KCdF3:Cr3+晶体EPR零场分裂参量与电荷补偿效应的研究

按照叠加模型与EPR零场分裂参量的三阶微扰理论,建立了KCdF₃:Cr³⁺晶体EPR零场分裂参量与四角对称Cr³⁺-Cd²⁺缺陷中心局域结构之间的定量关系.证实Cd²⁺空位与晶格畸变的存在,我们获得:围绕Cr³⁺离子的六个F^-配体分别向中心Cr³⁺移动Δ₁=0.00294nm,Δ₂=0.0010nm,Δ₃=0.0028nm (参见图2).EPR零场分裂参量与实验一致表明:Cd²⁺空位与晶格畸变的假设是合理的.尽管四角晶场主要来自Cd²⁺空位,但晶格畸变的贡献不可忽略.     

一类非公度体系的电子动力学

研究了格点势能V_n=λtanh［Acos(2πσn)］/tanhA,σ=(5-1)/2的一类非公度体系的电子动力学性质-分析了体系中不同相区内波函数的特征,发现延展态的波函数类似于Bloch波,在一定范围内体系中存在临界态-计算了自关联函数C(t)和均方位移d(t),分析了它们的长时行为,发现当体系各态均为延展态时,C(t)～t^-1,d(t)～t¹;当体系各态均为局域态时,C(t)～t⁰,d(t)～t^{0关键词：}     

C2H221分子转动角动量定向分布(Orientation)及其碰撞弛豫和转移

利用圆偏振激光受激Raman抽运,以 C₂H₂分子为样品选择性地制备了它的电子基态单一转动态(X¹Σ⁺_g,ν″₂＝1,J″的角动量定向布居(orientation)．并从圆偏振紫外激光诱导的A¹A_u(ν′₃＝1)←X¹Σ⁺_g(ν″₂＝1)的荧光（谱）,直接测定了 C₂H₂(X¹Σ^＋_g,ν″₂＝1,J″＝4,7,8,…,13）的角动量定向布居值．从时间分辨的荧光信号谱测定了角动量定向布居的碰撞弛豫速率常数,同时还研究了由各初始激励的转动态向其他邻近转动态碰撞诱导的角动量定向布居转移． 关键词：     

KMgF3:Cr3+的EPR参量和K+空位研究

EPR和ENDOR表明:顺磁离子取代KMgF₃中的Mg²⁺离子后,所处的环境具有轴对称。本文研究了KMgF₃:Cr³⁺的Δg=g_∥-g_⊥和零场分裂值D,证实KMgF₃:Cr³⁺中K⁺空位的存在。并且指出:由于K⁺空位,使Cr³⁺的最近邻F^-离子分 关键词：     

HF分子电子基态和激发态的MRCI计算

应用舍Davidson修正的多参考组态相互作用（MRCI）方法,在aug-cc-pVTZ基组水平上对HF基态及最低的多个单重和三重电子激发态进行了势能扫描计算.结合群论原理及分子的离解极限,分析了电子态势能曲线的特征,得出激发态B¹S⁺对应的离解极限为H⁺+F^-（¹S）.基于势能曲线,数值求解核运动的径向Schrodinge方程,得到J=0时束缚电子态X¹S⁺,B¹S⁺,C¹P和D¹S⁺的振动能级和转动常数,继而进行数据拟合得到电子态的光谱常数,基态X¹S⁺:ω_e=4146.94 cm^-1,ω_ez_e=88.08 cm^-1,B_e=21.22 cm^-1,a=0.785 cm^-1;B¹S⁺态:ω_e=1131.37 cm^-1,ω_ex_e=17.28 cm^-1,B_e=3.96 cm^-1,a_e=0.0215 cm^-1,C¹P态:ω_e=2696.37 cm^-1,ω_ex_e=73.43 cm^-1,B_e=15.91 cm^-1,a_e=0.776 cm^-1,D¹S⁺态:ω_e=3104.22 cm^-1,ω_ex_e=118.92 cm^-1,B_e=17.25 cm^-1,a_e=0.992 cm^-1,拟合结果与实验值吻合的较好.     

