heating and reradiation of gas. Results of 2D numerical simulation of the heating and reradiation of gas in various variants of the relative disposition of GRB and molecular clouds are presented. In conclusion, the possible relation between the so-called short GRBs and recurrent sources of soft γ rays in our Galaxy, the so-called “soft gamma repeaters,” is discussed.     

用羊八井ASγ实验数据寻找TeV能区的γ射线暴

γ射线暴的TeV能区辐射对研究其起源、辐射机制等是非常重要的.利用西藏羊八井ASγ实验三期阵列的重建数据,通过在给定的小天区和时间间隔内寻找较高显著性事例团的方法对TeV能区的γ射线暴进行了寻找,在计算过程中采用“等天顶角法”来估计背景.工作中采用了两种途径来寻找γ射线暴,一种是与卫星γ射线暴的符合寻找,另一种是全天区独立寻找.结果发现少量事例团对背景有明显超出,考虑试验次数后,其超出还不足以认定为γ射线暴.通过Monte Carlo模拟,给出了在95%置信水平下,到达大气顶部流强上限的估计值为3.32×10^-9—1.24×10^-7 cm^-2s^-1. 关键词： γ射线暴 TeV能区 ASγ实验 宇宙射线     

Search for strong gravitational lensing effect in the current GRB data of BATSE

Because gamma-ray bursts(GRBs)trace the high-z universe,there is an appreciable probability for a GRB to be gravitational lensed by galaxies in the universe.Herein we consider the gravitational lensing effect of GRBs contributed by the dark matter halos in galaxies.Assuming that all halos have the singular isothermal sphere(SIS)mass profile in the mass range 1010h?1M?M2×1013h?1M?and all GRB samples follow the intrinsic redshift distribution and luminosity function derived from the Swift LGRBs sample,we calculated the gravitational lensing probability in BATSE,Swift/BAT and Fermi/GBM GRBs,respectively.With an derived probability result in BATSE GRBs,we searched for lensed GRB pairs in the BATSE5B GRB Spectral catalog.The search did not find any convincing gravitationally lensed events.We discuss our result and future observations for GRB lensing observation.     

Correlation between pulse-width and peak luminosity of single-pulse long gamma-ray bursts

With a sample of 21 single-pulse GRBs with redshift measurement and a sample of 65 single-pulse GRBs with pseudo redshift estimated by the luminosity-spectral lag relation, we show that the peak luminosities of GRB pulses are anti-correlated with pulse width, indicating that longer-pulses tend to be dimmer in the gamma-ray band. The anti-correlation is accessible with an internal shock scenario.     

GRB 090423: Marking the death of a massive star at z=8.2

GRB 090423 is the new high-z record holder of Gamma-ray bursts (GRBs) with z～ 8.2. We present a detailed analysis of both the spectral and temporal features of GRB 090423 observed with Swift/BAT and Fermi/GBM. We find that the T ₉₀ observed with BAT in the 15–150 keV band is 13.2 s, corresponding to ～ 1.4 s at z=8.2. It once again gives rise to the issue of whether the progenitors of high-z GRBs are massive stars or mergers since the discovery of GRB 080913 at z=6.7. In comparison with the T ₉₀ distribution in the burst frame of the current redshift-known GRB sample, we find that it is marginally grouped into the long group (Type II GRBs). The spectrum observed with both BAT and GBM is well fitted by a power-law with exponential cutoff, which yields an E _p=(50.4±7.0) keV. The event satisfies the Amati-relation well for Type II GRBs within their 3σ uncertainty range. Our results indicate that this event would be produced by the death of a massive star. Based on the Amati-relation, we derive its distance modulus, which follows the Hubble diagram of the concordance cosmology model at a redshift of ～8.2.     

Reconstructing the cosmic expansion history up to redshift <Emphasis Type="Italic">z</Emphasis>=6.29 with the calibrated gamma-ray bursts

Recently, Gamma-Ray Bursts (GRBs) were proposed to be a complementary cosmological probe to type Ia supernovae (SNIa). GRBs have been advocated to be standard candles since several empirical GRB luminosity relations were proposed as distance indicators. However, there is a so-called circularity problem in the direct use of GRBs. Recently, a new idea to calibrate GRBs in a completely cosmology independent manner has been proposed, and the circularity problem can be solved. In the present work, following the method proposed by Liang et al., we calibrate 70 GRBs with the Amati relation using 307 SNIa. Then, following the method proposed by Shafieloo et al., we smoothly reconstruct the cosmic expansion history up to redshift z=6.29 with the calibrated GRBs. We find some new features in the reconstructed results.     

