非晶态(Fe1-xVx)84B16合金的磁性和电性

用单辊急冷法制备了非晶态(Fe_1-xV_x)₈₄B₁₆(x=0,0.02,0.04,0.06,0.10)合金的薄带,分别用磁天平和四端引线法测量了饱和磁化强度和高温电阻率的温度关系。得到平均每个磁性原子的磁矩随V含量的增加近似线性下降,计算出每个Fe原子和每个V原子的平均磁矩分别为2.08μ_B和-5.08μ_B。居里温度T_c从x=0时的622K下降到x=0.10时的478K。利用自旋波激发公式:σ(T)=σ(0)(1-BT^* 关键词：     

McMillan Tc公式的解析推导(Ⅰ)——μ*=0情形

作者在μ^*=0情形,从Eliashberg方程解析地导出如下的T_c公式:T_c=αω_logexp{-b((1+cλ)/λ)},式中α=2γ/π,b=c=1;Inγ=C=0.5772是Euler常数。这个T_c公式只有在T_c=0.36/α(k)以下才是正确的,α是个大于1并随材料而异的常数。我们推测,当T_c超过上述范围后,T_c公式的函数结构很可能不同于McMillan T_c公式,至少α,b和c等参量不再是些不依赖于材料的常数了。 关键词：     

Lifshitz时空s波超导模型的关联长度和穿透深度

在D=d+2维各向异性的Lifshitz黑洞时空背景中, 在探子极限下, 用解析方法研究了临界温度附近引力系统的微扰, 计算出超导的关联长度ξα(1/T_c)(1-(T/T_c)^-1/2, 这与平均场论的结果一致. 进一步, 考虑在该系统中加一个均匀外磁场, 计算出穿透深度λα(T_c-T)^-1/2, 该结果与Ginzburg-Landau理论相符.     

高Tc超导体的反铁磁理论计算

考虑到高T_c超导材料层状桔构特点,从三维各向异性Hubbard模型出发,在大U时导出了自旋波线性近似下的有效哈密顿量。利用格林函数运动方程技术,计算了系统的子晶格磁化强度、内能、比热、平行磁化率和垂直磁化率等物理量。结果表明,层间和层内反铁磁耦合强度之比δ=J_⊥/J对这些物理量有重要影响。在低温T?2J(2+δ)/k_B下,确定了特征温度T₀=2J(2δ(2+δ))^1/2/k_B,分别给出了T?T₀和T?T₀时,这些物理量的渐近表达式。 关键词：     

超导临界温度级数公式的应用

从超导临界温度级数公式(1)出发,得到了B类超导体同位素效应的表达。在μ^*=0及μ^*≠0时,我们得到了T_c上限的级数表达式,其结果与数值解符合甚好。最后,在μ^*=0时,讨论了T_c与各种谱参数的关系。由级数表达式可以清楚地看出,当固定不同参数时,T_c的行为完全不同,从而统一解释了不同作者关于声子谱对T_c影响的不同结论。我们着重指出,在大多数情况下,级数公式(1)仅取前两项,可作为数值解的很好近似,从而大大简化了级数公式。预计可以由此简化公式出发研究其它问题。 关键词：     

FeSi/Si成份调制膜的二维磁性和死层效应

成功地制成了FeSi/Si非晶态成分调制膜,固定Si层厚度,系统改变FeSi层厚度d_m发现,随d_m下降,饱和磁化强度M_s单调减小,这主要是死层效应的贡献,OK下死层厚度约为5.8?,随d_m减小,调制膜逐渐由三维磁性转变为二维磁性,表现为出现低磁滞和无磁滞效应,线性的M_s(T)-T关系和Curie温度T_c的单调下降,T_c满足如下关系:△T_{c关键词：}     