Co2+-Ti4+和Cu2+-Nb5+离子取代BaFe12O19单晶体中Fe3+离子的研究

本文对用Co²⁺-Ti⁴⁺和Cu²⁺-Nb⁵⁺离子取代BaFe₁₂O₁₉单晶体中Fe³⁺离子进行了研究,以Bi₂O₃作为助熔剂生长出了BaFe_12-2xCo_x²⁺Ti_x⁴⁺O₁₉(x=0;0.04;0.09;0.13;0.27和0.68)以及BaFe_12-x[Nb_1/3⁵⁺Cu_2/3²⁺]_xO₁₉(x=0;0.28;0.44和0.60)这两系列的单晶体,测定了100—300K温度范围内样品的磁化强度σ与单轴各向异性常数K₁,我们发现,对Co²⁺-Ti⁴⁺取代的样品,当x≤0.09时,其σ与K₁随x的增加而缓慢增加;当x>0.09时,其σ与K₁随x的增加而迅速降低,至x=1.1时,K₁变为零,对Nb⁵⁺-Cu²⁺取代的样品,其σ值在整个成份范围内基本保持不变,且有缓慢增加趋势;而K₁值则随x增加而单调下降,提出了取代离子在M型六角铁氧体中可能的分布模型来解释我们的结果。 关键词：     

试验优化设计Er3+/Yb3+共掺BaGd2ZnO5荧光粉及其上转换发光性质

为得到绿光和红光最大发光强度的Er³⁺/Yb³⁺共掺BaGd₂ZnO₅上转换材料荧光粉, 首先采用试验优化设计中的均匀设计初步寻找Er³⁺/Yb³⁺合理的掺杂浓度; 其次通过二次通用旋转组合设计进一步优化实验, 建立起Er³⁺/Yb³⁺掺杂浓度与绿光和红光发光强度的回归方程; 最后通过遗传算法计算出方程的最优解, 即绿光和红光最大发光强度时对应的Er³⁺/Yb³⁺掺杂浓度. 利用传统的高温固相法分别制备出最优样品. 采用X 射线衍射对得到荧光粉的晶体结构进行了分析, 证明了所有产物均为纯相BaGd₂ZnO₅. 采用980 nm抽运激光作为激发源, 在同样的条件下测量了样品的上转换荧光发射光谱, 从中可见样品有较强的红光发射和绿光发射, 发光中心位于662, 551和527 nm, 分别对应于⁴F_9/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 及²H_11/2→⁴I_15/2能级跃迁. 研究了绿光和红光最优样品的上转换发光强度与激光器工作电流之间的关系, 通过分析发现红色和绿色上转换发光均为双光子过程. 由归一化的绿色上转换发射光谱可以看出, 激光器工作电流导致的样品温度变化可以忽略不计. 由于能级²H_11/2和⁴S_3/2之间存在热平衡, 并满足玻尔兹曼分布, 由此探讨了绿光最优样品上转换发射光谱中的绿色发射与温度的关系, 计算出²H_11/2和⁴S_3/2之间的能级差为ΔE=926.11 cm^-1. 研究了绿光最优样品的温度效应, 随着温度的升高, 发射强度逐渐变小, 出现了温度猝灭现象. 并计算了样品的激活能, 分别为总体激活能ΔE_总=0.45 eV, 绿光激活能ΔE_绿=0.45 eV, 红光激活能ΔE_红=0.46 eV.     

PrNi$lt;sub$gt;2$lt;/sub$gt;的磁和磁热性能研究

在一些磁性材料内, 磁性离子间交换作用和磁性离子的自旋涨落对材料磁性有影响. 本文根据磁比热实验值确定了晶场参数后, 利用包含自旋涨落的交换作用有效场H_m= n₀ (1 + γ T + β e^{ω T})M, 计算了PrNi₂晶体晶场能级的Zeeman劈裂. 在温度为3.8 K ≤T≤ 30 K范围内, 计算了该晶体多晶磁矩随外磁场的变化, 以及外磁场H=5000 Oe时磁化率倒数随温度的变化, 计算结果和实验值符合较好. 当外磁场在0–50000 Oe时, 计算的该晶体的磁熵变与已有文献的理论结果相似. 计算结果说明, 提出的包含自旋涨落的交换作用有效场不仅适合亚铁磁性晶体, 而且也适合顺磁性晶体. 关键词： 2')" href="#">PrNi₂ 磁比热 交换作用有效场 磁矩 磁熵变     