Constraining Lorentz invariance violation from the continuous spectra of short gamma-ray bursts

In some quantum gravity theories, a foamy structure of space-time may lead to Lorentz invariance violation(LIV). As the most energetic explosions in the Universe, gamma-ray bursts(GRBs) provide an effect way to probe quantum gravity effects. In this paper, we use the continuous spectra of 20 short GRBs detected by the Swift satellite to give a conservative lower limit of quantum gravity energy scale MQG. Due to the LIV effect, photons with different energy have different velocities. This will lead to the delayed arrival of high energy photons relative to low energy ones. Based on the fact that the LIV-induced time delay cannot be longer than the duration of a GRB,we present the most conservative estimate of the quantum gravity energy scales from 20 short GRBs. The strictest constraint, M_(QG) 5.05 × 10~(14) GeV in the linearly corrected case, is from GRB 140622 A. Our constraint on MQG,although not as tight as previous results, is the safest and most reliable so far.     

Modeling the radio and optical/NIR afterglows of GRB 980703: A numerical study

Extensive multi-band afterglow data are available for GRB 980703. Especially, its radio afterglow was very bright and was monitored until more than 1000 days after the trigger time. Additionally, there is no obvious special feature, i.e., no rebrightenings, no plateau, and no special steep decay or slow decay in the multi-band afterglow light curves. All these conditions make GRB 980703 a precious sample in gamma-ray burst research. Here we use the observational data of GRB 980703 to test the standard fireball model in depth. It is found that the model can give a satisfactory explanation to the multi-band and overall afterglow light curves. The beaming angle of GRB 980703 is derived as ∼ 0.23 radian, and the circum-burst medium density is ∼ 27 cm⁻³. The total isotropic equivalent kinetic energy of the ejecta is ∼ 3.8 × 10⁵² ergs. A rest-frame extinction of A _V ∼ 2.5 mag in the host galaxy is also derived.     

The flux-E p relation within GRB060218 in comparison with typical GRB pulses

The prompt gamma-ray/X-ray emission of gamma-ray burst (GRB) 060218 was simultaneously observed by the Burst Alert Telescope (BAT) and X-ray Telescope (XRT) onboard Swift. Its peak energy of the joint vf _v spectrum (E _p) clearly evolves with time from tens of keV to ∼1 keV, crossing both the BAT and XRT bands. The best fit yields log E _p=(4.61±0.23)+(−1.29±0.08) log t, with a correlation coefficient of 0.98 and a chance probability of p<10⁻⁴. We derive its bolometric flux (F) in the 0.01–10⁴ keV band, and find that its F-E _p relation, with a power-law index of 0.37, is much shallower than that observed in typical GRB pulses. Discussion of this shallowness is presented.

     

Relationship between width of pulses and Lorentz factor expected from the light curve of fireball sources

Time profiles of many gamma-ray bursts consist of distinct pulses, which provides a possibility of characterizing the temporal structure of these bursts. We employ a simple model of highly symmetric fireballs to analyse the effect of the expansion speed on the light curve arising from different forms of local pulses. The relationship between the ratio r of the FWHM width of the rising phase of the light curve to that of the decaying phase and the Lorentz factor is investigated. The analysis shows that, when the rest frame radiation form is ignored, temporal profiles of the light curve arising from pulses of fireballs will not be affected by the expansion speed （that is, r is almost a constant） as long as the fireball expands relativistically. When the rest frame radiation form is taken into account, there will be a break in the curves of r - log F. The location of the break depends mainly on the adopted value of the rest frame peak frequency VO,p. One would reach almost the same result when a jet is considered. In addition, we utilize a sample of 48 individual GRB pulses to check the relationship between the ratio r and the expansion speed F. We find no significant correlation between them, and this is consistent with the theoretical analysis.