(BiPb)-Sr-Ca-Cu-O2223相高Tc超导体磁通钉扎与蠕动

本文介绍了(BiPb)-Sr-Ca-Cu-O2223相高T_c超导体烧结工艺,用振动样品磁强计测量了磁化强度地豫率随外磁场的变化,并相应求得钉扎势U₀随外磁场的依赖关系。发现在0—0.23T外磁场范围内U₀与外磁场B的关系为U₀-B^-1/2与Palstra对单晶用电阻展宽实验所得结果有类似的函数形式。 关键词：     

钼酸钆晶体铁弹相变的超声衰减和内耗

用四个频率(f=31,50,72,93MHz)超声纵波测量了钼酸钆晶体铁弹铁电相变(T_c=160.90℃)引起的超声衰减(α_s)峰。得到较准确的α_s正比f²的关系,求得T_c以上α_s的临界指数从1.52±0.12变到0.89±0.12。有渡越现象。用低频(f=1Hz和150kHz)测出了相变内耗峰,肯定其为相界面运动所引起。并求得界面运动的临界应力,从而确定了本文中测得的超声衰 关键词：     

McMillan Tc公式的解析推导(Ⅲ)

本文把作者在前面两篇文章导出的T_c公式推广成下面形式:T_c=αω_log(ω_log/ω_c)^{(μ*/(λ-μ*))exp{-(1+λ)/(λ-μ*)},并从线性Eliashberg方程出发,导出了计算α的方程组。α一般是λ和μ*的函数。在弱耦合极限下,由上述方程组解得,α=2γ/π,其中lnγ=C=0.5772是Euler常数。这个结果表明了,前面两篇文章得到的T_c公式在弱耦合极限下是正确的。作者进而在Einstein谱和μ*=0情形,用数值计算方法从定α的方程组算出当λ=0.23,0.25,0.38和0.48时,a的数值。结果表明,至少在0.23≤λ≤0.45区间中,α变化很小,近似等于1/1.30。此时,本文的T_c公式实际上就是Allen及Dynes修改后的经验的McMillan T_c公式。 关键词：}     

非晶态超导体转变温度Tc的经验公式

本文提出一个非晶态非过渡金属超导体的T_c经验公式,T_c=Aλ<ω>^1/2/(<ω>/ω₀+(1+λ)/20),式中A=(1/5)^(K1/2)。计算值和实验值,以及和Garland理论值的比较表明,T_c经验公式能很好地描述非晶态超导体的T_c值。 关键词：     

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe,Ru, Os)

The superconducting transition temperature, T_c, of the LaT₄P₁₂ compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of T_c from T_c (P = 0) = 4.1 K at a rate (dT_c/dP)_P=0= +7.2 x 10^-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of T_c from T_c (P = 0) = 7.2 K and 1.8 K with (dT_c/dP)_P=0= -1.6 x 10^-1 K/GPa and -9.5 x 10^-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of T_c(P) in terms of the structural characteristics of the RT₄X₁₂ class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that T_c(P) for these materials consists of two competing contributions: a depression of T_c due to the compression of the lattice (i.e., decrease in volume), and an enhancement of T_c due to the effect of pressure on La itself.     

Thermodynamics of SU(3) lattice gauge theory

The pressure and the energy density of the SU(3) gauge theory are calculated on lattices with temporal extent N_τ = 4, 6 and 8 and spatial extent N_σ = 16 and 32. The results are then extrapolated to the continuum limit. In the investigated temperature range up to five times T_c we observe a 15% deviation from the ideal gas limit. We also present new results for the critical temperature on lattices with temporal extent N_τ = 8 and 12. At the corresponding critical couplings the string tension is calculated on 32⁴ lattices to fix the temperature scale. An extrapolation to the continuum limit yields T_c/√σ = 0.629(3). We furthermore present results on the electric and magnetic condensates as well as the temperature dependence of the spatial string tension. These observables suggest that the temperature dependent running coupling remains large even at T ≅ 5T_c. For the spatial string tension we find √σ_s/T=0.566(13)g²2(T) with g² (5T_c) ≅ 1.5.     