CaBa-铝酸盐玻璃中Cr4+谱线宽度随温度的变化

测量了15~300K温度范围内57.5%Al_1.5O-35%CaO-7.5%BaO玻璃中四价铬的发射光谱.这种材料中铬离子的能级处于Tanabe-Sugano图上弱场范围中,最低的激发态是³T₂,发射谱是一个宽带.按照单频近似理论拟合低温下的光谱,得到³T₂能级的零声子线位置E_zp=8400cm^-1,声子能量tω=320cm^-1,黄昆因子S=358.尽管单频近似能够较好地描述低温下的线形,发射光谱宽度随温度的变化却与单频近似理论的结果不符.讨论了这种差别的原因,认为可能的解释是与激发态耦合的声子能量大于与基态耦合的声子能量.     

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。     

Spontaneous scale symmetry breaking in 2+1-dimensional QED at both zero and finite temperature

A complete analysis of dynamical scale symmetry breaking in 2+1-dimensional QED at both zero and finite temperature is presented by looking at solutions to the Schwinger-Dyson equation. In different kinetic energy regimes we use various numerical and analytic techniques (including an expansion in large flavour number). It is confirmed that, contrary to the case of 3+1 dimensions, there is no dynamical scale symmetry breaking at zero temperature, despite the fact that chiral symmetry breaking can occur dynamically. At finite temperature, such breaking of scale symmetry may take place. Received: 17 August 2000 / Revised version: 24 November 2000 / Published online: 23 January 2001     

Double cascade turbulence and Richardson dispersion in a horizontal fluid flow induced by Faraday waves

We report the experimental observation of Richardson dispersion and a double cascade in a thin horizontal fluid flow induced by Faraday waves. The energy spectra and the mean spectral energy flux obtained from particle image velocimetry data suggest an inverse energy cascade with Kolmogorov type scaling E(k) ∝ k(γ), γ ≈ -5/3 and an E(k) ∝ k(γ), γ ≈ -3 enstrophy cascade. Particle transport is studied analyzing absolute and relative dispersion as well as the finite size Lyapunov exponent (FSLE) via the direct tracking of real particles and numerical advection of virtual particles. Richardson dispersion with <ΔR(2)(t)> ∝ t(3) is observed and is also reflected in the slopes of the FSLE (Λ ∝ ΔR(-2/3)) for virtual and real particles.     

Thermodynamics of a Charged Hairy Black Holein (2+1) Dimensions

In this paper we study thermodynamics, statistics and spectroscopic aspects of a charged black hole with a scalar hair coupled to the gravity in (2+1) dimensions. We obtained effects of the black hole charge and scalar field on the thermodynamical and statistical quantities. We find that scalar charge may increase entropy, temperature and probability, while may decrease black hole mass, free and internal energy. Also electric charge increases probability and decreases temperature and internal energy. Also we investigate stability of the system and find that the thermodynamical stability exists.     

1+1 dimensional compactifications of string theory

We argue that stable, maximally symmetric compactifications of string theory to 1+1 dimensions are in conflict with holography. In particular, the finite horizon entropies of the Rindler wedge in 1+1 dimensional Minkowski and anti-de Sitter space, and of the de Sitter horizon in any dimension, are inconsistent with the symmetries of these spaces. The argument parallels one made recently by the same authors, in which we demonstrated the incompatibility of the finiteness of the entropy and the symmetries of de Sitter space in any dimension. If the horizon entropy is either infinite or zero, the conflict is resolved.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Linked cluster expansions for U(1) lattice gauge theory in 2 + 1 and 3 + 1 dimensions

Compact U(1) lattice gauge theory is studied in 2 + 1 and 3 + 1 dimensions using strong coupling series expansions and the recently proposed exact linled cluster expansion alborithm Results for the vacuum energy, specific heat and axial string tension in 2 + 1 dimensions are in agreement with previous finite lattice estimates. In 3 + 1 dimensions, we present new strong coupling series results (order g^?40) which together with the ELCE estimates show evidence of a continuous phase transition at x = 1/g⁴ = 0.72 ± 0.08. The associated critical index for the vanishing string tension is μ = 0.65 ± 0.12. The axial string tension in D = 3 + 1 appears to undergo a non-deconfining roughening transition at smaller x (0.56 ± 0.07).     

Predicting spinor condensate dynamics from simple principles

We study the spin dynamics of quasi-one-dimensional F=1 condensates both at zero and finite temperatures for arbitrary initial spin configurations. The rich dynamical evolution exhibited by these nonlinear systems is explained by surprisingly simple principles: minimization of energy at zero temperature and maximization of entropy at high temperature. Our analytical results for the homogeneous case are corroborated by numerical simulations for confined condensates in a wide variety of initial conditions. These predictions compare qualitatively well with recent experimental observations and can, therefore, serve as a guidance for ongoing experiments.     

Finite-temperature perturbation theory for the random directed polymer problem

We study the random directed polymer problem—the short-scale behavior of an elastic string (or polymer) in one transverse dimension subject to a disorder potential and finite temperature fluctuations. We are interested in the polymer short-scale wandering expressed through the displacement correlator 〈[δu(X)]²〉, with δu(X) being the difference in the displacements at two points separated by a distance X. While this object can be calculated at short scales using the perturbation theory in higher dimensions d > 2, this approach becomes ill-defined and the problem turns out to be nonperturbative in the lower dimensions and for an infinite-length polymer. In order to make progress, we redefine the task and analyze the wandering of a string of a finite length L. At zero temperature, we find that the displacement fluctuations 〈[δu(X)]²〉 ∝ LX ² depend on L and scale with the square of the segment length X, which differs from a straightforward Larkin-type scaling. The result is best understood in terms of a typical squared angle 〈α²〉 ∝ L, where α = ? _x u, from which the displacement scaling for the segment X follows naturally, 〈[δu(X)]²〉 ∝ 〈α²〉 X ². At high temperatures, thermal fluctuations smear the disorder potential and the lowest-order results for disorder-induced fluctuations in both the displacement field and the angle vanish in the thermodynamic limit L → ∞. The calculation up to the second order allows us to identify the regime of validity of the perturbative approach and provides a finite expression for the displacement correlator, albeit depending on the boundary conditions and the location relative to the boundaries.     

Temperature fluctuations for a finite system of classical spin-1/2 particles

For sufficiently large but finite N independent spin-1/2 paramagnets in a constant magnetic field, the orientation of every spin has the same probability to be parallel or anti-parallel to that of the magnetic field. In our statistical ensemble treatment, all fundamental thermodynamic quantities fluctuate, including the entropy and temperature, and all fluctuations are gradually vanishing as the temperature itself approaches zero. Our approach presents a remedy of infinite temperature fluctuations and reproduces other reasonable results that are given by standard theory.     

Numerical study of the directed polymer in a 1 + 3 dimensional random medium

The directed polymer in a 1+3 dimensional random medium is known to present a disorder-induced phase transition. For a polymer of length L, the high temperature phase is characterized by a diffusive behavior for the end-point displacement R² ∼L and by free-energy fluctuations of order ΔF(L) ∼O(1). The low-temperature phase is characterized by an anomalous wandering exponent R²/L ∼L^ω and by free-energy fluctuations of order ΔF(L) ∼L^ω where ω∼0.18. In this paper, we first study the scaling behavior of various properties to localize the critical temperature T_c. Our results concerning R²/L and ΔF(L) point towards 0.76 < T_c ≤T₂=0.79, so our conclusion is that T_c is equal or very close to the upper bound T₂ derived by Derrida and coworkers (T₂ corresponds to the temperature above which the ratio remains finite as L ↦ ∞). We then present histograms for the free-energy, energy and entropy over disorder samples. For T ≫T_c, the free-energy distribution is found to be Gaussian. For T ≪T_c, the free-energy distribution coincides with the ground state energy distribution, in agreement with the zero-temperature fixed point picture. Moreover the entropy fluctuations are of order ΔS ∼L^1/2 and follow a Gaussian distribution, in agreement with the droplet predictions, where the free-energy term ΔF ∼L^ω is a near cancellation of energy and entropy contributions of order L^1/2.