Relaxation of the ideal Bose gas

For the ideal Bose gas we study the approach to equilibrium. Above the critical temperature we prove exponential behaviour, with a relaxation time of the order (T-T _c)^-2 around T _c. For T _c we find the t ^-1 law for the two-point function.     

非晶态FeZrB合金的电阻率与温度的关系

本文研究了非晶态(Fe_1-xZr_x)_84.5B_15.5(x=0,0.02,0.04,0.06,0.08,0.1,0.15)和Fe_90-xB_xZr₁₀(x=0,4,10,16,20)合金的电阻率ρ与温度T的关系。实验结果表明,当Zr含量在0.02≤x≤0.08时,ρ-T曲线出现两个线性斜率,在略高于居里温度T_c处出现转折,在T关键词：     

非晶态Fe87-xSixB13合金的磁化强度、电阻与温度的关系

本文研究了非晶态Fe_87-xSi_xB₁₃(x=0,9.6,14.5)合金的饱和磁化强度、电阻率与温度的关系。得到样品的居里温度T_C和晶化温度T_cr随Si含量的增加而明显提高。低温下的热磁关系符合布洛赫的T^3/2定律,计算出自旋波劲度系数D从x=0时的62meV·A²增加到x=14.5时的111meV·A²。从D值和Handrich理 关键词：     

A型超导体的近似临界温度公式(Ⅰ)——μ*=0情形

本文用数值解方法从Eliashberg方程计算出超导临界温度T_c,并考察T_c对有效声子谱的依赖关系。在这个研究中,a²F(ω)被取为双δ函数谱,并允许其中的谱参数可以在很宽范围内改变。作者发现在λ<∧区域(即在T_c级数解的收敛圆外),T_c除了依赖λ和矩比外,还依赖T_c级数解的收敛半径倒数Λ;它们之间的关系是有规律的。在这些结果的启示下,本文在μ^*=0情形,用弥合数值解的方法得到一个适用于λ<Λ区域的T_c近似公式。接着,本文作者对吉光达和吴杭生的一篇文章进行了研究,指出:该文提出的超导体分类建议及其工作的主要结论是对的。但其中对决定A型超导体临界温度主要参量问题进行的分析,只适用于这样一些A型超导体,它们的收敛半径倒数Λ或者比λ₀小,或者虽比λ₀大、但λ又小于λ₀,其中λ₀是个依赖谱形状的参量,它的定义在正文中给出。对另一些A型超导体(λ₀<λ<Λ),决定T_c的主要参量不再是λ,而是δ=1/∧^0.5(ω_1/2/ω_log)^5.5λ^1.55。 关键词：     

Physical quantities for two coupled Hubbard planes and comparison with experiments on Bilayer high-Tc cuprates

For a bilayer model of the high-T_c cuprates consisting of two coupled Hubbard planes we calculate within the FLEX (fluctuation exchange) approximation the quasiparticle self-energies and dynamic spin susceptibilities. We consider a Fermi surface similar to those of the YBaCuO cuprates, an on-site Coulomb repulsion U = 6 t (where t is the intraplane hopping), and interplane hoppings t’ up to t’ = 0.7 t. For sufficiently large t'(?0.3 t) the spin susceptibility gC^? due to transitions between the bonding and antibonding states becomes much larger than the susceptibility gC⁺ for transitions within the same band. The spectral density Im gC^?(q, ω) exhibits a peak centered at the antiferromagnetic wavevector q = Q = (π, π) and at the antiparamagnon frequency ω = ω_sf. For decreasing temperature T and increasing t’ the peak becomes more and more pronounced. We calculate the Fermi lines of the bonding and antibonding bands for different t'. The superconducting transition temperature T_c for d_x ²?y²-wave pairing decreases as t' increases. Our results are in qualitative agreement with magnetic neutron scattering, magnetic resonance, and photoemission experiments on bilayer YBaCuO cuprates. They are also consistent with the results of quantum Monte Carlo simulations.     

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.